During transient running of an axial flow compressor, considerable temperature differences arc set up between the met surfaces and the adjacent airstream. It was suggested that these temperature differences might have an adverse effect upon the aerodynamic performance of the compressor, and increase the likelihood of stall. Roils-Royce Ltd. expressed interest in this hypothesis, and offered to sponsor a three-year programme of investigation. The essential study was considered to be that of turbulent separation of boundary layers from heated surfaces, but an initial experimental investigation of laminar transition was carried out for low-speed flow over a flat plate. This indicated that the transition point is generally advanced by wall heating. and confirmed the results of other authors. The turbulent separation measurements were made on the inner wall of a curved duct, again at low air velocities. Advancement of the separation point in response to wall heating was observed, but dependence on flow velocity was evident. Attempts were made to relate separation point movement to a single parameter, but these were unsuccessful. The curved duct results were compared with predictions from a'. integral- type boundary layer 6alculation program, which had been developed by staff at Rolls-Royce. This after modification by the author, gave acceptable results for the majority of the experimental data. The predictions were then extended to cover flow over compressor blades. It was concluded that premature separation of the boundary layer in a compressor is most likely to occur on the aerofoil suction surfaces, although separation from the drum is a further possibility. On the aerofoils, separation point movements as a result of thermal transients are likely to be small (less than of the chord). however, a small departure from design conditions is likely to produce a larger one further downstream. Thus it was considered that the aerodynamic consequences of heat transfer from the blading can be a major factor in reducing compressor stall margins during transient running
