King: Fluid Dynamics and Impeller Essay

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Copyright © 2010 ICCES

ICCES, vol.14, no.1, pp.1-22

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

2

Copyright © 2010 ICCES

ICCES, vol.14, no.1, pp.1-22

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