Physics
Thermodynamics and Engines
Thermodynamics is the study of energy and heat transfer. It explores how energy moves and changes within a system. Engines, which are devices that convert energy into mechanical work, are often analyzed using thermodynamic principles to understand their efficiency and performance. This field of study has led to the development of various types of engines, such as steam engines and internal combustion engines.
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12 Key excerpts on "Thermodynamics and Engines"
eBook - PDF- Myron Kaufman(Author)
- 2002(Publication Date)
- CRC Press(Publisher)
The efficiency of a Carnot cycle engine (a reversible engine that transforms heat to work on a continuous basis) is analyzed using the first law of thermodynamics. 2.1 The Nature of Thermodynamics Few endeavors reward precise thinking and penalize sloppy thinking as much as the study of thermodynamics. In this subject, we take familiar concepts, such as heat and energy, rigorously define them, and derive a multitude of relationships that are useful in many branches of science. By the above criteria of Einstein, thermodynamics is impressive indeed! Thermodynamics is applicable to the types of substances and occurrences that we are familiar with in our daily lives or can easily create in the laboratory. As a result, it is easy to compare the predictions of thermodynamics with ‘‘real- world’’ behavior. Needless to say, there are impressive amounts of data that verify the conclusions of this subject. Relatively few examples of the agreement of experiment with thermodynamic theory are given in this volume. The reader is referred elsewhere for additional examples of the conformity of thermodynamic theory and experiment. 1 Thermodynamics is not unique in dealing with energy. Energy considera- tions are also of paramount importance in fields such as mechanics, electricity and magnetism, and atomic and molecular structures. In these other fields, the energy of individual particles are discussed. The types of energy that they recognize are kinetic energy (the energy of motion), potential energy (the energy of position), and the energy of electric and magnetic fields. Thermo- dynamics is different from these other fields in that it considers the energy of entire systems, consisting of huge numbers of particles (and perhaps radiation fields). The approach of thermodynamics is totally macroscopic and its conclu- sions are not based on any particular model for the behavior and nature of the microscopic particles.
eBook - ePub- Callum Hill(Author)
- 2012(Publication Date)
- Routledge(Publisher)
2
Thermodynamics
(the science of energy and change)
Introduction
Thermodynamics is one of the most powerful tools at our disposal for the study of the sustainability of processes. Although a development that was associated with the invention of the steam engine, it has proved to be of much wider application in the fields of chemistry, ecology, biochemistry, cosmology and information theory. If we wish to understand anything about the sustainability of a process, it is clearly important to know about energy use, recycling and other facets concerned with the management or utilization of resources and it is thermodynamics that provides the basis for the answers to many of the questions. Although this book concerns physical resources and their exploitation, it is necessary to use energy in the processing of these materials and so it follows that we have to understand the properties of energy and how it interacts with matter.What is energy?
Energy is used to move things, to drive machinery, to provide heat and electricity, and to stay alive. Without energy there would be no change and time would not exist. But what exactly is it? For many years energy was seen as a mysterious force that acted upon matter and somehow gave it ‘life’ and it was given a name that reflected this idea, ‘vis viva’. Even now, much misunderstanding exists; it is common to see the words ‘energy’ and ‘power’ used interchangeably although they mean different things, or references to energy ‘consumption’, whereas energy is always conserved. There is no doubt that something is being consumed when we use energy and we shall examine later in this chapter what this something is.
- 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 - 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.
eBook - PDFThermo and Fluid Dynamics
Recent Advances
- Dritan Hoxha(Author)
- 2019(Publication Date)
- Arcler Press(Publisher)
A GENERAL VIEW ON THERMODYNAMICS CHAPTER 1 CONTENTS 1.1. What Is Thermodynamics? ....................................................................................... 2 1.2. Etymology of Thermodynamics ................................................................................ 3 1.3. Why is Thermodynamics Important? ........................................................................ 8 1.4. A Short View of The Quantities Involved in Thermodynamics ................................. 10 1.5. A Short View of the Theories and Laws Contained in Thermodynamics .................. 12 References .................................................................................................................... 17 Thermo and Fluid Dynamics: Recent Advances 2 1.1. WHAT IS THERMODYNAMICS? All of us would have heard at least once the words “thermo” or “thermodynamics” but only a few people know what do they really stand for. From the Greek language, “thermo” means heat and “dynamics” means something in motion. Thus, by definition, “ Thermodynamics is the branch of Physics that deals with the relationships and interaction between heat and other forms of energy. In particular, it describes how thermal energy turns into other forms of energy and vice-versa and how it affects the behavior of matter” [1]. The type of energy involved in Thermodynamics is known as “thermal energy.” It is the kind of energy that a certain substance or system owns due to its temperature [1]. As you may know, temperature is a physical quantity related to the particles’ motion, thus thermal energy represents the energy of molecules motion or vibration. Here I will make a parenthesis. People often confuse heat and thermal energy or worse, they consider them as the same thing. Heat and thermal energy, earlier, used to be considered as synonyms. But they differ as follows: Heat is the thermal energy transferred across a boundary of one region of matter to another.
