Final Major Project Report

C.F.D Analysis of Heat Transfer in Solar Collector by Using Diamond Shaped Roughened Absorber Plate

Chapter-1

IntroductionIn general the new energy production and radiated by the sun, more specifically the term

refers to the sun’s energy that reaches the earth. Solar energy, received in the form of energy, such as heat and electricity, which can be utilized by man. Since the sun is expected to radiate at an essentially constant rate for a few billion years, it may be regarded as an in-exhaustible source of useful energy. The major drawbacks to the extensive application of solar energy are:

1. The intermittent and variable manner in which it arrives at the earth’s surface and2. The large area required to collect the energy at a useful rate.

Experiments are underway to use this energy for power production, house heating, air conditioning, cooking and high temperature melting of metals.

Energy is radiated by sun as electromagnetic waves of which 99 percent have wave lengths in the range of 0.2 to 4.0 micrometers (1 micrometer =10-6 meter). Solar energy reaching the top of the earth’s atmosphere consists of about 8 percent ultraviolet radiation (short wave length, less than 0.39 micrometer ), 46 percent infrared radiation (long wave length more than 0.78 micrometer).

Diagram Direct, diffuse and total radiation

A solar collector is a device for collecting solar radiation and transfers the energy to a fluid passing in contact with it. Utilization of solar energy requires solar collectors. These are general of two types:

(i) Non concentrating or flat plate type solar collector.(ii) Concentrating (focusing) type solar collector.

The solar energy collector, with its associated absorber, is the essential component of any of any system for the conversion of solar radiation energy into more usable form (e.g. heat or electricity). In the non-concentration type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e. the area absorbing the radiation). On the other hand, in concentrating collectors, the area intercepting the solar radiation is greater, sometimes hundreds of times greater than the absorber area. By means of concentrating collectors, much higher temperatures can be obtained than with the non-concentrating type. Concentrating collectors may be used to generate medium pressure steam. They use many be used to generate medium pressure steam. They use many be used to generate medium pressure steam. They use many different arrangements of mirrors pressure steam. They use many be different arrangements of mirrors and lenses to concentrating collectors may be used

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to generate medium pressure steam. They use many different arrangements of mirrors and lenses to concentrate the sun’s rays on the boiler. This type shows better efficiency than the flat plate type. For best efficiency, collectors should be mounted to face the sun as it moves through the sky.

Heat transfer enhancement is a subject of considerable interest to researchers as it leads to saving in energy and cost. Because of the rapid increase in energy demand in all over the world, both reducing energy lost related with ineffective use and enhancement of energy in the meaning of heat have become an increasingly significant task for design and operation engineers for many systems. In the past few decades numerous researches have been performed on heat transfer enhancement. These researches focused on finding a technique not only increasing heat transfer, but also achieving high efficiency. Achieving higher heat transfer rates through various enhancement techniques can result in substantial energy savings, more compact and less expensive equipment with higher thermal efficiency. Heat transfer enhancement technology has been improved and widely used in heat exchanger applications; such as refrigeration, automotive, process industry, chemical industry, etc. One of the widely-used heat transfer enhancement technique is inserting different shaped elements with different geometries in channel flow.

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Chaper-2

Literature ReviewThe use of artificial roughness in the form of repeated ribs has been found to be an efficient

method of enhancing the heat transfer to fluid flowing in the duct. Detailed information about

the heat transfer and flow characteristics in ribbed ducts is very important in designing Solar

air Heater Ducts, Heat Exchangers and cooling systems of gas turbine engines. The

application of artificial roughness in the form of fine wires and ribs of different shapes has

been recommended to enhance the heat transfer coefficient by several investigators. It has

been found that the main thermal resistance to the convective heat transfer is due to the

presence of laminar sub layer on the heat-transferring surface. The ribs break the laminar sub

layer and create local wall turbulence due to flow separation and reattachment between

consecutive ribs, which reduce the thermal resistance and greatly enhance the heat transfer.

However, the use of artificial roughness results in higher friction and hence higher pumping

power requirements. Therefore, it is desirable that the turbulence should be created in the

vicinity of the wall, i.e. only in the laminar sub-layer region, which is responsible for thermal

resistance. Hence, the efforts of researchers have been directed towards finding the roughness

shape and arrangement, which break the laminar sublayer, enhance the heat transfer

coefficient most with minimum pumping power penalty.

2.1. J.C. Han et al [1] investigated the developing heat transfer in rectangular

channels with rib turbulators for rib angle varying from 90° to 30°. The combined effects of

rib angle and channel aspect ratio on local heat transfer coefficient were studied. The results

indicate that the best heat transfer in square channel was obtained with angled ribs at 30- 45°

and was about 30% higher than the 90° transverse ribs for constant pumping power.

However, for rectangular channel with aspect ratio of 2 and 4, the heat transfer enhancement

using 30°-45° ribs was only 5% more than the 90° transverse rib. In general, it was noted that

in square channel the heat transfer increased with decrease in rib angle whereas in rectangular

channel the dependence of heat transfer on rib angle was negligible.

2.1.1. Y.M. Zhang et al [2] observed that deploying of groove in between the ribs

enhances the turbulences as well as reattaches the free shear layer nearer to the rib. They have

reported that the addition of grooves in between adjacent square ribs enhances the heat

transfer capability of the surface considerably with nearly same pressure drop penalty. It

appears that it will be fruitful to investigate an artificially roughened surface with optimally

chamfered rib combined with grooves present between two ribs in order to achieve further

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decrease in relative roughness pitch and enhancement of heat transfer rate from such a

surface. In view of the above an experimental

Investigation has been planned to investigate the heat and fluid flow characteristics of

artificially roughened surface with chamfered rib-grooved roughness.

2.1.2. Liou and Hwang et al [3] investigated the fully developed flow in channels

roughened with three rib shapes, namely square, semicircular and triangular cross section.

The results showed that the three types of rib channels had comparable thermal performance,

but the square-ribbed geometry is the most likely the one to yield hot spots behind the rib.

Gupta et al [4] carried out an experimental investigation on solar air heater with

angled ribs with circular cross-section. They have investigated the effect of relative

roughness height (e/D), inclination of rib with respect to flow direction and Reynolds number

on fluid flow characteristics in transitionally rough flow region and evaluated the thermo

hydraulic performance of solar air heaters.

2.1.3. R. Kamali , A.R. Binesh et al [5] investigated the flow over two-dimensional

ribs of different shapes is studied to examine the heat transfer characteristics as well as the

friction characteristics. The simulations were performed for four rib shapes, i.e., square,

triangular, trapezoidal with decreasing height in the flow direction, and trapezoidal with

increasing height in the flow direction. The recirculation zones were clearly identified and the

flow is seen to reattach before the following in all cases. It is found that features of the inter-

rib distribution of the heat transfer coefficient are strongly affected by the rib shape. For the

range of Reynolds number studied, the trapezoidal shaped rib with decreasing height in the

flow direction (case C) has the highest value of heat transfer, meanwhile, the trapezoidal

shaped rib with increasing height in the flow direction (case D) has the lowest friction factor.

Also the simulations were performed for various P/e ratios to investigate case C to highlight

the effect of the rib pitch. The P/e ratio 12 provides the highest enhancement factor among

the four pitch ratios investigated. However, they found that in the recirculating region just

behind the upstream rib, the heat transfer coefficients seem to be less sensitive to the rib

spacing.

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Fig 2.1 Friction factor ratios for different rib shape Fig. 2.2 Nusselt number

ratios for different rib at p/e = 12

at Shapes at P/e=12

2.1.4. Han, Chandra et al. [6] studied a square channel with two ribbed walls for five

different rib profiles.Their study illustrated that rib turbulators with greater number of sharp

corners yield increasingly higher heat transfer coefficient as well as pressure drop.

2.1.5. Sahu and Bhagoria [7] have investigated the effect of 90° broken wire ribs on

heat transfer coefficient of a solar air heater duct. A pitch of 20 mm gives the highest thermal

efficiency of 83.5% for e = 1.5 mm and reported heat transfer coefficient of roughened duct

improves 1.25–1.4 times compared to smooth duct under similar operating conditions at

higher Reynolds number.

2.1.6. Taslim et al. [8] studied the effects of turbulator profile and spacing on heat

transfer and friction in a channel with traverse ribs. It was concluded that the heat transfer

coefficient was higher for aspect ratios greater than unity but resulted in higher pressure loss.

The trapezoidal shaped ribs spaced properly were found to be effective in heat removal. The

optimum pitch to height ratio for the 90° square turbulator was found to be around 8. The

sensitivity of Nusselt number was found to decrease with decrease in the blockage ratio

(e/Dh).

2.1.7. Taslim et al. [9] experimentally investigated the heat transfer and friction in

channel roughened with angled V-shaped and discrete ribs on two opposite walls for

Reynolds number raging from 5,000 to 30,000. The results showed that the 90° transverse

ribs produced the lowest heat transfer performance. The 45° angled V-shaped ribs produced

the highest heat transfer performance in comparison to other rib configurations. For V-shaped

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ribs facing downstream of flow, the one with lowest blockage ratio had better heat removal

rate. The discrete ribs also produced better performance in comparison to the transverse ribs.

2.1.8. Ryu et al. [10] have studied numerically friction and heat transfer in the flow in

rib-roughened channels with one smooth wall .Reynolds-averaged Navier–Stokes equations,

coupled with the k–ω turbulence model with a special near-wall treatment, were solved by a

finite-volume method. The roughness elements cross-section was square, triangle, semicircle

and cosine wave. The roughness function was found to be a function of the rib shape and

pitch ratio but was independent of the absolute rib size.

