Effect of pressurization on tip leakage losses in micro-scale centrifugal compressors

Abstract

Turbomachinery miniaturization has been an emerging research field during the last decades and, especially, micro-scale radial compressors are becoming increasingly relevant due to the development of high-speed electric motors. They can be found in a wide range of applications such as micro gas turbines, aerial unmanned vehicles propulsion, fuel cells and even medical ventilators. Furthermore, the use of micro-scale centrifugal compressors has been recently proposed for heat pump applications using working fluids different than air which need to be pressurized. Since this down-scaling implies low inlet Reynolds-number, the use of these pressurized fluids entails beneficial effects on the internal efficiency of the machine due to the increase in Reynolds-number within a laminar-to-turbulent regime. Nevertheless, one of the main challenges in micro-turbomachinery is the large relative tip clearances due to assembly and manufacturing tolerances. These large clearance ratios lead to increased tip leakage losses that limit drastically the efficiency in the considered reduced-scale compressors. Besides, pressurized working fluids used in the aforementioned heat pump applications result in an increase in the pressure difference between the pressure and suction sides of the blade. This is precisely why the main goal of this work is to quantify the effect of inlet pressure on the tip leakage losses in micro-scale centrifugal compressors. The present paper proposes the assessment of the tip leakage losses by means of both analytical and numerical approaches. First, new operating conditions when using different working fluids are calculated considering the corresponding corrections due to Reynolds-number-dependent losses. Then, one-dimensional methods are applied to estimate the expected reduction in internal efficiency and the clearance ratio. On the other hand, a numerical model of the impeller-diffuser reference micro-compressor stage is developed using ANSYS CFX 19 in order to characterize the complex three-dimensional interactions within the tip leakage phenomena. Results show the effect of pressurization in four selected working fluids (air, carbon dioxide, isobutane and propane) in the range of desired inlet pressures for heat pump applications. A comparison is presented between analytical and numerical results. While slight differences in clearance ratio are found, expected reduction in efficiency provided by analytical methods are not aligned to the numerical results. However, both methods agree in the fact that inlet pressure has low influence on the internal efficiency for all working fluids simulated.