Cavitation Visualizes the Flow Structure in the Tip Region of a Waterjet Pump Rotor Blade

摘要

In this paper, we examine the occurrence of cavitation in the tip region of a waterjet pump, and use the observations to identify key features of the flow structure. The rotor, stator and pump casing in our recently upgraded facility are made of acrylic, whose refractive index matches that of the working fluid, a concentrated aqueous solution of sodium iodide. Such matching makes the blades invisible, enabling flow structure visualization and optical measurements without any obstructions. Our initial tests with fresh water focus on cavitation in the vicinity of the tip region close to design conditions. Near the leading edge, cavitation inception near the tip corner of the pressure side causes accumulation of bubbles along the pressure side of the corner until mid blade. As roll-up of the tip leakage vortex (TLV) starts in the mid blade region, these bubbles travel across the tip-clearance to the suction side, and become nuclei for cavitation inception in the TLV. As the TLV migrates to the vicinity of the pressure side of the neighboring blade it bursts, generating a cloud of bubbles that spreads over most of the aft section of the passage. The leakage flow in the tip gap along the aft side of the blade is strong enough to cause sheet cavitation within the gap, starting from the pressure side corner. The paper also presents preliminary Particle Image Velocimetry (PIV) data concentrating on the roll-up of TLV. Results are consistent with observed cavitation phenomena.