Hydrodynamic Design of a Turbocharger Pump and Turbine for Reverse Osmosis Desalination Plant Application

The meridional shape, number of blades and component specific work rVt (ie. impeller head) were loaded from TURBOdesign Pre into TURBOdesign1 for the detailed 3D design of the pump impeller. TURBOdesign1 is a unique blade design software that uses the 3D Inverse Design approach to compute the blade shape for a given blade loading distribution.

It is well known that the main source of hydrodynamic loss in medium specific speed centrifugal pump impellers is the formation of secondary flows in the impeller, so ADT engineers started the impeller design with a spanwise loading distribution known to systematically suppress secondary flows. Such blade loading, represented in Fig. 1, features an aft-loaded hub that increases surface velocities on suction surface of hub and a fore-loaded shroud that removes surface velocities on suction surface of shroud. This spanwise loading distribution results in net reduction of reduced static pressure gradients on suction surface and hence suppresses secondary flows resulting in high impeller efficiency and the more uniform exit flow ensures very good stage efficiency.

A number of design iterations were performed directly in TURBOdesign1 with the help of the 3D inviscid flow solution and performance data provided by the code to fine tune the meridional shape and blade loading, before assessing the impeller performance via 3D CFD. The initial design loop in TURBOdesign1 taking less than 30 seconds per design iteration, it ensures large areas of the design space are covered with minimal number of CFD iterations, making it possible to achieve breakthrough designs without increasing product development costs.

3D Turbine Impeller Design

The turbine impeller was also designed in TURBOdesign1 following a similar process and principles as the pump impeller design, the aim being to achieve high total-to-static efficiency by reducing the accumulation of low momentum flows and exit swirl at the impeller trailing edge. To achieve this target the initial turbine impeller was designed with a fore-loaded hub and mid to aft-loaded shroud as presented in Fig. 2.

A design system based on TURBOdesign1 facilitates development and transfer of design know-how across designers, projects and teams, resulting in improved engineering productivity and company’s industry competitiveness. These unique advantages are only possible thanks to the 3D Inverse Design approach implemented within TURBOdesign1 as it ensures the design inputs are based on aerodynamic / hydrodynamic design parameters instead of geometrical blade angles.

The pump and turbine volutes also play a critical role in ensuring the system can achieve very high efficiency at design and off-design conditions within a compact packaging. Volutes are traditionally designed assuming a uniform flow coming into the volute, in reality however; there is significant circumferential variation of velocities within the volute inlet particularly at off-design conditions at higher flow than the design point. Most volute designers are well aware of this issue and tend to oversize the volute, shifting its best efficiency point at higher than design point, to counter the effect of these high losses. Using this practice however the volute sizing increases and creates a mismatch between best efficiency point of the impeller and the volute, resulting in overall lower efficiency at design point.

For this project ADT engineers have used TURBOdesign Volute to design both the pump exit volute and turbine inlet volute. TURBOdesign Volute is a unique volute design code which allows designers to specify non-uniform circumferential flow variation at the inlet of the volute, the code then uses the streamline curvature method to calculate the outer volute flow boundary and automatically provides an optimal volute area distribution. The final output is a clean 3D surface IGES file which is ready for meshing and 3D CFD analysis. The resulting pump and turbine stages can be seen in Figs. 3 & 4.

Conclusion

During the course of this project ADT engineers have used the proprietary tools TURBOdesign Pre, TURBOdesign1 and TURBOdesign Volute to design and optimise all the components of a hydraulic turbocharger plant system for reverse osmosis desalination.

The unique features provided by ADT’s TURBOdesign Suite allowed ADT engineers to explore large areas of the design space within a very short product development timeframe and ensured the best performances could be achieved by the hydraulic turbocharger system. The full system was finally analysed with 3D CFD and the results confirmed all the customer duty points requirements were met with a target efficiency exceeded by 5 points.