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most of the suction surface in the exducer, there are relatively strong secondary flows towards the shroud where the reduced static pressure is at a minimum. Very near to the shroud, the surface flow is directed toward the hub. The resulting herringbone pattern indicates the presence of a lift-off line. This feature is probably associated with the scraping vortex and suggests that the tip clearance vortex is relatively small in this machine. The pressure surface flow visualisation of figure 11 shows that on the rotor, there is a large low momentum region close to the leading edge. Indeed, at the leading edge, some reversal of the flow direction may be observed. The reversed flow and pressure surface stagnation will be largely an inviscid effect and are to be anticipated since the turbine rotor is operating at an incidence which is greater than the Stanitz value. There is no evidence that this reversed flow region has a significant effect on the overall flow which is consistent with the observations of many researchers that reducing the blade numbers below the limit of pressure surface reversed flow does not necessarily inhibit the performance (e.g. Futral and Wasserbauer (1970)). Downstream of this low momentum region, the flow responds to the meridional curvature and secondary movements occur towards the shroud. In the exducer, the influence of the reduced static pressure gradient means that the secondary flow is towards the hub as predicted by the Dawes code and by Choo and Civinskas (1985). The results of traversing a 5-hole pyramidal probe at rotor exit are presented in figure 12. These plots show the mass flow-weighted pitchwise-average of the stagnation pressure loss and flow angle when the turbine is operating at the design flow coefficient. The overall total pressure loss coefficient for the turbine is 0.10. Using the mass averaged values of total pressure, the total-total efficiency derived using the Euler work equation is of the order of 93 percent. This value is approximately 5 percent higher than expected, a result which may be attributed to the difficulty of measuring the stator and rotor exit flow angle and mass flow distributions with sufficient accuracy. These problems will be resolved for future work. The spanwise variations in total pressure loss (Yp) and yaw angle shown in figure 12 follow the trends observed in other radial turbines (eg Futral and Holeski (1970)), except at the shroud where the loss is reduced and the yaw angle indicates the overturning which is consistent with the existence of a scraping vortex. These differences are probably due to the relatively small tip clearance of the model turbine. If this is indeed the case, then the region of high loss near to the shroud is probably due to the accumulation of suction surface low momentum fluid and is not a direct result of shroud clearance flows. This observation, which is consistent with the rotor flow visualisation (figure 11), is in agreement with the suggestions of Zangeneh et al (1988). Future investigations will reveal the true origins of this region of loss. L.E. T.E. Suction surface T.E. L.E. Pressure surface Span Experimental results Span Predicted -7.6 0.0 0.5 1.0 Fractionof span Figure 12:Measured rotor exit pitchwise averaged results CONCLUSIONS A gas generator radial inflow turbine has been designed to operate at a total-total pressure ratio of 4.7. This turbine has then been modeled and placed in a large scale, low speed test rig at the Whittle Laboratory. The model stator blades show only weak secondary flow movements and have laminar blade surface boundary layers over the whole blade surface. The overall loss coefficient based on rotor inlet dynamic head is 0.033. The rotor blades show strong secondary flow movements on both surfaces and a low momentum region on the pressure surface at inlet when at the design flow coefficient. Exit traverses display similar results to those obtained by other organisations but lower loss and yaw angle is to be observed towards the casing of the machine, suggesting that high shroud losses are due to an accumulation of low momentum fluid and not due to strong tip clearance flows. A simple model has been developed to determine the effects of rotor blade inlet lean. ACKNOWLEDGEMENTS The authors would like to acknowledge Rolls Royce plc for support of the research work and also Rolls Royce Business Ventures Ltd for initiating and funding the original high speed turbine design work. Figure 10: Comparison of experimental and predicted (Dawes, 1986) stator exit total pressure contours Figure 11: Flow visualisation on the rotor blade using ammonia and Ozalid paper 0.28 Y p 0.14 Ypmean=0.111 0.00 0.0 0.5 1.0 15.2 Yaw mean = -22.6 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1991/78989/V001T01A077/2400491/v001t01a077-91-gt-220.pdf by guest on 23 January 2021PDF Image | Design and Testing of a Radial Flow Turbine for Aerodynamic Research
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