Flow through the first stage is much like a 2D diffusing cascade, with a reduction in relative velocity from inlet to outlet, however, there is an increase in radial velocity component. Conversely, the second stage behaves somewhat like the blades in a forward curved centrifugal fan except the flow is accelerated by contraction in the impeller interior, leading to relatively low flow incidence. Flow contraction given by the ratio of suction to discharge arc lengths S1/S4 is frequently noted in the literature, yet its affect on second stage incidence and loading is often overlooked in simple models of the fan. For reference, the dotted line velocity triangles in Fig. 11 represent zero flow contraction. In high-speed fans the accelerated through-flow can lead to high Mach number and choking, presenting a potential limitation for high-speed aircraft application. Note that the velocity triangles make it clear that an inner blade angle near 90° is appropriate from a second stage flow incidence standpoint.
By inspection of the velocity triangles and assuming cθ3=cθ2 (conservation of angular momentum in the interior region), we can write the non-dimensional ideal total pressure rise across the two stages as