Off-design analysis of centrifugal compressor of micro turbojet engine 400 N using computational fluid dynamics

Off-design analysis of centrifugal compressor of micro turbojet engine 400 N is carried out to determine the compressor performance beyond its design point. The compressor has been designed and optimized to produce a pressure and efficiency ratio of 3.8 and 0.75 at 98,000 RPM, respectively, with the dimensions of the compressor having eye root and eye tip diameters of 13 mm and 76.3 mm, the impeller diameter is 100.6 mm and the backsweep angle is 30.3°. This analysis is done by varying the mass flow rate of air entering the compressor and the rotational speed of the compressor impeller which is displayed in the compressor performance map. Off-design analysis was carried out at steady state using the Spallart Allmaras (SA) and Shear Stress Transport (SST) turbulence models using Numeca software. The boundary conditions used are the total pressure and the total temperature at the inlet of 101.325 Pa and 288.15 K, respectively, the static pressure at the outlet which varies in magnitude, and the non-slip condition on the surface of the object. This off-design analysis was carried out under conditions of minimum to the maximum flow rate with the impeller speed varied at 70%, 80%, 90%, 100%, 110%, and 120% of the design rotational speed. From the results of the off-design analysis, the maximum pressure ratio and efficiency isentropic can occur at the minimum mass flow rate or near the minimum mass flow rate. But this phenomenon is different for other rotating speeds where the point of maximum efficiency occurs between the maximum and minimum flow rates. The results of the analysis using the SST turbulence model showed lower performance than those using the SA model. This is because the SST model can provide better prediction results for the separation of the diffuser. Therefore, it is necessary to optimize the diffuser so that the design target can be achieved.