Evaluation of the entropy integral for a real gas

Gas turbines will pass a mixture of gases normally dominated by air but also containing the products of hydrocarbon combustion. It is found that the specific heat is a function of temperature only, and the gas compressibility factor stays very close to unity, Z % 1, for sdl industrial applications. This latter fact idlows the second term on the right-hand side of equation (16.40) to be integrated analytically  

We may express the remaining definite integral as the difference of integrals between 0 and Tb and 0 and 

Let us now introduce a new thermodynamic variable, which is a function of temperature only  

This variable is the temperature-dependent component of specific entropy, defined to be zero at a temperature of absolute zero. We will call it simply phi , J/(kgK). Substituting back into equation (16.47) gives  

It has been found more convenient to list phi rather than specific entropy in gas tables, which are thereby made independent of pressure. 

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