WHAT IS THERMAL SYSTEMS - WordPress.com Chapter 1. What Is Thermal Systems Engineering? Coal Air Condensate Cooling water Ash Stack Steam generator Condenser Generator Cooling tower

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Introduction…

The objective of this chapter is to introduce you to thermal systems engineeringusing several contemporary applications. Our discussions use certain terms thatwe assume are familiar from your background in physics and chemistry. Theroles of thermodynamics, fluid mechanics, and heat transfer in thermal systemsengineering and their relationship to one another also are described. Thepresentation concludes with tips on the effective use of the book.

Getting StartedThermal systems engineering is concerned with how energy is utilized to accomplish bene-ficial functions in industry, transportation, and the home, and also the role energy plays inthe study of human, animal, and plant life. In industry, thermal systems are found in electricpower generating plants, chemical processing plants, and in manufacturing facilities. Ourtransportation needs are met by various types of engines, power converters, and cooling equip-ment. In the home, appliances such as ovens, refrigerators, and furnaces represent thermalsystems. Ice rinks, snow-making machines, and other recreational uses involve thermal sys-tems. In living things, the respiratory and circulatory systems are thermal systems, as areequipment for life support and surgical procedures.

Thermal systems involve the storage, transfer, and conversion of energy. Energy can bestored within a system in different forms, such as kinetic energy and gravitational potentialenergy. Energy also can be stored within the matter making up the system. Energy can betransferred between a system and its surroundings by work, heat transfer, and the flow ofhot or cold streams of matter. Energy also can be converted from one form to another. Forexample, energy stored in the chemical bonds of fuels can be converted to electrical or me-chanical power in fuel cells and internal combustion engines.

The sunflowers shown on the cover of this book can be thought of as thermal systems.Solar energy aids the production of chemical substances within the plant required for life(photosynthesis). Plants also draw in water and nutrients through their root system. Plantsinteract with their environments in other ways as well.

Selected areas of application that involve the engineering of thermal systems are listedin Fig. 1.1, along with six specific illustrations. The turbojet engine, jet ski, and electricalpower plant represent thermal systems involving conversion of energy in fossil fuels toachieve a desired outcome. Components of these systems also involve work and heat trans-fer. For life support on the International Space Station, solar energy is converted to electricalenergy and provides energy for plant growth experimentation and other purposes. Semi-conductor manufacturing processes such as high temperature annealing of silicon wafersinvolve energy conversion and significant heat transfer effects. The human cardiovascular

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chapter objective

WHAT IS THERMAL SYSTEMSENGINEERING?1

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2 Chapter 1. What Is Thermal Systems Engineering?

Coal Air

CondensateCooling water

Ash

Stack

Steam generator

Condenser

GeneratorCoolingtower

Electricpower

Electrical power plant

Combustiongas cleanup

TurbineSteam

Turbojet engine

Solar-cell arrays

Compressor Turbine

Air in Hot gasesout

CombustorFuel in

Thorax

Quartz-tube furnace

Lung

Heart

Surfaces with thermalcontrol coatings

International Space Station

Jet ski water =-pump propulsion

Human cardiovascular system

Wafer boat

High-temperature annealing of silicon wafers

3.5 in. diameteroutlet jet

30°25 in.2 inlet area

Figure 1.1 Selected areas of applications for thermal systems engineering.

Prime movers: internal-combustion engines, turbinesFluid machinery: pumps, compressorsFossil- and nuclear-fueled power stationsAlternative energy systems

Fuel cellsSolar heating, cooling and power generation

Heating, ventilating, and air-conditioning equipmentBiomedical applications

Life support and surgical equipmentArtificial organs

Air and water pollution control equipmentAerodynamics: airplanes, automobiles, buildingsPipe flow: distribution networks, chemical plantsCooling of electronic equipmentMaterials processing: metals, plastics, semiconductorsManufacturing: machining, joining, laser cuttingThermal control of spacecraft

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1.2 Thermal System Case Studies 3

system is a complex combination of fluid flow and heat transfer components that regulatesthe flow of blood and air to within the relatively narrow range of conditions required tomaintain life.

