Stall Inception in Low Pressure Ratio Fans

Abstract

A combined experimental and computational test programme, with two low pressure ratio aero-engine fans, has been used to identify the flow mechanisms at stall inception and the subsequent stall cell growth. The two fans have the same rotor tip clearance, annulus design and downstream stators, but different levels of tip loading. The measurement data show that both fans stall via spike-type inception, but that the growth of the stall cell, and the final cell size, is different in each fan. The computations, reproducing both the qualitative and quantitative behaviour of the steady-state and transient measurements, are used to identify the flow mechanisms at the origin of stall inception. In one fan, spillage of tip leakage flow upstream of the leading edge plane is responsible. In the other, sudden growth of casing corner separation blockage leads to stall. These two mechanisms are in accord with the findings from core compressors. However, the transonic aerodynamics and low hub-to-tip radius ratio of the fans leads to the following two findings: first, the casing corner separation is driven by shock-boundary layer interaction; second, the spanwise loading distribution of the fan determines whether the spike develops into full-span or part-span stall and both types of behaviour are represented in the present work. Finally, the axial momentum flux of the tip clearance flow is shown to be a useful indicator of the leakage jet spillage mechanism. A simple model is provided that links the tip loading, stagger and solidity with the tip clearance axial momentum flux, thereby allowing the aerodynamicist to connect, qualitatively, design parameters with the stall behaviour of the fan.

Low pressure ratio transonic fan stall with radial distortion

“…Decreasing the negative axial momentum flux in the leakage flow increased stable operating range until stall initiated through the passage separation mechanism. Kim, et al (2019) observed a similar distinction in inception mechanism for two low speed fan designs. Supersonic flow at the tip produced suction surface boundary layer separation downstream of the shock, contrasting the corner separation in the subsonic cases.…”

“…values for wall cells were kept below 5 everywhere other than near stagnation points. Solver settings mirrored those calibrated and validated by Kim, et al (2019) for Fan A with clean inflow. Jameson's dual time-stepping method (Jameson, 1991) gave unsteady calculations second order accuracy in time.…”

“…As well as a lower total pressure ratio, low pressure ratio fans are distinguished by reduced rotor inlet relative Mach numbers, typically below 1.4 at the tip and with subsonic relative inflow towards the hub. Analysis of single-stage, low hub-to-tip ratio subsonic compressors (Pullan, et al, 2015;Hewkin-Smith, et al, 2019) and transonic compressors (Hah, et al, 2006;Kim, et al, 2019) observe spike-type stall inception, or the absence of significant low-frequency disturbance preceding stall. Hewkin-Smith, et al (2019) attributed the sudden drop in pressure rise from local sectors of the rotor row to either spillage of tip leakage flow ahead of the leading edge, or separation from the suction surface within the rotor passage.…”

“…This reduces the stability margin at takeoff and climb, leading to a greater demand for accuracy of fan stability predictions during fan design. Full-annulus Unsteady Reynolds-averaged Navier Stokes (URANS) calculations by Kim, et al (2019) accurately predicted the operating point, inception mechanism, and developed stall cell characteristics for two low pressure ratio, transonic fans. The work also resolved the contrasting flow physics and their link to the fans' designs.…”

Investigation of the Unsteady Flow Features in a Tip Leakage Flow Model Using Large-Eddy Simulation

“…Direct numerical simulation (DNS), large eddy simulation (LES), and hybrid RANS/LES have been demonstrated to yield much more accurate results (You et al, 2007;Decaix et al, 2015;Scillitoe et al, 2017;Liu et al, 2017;Yan et al, 2018). Higher fidelity methods could be used to study turbulence physics and to educate RANS turbulence modelling (Liu et al, 2011;Lee et al, 2018;Kim et al, 2018;Tang et al, 2018). LES is a favorite and powerful tool for turbulence research because of much higher resolution compared with hybrid RANS/LES and much lower cost compared with DNS.…”