Study of Convective Flow Effects in Endwall Casing Treatments in Transonic Compressor Rotors

Abstract

The unsteady convective flow effects in a transonic compressor rotor with a circumferential-groove casing treatment are investigated in this paper. Experimental results show that the circumferential-groove casing treatment increases the compressor stall margin by almost 50% for the current transonic compressor rotor. Steady flow simulation of the current casing treatment, however, yields only a 15% gain in stall margin. The flow field at near-stall operation is highly unsteady due to several self-induced flow phenomena. These include shock oscillation, vortex shedding at the trailing edge, and interaction between the passage shock and the tip clearance vortex. The primary focus of the current investigation is to assess the effects of flow unsteadiness and unsteady flow convection on the circumferential-groove casing treatment. Unsteady Reynolds-averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) techniques were applied in addition to steady Reynolds-averaged Navier-Stokes (RANS) to simulate the flow field at near-stall operation and to determine changes in stall margin. The current investigation reveals that unsteady flow effects are as important as steady flow effects on the performance of the circumferential grooves casing treatment in extending the stall margin of the current transonic compressor rotor. The primary unsteady flow mechanism is unsteady flow injection from the grooves into the main flow near the casing. Flows moving into and out of the grooves are caused due to local pressure difference near the grooves. As the pressure field becomes transient due to self-induced flow oscillation, flow injection from the grooves also becomes unsteady. The unsteady flow simulation shows that this unsteady flow injection from the grooves is substantial and contributes significantly to extending the compressor stall margin. Unsteady flows into and out of the grooves have as large a role as steady flows in the circumferential grooves. While the circumferentialgroove casing treatment seems to be a steady flow device, unsteady flow effects should be included to accurately assess its performance as the flow is transient at near-stall operation.

Large eddy simulation of flows in industrial compressors: a path from 2015 to 2035

“…The design of flow control devices is a key point to improve the performance of compressors and better manage surge margin [33]. A lot of solutions have been proposed in the literature such as active flow control [34,35] and casing treatments [36][37][38], but the flow control mechanisms are not yet completely understood [39,40]. The prediction of a control devices effectiveness is a challenge in itself due to the complexity of the flow that develops in a compressor equipped with a control solution (figure 2).…”

Evaluation of the effectiveness of typical casing treatments for a low-speed compressor by an integral method

“…Even for the same compressor, stall margin extensions are usually irregular at different rotating speed. The modern powerful computers make it possible to utilize numerical simulations, especially with the advanced high-fidelity computational models, to explore in details the flow structures and the stalling mechanism with and without casing treatments [6]. However, due to the complexity of the stall phenomena, the mechanism of the stall margin improvement by CT is still not completely clear.…”

Effect of axial skewed slot casing treatment in a transonic fan

“…Most of the detailed investigations reported on circumferential groove casing treatment are based on steady-state numerical simulations and emphasize the important role of the grooves on the strength and position of the leakage vortex. [5][6][7][8][9] However, Hah et al 10 showed both experimentally and numerically that the flow field near the tip of a transonic compressor rotor with circumferential grooves can be ''highly unsteady'', implying that steady calculations are inadequate.…”