THE EFFECT OF THE ISENTROPIC EXPONENT ON TURBINE CHARACTERISTICS

It's common practice to use characteristic maps based on similarity la ws to describe the performance of turbomachines for the cycle analysis of gas turbine engines or power plants. While the similarity laws tak e into account different gas constants a correct presentation allowing for different isentropic exponents is not possible. Due to the increa sing accuracy requirements the influence of isentropic exponent must n ot be ignored. As the errors associated with varying isentropic expone nts depend on the method used to describe the characteristics of turbi nes, various methods were investigated to evaluate the effect of a cha nging isentropic exponent on the dependent par meters. Using a mean di ameter calculation procedure the characteristics of three different tu rbines were calculated over a wide range of the independent parameters for different isentropic exponents. As it is not possible to allow fo r the influence of the isentropic exponent exactly, all investigated m ethods to describe the turbine characteristics are more or less incorr ect. No method was found to be superior. The errors of the dependent p arameters were larger than tolerable regarding the accuracy demanded. Therefore as a result of this investigation, a turbine performance des cription is proposed using a correction procedure for different isentr opic exponents.