Contra-flow turbo-compressor tests

Abstract

Several methods of constructing contra-flow turbo-compiessor wheels have been investigated by mechanical tests on single-stage wheels. The results have been incorporated in a complete unit which has been designed and tested at the Royal Aircraft Establishment for research purposes. It was designed to pass 200 lb/min of air at 25,000 It with a compression ratio of 2.7 : 1 and a temperature at inlet to the turbine of 145 deg C. In designing, the compressor results from aerofoil cascade tests were extrapolated beyond the limits then covered (1938). Subsequent cascade experiments showed that the compressor efficiency would be low and that the blading used would be stalled under design conditions. Tests on the unit confirmed this, indicating that a compressor efficiency of about 70 per cent was the maximum obtainable, whereas the designed efficiency was 83 per cent, a figure which with present day knowledge is easily obtainable. A slight modification to the compressor-blade heights improved the efficiency and enabled the range of operation to be extended. In the contra-flow unit the leakage between the shrouds separating the compressor and turbine annuli is a special problem. Owing to the departure from design conditions and the intake air boost the leakage observed on the unit was at times as much as 50 per cent of the entering air. The leakage likely to be obtained in a unit operating under designed conditions is estimated at 4 per cent. Most Of the remainder of the running time was devoted to investigation of mechanical problems. These included the temperature gradients in the wheels, bearing cooling and lubrication, and constructional features. At a gas temperature of 400 deg C. the constricting section in the wheel disc caused a drop of temperature of 150 deg C. above the high pressure bearing housing. By increasing the cooling air mass flow this drop was increased to 250 deg C. The bearings were found to be satisfactory provided their temperature could be maintained at less than 200 deg C., but the oil metering supply was unsatisfactory. Some movement of the blades in the rotors was observed and relative axial expansion of t.he rotors andcasing led to rubbing at the high-pressure end. Trouble was also experienced with the large gland leakage areas at the shrouds and around the bearing housings. It was concluded that, in spite of the poor aerodynamic performance, there was no fundamental reason why similar units should not operate efficiently and why a good mechanical performance should not be obtained.