Advances in Research on Rotor Stall of Compressors in Engineering Thermophysics

Rotary stall is a key factor that restricts the stable working range of the compressor. How to quantitatively predict the stall boundary in the design phase has always been a key issue for designers. There are two main methods for predicting stall boundaries: First, numerical simulation and second, analysis model. Although the former can capture more details of the flow field, it requires high computational resources, especially unsteady value simulation, which is still too time-consuming for current industrial applications. The latter can quickly predict the stall boundary during the design phase, but due to the introduction of approximate assumptions and empirical coefficients, its accuracy and application range are limited. In transonic compressors, the tip leakage flow and its interaction with the shock wave play a dominant role in the tip flow field. Previous studies have shown that this interaction is closely related to the precursor of the compressor's spike-type stall.

In order to evaluate the effect of tip clearance on the stall boundary during the design phase, researchers at the National Energy Wind Blade Research and Development (Laboratory) Center of the Institute of Engineering Thermophysics, Chinese Academy of Sciences developed a method suitable for transonic gas injection based on the tip leakage flow frontage criteria. The machine's stall boundary prediction method. Firstly, through the simulation of the unsteady value of a certain transonic compressor rotor, the relationship between the unsteady fluctuations of the tip flow field and the threat of stall was explored. By analyzing the tip flow field fluctuation at the boundary point of the stall, it is found that the interface between the leakage flow and the mainstream oscillates periodically with time, and its frequency is approximately 0.5 times the blade passing frequency. The change of the tip relative to the total pressure distribution indicates that the interface near the low relative total pressure area deflects upstream and the interface oscillates with the periodic variation of the relative total pressure distribution; at some moments, the interface is already at the leading edge of the blade. Upstream, that is to say the leakage flow overflows from the leading edge of the adjacent blade. The distribution and changing trend of the axial velocity of the leakage flow is consistent with the distribution of the relative total pressure. In summary, the low relative total pressure causes the negative axial velocity of the leakage flow to increase, which causes the interface of the corresponding position to deflect upstream. Therefore, how to establish the quantitative relationship between the leakage flow volume and the interface position is the key to modeling.

In a transonic compressor, due to the influence of shock waves, the interface exhibits different forms in the wavefront and the wavefront. In the upper boundary of the shock wave, the interface is approximately linear, and the interface downstream of the shock wave is deflected upstream. This change is caused by the change of the momentum balance between the leakage flow and the mainstream. In the upstream of the shock wave, the area of ​​the leakage flow and the mainstream is a three-dimensional spiral vortex structure. In the direction perpendicular to the interface, the momentum of the leakage flow and the mainstream flow balance. In order to analyze the momentum exchange relationship, the researchers simplified the role of the leakage flow and the mainstream into a control body model as shown in FIG. 1 , in which the leakage flow surrounds the upstream leakage vortex and exchanges momentum with the mainstream to form an interface. Assuming that the region of action is inviscid and incompressible, an approximation of the radius of the region of interest in the plot can be obtained based on the mass of the control body and the momentum conservation constraint. Then, the position of the suction surface of the interface relative to the suction surface of the blade is obtained by superimposing the position of the leakage vortex core track and the radius of the action area.

Figure 2 compares the predicted effects of the new model and Other models studied. The results show that the new model can better predict the size of the leakage flow and the main interaction area, which is due to the cumulative effect of the upstream leakage vortex in the new model. Based on the ability of the new model to predict the position of the interface, the researchers modeled the interface of the critical point of stall, as shown in Figure 3. At the critical point of stalling, the interface is deflected upstream through the shock, just past the leading edge of the adjacent blade. According to the geometric relationship in the figure, the relationship between the critical point leakage flow velocity and the mainstream velocity can be obtained. When the leakage flow velocity distribution is known, the mainstream velocity can be obtained based on the model in this study to estimate the flow coefficient at the stall boundary point. In addition, if we consider the fluctuation of the leakage flow velocity distribution over time, we can also get the mainstream speed fluctuations based on the model to analyze the unsteady flow effects of near-stall conditions. Comparing the results of model prediction and numerical simulation of the stall boundary point when comparing different gap sizes, we found that the model proposed in this study can well evaluate the impact of the leakage flow between the blades on the stall boundary. Due to the reasonable consideration of the morphological changes at the interface between the shock upstream and downstream, the model can capture the non-linear change of the flow coefficient of the stall boundary point with the gap size.

The model proposed in the study can be used to evaluate the effect of tip clearance on the stability of the compressor during the design phase, or integrated into the engine control system as a stall prevention module to ensure safe and reliable operation of the engine. At present, the modeling and design of the model verification under the rotational speed conditions have been completed, and the analysis and model validation during the partial rotational speed operating conditions are in progress, and the application of the model is extended to the engine under variable operating conditions.

Some related research results have been published at the international conference ASME Turbo Expo 2016. In order to fully evaluate the effect of tip clearance, provide the basis for rotor optimization design and tip flow control, the center researchers are also studying the impact of tip leakage flow on loss and pressure rise.

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