centrifugal compressors, modellung performances, Supersonic Axial Compressor Stage
Application of supersonic axial compressor stages is an effective way to decrease mass and volume of gas turbines. It is reported that stages with pressure ratio up to 2,8 and blade velocity 450 m/s can operate quite satisfactory. There is demand for stages with pressure ratio that exceeds 3.0. The possible efficiency of stages is vital for successful application. As a velocity coefficient at an impeller inlet of stages can be about 1.5 and more, shock wave losses can limit overall stage efficiency. The simplified model of a stage was used for calculations. Supersonic flow in elementary blade cascade with sharp leading edges of blades produces oblique shock wave with sub – or supersonic flow after it. This depends on an inlet velocity coefficient and an angle between shock wave front and flow direction. The normal shock wave follows if a flow is still supersonic after an oblique shock. The known equations are used to calculate head losses in shock waves. Losses in a subsonic part of a stage are estimated by an arbitrary appointed loss coefficient. The calculations were made for a gas with = 1,4 in a range of velocity coefficient =1,1 – 1,8. Shock wave angle was varied in a range = 900 – ?0, where ?0 is an angle of a sonic wave. There were calculated static pressure ratios, static polytrophic efficiency, loss coefficient, velocity coefficients after shock waves. There are two zones of an operation: – subsonic flow after an oblique shock wave with angles bigger than 720-620 (the bigger value corresponds to a smaller velocity coefficient); – supersonic flow after an oblique shock wave with angles smaller than 720-620 and a normal shock wave after. An efficiency of a calculated stage model with subsonic flow after an oblique shock wave is not less 0,88 when velocity coefficient is no more than 1,6. The corresponding pressure ratio is about 4,7. The problem is how far is the simplified model from a real 3-D stage with all her complications. The system of an oblique and following normal shock waves transforms kinetic energy in a pressure rise formally most effective. An optimal angle of an oblique shock wave lies in a range = 550 - 450 (the bigger value corresponds to a smaller velocity coefficient); The efficiency is about 88% for the velocity coefficient 1,8 and = 450. There is a fantastic pressure ratio 11 ion this case. In can be concluded that for stages with pressure ratio about 3,0 shock wave losses do not limit efficiency. The problem returns to trivial problems of preventing excessive separation after shock waves and to effective 3-D design.