Numerical investigation of the 4-Quadrant behaviour of different mixed flow diffuser pumps with experimental verification

The complete pump characteristics, often called the 4-quadrant (4Q) behaviour of a pump, are of essential interest in case of off-design operating conditions such as pump trip. At this exceptional load case the pump enters the dissipation zone and moves further into the turbine zone whilst changing the direction of rotation and the flow direction.
The time-consuming and expensive experimental investigation of the 4Q-behaviour requires a specific test rig, allowing the flow direction as well as the rotation direction to be reverted. By measuring the pump performance (head and efficiency) at variable positive and negative discharge in both directions of rotation, the complete pump characteristics are evaluated.
Nowadays Computational Fluid Dynamics (CFD) - analyse allow the reliable prediction of the hydraulic performance of a pump around the design point. However, abnormal operating conditions lead to complex and unsteady flow phenomena inside the pump. Besides steady-state calculations in the normal operating conditions, lots of transient CFD-investigations are therefore necessary to simulate the whole pump characteristics accurately.
In the present study two different types of mixed flow diffuser pumps, one equipped with adjustable impeller blades, the second one with comparable low specific speed, were investigated. It can be shown that it is possible to reliably simulate the 4Q behaviour of mixed flow diffuser pumps based on CFD-methods as experimental verifications have shown a remarkably good agreement for both investigated pumps.
Keywords: Mixed flow oump, CFD, 4-quadrants, comparison of measurement and simulation, model test, transient calculations