INVESTIGATION OF AN AIR SUPPLY CENTRIFUGAL FAN FOR AIR CUSHION VEHICLE: IMPELLER DESIGN AND VALIDATION

A design method is presented for re-designing the double-discharge, double-width, double-inlet (DWDI) centrifugal impeller for the lift fans of a hovercraft. Given the current high performance of impellers, the design strategy uses a computational method, which is capable of predicting flow separation and vortex-dominated flow fields, enabling a detailed comparison of all aerodynamic losses. The design method, assuming a weak interaction between the impeller and the volute, employs a blade optimization procedure and several effective flow path modifications. Simplified CFD calculations were performed on fans with two existing impellers and the newly designed impeller to evaluate the impeller design criterion. The calculation was made with the impeller/volute coupling calculation and a frozen impeller assumption. Further refined CFD calculations, including the gap between the stationary bellmouth and the rotating shroud, revealed a reduction in the new impeller's gain in efficiency due to the gap. The calculations also further supported the necessity of matching the volute and the impeller to improve the fan's overall efficiency. Measured data of three fans validated CFD predictions in pressure rise at design and off-design conditions. CFD calculations also demonstrated the Reynolds number effect between the model- and full-scale fans. Power reduction data were compared between the measurements and the predictions along with the original design requirements.