Polytropic Process


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Polytropic process

A process which occurs with an interchange of both heat and work between the system and its surroundings. The nonadiabatic expansion or compression of a fluid is an example of a polytropic process. The interrelationships between the pressure (P) and volume (V) and pressure and temperature (T) for a gas undergoing a polytropic process are given by Eqs. (1) and

(1) 
(2) 
(2), where a and b are the polytropic constants for the process of interest. These constants, which are usually determined from experiment, depend upon the equation of state of the gas, the amount of heat transferred, and the extent of irreversibility in the process. See Isentropic process, Isothermal process, Thermodynamic processes

Polytropic Process

 

a change in the state of a physical system where the system’s specific heat C remains constant. The curve representing a polytropic process on a thermodynamic diagram is called a polytropic curve.

A simple example of a reversible polytropic process is the polytropic change of an ideal gas. This process is defined by the equation pVn= const, where p is the pressure of the gas, V is the volume of the gas, and n = (C— CP)/(C— Cv) is the polytropic exponent (Cp and Cv are the specific heats of the gas at constant pressure and constant volume, respectively). By using the equation of state for an ideal gas, the equation of a polytropic curve can be written in a different form: pTn/(1-n)= const or VT1(1-n) = const, here T is the absolute temperature. Special cases of the equation of a polytropic process for an ideal gas are the equations for an isentropic curve, where C = 0 and n = Cp/cv (this ratio of specific heats is designated γ); an isobar, where C— Cp and n = 0; an isochor, where C= Cv and n = ∞; and an isotherm, where C= ∞ and n = 1. The work A done by an ideal gas against the ambient pressure is determined from the formula

where the subscripts 1 and 2 refer to the initial and final states of the gas.

Engineering thermodynamics makes extensive use of the concept of polytropic processes in investigating the operating cycles of heat engines.

polytropic process

[¦päl·i¦träp·ik ′prä·səs]
(thermodynamics)
An expansion or compression of a gas in which the quantity pV n is held constant, where p and V are the pressure and volume of the gas, and n is some constant.
References in periodicals archive ?
The assumption of a polytropic process is quite good during the compression and expansion strokes, but is obviously incorrect during the combustion process.
Compression with simultaneous heat transfer is frequently modeled as a polytropic process, [Pv.sup.n] = constant, in which the polytropic exponent, n, is a function of refrigerant properties, pressure ratio, ([P.sub.o]/[P.sub.i]), and compressor design (Gosling 1980; Moran and Shapiro 1995; Popovic and Shapiro 1995; Stoecker 1982; Threlkeld 1970).
However, for a spark-ignited engine, the end gas is compressed by the propagating flame and the temperature rises following a polytropic process. Livengood and Wu [19] proposed that the end gas auto-ignition chemistry is cumulative and can be predicted by integrating the reaction rate of the end gas at discretized pressure and temperature time steps until the critical time when the integral value is equal to one (L-W knock integral).