Effects of Recessed Blade Tips on the Performance and Flow Characteristics in a Centrifugal Compressor

DoctorCentrifugal compressors are widely used in a wide range of aeronautical and industrial applications with high pressure ratio and small mass flow. Recently, the application of centrifugal compressors in distributed power generation systems and micro-turbines as well as turbo-chargers for internal combustion engines and gas turbines of helicopters has been discussed. Centrifugal compressors are required to function at high efficiencies and over wide operating ranges. Improvement of performance and extension of operating rage have been achieved by using measurements or modern methods such as inverse direct design based on computational fluid dynamics (CFD). In addition, a better understanding of the flow physics inside compressors is essential for the design of centrifugal compressors with improved performance and extended operational ranges.The tip clearance between the blade tip and the casing induces leakage flow from the pressure side to the suction side because of the pressure difference between these sides. The tip leakage flow mixes into the passage flow and causes a large aerodynamic loss in an impeller. Therefore, the tip leakage flow significantly affects the performance and flow field in a centrifugal compressor. Minimizing the tip clearance height is the best way to reduce the performance degradation caused by the tip leakage flow. However, a small tip clearance height increases the risk of abration between the impeller and the casing. Several strategies have been tested to reduce the tip leakage flow and minimize the risk of the impeller blade scratching the casing during rotation. Using a recessed blade tip instead of a flat tip is one possible solution for this problem. The objective of the present study is to investigate the effects of recessed blade tips on the performance and flow characteristics in a centrifugal compressor using CFD. The numerical flow solver used for this study is a commercially available flow solver, ANSYS CFX 12.0, which has been widely used in many industrial areas handling turbomachines.In order to design a more effective recessed blade tip, it is important to understand the characteristics of tip leakage flow. Therefore, the influence of tip leakage flow on the performance was firstly investigated by considering a non-uniform tip clearance profile. Six impellers with different tip clearance profiles were tested in the flow simulations. The accuracy of the numerical simulations was assessed by comparing the experimental data with the computational results for a system characterized by the original tip clearance. Although the performance improved for low tip clearances, a low tip clearance at the trailing edge improved the compressor performance more significantly than a low tip clearance at the leading edge. The flow field calculated for a system characterized by a low tip clearance at the trailing edge produced a more uniform velocity distribution both in the circumferential and in the axial directions at the impeller exit because the wake magnitude was reduced. As a consequence, this impeller provided a better potential for diffusion processes inside a vaneless diffuser.Steady computations were performed for three different blade tip geometries over their whole operating ranges to investigate the effect of a recessed blade tip on the flow characteristics. A flat tip blade was used as a baseline case to assess two different recessed blade geometries. It was found that recessed blade tip designs could improve the total-to-total pressure ratio and efficiency over whole operating ranges. The recessed tips were also effective in reducing the tip leakage flow. However, the recess cavity had an adverse effect on the efficiency due to the generation of a vortex in the cavity. As a result, the overall stage loss was reduced in the recessed cases because the positive effect of the reduced tip leakage flow ended up being superior to the negative effect of a strong vortex in the cavity. In addition, the diffuser performance of the centrifugal compressor could be enhanced with the recess cavity, because the pressure recovery coefficient was higher in the recessed cases than in the flat tip case. A parametric study was conducted to determine the geometry of a recess cavity with improved performance. This study revealed that as the cavity volume increases, the tip leakage flow continuously decreases but the strength of the cavity vortex increases. Therefore, it may be possible to optimize the recess cavity geometry further