WHAT IS THERMAL SYSTEMS ENGINEERING?

Transcription

1 1 WHAT IS THERMAL SYSTEMS ENGINEERING? Introduction The objective of this chapter is to introduce you to thermal systems engineering using several contemporary applications. Our discussions use certain terms that we assume are familiar from your background in physics and chemistry. The roles of thermodynamics, fluid mechanics, and heat transfer in thermal systems engineering and their relationship to one another also are described. The presentation concludes with tips on the effective use of the book. chapter objective 1.1 Getting Started Thermal systems engineering is concerned with how energy is utilized to accomplish beneficial functions in industry, transportation, and the home, and also the role energy plays in the study of human, animal, and plant life. In industry, thermal systems are found in electric power generating plants, chemical processing plants, and in manufacturing facilities. Our transportation needs are met by various types of engines, power converters, and cooling equipment. In the home, appliances such as ovens, refrigerators, and furnaces represent thermal systems. Ice rinks, snow-making machines, and other recreational uses involve thermal systems. In living things, the respiratory and circulatory systems are thermal systems, as are equipment for life support and surgical procedures. Thermal systems involve the storage, transfer, and conversion of energy. Energy can be stored within a system in different forms, such as kinetic energy and gravitational potential energy. Energy also can be stored within the matter making up the system. Energy can be transferred between a system and its surroundings by work, heat transfer, and the flow of hot or cold streams of matter. Energy also can be converted from one form to another. For example, energy stored in the chemical bonds of fuels can be converted to electrical or mechanical 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. Plants interact with their environments in other ways as well. Selected areas of application that involve the engineering of thermal systems are listed in Fig. 1.1, along with six specific illustrations. The turbojet engine, jet ski, and electrical power plant represent thermal systems involving conversion of energy in fossil fuels to achieve a desired outcome. Components of these systems also involve work and heat transfer. For life support on the International Space Station, solar energy is converted to electrical energy and provides energy for plant growth experimentation and other purposes. Semiconductor manufacturing processes such as high temperature annealing of silicon wafers involve energy conversion and significant heat transfer effects. The human cardiovascular 1

2 2 Chapter 1. What Is Thermal Systems Engineering? Prime movers: internal-combustion engines, turbines Fluid machinery: pumps, compressors Fossil- and nuclear-fueled power stations Alternative energy systems Fuel cells Solar heating, cooling and power generation Heating, ventilating, and air-conditioning equipment Biomedical applications Life support and surgical equipment Artificial organs Air and water pollution control equipment Aerodynamics: airplanes, automobiles, buildings Pipe flow: distribution networks, chemical plants Cooling of electronic equipment Materials processing: metals, plastics, semiconductors Manufacturing: machining, joining, laser cutting Thermal control of spacecraft Air in Fuel in Compressor Combustor Turbojet engine Turbine Solar-cell arrays Hot gases out Surfaces with thermal control coatings International Space Station Quartz-tube furnace 3.5 in. diameter outlet jet in. 2 inlet area Wafer boat Jet ski water =-pump propulsion High-temperature annealing of silicon wafers Thorax Lung Steam generator Combustion gas cleanup Stack Electric power Heart Human cardiovascular system Coal Ash Air Steam Turbine Generator Condenser Condensate Cooling water Cooling tower Electrical power plant Figure 1.1 Selected areas of applications for thermal systems engineering.

