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 Natural convection: if the fluid motion is caused by buoyancy forces induced by density differences due to the variation of temperature in the fluid lewton '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 0=5.67x10-8 W/(m2K4)is the Stefan-Bolteman 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 urface emits radiation at a lower value is called a graybody The expression for a real radiator e 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 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 singl 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 NotesConvection 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 forcesinduced 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) ]; ýThe 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. ýA 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