Heat and Temperature 2

8/8/2019 Heat and Temperature 2

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Heat Transfer

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Heat Transfer

heat exchanger fluids and the thermal conductivity of the heat exchanger tubes. (Uo)

is specific to the heat exchanger and the fluids that are used in the heat exchanger.

Where: = the rate heat of transfer (Btu/hr) (calorie/sec) = the overall heat transfer coefficient (Btu/hr -

-) (calorie /K)

= the overall cross-sectional area for heat transfer () (

= the overall temperature difference () (K)

Conduction

Conduction is the transfer of heat by the direct contact of particles of matter. It occurs

between two materials at different temperatures when they are touching each other.

Conduction can also occur through one material, if one part of the material is hotter

than another part. For example, a metal spoon placed in hot water quickly transmits

the heat to the hand.

Conduction heat transfer is the transfer of thermal energy by interactions between

adjacent atoms and molecules of a solid.

Conduction involves the transfer of heat by the interaction between adjacent

molecules of a material.Heat transfer by conduction is dependent upon the driving "force" of temperature

difference and the resistance to heat transfer. The resistance to heat transfer is

dependent upon the nature and dimensions of the heat transfer medium.

There are three different experimental models to study the heat transfer by conduction

Linear Heat Conduction. Radial Heat Conduction. Extended surface heat transfer.

How is thermal energy transferred?

In solids the molecules and atoms vibrate in place, in liquids they move over and

around each other, and in gases they shoot around.


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Heat Transfer

Collisions are occurring everywhere as atoms and molecules vibrate and zoom. All

these moving atoms and molecules have kinetic energy. When a warmer material

comes in contact with a cooler material. The atoms and molecules of the warmer

material are moving around faster than the atoms and molecules of the cooler

material. Where the two materials are in contact there are lots of collisions betweenthe atoms and molecules of each.

Conduction can take place in solids, liquids and gases. However, the more densely

packed atoms or molecules of a solid can conduct more heat because there are many

more collisions taking place. The low density of gases mean that relatively few

collisions take place per second therefore air is a poor conductor of heat. This

explains why many things we use to keep things warm or cold, such as foam, fiber

glass insulation and down jackets contain air pockets that slow down the transfer of

heat.

The rate of heat conduction through a medium in a specifieddirection (say, in the x-direction) is expressed by Fouriers law

of heat conduction for one-dimensional heat conduction as:

(W)

Where:

= heat flow vector (W).K = thermal conductivity, a thermodynamic property of the

material. (W/m K)A = Cross sectional area in direction of heat flow. (m2)

T = Gradient of temperature (K/m)

= T/x + T/y + T/z

Heat is conducted in the direction of decreasing temperature, and thus the

temperature gradient is negative when heat is conducted in the positive x-direction.

General Relation for Fouriers Law ofHeat Conduction

The heat flux vector at a point P on the surface of the

figure must be perpendicular to the surface, and it mustpoint in the direction of decreasing temperature.

If n is the normal of the isothermal surface at point P,

the rate of heat conduction at that point can be expressed

by Fouriers law as:

(W)


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Heat Transfer

Thermal conductivity in the building and manufacturing industries

Thermal conductivity is a measure of how well a material conducts heat. Although

solids in general are better conductors of heat than liquids or gases, each material

conducts heat at a different rate. We can compare thermal conductivities by

measuring how fast a certain amount of thermal energy flows through uniformly sized

pieces of various materials. Measuring the thermal conductivity of different materialsis important in the building and manufacturing industries. We dont want hot air to

leave our home when its cold outside and we also dont want hot air to enter our

home when its very hot outside. A home built with good insulators lessens heat

transfer in both directions.

Applications of conductiono Trapping air as insulation.o Different sensations from conductors and insulators.o Uses of good conductors: cooking utensils.o Uses of good insulators: table mats, handles.