3 1.2 Thermal System Case Studies 3 system is a complex combination of fluid flow and heat transfer components that regulates the flow of blood and air to within the relatively narrow range of conditions required to maintain life. In the next section, three case studies are discussed that bring out important features of thermal systems engineering. The case studies also suggest the breadth of this field. 1.2 Thermal System Case Studies Three cases are now considered to provide you with background for your study of thermal systems engineering. In each case, the message is the same: Thermal systems typically consist of a combination of components that function together as a whole. The components themselves and the overall system can be analyzed using principles drawn from three disciplines: thermodynamics, fluid mechanics, and heat transfer. The nature of an analysis depends on what needs to be understood to evaluate system performance or to design or upgrade a system. Engineers who perform such work need to learn thermal systems principles and how they are applied in different situations Domestic Hot Water Supply The installation that provides hot water for your shower is an everyday example of a thermal 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 temperature. Clearly the temperature of the water changes from when it enters your house until it exits the shower head. Cold water enters from the supply pipe with a pressure greater than the atmosphere, at low velocity and an elevation below ground level. Water exits the shower head 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 by heating elements (electrical or gas) in the hot water heater. The energy added can be evaluated using principles from thermodynamics and heat transfer. The relationships among the 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 mechanics principles. Water heaters are designed to achieve appropriate heat transfer characteristics so that the energy 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 demand. Accordingly, appropriate insulation on the tank is required to reduce energy losses to the 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, it also may be advantageous to insulate the pipes. The flow from the supply pipe to the shower head involves several fluid mechanics principles. The pipe diameter must be sized to provide the proper flow rate too small a diameter and there will not be enough water for a comfortable shower; too large a diameter and the material costs will be too high. The flow rate also depends on the length of the pipes and

4 4 Chapter 1. What Is Thermal Systems Engineering? Shower head Shower head Hot Cold Diverter valve Cold water faucet To shower head Cold water Cold water supply line (a) Water heater Hot water faucet Tub spout (b) Hot water To tub spout Valve stem Figure 1.2 Home hot water supply. (a) Overview. (b) Faucet and shower head. the number of valves, elbows, and other fittings required. As shown in Fig. 1.2b, the faucet and the shower head must be designed to provide the desired flow rate while mixing hot and cold water appropriately. From this example we see some important ideas relating to the analysis and design of thermal systems. The everyday system that delivers hot water for your shower is composed of various components. Yet their individual features and the way they work together as a whole involve a broad spectrum of thermodynamics, fluid mechanics, and heat transfer principles Hybrid Electric Vehicle Automobile manufacturers are producing hybrid cars that utilize two or more sources of power within a single vehicle to achieve fuel economy up to miles per gallon. Illustrated in Fig. 1.3a is a hybrid electric vehicle (HEV) that combines a gasoline-fueled engine with a set of batteries that power an electric motor. The gasoline engine and the electric motor are each connected to the transmission and are capable of running the car by themselves 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 features in 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 power needs by the battery alone, special design to reduce aerodynamic drag and the use of tires that have very low rolling resistance (friction), and the use of lightweight composite materials such as carbon fiber and the increased use of lightweight metals such as aluminum and magnesium.

5 1.2 Thermal System Case Studies 5 Generator Inverter Gasoline engine Electric motor Batteries (a) Overview of the vehicle showing key thermal systems (b) Regenerative braking mode with energy flow from wheels to battery Figure 1.3 Hybrid electric vehicle combining gasoline-fueled engine, storage batteries, and electric motor. (Illustrations by George Retseck.) The energy source for such hybrid vehicles is gasoline burned in the engine. Because of the ability to store energy in the batteries and use that energy to run the electric motor, the gasoline engine does not have to operate continuously. Some HEVs use only the electric motor to accelerate from rest up to about 15 miles per hour, and then switch to the gasoline engine. A specially designed transmission provides the optimal power split between the gasoline engine and the electric motor to keep the fuel use to a minimum and still provide the needed power. Most HEVs use regenerative braking, as shown in Fig. 1.3b. In conventional cars, stepping on the brakes to slow down or stop dissipates the kinetic energy of motion through the frictional action of the brake. Starting again requires fuel to re-establish the kinetic energy of the vehicle. The hybrid car allows some of the kinetic energy to be converted during braking to electricity that is stored in the batteries. This is accomplished by the electric motor serving as a generator during the braking process. The net result is a significant improvement in fuel economy and the ability to use a smaller-sized gasoline engine than would be possible to achieve comparable performance in a conventional vehicle. The overall energy notions considered thus far are important aspects of thermodynamics, which deals with energy conversion, energy accounting, and the limitations on how energy is converted from one form to another. In addition, there are numerous examples of fluid mechanics and heat transfer applications in a hybrid vehicle. Within the engine, air,

