A Numerical Investigation of the Influence of Casing Treatments on the Tip Leakage Flow in a HPC Front Stage

Abstract

This paper describes the influence of casing treatments on the tip leakage flow and its resulting vortex. The presented results and conclusions are based on steady state numerical simulations of a high pressure compressor stage. Without casing treatments a significant change of behavior of the tip leakage flow can be observed near surge. This change is termed as vortex breakdown and occurs after passing the shock in the blade passage. The simulations indicate the losses in total pressure inside the vortex core as the main reason for the vortex breakdown. These losses mainly depend on the blade loading. Running the compressor stage at high pressure ratios these losses can reach such a high level that the total pressure inside the vortex measured in the rotating system of the rotor falls below the static pressure after the shock. This pressure difference works as a physical barrier for the low energy vortex core and prevents it from reaching the high pressure rotor outlet. Consequently, this blockage must lead to the onset of recirculation zones — the so called vortex breakdown. Different casing treatments have been tested on their ability to delay vortex breakdown and to move the surge line to lower mass flows. Numerical simulations show that configurations with axial slots as well as circumferential grooves weaken or even destroy the characteristic tip leakage vortex and reduce its resulting losses in total pressure. This reduction in losses delays or prevents the onset of vortex breakdown compared to the untreated case explaining the effectiveness of casing treatments. Observations indicate that casing treatments do not interfere with the vortex directly. The key mechanism seems to lay mainly in the interaction with the tip leakage alone. Taking advantage of existing pressure differences in the rotor blade row casing treatments remove tip leakage flow in zones of high pressure and interrupt temporarily the feeding of the vortex. The separated tip leakage reenters the main flow in zones of low pressure again. The way how this tip leakage bypass is realized defines the influence of casing treatments on efficiency and surge line.