PARAMETRIC STUDY OF TIP INJECTION ON STABILITY OF TRANSONIC AXIAL FLOW COMPRESSOR

Abstract

The performance of a gas turbine engine highly depends on the stability of its compressor. The objective of a designer is to provide sufficient stall margin to enhance operating range. The stall of a compressor is associated with the low energy tip leakage flow, which leads to the flow blockage and thermodynamic losses. The effects are more detrimental in transonic axial flow compressors due to the interaction of shock tip leakage vortex. Many researchers have studied different techniques to enhance the stable operating range of a compressor. Flow injection upstream of the rotor tip is an efficient technique to increase the stable operating range of a compressor and therefore the safety of the system but generally with some efficiency penalties. Therefore, with the advance of computational techniques and increasing availability of low cost computing, considerable amount of numerical and modeling activity has been devoted in recent years. This work presents a parametric study of tip injection for a transonic axial flow compressor NASA Rotor 37. Numerically obtained flow fields have been interrogated for the effects of injector mass flows (2.5%, 3.5% and 5%) and injection flow angles (0; 15'and 45') to identify the physical mechanism responsible for the improvement in the stall margin due to these treatments and highlight the parameters responsible for the efficiency penalties. The steady state CFD simulations were performed at 100% design speed and results were compared with those obtained on the base line compressor with smooth casing. It is shown that the presence of flow injection significantly alters the stall margin and rotor efficiency compared to the smooth casing. For an injection mass flow 5% of the annulus mass xiii flow gives maximum stall margin increase with performance penalties. Injection flow aligned with blade camber 45° gives maximum stall improvement for a given mass flow rate. Minimum injection mass flow rate aligned with the blade camber gives maximum stability and minimum performance penalties. The optimum configuration obtained from this work is 5% of the annulus chocked mass flow injected at 45' This configuration gives a stable range extension of 119.16% and 0.7% peak efficiency improvement without any drop in pressure ratio.