The Experimental Researches on Improving Operating Stability of a Single Stage Transonic Fan

Abstract

High thrust-weight ratio aircraft engine means high single stage load and high tangential speed of the rotor blade, while strong shock waves, secondary flow, boundary layer development and mutual interaction among shock, boundary layer and leakage flow occur in the rotor tip region. All of these have great influence on the performance of compressor and decrease operating stability of compressor. Casing treatment is a practical technique that is often used to enlarge compressor stable operating region, but normally this benefit is achieved at the sacrifice of efficiency of the compressor. In this paper, three kinds of casing treatment have been tested on a high pressure-ratio, high through-flow single-stage transonic fan. The performances of the transonic axial fan have been measured with and without casing treatments at different operating speeds. It is obvious that the three types of casing treatments — Reversed Vortex Style Casing Treatment (RVSCT), Axial Slope Slot Casing Treatment (ASSCT) and Arc-Curve Skewed Slot Casing Treatment (ACSSCT), are not same effective on the operating stability and efficiency. It is very exciting that the peak efficiency of the single-stage transonic fan was improved as well as the stable operating range was increased with ACSSCT about 1% at low operating speeds. Unfortunately at the design speed the stall margin was increased but the adibatic efficiency was decreased 2% compared with the original fan in this case. In order to get detailed understanding of the flow structure with casing treatments near the end wall, three-dimensional velocity was measured using a small five-hole probe at outlet of the rotor passage at the lower operating speeds. The results indicated that the casing treatments with plenum chamber delayed the formation of rotating stall and improved the flow near the end wall region. That means at these lower operating speeds the boundary layer of the end wall was decreased and the efficiency of the fan was increased.