Theoretically Estimating the Performance of Gas Turbines Under Varying Atmospheric Conditions

It is well known that the performance of internal combustion engines is affected by varying atmospheric conditions, namely by the atmospheric temperature, pressure, and humidity. However, except for variable-speed gas turbines, performance correction formulas have not been established for gas turbines as they have for reciprocating engines. Although performance correction formulas, based on dimensional analysis, have been proposed for variable-speed gas turbines, these formulas are not applicable to the constant-speed gas turbines which are commonly used to drive generators. The correction quantities of gas turbines must be obtained experimentally. But, in general, a gas turbine, compared to a reciprocating engines, consumes such a large amount of air that conducting experiments at various atmospheric conditions in an environmental laboratory is hardly feasible. Therefore it is necessary to establish highly accurate performance correction formulas theoretically. The purpose of this paper, as a basic study in the development of correction formulas, is to analyze theoretically the effects of atmospheric conditions on the performance of gas turbines. For the purpose of analysis, the following concepts are introduced: (1) the use of moist air in place of dry air as the working medium, thus enabling one to estimate the effects of humidity; (2) the idea of “the specific output per unit volume” in addition to the specific output per unit mass and the thermal efficiency; (3) the effect of air temperature on the pressure ratio and the efficiency of the compressor. In the performance analysis, the following two methods are adopted: first, the changes in the efficiencies of the components are neglected while varying atmospheric conditions; second, as a more precise method, the changes in the efficiencies of the components are also considered. Finally, in order to confirm the validity of the assumptions and the calculation methods, the theoretical values obtained are compared with actual gas turbine data. As the result of this paper, the effects of varying atmospheric conditions on the performance of gas turbines are described quantitatively, so that the resultant change in performance can be estimated without experiment.