Investigations of the Performance on Vaned Diffusers for Low Specific Speed Centrifugal Compressor
In this paper, the performance of vaned diffusers on low specific speed centrifugal compressors, was investigated experimentally and analytically. There is a problem of non uniform distribution of the flow at impeller exit, as a factor to deteriorate the performance of the diffuser. This problem appears remarkably on the low specific speed type, because the blade height is relatively small and the flow inclines toward circumferential direction. The experiment was carried out with 2nd stage compressor of two stage centrifugal type, focusing on the effect of number of diffuser vanes, vaneless ratio and throat area. Furthermore, unsteady fluid analysis was carried out by using Non Linear Harmonic method in order to understand about the phenomenon associated with the problem of non uniform distribution of the flow. NOMENCLATURE Ath throat area of diffuser AR area ratio of diffuser from throat to exit b blade height BL blockage factor 1effective area geometrical area) Cp static pressure recovery coefficient, Cp p p2 (P2 p2 cv Specific heat at constant volume D diameter G mass flow non dimensional flow, G R T1 D2 2 P1 ad Non dimensional adiabatic head adiabatic head (RT1) ad 1) 1) 1) M absolute Mach number Non dimensional revolution speed D2 (R T1) 0.5 S specific speed (non dimensional value) S ad 3/4 P total pressure p static pressure Q volume flow R gas constant S entropy (meridional mass average value per unit mass) S cv ln{ p pin in T total temperature u blade speed ZB number of impeller blades Zv number of diffuser vanes 3V Inlet angle of diffuser (from tangential direction) 2B backswept angle of impeller (from radial direction) adiabatic efficiency (total to static)