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.
- Michael Clifford, Kathy Simmons, Philip Shipway(Authors)
- 2009(Publication Date)
- CRC Press(Publisher)
An Introduction to Mechanical Engineering: Part 1 232 ✔ By the end of this section you should understand that the second law of thermodynamics distinguishes between work and heat transfer and recognize that work transfer is the more valuable of these. This does not contradict the first law; energy transferred by one is indistinguishable from energy transferred by the other, and the principle of energy conservation is not violated. There are, however, limits on how efficiently heat can be drawn from a source and converted into work output using a system which operates in a cycle: ✔ As a consequence of the second law, a system operating in a cycle and producing work output must be exchanging heat with at least two reservoirs at different temperatures. If a system is operating in a cycle while exchanging heat with only one reservoir, if a net transfer of work occurs it must be to the system. Work can be converted continuously and completely into heat, but heat cannot be converted completely and continuously into work. The efficiency of a heat engine designed to produce a net work output is defined as n h et ea w t o s r u k p o p u li t e p d ut Q W 1 ✔ Note that it is the heat supplied that we are trying to convert to work output, not the net heat transfer. ✔ Efficiency is the measure of success in achieving this. The highest possible efficiency that can be achieved is the Carnot efficiency. ✔ carnot 1 T T 2 1 ✔ The existence of entropy is a corollary of the second law. It is important to remember that entropy is a property, like pressure or temperature, but also that it provides a measure of order and irreversibility. It is not conserved, like mass of energy, and the entropy of the universe is increasing continuously as the result of the myriad irreversible processes taking place: an implication of the Clausius inequality is that entropy is created during an irreversible process; this may result in an increase in entropy of the system and/or of the surroundings.
eBook - ePub- Brian F. Towler(Author)
- 2014(Publication Date)
- Academic Press(Publisher)
How much heat can be transferred between two bodies depends on their temperatures and the method of heat transfer. Heat energy cannot be converted entirely into work or mechanical energy, electricity or any other form of energy. This has a big impact on the efficiency of heat engines. When fuel is burned in a heat engine, the chemical energy that is stored in the chemical bonds of the fuel molecules is released. This energy is transferred to the engine as heat; however, not all of that heat can be converted into mechanical energy. When people were developing and trying to perfect heat engines, such as the internal combustion engine, they discovered this limitation and had to understand it. The result was the formulation of the second law of thermodynamics.The seminal work in this area was due to a French engineer called Sadi Carnot. In 1824, he published a paper entitled, Reflections on the Motive Power of Fire and the Machines Needed to Develop This Power . This paper presented the idea that the amount of work done by a heat engine is due to the flow of heat from a hot to a cold body. Carnot’s understanding of heat was still mired in the incorrect caloric theory of heat, but his conclusions were still valid. His analysis determined that the theoretical heat that could be transferred to the heat engine was proportional to the temperature difference between the heat source (the hot body) and the heat sink (the cold body). This analysis allowed him to calculate the theoretical efficiency of a heat engine, which turned out to be much lower than the efficiency of other energy conversion processes.Using Carnot’s analysis, several people were able to deduce different statements of the second law of thermodynamics. Some of these are:
- S. Bobby Rauf(Author)
- 2021(Publication Date)
- River Publishers(Publisher)
