Session: 40-02 Axial Compressor Instabilities and Stall

Paper Number: 122609

122609 - Unsteady Flow Structure of Rotating Instability in a 1.5-Stage Axial Compressor 

The unsteady flow structure of rotating instability, abbreviated to RI, in a 1.5-stege axial compressor is investigated by experimental and numerical analysis. The test compressor was a 1.5-stage axial-flow compressor comprising 50 inlet guide vanes, 38 rotor blades, and 59 stator vanes. The rotational speed was 4500 rpm. Two tip clearances were implemented: 0.57 mm design clearance (DC) and 1.14 mm wide clearance (WC). Mass flow was measured using a three-pitot tube located at the compressor inlet. The pressure increase of the blade passage was measured at 21 points along the span using three-hole yaw meters installed ahead of the rotor blades and downstream of the rotor blade. Additionally, the velocity distribution in the span direction was measured using a hotwire anemometer. Furthermore, the casing-wall pressure traces within the rotor passages were measured at seven points along the axial direction. An in-house DES code was developed for unsteady CFD analysis. The finite-volume method was used for discretization. The convective flux was evaluated using SLAU and extended to a third-order MUSCL interpolation. The viscous flux was evaluated as a second-order central difference. The MFGS implicit algorithm was employed for time integration. The SST k–ω turbulence model was used in this code.

As a result of the total pressure rise characteristics of the rotor blades, it was confirmed that the kink point occurs in the case of wide clearance, and RI occurs below the kink point. RI appears in the power spectrum as a gentle hump of about 20~40% of the blade-passing frequency at the blade tips. The mode of the RI disturbance increases with decreasing flow rate, but the propagation velocity remains constant. This causes the RI frequency to move to higher frequencies. The Double-phase-locked averaging method was used to visualize the flow field inside the rotor blade when RI occurred. The results show that as the flow rate decreased, the spacing of the pressure patterns inside the rotor blade became narrower and the mode of RI increased. From numerical analysis, the unsteady flow structure of RI was changed as the mass flow rate was decreased. First, RI was formed by repeated collision and separation of the tip leakage vortex onto the pressure surface of the adjacent blade at the onset point of RI. A pattern of this vortex structure has been confirmed within the two blade passages. Then, RI was caused by the vortex breakdown of the tip leakage vortex as the mass flow rate was decreased. The fluctuation of vortex breakdown influenced on the blade loading near tip side of the next blade and propagated to next blade passage. A pattern of this vortex structure has been confirmed within the four blade passages. The vortex shedding from the blade tips, which caused RI, occurred between multiple blades in the circumferential direction, because the position of vortex shedding rotated to rotor rotational direction in a rotor rotational frame. As the mass flow rate decreased, the phase difference in the vortex shedding phenomenon between each blade became smaller. It means that the frequency of vortex shedding was high and the number of RI modes increased. The unsteady structure of RI was caused by the fluctuation of the inflow angle of the vortex into the adjacent blade changed its blade-tip loading. The changes of these vortex pattern, as the mass flow rate decrease, affected on the frequency of RI fluctuation.

Presenting Author: Nobumichi Fujisawa Waseda University

Presenting Author Biography: I'm an Assistant Professor at Waseda Univ.

Authors:

Nobumichi Fujisawa Waseda University
Yutaka Ohta Waseda University
Mai Yamagami IHI Corporation
Takashi Goto IHI Corporation
Dai Kato IHI Corporation