In the next section, three case studies are discussed that bring out important features ofthermal systems engineering. The case studies also suggest the breadth of this field.

Thermal System Case StudiesThree cases are now considered to provide you with background for your study of thermalsystems engineering. In each case, the message is the same: Thermal systems typically con-sist of a combination of components that function together as a whole. The componentsthemselves and the overall system can be analyzed using principles drawn from three dis-ciplines: thermodynamics, fluid mechanics, and heat transfer. The nature of an analysisdepends on what needs to be understood to evaluate system performance or to design orupgrade a system. Engineers who perform such work need to learn thermal systems prin-ciples and how they are applied in different situations.

1.2.1 Domestic Hot Water Supply

The installation that provides hot water for your shower is an everyday example of a ther-mal system. As illustrated schematically in Fig. 1.2a, a typical system includes:

• a water supply

• a hot-water heater

• hot-water and cold-water delivery pipes

• a faucet and a shower head

The function of the system is to deliver a water stream with the desired flow rate and tem-perature.

Clearly the temperature of the water changes from when it enters your house until itexits the shower head. Cold water enters from the supply pipe with a pressure greater thanthe atmosphere, at low velocity and an elevation below ground level. Water exits the showerhead at atmospheric pressure, with higher velocity and elevation, and it is comfortably hot.The increase in temperature from inlet to outlet depends on energy added to the water byheating elements (electrical or gas) in the hot water heater. The energy added can be eval-uated using principles from thermodynamics and heat transfer. The relationships amongthe values of pressure, velocity, and elevation are affected by the pipe sizes, pipe lengths,and the types of fittings used. Such relationships can be evaluated using fluid mechanicsprinciples.

Water heaters are designed to achieve appropriate heat transfer characteristics so that theenergy supplied is transferred to the water in the tank rather than lost to the surrounding air.The hot water also must be maintained at the desired temperature, ready to be used on de-mand. Accordingly, appropriate insulation on the tank is required to reduce energy losses tothe surroundings. Also required is a thermostat to call for further heating when necessary.When there are long lengths of pipe between the hot water heater and the shower head, italso may be advantageous to insulate the pipes.

The flow from the supply pipe to the shower head involves several fluid mechanics prin-ciples. The pipe diameter must be sized to provide the proper flow rate—too small a diam-eter and there will not be enough water for a comfortable shower; too large a diameter andthe material costs will be too high. The flow rate also depends on the length of the pipes and

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4 Chapter 1. What Is Thermal Systems Engineering?

the number of valves, elbows, and other fittings required. As shown in Fig. 1.2b, the faucetand the shower head must be designed to provide the desired flow rate while mixing hot andcold water appropriately.

From this example we see some important ideas relating to the analysis and design ofthermal systems. The everyday system that delivers hot water for your shower is composedof various components. Yet their individual features and the way they work together as awhole involve a broad spectrum of thermodynamics, fluid mechanics, and heat transfer prin-ciples.

1.2.2 Hybrid Electric Vehicle

Automobile manufacturers are producing hybrid cars that utilize two or more sources ofpower within a single vehicle to achieve fuel economy up to 60–70 miles per gallon.Illustrated in Fig. 1.3a is a hybrid electric vehicle (HEV) that combines a gasoline-fueledengine with a set of batteries that power an electric motor. The gasoline engine and the elec-tric motor are each connected to the transmission and are capable of running the car bythemselves or in combination depending on which is more effective in powering the vehicle.What makes this type of hybrid particularly fuel efficient is the inclusion of several featuresin the design:

• the ability to recover energy during braking and to store it in the electric batteries,

• the ability to shut off the gasoline engine when stopped in traffic and meet powerneeds by the battery alone,

• special design to reduce aerodynamic drag and the use of tires that have very lowrolling resistance (friction), and

• the use of lightweight composite materials such as carbon fiber and the increased useof lightweight metals such as aluminum and magnesium.

Figure 1.2 Home hot water supply. (a) Overview. (b) Faucet and shower head.