6 6 Chapter 1. What Is Thermal Systems Engineering? fuel, engine coolant, and oil are circulated through passageways, hoses, ducts, and manifolds. These must be designed to ensure that adequate flow is obtained. The fuel pump and water pump also must be designed to achieve the desired fluid flows. Heat transfer principles guide the design of the cooling system, the braking system, the lubrication system, and numerous other aspects of the vehicle. Coolant circulating through passageways in the engine block must absorb energy transferred from hot combustion gases to the cylinder surfaces so those surfaces do not become too hot. Engine oil and other viscous fluids in the transmission and braking systems also can reach high temperatures and thus must be carefully managed. Hybrid electric vehicles provide examples of complex thermal systems. As in the case of hot water systems, the principles of thermodynamics, fluid mechanics, and heat transfer apply to the analysis and design of individual parts, components, and to the entire vehicle Microelectronics Manufacturing: Soldering Printed-Circuit Boards Printed-circuit boards (PCBs) found in computers, cell phones, and many other products, are composed of integrated circuits and electronic devices mounted on epoxy-filled fiberglass boards. The boards have been metallized to provide interconnections, as illustrated in Fig. 1.4a. The pins of the integrated circuits and electronic devices are fitted into holes, and a 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 to a temperature above the solder melting temperature; this is known as the reflow process. The Integrated circuit (IC) Pin lead Metal film Pre-form solder paste (b) (c) (a) (d) Figure 1.4 Soldering printed-circuit boards (a) with pre-form solder paste applied to integrated circuit pins and terminal pads (b) enter the solder-reflow oven (c) on a conveyor and are heated to the solder melting temperature by impinging hot air jets (d ).

7 1.3 Analysis of Thermal Systems 7 PCB and its components must be gradually and uniformly heated to avoid inducing thermal stresses and localized overheating. The PCB is then cooled to near-room temperature for subsequent safe handling. The PCB prepared for soldering is placed on a conveyor belt and enters the first zone within the solder reflow oven, Fig. 1.4c. In passing through this zone, the temperature of the PCB 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 temperature is reduced by exposure to air that has been cooled by passing through a water-cooled heat exchanger. From the foregoing discussion, we recognize that there are many aspects of this manufacturing process that involve electric power, flow of fluids, air-handling equipment, heat transfer, and thermal aspects of material behavior. In thermal systems engineering, we perform analyses on systems such as the solder-reflow oven to evaluate system performance or to design or upgrade the system. For example, suppose you were the operations manager of a factory concerned with providing electrical power and chilled water for an oven that a vendor 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 might be interested in determining what air flow patterns and heating element arrangements would allow the fastest flow of product through the oven while maintaining necessary uniformity of heating. How would you approach obtaining such information? Through your study of thermodynamics, fluid mechanics, and heat transfer you will learn how to deal with questions such as these. 1.3 Analysis of Thermal Systems In this section, we introduce the basic laws that govern the analysis of thermal systems of all kinds, including the three cases considered in Sec We also consider further the roles of thermodynamics, fluid mechanics, and heat transfer in thermal systems engineering and their relationship to one another. Important engineering functions are to design and analyze things intended to meet human needs. Engineering design is a decision-making process in which principles drawn from engineering and other fields such as economics and statistics are applied to devise a system, system component, or process. Fundamental elements of design include establishing objectives, analysis, synthesis, construction, testing, and evaluation. Engineering analysis frequently aims at developing an engineering model to obtain a simplified mathematical representation of system behavior that is sufficiently faithful to reality, even if some aspects exhibited by the actual system are not considered. For example, idealizations often used in mechanics to simplify an analysis include the assumptions of point masses, frictionless pulleys, and rigid beams. Satisfactory modeling takes experience and is a part of the art of engineering. Engineering analysis is featured in this book. The first step in analysis is the identification of the system and how it interacts with its surroundings. Attention then turns to the pertinent physical laws and relationships that allow system behavior to be described. Analysis of thermal systems uses, directly or indirectly, one or more of four basic laws: Conservation of mass Conservation of energy Conservation of momentum Second law of thermodynamics

