The calculated effect of flow region variation during throttling is given by the solid lines in Fig. 17, and referring to the corresponding predicted flow incidence in Fig. 18, it becomes apparent why the performance is stabilized for this type of vortex motion. In particular, contraction of the through-flow leads to increased relative velocity and reduced incidence at the second stage, thereby delaying second stage stall and increasing work capacity. In this case, a self-regulating process occurs involving non-linear variation in vortex growth with flow coefficient, as well as growth of region B at condition (5). Although it is not shown here, additional growth of region B tends to occur at even lower flow rates, which then tends to further unload the first stage blading. Another observation from Fig. 17 is that the fraction of vortex power increases as the vortex grows. The vortex power ratio of approximately 33% at high flow rates is in agreement with the experimental findings of Mazur and Singh [40], where vortex power was determined for a similar fan using hot-wire anemometer flow field measurements and control volume analysis.