Diverter valve

Hot

Cold

Waterheater

Cold watersupply line

Shower head

Shower head

To showerhead

Coldwater

Valvestem

To tubspout

Tub spout

Hotwater

Hot waterfaucet

Cold waterfaucet

(a) (b)

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1.2 Thermal System Case Studies 5

The energy source for such hybrid vehicles is gasoline burned in the engine. Because ofthe ability to store energy in the batteries and use that energy to run the electric motor, thegasoline engine does not have to operate continuously. Some HEVs use only the electricmotor to accelerate from rest up to about 15 miles per hour, and then switch to the gasolineengine. A specially designed transmission provides the optimal power split between the gaso-line engine and the electric motor to keep the fuel use to a minimum and still provide theneeded power.

Most HEVs use regenerative braking, as shown in Fig. 1.3b. In conventional cars, step-ping on the brakes to slow down or stop dissipates the kinetic energy of motion throughthe frictional action of the brake. Starting again requires fuel to re-establish the kineticenergy of the vehicle. The hybrid car allows some of the kinetic energy to be convertedduring braking to electricity that is stored in the batteries. This is accomplished by theelectric motor serving as a generator during the braking process. The net result is asignificant improvement in fuel economy and the ability to use a smaller-sized gasolineengine than would be possible to achieve comparable performance in a conventionalvehicle.

The overall energy notions considered thus far are important aspects of thermodynam-ics, which deals with energy conversion, energy accounting, and the limitations on how en-ergy is converted from one form to another. In addition, there are numerous examples offluid mechanics and heat transfer applications in a hybrid vehicle. Within the engine, air,

Figure 1.3 Hybrid electric vehicle combining gasoline-fueled engine, storage batteries, andelectric motor. (Illustrations by George Retseck.)

Generator

Inverter

Gasoline engine

BatteriesElectric motor

(a) Overview of the vehicle showing key thermal systems

(b) Regenerative braking mode with energy flow from wheels to battery

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fuel, engine coolant, and oil are circulated through passageways, hoses, ducts, and mani-folds. These must be designed to ensure that adequate flow is obtained. The fuel pump andwater pump also must be designed to achieve the desired fluid flows. Heat transfer princi-ples guide the design of the cooling system, the braking system, the lubrication system, andnumerous other aspects of the vehicle. Coolant circulating through passageways in the engineblock must absorb energy transferred from hot combustion gases to the cylinder surfaces sothose surfaces do not become too hot. Engine oil and other viscous fluids in the transmis-sion and braking systems also can reach high temperatures and thus must be carefullymanaged.

Hybrid electric vehicles provide examples of complex thermal systems. As in the case ofhot water systems, the principles of thermodynamics, fluid mechanics, and heat transfer ap-ply to the analysis and design of individual parts, components, and to the entire vehicle.

1.2.3 Microelectronics Manufacturing: Soldering Printed-Circuit Boards

Printed-circuit boards (PCBs) found in computers, cell phones, and many other products, arecomposed of integrated circuits and electronic devices mounted on epoxy-filled fiberglassboards. The boards have been metallized to provide interconnections, as illustrated inFig. 1.4a. The pins of the integrated circuits and electronic devices are fitted into holes, anda droplet of powdered solder and flux in paste form is applied to the pin-pad region, Fig. 1.4b.To achieve reliable mechanical and electrical connections, the PCB is heated in an oven toa temperature above the solder melting temperature; this is known as the reflow process. The

(b)

Integrated circuit (IC)

Pin lead

Metal filmPre-form solder paste

(a) (d)

(c)

Figure 1.4 Soldering printed-circuit boards (a) with pre-form solder paste applied to integratedcircuit pins and terminal pads (b) enter the solder-reflow oven (c) on a conveyor and are heated tothe solder melting temperature by impinging hot air jets (d ).

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1.3 Analysis of Thermal Systems 7

PCB and its components must be gradually and uniformly heated to avoid inducing thermalstresses and localized overheating. The PCB is then cooled to near-room temperature forsubsequent safe handling.

