Thermal Sciences and Mechanical Engineering Sections 1. Introduction、2. The Concept of Temperature 、3. Heat Transfer 4. Thermodynamics 、5. Thermal Deformation、6. Heat Treatment 7. Energy and Environment、8. Summary Objectives After learning this chapter, you should be able to do the following : Understand the concept of temperature and convert a temperature from one scale to another. . Discuss the different modes of heat transfer. . Understand that the design or analysis of a machine operating in a thermal cycle is governed by the laws of thermodynamics. 1、Introduction Thermal science is an area of scientific thought encompassing heat transfer and thermodynamics. . ( Heat transfer deals with the transfer of thermal energy. ( Thermodynamics deals with converting heat to work and understanding the role of energy and other properties of matter in this conversion process. Thermal Machinery Thermal Machinery wherein the working fluid or substance undergoes a thermal cycle includes: Power system ( Heat engine ) Refrigeration system Some systems produce work, such as internal combustion engines, fossil-fuel power plants; . (power system) The thermal efficiency ηth of a power system : @Thermal efficiency is a measure of how efficiently a heat engine converts the heat that it received to work.. Other systems require work input to produce other effects , such as refrigerators, air-conditioning systems. (refrigeration system) Coefficient of performance (COP) for a refrigeration system : An objective of a mechanical engineer working in designing thermal machinery is to improve the efficiency or COP of these devices. 2、 The Concept of Temperature Temperature is a measurement of the energy of molecules due to their rapid movement. (the degree of hotness or coldness of a substance.) . Thermometer measures the temperature by the expansion and contraction of a liquid (usually mercury) in a glass tube. The equality of temperature is the only requirement for thermal equilibrium. 2.1 The 0th Law of Thermodynamics If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. Notes It may seem silly, but it cannot be concluded from the other laws of thermodynamics, and it serves as a basis for the validity of temperature measurement. . . ( Its value as a fundamental physical principle was recognized more than half a century after the formulation of the 1st and the 2nd laws. It is so named because it logically precedes the 1st and the 2nd laws. 2.2 Temperature Scales The Fahrenheit scale places the freezing point of water at 32 degrees and the boiling point of water at 212 degrees. The Celsius scale places the freezing point at zero degrees and the boiling point at 100 degrees. The relationship between the Celsius scale and the Fahrenheit scale : The Kelvin scale is an absolute scale, with zero defined as when the motion of all molecules ceases. This point occurs at . The Kelvin scale is independent of the properties of any substance. The magnitude of each division of 1K and 1℃ are identical, i.e., the temperature interval on both scales is the same. 2.3 Specific Heat The specific heat is defined as the amount of heat required to raise the temperature of a unit mass (or unit quantity, such as mole) of a substance by one degree Celsius. . The relationship between heat and temperature change is usually expressed in the form shown below where c is the specific heat. . Notes The relationship does not apply if a phase change is encountered, because the heat added or removed during a phase change does not change the temperature. . The specific heats of most solids at room temperature and above are nearly constant. At lower temperatures the specific heats drop as quantum processes become significant. . 2.4 Calorie calorie, abbr. cal, is the unit of heat energy in the metric system. The calorie, is the quantity of heat required to raise the temperature of 1 gram of pure water 1°C. Heat is commonly expressed in the calorie, an older metric unit. . Calorie is usually used in describing the energy content of food. . Scientists express heat in terms of the joule, a unit used for all forms of energy. 3. Heat Transfer Heat transfer will occur whenever there is a temperature difference. And heat transfer is from the high-temperature medium to a lower-temperature one. . Heat is transferred by three mechanisms: conduction, convection, and radiation. Heat transfer is quantified in terms of the heat Q (in Joules. J), heat rate (in Watts, W), or heat per unit area (W/m2), called the heat flux. Conduction Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. Conduction can take place in solids, liquids, or gases. Metals and metallic alloys are good conductors of heat whereas air, wood, plastic, glass are poor conductors. |Fourier’s law of heat conduction : k is the thermal conductivity of the material, ▽T is the temperature gradient. It indicates that the heat flux of conduction in a direction is proportional to the temperature gradient in that direction. Example 1 On a cold day, the temperature outside of a house reaches -30℉, whereas the temperature inside the house is 70 ℉. What is the heat transferred through a wall of the house with thickness of 4.5 inches if the wall is made of concrete with a thermal conductivity of 1 W/(M·K) ? Solution First, the temperature values are converted to Celsius. We find that –30 ℉ = – 34.4 ℃ and 70 ℉ = 21 ℃. Also, 4.5 inches = 0.1143 m. Then, by substitution into Equation 7-4, we get : Convection Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion. The faster the fluid motion, the greater the convection heat transfer. In the absence of fluid motion, heat transfer between a solid surface