In making cycle performance calculation, it is first necessary to establish data relating to the properties of the working fluid of turbo-machinery. Because the composition of natural gas combustion products varies, along with the fuel chemistry, unique formulae for their gas properties do not exist, hence the calculation becomes more complex. Thermodynamic properties depend not only on composition of gases but also on assumptions concerning the change of composition during each process in which temperature, pressure or some other thermodynamic state functions may be shifting. In the present study, a calculation method of gas constants, heat capacity, enthalpy of dry, humid air and combustion products of organic fuel are studied. A theoretical base of thermodynamic properties of gas mixtures constructed on a model of ideal gas is formulated. The composition products are represented by the mixture of ideal gases, humid air and steam directly participating in the burning process and the products of stoichiometric combustion of any organic fuel. The developed calculation procedure is validated through comparison with tabulated data in open literatures and it presented satisfactory results. Recommendation for the application of the suggested method with restriction in relation to range of temperature for static and dynamic calculation is also given. It is supposed that the present method and its result can be used in the preliminary design stage of heat exchangers, high temperature gas turbine units and boilers.

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