OFF-DESIGN PERFORMANCE ANALYSIS OF CENTRIFUGAL COMPRESSOR OF A SMALL GAS TURBINE ENGINE

This paper describes the work carried out to evaluate the off-design performance of a centrifugal compressor which consists of an impeller, a vaneless diffuser, a radial vaned diffuser, a radial-to-axial 900 bend and an axial diffuser (also called as de-swirl vanes), used for small gas turbine engine applications. As the flow through the centrifugal impeller is three dimensional, inviscid, and highly complex in nature, an effort is made to predict the performance as accurate as possible by exploring various flow and loss models from the literature. Impeller flow is analyzed using Jet-Wake theory (or Two-zone modeling). Losses due to mixing at impeller exit, clearance and leakage, and losses due to friction are included. Vaneless diffuser is analyzed by solving momentum integral equation in radial and tangential directions. Performance of the vaned diffuser is estimated based on the Runstadler's data plots. Radial-to-axial bend and De-swirl vanes are analyzed based on fluid dynamics relations and necessary empirical correlations. A computer program is developed, to calculate aero-thermodynamic properties of each component and the overall stage performance characteristics at different speeds ranging from 60 percent to 100 percent of design speed. A performance map is generated which consists of adiabatic efficiency and total-total pressure ratio values with respect to mass flow function. Predicted results are validated with performance characteristics of a 3.5 pressure ratio centrifugal compressor used for aero-engine applications. The results are very encouraging when compared with the experimental values. Choking flow is well predicted and the estimation of surge flow is not included in this method and is to be developed as a continuation of this research work.