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Heat Transfer


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Heat Transfer


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Heat Transfer

Convection

An energy transfer across a system boundary due to a temperature difference by the

combined mechanisms of intermolecular interactions and bulk transport. Convection

needs fluid matter.

The convection heat transfer mode is comprised of two mechanisms, random

molecular motion and energy transferred by bulk or macroscopic motion of the fluid.The convection heat transfer occurs when a cool fluid flows past the warm body. The

fluid adjacent to the body forms a thin slowed down region called the boundary layer.

The velocity of the fluid at the surface of the body is reduced to zero due to the

viscous action. Therefore, at this point, the heat is transferred only by conduction.

The moving fluid then carries the heat away. The temperature gradient at the surface

of the body depends on the rate at which the fluid carries the heat away.

Newtons law of cooling expresses the overall effect of convection:

= h A ( Where:

= heat flow from surface, a scalar (W)A = surface area from which convection is occurring (m2)

= wall (surface) temperature

= fluid temperature.h = convection heat transfer coefficient (which is not a thermodynamic property of thematerial, but may depend on geometry of surface, flow characteristics,

thermodynamic properties of the fluid, etc.) (W/m2K).

Convection heat transfer coefficient (h) strongly depends on the following fluid

properties:

By decreasing dynamic viscosity, convection heat transfer coefficient can beincreased.

By increasing thermal conduction, K convection heat transfer coefficient canbe increased.

By increasing specific heat, Cp convection heat transfer coefficient can beincreased.

By increasing fluid velocity, V convection heat transfer coefficient can beincreased.

Convection heat transfer coefficient also depends on: (Surface geometry, Surface

roughness, Type of fluid flow).

Types of convection heat transfer:

Convection heat transfer depends on how the fluid motion is initiated.(i) Natural or free convection:


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Heat Transfer

In natural convection, any fluid motion is caused by natural means such as the buoyan

effect, which manifests itself as the rise of warmer fluid and the fall of cooler fluid.

(ii) Forced convection:

In forced convection, the fluid is forced to flow over a surface or in a tube by external

means such as a pump, blower, or a fan.

Heat transfer by convection is more difficult to analyze than heat transfer by

conduction because no single property of the heat transfer medium, such as thermal

conductivity, can be defined to describe the mechanism. Heat transfer by convection

varies from situation to situation (upon the fluid flow conditions), and it is frequently

coupled with the mode of fluid flow. In practice, analysis of heat transfer by

convection is treated empirically (by direct observation).

Convection heat transfer is treated empirically because of the factors that affect the

stagnant film thickness: (Fluid velocity, Fluid viscosity, Heat flux, Surface roughness,

Type of flow (single-phase/two-phase)

Applications of convection:

oAir con is usually placed at the top of a room.

o Heating coil of a kettle is usually at the bottomo Formation of land and sea breezeso Pipes, tubes, or some similar cylindrical device.

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Heat Transfer

Radiation

Radiant heat transfer is thermal energy transferred by means of

electromagnetic Waves or particles.

Radiant heat transfer involves the transfer of heat by electromagnetic

radiation that arises due to the temperature of a body. Most energy of thistype is in the infra-red region of the Electromagnetic spectrum although

some of it is in the visible region. The term thermal radiation is frequently

used to distinguish this form of electromagnetic radiation from other forms,

such as radio waves, x-rays, or gamma rays. The transfer of heat from a fire

place across a room in the line of sight is an example of radiant heat transfer.

Radiant heat transfer does not need a medium, such as air or metal, to take

place. Any material that has a temperature above absolute zero gives off

some radiant energy. When a cloud covers the sun, both its heat and light

diminish. This is one of the most familiar examples of heat transfer by

thermal radiation.

Radiation Facts

Radiant heat transfer can be reduced or blocked by interveningmaterials .

Releases energy in all matter, above absolute zero. Hotter matterreleases more radiant energy than cooler matter.

Rough surfaces absorb radiant heat very well, therefore are easilyheated by radiation. Smooth polished surfaces are usually good reflectors that

do not hold heat efficiently. Objects that absorb well often emit heat well.