8 8 Chapter 1. What Is Thermal Systems Engineering? In your earlier studies in physics and chemistry, you were introduced to these laws. In this book, we place the laws in forms especially well suited for use in thermal systems engineering and help you learn how to apply them The Three Thermal Science Disciplines As we have observed, thermal systems engineering typically requires the use of three thermal science disciplines: thermodynamics, fluid mechanics, and heat transfer. Figure 1.5 shows the roles of these disciplines in thermal system engineering and their relationship to one another. Associated with each discipline is a list of principles featured in the part of the book devoted to that discipline. Thermodynamics provides the foundation for analysis of thermal systems through the conservation of mass and conservation of energy principles, the second law of thermodynamics, and property relations. Fluid mechanics and heat transfer provide additional concepts, including the empirical laws necessary to specify, for instance, material choices, component sizing, and fluid medium characteristics. For example, thermodynamic analysis can tell you the final temperature of a hot workpiece quenched in an oil, but the rate at which it will cool is predicted using a heat transfer analysis. Fluid mechanics is concerned with the behavior of fluids at rest or in motion. As shown in Fig. 1.5, two fundamentals that play central roles in our discussion of fluid mechanics are the conservation of momentum principle that stems from Newton s second law of motion and the mechanical energy equation. Principles of fluid mechanics allow the study of fluids flowing inside pipes (internal flows) and over surfaces (external flows) with consideration of frictional Thermodynamics Conservation of mass Conservation of energy Second law of thermodynamics Properties Thermo Thermal Systems Engineering Analysis directed to Design Operations/Maintenance Marketing/Sales Costing Heat transfer Heat Transfer Conduction Convection Radiation Multiple Modes Fluid Mechanics Fluid statics Conservation of momentum Mechanical energy equation Similitude and modeling Fluids Figure 1.5 The disciplines of thermodynamics, fluid mechanics, and heat transfer involve fundamentals and principles essential for the practice of thermal systems engineering.

9 1.4 How to Use This Book Effectively 9 effects and lift/drag forces. The concept of similitude is used extensively in scaling measurements on laboratory-sized models to full-scale systems. Heat transfer is concerned with energy transfer as a consequence of a temperature difference. As shown in Fig. 1.5, there are three modes of heat transfer. Conduction refers to heat transfer through a medium across which a temperature difference exists. Convection refers to heat transfer between a surface and a moving or still fluid having a different temperature. The third mode of heat transfer is termed thermal radiation and represents the net exchange of energy between surfaces at different temperatures by electromagnetic waves independent of any intervening medium. For these modes, the heat transfer rates depend on the transport properties of substances, geometrical parameters, and temperatures. Many applications involve more than one of these modes; this is called multimode heat transfer. Returning again to Fig. 1.5, in the thermal systems engineering box we have identified some application areas involving analysis. Earlier we mentioned that design requires analysis. Engineers also perform analysis for many other reasons, as for example in the operation of systems and determining when systems require maintenance. Because of the complexity of many thermal systems, engineers who provide marketing and sales services need analysis skills to determine whether their product will meet a customer s specifications. As engineers, we are always challenged to optimize the use of financial resources, which frequently requires costing analyses to justify our recommendations The Practice of Thermal Systems Engineering Seldom do practical applications involve only one aspect of the three thermal sciences disciplines. Practicing engineers usually are required to combine the basic concepts, laws, and principles. 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 because of ties to other important issues such as controls, manufacturing, vibration, and materials that are likely to be present in real-world situations. Thermal systems engineering not only has played an important role in the development of a wide range of products and services that touch our lives daily, it also has become an enabling technology for evolving fields such as nanotechnology, biotechnology, food processing, health services, and bioengineering. This textbook will prepare you to work in both traditional 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 more experience in model building and solving applications-driven problems. 1.4 How to Use This Book Effectively This book has several features and learning resources that facilitate study and contribute further to understanding.

