Design Criteria Of Radial Gap Between Impeller And Vaned Diffuser In Centrifugal Compressors

Although the interaction between the impeller and diffuser plays a significant role in flow behavior and compressor performance,studies on this phenomenon for thorough understanding are still not satisfying.So that,designating an appropriate radial gap regarding the tip-speed Mach number may be an important and notable factor for designing a centrifugal compressor.Therefore,effects of the radial gap between impeller trailing edge and diffuser leading edge,as a significant section of impellerdiffuser interaction,on the performance of high pressure ratio centrifugal compressors are investigated in this research.Afterward,according to the performance of the compressor with different radial gaps,the optimum radial gaps for a different range of tip-speed Mach number(Mu)are represented and the design criteria are established.Due to a better understanding of flow structures in the compressors with different radial gaps,vast studies,using steady three-dimensional Reynolds-averaged Navier-Stokes simulations,have been done.Simulations were done at six different radial gaps for five rotational speeds.The results show that radial gap considerably affects the choke mass flow rate and also the performance of diffuser in terms of pressure recovery and loss coefficient.Choke mass flow rate decreases by reducing the radial gap.Pressure recovery and loss coefficient have different behavior at the high and low amount of tipspeed Mach number.When Mu is smaller than 1.0,the pressure recovery and the loss coefficient are not sensitive to the radial gap within the studied range,while when Mu is larger than 1.0,the best gap ratio for these two parameters is 1.08.Besides,the effect of a radial gap on impeller enthalpy is negligible.However,impeller pressure ratio and efficiency are reduced by enhancing the radial gap at a same mass flow rate.Eventually,taking the whole stage efficiency as well as the diffuser performance into account,a design criteria of radial gap for centrifugal compressor is established as follows: When the tip-speed Mach number is smaller than one,the radial gap could be within a wide range,while when it is larger than 1.0,the radial gap should be about 1.08 to obtain a better compressor performance.