Session: 37-03 Radial Compressor Unsteady Flow

Paper Number: 128457

128457 - Unsteady Flow Mechanism of Impeller Stall Inception in a High-Pressure Ratio Centrifugal Compressor With Bleed Slots 

The impeller stall inception in a high-pressure ratio centrifugal compressor with bleed slots was investigated by the experiments and the unsteady numerical analysis. Our paper reported that the flow stability of the internal flow was first initiated around the leading-edge of splitter blades and finally results in tip-leakage flow instability at the leading-edge of main blades. It is because that the reverse flow, which was caused by the separation vortex near the splitter blades, induced the flow instability of the impeller. The investigated centrifugal compressor was developed as the high-pressure compressor of a small aviation gas turbine and is a sub-scale model. Two bleed air extraction slots are located on around the leading edge of the splitter blade, and the bleed air flow rate from the impeller passages can be changed by adjusting the opening degree of a valve installed downstream of the bleed slots. In the tests and numerical analysis, the compressor speed was set to 80% of the design speed and the bleed mass flow rate to 8.7% of the compressor inlet flow rate, about half of the normal flow rate, in order to cause impeller stall. The multi-point pressure measurements were conducted using unsteady pressure sensors in the experiments. The Detached Eddy Simulation (DES) in-house code was used to obtain the detailed internal flow field of the tested compressor.

The both experimental and CFD results showed that the impeller stall was consisted of a stall cell and rotated about 70% of the impeller rotational speed. Furthermore, a pressure disturbance occurs first at the splitter blade leading edge before the main blade leading edge. The numerical results showed that the blockage within the passage of the suction side at the leading edge of the splitter blade was first increased, because the leading-edge separation on the suction surface of splitter blade was enlarged. Then, the blockage within the passage of the pressure side was increased after that of suction side stopped increasing. It is because the flow angle gradually became positive within the splitter blade pressure side passage and the stagnation region, which the strong pressure gradient was induced at the leading edge of the splitter blade, was formed. Therefore, the blockage development of the both pressure and suction passage at the leading-edge of the splitter blade was the main cause of the stall inception. Then, the reverse flow region in the inducer was developed and grown in the circumferential direction before the stall cell was extended to the impeller inlet. The development of the reverse flow region resulted from the growth of the blockage at a leading edge of a splitter blade to upstream. After that, a longitudinal vortex was formed near the bleed slots and blocked the main stream neat the tip side. Therefore, the flow angle to a main blade increased. As a result, the spillage occurred at a leading edge of a main blade and the rotating stall was generated.

Presenting Author: Ryo Nakayama Honda R&D Co., Ltd.

Presenting Author Biography: My work is aerodynamics engineer.
I mainly design centrifugal compressor and diffuser.
I am interested in efficient development methods such as optimization methods.

Authors:

Nobumichi Fujisawa Waseda University
Yuta Yamamoto Waseda University
Yutaka Ohta Waseda University
Atsushi Ogino Honda R&D Co., Ltd.
Ryo Nakayama Honda R&D Co., Ltd.
Eijiro Kitamura Honda R&D Co., Ltd.