and adjacent fluid is by pure conduction. . Forced convection : if the fluid is forced to flow in a tube or over a surface by external means such as a fan, pump, or the wind. . Natural convection : if the fluid motion is caused by buoyancy forces induced by density differences due to the variation of temperature in the fluid. . (free convection) |Newton’s law of heat convection : h is called the convection coefficient [W/(m2·K) ]; yThe convection coefficient h is not a property of the fluid. Its value depends on all the variables that influence convection. Radiation Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules. The transfer of energy by radiation does not require the presence of an intervening medium. . |The Stefan-Boltzman law gives the maximum amount of energy that may be transmitted : σ= 5.67×10-8 W/(m2·K4) is the Stefan-Boltzman constant Ts is the temperature of the radiating surface. yA surface that radiates energy according to the Stefan-Boltzmann law is called an ideal radiator, or blackbody. A real surface emits radiation at a lower value is called a graybody. . The expression for a real radiator : ε is called the emissivity of the surface, which has a value between zero and one. 4. Thermodynamics In the most general sense thermodynamics is the study of energy—its transformations and its relationship to the properties of matter. . In its engineering applications thermodynamics has two major objectives. . One is to describe the properties of matter when it exists in what is called an equilibrium state, a condition in which its properties show no tendency to change. . The other objective is to describe processes in which the properties of matter undergo changes and to relate these changes to the energy transfers in the form of heat and work which accompany them. 4.1 The 1st Law of Thermodynamics The principle of energy conservation for a system with a thermodynamic state: 4.2 The 2nd Law of Thermodynamics There are two common statements for the 2nd law of thermodynamics. . The Kelvin-Planck statement : It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. . No heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment. The Clausius statement : It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body. In other words, the spontaneous flow of heat from hot to cold bodies is reversible only with the expenditure of mechanical or other nonthermal energy. If we want to transfer heat from a cooler temperature to a warmer temperature, then work input is required. This is how a refrigerator works.. Notes Both the two statements of the 2nd law are negative statements, and a negative statement cannot be proved. . The two statements are equivalent in their consequences, and either statement can be used as the expression of the 2nd law. . Any device that violates the Kelvin-Planck statement also violates the Clausius statement, and vice versa. . Any device that violates either 1st law or 2nd law of thermodynamics is called a perpetual-motion machine. . Example 3 A car engine produces 136 hp on the output shaft with a thermal efficiency of 30%. The fuel it burns gives 35 000 kJ/kg as energy release. Find the total rate of energy rejected to the ambient and the rate of fuel consumption in kg/s. Solution From the definition of a heat engine efficiency and the conversion of hp we have: The energy equation for the overall engine gives: From the energy release in the burning we have: Notes An actual engine rejects energy to the ambient through the radiator cooled by atmospheric air, as heat transfer from the exhaust system and the exhaust flow of hot gases. Entropy （熵） The second law is expressed mathematically in terms of the concept of entropy. When a body absorbs an amount of heat Q from a reservoir at temperature T, the body gains and the reservoir loses an amount of entropy S = Q / T . If an amount of heat Q flows from a hot to a cold body, the total entropy increases; because S=Q/T is larger for smaller values of T, the cold body gains more entropy than the hot body loses. . The statement that heat never flows from a cold to a hot body can be generalized by saying that in no spontaneous process does the total entropy decrease. 4.3 Reversible & Irreversible Process A process is reversible if the system and its surroundings can be returned to their initial states. . The system and its surroundings cannot be restored to their initial states if the process is irreversible. . Notes All real processes are irreversible but many of our calculations ignore that fact because the assumption of reversibility makes the calculations easier. . We go ahead assuming most processes are reversible and then correct our calculations to get approximate answers. The corrections are based on experience. . Our goal as engineers is to minimize the degree of irreversibility. 4.4 The 3rd Law of Thermodynamics A postulate related to but independent of the 2nd law is that it is impossible to cool a body to absolute zero by any finite process. Although one can approach absolute zero as closely as one desires, one cannot actually reach this limit. . The 3rd law of thermodynamics, formulated by Walter Nernst and also known as the Nernst heat theorem, states : . The limiting value of the entropy of a system can be taken as zero as the absolute value of temperature approaches zero. 5. Thermal Deformation Thermal deformation means that as the thermal energy (and temperature) of a material increases, so does the vibration of its atoms/molecules ; . and this increased vibration results in a stretching of the molecular bonds - which causes the material to expand. Thermal expansion Thermal contraction 5.1 Linear expansion For a long rod the main thermal deformation occurs along the length of the rod, where α is the linear coefficient of expansion for the material, and is the fractional change in length per degree change in temperature. . The term 'L' represents the initial length of the rod.. Over small temperature ranges, the thermal expansion is described by the coefficient of linear expansion. 