The PCB prepared for soldering is placed on a conveyor belt and enters the first zonewithin the solder reflow oven, Fig. 1.4c. In passing through this zone, the temperature of thePCB is increased by exposure to hot air jets heated by electrical resistance elements, Fig. 1.4d.In the final zone of the oven, the PCB passes through a cooling section where its tempera-ture is reduced by exposure to air that has been cooled by passing through a water-cooledheat exchanger.

From the foregoing discussion, we recognize that there are many aspects of this manu-facturing process that involve electric power, flow of fluids, air-handling equipment, heattransfer, and thermal aspects of material behavior. In thermal systems engineering, we per-form analyses on systems such as the solder-reflow oven to evaluate system performance orto design or upgrade the system. For example, suppose you were the operations manager ofa factory concerned with providing electrical power and chilled water for an oven that a ven-dor claims will meet your requirements. What information would you ask of the vendor? Or,suppose you were the oven designer seeking to maximize the production of PCBs. You mightbe interested in determining what air flow patterns and heating element arrangements wouldallow the fastest flow of product through the oven while maintaining necessary uniformityof heating. How would you approach obtaining such information? Through your study ofthermodynamics, fluid mechanics, and heat transfer you will learn how to deal with ques-tions such as these.

Analysis of Thermal SystemsIn this section, we introduce the basic laws that govern the analysis of thermal systems ofall kinds, including the three cases considered in Sec. 1.2. We also consider further the rolesof thermodynamics, fluid mechanics, and heat transfer in thermal systems engineering andtheir relationship to one another.

Important engineering functions are to design and analyze things intended to meet humanneeds. Engineering design is a decision-making process in which principles drawn fromengineering and other fields such as economics and statistics are applied to devise a system,system component, or process. Fundamental elements of design include establishingobjectives, analysis, synthesis, construction, testing, and evaluation.

Engineering analysis frequently aims at developing an engineering model to obtain asimplified mathematical representation of system behavior that is sufficiently faithful toreality, even if some aspects exhibited by the actual system are not considered. For ex-ample, idealizations often used in mechanics to simplify an analysis include the assump-tions of point masses, frictionless pulleys, and rigid beams. Satisfactory modeling takesexperience and is a part of the art of engineering. Engineering analysis is featured in thisbook.

The first step in analysis is the identification of the system and how it interacts with itssurroundings. Attention then turns to the pertinent physical laws and relationships that allowsystem behavior to be described. Analysis of thermal systems uses, directly or indirectly, oneor more of four basic laws:

• Conservation of mass

• Conservation of energy

• Conservation of momentum

• Second law of thermodynamics

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In your earlier studies in physics and chemistry, you were introduced to these laws. Inthis book, we place the laws in forms especially well suited for use in thermal systemsengineering and help you learn how to apply them.

1.3.1 The Three Thermal Science Disciplines

As we have observed, thermal systems engineering typically requires the use of three ther-mal science disciplines: thermodynamics, fluid mechanics, and heat transfer. Figure 1.5 showsthe roles of these disciplines in thermal system engineering and their relationship to oneanother. Associated with each discipline is a list of principles featured in the part of the bookdevoted to that discipline.

Thermodynamics provides the foundation for analysis of thermal systems through the con-servation of mass and conservation of energy principles, the second law of thermodynamics,and property relations. Fluid mechanics and heat transfer provide additional concepts, in-cluding the empirical laws necessary to specify, for instance, material choices, componentsizing, and fluid medium characteristics. For example, thermodynamic analysis can tell youthe final temperature of a hot workpiece quenched in an oil, but the rate at which it will coolis predicted using a heat transfer analysis.

Fluid mechanics is concerned with the behavior of fluids at rest or in motion. As shownin Fig. 1.5, two fundamentals that play central roles in our discussion of fluid mechanics arethe conservation of momentum principle that stems from Newton’s second law of motion andthe mechanical energy equation. Principles of fluid mechanics allow the study of fluids flowinginside pipes (internal flows) and over surfaces (external flows) with consideration of frictional

Thermal Systems EngineeringAnalysis directed to

DesignOperations/MaintenanceMarketing/SalesCosting•••

Conservation of massConservation of energySecond law of thermodynamicsProperties

Thermodynamics

Fluid MechanicsFluid staticsConservation of momentumMechanical energy equationSimilitude and modeling

Heat TransferConductionConvectionRadiationMultiple Modes

Fluids

Heat transfer

Thermo

Figure 1.5 The disciplines of thermodynamics, fluid mechanics, and heat transfer involvefundamentals and principles essential for the practice of thermal systems engineering.

