Numerical investigation of effect of recess vane casing treatments on an axial lift fan performance

Lift fans fitted on hovercraft are often subjected to pressure pulse generated by the sea waves. With a high pressure from the pressure pulse, the fan is driven transiently to a low mass flow rate operating point. The probability that a stall can happen is relatively high. The recess vane casing treatment (RVCT) is used to improve the axial lift fan's stall margin in this paper. Using the NUMECA software, the fan with solid casing and different RVTC geometry and its flow field are analyzed. The geometry modifications include blade chord exposure variation and cavity outlet axial span. Compared with the solid case, all casing treatments result in a reduction in efficiency. The blade chord exposure is a key factor that affects the efficiency. The RVCT with minimum blade chord exposure provides an inferior stall margin of -0.293% while the others provide 6% to 15% stall margin improvement, respectively. In the study of the physical flow mechanisms, visualization can provide an insight into the flow field. This reveals that characteristics of the mainstream flow are different between near stall point and design point for the solid casing fan. The three-dimensional (3D) flow field suggests that the flow capacity near the blade tip is damaged by the blockage. The rotor blade is considered as a critical tip based on its stalling behavior. By applying RVCT, the flow field near blade tip is modified, and local mass flow ahead of blade leading edge increases while flow distribution of blade downstream along spanwise is almost the same with the solid casing fan. Also, the flow exchange between RVCT and mainstream is established through the introduction of RVCT. In quantitative analysis, the flow exchange is quantified based on the mass flow passing through the cavity. The ability of RVCT to stabilize the fan is based on the size of cavity, the more mass flow passes through cavity, the more stall margin enhancement can be obtained by the fan. However, the flow exchange between RVCT and mainstream can cause intense mixing, which can lead to efficiency loss.