Design High Performance Industrial 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).

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 3D Inverse Design method, enables direct control of the Mach number distribution in the impeller by specifying the blade loading distribution.

Loading Distribution specified in TURBOdesign1 for the design of the impeller


The graph above shows the loading distribution specified in TURBOdesign1. Below we can see the resulting Mach number distribution, which indicates considerable reduction in the spanwise Mach number gradients as compared to the conventional impeller, which should help to control the secondary flows.

Mach number distribution of the Conventional Impeller vs TURBOdesign1. 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.

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.

Blade angle of the conventional impeller vs TURBOdesign1


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.

Comparison of head flow characteristics (left) and measured efficiency


The comparison below shows the three-dimensional CFD prediction for a state-of-the-art conventionally designed impeller compared to the geometry computed by TURBOdesign1. 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 in the conventional design.

This flow non-uniformity has an adverse effect on the diffuser performance and results in considerable mixing losses. In the impeller designed by  TURBOdesign1, the blades have straight filaments for ease of manufacture.

Predicted particle paths for the impeller, conventional design vs TURBOdesign1

The results of the inversely designed impeller show very little secondary flows on the suction surface. 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.

On the Design Criteria for Suppression of Secondary Flows in Centrifugal and Mixed Flow Impellers

In this paper, for the first time, a set of guide-lines are presented for the systematic design of mixed flow and centrifugal compressors and pumps with suppressed secondary flows and a uniform exit flow field.

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