Effects of the inlet boundary layer thickness on the loss characteristics in an axial compressor

Abstract

A three-dimensional computation was conducted to understand effects of the inlet boundary layer thickness on the loss characteristics in a low-speed axial compressor operating at the design condition (phi=85%) and near stall condition (phi=65%). At the design condition, flows in the axial compressor show, independent of the inlet boundary layer thickness on the hub and casing, similar characteristics such as the pressure distribution, size of hub corner stall, tip leakage flow trajectory and limiting streamlines on the blade suction surface and hub. However, as the load is increased, the difference in internal flows is developed by inlet conditions of different boundary layer thickness. For the thick inlet boundary layer, the hub corner stall grows to make a large separation region in the junction of hub and suction surfaces. The tip leakage flow is more vortical than that observed for the thin inlet boundary layer and has a particular point where the trajectory of the tip leakage flow is abruptly turned into the downstream. For the thin inlet boundary layer, the hub corner stall remains barely unchanged. The tip leakage flow leans to the circumferential direction more than that at the design condition but has no such particular point. Moreover, a large separation region is formed upon the suction surface near the casing and acts as the blockage of news, resulting in a heavy loss. As a result of these differences in the internal flow induced by different inlet conditions, the spanwise distribution of the total loss is changed dramatically. At the design condition, total pressure losses in two cases are almost alike in the core flow region but the larger losses are generated both at hub and tip when the inlet boundary layer is thin. At the near stall condition, however, total loss for the thick inlet boundary layer is found to be greater than that for the thin inlet boundary layer on most of the span except regions near the hub and casing. In order to analyze effects of the inlet boundary layer thickness on the total loss in detail, the total loss is scrutinized through three major loss categories available in a subsonic axial compressor such as profile loss, tip leakage loss and endwall loss.