A method for evaluating the effect induced by vortices on the moment and efficiency of the fluid machinery

Up to now, there is no appropriate method to quantitatively evaluate the effect of local flow on the fluid machinery's performance, especially the moment and efficiency. In this paper, a moment decomposition method (MDM) is developed for this purpose. The major advantage of this method is the ability to calculate the contribution of local flow to the blade's moment and the applicability to simulations with non-constant density flows. The method is applied to a centrifugal pump under different flow rates. The relative error is around 10% under 0.5Qd∼1.0Qd (Qd is the design flow rate), showing satisfying accuracy. By analyzing the flow angle, it is found that the flow impact at blade inlet triggers the flow rate imbalance and alternative vortex generation in adjacent passages. The MDM results show that vorticity effect dominates under different flow rates. Negative kinematic effect in the passage with stronger vortices leads to lower moment contribution compared to another passage. By decomposing the passages, it is found that the wake region is the main source of moment. In addition, the hydraulic efficiency decrease in the impeller due to flow rate decrease is the result of wake suppression in the passage with weaker vortices.