EFFECT OF MODIFICATION TO TONGUE AND IMPELLER GEOMETRY ON UNSTEADY-FLOW, PRESSURE-FLUCTUATIONS, AND NOISE IN A CENTRIFUGAL PUMP

Particle Image Velocimetry (PN), pressure, and noise measurements are used to study the effect of modifications to tongue and impeller geome tries on the flow structure and resulting noise in a centrifugal pump. It is demonstrated that the primary sources of noise are associated w ith interactions of the nonuniform outflux from the impeller (jet/wake phenomenon) with the tongue. Consequently, significant reduction of n oise is achieved by increasing the gap between the tongue and the impe ller up to about 20 percent of the impeller radius. Further increase i n the gap affects the performance adversely with minimal impact on the noise level. When the gap is narrow, the primary sources of noise are impingement of the wake on the tip of the tongue, and tongue oscillat ions when the pressure difference across it is high. At about 20 perce nt gap, the entire wake and its associated vorticity trains miss the t ongue, and the only (quite weak) effect of nonuniform outflux is the i mpingement of the jet on the tongue. An attempt is also made to reduce the nonuniformity in outflux from the impeller by inserting short van es between the blades. They cause reduction in the size of the origina l wakes, but generate an additional jet/wake phenomenon of their own. Both wakes are weak to a level that their impacts on local pressure fl uctuations and noise are insignificant. The only remaining major contr ibutor to noise is tongue oscillations. This effect is shown to be dep endent on the stiffness of the tongue.