5.2 Thermal Stress If the structure or members of the structure are constrained such that the thermal expansion can not occur, then a significant thermal stress may arise which can effect the structure substantially. . There are many cases where structures and materials are near or at their allowable stresses. In that case, if a thermal stress develops, the total stress may well exceed the allowable stress and cause the structure to fail. This is the reason bridges are built with expansion joints which allow the structure to expand and contract freely and thus avoid thermal stresses. . Additionally, this is why concrete sidewalks are built with spaces separating adjacent slabs, allowing expansions to avoid thermal stresses. 6. Heat Treatment Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. There are five basic heat treating processes: hardening (quenching), tempering, annealing, normalizing, and case hardening. . Although each of these processes bring about different results in metal, all of them involve three basic steps: heating, soaking, and cooling. Basic Steps Heating is the first step in a heat-treating process. Many alloys change structure when they are heated to specific temperatures. . Once a metal part has been heated to the desired temperature, it must remain at that temperature until the entire part has been evenly heated throughout. This is known as soaking. . The third step is cooling. Metals can be made to conform to specific structures in order to increase their hardness, toughness, ductility, tensile strength, and so forth. . Heat Treatment of Ferrous Metals Hardening ( Quenching ) 淬火 A ferrous metal is normally hardened by heating the metal to the required temperature and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil, water, or brine. . The hardening process increases the hardness and strength of metal, but also increases its brittleness. Tempering 回火 Severe internal stresses are set up during the rapid cooling of the metal. Steel is tempered after being hardened to relieve the internal stresses and reduce its brittleness. . Tempering consists of heating the metal to a specified temperature and then permitting the metal to cool in still air. . Temperatures used for tempering are normally much lower than the hardening temperatures. Annealing 退火 Metal is annealed by heating it to a prescribed temperature, holding it at that temperature for the required time, and then cooling it slowly back to room temperature. Annealing is used to relieve internal stresses, soften them, make them more ductile, and refine their grain structures Normalizing 正火 Normalizing is achieved by heating the metal to a specified temperature (which is higher than either the hardening or annealing temperatures), soaking the metal until it is uniformly heated, and cooling it in still air. Ferrous metals are normalized to relieve the internal stresses produced by machining, forging, or welding. . Steel is much tougher in the normalized condition than in any other condition. Parts that will be subjected to impact and parts that require maximum toughness and resistance to external stresses are usually normalized. Case Hardening 表面强化 During the case-hardening process, a low-carbon steel is heated to a specific temperature in the presence of a material which decomposesand deposits more carbon into the surface of a steel. Then, when the part is cooled rapidly, the outer surface or case becomes hard, leaving the inside of the piece soft but very tough. Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and a tough core, such as gears, cams, cylinder sleeves, and so forth. The most common case-hardening processes are carburizing and nitriding. Carburizing is a process of diffusing Carbon into the surface of steel. (渗碳) . Nitriding is a process of diffusing Nitrogen into the surface of steel. (渗氮) 7. Energy and Environment Pollutants emitted during the combustion of fossil fuels are responsible for smog, acid rain, and global warming and climate change (the greenhouse effects). . The environmental pollution has reached such high levels that it became a serious threat to vegetation, wild life, and human health. Air pollution has been the cause of numerous health problems including asthma and cancer. Acid Rain The sulfur oxides and nitric oxides react with water vapor and other chemicals high in the atmosphere in the presence of sunlight to form sulfuric and nitric acids. . The acids formed usually dissolve in the suspended water droplets in clouds or fog. These acid-laden droplets, are washed from the air on to the soil by rain. 8. Summary Mechanical engineers involved in the design and analysis of machinery must be aware of the change in the thermodynamic state during the cycle of the machinery. . The total energy must be conserved, as stated by the first law of thermodynamics. Real processes always obey the first law of thermodynamics. As the second law of thermodynamics states, not all the available heat can be converted to work, since some heat is always lost to the surroundings. . Natural heat transfer is from a body at a high temperature to a body at a low temperature. The potential of heat to go naturally from high to low temperatures can be utilized to produce work through a heat engine. .