Physics
Heat Engines
Heat engines are devices that convert thermal energy into mechanical work. They operate on the principle of the second law of thermodynamics, which states that heat naturally flows from a hot reservoir to a cold reservoir. Heat engines utilize this heat flow to produce useful work, such as powering vehicles or generating electricity.
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10 Key excerpts on "Heat Engines"
- Raymond Serway, John Jewett(Authors)
- 2018(Publication Date)
- Cengage Learning EMEA(Publisher)
21.1 Heat Engines and the Second Law of Thermodynamics A heat engine is a device that takes in energy by heat 2 and, operating in a cyclic process, expels a fraction of that energy by means of work. For instance, in a typi- cal process by which a power plant produces electricity, a fuel such as natural gas is burned and the high-temperature gases produced are used to convert liquid water to steam. This steam is directed at the blades of a turbine. The steam does work on the blades of the turbine, setting it into rotation. The mechanical energy associated with this rotation is used to drive an electric generator. Another device that can be modeled as a heat engine is the internal combustion engine in an automobile. This device uses energy from a burning fuel to perform work on pistons that results in the motion of the automobile. Let us consider the fundamental operation of a heat engine in more detail. A heat engine carries some working substance through a cyclic process during which (1) the working substance absorbs energy by heat from a high-temperature energy reservoir, (2) work is done by the engine, and (3) energy is expelled by heat to a lower-temperature reservoir. As an example, consider the operation of a steam engine (Fig. 21.1), which uses water as the working substance. The water in a boiler absorbs energy from burning fuel and evaporates to steam, which then does work by expanding against a piston. After the steam cools and condenses, the liquid water produced returns to the boiler and the cycle repeats. It is useful to represent a heat engine schematically as in Figure 21.2 (page 558). The engine absorbs a quantity of energy | Q h | from the hot reservoir. For the mathematical discussion of Heat Engines, we use absolute values to make all energy transfers by heat positive, and the direction of transfer is indicated with an explicit positive or negative sign.
eBook - PDF- Claus Borgnakke, Richard E. Sonntag(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
.................................................................................................................................................................... ............. Heat Engines AND REFRIGERATORS 145 during the cycle. From the first law we conclude that the net heat transfer is positive and equals the work done during the cycle. A heat engine may be defined as a device that operates in a thermodynamic cycle and produces a certain amount of net positive work through the transfer of heat from a high-temperature body to a low-temperature body. Often the term heat engine is used in a broader sense to include all devices that produce work, either through heat transfer or through combustion, even though the device does not operate in a thermodynamic cycle. The internal combustion engine and the gas turbine are examples of such devices, and call- ing them Heat Engines is an acceptable use of the term. In this chapter, however, we are concerned with the more restricted form of heat engine, as just defined, one that operates on a thermodynamic cycle. A simple steam power plant is an example of a heat engine in this restricted sense. Each component in this plant may be analyzed individually as a steady-state, steady-flow process, but as a whole it may be considered a heat engine (Fig. 5.4) in which water (steam) is the working fluid. An amount of heat, Q H , is transferred from a high-temperature body, which may be the products of combustion in a furnace, a reactor, or a secondary fluid that in turn has been heated in a reactor. In Fig. 5.4 the turbine is shown schematically as driving the pump. What is significant, however, is the net work that is delivered during the cycle. The quantity of heat Q L is rejected to a low-temperature body, which is usually the cooling water in a condenser.
eBook - PDF- Donald Olander(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
109 4 Heat Engines, Power Cycles, and the Thermodynamics of Open Systems 4.1 Heat Engines In Section 1.9, it was noted that the first law regards heat and work as completely interchangeable; if a certain number of Joules of heat added to a system increases the internal energy of a body by, say, Δ U , the same number of Joules of work performed on the body would produce the same Δ U . In addition, work can be completely converted to heat, as everyday experience with friction attests. However, the reverse is not true; heat cannot be completely transformed into work. This limitation, which is a consequence of the second law, is best demonstrated by studying the properties of Heat Engines . A heat engine is a system operating in a cycle that receives heat from a high-temperature source (called a thermal reservoir) and produces useful work. However, since the efficiency of conversion must be less than 100%, some of the input heat is rejected to a cold reservoir. Figure 4.1 shows a schematic of a heat engine/heat pump and their associated thermal reservoirs. The reservoirs supply or receive heat without alteration of their temperatures. Heat flows in the reservoirs are reversible whether or not the engine is. FIGURE 4.1 A schematic of a heat engine or heat pump. The heat pump is a heat engine running in reverse. Hot Reservoir T H Cold Reservoir T L Heat Pump Heat Engine Cold Reservoir T L Hot Reservoir T H W W Q H Q L Q L Q H 110 General Thermodynamics The circle with the arrows in Figure 4.1 is a shorthand representation of the heat engine. It is intended to signify that the working substance (a fluid such as an ideal gas or water) moves through many thermodynamic states in a never-ending cyclic process. The detailed structure of the heat engine can vary greatly, but the simplest version contains the following four steps: 1. One in which heat is absorbed isothermally from the high-temperature reservoir. 2. The next, in which work is produced adiabatically.