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1.4 How to Use This Book Effectively 9

effects and lift/drag forces. The concept of similitude is used extensively in scaling measure-ments on laboratory-sized models to full-scale systems.

Heat transfer is concerned with energy transfer as a consequence of a temperature dif-ference. As shown in Fig. 1.5, there are three modes of heat transfer. Conduction refersto heat transfer through a medium across which a temperature difference exists. Convectionrefers to heat transfer between a surface and a moving or still fluid having a differenttemperature. The third mode of heat transfer is termed thermal radiation and representsthe net exchange of energy between surfaces at different temperatures by electromagneticwaves independent of any intervening medium. For these modes, the heat transfer ratesdepend on the transport properties of substances, geometrical parameters, and tempera-tures. Many applications involve more than one of these modes; this is called multimodeheat transfer.

Returning again to Fig. 1.5, in the thermal systems engineering box we have identifiedsome application areas involving analysis. Earlier we mentioned that design requires analy-sis. Engineers also perform analysis for many other reasons, as for example in the operationof systems and determining when systems require maintenance. Because of the complexityof many thermal systems, engineers who provide marketing and sales services need analy-sis skills to determine whether their product will meet a customer’s specifications. As engi-neers, we are always challenged to optimize the use of financial resources, which frequentlyrequires costing analyses to justify our recommendations.

1.3.2 The Practice of Thermal Systems Engineering

Seldom do practical applications involve only one aspect of the three thermal sciences disci-plines. Practicing engineers usually are required to combine the basic concepts, laws, and prin-ciples. Accordingly, as you proceed through this text, you should recognize that thermodynamics,fluid mechanics, and heat transfer provide powerful analysis tools that are complementary.Thermal systems engineering is interdisciplinary in nature, not only for this reason, but becauseof ties to other important issues such as controls, manufacturing, vibration, and materials thatare likely to be present in real-world situations.

Thermal systems engineering not only has played an important role in the developmentof a wide range of products and services that touch our lives daily, it also has become anenabling technology for evolving fields such as nanotechnology, biotechnology, food pro-cessing, health services, and bioengineering. This textbook will prepare you to work in bothtraditional and emerging energy-related fields.

Your background should enable you to

• contribute to teams working on thermal systems applications.

• specify equipment to meet prescribed needs.

• implement energy policy.

• perform economic assessments involving energy.

• manage technical operations.

This textbook also will prepare you for further study in thermodynamics, fluid mechanics,and heat transfer to strengthen your understanding of fundamentals and to acquire moreexperience in model building and solving applications-driven problems.

How to Use This Book EffectivelyThis book has several features and learning resources that facilitate study and contributefurther to understanding.

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E T H O D O L O G YU P D A T E

M

Core Study Features

Examples and Problems . . .

• Numerous annotated solved examples are provided that feature the solutionmethodology presented in Sec. 2.6, and illustrated initially in Example 2.1. Weencourage you to study these examples, including the accompanying comments.

• Less formal examples are given throughout the text. They open with the words ForExample… and close with the symbol �. These examples also should be studied.

• A large number of end-of-chapter problems are provided. The problems are se-quenced to coordinate with the subject matter and are listed in increasing order ofdifficulty. The problems are classified under headings to expedite the process ofselecting review problems to solve.

Other Study Aids . . .

• Each chapter begins with an introduction stating the chapter objective and con-cludes with a summary and study guide.

• Key words are listed in the margins and coordinated with the text material at thoselocations.

• Key equations are set off by a double horizontal bar.

• Methodology Update in the margin identifies where we refine our problem-solvingmethodology, introduce conventions, or sharpen our understanding of specificconcepts.