Chapter 1 Introduction to Energy, Heat and Thermodyna mics INTRODUCTION The term “thermodynamics” comes from two root words: “ther-mo,” which means heat, and “ dynamic, ” meaning energy in motion, or power. This also explains why the Laws of Thermodynamics are some-times viewed as Laws of “Heat Power.” Since heat is simply thermal energy, in this chapter, we will review energy basics and lay the foundation for in depth discussion on heat en-ergy and set the tone for discussion on more complex topics in thermody-namics. ENERGY The capacity of an, object, entity or a system to perform work is called energy. Energy is a scalar physical quantity. In the International System of Units (SI), energy is measured in Newton-meters (N-m) or Joules, while in the US system of units, energy is measured in ft-lbf, Btu’s, therms or calories. In the feld of electricity, energy is measured in watt-hours, (Wh), kilowatt-hours (kWh), Gigawatt-hours (GWh), Terawatt-hours (TWh), etc. Units for energy, such as ft-lbs and N-m, point to the equivalence of energy with torque (moment) and work . This point will be discussed later in this chapter. Energy exists in many forms. Some of the more common forms of energy, and associated units, are as follows: 1) Kinetic Energy 1 ; measured in ft-lbf, Btus, Joules, N-m (1 N-m = 1 Joule), etc. Where, Btu stands for British thermal units 1 2 Thermodynamics Made Simple for Energy Engineers 2) Potential Energy 1 ; measured in ft-lbf, Btus, Joules, N-m, etc. 3) Thermal Energy 1 , or heat (Q); commonly measured in calories, Btus, Joules, therms, etc. 4) Internal Energy 1 , (U); commonly measured in Btu’s, calories or Joules. 5) Electrical Energy ; measured in Watt-hours (Wh), killowatt-hours (kWh) and horsepower-hours (hp-hrs), etc. 6) Gravitational Energy ; measured in ft-lbf, Joules, N-m, etc. 7) Sound Energy ; measured in Joules. 8) Light Energy ; measured in Joules. 9) Elastic Energy ; measured in ft-lbf, Btus, Joules, N-m, etc.
eBook - ePubA Mole of Chemistry
An Historical and Conceptual Approach to Fundamental Ideas in Chemistry
- Caroline Desgranges, Jerome Delhommelle(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
In 1848, Thomson defines a new scale of temperature, which now bears his name: the Kelvin scale. This absolute scale aims to replace the old scales (Fahrenheit, Celsius, etc.), which are based on the behavior of a fluid (mercury, alcohol, water, etc.) in a tube, therefore limiting their use. Therefore, Lord Kelvin decides to use the efficiency of a Carnot engine as a measure of temperature. Since the calculation of the efficiency does not rely on the nature of the fuel, engine configuration or even its power, it is thus possible to think of a scale of absolute temperature that would be universal! He proposes an absolute temperature scale in which “a unit of heat descending from a body A at the temperature T of this scale, to a body B at the temperature (T-1), would give out the same mechanical effect, whatever be the number T”. In other words, he proposes that the ratio of the heat received from the hot source to the heat yielded to the cold source be equal to the ratio of the absolute temperature of the hot source to that of the cold source; in other words, a Carnot engine, operating between 1,000K and 500K, rejects 500/1,000 = 50% of the heat it receives. If the low temperature is four times lower, the heat rejected is four times less (1,000/250) than the heat received. He also introduces the “absolute zero”, corresponding to the temperature for which no more heat can be transferred. This theoretical temperature is now fixed, following an international agreement, at −273.15°C. Let us add that the temperature scale used nowadays is the Kelvin scale (K) and is not referred to as degrees. For example, 0K = −273.15°C. It is the base unit of temperature in the International System of Units (SI).The second principle of thermodynamics starts from a historical statement; Thomson states “it is impossible, by means of an inanimate external agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects”. As we have seen before, according to Carnot, a thermal machine undergoes a caloric drop from the hot source to the cold source and it provides a mechanical work. However, the first principle tells us that there is energy conservation and work–heat equivalence. It is Clausius again, in 1856, who introduces another version of the second principle in his paper “On the moving force of heat”: “Heat cannot, of itself, pass from a colder to a hotter body”. A good example is the refrigerator, for which heat flows from cold to hot, but only when forced by an external agent, the refrigeration system!Nevertheless, an issue remains. Machines, such as the Newcomen engine or other engines, are rather inefficient. Only 2% of the input energy is converted into a useful work output and a great amount of useful energy is dissipated or lost. To better understand this energy loss, Clausius introduces in 1865 the concept of entropy, S (Greek: τροπή = transformation). He uses the unit “Clausius” (Cl) for entropy. In nature, entropy is also revealed by the fact that heat always flows spontaneously from a hot body to a cold body. It never goes in reverse (cold → hot) spontaneously, except if work is done on the system. In other words, the total entropy (S) never decreases: It stays the same or increases! Mathematically, it reads as ΔS ≥ ΔQ/T. At the end of Clausius’ book “The Mechanical Theory of Heat: With Its Applications to the Steam-engine and to the Physical Properties of Bodies”, we find the following two fundamental theorems of the mechanical theory of heat:- 1. “Die Energie der Welt ist constant
