In high pressure centrifugal compressors, the overall stage performance is greatly influenced by its diffuser performance. Extremely complicated non-uniform and unsteady flow exists in the region between the impeller exit and the diffuser inlet. Furthermore, in the case of supersonic diffuser, shock waves can be observed near the diffuser inlet. These can cause aerodynamic losses. Therefore, it is essential to recognize such complicated flow to realize an appropriate diffuser design. An investigation into the performance of supersonic diffuser was carried out using a high pressure compressor test rig for a small industrial gas turbine with a high back swept impeller and a quasi pipe-shaped channel diffuser. In addition, 3D quasi-unsteady flow analyses of the entire compressor by a RANS code with Non Linear Harmonic method at several operating conditions between surge and choke were conducted to investigate the details of unsteady flow between the impeller exit and the diffuser exit. The results of the performance test and that of the 3D unsteady flow analyses have shown good agreement in the pressure rise and the isentropic efficiency at several operating conditions. These support high accuracy of the flow analyses and the performance measurements.

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 A th throat area of diffuser AR area ratio of diffuser from throat to exit b blade height BL blockage factor (= 1-effective area / geometrical area) Cp static pressure recovery coefficient, Cp = (p-p 2) /(P 2-p 2) c v Specific heat at constant volume D diameter G mass flow ' non-dimensional flow, = G (R T 1) 0.5 / (D 2 2 P 1) ad Non-dimensional adiabatic head = adiabatic head / (RT 1) ad = { /(1)}(/(1) 1) M absolute Mach number ' Non-dimensional revolution speed ' = D 2 /(R T 1) 0.5 S specific speed (non-dimensional value) S = ' '0.5 / ad 3/4

Research and development of ceramic gas turbines (CGT), which is promoted by the Japanese Ministry of International Trade and Industry (MITI), was started in 1988. The target of the CGT project is development of a 300kW-class ceramic gas turbine with a 42 % thermal efficiency and a turbine inlet temperature (TIT) of 1350°C. Two types of CGT engines are developed in this project. One of the CGT engines, which is called CGT302, is a recuperated two-shaft gas turbine with a compressor, a gas-generator turbine, and a power turbine for cogeneration. In this paper, we describe the research and development of a compressor for the CGT302. Specification of this compressor is 0.89 kg/sec air flow rate and 8:1 pressure ratio. The intermediary target efficiency is 78% and the final target efficiency is 82%, which is the highest level in email centrifugal compressors like this one. We measured impeller inlet and exit flow distribution using three-hole yaw probes which were traversed from the shroud to the hub. Based on the measurement of the impeller exit flow, diffusers with a leading edge angle distribution adjusted to the inflow angle were designed and manufactured. Using this diffuser, we were able to attain a high efficiency (8:1 pressure ratio and 78% adiabatic efficiency).

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