On the Assessment of Turbine Efficiency From Experimental Data for Low Pressure Turbines at Low Reynolds Numbers

The isentropic efficiency of a turbo machinery device is assessed using a defined control volume. Representative stagnation pressures and temperatures are measured at inlet and exit of the volume. This task is mainly accomplished by radial rake measurements. It is assumed to measure a representative flow quantity for an area with a defined number of circumferentially and radially distributed probes. The assumption incorporates biases into the assessed efficiency. The sensitivities of a radial rake measurement at the exit of a two-stage turbine to the unresolved area, the spatial resolution, and the averaging method are studied at a low Reynolds number working point (ReV1 = 75000). A method of analysis has been developed to assess the changes in turbine efficiency. For the case studied, the results show that the applied averaging method represents a minor contributor to the overall uncertainty. The effects of the spatial resolution and the unresolved area constitute up to ∼0.6% of turbine efficiency. Methods are presented to reduce the bias of the unresolved area significantly. The circumferential and the radial direction are observed separately which allows drawing conclusions for the design of rake measurement systems. The method of analysis is based on CFD completely, and thus it can be used to optimize or to design rake measurement systems prior to turbine testing. Additionally, the knowledge gained of the remaining biases can be used for an improved uncertainty analysis.Copyright © 2012 by ASME