- Alexander V. Dimitrov(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
2Conversion of Thermal Energy into Mechanical Work (Thermal Engines)Energy-related (power) technologies may be treated as a combination of engineering-technical methods of energy and work conversion employed to facilitate human life. They are divided into two main groups. The first group comprises technologies of heat conversion into another type of energy (mechanical, electrical, electromagnetic, etc.) while the second one comprises technologies of heat transfer, accumulation, and regeneration. Each thermal technology discussed herein will be illustrated by specific physical schemes and devices. We shall consider them in the following order:•Technologies of mechanical work performance (so called thermomechanical technologies) •Technologies of generation of electrical energy (thermoelectric technologies) •Technologies of heat transformation (regeneration and recuperation) •Technologies of heat transfer and collection (transfer and accumulation) •Technologies creating comfortable environment (air conditioning and ventilation)Thus, we will treat a certain technology as an object of study of respective scientific-applied research fields, on one hand, and we will follow the teaching programs on “Power engineering,” “Transport management” and “General mechanical engineering,” on the other hand.2.1 Evolution of Engine TechnologiesAs is known from physics, energy conversion follows a natural course, that is, energy of motion of macro- and microbodies (popular as mechanical energy) is converted into heat by mechanisms that are studied by tribology (including dry, semi-dry, viscous, or turbulent friction). No opposite transformation is observed in nature. Heat conversion into energy needed for the operation of machines and mechanisms was an impossible task for primitive people as well as for those living in slave-holding* and feudal†
eBook - PDFThermodynamics
From Concepts to Applications, Second Edition
- Arthur Shavit, Chaim Gutfinger(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
141 7 Heat Engines and Second Law of Thermodynamics In 1712, Thomas Newcomen invented the steam engine. James Watt took out a patent in 1769 for a separate condenser, which greatly improved the performance of the steam engine. The development of the steam engine opened the way to the industrial revolution. The engine used steam, which is raised in a boiler, to produce work for pumping water. This invention pointed the way to converting heat into work. One of the first questions that arose with the invention of the heat engine was how much work could an engine produce per unit heat input. Another question was could it be improved, and by how much. In this chapter we consider these questions and develop a theory that is the basis for the operation of Heat Engines. 7.1 Heat Engines A heat engine is a closed system operating in a cycle while undergoing work and heat interactions. We distinguish between two types of Heat Engines as follows: Those that use heat to produce work, ∮ W > 0 Those that use work to produce cooling or heating, ∮ W ≤ 0 A steam power plant is an example of the first type of engine, whereas a household refrig-erator is an example of the second type. A work-producing engine is usually called a heat engine, whereas one for cooling or heating is called a refrigerator or a heat pump, respectively. The term heat engine has two meanings describing (1) the entire class of cyclic devices that have heat and work interac-tions and (2) a subclass of those devices that produce positive work. 1 A simple example of a work–producing heat engine is a cylinder–piston assembly that contains a gas (the working fluid) in which the piston can move between two stops. Such a system is shown schematically in Figure 7.1. This assembly can be used to lift carts, that is, weights, from a low level A to a higher level B. The operation of the heat engine begins at state 1 where the piston is at its lowest level.
