# THERMODYNAMICS 02

version from 2018-03-28 12:24

## Section

Closed system (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary. But energy, in the form of heat or work, can cross the boundary.
Coefficient of performance(COP) is the measure of performance of refrigerators and heat pumps. It is expressed in terms of the desired result for each device (heat absorbed from the refrigerated space for the refrigerator or heat added to the hot space by the heat pump) divided by the input, the energy expended to accomplish the energy transfer (usually work input).
Cogenerationis the production of more than one useful form of energy (such as process heat and electric power) from the same energy source.
Cold-air-standard assumptioncombines the air-standard assumptions with the assumption that the air has constant specific heats whose values are determined at room temperature (25°C, or 77°F).
Combined gas-vapour cycle, or just the combined cycleis the gas-turbine (Brayton) cycle topping a steam-turbine (Rankine) cycle, which has a higher thermal efficiency than either of the cycles executed individually.
Combustionis a chemical reaction during which a fuel is oxidized and a large quantity of energy is released.
Combustion airis dry air which can be approximated as 21 percent oxygen and 79 percent nitrogen by mole numbers. Therefore, each mole of oxygen entering a combustion chamber will be accompanied by 0.79/0.21 = 3.76 mol of nitrogen. To supply one mole of oxygen to a combustion process, 4.76 mol of combustion air are required.
Combustion efficiencycombustion equipment is defined as the amount of heat released during combustion divided by the heating value of the fuel. A combustion efficiency of 100 percent indicates that the fuel is burned completely and the stack gases leave the combustion chamber at room temperature, and thus the amount of heat released during a combustion process is equal to the heating value of the fuel.
Complete combustionis a combustion process in which all the carbon in the fuel burns to CO2, all the hydrogen burns to H2O, and all the sulphur (if any) burns to SO2. That is, all the combustible components of a fuel are burned to completion during a complete combustion process.
Component pressureis the pressure a component in a gas mixture would have if it existed alone at the volume and temperature of the mixture.
Component volumeis the volume a component in a gas mixture would occupy if it existed alone at the temperature and pressure of the mixture.
Compressed liquidhas a pressure greater than the saturation pressure corresponding to the temperature.
Compressed liquid regionis all compressed liquid states located in the region to the left of the saturated liquid line and below the critical temperature line. In the absence of compressed liquid data, a general approximation is to treat compressed liquid as saturated liquid at the given temperature.
Compressibility factorZ is a correction factor to account for deviation from ideal-gas behaviour at a given temperature and pressure. Z = Pv/RT.
Compression-ignition (CI) engines are reciprocating engines in which the combustion of the air-fuel mixture is self-ignited as a result of compressing the mixture above its self-ignition temperature.
Compression ratio r of an engine is the ratio of the maximum volume formed in the cylinder to the minimum (clearance) volume. Notice that the compression ratio is a volume ratio and should not be confused with the pressure ratio.
Compressor is a device that increases the pressure of a gas to very high pressures.
Condenseris a heat exchanger in which the working fluid condenses as it rejects heat to the surroundings.
Condenser is a heat exchanger in which a vapour, such as steam, condenses to the saturated liquid state as the result of heat transfer from the vapour to a cooling medium such as a lake, a river, or the atmosphere.
Conductionis the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles.
Conservation of energy principleor energy balance based on the first law of thermodynamics may be expressed as follows: Energy can be neither created nor destroyed; it can only change forms. The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. The energy balance can be written explicitly as Ein - Eout =(Qin -Qout ) + (Win -Wout ) + (Emass, in- Emass, out ) = DEsystem
Conservation of mass principleis expressed as net mass transfer to or from a system during a process equal to the net change (increase or decrease) in the total mass of the system during that process.
Conservation of mass principle (or the mass balance)is the principle used to balance chemical reaction equations. It can be stated as follows: The total mass of each element is conserved during a chemical reaction. The total mass of each element on the right-hand side of the reaction equation (the products) must be equal to the total mass of that element on the left-hand side (the reactants) even though the elements exist in different chemical compounds in the reactants and products. Even though the mass must be conserved, the total number of moles is not conserved during a chemical reaction.
Continuity equation is the conservation of mass equation as it is often referred to in fluid mechanics.
Continuum is a view of mass as continuous, homogeneous matter with no holes. Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance. The continuum idealization allows us to treat properties as point functions, and to assume the properties to vary continually in space with no jump discontinuities. This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules. This is the case practically in all problems, except some specialized ones.
Control surfaceis the boundary of a control volume, and it can be real or imaginary.
Control volume, or open systemis any arbitrary region in space through which mass and energy can pass across the boundary. Most control volumes have fixed boundaries and thus do not involve any moving boundaries. A control volume may also involve heat and work interactions just as a closed system, in addition to mass interaction.
Convection is the transfer of energy between a solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion.
Converging-diverging nozzlesare ducts in which the flow area first decreases and then increases in the direction of the flow.
Cooling capacityis the rate of heat removal from the refrigerated space by a refrigeration system.
Cooling pondis a large lake open to the atmosphere into which warm water containing waste heat is pumped. Heat transfer from the pond surface to the atmosphere is very slow, thus about 20 times the area of a spray pond is needed in this case to achieve the same cooling.
Criterion for chemical equilibriumis the equation set equal to zero that involves the stoichiometric coefficients and the molar Gibbs functions of the reactants and the products in the equilibrium reaction.
Critical pointis defined as the point at which the saturated liquid and saturated vapour states are identical.
Critical pressurePcr is the pressure of a substance at the critical point.
Critical propertiesare the properties of a fluid at a location where the Mach number is unity.
Critical ratiosare the ratios of the stagnation to static properties when the Mach number is unity.
Critical temperatureTcr is the temperature of a substance at the critical point.
Critical volume vcr is the volume of a substance at the critical point.
Cutoff ratio rc is the ratio of the cylinder volumes after and before the combustion process in the Diesel cycle.
Cycleis a process, or series of processes, that allows a system to undergo state changes and returns the system to the initial state at the end of the process. That is, for a cycle the initial and final states are identical.
Cyclic relation of partial derivatives shows that the derivatives of a function of two variables are related in a cyclic manner.

