Tip clearance control concepts in gas turbine H.P. compressors

This thesis describes the development of concept and the evaluation for the reduction and control of tip clearance in HP compressors. The potential method of tip clearance improvement was the reduction in the disc time constant by improving drum heat transfer using radial inflow of air. A passive clearance control scheme was employed in this research. This involves the control of disc and casing thermal response during engine transient by increasing the heat transfer coefficient of the drum. This will speed up the thermal response of the drum hence controlling the clearance between casing and the blade tip during engine transient. A sensitivity analyses were performed to determine the quantitative effect of heat transfer coefficient on the time constant of the disc hence the effect on tip clearance during engine transient in a square cycle using a finite element compressor drum and casing models of RB211 524 and Trent 1000 engines. Measured disc surface temperatures were used to analyse the disc time constant. The results show ix that an increase in heat transfer coefficient reduces the drum time constant. It produces a reduction in cruise clearance and impacts positively on compressor operability. The effect was improved significantly with radial inflow injection on the time constant of the disc and hence the effect on tip gap from a transient cycle. A finite element model of the multiple cavity rig which incorporates a rotor and an inner shaft scaled down from a Rolls Royce Trent aero-engine to a ratio of 0.7:1 was also used to simulate 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  Re  1.0 107 . The project involves modelling of radial inflow regimes of 1.6%, 2%, 3%, 4%, and 6% and the use of a lumped parameter model and experimental data to demonstrate proof of concept. The results shows that 6% radial inflow is capable of reducing the cavity disc time constant by approximately 44% during acceleration at high power and 39% during deceleration at low power with a time constant reduction factor of 2 during acceleration and 1.8 during deceleration.