2.1.9. Karwa et al. [14] experimented on integral chamfered rib roughness on the

heated wall and reported that the chamfer angle of 15° gives the maximum heat transfer.

Most of the investigations carried out so far have been with ducts of circular cross-section or

of rectangular section having two opposite roughened walls and with all the four walls

heated. It needs to be mentioned that for the application of this concept of enhancement of

heat transfer in the case of solar air heaters, roughness elements have to be considered only

on one broad wall, which is the only heated wall. This application makes the fluid flow and

heat transfer characteristics distinctly different from those found in the case of two roughened

walls and four heated wall ducts. In solar air heaters, only one wall of the rectangular air

passage is subjected to uniform heat flux (isolation) while the remaining three walls are

insulated. It has recently been proposed by several investigators that providing artificial

roughness on the absorber plate could substantially enhance the heat transfer capability of a

solar air heater.

2.1.10. Bhagoria et al. [15] used wedge shaped ribs to study enhancement of heat

transfer coefficient and they have shown experimentally that a maximum enhancement of

heat transfer occurs at a wedge angle of about 10° while on either side of this wedge angle,

Nusselt number decreases. The friction factor increases as the wedge angle increases.

2.1.11. Prasad and Mullick [21] recommended protruding wires on the underside of

the absorber plate of an unglazed solar air heater used for cereal grains drying to improve the

heat transfer characteristics and hence the plate efficiency factor

2.1.12. Prasad and Saini [22] developed the relations to calculate the average friction

factor and Stanton number for artificial roughness of absorber plate by small diameter

protrusion wire. They used these relations to compare the effect of height and pitch of

roughness element on heat transfer and friction factor with already available experimental

data. The friction factor for one side rough duct is determined by assuming that the total shear

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force in the one side rough duct is approximately equal to the combined shear force from

three smooth walls in a four-sided smooth duct and the shear force from one rough wall in a

four-sided rough duct. They used the friction similarity law and heat–momentum transfer

analogy

2.1.13. R.P. Saini, Jitendra Verma [25] investigated and concluded that heat transfer

can be enhanced considerably as a result of providing dimple-shape roughness geometry on

the absorber plate of a solar air heater duct. Nusselt number and friction factor are the strong

function of the system and operating parameters. The maximum value of Nusselt number has

been found corresponds to relative roughness height (e/D) of 0.0379 and relative pitch (p/e)

of 10. While minimum value of friction factor has been found correspond to relative

roughness height (e/D) of 0.0289 and relative pitch (p/e) of 10. It is therefore, roughness

parameters of the geometry can be selected by considering the net heat gain and

corresponding power required to propel air through the duct. Further different arrangement

mode of the dimple-shape artificial roughness on the absorbing plate may be investigated in

order to get the optimal arrangement mode of such artificial Roughness.

Fig 2.3. Comparison of Experimental values and predicted values

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Fig 2.4. Variation of Nusselt number with Reynolds number and for Fig. 2.5 Variation of Nusselt number with Reynolds number for different values of e/D and for fixed value of p/e. For different p/e and for fixed value of e/D.

2.1.14. Apurba layek , J.S. Saini, S.C. Solanki [29] studied and found that the

artificial roughness in the form of chamfered rib groove on the absorber plate results in

considerable enhancement of heat transfer. This enhancement is, however, accompanied by a

substantial increase in the friction factor. It is, therefore, desirable to select the roughness

geometry such that the heat transfer coefficient is maximized while keeping the friction

losses at the minimum possible value. Considering the heat transfer and friction

characteristics can fulfill this requirement of the collector simultaneously.

Fig.2.6 (a) Nusselt number and (b) friction factor as a function of Reynolds number for Smooth ducts.

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Chapter-3

Pyranometers A pyranometer is an instrument which measures total or global radiation over a

hemispherical field of view. If a shading ring is attached, the beam radiation is prevented from falling on the instrument sensor and in then measures only the diffuse component of the radiation. In most pyranometers, the sun’s radiation is allowed to fall on a black surface to which the hot junctions of a thermopile are attached. The cold junctions of the thermopile are located in such a way that they do not receive the radiation. As a result an e.m.f. proportional to the solar radiation is generated. The e.m.f. which is usually in the range of 0 to 10mV calibration of about ±2 percent can be obtained.There are following types of pyranometers: (i) Eppley pyranometer, (ii) Yellot solarimeter (photo-voltaic solar cell, (iii) Moll-Gorczyhski solarimeter, (iv) Bimetallic Actionographs of Rabitzsch type (v) Velochme pyranometer, (vi) Thermoelectric pyranometer etc.

First two types are described briefly in the following paragraphs.

3.1. Eppley Pyranometer It is based on the principle as stated above that the there is a difference between the temperature of black surface (which absorb most solar radiation) and white surface (which reflect most solar radiation). The detection of temperature difference is achieved by thermopile. It uses concentric silver rings 0.25 mm thick, appropriate coated black and white, with either 10 or 50 thermocouple junctions to detect temperature differences between coated rings. Later models use wedges arranged in a circular pattern, with alternate black and white coatings. The disks or wedges are enclosed in a hemispherical glass cover. Similar instruments are manufactured in Europe under the name Kipp. The Eppley pyranometers and similar instruments are calibrated in a horizontal position. Calibration of these instruments will vary to some degree if the instrument is inclined to measure radiation on other than a horizontal surface.

Fig.3.1. Pyranometer with alternate black and white sensor segments.10

3.2 Yellot Solarimeter (Photovoltaic solar cell) Pyranometers have also been used on photovoltaic (solar cell) detectors. Silicon cells are the most common for solar energy. Silicon solar cells have the property that their light current (approximately equal to the short circuit current at normal radiation levels) in a linear function of the incident solar radiation. They have the disadvantages that the spectral response is not linear, so instrument calibration is a function of the spectral distribution of the incident radiation.

Fig.3.2. Photovoltaic solar cell

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Chapter-4

Solar Energy The fundamental process now in general use for heat conversion is the green house effect. The name come from its first use in green houses, in which it is possible to grow exotic plants in cold climates through better utilization of the available sunlight.

Most of the energy we receive from the sun comes in the form of light, a shortwave radiation, not all of which is visible to the human eye. When this radiation strikes a solid or liquid, it is absorbed and transformed into heat energy; the material becomes warm and stores the heat, conducts it to surrounding materials (air water, other solids or liquids) or reradiates it to other materials of lower temperature. This reradiation is a long wave radiation.

Fig.4. shows how temperature on earth is affected by the ‘green house effect’. Visible sunlight is absorbed on the ground, at a

Fig.4.1. the green house effect radiated to Co2 content of atmosphere.

Temperature of 20ºC, for example emits infra-red light at wavelength of about 10μm, but CO2 in atmosphere absorbs light of that wavelength and back radiates part of it to earth. (CO2

does not absorb the incoming sunlight which has a shorter wavelength).

Hence the green house effect brings about an accumulation of energy of the ground.Glass easily transmits short-wave radiation, which means that it poses little interference

to incoming solar energy, but it is a very poor transmitter of long-wave radiation. Once the sun’s energy has passed through the glass windows and has been absorbed by some material inside, the heat will not be reradiated back outside. Glass therefore, act as a heat trap, a phenomenon which has been recognized for sometime in the construction of green houses, which can get quite warm on sunny days, even in the middle of winter ; this has come to be known in fact, as the ‘green house effect’. Solar collectors for home heating usually called flat plate collectors; almost have one or more glass covers, although various plastic and other transparent materials are often used instead of glass.

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In Fig.4.2. A black-painted plate absorbs the incoming sunlight. About it is fixed a plate of ordinary window glass. When the temperature of the black plate increases, its emits an increment of thermal heat in the form of infra-red light. The black absorber has the properties of a black body; ideal black bodies have not only the highest absorption rate but also the highest emission coefficient for all wavelengths of light. Emission increases with temperature, following T4 law. The re-emitted light if so progressively shorter wavelength and greater energy as the

Fig.4.2. Principle of green house effect. Temperature of the black body increases. This is expressed by Wien’s law, which may be

written as:

λ max .T = constant = 2989 μm Kelvin

T being the surface temperature of the black body and λ max the wave-length at which light emission reaches a maximum.

The sun emits radiation like a “black body” whose surface temperature is about 5700ºC, this corresponds to maximum emission of 0.5 μm. A black body at a room temperature emits radiation with a maximum at about 10 μm, which is within the spectrum of invisible of infra-red light. The ordinary glass plate fixed above the black plate in a green house has a spectral absorption which is relatively transparent for visible light is absorbent for the infra-red light

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emitted by the black plate when it evacuates its thermal energy. The infra-red light absorbed by the glass is remitted in all directions; half of it is emitted to the outside and lost, the other half re-emitted towards the black plate which absorbs it again. More and more heat is accumulated in the way in the black plate, whose temperature thus increases. Equilibrium is reached when the energy gain by absorption of visible light is exactly balanced by loss of energy through infra-red emission of the glass plate. With rising temperature, the wavelength of the infra-red emission becomes shorter. At 200ºC (473ºK) the maximum radiation is emitted at about 6 μm, compared with 10 μm at room temperature. Finally at about 500ºC (773ºK) the bulk of the radiation would be emitted at 4 μm , at which wavelength, glass is partially transparent for infra-red light.

It follows that an efficient green house effect is possible only below 500ºC. However, unless concentration of sunlight is combined with the green house effect, the equilibrium temperature achieved are much lower because, practice, the equilibrium temperature is further reduced by heat losses from the black plate due to thermal conductivity and air convection.