10 10 Chapter 1. What Is Thermal Systems Engineering? Core Study Features M ETHODOLOGY UPDATE Examples and Problems... Numerous annotated solved examples are provided that feature the solution methodology presented in Sec. 2.6, and illustrated initially in Example 2.1. We encourage you to study these examples, including the accompanying comments. Less formal examples are given throughout the text. They open with the words For Example and close with the symbol. These examples also should be studied. A large number of end-of-chapter problems are provided. The problems are sequenced to coordinate with the subject matter and are listed in increasing order of difficulty. The problems are classified under headings to expedite the process of selecting review problems to solve. Other Study Aids... Each chapter begins with an introduction stating the chapter objective and concludes with a summary and study guide. Key words are listed in the margins and coordinated with the text material at those locations. Key equations are set off by a double horizontal bar. Methodology Update in the margin identifies where we refine our problem-solving methodology, introduce conventions, or sharpen our understanding of specific concepts. For quick reference, conversion factors and important constants are provided on the 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 material and learning resources are provided. Icons... identifies locations where the use of appropriate computer software is recommended. 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 thermal engineering problems, instructional material and computer-type problems are provided on the CD for Interactive Thermodynamics (IT) and Interactive Heat Transfer (IHT). These programs are built around equation solvers enhanced with property data and other valuable features. With the IT and IHT software you can obtain a single numerical solution 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 software or spreadsheet to generate reports. Tutorials are available from the Help menu, and both programs include several worked examples. Accompanying CD... The CD contains the entire print version of the book plus the following additional content and resources: answers to selected end-of-chapter problems additional text material not included in the print version of the book

11 Problems 11 the computer software Interactive Thermodynamics (IT) and Interactive Heat Transfer (IHT), including a directory entitled Things You Should Know About IT and 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 examples that are not included in the print version of the book. Problems 1.1 List thermal systems that you might encounter in everyday activities such as cooking, heating or cooling a house, and operating an automobile. 1.2 Using the Internet, obtain information about the operation of a thermal system of your choice or one of those listed or shown in Fig Obtain sufficient information to provide a description to your class on the function of the system and relevant thermodynamics, fluid mechanics, and heat transfer aspects. 1.3 Referring to the thermal systems of Fig. 1.1, in cases assigned by your instructor or selected by you, explain how energy is converted from one form to another and how energy is stored. 1.4 Consider a rocket leaving its launch pad. Briefly discuss the conversion of energy stored in the rocket s fuel tanks into other forms as the rocket lifts off. 1.6 Contact your local utility for the amount you pay for electricity, in cents per kilowatt-hour. What are the major contributors to this cost? 1.7 A newspaper article lists solar, wind, hydroelectric, geothermal, and biomass as important renewable energy resources. What is meant by renewable? List some energy resources that are not considered renewable. 1.8 Reconsider the energy resources of Problem 1.7. Give specific examples of how each is used to meet human needs. 1.9 Our energy needs are met today primarily by use of fossil fuels. What fossil fuels are most commonly used for (a) transportation, (b) home heating, and (c) electricity generation? 1.10 List some of the roles that coal, natural gas, and petroleum play in our lives. In a memorandum, discuss environmental, political, and social concerns regarding the continued use of these fossil fuels. Repeat for nuclear energy A utility advertises that it is less expensive to heat water for domestic use with natural gas than with electricity. Determine if this claim is correct in your locale. What issues determine the relative costs? 1.12 A news report speaks of greenhouse gases. What is meant by greenhouse in this context? What are some of the most prevalent greenhouse gases and why have many observers expressed concern about those gases being emitted into the atmosphere? 1.13 Consider the following household appliances: desktop computer, toaster, and hair dryer. For each, what is its function and what is the typical power requirement, in Watts? Can it be considered a thermal system? Explain. Figure P Referring to the U.S. patent office Website, obtain a copy of a patent granted in the last five years for a thermal system. Describe the function of the thermal system and explain the claims presented in the patent that relate to thermodynamics, fluid mechanics, and heat transfer. Figure P1.13

