Tip Gap Variation on a Transonic Rotor in the Presence of Tip Blowing

Abstract

High stability and efficiency are the main two objectives in the design of an axial-flow compressor. Stability usually reduces at higher stage loading, and the stability margin critically drops in transient operation and through the life cycle of an engine. A major reason for this to happen is the growing tip gap. A recirculating tip blowing casing treatment has shown the ability to enhance stability. To be able to use it as a stability control system at varying tip clearances in aircraft engines, the behavior of this casing treatment at different tip clearances was considered important and investigated in this paper. The present study investigates in depth the ability of a tip blowing casing treatment to postpone stall at three different tip clearances. The results prove a substantial beneficial effect for design and increased tip gaps and show some negative impact of the casing treatment for a small tip gap. The study is carried out on a 1.5 stage research compressor. The investigated rotor was already investigated with an axial-slot casing treatment for different tip gap heights at the Institute for Flight Propulsion. The design of a recirculating tip blowing casing treatment is simulated with an equivalent numerical setup. A tip blowing casing treatment consists of a bleed port connected to a tip blowing upstream of the rotor. The streamwise pressure gradient drives the tip blowing with a high injection velocity. A design speed line is simulated for three tip clearance values with and without the tip blowing casing treatment. The impact of the interaction between the tip blowing and the tip gap vortex is analyzed. A detailed analysis of the passage flow is conducted. A comparison of the stall margin is made. The study is carried out using URANS simulations.