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4.1. Solar Cooking In our country energy consumed for cooking shares a major portion of the total energy consumed in a year. In villages 95% of the consumption goes only to cooking. Variety of fuel like coal, Kerosene, cooking gas, firewood, dung cakes and agricultural waste are used. The energy crisis is affecting everyone. It is affecting the fuel bills for those who use it for heating the houses and cooking their food. The poor of the developing countries who have been using dry wood, picked up from the fields and forests as domestic fuel in the rural areas. At present, firewood and cow dung cakes are the most important sources of fuel to cook food. Cow dung too precious to be allowed to be used for burning and cooking. It is very useful to improve the fertility of the soil; it should be used in proper way. The supply of wood is also fast depleting because of the indiscriminate felling of trees in the rural areas and the denudation of forests. There is a rapid deterioration in the supply of these fossil fuels like coal, kerosene or cooking gas. The solution for the above problem is the harnessing of solar energy for cooking purposes.

Thus solar cookers have a very relevant place in the present fuel consumption pattern. Various designs of solar cookers have been developed in our country. The first solar cooker was developed in the year 1945 by Mr. M.K.Ghosh of Jamshedpur a freedom fighter. He developed a box type solar cooker with a reflecting mirror and a copper coil inside, on which the food materials used to be placed in pots. Mr. Ghosh also designed a parabolic reflector which was used for sometime as a boiler of Neera (palm juice). Later in 1953 NPL of India developed a parabolic solar cooker. The main reason for non-acceptance of these devices was the cheap availability of cooking fuel during these days. The problem of harnessing and utilization of solar energy arise after the fuel crisis of the 1970s, which also affected the rural areas.

Basically there are three designs of solar cooker:(i) Flat plate box type solar cooker with or without reflector, (ii) Multi reflector type solar oven and,(iii) Parabolic disc concentrator type solar cooker.

Flat plate box type design is the simplest of all the designs. Maximum no load temperature with a single reflector reaches upto 160ºC. In multi reflector oven four square or triangular or rectangular reflectors are mounted on the oven body. They all reflect the solar radiations into the cooking zone in which cooking utensils are placed. Temperature obtained is of the order of 200ºC. The maximum temperature can reach to 250ºC, if the compound cone reflector system is used. With parabolic disc concentrator type solar cooker, temperatures of the order of 450ºC can be obtained in which solar radiations are concentrated onto a focal point.

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(a) Principle of box type cooker

(b) Reflector type solar cooker

(c) Principle of concentrating type cooker.

Fig. 4.1. Principle of operation of solar cookers.

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4.2 Design Principle and Constructional Details of a Box Type Solar Cooker.

The principle of operation of box type solar cooker is illustrated in Fig (8.1.). The solar rays penetrate through the glass covers and absorbed by a blackened metal tray kept inside the solar box. The solar radiations entering the box are of short wavelength. The higher wavelength radiation is not able to pass through the glass

cover , reradiation the glass cover. Two glass cover are provided to gain minimize

the heat loss. The loss due to convection is minimized by

Fig.4.2. Details of box type cooker.Making the box air tight by providing a rubber strip all round between the upper lid and the box. Insulating material like glass wool, paddy husk, saw dust or any other material is filled in space between blackened tray and outer cover of the box. These minimize heat loss due to conduction. With this type of cooker is placed in the sun, the blackened surface starts absorbing sunrays and temperature inside the box starts rising. The cooking pots, which are also blackened, are placed inside with food material, get heat energy and food will be cooked in a certain period of time depending upon the actual temperature attained inside. The temperature attained depends upon the intensity of solar radiation and material of insulation provided. The amount of solar radiation intensity can be increased by provided mirror or mirrors. The solar cooker is made up of inner and outer metal or wooden box with double glass sheet on it. Absorber tray (blackened tray) is painted black with suitable black paint like boiler interior paint. This paint should be dull in colour so that it can

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withstand the maximum temperature attained inside the cooker as well as water vapour coming out of the cooking utensils. The top cover contains two plain 20 mm distance between them. The entire top cover can be made tight with padlock hasp. Neoprene rubber sealing is provided around the contact surfaces of the glass cover and the cooker box. A small vent for vapour escape, in provided in the sealing. Collector area of the solar cooker is increased by providing a plane reflecting mirror equal to the size of the box, and hinged on one side of the glass frame. A mechanism (guide for adjusting mirror) is provided to adjust the reflector at different angles with the cooker box. A 15 to 25ºC rise in temperature is achieved inside the box when reflector is adjusted to reflect the sun rays into the box. In winter, when sun rays are mush inclined to horizontal surface, reflector is a most useful addition.

Overall dimensions of a typical model are height. This type of

cooker is termed as family solar cooker as it cooks sufficient dry food material for family of 5 to 7 people.

The temperature inside the solar cooker with a single reflector is maintained from 70 to 110ºC above the ambient temperature. This temperature is enough to cook food slowly, steadily and surely with delicious taste and preservation of nutrients. Maximum air temperature obtained inside the cooker box (without load) is 140ºC, in winter and 160ºC in summer. Depending upon the factors such as season and time of the day, type of the food and depth of the food layer, time of the cooking with this cooker ranges from 1 hr to 4 hrs. Meat should be allowed to stay for 3-4 hours. Vegetables take from ½ to 2½ hours. All types of Dals can cooked between 1½ to 2 hours. Rice is cooked between 30 minutes and 2 hours. The best of the day for cooking is between 11 am and 2 pm. Cooking is faster in summer than in winter due to high ambient temperature.Following are the some merits of solar cooker:(i) No attention is needed during cooking as in other devices.(ii) No fuel is required.(iii) Negligible maintenance cost.(iv)No pollution.(v) Vitamins of the food are not destroyed and food cooked is nutritive and delicious

with natural taste.(vi)No problem of charring of food and no over flowing.Limitations of solar cooker are:(i) One has to cook according to the sun shine; the menu has to be preplanned.(ii) One cannot cook at short notice and food cannot be cooked in the night or during

cloudy days.(iii) It takes comparatively more time.(iv) Chapaties are not cooked because high temperature for baking is required and

also needs manipulation at the time of baking.

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Chapter-5

Collectors A device used to collect, absorb, and transfer solar energy to a working fluid, such as

water or air. The solar heat can be used for heating water, to back up heating systems, or for heating swimming pools. The heart of a solar collector is the absorber, which is usually composed of several narrow metal strips. The carrier fluid for heat transfer flows through a heat-carrying pipe, which is connected to the absorber strip. In flat-plate absorbers, two sheets are sandwiched together allowing the medium to flow between the two sheets.

5.1 Types of solar collector include: (i) Flat-Plate Collectors(ii) Typical Liquid Collector(iii) Typical Air Collectors or Solar Air Heaters(iv) Non-porous absorber plate type collectors. (v) Collectors with porous absorbers. (vi) Concentrating Collector.

Types of concentrating collectors. (a) Focusing Type (b) Line Focusing Collectors: Parabolic Trough Reflector. (c) Mirror-Strip Reflector. (d) Fresnel lens Collector. (e) Receiver pipe. (f) Point Focusing Collector (Paraboloidal Type). (g) Concentrating Collectors: Non-Focusing Type. (h) Compound Parabolic Concentrator (CPC).

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5.1.1 Flat-Plate CollectorsWhere temperature below about 90ºC are adequate, as they are for space and service

water heating flat plate collectors, which are of the non-concentrating type, are particularly convenient. They are made in rectangular panels, from about 1.7 to 2.9sq. m, in area, and are relatively simple to construct and erect. Flat plates can collect and absorb both direct and diffuse solar radiation; they are consequently partially effective even on cloudy days when there is no direct radiation.

Flat-plate solar collectors may be divided into two main classifications based on the type of heat transfer fluid used.

Liquid heating collectors are used for heating water and non-freezing aqueous solutions and occasionally for non-aqueous heat transfer fluids. Air or gas heating collectors are employed as solar air heaters.

The principal difference between the two types is design of the passages for the heat for the transfer fluid.

The majority of the flat-plate collectors have five main components as follows:

(i) A transparent cover which may be one or more sheets of glass or radiation transmitting plastic film or sheet.

(ii) Tubes, fins, passages or channels are integral with the collector absorber plate or connected to it, which carry the water, air or other fluid.

(iii) The absorber plate, normally metallic or with a black, surface, although a wide variety of other materials can be used with air heaters.

(iv) Insulation, which should be provided at the back and sides to minimize the heat losses. Standard insulating materials such as fiber glass or styro-foam are used for the weather.

Advantages of Flat-plate Collector

(i) They have the advantages of using both beam and diffuse solar radiation.

(ii) They do not require orientation towards the sun.

(iii) They require little maintenance.

(iv) They are mechanically simpler than the concentrating reflectors, absorbing surface and orientation devices of focusing collectors.

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5.1.2 A Typical Liquid CollectorThere are many flat-plate collector designs, but most are based on the principle shown

in Fig.5.1.2. It is the plate and the tube type Collector.

Fig5.1.2. Selection through typical flat-plate collector.

It basically consists of a flat surface with high absorptivity for solar radiation, called the absorbing surface. Typically metal plates, usually of copper, steel or aluminum material with tubing of copper in thermal contact with the plates, are the most commonly used materials. The absorber plate is usually made from a metal sheet 1 to 2 mm in thickness, while the tubes, which are also of metal, and range in diameter from 1 to 1.5 cm. They are soldered, brazed or clamped to the bottom (in some cases, to the top) of the absorber plate with the pitch ranging from 5 to 15 cm. In some designs, the tubes are also in line and integral with the absorber plate. For the absorber plate corrugated galvanized sheet is a material widely available throughout the world, show two ways in which it has been used.