12 12 Chapter 1. What Is Thermal Systems Engineering? 1.14 A person adjusts the faucet of a shower as shown in Figure P1.14 to a desired water temperature. Part way through the shower the dishwasher in the kitchen is turned on and the temperature of the shower becomes too cold. Why? 1.21 Automobile designers have worked to reduce the aerodynamic drag and rolling resistance of cars, thereby increasing the fuel economy, especially at highway speeds. Compare the sketch of the 1920s car shown in Figure P1.21 with the appearance of present-day automobiles. Discuss any differences that have contributed to the increased fuel economy of modern cars. Hot Shower Dishwasher Figure P1.21 Water meter Cold Cold Hot water heater Figure P The everyday operation of your car involves the use of various gases or liquids. Make a list of such fluids and indicate how they are used in your car Your car contains various fans or pumps, including the radiator fan, the heater fan, the water pump, the power steering pump, and the windshield washer pump. Obtain approximate values for the power (horsepower or kilowatts) required to operate each of these fans or pumps When a hybrid electric vehicle such as the one described in Section is braked to rest, only a fraction of the vehicle s kinetic energy is stored chemically in the batteries. Why only a fraction? 1.18 Discuss how a person s driving habits would affect the fuel economy of an automobile in stop-and-go traffic and on a freeway The solder-reflow oven considered in Section operates with the conveyer speed and hot air supply parameters adjusted so that the PCB soldering process is performed slightly above the solder melting temperature as required for quality joints. The PCB also is cooled to a safe temperature by the time it reaches the oven exit. The operations manager wants to increase 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 Section 1.2.3, we introduced the requirement that the PCB and its components be gradually and uniformly heated to avoid thermal stresses and localized overheating. Give examples from your personal experience where detrimental effects have been caused to objects heated too rapidly, or very nonuniformly Considering the hot water supply, hybrid electric vehicle, and solder-reflow applications of Sec. 1.2; give examples of conduction, convection, and radiation modes of heat transfer A central furnace or air conditioner in a building uses a fan to distribute air through a duct system to each room as shown in Fig. P1.23. List some reasons why the temperatures might vary significantly from room to room, even though each room is provided with conditioned air. Conditioned air supply duct Cooling Heating and fan Air return Outdoor air intake Figure P Figure P1.24 shows a wind turbine-electric generator mounted atop a tower. Wind blows steadily across the turbine blades, and electricity is generated. The electrical output of the generator is fed to a storage battery. For the overall thermal system consisting of the wind-turbine generator and storage battery, list the sequence of processes that convert the energy of the wind to energy stored in the battery.

13 Problems 13 Figure P A plastic workpiece in the form of a thin, square, flat plate removed from a hot injection molding press at 150 C must be cooled to a safe-to-handle temperature. Figure P1.25 shows two arrangements for the cooling process: The workpiece is suspended vertically from an overhead support, or positioned horizontally on a wire rack, each in the presence of ambient air. Calling on your experience and physical intuition, answer the following: (a) Will the workpiece cool more quickly in the vertical or horizontal arrangement if the only air motion that occurs is due to buoyancy of the air near the hot surfaces of the workpiece (referred to as free or natural convection)? (b) If a fan blows air over the workpiece (referred to as forced convection), would you expect the cooling rate to increase or decrease? Why? 1.26 An automobile engine normally has a coolant circulating through passageways in the engine block and then through a finned-tube radiator. Lawn mower engines normally have finned surfaces directly attached to the engine block, with no radiator, in order to achieve the required cooling. Why might the cooling strategies be different in these two applications? Still, ambient air Figure P1.25 Figure P1.26