Recess Vane Passive Stall Control

Abstract

An experimental investigation has been carried out on the influence of a vaned recessed casing treatment on the stall margin improvement of axial flow fans with different hub to tip ratio, with and without inlet distortion. The inlet distortion tests were conducted on a 0.5 hub to tip ratio fan and significant increases in the flow range with only small drops in operating efficiency were observed. The clean flow tests were conducted on higher hub to tip ratio fans (0.7 and 0.9). In each case the stage characteristic was compared with the results obtained with a solid casing. Significant increases in the flow range, with only modest or no loss in operating efficiency, were observed for optimum configurations at both diameter ratios.

Application of casing treatment to an industrial axial-flow fan

“…This results in a tendency towards an early stalling regime. Ziabasharhagh et al [9] also showed that scaling of the casing treatment in proportion to the blade chord was a reliable design criteria. Further analysis of these results indicated that a depth of 50 mm would be sufficient to expect a reasonable stall margin improvement.…”

“…It also appears that axial chord exposure is independent of loading. Miyake and Inaba [7], for instance, used an industrial fan with 15 blades and low loading, whereas Azimian et al [8], Ziabasharhagh et al [9] and Kang et al [4] all conducted experiments on a fan with 27 blades and aero-engine loading. With this knowledge, a casing treatment was built for the experimental fan with the facility to vary the amount of rotor axial chord exposure.…”

“…Therefore no analysis of the effect of treatment height above 50 mm was made. However, Azimian et al [8] and Ziabasharhagh et al [9] all varied the height of the recess and showed that a minimum recess volume for flow exchange is essential for optimal casing treatment operation. A volume of less than the minimum causes an internal flow blockage inside the recess vane passage and a reduced flow exchange between the main and recess flow.…”

Recessed Casing Treatment Effects on Fan Performance and Flow Field

“…The results were very encouraging with superior stall margin and less efficiency sacrifice when compared to conventional slot type treatments. In addition , Ziabasharhagh et al(1992) casing treatments were capable of increasing stall margin for a range of machines and also able to increase tolerance to inlet flow distortion.…”

“…Takata et al(1977), Smith et al (1984( ), Johnson et al(1987 and Crook et al(1993), but few on vaned recess casing treatments. Ziabasharhagh et al(1992), however, based on the 'wool tuft flow study provided a hypothesis of the operation from which the following physical description of the flow may be deduced as sketched in Fig.l. The low momentum fluid, associated with stalling flow is centrifuged towards the blade tip and then recirculates axially as shown in Fig.1-a and also circumferentially in the untreated fan.…”