eBook - PDF- Bernard Desmet(Author)
- 2022(Publication Date)
- Wiley-ISTE(Publisher)
1 Energy Conversion: Thermodynamic Basics Georges DESCOMBES 1 and Bernard DESMET 2 1 CNAM, Paris, France 2 INSA – HdF, Université Polytechnique Hauts-de-France, Valenciennes, France 1.1. Introduction We are interested here in the conversion of heat into mechanical work via machines using a fluid medium in a continuous flow, or functioning in a cyclic manner. This first chapter succinctly presents the main concepts of thermodynamic used in this context. For a more in-depth study, the reader may consult the specialized works of Borgnakke and Sonntag (2013), Feidt (2014), Foussard et al. (2021) and Çengel et al. (2019). Classical sign conventions will be used: the quantities of heat and work exchanged between a system and its exterior will be positive while they are received by the system. Work, quantities of heat and extensive state quantities – quantities of which the value is proportional to the quantity of matter of the system – will be denoted in uppercase when they refer to the whole system and in lowercase when they are expressed per unit mass. Therefore, W, Q, U, etc. refer to work exchange, heat, internal energy, etc. for the considered system, and w, q, u, etc. are the corresponding specific quantities. Thermodynamics of Heat Engines, coordinated by Bernard DESMET. © ISTE Ltd 2022. 2 Thermodynamics of Heat Engines 1.2. Principles of thermodynamics 1.2.1. Notion of a thermodynamic system In the strict sense of the term, a thermodynamic system or even a closed system does not exchange matter with its exterior. Its boundary is impermeable to the exchange of matter. In technical thermodynamics, we are interested more often in the equipment (heat exchangers, turbines, compressors, etc.) through which one or more fluids flow. Generally, we look for characteristics (pressure, temperature, mass flow rate, etc.) in the fixed sections located on either side of the component being studied and defined as the inlet and output of this component.
eBook - PDF- Robert A. Ristinen, Jack J. Kraushaar, Jeffrey T. Brack(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
3 Heat Engines Hulton Archive/Strin g er/Gett y Ima g es 3.1 The Mechanical Equivalent of Heat If we take a look at the energy units, it becomes apparent that what we think of as a rather small unit of heat energy, for instance one Btu is in fact a very large amount of energy com-pared to what we see in a mechanical unit of energy, the foot-pound. One Btu is the same as 778 foot -pounds. The thought of lifting a one-pound weight 778 feet into the air with the energy released by the burning of only one match can certainly be appealing to those who do heavy lifting for a living. To realize this goal, we must first find a way to capture the heat energy of the fuel and turn it into mechanical energy. The possibility of easing human labor by utilizing heat sources has been the driving force behind a long history of development of what we now call Heat Engines . This is the main sub-ject of this chapter. 65 Energy and the Environment , Fourth Edition. Robert A. Ristinen, Jack J. Kraushaar, and Jeffrey T. Brack. © 2022 John Wiley & Sons, Inc. Published 2022 by John Wiley & Sons, Inc. Companion website: www.wiley.com/go/brack/energyandenvironment4thedition 3.2 The Energy Content of Fuels We go to a great deal of effort to obtain fossil fuels and other types of fuel for two basic pur-poses: to provide direct heating and lighting, and to power Heat Engines. The general path-ways by which we do this are shown in Figure 3.1. Heat Engines and the thermodynamic principles that govern their operation will be dis-cussed in detail later. First, let us look at how heat is derived from fuels. In simplified form, the burning of hydrocarbon fuels is merely the combining of the carbon and the hydrogen from the fuel with oxygen from the air.
- John R. Fanchi, John R. Fanchi, (Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
72 Energy: Technology and Directions for the Future Table 3-2 Equilibrium conditions Variable Extremum Subject to constant S Maximize U , V , n i for all i U Minimize S , V , n i for all i G Minimize T , P , n i for all i An isolated system reaches equilibrium when entropy is maximized, that is, when entropy satisfies the relation dS = 0 (3.4.15) subject to the conditions that U , V , { n i } are constant: dU = 0 dV = 0 (3.4.16) dn i = 0 ∀ i = 1, . . . , N c Equilibrium conditions can be determined for the thermodynamic quantities U, G if U, G are minimized. The equilibrium conditions are summarized in Table 3-2. 3.5 Heat Engines A heat engine is a device that transforms heat into other forms of energy, such as mechanical or electrical energy. For comparison, a heat pump is a device that transfers heat from one location to another. Diagrams of a heat engine and a heat pump are presented in Figure 3-1. In a heat engine, the engine transforms heat Q 2 from the hot reservoir to work W and expels heat Q 1 to the cold reservoir. The heat pump, on the other hand, combines heat Q 1 from the cold reservoir and work W to provide heat Q 2 to the hot reservoir. The thermal efficiency of the heat engine is the ratio of the net work done to the heat absorbed: η = W Q 2 = Q 2 − Q 1 Q 2 = 1 − Q 1 Q 2 (3.5.1) Heat Engines and Heat Exchangers 73 Heat Engine Heat Pump Hot Reservoir T 2 Cold Reservoir T 1 W Q 2 Engine Hot Reservoir T 2 Cold Reservoir T 1 W Q 2 Q 1 Q 1 Pump Figure 3-1. Heat engine and heat pump. The coefficient of performance of a heat pump is the ratio of the heat transferred to the work done by the pump: COP = Q 2 W (3.5.2) CARNOT CYCLE Frenchman Sadi Carnot (1796–1832) presented a theoretical model of a heat engine in 1824. He used an ideal gas as the working material. The gas was expanded and compressed in four successive, reversible stages. The stages are listed in Table 3-3 and schematically illustrated in Figure 3-2.
