Numerically simulating effect of location of slot of axial-and-skewed slot casing treatment on extending the fan stage stable operating range

For the purpose of gaining some insight into the mechanism of the effect mentioned in the title, we studied by numerical simulation the axial-and-skewed slot casing treatment on an axial-flow transonic fan stage designed by A. J. Wennerstrom^[1]. Two axial-and-skewed casings were designed. The slot lengths of both casings were about 50% blade chord. The slots of Casing 1 were ahead of the blade, and were overlapped with the blade tip about 30% blade chord. The slots of Casing 2 were comparatively shifted downstream and were overlapped with the blade tip about 50% blade chord. Numerical simulation work was done for both casings and at two different speeds: 95% and 70% of design speed. The simulation results, presented in Figs.5 through 12 and Table 2, show preliminarily that, at both the 95% and 70% design speeds, Casing 2 extends the fan stage stable operating range more significantly than Casing 1 at the cost of the reduction of some efficiency. We found an unexpected phenomenon that, at the 70% design speed, not only the stable operating range is extended but also the pressure ratio and efficiency of the fan stage are improved by both casings. The reasons for this phenomenon were analyzed in the last paragraph of section 4 of the full paper.