The use of conventional standard panel radiators shown in Fig. 5.2.1 (c) is one of the simplest practical applications. The methods of bonding and clamping tubes to flat or corrugated sheet are shown in Fig5.2.1 (d) and (e) while Fig.5.2.1 (f) is the “tube in strip” or roll bond design in which the tubes are formed in the sheet, ensuring a good thermal bond between the sheet and the tube.

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(a)

(b)

(c)

(d)

(e)

(f)22

Fig.5.1.2.Cross-section through collector plates.

Heat is transferred from the absorber plate to a point of use by circulation of fluid (usually water) across the solar heated surface. Thermal insulation of 5 to 10 cm thickness is usually placed behind the absorber plate to prevent the heat losses from the rear surface. Insulation a material is generally mineral wools or glass wool or fiberglass as stated above.

The front covers are generally glass (may be one or more) that is transparent to in-coming solar radiation and opaque to the infra-red rays. The glass covers act as convection shield to reduce the losses from the absorber plate beneath. Glass is generally used for the transparent covers but certain plastic films may be satisfactory. Glass is the most favorable material. Thickness of 3 and 4 mm are commonly used. The usual practices is to have 1or 2 covers with a specific ranging from 1.5 to 3 cm.

Advantages of second glass which is added above the first one are:

(i) Losses due to air convection are further reduced. This is important in windy areas. (ii) Radiation losses in the infra-red spectrum are reduced by a further 25%, because half of the

50% which is emitted outwards from the first glass plates is back radiated. It is not worthwhile to use more than two glass plates. This is due to fact that each plate reflects about 15% of the incoming sunlight.

As we know that main purpose of the transparent cover of the flat-plate collector is to decrease heat loss without significantly reducing the incoming solar radiation. In the first place, the relatively still (or stagnant) air space between the cover and the absorber plate largely prevents loss of heat from the plate by convection.

Furthermore, if the cover is made of glass, it permits the passage of solar radiations with wavelengths less than 2 micrometer (µm) but it is largely opaque to the longer wavelength thermal infra-red. As a result, heat is trapped in the air space between the cover and the absorber plates in a manner similar to green house. The effect is to reduce the loss of heat from the absorber. However, since the enclosed air is inevitably warmer than the ambient air, there is some loss of heat to the surroundings from the top of the cover by convection, conduction and radiation. The rate of heat loss increases as the temperature of the air space rises; as will be seen shortly, this affects the overall efficiency of the solar collector.

A certain proportion of the incident solar radiation is lost by absorption in the glass cover plates, but the loss can be kept small by using a clear (“water white”) glass with low iron content. A much larger loss occurs as a result of partial reflection. Two glass plates may reflect some 15 percent of solar radiation coming from a perpendicular direction. The reflection loss increases as the direction of incidence departs from the perpendicular. The reflection of glass covers may be reduced by coating with thin films of certain substances (e.g., magnesium fluoride) or by gentle etching with a solution of hydrofluoric acid. Such antireflective coatings add to the cost of the collectors but make them more efficient.

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Transparent plastics have been used in place of glass, but they have some drawbacks. Most plastics are not as opaque as glass to the thermal infra-red radiation and so permit greater loss of heat from the absorber. They also suffer a decrease in transparency and sometimes breakup in the course of time due to heating and the action of solar ultraviolet radiation. Efforts are being made to develop better plastic material that might be used in solar collectors.

For water streams the absorber plate can be any metal, plastic or rubber sheet that incorporates water channels, while for air systems the space above or below the collector plate serves as the conduit. The surface finish of the absorber plates may be a flat black paint with an appropriate primer. The primer coat should preferable be thin since a thick under coat of paint would increase the resistance to heat transfer. The primer should be of etching type. If the primer is not a self etching type, the repeated thermal expansion and contraction of the plate may cause the paint to peel after a year or so. Several types of backed on or chemical finishes are also available. Black painted absorbers are preferred because they are considerably cheaper. The coatings applied on absorbed plate are called “selective coatings” which reduces the amount of energy emitted by thermal infra-red radiation. These are poor emitter for longer wavelengths. A promising selecting coating is “black chrome” form of chromium metal, in a layer 0.15 to 2 µm thick, electrodeposited on a nickel base.

The liquid heated is generally water. However sometimes mixtures of water and ethylene glycol are used, if ambient temperatures below 0ºC are likely to be encountered.

Typical collector dimensions are 2m × 1m × 15cm.

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5.1.3 Typical Air Collectors or Solar Air HeatersFig. 5.1.3. Shows a schematic flat-plate collector where an air stream is heated by the

black side of the collector plate. Fins attached to the plate increase the contact surface. The back side of the collector

Fig.5.1.3. (i) A Typical Solar Air Collector.

is heavily insulated with minerals wool or some other material. The most favourable orientation of a collector, for heating only is facing due south at an inclination angle to the horizontal equal to the latitude plus 15º.

Air has been used so for to a lesser extent as the heat-transport medium in solar collectors, but it may have some advantages over water. To decrease the power required to pump the necessary volume of air through tubes, wider flow channels are used. For example, the air may be passed through a space between the absorber plate and insulator with baffles arranged to provide a long (zig-zag) flow path

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Fig.5.1.3. (ii) Zig-zag air flow path in flat-plate collector

The use of air as the heat-transport fluid eliminates both freezing and corrosion problems, and small air leaks are of less concern than water leaks. Moreover, the heated air can be used directly (or by way of heat storage) for space heating. On the other hand, larger duct sizes air than when water is heat transport medium. Another drawback is that transfer of heat from air to water supply system is inefficient.

But solar air heater has an important place among solar heat collectors. It can be used as subsystems in many systems meant for the utilization of solar energy. Possible applications of solar air heaters are drying or curing of agricultural products, space heating for comfort, regeneration of dehumidifying agents, seasoning of timber, curing of industrial products such as plastics.

Numerous variations is the design of collectors for heating air by solar energy are shown in Air can be passed in contact with black solar absorbing surface such as finned plates or ducts as mentioned above, corrugated or roughened plates of various materials, several layer of metal screening and overlapped glass plates. Flow may be straight through, serpentine, above or below or on both direction of the absorber plate, or through a porous absorber material.

Basically air heaters are classified in the following two categories.

1. The first type has a non-porous absorber in which the air stream does not flow through the absorber plate. Air may flow above and or behind the absorber plate, as shown in

2. The second type has a porous absorber that includes slit and expanded metal, transpired honey comb and over-lapped glass plate absorber.

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(a)

(b)

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(c)Fig.5.1.3.(iii) Non-porous type air heater

5.1.4 Non-porous absorber plate type collectors. A non-porous absorber may be cooled by the air stream flowing over both sides of the

plate as shown in most common design the air flows behind the absorbing surface. Air flow the cover plate and therefore is not recommended if the air inlet temperature rise at the collector are large, it is shown in

Transmission of the solar radiation through the transparent cover system and its absorption is identical to that of a liquid type flat-plate collector. To improve collection efficiency selective coating may be applies provided there is no much cost. Due to low heat transfer rates, efficiencies are lower than liquid solar heaters under the same radiation intensity and temperature conditions.

Performance of air heaters is improved by:

(a) Roughing the rear of the plate to promote turbulence and improve the convective heat transfer coefficient, or

(b) Adding fins to increase the heat transfer surface. Usually turbulence is also increased which enhances the convective heat transfer.

A solar collector with V-corrugated copper foil is illustrated in. Absorption of solar radiation is improved due to surface radioactive characteristics and the geometry of the corrugations, which help in trapping the reflected radiation.

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5.1.5 Collectors with porous absorbers.The main drawback of the non-porous absorber plate is the necessity of absorbing all

incoming radiation over the projected area from a thin layer over the surface, which is in the order of few microns. Unless selective coatings are used, radiation losses from the absorber plate are excessive; therefore the collection efficiency cannot be improved. The pressure drop along the duct formed between the absorber plate and the rear insulation may also be prohibitive especially in the case of added fins to increase the heat transfer and turbulence rate. The difficulty with turbulence is the pressure drop across the collector. Too many surfaces and too much restriction to air flow will require a larger fan and a larger amount of energy to push the air through. The energy required for this cancels out saving from using solar energy, particularly if fan is electrical and it the amount of energy which is burned at the power plant to produce the electrical energy is included.

These defects are eliminated in porous absorber type collectors in two ways.

(a). The solar radiation penetrates to greater depths and is absorbed gradually depending on the matrix density. The cool air stream introduced from the upper surface of the matrix is first heated by upper layers which are cooler than the bottom layers. The air stream warm up, while traversing the matrix layers. The lower matrix layers are hotter than the upper ones; therefore, the air stream can effectively transfer heat from the matrix. Improper selection of the

(a) Slit or expanded metal

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(b) Transpired Honey Comb

(c) Broken bottle absorber.

(d) Over-lapped glass plate air-heating collector.Fig.5.1.5. Sketches of porous absorber-type air heaters

matrix porosity and the thickness may result in reduced efficiencies since the additional matrix layers beyond on optimum may no longer absorb the solar radiation and heat the air stream further.

(b). The pressure drop for the matrix is usually lower than the non-porous absorber with flow behind the plate since flow per unit cross-section would be much lower. Although the matrix

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hinders the flow, the pressure drops reported for porous absorbers. The solar air heating utilizing a transpired honey comb is also very favourable from the pressure drop stand point since the flow cross-section is much larger.

