Session: 01-04-01 Fluid Machinery Symposium

Paper Number: 86728

86728 - Diffuser Vane Slit Width Optimization Using Simple Prediction Method of Diffuser Rotating Stall Onset in a Centrifugal Pump 

■Background
  The diffuser rotating stall (DRS) is one of the biggest problems in pumps because unstable flow and pressure fluctuations are induced and the pump operating range is severely limited. Previous studies have confirmed that rotating stall is possible to occur both in the diffuser and impeller, causing noise, vibration and bearing damage. In order to suppress DRS, casing treatment has been investigated but the complete method has not been established and the mechanism of DRS onset is not fully understood yet.
  The authors have already confirmed that the DRS onset flow rate is shifted to lower flow rate by slitting to the diffuser vanes in the experiment and Computational Fluid Dynamics (CFD) of previous research work. However, it has not yet been shown as an effective slit design guideline.

■Objective
  The objective of this study is suppressing the DRS for wide range from larger flow rate to shut off without reducing the maximum efficiency.
  Another objective is to confirm the effectiveness of the optimized diffuser vane slit by experiments and to clarify the mechanism of the DRS suppression by analyzing the flow in the vicinity of the slit, based on the Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculation results.

■Method
  We propose a new simple method to predict the flow rate of the DRS onset by evaluating the non-uniformity of the flow rate in each diffuser passage using the steady-state calculation results.
  A multi-objective optimization using a surrogate model was conducted for the diffuser vane slit. In the process, the total pressure loss at diffuser, static pressure recovery ratio and the deviation of the flow rate in each diffuser passage were adopted as objective functions.
  Experiments and CFD calculations were conducted for two types of the diffusers, one with the original slit width W=2mm and another with the optimized slit width. In the experiment, the pump performance and the onset flow rate of the positive slope of a head-flow performance curve were compared for the two types of diffusers. The time history of static pressure signals were measured by four pressure sensors mounted at the inlet and the center of the diffuser for two passages, and the post processing of the wave form was conducted by FFT analyzer to identify the number of stall cells and its propagating velocity.
  In the numerical analysis, URANS calculation using a commercial software package (ANSYS CFX 2019R3) was performed for two types of the diffusers to compare the internal flow.

■Result and Conclusion
  As a result of a multi-objective optimization for the diffuser vane slit, the slit width, W=9mm was comprehensively determined. As for the pump performance test, the diffuser with the optimized vane slit showed the stable head-flow performance curve compared to the diffuser with the original vane. However, it was clarified that the DRS was caused at partial flow rate, based on the time history of the static pressure signals. As a result, the flow rate of the DRS onset could be shifted to the lower flow rate by 12%, and the operating range could be widened, though the pump efficiency decreased by 2.7% at the design flow rate. In the URANS calculation, it was found that the velocity of the jet flow from the slit depends on the slit width, and its effect on the backflow from the diffuser outlet was clarified.

Presenting Author: Sota Nakayama Osaka Institute of Technology