Improving of the micro-turbine’s centrifugal impeller performance by changing the blade angles
R. A. Tough1,2 , A. M. Tousi2 , J. Ghaffari2
Summary
In this paper, micro-turbine centrifugal impeller with three different blade angles was investigated by using Computational Fluid Dynamics (CFD) method. The other basic geometric parameters are held constant. The influence of the blade angles change on the observed values was determined from numerical solution of the flow in the impeller with help of the FLUENT software. The numerical simulation focused on the air flow from compressor impeller inlet to exit, and the performance of impeller is predicted. The numerical solution was performed for original impeller geometry and for two other cases, in which blade inlet angle and backward sweep was changed. The standard k − ε turbulence model was used to obtain the eddy viscosity. Performance of the code was verified using measured data for the
Eckardt impeller.
Keywords: micro-turbine centrifugal compressor, CFD, blade inlet angle, backward sweep
Introduction
Microturbines are gas turbines with a power ranging approximately from 10 to 200 kW. These devices can be used in stationary, transport or auxiliary power applications. In a micro-turbine, a centrifugal compressor compresses the inlet air and a radial turbine change the hot gas kinetic energy to the rotary work. Both turbomachinery components are radial. Micro-turbine compressors have been widely accepted as effective devices to improve the performance of these engines. The flow through the centrifugal compressor is complex due to growth of boundary layers and flow separation on blade surfaces, the formation of secondary flows due to rotation and passage curvature and tip leakage in the impeller region. Resulting jet-wake formation is associated with high viscous losses and affects the operating range of the rotating impeller downstream. To improve the aerodynamic performance of centrifugal compressor it is necessary to suppress the separation and wake formation maintaining high level of diffusion within the impeller. It is essential to understand the flow structure to achieve these objectives within the passages.
The complexity of the flow in a centrifugal impeller impacts the performance of impeller and makes it difficult to predict flow field correctly. Methods for accurate
1 Corresponding
2 Department
author. E-mail address: reza_tog@yahoo.com of Aerospace Engineering, Amir Kabir University of Technology, Tehran, Iran
prediction of turbulent flow in centrifugal impellers are important in order to get good estimate of the fluctuating loads in impeller.
Significant progress has been made in understanding impeller aerodynamic performance and also in predicting certain local flow details. The most popular guide to impeller design is diffusion parameter of some sort. Dean [1] discussed the influence of internal diffusion on impeller efficiency. His results, showed a trend of increasing efficiency with an increased overall diffusion ratio. Overall diffusion ratio is defined as the ratio of impeller inlet relative velocity, usually taken at the shroud, to impeller discharge relative velocity (w1 /w2 ). Kano et al. [2] presented results showing that in addition to the overall diffusion ratio, the rate of diffusion and maximum loading can significantly impact impeller peak efficiency and range.
Moore et al. [3] used a three-dimensional viscous CFD method to examine the flow in a medium pressure ratio impeller. The CFD results showed several aspects of loss production in the impeller. Loss production was high over most of the shroud particularly within the clearance flow region. In most measurements of impeller efficiency, the inefficiency of the internal diffusion process is hidden by the large centrifugal pressure ratio. Detailed
