An investigation has been carried out in order to define the process of rotating stall inception in a low speed, low hub-tip ratio fan. Based on elementary cascade analysis, the fan would be expected to stall from the root, however, considerable experimental evidence indicates that tip stall is more frequently incurred. Although the analysis has been undertaken for a specific fan it is considerd to be representative of a broad range of machines. The analysis has involved two primary considerations, first the effect of streamtube contraction which has been studied theoretically and secondly real flow effects (those not contained in the theoretical model) which have been studied experimentally.
The study of streamtube contraction indicates that the root rematches to a more stable operating point thus alleviating some of the problems in that region. The experimental investigation was undertaken on an isolated rotor, with successive build modifications to increase the likelihood of rotating stall inception at the root. It was apparent that real fluid effects tended to steepen the root pressure rise characteristic, thus enhancing the stability in that region. The performance of the fan at the tip tended to be poor providing a pressure characteristic with a lower negative gradient than anticipated indicating less stability than simple flow models would suggest. Hot wire flow mapping at the rotor exit supported the overall conclusion that the rotor showed a strong reluctance to stall at the root apparently due to “centrifuging” of the boundary layer towards the tip.