• For quick reference, conversion factors and important constants are provided onthe inside front cover and facing page.

• A list of symbols is provided on the inside back cover and facing page.

• (CD-ROM) directs you to the accompanying CD where supplemental text materialand learning resources are provided.

Icons . . .

identifies locations where the use of appropriate computer software isrecommended.

directs you to short fluid mechanics video segments.

Enhanced Study Features

Computer Software . . .

To allow you to retrieve appropriate data electronically and model and solve complex ther-mal engineering problems, instructional material and computer-type problems are pro-vided on the CD for Interactive Thermodynamics (IT) and Interactive Heat Transfer (IHT).These programs are built around equation solvers enhanced with property data and othervaluable features. With the IT and IHT software you can obtain a single numerical solu-tion or vary parameters to investigate their effects. You also can obtain graphical output,and the Windows-based format allows you to use any Windows word-processing softwareor spreadsheet to generate reports. Tutorials are available from the ‘Help’ menu, and bothprograms include several worked examples.

Accompanying CD . . .

The CD contains the entire print version of the book plus the following additional con-tent and resources:

• answers to selected end-of-chapter problems

• additional text material not included in the print version of the book

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Problems 11

• the computer software Interactive Thermodynamics (IT) and Interactive HeatTransfer (IHT), including a directory entitled Things You Should Know About ITand IHT that contains helpful information for using the software with this book.

• short video segments that illustrate fluid mechanics principles

• built-in hyperlinks to show connections between topics

Special Note: Content provided on the CD may involve equations, figures, and examplesthat are not included in the print version of the book.

Problems

1.1 List thermal systems that you might encounter in everydayactivities such as cooking, heating or cooling a house, andoperating an automobile.

1.2 Using the Internet, obtain information about the operation ofa thermal system of your choice or one of those listed or shownin Fig. 1.1. Obtain sufficient information to provide a descrip-tion to your class on the function of the system and relevant ther-modynamics, fluid mechanics, and heat transfer aspects.

1.3 Referring to the thermal systems of Fig. 1.1, in cases assignedby your instructor or selected by you, explain how energy isconverted from one form to another and how energy is stored.

1.4 Consider a rocket leaving its launch pad. Briefly discuss theconversion of energy stored in the rocket’s fuel tanks into otherforms as the rocket lifts off.

1.6 Contact your local utility for the amount you pay for elec-tricity, in cents per kilowatt-hour. What are the major contrib-utors to this cost?

1.7 A newspaper article lists solar, wind, hydroelectric, geo-thermal, and biomass as important renewable energy resources.What is meant by renewable? List some energy resources thatare not considered renewable.

1.8 Reconsider the energy resources of Problem 1.7. Givespecific examples of how each is used to meet human needs.

1.9 Our energy needs are met today primarily by use of fossilfuels. What fossil fuels are most commonly used for (a) trans-portation, (b) home heating, and (c) electricity generation?

1.10 List some of the roles that coal, natural gas, and petroleumplay in our lives. In a memorandum, discuss environmental,political, and social concerns regarding the continued use ofthese fossil fuels. Repeat for nuclear energy.

1.11 A utility advertises that it is less expensive to heat waterfor domestic use with natural gas than with electricity. Deter-mine if this claim is correct in your locale. What issues deter-mine the relative costs?

1.12 A news report speaks of greenhouse gases. What is meantby greenhouse in this context? What are some of the most preva-lent greenhouse gases and why have many observers expressedconcern about those gases being emitted into the atmosphere?

1.13 Consider the following household appliances: desktopcomputer, toaster, and hair dryer. For each, what is its func-tion and what is the typical power requirement, in Watts? Canit be considered a thermal system? Explain.

1.5 Referring to the U.S. patent office Website, obtain a copyof a patent granted in the last five years for a thermal system.Describe the function of the thermal system and explain theclaims presented in the patent that relate to thermodynamics,fluid mechanics, and heat transfer. Figure P1.13

Figure P1.4

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12 Chapter 1. What Is Thermal Systems Engineering?

