Stall Margin Research Articles

Flow Mechanism for Stall Margin Improvement due to Circumferential Casing Grooves on Axial Compressors

A computational study is carried out to understand the physical mechanism responsible for the improvement in stall margin of an axial flow rotor due to the circumferential casing grooves. Computational fluid dynamics simulations show an increase in operating range of the low speed rotor in the presence of casing grooves. A budget of the axial momentum equation is carried out at the rotor casing in the tip gap in order to understand the physical process behind this stall margin improvement. It is shown that for the smooth casing the net axial pressure force at the rotor casing in the tip gap is balanced by the net axial shear stress force. However, for the grooved casing the net axial shear stress force acting at the casing is augmented by the axial force due to the radial transport of axial momentum, which occurs across the grooves and power stream interface. This additional force adds to the net axial viscous shear force and thus leads to an increase in the stall margin of the rotor.

Stall Margin Improvement of an Axial Flow Fan with End Wall Injection and Suction. 2nd Report. A Study of Stall Recovery Margin Improvement.

This paper presents the experimental results of stall recovery margin improvements. Studies have been conducted on a low speed axial flow fan to investigate the stall cessation flow rate with several kinds of injection or suction holes on the end wall. First of all, the fan characteristics for the solid wall have been measured. The hysteresis loop has been observed, but the size of hysteresis loop was not large, so that the stall cessation flow rate was about 17% smaller than the stall onset flow rate for several rotor speeds. Stall recovery margin improvements for both injection and suction were about 3 to 5% larger than stall margin improvements. For the case of injection, the position of maximum stall recovery margin improvements has moved to downstream direction a little from the position of maximum stall margin improvements. For the case of suction, the largest improvement was found at the position of just upstream of leading edge of blade. And negative improvement was not found except very large suction. For the case of injection, comparing with stall recovery margin improvements and stall margin improvements, both values are almost the same with regard to the injection momentum relative to the chordwise.

Stall Margin Improvement of an Axial Flow Fan with End Wall Injection and Suction.

The experimental studies are conducted to reveal the mechanism of stall margin improvement of an axial flow fan by injection or suction from the end wall. In case of injection, the largest improvement is obtained by the injection at about 0.14∼0.21 times axial chord length downstream from leading edge. The reason for large improvement is that stall vortex, shed intermittent separation vortex and tip leakage vortex are dissipated by this injection, and also that this blowing suppresses the separation of boundary layer. In case of suction, the largest improvement is found for the suction from the end wall near leading edge. The amplitude of periodic static pressure after stall inception becomes smaller in comparison with injection cases. These effects are increased with the increase of suction flow rate, because the discharge of the vortex occurs more easily. On the other hand, the suction from the upstream of leading edge reduces the axial velocity near rotor tip, and then it induces stall. Also we tried to visualize the tip region flow. The suppression mechanism is discussed based on the visualization. The suppression of stall is successfully photographed.

Stall Margin Improvements of Conventional Axial Flow Fans with the Reformed Casingtreatments.

At the first part of this paper, the experiments and results to the blowing and suction on a fan are presented, and it is found that blowing toward the down stream direction are more effective and blowing location is suitable on the leading edge side of blades. On basis of these results, the reformed casingtreatments which the blowing air from the slots makes the large relative momentum for blade chord are build, and the experiments using the two conventional fans equiped with the reformed casingtreatment and conventional one are performed. So, the very large stall margin improvements (about 1.7 times) and about 0.5% efficiency penalty of 10% stall margin improvement at design flow rate are obtained for the reformed casingtreatment.

Stall Margin Improvement by Casing Treatment—Its Mechanism and Effectiveness

Experiments on the effect of casing treatment were carried out using low-speed axial-flow compressors. Results on the overall compressor performance and on the flow through the blade row as well as the flow within the treatment slots are presented. Then, based on the experiments, a possible mechanism of the stall margin improvement is suggested.