To what extent is centrifugal compressor performance enhanced by free-form 3D geometry? Centrifugal impellers are conventionally designed using ‘ruled' or straight line elements which linearly connect point pairs on the hub and shroud profiles, allowing for manufactured by flank milling process. In general, the ruled elements do not need to coincide with the leading and trailing edge definition curves of the impeller which allows for more complex geometrical shapes to be generated. This approach has been used across the turbocharging industry to produce highly efficient impeller designs over past decades. Apart from the cost benefits, the method also simplifies the design process by reducing the degrees of freedom, as the blade angle and thickness distributions need to be specified only on the hub and shroud sections.

Under pressure to pursue gains in efficiency, an alternative and more costly approach is to manufacture impellers by point milling, which removes constraints on intermediate blade sections and allows the use of ‘free-form' vane designs. In high-speed impellers, where a large portion of the loss is generated by the 3D shock at the entrance to the passage and its interaction with leakage and secondary flows, independent design of the intermediate blade sections is believed to be beneficial. In addition to a more precise control of the flow at different spanwise locations, a free-form inducer design allows for non-linear application of thickness, sweep and lean at the leading edge of the blades and the ever-present drive for higher efficiency levels has increased interest in this concept.

For the study reported here, PCA Engineers carried out back-to-back designs for high Mach number stages of high pressure ratio which were then tested by ABB Turbo Systems with identical static components. The ASME Paper GT2017-63538 reports the results in detail, a summary can be found in the presentation on our Publications page.