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Design High Performance Centrifugal Compressor Impellers
The flow in centrifugal compressor impellers is dominated by secondary flows which result in the formation of the impeller exit non-uniformity (the so-called “jet-wake flow” effect). This can be clearly seen in Figure 1, which shows the three-dimensional CFD prediction for a state-of-the-art conventionally designed impeller.
The particle paths, shown in blue, clearly indicate the accumulation of the low momentum fluid present on the suction surface at the shroud/suction surface corner resulting in the formation of a low momentum region at the exit of the impeller. This flow non-uniformity has an adverse effect on the diffuser performance and results in considerable mixing losses.
The spanwise Mach number gradients (i.e. difference in relative Mach number between the hub and shroud), especially from 40% of chord to trailing edge, are the main driving force of secondary flows on the suction surface. TURBOdesign1, being a threedimensional inverse design method, enables the direct control of the Mach number distribution in the impeller by specifying the blade loading distribution.
Figure 5, shows the loading distribution specified in TURBOdesign1 and the resulting Mach number distribution (Figure 4) which indicates considerable reduction in the spanwise Mach number gradients as compared to the conventional impeller (Figure 3), which should help to control the secondary flows.
The geometry of the impeller computed by TURBOdesign1 is shown in Figure 2. In this case the blades have straight filaments for ease of manufacture. The results of three-dimensional viscous flow prediction in this impeller were used to obtain the particle paths, shown in blue. The results show very little secondary flows on the suction surface and as a result the flow field at the exit from the impeller is more uniform.
This important design objective was achieved quite easily by using a blade loading distribution which was fore-loaded at the shroud and aft-loaded at the hub. This optimum design specification is general enough to be applicable to other impellers with different sizes or design conditions. TURBOdesign1 enables the systematic design of centrifugal compressor impellers with uniform exit flow.
Fig.3-5: The difference in relative Mach number between the hub and shroud is the main driving force of secondary flows on the suction surface. Usually it is quite difficult to control the three-dimensional Mach number distribution in conventional design.
Left: The blade geometry computed by TURBOdesign1 is significantly different from that obtained from conventional design. This can be seen clearly by comparing the blade angle distributions shown opposite.
Left: The performance of the conventional and TURBOdesign1 impeller were measured in the same closed loop test stand with the same vane-less diffuser and de-swirl vane. The resulting stage efficiency was found to be 5% higher for the TURBOdesign1 impeller at the design point with appreciable improvements at off-design.
Reference: Zangeneh, M., Goto, A., and Harada, H., 1998,“On the Design Criteria for Suppression of Secondary Flows in Centrifugal and Mixed Flow Impellers” ASME Journal of Turbomachinery, Vol. 120, pp.723-735.