| Among several casing treatment methods, circumferential grooves (CG) are well recognized as a means for stall margin extension with the best structure integrity and the minimum negative impact on the thermal efficiency for axial compressors. However, their design rule remains undecided due to the controversies in associate with the underlining physical mechanisms. A long lasting topic in this area is to accomplish a proper circumferential groove configuration for real compressors. The most common approach is to establish a database of casing grooves through extensive experimentation arid elaborate measurements in the tip region. A new control volume analysis is developed in this paper aiming at assessing the circumferential grooves effectiveness on stability improvement. The underlying mechanism for this approach is based on the hypothesis that the spike stall precursors can be triggered by the forward spillage of the rotor tip leakage flow and the onset condition of such a spillage is determined by the axial momentum balance within the rotor tip region. Control volumes are defined to quantify the axial momentum balance of the whole region where the grooves influence the flow at the rotor tip. Through comparing the axial momentum distribution of the blade tip region of circumferential groove configurations, its stall margin extension effeteness will be identified quantitatively, which can be very helpful during the design processing. Experiments are carried out to validate this approach both in low speed compressors and transonic compressors.The control volume method provides a brief yet quantitative approach for such a prescreening purpose because it is capable of providing an insight in the flow physics related to fluid forces, momentum, energy and entropy. NASA Rotor67is first taken as an example to develop control volumes in this paper. The compressor field with and without grooves are firstly simulated by using the steady/unsteady single passage simulation. A series of control volumes are set on the rotor tip region. Each of those control volumes extends one blade pitch in the tangential direction, aiming at catching all of the flow physics of the whole pitch in the tip region. Periodic boundary conditions apply to both control surfaces in the tangential direction. The cumulative axial momentum distribution are formed to describe the flow status in the rotor tip region, whose distribution resemble bell curves. It is an integral result to reflect the final balance between the main flow and TLF and it is extremely important information on stability. Each point on the bell curves represents the value of the cumulative axial momentum from the inlet up to this local position, which is equal to the net axial force acting on the flow by this part of the blade. The value and the axial location of the peak are crucial parameters to weight the potential of flow instability. While the peak value indicates how strong the positive axial force can reach, the axial location of the peak indicates how such a force is distributed along the blade chord. Therefore, if such a force becomes increasingly stronger and concentrated towards the blade leading edge further, the flow should become less stable.Two examples are conducted to validate this control volume approach experimentally. For the low speed compressor case, three different double-groove schemes are studied. The effectiveness of these groove-schemes are compared by using the bell curves, and then validated with experiments, which shows a good agreement. Another example is the series of circumferential groove VCT3of transonic rotor ND-TAC in university of Notre Dame are studied. Their stall margin extensions are analyzed by the control volume method. It has then been proven that the bell curves get the correct stall margin extension tendency via experiments.Another application for control volume method is the analysis of the influence of grooves on compressor efficiency. Entropy generation is indentified as the critical parameters of the irreversible flow loss. The entropy generation changed by three different grooved casings are studied. Control volumes are settled down at the rotor tip region as well as the inside of each groove. Comparing with the quantitative entropy value in and out of the groove region, it is found that the groove improved the local tip flow structure and ameliorated the flow loss, besides, the inner groove produce extra flow loss. Hence, the peak efficiency changed by grooves are the final effect of these two factors.Groove depth, groove number as well as groove axial locations are studied as they are the most important groove geometries that influence the stall margin extension. By using the control volume method, the effectiveness of the grooves with different geometries are compared for a transonic compressor rotor J69. The groove axial location show a significant effect on stall margin improvement. With the control volume approach proposed in this paper, an optimized groove scheme is finally select for this rotor, which has a considerable stall margin improvement as well as a slighter negative influence on efficiency. |
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