Whillier has suggested a method of using crushed glass layers to absorb solar radiation and the heat the air. A porous bed, as shown in made by forming layers of broken bottles (bottom dark top clear glass), may be readily used for agricultural drying purposes with minimal expenditure.

The overlapped glass plate air heater as shown in can be considered as a form of porous matrix, although overall flow direction is along the absorber glass plates instead of being across the matrix. Plate and air stream temperature increase gradually along the collector length and across from top to bottom. Thus thermal losses could be significantly reduced. The pressure drop is also significantly less than the non-porous flat-plate absorber design.

5.1.6 Concentrating Collector.Concentrating collectors use mirrored surfaces to concentrate the sun's energy on an

absorber called a receiver. Concentrating collectors also achieve high temperatures, but unlike evacuated-tube collectors, they can do so only when direct sunlight is available. The mirrored surface focuses sunlight collected over a large area onto a smaller absorber area to achieve high temperatures. Some designs concentrate solar energy onto a focal point, while others concentrate the sun's rays along a thin line called the focal line. The receiver is located at the focal point or along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat. These collectors reach much higher temperatures than flat-plate collectors. However, concentrators can only focus direct solar radiation, with the result being that their performance is poor on hazy or cloudy days. Concentrators are most practical in areas of high insulation (exposure to the sun's rays), such as those close to the equator and in the desert southwest United States. Concentrators perform best when pointed directly at the sun. To do this, these systems use tracking mechanisms to move the collectors during the day to keep them focused on the sun. Single-axis trackers move east to west; dual-axis trackers move east and west and north and south (to follow the sun throughout the year). In addition to these mechanical trackers, there are passive trackers that use Freon to supply the movement. While not widely used, they do provide low-maintenance alternatives to mechanical systems. Concentrators are used mostly in commercial applications because they are expensive and because the trackers need frequent maintenance. Some residential solar energy systems use parabolic-trough concentrating systems. These installations can provide hot water, space heating, and water purification. Most residential systems use single-axis trackers, which are less expensive and simpler than dual-axis trackers.

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Fig. 5.1.6.A Concentrating Collectors

5.1.6 (A) Focusing Type. Focusing collector is a device to collect solar energy with high intensity of solar radiation

on the energy absorbing surface. Such collectors generally use optical system in the form of reflectors or refractors. A focusing collector is a special form of flat-plate (concentrator) between the solar radiation increases from low value of 1.5-2 to high values of the order of 10,000. In these collectors radiation falling on a relatively large area is focused on to a receiver (or absorber) of considerably smaller area. As a result of the energy concentration, fluids can be heated to temperatures of 500ºC or more.

An important difference between collectors of the non-focusing and focusing types in that the latter concentrate only direct radiation coming from a specific direction, since diffuse radiation arrives from all directions, only a very small proportion is form the direction for which focusing occurs. The optical system directs the solar radiation on to an absorber of smaller area which is usually surrounded by a transparent cover. Because of the optical system, certain losses (in addition to those which occur while radiation id transmitted through the cover) are introduced. These include reflection or absorption losses in the mirrors or lenses and losses due to geometrical imperfections in the optical system. The combined effect of all losses is indicating through the introduction of term called the optical efficiency. The introduction of more optical losses is compensated for by the fact that the flux incident on the absorber surface is concentrated on a smaller area. As a result, the thermal loss terms do not dominate to the same extent as in a flat-plate collector and the collection efficiency is usually higher.

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5.1.6 (B) Line Focusing Collectors: Parabolic Trough Reflector. The principle of the parabolic trough collector, which is often used in concentration

collectors, is shown by the cross-section in, solar radiation coming from the particular direction is collected over the area

Fig.5.1.6. (B) 1.Cross-section of parabolic-trough collector.

Of the reflection surface and is concentrated at the focus of the parabola, if the reflector is in the form of a trough with parabolic cross-section, the solar radiation is focused alone a line. Mostly cylindrical parabolic concentrators are used, in which absorber is placed along focus axis. The collector pipe, preferably with a selective absorber coating, is used as an absorber. The dimension of parabolic trough or parabolic cylindrical collector can be vary over a wide range the length of a reflector unit may be roughly 3 to 5m, and the width about 1.5 to 2.4 m, Ten or more such units are often connected end to end in a row, several rows may also be connected in parallel. Parabolic trough reflectors have been made of highly polished aluminum, of silvered glass or of a thin film of aluminized plastic on a firm base. Instead of having a continuous form, the reflector may be constructed from a number of long flat strips on a parabolic base.For the solar radiation to be brought to a focus by parabolic trough reflector, the sun must be in such a direction that it lies on the

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Fig.5.1.6. (B) 2. A typical cylindrical parabolic system

plane passing through the focal line and the vertex ( the base) of the parabola. Since the

elevation of the sun is always changing, either the reflector trough or the collector pipe (absorber) must be turn continuously about its long axis to maintain the required orientation. Both schemes are used in different practical-designs. The trough/cylindrical reflector or the pipe is turned by partial rotation around a single axis parallel to the east-west or north-south directions. For the east-west orientation, the collector are laid flat on (or parallel, to) the ground. For the north-south orientation, however, the north end of the trough is raised so the collectors are sloped facing south just like flat-plate collectors. Ideally, the slope angle should be changed periodically; it is simpler, but less efficient, however to used a fixed angle design.

The north-south orientation permits more solar energy to be collected than the east-west arrangement, except around the winter equinox. On the other hand, construction costs are higher for the north-south (sloping) type. Moreover, a system of such collectors requires a larger land area to allow for the shadowing effect of the sloping troughs. The increased separation distance between rows of collectors also results in increased pipe line costs and greater pumping and thermal losses. Finally the sun set position of an east-west reflector is essentially the same as the sunrise position and little or no ever night adjustment is required. For the north-south orientation, however, the trough (or receiver) must be turned through a larger angle from sunset to sunrise. The choice of orientation in any particular instance depends on the foregoing and other considerations.

5.1.6 (C) Mirror-Strip Reflector.

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In another kind of focusing collector, a number of plane or slightly curved (concave) mirror stripe are mounted on a flat base. The angles of the individual mirrors are such that they reflect solar radiation from a specific direction on to the same focal line. The angles of the mirrors must be adjusted to

Fig.5.1.6. (C) Mirror-strip solar collector.

Allow for changes in the sun’s elevation, while the focal line (for collector pipe) remains in a fixed position. Alternatively, as mentioned for parabolic trough collectors, the mirror strips may be fixed and the collector pipe moved continuously so as to remain on the focal line.

5.1.6 (D) Fresnel lens Collector. In addition to the reflecting collectors described above, a refraction type of focusing

collectors has been developed. It utilizes the focusing effect of a Fresnel lens, as represented in cross-section in Fig. 3.7.4. For a trough-type collector, the lens is

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Fig 5.1.6. (D).1. Cross-section of Fresnel lens through collector.

rectangle, about 4.7 m in overall length and 0.95 m in width. It is mad in sections from cost acrylic plastic and can probably be produced in quantity at low cost. The rounded triangular trough serves only as a container and plays no role in concentrating the solar energy.

To be fully effective, the Fresnel lens must be continuously aligned with the sun in two directions namely, both along and perpendicular to its length. This is achieved by orienting the troughs in the north-south direction with rotation about the length wise axis; in addition, the north ends of the troughs are raised to increase the slope as the sun’s elevation decreases (and vice versa). The total solar radiation energy that can be collected annually is about 30 percent greater than for an east-west orientation.

In a Fresnel lens collector, the solar radiation is focused into the absorber from the top, rather than from the bottom as in the parabolic (reflection) type. A modified absorber design is then possible.

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Fig.5.1.6. (D).2. Receiver for Fresnel lens collector.

Insulation at the bottom and sides of the absorber pipe and a flat-plate over the top reduce thermal losses. A stainless steel reflector adjacent to the pipe (absorber or receiver) reflects back emitted thermal radiation.

5.1.6 (E) Point Focusing Collector (Paraboloidal Type). A paraboloidal dish collector brings solar radiation to a focus at a point actually a small

central volume. A dish 6.6 m in diameter has been made from about 200 curved mirror segments forming a Paraboloidal surface. The absorber, located at the focus, is a cavity made

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of a zirconium-copper alloy with a black chrome selective coating. The heat-transport fluid flows into and out of the absorber cavity through pipes bonded to the interior. The dish can be turned automatically about two axes (up-down and left-right) so that the sum is always kept in a line with the focus and the base (vertex) of the Paraboloidal dish. Thus, the sun can be fully tracked at essentially all times.

The concentration ratios (concentration ratio is the ratio of the area of the concentrator aperture to the energy absorbing area of the receiver, it determines the effectiveness of a concentrator), are very high in the case of parabolic system and therefore can be used where high temperatures are required. In a cylindrical parabolic system, the concentration ratio is

lower than paraboloid counter-parts. In both the cases, the receiver is placed at the focus along the focal line in cylindrical parabolic or parabolic trough system and the focus point in Paraboloidal system.

Fig.5.1.6. (E).1. Point focus solar collector (Paraboloid)

Concentration ratios of about 30 to 100 or higher would be needed to achieve temperatures in the range 300 to 500ºC or higher. Collectors designed for such high concentration ratio necessarily have small angles of field of view and hence need to track the sun continuously. A broad classification of such collector is:

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(i) The linear focus collector in the form of a parabolic through or the ones employing faceted mirror strips.

(ii) Spherical and conical mirror (Axicon) with aberrated foci. The physical upper limit to the concentration ratios achievable with paraboloids and parabolic troughs is determined

by their ratios (focal length/diameter) and are about 10,000 and 100 respectively for

the two cases. The concentration ratios achieved in practice are about of the above

values because of surface irregularities of the reflector, tracking errors etc.

