# THERMODYNAMICS 04

version from 2018-03-28 12:03

## Section

Gage pressureis the difference between the absolute pressure and the local atmospheric pressure.
Gas constantR is different for each gas and is determined from R = Ru/M.
Gas phase of a substancehas molecules that are far apart from each other, and a molecular order is nonexistent. Gas molecules move about at random, continually colliding with each other and the walls of the container they are in.
Gas power cyclesare cycles where the working fluid remains a gas throughout the entire cycle. Spark-ignition automobile engines, diesel engines, and conventional gas turbines are familiar examples of devices that operate on gas cycles.
Gas refrigeration cycleis based on the reversed Brayton cycle where the compressor exit gases are cooled and then expanded in a turbine to further reduce the temperature of the working fluid. The lower-temperature fluid is used to produce the refrigeration effect.
Generalized compressibility chartshows that by curve-fitting all the data, gases seem to obey the principle of corresponding states reasonably well.
Generalized enthalpy departure chartis a plot of the enthalpy departure factor as a function of reduced pressure and reduced temperature. It is used to determine the deviation of the enthalpy of a gas at a given P and T from the enthalpy of an ideal gas at the same T.
Generalized entropy departure chartis a plot of the entropy departure factor as a function of reduced pressure and reduced temperature. It is used to determine the deviation of the entropy of a gas at a given P and T from the entropy of an ideal gas at the same P and T.
Generatoris a device that converts mechanical energy to electrical energy.
Generator efficiencyis the ratio of the electrical power output to the mechanical power input.
Geothermal heat pumps(also called ground-source heat pumps) use the ground as the heat source.
Gibbs-Dalton lawan extension of Dalton's law of additive pressures, states that under the ideal-gas approximation, the properties of a gas in a mixture are not influenced by the presence of other gases, and each gas component in the mixture behaves as if it exists alone at the mixture temperature and mixture volume.
Gibbs function g is defined as g = h - Ts.
Gibbs phase ruleprovides the number of independent variables associated with a multicomponent, multiphase system.
Gravimetric analysisis one way to describe the composition of a mixture that is accomplished by specifying the mass of each component.
Gravitational acceleration g is 9.807 m/s2 at sea level and varies by less than 1 percent up to 30,000 m. Therefore, g can be assumed to be constant at 9.81 m/s2.
Heatis defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.
Heat-driven systemsare refrigeration systems whose energy input is based on heat transfer from an external source. Absorption refrigeration systems are often classified as heat-driven systems.
Heat enginesare devices designed for the purpose of converting other forms of energy (usually in the form of heat) to work.
Heat exchangersare devices where two moving fluid streams exchange heat without mixing. Heat exchangers are widely used in various industries, and they come in various designs. The simplest form of a heat exchanger is a double-tube (also called tube-and-shell) heat exchanger composed of two concentric pipes of different diameters. One fluid flows in the inner pipe, and the other in the annular space between the two pipes. Heat is transferred from the hot fluid to the cold one through the wall separating them. Sometimes the inner tube makes a couple of turns inside the shell to increase the heat transfer area, and thus the rate of heat transfer.
Heating valueof a fuel is defined as the amount of heat released when a fuel is burned completely in a steady-flow process and the products are returned to the state of the reactants. In other words, the heating value of a fuel is equal to the absolute value of the enthalpy of combustion of the fuel.
Heating value of a fuelis the amount of heat released when a specified amount of fuel (usually a unit mass) at room temperature is completely burned and the combustion products are cooled to the room temperature.
Heat pumpis a cyclic device which causes the transfer of heat from a low-temperature region to a high-temperature region. The objective of a heat pump is to maintain the heated space at a high temperature by supplying heat to it.
Heat pump coefficient of performanceis the efficiency of a heat pump, denoted by COPHP, and expressed as desired output divided by required input or COPHP = QH/Wnet, in.
Heat rateis the expression of the conversion efficiency of power plants in the United States and is the amount of heat supplied, in Btu's, to generate 1 kWh of electricity. The smaller the heat rate, the greater the efficiency.
Heat reservoiris a thermal energy reservoir since it can supply or absorb energy in the form of heat.
Heat sinkis a heat reservoir that absorbs energy in the form of heat.
Heat sourceis a heat reservoir that supplies energy in the form of heat.
Heat transferis the area under the process curve on a T-S diagram during an internally reversible process. The area has no meaning for irreversible processes.
Helmholtz function a is defined as a = u - Ts
Henry's lawstates that the mole fraction of a weakly soluble gas in the liquid is equal to the partial pressure of the gas outside the liquid divided by Henry's constant.
Higher heating value or HHV, is the heating value of the fuel when the water in the combustion gases is completely condensed and thus the heat of vaporization is also recovered. Efficiencies of furnaces are based on higher heating values.
Higher heating valueHHV) is the heating value when the H2O in the products is in the liquid form.
Humidifyingis the process of adding moisture to atmospheric air.
Hydrocarbon fuelsare the most familiar fuels and consist primarily of hydrogen and carbon. They are denoted by the general formula CnHm. Hydrocarbon fuels exist in all phases, some examples being coal, gasoline, and natural gas.
Hypersonic flowoccurs when a flow has a Mach number M >>1.
Ideal cycleis an actual cycle stripped of all the internal irreversibilities and complexities. The ideal cycle resembles the actual cycle closely but is made up totally of internally reversible processes.
Ideal gasis a gas that obeys the ideal-gas equation of state.