1.21 Automobile designers have worked to reduce the aerody-namic drag and rolling resistance of cars, thereby increasingthe fuel economy, especially at highway speeds. Compare thesketch of the 1920s car shown in Figure P1.21 with the ap-pearance of present-day automobiles. Discuss any differencesthat have contributed to the increased fuel economy of mod-ern cars.

1.15 The everyday operation of your car involves the use ofvarious gases or liquids. Make a list of such fluids and indi-cate how they are used in your car.

1.16 Your car contains various fans or pumps, including theradiator fan, the heater fan, the water pump, the power steeringpump, and the windshield washer pump. Obtain approximatevalues for the power (horsepower or kilowatts) required tooperate each of these fans or pumps.

1.17 When a hybrid electric vehicle such as the one describedin Section 1.2.2 is braked to rest, only a fraction of the vehi-cle’s kinetic energy is stored chemically in the batteries. Whyonly a fraction?

1.18 Discuss how a person’s driving habits would affect the fueleconomy of an automobile in stop-and-go traffic and on afreeway.

1.19 The solder-reflow oven considered in Section 1.2.3 oper-ates with the conveyer speed and hot air supply parameters ad-justed so that the PCB soldering process is performed slightlyabove the solder melting temperature as required for qualityjoints. The PCB also is cooled to a safe temperature by thetime it reaches the oven exit. The operations manager wants toincrease the rate per unit time that PCBs pass through the oven.How might this be accomplished?

1.20 In the discussion of the soldering process in Section1.2.3, we introduced the requirement that the PCB and itscomponents be gradually and uniformly heated to avoid ther-mal stresses and localized overheating. Give examples fromyour personal experience where detrimental effects have beencaused to objects heated too rapidly, or very nonuniformly.

Figure P1.21

Figure P1.23

1.22 Considering the hot water supply, hybrid electric vehi-cle, and solder-reflow applications of Sec. 1.2; give exam-ples of conduction, convection, and radiation modes of heattransfer.

1.23 A central furnace or air conditioner in a building uses afan to distribute air through a duct system to each room asshown in Fig. P1.23. List some reasons why the temperaturesmight vary significantly from room to room, even though eachroom is provided with conditioned air.

1.24 Figure P1.24 shows a wind turbine-electric generatormounted atop a tower. Wind blows steadily across the turbineblades, and electricity is generated. The electrical output of thegenerator is fed to a storage battery. For the overall thermalsystem consisting of the wind-turbine generator and storagebattery, list the sequence of processes that convert the energyof the wind to energy stored in the battery.

CoolingHeating and fan

Outdoor airintake

Air return

Conditioned airsupply duct

1.14 A person adjusts the faucet of a shower as shown in Fig-ure P1.14 to a desired water temperature. Part way through theshower the dishwasher in the kitchen is turned on and the tem-perature of the shower becomes too cold. Why?

Watermeter

Cold

Cold Hot waterheater

Hot

Dishwasher

Shower

Figure P1.14

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Problems 13

1.25 A plastic workpiece in the form of a thin, square, flat plateremoved from a hot injection molding press at 150�C must becooled to a safe-to-handle temperature. Figure P1.25 showstwo arrangements for the cooling process: The workpiece is

Figure P1.24

Figure P1.25

suspended vertically from an overhead support, or positionedhorizontally on a wire rack, each in the presence of ambientair. Calling on your experience and physical intuition, answerthe following:(a) Will the workpiece cool more quickly in the vertical or

horizontal arrangement if the only air motion that occursis due to buoyancy of the air near the hot surfaces of theworkpiece (referred to as free or natural convection)?

(b) If a fan blows air over the workpiece (referred to as forcedconvection), would you expect the cooling rate to increaseor decrease? Why?

1.26 An automobile engine normally has a coolant circulatingthrough passageways in the engine block and then througha finned-tube radiator. Lawn mower engines normally havefinned surfaces directly attached to the engine block, withno radiator, in order to achieve the required cooling. Whymight the cooling strategies be different in these two appli-cations?

Still, ambientair

Figure P1.26

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