(iii) Central receiver collector, such as the Paraboloidal mirror and the tower power plant using heliostat mirrors.

A system equivalent to a very large Paraboloidal reflector consists of a considerable number of mirrors distributed over an area on the ground. Each mirror, called a heliostat, can be steered independently about two axes so that the reflected solar radiation is always directed towards an absorber mounted on a tower (Fig.3.7.8). This type of collector is classified as Central Receiver Collector. This is mostly used in tower power plant for generation of electrical energy.

Fig. 5.1.6.(E).1. Distributed heliostat point-focusing reflector.

In the typical central receiver, the mirror is composed of many small mirrors; each with its own heliostat to follow the sun. The heliostats are generally located in the horizontal plane, but when the situation is favourable, can simply follow the existing terrain. The basic

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difference between a single mirror concentrator and the heliostat system is that the heliostat system has a dilute mirror. This means that the entire surface within the system is not covered with mirror surface. This diluteness is generally termed as the fill factor. A central receiver with a fill factor of 40% of the land area is covered by the mirrors.

In a central receiver optical system as shown in figure many small mirrors are separately mounted to act together like a dilute paraboloid. The basic problem associated with the central receiver is that the heliostat mirrors require non-linear drive rate in two co-ordinates to achieve the requirement of keeping the reflected image point on a fixed receiver. Along with the problem is the requirement that the heliostat be rugged enough to survive storm and operate successfully in a moderate wind.

Among all the steerable concentrators mentioned above paraboloids have the highest efficiency in terms of the utilization of the reflector area because in a fully steerable paraboloid there are no losses due to aperture effects. Also radiation losses are small because of the small area of the absorber at the focus. Both then they are the most difficult to fabricate and operate too. A practical size for apertures area would be about 50 m2 from which 15 to 20 kW of useful energy could be extracted by thermal conversion processes.

5.1.6 (F) Concentrating Collectors: Non-Focusing Type. The simplest type of concentrating collector is the mirror-boosted, flat plate collector. It

consists of a flat plate facing south with mirrors attached to its north and south edge. If the mirrors are set at the proper angle, they reflect solar radiation on to the absorber plate. Thus,

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the latter receives reflected radiation in addition to that normally falling on it. The mirrors cut off part of the scattered radiation that would otherwise have reaches the absorber plate and only part of the scattered radiation falling on the mirrors will be reflected onto the absorber.

Fig.5.1.6. (F). Flat-plate collector augmented with mirror

Thus the concentration effects arise mainly from the increase in direct radiation reaching the absorber plate.

When a number of collectors are combined in two or more rows, as they often are, the rows must be set further apart in the north-south directions to allow for the additional sun shading caused by the mirror extensions. Furthermore, in order for the mirror to be effective, the angles should be adjusted continuously as the sun’s attitude changes. For these reasons, and they can provide only a relatively small increase in the solar radiation falling on the absorber, flat-plate collectors with mirrors are not widely used.

5.1.6 (G) Compound Parabolic Concentrator (CPC). The CPC (or Winston Collector) is a trough-like arrangement of the two facing parabolic

mirrors. Unlike the single parabolic trough reflector described earlier, the CPC is non-

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focusing, but solar radiation form many directions are reflected toward the bottom of the trough.

Fig5.1.6.(G). Compound Parabolic Concentrator.

Because of this characteristic; a larger proportion of the solar radiation, including diffuse (scattered) radiation, entering the trough opening is collected (and concentrated) on a small area. In addition to collecting both direct and diffuse radiations, an advantage of the CPC is that it provides moderately good concentration, although less than a focusing collector, in an east-west direction without (or only seasonal) adjustment for sun tracking.

It is possible to concentrate solar radiation by a factor of 10 without diurnal tracking, using this type of collector.

CPC reflectors can be designed for any absorber shapes: For example:

(a) Flat one side absorber,

(b) Flat two sided absorbers (fin),

(c) Wedge-like absorbers, or

(d) Tubular absorbers.

For economic as well as for thermal reasons the fin and the tubular type of absorbers are preferable. With a concentric tubular absorber with an evacuated jacket, temperatures of about 200ºC are achievable with Winston collectors. They are suitable for the temperature range of 100-150ºC even if the absorber is not surrounded by a vacuum. It is claimed that Winston Collectors are capable of competitive performance at high temperatures of about 300ºC required for power generation, if they are used with selectively coated, vacuum enclosed receivers which decrease thermal losses from the collector. The advantages of this new type are:

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(i) There is no need for tracking as it has high acceptance angle only seasonal adjustments

are required. For concentration ratios of , even seasonal adjustments may not be

required.

(ii) The efficiency for accepting diffuse radiation is much larger than conventional concentrators, and

(iii) Its concentration ratio is equal to the maximum value possible for a given acceptance angle.

The parabolic trough or a linear parabolic collector is also more commonly known as the cylindrical parabolic collector. It has many commercial ratios are available with Paraboloidal system; it is of the order of 10,000.

5.2 Receiver pipe.

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The receiver pipe of a parabolic line focusing collector, shown in cross-section in, has the same general

Fig.4.3.1. Cross-section of solar energy pipe receiver

characteristics as a flat-plate collector. The solar radiation absorber is a central steel pipe with a treated surface. A selective absorber surface, such as the black chrome referred to earlier, may be advantageous. A hollow steel plug within the absorber pipe restricts the flow of the heat-transfer fluid to a narrow annular region. This results in high flow velocity of the fluid and consequently a high rate of heat transfer from the absorber.

The absorber pipe is usually enclosed in a glass (Pyrex) jacket in order to decrease thermal losses by convection and radiation. The space between the pipe and the jacket is sometimes evacuated to reduce convection losses. The diameter of the glass jacket may be about 5cm and that of absorber pipe about 3 cm. The annuls between this pipe and the plug may be as little as 2.5 mm wide.

5.3 Advantages and Disadvantages of Concentrating Collectors over Flat-plate Type Collectors

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5.3.1 Advantages. The main advantages of concentrator systems over flat-plate type collectors are:1. Reflecting surface required less material and are structurally simpler than flat-plate

collectors. For a concentrator system the cost per unit area of solar collecting surface is therefore potentially less than that for flat-plate collectors.

2. The absorber area of a concentrator system is smaller than that of a flat-plate system for same solar energy collection and therefore the insulation intensity is greater.

3. Because of the area from which heat is lost to the surroundings per unit of solar energy collecting area is less than that for flat-plate collector and because the insulation on the absorber is more concentrated, the working fluid can attain higher temperatures in a concentrating system than in a flat-plate collector of the same solar energy collecting surface.

4. Owing to the small area of absorber per unit of solar energy collecting area, selective surface treatment and/or vacuum insulation to reduce heat losses and improve collector efficiency are economically feasible.

5. Focusing or concentrating systems can be used for electric power generation when not used for heating or cooling. The total useful operating time per year can therefore be larger for a concentrator system than for a flat-plate collector and the initial installation cost of the system can be regained by saving in energy in shorter period of time.

6. Because the temperature attainable with concentrating collector system is higher, the amount of heat which can be stored per unit volume is larger and consequently the heat storage costs are less for concentrator systems than for flat-plate collectors.

7. In solar heating and cooling applications, the higher temperature of the working fluid attainable with a concentrating system makes it possible to attain higher efficiencies, in the cooling cycle and lower cost for air conditioning with concentrator systems than with flat-plate collectors.

8. Little or no anti-freeze is required to protect the absorber in a concentrator system whereas the entire solar energy collection surface requires anti-freeze protection in a flat-plate collector.

5.3.2 Disadvantages

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1. Out of the beam and diffuse solar radiation components, only beam component is collected in case of focusing collectors because diffuse component cannot be reflected and is thus lost.

2. In some stationary reflecting systems it is necessary to have a small absorber to track the sun image; in others the reflector may have to be adjustable more than one position if year round operation is desired; in other words costly orienting systems have to be used to track the sun.

3. Additional requirements of maintenance particular to retain the quality of reflecting surface against dirt, weather, oxidation etc.

4. Non-uniform flux on the absorber whereas flux in flat-plate collectors is uniform.5. Additional optical losses such as reflectance loss and the intercept loss, so they

introduce additional factors in energy balances.6. High initial cost.

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5.4 Heat Transport System.

The heat generated in the absorber is removed by continuous flow of heat-transport (or heat transfer) medium, either water or air. It is mainly in the design of the heat transfer system that plate collectors differ. When water is used, it is most commonly passed through metal tubes with either circular or rectangular cross-section; the tubes are welded to the absorber plate (or from integral part of it) so as to assure effective heat transfer of heat to the fluid. Some examples are already represented. The tubes are connected to common headers at each end of the collector. In order to maximize the exposure to solar radiation, collectors are almost invariably sloped. Cooler water then enters at the bottom header, flows upward through the tubes where then enters at the bottom header, flows upward through the tubes where it is warmed by the absorber, and leaves by way of the top header. Fig.5.4.

In one simple type of flat-plate collector, the absorber is a blackened sheet with close corrugations running from top to bottom through the grooves formed by the corrugations. A problem with this design is that in cold weather, moisture may condense on the inside of the transparent cover plate and thus decrease the transmission of solar radiation.

Fig.5.4.Water Flow in flat-plate collectorWater is very effective heat-transport medium, but it suffers from certain drawbacks, one

is possibility of freezing in the collector tubes in cold climates during cold nights. As stated earlier ethylene glycol is added to prevent freezing, but this generally adds to the complexity of the heating system. Furthermore, the antifreeze solution is less effective than water for heat removed from the absorber. In some cases, the water is drained from the collector tubes if freezing is expected, but difficulties have been experienced in refilling all the tubes in the morning.

