Editorial

Abstract

This volume contains eight papers on turbomachinery covering CFD applications, detailed flow measurements on unsteady flow, new design methods and control devices, and conventional empirical approaches. Shi and Tsukamoto succeeded in predicting pressure fluctuations caused by the interaction between a centrifugal impeller and diffuser vanes, using a Navier-Stokes code with standard k−ε turbulence model. Using a commercial code, Gu et al. succeeded in predicting the performance of a single-stage centrifugal compressor and studied the volute/diffuser flow in detail at design and off design conditions. By combining a scheme called the Single-Sweep Method with an algebraic eddy viscosity model, Kochevsky made a parametric study of the energy loss in annular diffusers with a rotating hub and inlet flow swirl, with the purpose of minimizing the loss in annular diffusers. These papers suggest that CFD is now becoming capable of predicting not only steady flows under design flow conditions, but also unsteady flows and off-design flows. Sinha, Pinarbasi and Katz, using PIV and pressure fluctuation measurements, made detailed observations of unsteady flow process during the onset and developed stages of rotating stall within a vaned diffuser of a centrifugal pump. This work is useful for refining CFDs so that they can predict local flow process, as well as for obtaining a physical understanding of the stalling process. The combination of those efforts in CFD and detailed flow measurements will lead CFDs to more useful design tools that require less experience and fewer empirical factors, and also to methods for clarifying the flow physics in turbomachinery. New production technologies can expand the applications of turbomachinery. Liu, Nishi and Yoshida have shown that a high efficiency mini turbopump is possible by employing a larger outlet blade angle and a larger number of blades with a smaller outlet/inlet area ratio, combined with high precision manufacturing technology. Two papers propose new control methods. Saha et al. propose the application of radial grooves on the casing walls between impeller exit and vaned diffuser throat, to control vaned diffuser rotating stall in radial centrifugal impellers. In many cases the suppression was realized at the cost of head decrease, but it was shown that shallow grooves can decrease the rotating stall onset flow rate with acceptable head decrease. A pitch-flap coupling mechanism to control the output power of a horizontal axis wind turbine was studied by Shimizu and Kamada to determine the optimum pitch angle to flap angle ratio and also the flow physics of control. Despite the developments of CFD as mentioned before, we still need to depend largely on empirical relations in various aspects of turbomachinery design. Lazzarotto et al. studied the influence of Reynolds number on the performance of cross flow fans with various casing geometry. Engin and Gur studied the effects of solid particles on the performance of open type centrifugal pump impellers with various tip clearance. Such empirical studies will remain a requirement for the practical design of various types of turbomachinery.