eBook - PDF- J. Edward Pope(Author)
- 1996(Publication Date)
- Gulf Professional Publishing(Publisher)
60 Rules of Thumb for Mechanical Engineers THERMODYNAMIC CYCLES A thermodynamic cycle can be either open or closed. In an open cycle, the working fluid is constantly input to the system (as in an aircraft jet engine); but in a closed cycle, the working fluid recirculates within the system (as in a re- frigerator). A vapor cycle is one in which there is a phase change in the working fluid. A gas cycle is one in which a gas or a mixture of gases is used, as the fluid that does not undergo phase change. Basic Systems and System Integration While, in theory, a cycle diagram explains the thermo- dynamic cycle, it takes a real device to achieve that ener- gy extraction process. The change of property equations for these devices can be derived from the steady-flow energy equation. Following is a list of these devices: 9Heat exchangers: transfer energy from one fluid to another. 9Pumps: considered adiabatic devices that elevate the total energy content of the fluid. 9 Turbines: adiabatic extraction of energy from the fluid with a drop in temperature and pressure. 9 Compressors: similar to pumps in principle. 9 Condensers: remove heat of evaporation from fluid and reject to the environment. 9Nozzles: convert the fluid energy to kinetic energy; adiabatic; no work is performed. Carnot Cycle The Camot cycle (Figure 4) is an ideal power cycle, but it cannot be implemented in practice. Its importance lies in the fact that it sets the maximum attainable thermal effi- ciency for any heat engine. The four processes involved are: 1-2 Isothermal expansion of saturated liquid to saturated gas 2-3 lsentropic expansion 3-4 Isothermal compression 4-1 Isentropic compression The heat flow in and out of the system and the turbine and compressor work terms are: Qin = Thigh (S2- Sl) = h2 - hi Qout = 1]o, (s3 - s4) = h3 - 114 Wturb = h2 - h3 Wcomp = h l - h4 The thermal efficiency of the cycle is: ~thermal = TH =..
eBook - PDF- Gerard Olivier, Vincent Lemort, Georges de Pelsemaeker(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
As depicted in Figure 1.18a, a heat engine comprises a working fluid that undergoes a cycle. During this evolution, the fluid receives and gives heat and work from/to its surroundings. The First Law expressed over one entire cyclic process can be written as Q su − Q ex = W ex − W su = W net,ex = −W net,su (1.106) The thermal efficiency of a heat engine is defined as the ratio of the net work produced by the system to the heat it receives from the heat source. 𝜂 = W net,ex Q su (1.107) The major example of heat engines covered in this book is the internal combustion engine. In the case of an internal combustion engine, the heat received by the engine corresponds to its fuel consumption m fuel multiplied by the low heating value LHV fuel of the fuel. In terms of rate of heat transfer and power, the thermal efficiency can be written as 𝜂 = ̇ W net,ex ̇ m fuel LHV fuel (1.108) Heat source T high Heat sink Heat engine Heat sink T high T low W ex W ex W su W su Q ex Q su Q ex Q su T low Heat source Heat pump Refrigerant Working fluid (a) (b) Figure 1.18 Coupling between a heat engine (a)/heat pump (b) and the heat source/heat sink. 1.6 Second Law of Thermodynamics 35 The low heating value LHV [J kg −1 ] is defined as the thermal energy released by the combustion of 1 kg of fuel if the water in the combustion gases stays in vapor phase. The high heating value HHV [J kg −1 ] is defined as the heat that is released by the combustion of 1 kg of fuel if the water in the combustion gases is condensed to liquid state. Hence, the high heating value of a fuel is higher than its low heating value. For instance, for gasoline, LHV = 44 [MJ kg −1 ]. 1.6.1.3 Refrigerators and Heat Pumps As explained previously, heat cannot flow spontaneously from a cold body to a hot body. To achieve such energy transfer, a refrigerator or heat pump is needed.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