Another problem arises from corrosion of the metal tubes by the water; this is aggravated if the water is drained at night thus allowing air to enter. The oxygen in air increases the rate of corrosion of most metals. Corrosion can be minimized by using copper tubing. Aluminum is less expensive alternative, although periodic chemical treatment of water is desirable. Finally, leaks in water (or anti freeze) circulation system require immediate attention.

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Chapter-6

FLUENTFLUENT is a state-of-the-art computer program for modeling fluid flow and heat transfer in complex geometries. FLUENT uses unstructured meshes in order to reduce the amount of time you spend generating meshes, simplify the geometry modeling and mesh generation process, model more-complex geometries than you can handle with conventional, multi-block structured meshes, and let you adapt the mesh to resolve the flow-field features. FLUENT provides complete mesh flexibility, solving your flow problems with unstructured meshes that can be generated about complex geometries with relative ease. Supported mesh types include 2D triangular/quadrilateral, 3D tetrahedral/ hexahedral/pyramid/wedge, and mixed (hybrid) meshes. FLUENT also allows you to refine or coarsen your grid based on the flow solution. FLUENT is written in the C computer language and makes full use of the flexibility and power offered by the language. Consequently, true dynamic memory allocation, efficient data structures, and flexible solver control are all made possible. In addition, FLUENT uses a client/server architecture, which allows it to run as separate simultaneous processes on client desktop workstations and powerful computer servers, for efficient execution, interactive control, and complete flexibility of machine or operating system type. All functions required to compute a solution and display the results are accessible in FLUENT through an interactive, menu-driven interface. The user interface is written in a language called Scheme, a dialect of LISP. The advanced user can customize and enhance the interface by writing menu macros and functions.

Fig.6. Schematic diagram of fluent

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You can create your geometry and grid using GAMBIT.Use TGrid to generate a triangular, tetrahedral, or hybrid volume mesh from an existing boundary mesh (created by GAMBIT or a third party CAD/CAE package). It is also possible to create grids for FLUENT using ANSYS (Swanson Analysis Systems, Inc.), CGNS (CFD general notation system), or I-DEAS (SDRC); or MSC/ARIES, MSC/PATRAN, or MSC/NASTRAN (all from MacNeal-Schwendler Corporation). Interfaces to other CAD/CAE packages may be made available in the future, based on requirements. Once a grid has been read into FLUENT, all remaining operations are performed within the solver. These include setting boundary conditions, defining fluid properties, executing the solution, refining the grid, and viewing and post processing the results. Note that preBFC and GeoMesh are the names of fluent preprocessors that were used before the introduction of GAMBIT.

The FLUENT solver has the following modeling capabilities:1. 2D planar, 2D axisymmetric, 2D axisymmetric with swirl (rotationally symmetric), and 3D flows.2. Quadrilateral, triangular, hexahedral (brick), tetrahedral, prism (wedge), pyramid, and mixed element meshes.3. Steady-state or transient flows.4. Incompressible or compressible flows, including all speed regimes (low subsonic, transonic, supersonic, and hypersonic flows).5. Inviscid, laminar, and turbulent flows.6. Newtonian or non-Newtonian flows.7. Heat transfer, including forced, natural, and mixed convection, conjugate (solid/fluid) heat transfer, and radiation.8. Chemical species mixing and reaction, including homogeneous and heterogeneous combustion models and surface deposition/reaction models.9. Lumped parameter models for fans, pumps, radiators, and heat exchangers.

49

6.1 Steps in Solving Heat Transfer ProblemsThe procedure for setting up a heat transfer problem is described below. (Note that this procedure includes only those steps necessary for the heat transfer model itself; you will need to set up other models, boundary conditions, etc. as usual.)

6.1.1. To activate the calculation of heat transfer, enable the Energy Equation option inThe Energy panel

6.1.2. (Optional, pressure-based solver only.) If you are modeling viscous flow and you want to include the viscous heating terms in the energy equation, enable the Viscous Heating option in the Viscous Model panel.

the viscous heating terms in the energy equation are (by default) ignored by FLUENT when the pressure-based solver is used. (They are always included for the density-based solver.) Viscous dissipation should be enabled when the shear stress in the fluid is large (e.g., in lubrication problems) and/or in high-velocity, compressible flows

6.1.3. Define thermal boundary conditions at flow inlets, flow outlets, and walls.

At flow inlets and exits we will have to set the temperature; at walls we have to use any of the following thermal conditions:-- Specified heat flux-- Specified temperature-- Convective heat transfer-- External radiation-- combined external radiation and external convective heat transfer

50

6.2. Determining Turbulence ParametersWhen the flow enters the domain at an inlet, outlet, or far field boundary, FLUENT requires specification of transported turbulence quantities. This section describes which quantities are needed for specific turbulence models and how they must be specified. It also provides guidelines for the most appropriate way of determining the inflow boundary values.

6.2.1. Uniform Specification of Turbulence QuantitiesIn some situations, it is appropriate to specify a uniform value of the turbulence quantity at the boundary where inflow occurs. Examples are fluid entering a duct, far-field boundaries, or even fully-developed duct flows where accurate profiles of turbulence quantities are unknown.

In most turbulent flows, higher levels of turbulence are generated within shear layers than enter the domain at flow boundaries, making the result of the calculation relatively insensitive to the inflow boundary values. Nevertheless, caution must be used to ensure that boundary values are not so unphysical as to contaminate your solution or impede convergence. This is particularly true of external flows where unphysical large values of effective viscosity in the free stream can swamp the boundary layers.

We can use the turbulence specification methods described above to enter uniform constant values instead of profiles. Alternatively, we can specify the turbulence quantities in terms of more convenient quantities such as turbulence intensity, turbulent viscosity ratio, hydraulic diameter, and turbulence length scale. These quantities are discussed further in the following sections.

6.2.2. Turbulence IntensityThe turbulence intensity, I, is defined as the ratio of the root-mean-square of the velocity fluctuations, u’ to the mean flow velocity, uavg.A turbulence intensity of 1% or less is generally considered low and turbulence intensities greater than 10% are considered high. Ideally, you will have a good estimate of the turbulence intensity at the inlet boundary from external, measured data for internal flows; the turbulence intensity at the inlets is totally dependent on the upstream history of the flow. If the flow upstream is under-developed and undisturbed, we can use low turbulence intensity. If the flow is fully developed, the turbulence intensity may be as high as a few percent. The turbulence intensity at the core of a fully-developed duct flow can be estimated from the following formula derived from an empirical correlation for pipe flows

6.2.3. Turbulent Intensity

Turbulent Intensity

For fully-developed internal flows, choose the Intensity and Hydraulic Diameter specification method and specify the hydraulic diameter L = DH in the Hydraulic Diameter field.

51

6.3 Setting Boundary Conditions6.3.1 Velocity Inlet Boundary Conditions

Velocity inlet boundary conditions are used to define the flow velocity, along with all other relevant scalar properties of the flow, at the flow inlets. The total (or stagnation) properties of the flow are not fixed, so they will rise to whatever value necessary to provide the required velocity distribution. This type of boundary condition at inlet is intended to be used in incompressible flow. It requires the specification of velocity magnitude and direction, the velocity components, or the velocity magnitude normal to the boundary. In this case the velocity normal to boundary specification method was used. There are several ways in which the code allows the definition of the turbulence parameters for turbulent calculations. The method of specifying the turbulent intensity and hydraulic diameter was used for turbulence modeling purposes.

6.3.2 Defining the Velocity(i) The procedure for defining the inflow velocity is as follows:(ii) Choose which method you will use to specify the flow direction by selecting

Magnitude and Direction, Components, or Magnitude, Normal to Boundary in the Velocity Specification Method drop-down list.

(iii) If the cell zone adjacent to the velocity inlet is moving, you can choose to specify relative or absolute velocities by selecting Relative to Adjacent Cell Zone or Absolute in the Reference Frame drop-down list. If the adjacent cell zone is not moving, absolute and Relative to Adjacent Cell Zone will be equivalent, so you need not visit the list.

(iv) If you are going to set the velocity magnitude and direction or the velocity components, and your geometry is 3D, you will next choose the coordinate system in which you will define the vector or velocity components. Choose Cartesian (X, Y, Z), Cylindrical (Radial, Tangential, Axial), or Local Cylindrical (Radial, Tangential, Axial) in the Coordinate System drop-down list. .

(v) Set the appropriate velocity parameters, as described below for each specification method.

6.4 Defining the TemperatureFor calculations in which the energy equation is being solved, you will set the static temperature of the flow at the velocity inlet boundary in the Temperature field.

6.5 Defining Turbulence ParametersFor turbulent calculations, there are several ways in which you can define the turbulence parameters as discussed in section 4.4.

52

6.6 Outflow Boundary ConditionsOutflow boundary conditions in FLUENT are used to model flow exits where the details of the flow velocity and pressure are not known prior to solution of the flow problem. We do not define any conditions at outflow boundaries (unless you are modeling radiative heat transfer, a discrete phase of particles, or split mass flow): FLUENT extrapolates the required information from the interior. Outflow boundary condition is obeyed in fully-developed flows where the diffusion fluxes for all flow variables in the exit direction are zero. However, you may also define outflow boundaries at physical boundaries where the flow is not fully developed and we can do so with confidence if the assumption of a zero diffusion flux at the exit is expected to have a small impact on your flow solution. The appropriate placement of an outflow boundary is described by example below.

