Numerical investigation of trailing edge losses in transonic ORC turbines

Abstract

Profile loss that contributes 1/3 of their total loss is one of the major sources of entropy within the turbine blades. Understanding loss mechanisms and accurate prediction of them play critical role in turbine design. In this study, the loss mechanisms within the two-dimensional axial flow turbine passage, whose working fluid is R134a, are computationally investigated. Firstly, the numerical model is experimentally validated with VKI LS89 and LS59 cascades for air flow by examining the variation of the total pressure loss for various exit Mach numbers of the cascades. Then, the real gas deviations of the aerodynamic loss generation for the case of R134a organic fluid from the ideal gas flow with air are investigated. It is observed that the results obtained using the ideal gas approach of R134a fluid do not provide significant deviations from the results obtained using air. The major difference appears when the inlet state of the real gas approaches the saturation line having low compressibility factors. As the compressibility factor decreases while going away from the ideal gas state, due to having high density, the Reynolds number increases. The shock losses behind the trailing edge decreases up to inlet compressibility factor of 0.7 but then they increase continuously with the decreasing compressibility factor. The compressibility factor and Reynolds number which are two driving mechanisms for the loss generation within turbine passage are investigated separately. It is observed that the total pressure loss decreases by 5% as the compressibility value goes from 1 to 0.7, and increases by about 50% as it goes from 0.7 to 0.4. According to the results obtained from the study, the loss correlations extracted for the ideal gas should be corrected when R134a gas is used. Finally, a curve that is fitted to the numerical results is offered to correct the profile loss coefficient for air flow.