eBook - PDF- Aldo Vieira da Rosa(Author)
- 2009(Publication Date)
- Academic Press(Publisher)
Most work has been done on the close-cycle configuration, which is regarded as more economical. However, the costs of the two versions may turn out to be comparable. 4.3 Turbines A turbine (Figure 4.4) generates mechanical energy from a difference in pressure. Usually, the state of the gas at the inlet and the pressure of the gas at the exhaust are specified. Let p in and T in be the pressure and the temperature at the inlet of the turbine and p out , T out the corresponding quantities at the exhaust. The output of the turbine is the mechanical work, W . The heat, Q , is exchanged with the environment by some means other than the circulating gases. Most practical turbines are sufficiently well insulated to be assumed adiabatic —that is, a condition in which Q = 0. The inlet gas carries an enthalpy, H in , into the turbine, while the exhaust removes H out from the device. Conservation of energy requires that † W = H in − H out . (4.1) Expressing the quantities on a per kilomole basis (quantities per kilomole are represented by lower case letters), we can write W = μ in h in − μ out h out = μ ( h in − h out ) (4.2) because, under steady-state conditions, μ in = μ out ≡ μ. Inlet T in , P in Energy out, W Exhaust T o u t , P o u t AVR Figure 4.4 A turbine. † Provided there is no appreciable change in kinetic, potential, magnetic, and other forms of energy. 4.3 Turbines 143 In a perfect gas, h in − h out = T in T out c p dT. (4.3) Assuming a constant specific heat, h in − h out = c p ( T in − T out ) , (4.4) and W = μc p ( T in − T out ) . (4.5) W = μc p T in T in − T out T in = μc p T in η CARNOT (4.6) Equation 4.6 looks similar to that which describes the behavior of a heat engine. However, the quantity, μc p T in , although having the dimensions of energy, is not the heat input to the device; rather, it is the enthalpy input. For a given input state and a given exhaust pressure, the mechanical energy output increases with decreasing exhaust temperature.
eBook - PDF- Aldo Vieira da Rosa(Author)
- 2005(Publication Date)
- Academic Press(Publisher)
4.14 Chapter 5 Thermoelectricity So far we have discussed Heat Engines in general and have exam-ined in some detail the heat engine used in Ocean Thermal Energy Converters. All these engines convert heat into mechanical energy. In this chapter and in the next two, we consider engines that transform heat directly into electricity: the thermoelectric, the thermionic, and the radio-noise converters. In the chapter on pho-tovoltaic cells, we will discuss the thermophotovoltaic converter that transforms heat into radiant energy and then into electricity. Other engines such as magnetohydrodynamic engine that converts heat into kinetic energy of a plasma and then into electricity will not be covered here. In most of the chapters of this book we have adopted the strategy of first introducing a simplified model that explains the phenomena that underlie the operation of the device being studied, and, from that, we deduce the way the device behaves. In this chapter, we will change the approach, describing first the behavior of thermocouples and only later will we examine the underlying phenomenology. The reason for this is that although the behavior of thermoelectric devices is easy to describe, there is no simple way of explaining how these effects come about. In fact, were we to use classical mechanics with Maxwellian electron energy distribution, we would prove that there is no Peltier effect and that the Thompson effect in metals should be two orders of magnitude larger than what it really is. The Peltier and Thompson effects are two of the effects associated with thermo-electricity. 5.1 Experimental Observations Consider a heat-conducting bar whose ends are at different tempera-tures, T H and T C . Clearly a certain amount of heat power, P H F , will enter through one face and an amount, P C F , will leave through the opposite face. If the bar has its remaining sides perfectly insulated so no heat can exit through them, then P H F = P C F = Λ( T H -T C ) (1)
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