Ideal-gas equation of stateor ideal-gas relation) predicts the P-v-T behaviour of a gas quite accurately within some properly selected region where Pv = RT.
Ideal gas specific heat relation is Cp = Cv + R.
Ideal gas temperature scale is a temperature scale that turns out to be identical to the Kelvin scale. The temperatures on this scale are measured using a constant-volume gas thermometer, which is basically a rigid vessel filled with a gas, usually hydrogen or helium, at low pressure.
Ideal mixture or ideal solution is a mixture where the effect of dissimilar molecules in a mixture on each other is negligible and the chemical potential of a component in such a mixture is simply taken to be the Gibbs function of the pure component.are devices designed for the purpose of converting other forms of energy (usually in the form of heat) to work.
Ignition temperatureis the minimum temperature to which a fuel must be brought to start the combustion.
Immediate surroundingsrefer to the portion of the surroundings that is affected by the process.
Incomplete combustionis a combustion process in which the combustion products contain any unburned fuel or components such as C, H2, CO, or OH.
Incompressible substancessuch as liquids and solids, have densities that have negligible variation with pressure.
Increase of entropy principle or second law of thermodynamics is expressed as the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. In other words, the entropy of an isolated system never decreases.
Independent propertiesexist when one property can be varied while another property is held constant.
Inert gasis a gaseous component in a chemical reaction that does not react chemically with the other components. The presence of inert gases affects the equilibrium composition (although it does not affect the equilibrium constant).
Intake valveis an inlet through which the air or air-fuel mixture is drawn into the cylinder.
Intensive propertiesare those that are independent of the size of a system, such as temperature, pressure, and density.
Intensive properties of a nonreacting ideal-or real-gas mixture are obtained by dividing the extensive properties by the mass or the mole number of the mixture in the gas mixture. The internal energy, enthalpy, and entropy of a gas mixture per unit mass or per unit mole of the mixture can be determined summing the products of the mass fractions and the specific property or summing the products of the mole fractions and the molar specific property. That is, the intensive properties of a gas mixture are determined by either a mass weighted average of the properties or a mole weighted average of the properties.
Internal combustion enginesare engines where the energy is provided by burning a fuel within the system boundaries.
Internal energy U of a system is the sum of all the microscopic forms of energy.
Internally reversible processhas no irreversibilities occurring within a system undergoing the process.
Internally reversible process hasno irreversibilities that occur within the boundaries of the system during the process. During an internally reversible process, a system proceeds through a series of equilibrium states, and when the process is reversed, the system passes through exactly the same equilibrium states while returning to its initial state.
Inversion lineis the line that passes through the points of zero slope of constant-enthalpy lines or zero Joule-Thomson coefficient on the T-P diagram. The slopes of the h = constant lines are negative (mJT< 0) at states to the right of the inversion line and positive (mJT> 0) to the left of the inversion line.
Inversion temperatureis the temperature at a point where a constant-enthalpy line intersects the inversion line.
Irreversibilitiesare the factors that cause a process to be irreversible. They include friction, unrestrained expansion, mixing of two gases, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions.
IrreversibilityI is any difference between the reversible work Wrev and the useful work Wu due to the irreversibilities present during the process. Irreversibility can be viewed as the wasted work potential or the lost opportunity to do work.
Irreversible processesare processes which, once having taken place in a system, cannot spontaneously reverse themselves and restore the system to its initial state.
Isentropic efficiency of a compressoris defined as the ratio of the work input required to raise the pressure of a gas to a specified value in an isentropic manner to the actual work input.
Isentropic efficiency of a nozzleis defined as the ratio of the actual kinetic energy of the fluid at the nozzle exit to the kinetic energy value at the exit of an isentropic nozzle for the same inlet state and exit pressure.
Isentropic efficiency of a turbineis defined as the ratio of the actual work output of the turbine to the work output that would be achieved if the process between the inlet state and the exit pressure were isentropic.
Isentropic processis an internally reversible and adiabatic process. In such a process the entropy remains constant.
Isentropic stagnation stateis the stagnation state when the stagnation process is reversible as well as adiabatic (i.e., isentropic). The entropy of a fluid remains constant during an isentropic stagnation process.
Isoprefix is often used to designate a process for which a particular property remains constant.
Isobaric processis a process during which the pressure P remains constant.
Isochoric (or isometric) processis a process during which the specific volume v remains constant.
Isolated systemis a closed system in which energy is not allowed to cross the boundary.
Isothermal compressibilityrelates how volume changes when pressure changes when temperature is held constant.
Isothermal efficiency of a compressoris defined as the ratio of the work input to a compressor for the reversible isothermal case and the work input to a compressor for the actual case.
Isothermal processis a process during which the temperature T remains constant.
Jet-propulsion cycleis the cycle used in aircraft gas turbines. The ideal jet-propulsion cycle differs from the simple ideal Brayton cycle in that the gases are not expanded to the ambient pressure in the turbine. Instead, they are expanded to a pressure such that the power produced by the turbine is just sufficient to drive the compressor and the auxiliary equipment. The gases that exit the turbine at a relatively high pressure are subsequently accelerated in a nozzle to provide the thrust to propel the aircraft.