Optimization and Validation of Secondary Flow Effect for Flowrange Enhancement in a Low Solidity Circular Cascade Diffuser

A Low Solidity circular cascade Diffuser (LSD) in a centrifugal blower is designed by means of multi-objective optimization technique. An optimization code with a meta-model assisted evolutionary algorithm is used with a commercial CFD code ANSYS-CFX. The optimization is aiming at improving the static pressure coefficient at design point and at small flow rate condition while constraining the slope of the lift coefficient curve. Seven detailed design parameters describing the shape and position of the LSD vane were introduced. Moreover, a small tip clearance of the LSD blade was applied in order to activate and to stabilize the secondary flow effect at small flow rate conditions. The optimized LSD blade has been manufactured and the characteristic of optimized LSD is confirmed by the experimental test rig. Optimized LSD has an extended operating range of 114 % towards smaller flow rate as compared to the baseline design without deteriorating the diffuser pressure recovery at design point. The detailed flow in the diffuser is also confirmed by means of a Particle Image Velocimetery (PIV). The flow was compared between in the cases with and without tip clearance. In spite of the fluctuating flow at the diffuser inlet, secondary flow spreads to the whole area of LSD blade pitch stably. Stable secondary flow suppresses flow separation at the suction surface of the LSD. It is found that the optimized LSD blade shows good improvement of the blade loading in the whole operating range, while at small flow rate the flow separation on the LSD blade has been successfully suppressed by the secondary flow effect.