Paper 13: The Performance of Turbojet Engines during the ‘Thermal Soak' Transient

Abstract

The changes are considered which might be present within a jet engine during the ‘thermal soak’ transient to cause the engine's performance to differ from the equilibrium performance. The effects of known values of these changes are given for a simple jet engine over a wide range of engine pressure ratios. Values of these changes are then predicted for a typical engine, and the observed loss in thrust of 1·7 per cent is satisfactorily explained. Two of the major factors giving loss of thrust in the simple jet engine were heat absorption in the turbine metal and heat absorption in the compressor metal. The effects of known values of these two factors on the performance of a typical high by-pass turbofan engine are given. For similar fractional heat absorptions, the losses in thrust of the turbofan engine are roughly double those of the simple jet engine.