Fig. 4a shows the flow visualization results of four impellers; it can be commented that the IMRs are clearly different in terms of shape, size and location, depending on the impeller type and Reynolds number. In the case of axial-flow propeller PR30, the IMR above the impeller is smaller than that located below the impeller, most likely owing to its downwards-pumping direction, whereas for other impellers, the IMRs are about the same size in all four quadrants.