Numerical and Experimental Analysis of Flow Phenomena in a Centrifugal Pump Operating Under Low Flow Rates

We describe an experimental investigation of two similar centrifugal pump impellers of low specific speed. Both impellers show rotating cavitation over a wide range of part load operating points. The occurrence of this phenomenon produces a characteristic shape of creeping head-drop compared to the more usual sudden head-drop at normal operation points. The onset of rotating cavitation can be assigned to a certain value of the parameter σ/2α meaning the cavity volume in relation to the incidence angle. Optical analysis by video and high-speed camera techniques illustrates the development of this instability mechanism which is mainly driven by an interaction of the cavity closure region and the following blade. Combining these observations and the results of a Fourier-transformation the characteristic propagation frequencies of rotating cavitation can be presented for one impeller

Rotating Cavitation in a Centrifugal Pump Impeller of Low Specific Speed

An analysis of alternate blade cavitation on flat plate cascade is made using a singularity method based on a closed cavity model. In the steady flow analysis, it was found that two kinds of steady cavitation patterns exist. One is equal length cavitation in which the cavity lengths of all blades are the same. The other is alternate blade cavitation in which the cavity length changes alternately from blade to blade. Although the present model fails to predict the range of cavitation number where alternate blade cavitation occurs, it predicts alternate blade cavitation fairly well in terms of cavity length. A parameter study shows that the development of alternate blade cavitation is quite different depending on the solidity of cascade. The stability of equal length and alternate blade cavitation is then examined allowing the cavity length freely to change. It was found that alternate blade cavitation is stable for the cascades with larger solidity and unstable for the cascades with smaller solidity. The equal length cavitation is stable in both cases only in the region of cavitation number larger than that where the alternate blade cavitation solution separates from the equal length cavitation

A Theoretical Analysis of Alternate Blade Cavitation in Inducers

We analyze, from experimental results, the influence of the shape of the leading edge and its sharpening on the cavitating behavior of an inducer. The studied inducer is designed according to a methodology developed at LEMFI. Successive cutting and sharpening (four cuts, which modify up to 20 percent of the blade chord at the tip), were made to modify the shape of the leading edge. For the various geometries, the experimental results obtained on the LEMFI test rig are presented as follows. Noncavitating Regime.- Overall performances at 1450 rpm. Cavitating Regime.- (1) The development of the cavitation versus the cavitation number, (2) the description of the various cavitation pictures, and (3) the pressure fluctuations measured at the wall at 150 mm downstream of the trailing edge for various flow: rates and inlet pressures. The CFD simulations carried out under CFX-Blade Gen + on this range of inducers are presented to explain certain aspects observed

Experimental Analysis of an Axial Inducer Influence of the Shape of the Blade Leading Edge on the Performances in Cavitating Regime

A new method is proposed for the stability analysis of cavitating flow. In combination with the singularity method, a closed cavity model is employed allowing the cavity length freely to oscillate. An eigen-value problem is constituted from the boundary and supplementary conditions. This method is applied for the analysis of rotating cavitation in a cascade with a finite pitch and a finite chordlength. Unlike previous semi-actuator disk analyses (Tsujimoto et al., 1993 and Watanabe et al., 1997a), it is not required to input any information about the unsteady cavitation characteristics such as mass flow gain factor and cavitation compliance. Various kinds of instability are predicted. One of them corresponds to the forward rotating cavitation, which is often observed in experiments. The propagation velocity ration of this mode agrees with that of experiments, while the onset range in terms of cavitation number is larger than that of experiments. The second solution corresponds to the backward mode, which is also found in semi-actuator disk analyses and identified in an experiments. Other solutions are found to be associated with higher order cavity shape fluctuations, which have not yet been identified in experiments.

Analysis of Rotating Cavitation in a Finite Pitch Cascade Using a Closed Cavity Model and a Singularity Method

Analysis of the suction performance, cavitation development, and internal flow fields of the impeller of a flat-helical inducer with two blades were performed. As the suction pressure is reduced, the balanced cavity development on both blades is destroyed. Alternate blade cavitation, in which cavity development evolves on one blade while it weakens on the other, can occur. When it does, the theoretical and actual pump heads can decrease quickly. Investigations were conducted to determine how this phenomenon develops.

Jian-Tong Zhang

Papers

Alternate Blade Cavitation on Inducer.