Dalton's law of additive pressuresThe pressure of a gas mixture is equal to the sum of the pressures each gas would exert if it existed alone at the mixture temperature and volume.
Dead stateis a state a system is said to be in when it is in thermodynamic equilibrium with its environment.
Decrease of exergy principlecan be expressed as the exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant. In other words, it never increases and exergy is destroyed during an actual process. For an isolated system, the decrease in exergy equals exergy destroyed.
Deficiency of airresults when the amounts of air are less than the stoichiometric amount.
Dehumidifyingis the process of removing moisture from atmospheric air.
Densityis defined as mass per unit volume.
Derivative of a function The derivative is equivalent to steepness of a curve at a point as measured by the slope of a line tangent to the curve at that point.
Dew-point temperatureis defined as the temperature at which condensation begins when the air is cooled at constant pressure.
Diffuseris a device that increases the pressure of a fluid by decreasing the fluid velocity.
Diffuser efficiencyis a measure of a diffuser's ability to increase the pressure of the fluid. It is expressed in terms of the ratio of the kinetic energy that can be converted to pressure rise if the fluid is discharged at the actual exit stagnation pressure to the maximum kinetic energy available for converting to pressure rise. These two quantities are identical for an isentropic diffuser since the actual exit stagnation pressure in this case becomes equal to the inlet stagnation pressure, yielding an efficiency of 100 percent.
Dimensionally homogeneousmeans that every term in an equation must have the same unit. To make sure that all terms in an engineering equation have the same units is the simplest error check one can perform.
Dimensionsare any physical characterizations of a quantity.Discharge coefficienta parameter that is used to express the performance of a nozzle, is defined as the ratio of the mass flow rate through the nozzle to the mass flow rate through the nozzle for isentropic flow from the same inlet state to the same exit pressure.
Displacement volumeis the volume displaced by the piston as it moves between top dead center and bottom dead center.
Domeis the saturation states located beneath the joined saturated liquid line and saturated vapour line.
Dry airis air that contains no water vapour.
Dry-bulb temperatureis the ordinary temperature of atmospheric air.
Dual cycleis the ideal cycle which models the combustion process in both gasoline and diesel engines as a combination of two heat-transfer processes, one at constant volume and the other at constant pressure.
Dynamic temperatureis the kinetic energy per unit mass divided by the constant pressure specific heat and corresponds to the temperature rise during the stagnation process.

Latent energyis the internal energy associated with the phase of a system.
Latent heatis the amount of energy absorbed or released during a phase-change process.
Latent heat of fusionis the amount of energy absorbed during melting and is equivalent to the amount of energy released during freezing.
Latent heat of vaporizationis the amount of energy absorbed during vaporization and is equivalent to the energy released during condensation.
Lighting efficacyis defined as the amount of light output in lumens per W of electricity consumed.
Liquefied petroleum gas(LPG) is a by-product of natural gas processing or crude oil refining. It consists mainly of propane (over 90 percent), and thus LPG is usually referred to as propane. However, it also contains varying amounts of butane, propylene, and butylenes.
Liquid-vapour saturation curveis a plot of saturation temperature Tsat versus saturation pressure Psat.
Lower heating valueor LHV, is the heating value of the fuel when the water in the combustion gases is a vapour. Efficiencies of cars and jet engines are normally based on lower heating values since water normally leaves as a vapour in the exhaust gases, and it is not practical to try to recuperate the heat of vaporization.
Lower heating value(LHV) is the heating value when the H2O in the products is in the vapour form.
Mach number is the ratio of the actual velocity of the fluid (or an object in still air) to the velocity of sound in the same fluid at the same state.
Macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies.
Manometeris a device based on the principle that an elevation change of Δ z of a fluid corresponds to a pressure change of ΔP/ ρg, which suggests that a fluid column can be used to measure pressure differences. The manometer is commonly used to measure small and moderate pressure differences.
Mass flow rateis the amount of mass flowing through a cross section per unit time.
Mass of a systemis equal to the product of its molar mass M and the mole number N.
Maximum inversion temperatureis the temperature at the intersection of the P= 0 line (ordinate) on the T-P diagram and the upper part of the inversion line.
Maxwell relationsare equations that relate the partial derivatives of properties P, v, T, and s of a simple compressible system to each other.
Mayer relationnamed in honour of the German physician and physicist J. R. Mayer (1814-1878, shows how the difference between the constant-pressure specific heat and constant-volume specific heat is related to the specific volume, temperature, isothermal compressibility, and volume expansivity.
Mean effective pressure (MEP) is a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle.