The effect of volute geometry on the flow structure, pressure fluctuation and noise in a centrifugal pump.

摘要

The characteristics of the flow structures within the volute of a centrifugal pump are investigated using Particle Image Velocimetry. The objective is to study the relationship between flow structure, pressure fluctuations and "far field" noise for different volute tongue geometries and to identify the sources of pressure fluctuation and "far field" noise. The research includes development of: (1) A quantitative flow visualization technique for studying flow structures within the volute of the pump; (2) An uncertainty analysis for verifying the accuracy of the technique; and (3) Detailed measurement of instantaneous velocity, noise and pressure.; The velocity is calculated by utilizing auto-correlation function to determine mean shift of all particles within a very small area covering about one to two thousandth of entire image. Uncertainty analysis shows that the relative error can be maintained within one percent by controlling the magnification and particle concentration.; The velocity, pressure and noise were measured for different impeller blade orientations, tongue geometries and flow conditions (on and off design conditions). The velocity measurements demonstrate that the flow is pulsating and depends on the orientation of the blade relative to the tongue.; A "jet/wake" phenomenon can be identified near the impeller, and it is evident especially at the cut-water region. Away from the tongue the distributions of tangential velocity, {dollar}Vsbtheta{dollar}, mostly agree with the assumption of {dollar}Vsbtheta propto 1/r{dollar}. A large elongated vortex train with negative vorticity is generated by a mismatch between instantaneous high {dollar}Vsbtheta{dollar} exiting from the impeller and local low tangential velocity in the volute. The flow structure and velocity distributions strongly depend on the geometry of the tongue and the clearance between the tongue and the impeller. The results show that the strength of the blade "wake" varies with tongue geometry.; The pressure distribution calculated from PIV data (velocity distributions) shows that blade-tongue interactions and non-uniform outflux from the impeller are the primary contributors to local pressure fluctuations. Furthermore, the vorticity distributions, particularly the large vortex trains associated with the "jet/wake" phenomenon, dominate the variations in total pressure.; Noise measurements show that noise level is reduced considerably by increasing the clearance between the tongue and the impeller, as well as shortening the tongue. This major reduction is the result of changes in flow structure, velocity and pressure distributions with different tongue geometries. A careful analysis of the flow structure and pressure distributions indicates that the impingement of the jet on the tip of the tongue and the oscillation of the tongue are also major sources of noise generation in the present centrifugal pump. (Abstract shortened by UMI.)