Tip Flow Control in Optimized Large Gas Turbine Compressor Blading

Abstract In transonic axial compressors of gas turbines, casing treatments (CTs) have the capability to improve the stability and delay stall. A full comprehension of the stall mechanism is essential to design a beneficial CT. Tip stall is a prerequisite. Most CT investigations and publications investigate flight propulsion applications. There is a need for detailed investigations in stationary large gas turbines concerning stability optimization. Typically, the tip gaps in the front stages are — relative to the blade span — by one magnitude smaller than for aero engines. This is due to the large absolute dimensions. The goal of the present study is to increase the operating range towards stall without losing efficiency at design point conditions. Axisymmetric active and passive CT types are investigated. The model is based on an optimized tip critical rotor design, which matches the boundary conditions of a large gas turbine. Most other investigations from the last decades are conducted for known airfoil shapes that were not automatically optimized for their operating conditions. A tip flow control was developed to improve the stability limits. The numerical investigations are carried out on a single rotor optimization using RANS simulations. The study investigates in depth the ability of axisymmetric grooves with and without mass injection to postpone stall. It is shown why passive circumferential grooves do not improve the stability for the specific blade geometry. It is essential to quantify the influence of the axisymmetric types of CT onto the rotor tip flow especially for near stall conditions. Of particular interest is how the circumferential grooves perform when used in combination with mass injection. The results show that for this aerodynamically optimized rotor, a mass injection is needed to energize the tip leakage and therefore reduce the blockage. Passive circumferential grooves do not increase the stability. The injection of 1% stall mass flow rises the stall margin by 15% only with little losses.