Objects that reflect heat well often emit heat poorly.

The rate at which heat is radiated by a body of surface area is strongly affectedby the distance between the radiator and the target area.

Electromagnetic radiation propagated as a result in a temperature difference istermed as thermal radiation.

An ideal radiator is called a black body. A black body emits energy at a rateproportional to the fourth power of the absolute temperature of the body and is

directly proportional to the surface area.

No object is a perfect black body, thus it cannot radiate all the heat given to it.To account for this gray nature we introduce a factor called emissivity. Anemissivity of one corresponds to a body which absorbs all radiations incident

upon it and reflects nothing. For solid and liquid surfaces, emissivity is

typically 0.8 0.2 for fire applications. For gases and flames, emissivity

depends on the thickness of the flame.

Black Body Radiation

A body that emits the maximum a mount of heat for its absolute temperature Is called

a black body. Radiant heat transfer rate from a black body to its surroundings can be

expressed by the following equation:


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Heat Transfer

Where:

=heat transfer rate (Btu/hr) = Stefan- Boltzmann constant (0.174Btu/hr-ft

2-Fo4) (=5.669 x 108 W/m2K4)

A=surface area(ft2)T= temperature(F)

Two black bodies that radiate toward each other have a net heat flux between them.

The net Flow rate of heat between them is given by an adaptation of Equation:

Where:

A=surface area(ft2)T1 =temperature of the first body (F)T2 =temperature of the second body(F)

All bodies above absolute zero temperature radiate some heat. The sun and earth both

radiate Heat toward each other. This seems to violate the Second Law of

Thermodynamics, which states that heat cannot flow from a cold body to a hot body.

The paradox is resolved by the fact that each body must be indirect line of sight of the

other to receive radiation from it. Therefore, When ever the cool body is radiating

heat to the hot body, the hot body must also be radiating Heat to the cool body. Since

the hot body radiates more heat (due to its higher temperature) than the cold body, the

net flow of heat is from hot to cold, and the second law is still satisfied.

Emissivity

Real objects do not radiate as much heat as a perfect black body. They radiate less

heat than a black body and are called gray bodies. To take into account the fact that

real objects are gray bodies, the last Equation is modified to be of the following form:

Where:

=emissivity of the gray body (dimensionless).

Emissivity is simply a factor by which we multiply the black body heat transfer to

take into Account that the black body is the ideal case .Emissivity is a dimension less

number and has a Maximum value of 1.0.

Radiation Configuration Factor

Radiative heat transfer rate between two gray bodies can be calculated by this

equation:


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Heat Transfer

Where:

= is the shape factor, which depends on the spatial arrangement of the two objects(Dimensionless)

= is the shape factor, which depends on the emissivity of the two objects(Dimensionless)

The two separate terms and can be combined and given the symbol f. The heatflow

Between two gray bodies can now be determined by the following equation:

The symbol (f) is a dimensionless factor some times called the radiation configuration

factor, which takes into account the emissivity of both bodies and their relative

geometry. The radiation Configuration factor is usually found in a text book for the

given situation. Once the Configuration factor is obtained, the over all net heat fluxcan be determined. Radiant heat flux should only be included in a problem when it is

greater than 20% of the problem.

Applications of radiation

o Teapots.o The greenhouse.o Color and texture of cloths.o Skin cancer.


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Heat Transfer

References:

- Heat Transfer Textbook. 3rd ed. Lienhard IV, John H. and Lienhard V, John H

phlogiston press 2002.

- Thermodynamics, Heat Transfer and Fluid Flow Fundamentals Handbook.

- ocw.mit.edu/courses/aeronautics-and-astronautics/16.../10_part3.pdf
http://www.4shared.com/document/xBcKwVu0/Thermodynamics_Heat_Transfer_a.htmhttp://www.4shared.com/document/xBcKwVu0/Thermodynamics_Heat_Transfer_a.htm

Heat and Temperature 2

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