Fig.6.6.Defining Outflow Boundary condition

53

6.7 Wall Boundary ConditionsWall boundary conditions are used to bound fluid and solid regions. In viscous flows, the no-slip boundary condition is enforced at walls by default, but you can specify a tangential velocity component in terms of the translational or rotational motion of the wall boundary, or model a slip wall by specifying shear

6.8 Thermal Boundary Conditions at WallsWhen you are solving the energy equation, you need to define thermal boundary conditions at wall boundaries. Five types of thermal conditions are available:

(i) Fixed heat flux(ii) Fixed temperature(iii) Convective heat transfer(iv) External radiation heat transfer(v) Combined external radiation and convection heat transfer

6.9 Heat Flux Boundary ConditionsFor a fixed heat flux condition, choose the Heat Flux option under Thermal Conditions. We will then need to set the appropriate value for the heat flux at the wall surface in the Heat Flux field. We can define an adiabatic wall by setting a zero heat flux condition. This is the default condition for all walls.

6.10 Temperature Boundary ConditionsTo select the fixed temperature condition, choose the Temperature option under Thermal Conditions in the Wall panel. You will need to specify the temperature at the wall surface (Temperature).

54

Chapter-7

7.1 Assumptions

(i) Steady Flow (ii) Incompressible flow(iii) Two-dimensional flow.(iv)Constant thermo physical properties of the fluid. (v) Neglecting conduction resistance of the heated absorber plate.(vi)Neglecting viscous dissipation in the energy equation as significance of viscous

dissipation only for flows at high velocities.

7.2 Properties of Materials(i) Fluid - Air(ii) Density - 1.225 kg/m3(iii) Specific Heat Cp - 1006.43 J/KgK (iv)Thermal Conductivity K - 0.0242 w/mK (v) Dynamic Viscosity µ - 1.7894e-5

55

Chapter-8

Details of Experimental Set-upThe experimental set up is an indoor open flow loop that consist of a test duct with

entrance and exit sections, a blower, control valve, orifice plate and various devices for measurement of temperature and fluid head. The test section is of length 1500 mm (33.75 Dh

). The entry and exit lengths are 177 mm (2.5√WH) and 354 mm (5√WH) respectively (ASHRAE, 1997). The tests section of carries the roughened absorber plate at the top. The exit section of 354 mm length is used after the test section in order to reduce the end effect in the test section. In exit section after 130 mm three equally spaced baffles are provided in 87 mm length for the purpose of mixing the hot air coming out of solar air duct to obtain a uniform temperature of air at the outlet. The outside of the entire set-up, from the inlet to the orifice plate is insulated with 25 mm thick thermocol. The heated plate is 1 mm thick G.I. sheet having W-shaped rib glued on its rear side by epoxy resin and this forms the top broad wall of the duct. The mass flow rate of air is measured by means of a orifice meter connected with an inclined manometer and the floe is controlled by the control valves provided in the lines. Calibrated copper-constantan thermocouples were used to measure the air and the heated plate temperatures at different locations. A digital micro-voltmeter is used to indicate the output of the thermocouples. The optimum value of p/e id reported to be 10 (Han et al., 1978; Momin et al., 2002) have reported an optimum rib angle of 45º to 60º. The schematic of the experimental set up is shown in fig. 1, Fig. 2 a and b show details of roughened plate.

1. Inlet section 6. G.I. pipe2. Test section 7. Inclined manometer3. Mixing section 8. Orificemeter4. Exit section 9. Control valve5. Transition section 10. Blower

Fig. 1 Schematic diagram of experimental setup56

Experimental ProcedureBefore starting all components of setup, the instruments have been checked for proper

operation. The blower is then switched on and joints have been checked for leakage. Flow control valve is adjusted to give a predetermined rate of airflow to the test section after switching on the blower. Under steady state conditions the test runs to collect relevant heat transfer data were conducted. For each rib configuration 7 runs have been conducted at air-flow rates corresponding to the flow change of flow rate, the system was allowed to attain steady state before the data were recorded. Table 1 gives range of parameters for investigation. The following parameters were measured during the experiments;

1. Pressure drop across the orifice plate2. Inlet air temperature of collectors3. Outlet air temperature of collectors4. Temperature of plate.

ResultsValidity test: Friction factor and Nusselt number determined from the experimental

dta on a smooth duct were compared with those obtained from the modified Dittus-Boelter (Sadik et al., 1987) eqn. (2) for Nusselt number.

Modified Blasius equation.

s-0.25 (1)

Modified Dittus-Boelter equation

0.8 0.4 (2Ray / De )-0.2 (2)

Where 2Ray / De = (1.156 + H/W – 1)/ (H/W) for rectangular channel.Where Fs is friction factor for smooth duct, Re is Reynolds number, Nus is Nusselt number for smooth duct, Pr is Prandtl number, H is duct height (m) and W is duct width (m). Comparison of experimental and predicted values of Nusselt number and friction factor is shown in Fig. 3 and Fig. 4 respectively. It is seen that the smooth plate data for friction factor and Nusselt number agree reasonably good agreement with predicted values the validity of the experimental results is ensured.

The variation of Stanton number with Reynolds number for W-down and W-up ribs is shown in Fig. 5. It reveals that W-up ribs for the entire range of Reynolds number studied indicating clearly the effect of parameters investigated. The results are in broad agreement with previous investigations on V-shaped ribs (Karwa, 2003).

57

ConclusionOn the basis of this investigation on heat transfer characteristics in solar air duct

having on absorber plate, following conclusions have been drawn: Roughened solar air heater having W-shaped ribs pointing downstream to the flow. The maximum enhancement in Stanton number is 2.39 for W-down and 2.21 for W-up ribs respectively over smooth plate.

58

Chapter-9

Formula Used1. Hydraulic Diameter ( Dh ):

2. Reynolds Number Re :

3. Temperature Difference ( ΔT) :

4. Heat Transfer Coefficient ( h ) : As we know that,

5. Nusselt Number For Roughned Duct ( Nu ):

6. Nusselt Number For Smooth duct ( Nuo) :This relation can be given by Dittus–Boelter correlation

7. Prandtl Number ( Pr) :

8.Friction Factor For Roughned Duct ( f ) :

9. Friction Factor For Smooth Duct ( fo ) :It can be expressed by the Blasius formula:

10.Thermal Performance :

Thermal Performance

59

SAMPLE CALCULATIONS

Sample calculations for Square ribs with Re = 5000 with duct size of 50× 50 mm2 .

11.Hydraulic Diameter ( Dh ):

12.Velocity ( V ) :

13.Temperature Difference ( ΔT) :

14.Heat Transfer Coefficient ( h ) :

15.Nusselt Number For Roughned Duct ( Nu ):

60

16.Prandtl Number ( Pr) :

17.Nusselt Number For Smooth duct ( Nuo) :

18.Friction Factor For Roughned Duct ( f ) :

19.Friction Factor For Smooth Duct ( fo ) :

20.Thermal Performance :

Thermal Performance

3.01

21.Turbulent Intensity :

61

Turbulent Intensity

Nuo for Re = 5000 = 19 Nuo for Re = 6000 = 21.52

Nuo for Re = 7000 = 24.34 Nuo for Re = 8000 = 27.09

Nuo for Re = 9000 = 29.78 Nuo for Re = 10000 = 32.39

fo for Re = 5000 = 0.00940 fo for Re = 6000 = 0.00898

fo for Re = 7000 = 0.00864 fo for Re = 8000 = 0.00836

fo for Re = 9000 = 0.00812 fo for Re = 10000 = 0.00791

Turbulent Intensity for Re = 5000 = 5.5

Turbulent Intensity for Re = 6000 = 5.39

Turbulent Intensity for Re = 7000 = 5.27

Turbulent Intensity for Re = 8000 = 5.2

Turbulent Intensity for Re = 9000 = 5.12

Turbulent Intensity for Re = 10000 = 5.2

20 mm Pitch

62

22. Calculating Average Temperture from Tempeartrure contour

plot

63

From the temperature contour plot take two readings at different location at the outlet.

Let 1st reading = x1

And 2nd reading = x2

Then, Avg. Outlet temperature =

Chapter-10Observation Table

64

65

Fig 10.1. COMPARISON OF EXPERIMENTAL AND PREDICTED VALUE OF NUSSELT

66

NUMBER vs. REYNOLD NUMBER (SMOOTH PLATE)

Reynolds no.(Re) Experimental Values of

Nusselt no. (Nu)

3000 11.42

3500 15.4

4000 21.7

4500 30.85

5000 37.61

6000 44.6

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Fig.10.2. COMPARISON OF EXPERIMENTAL AND PREDICTED VALUE OF NUSSELT

NUMBER vs. REYNOLD NUMBER (ROUGH PLATE)

Reynolds no.(Re) Experimental Values of

Nusselt no. (Nu)

3000 13.1

3500 20.83

4000 34.42

4500 47.11

5000 61.03

6000 82.51

68

Fig10.3. VARIATION OF THERMAL EFFICIENCY WITH REYNOLDS NUMBER FOR DIFFERENT VALUES OF e/d

Chapter-11Reattachment Point for 15mm pitch

69

Reynolds no.

(Re)

Thermal efficiency in %

Smooth Plate Rough Plate

(e/D=0.0225)

Rough Plate

(e/D=0.0315)

3000 44.3 45.54 46.68

3500 50.3 52.3 55.3

4000 60.83 64.33 68.5

4500 63.33 69.33 72.97

5000 67.03 74 79.3

6000 72 80.3 85.66

70

71

72

73

Reattachment Point for 20mm pitch

74

75

76

Chapter-11

77

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