To improve the thermodynamic efficiency of aircraft engine and other gas turbine engines, higher and higher pressure ratios are desired in conjunction with more refined engine cycles. In the high pressure compressor, higher pressure ratios result in lower aspect blades and enhanced sensitivity of the engine design to radial clearance effects.
The tip clearance in the axial flow compressor of modern commercial civil aero-engines is of critical importance in terms of both mechanical integrity and performance. Typically as the clearance between the compressor blade tips and the casing increases, the aerodynamic efficiency will decrease and therefore the specific fuel consumption and operating costs will increase, and the clearance is therefore of critical importance to civil airline operators and their customers alike. A design exercise was performed and a series of conceptual solutions were developed using the theory of inventive problem solving (TRIZ) process and their potential viability in clearance control was investigated with thermal modelling. TRIZ was selected as an appropriate tool as the issue was long-standing having been the focus of previous projects, and robust design solutions were being sought.
In order to validate the concepts, use was made of a test facility developed at the University of Sussex, incorporating a rotor and an inner shaft scaled down from a Rolls Royce Trent aeroengine to a ratio of 0.7:1. The mechanical design of the test facility allows the simulation of flow conditions in the HP compressor cavity equivalent to the Trent 1000 aero-engine, with a rotational speed of up to 10000 rpm. The idle and maximum take-off conditions in the square cycle correspond to in-cavity rotational Reynolds numbers of 3.1×106 ≤ Reφ ≤ 1.0×107. The finite element thermomechanical model has been built to validate the engine measurements. This paper describes the use of TRIZ and the development of a selected concept and the detailed evaluation for reduction and control of tip clearance in HP compressors. This was achieved through the reduction in the compressor disc heat expansion time constant by improving drum heat transfer using bleed air from the compressor core flow. This paper explores the trade-offs between clearance and efficiency and develops and explores concepts to control the compressor tip clearance throughout the engine operating cycle. The project involved modelling of potential solutions and use of experimental facilities, a rotating compressor cavity rig, in order to explore the physical principles and demonstrate proof of concept for controlling tip clearance in HP compressors of gas turbine engines.
