Research On The Low-speed Modeling Similarity Criteria For High-speed Compressors And The Flow Mechanisms Of Axial-slot Casing Treatments

The performance and stability of axial compressor have significant impact on the overall performance of the aeroengine. Nowadays, advanced blade design techniques have been able to enhance the compressor aerodynamic performance. However, stall margin still limits the development of high-performance compressor. As a simple but effective passive control method, casing treatments are widely used in engineering practice. Nevertheless, due to the complex flow mechanisms in compressors, it is difficult to form a universal design approach for casing treatments. Thus, for a certain compressor, it is usually time-and cost- consuming to conduct numerous tests to establish a database. In addition, there exists a significant risk to conduct casing treatment tests on a high-speed compressor.Aiming at lowering test duration and risk, low-speed modeling method which has been successfully applied to improve the design techniques for highly-loaded compressors will be used in this paper. However, there still exist some challenges when it comes to simulating flows inside the casing treatments. On one hand, similarity criteria should be established to produce similar flow characteristics inside the casing treatments on both low-and high-speed compressors. On the other hand, whether an optimum casing treatment configuration on the low-speed model can achieve comparable effects on high-speed compressor needs to be validated. For these reasons, a low-speed model compressor is designed to simulate the flow at partial design speed condition of a high-speed compressor rotor based on similarity criteria. Numerical and experimental investigations are performed on the low-speed model compressor to study the influence mechanisms of casing treatments on flow loss and stability margin. Based on the results of the parametric study, the geometry of the casing treatment is optimized. Then, the refined scheme is validated on the low-speed compressor and finally applied on the high-speed compressor.Firstly, a low-speed model compressor was designed to simulate the peak-efficiency operating condition at 65% design speed of J69 transonic compressor rotor, referring to the procedure of the low-speed repeating-stage model testing program for the rear stages of a high-speed core compressor. The low-speed airfoils were designed to achieve the normalized pressure/velocity distributions of the high-speed rotor blade which were determined from a numerical simulation. Then, the low-speed compressor test rig was built and equipped with comparatively complete measurement system. Both the computational and experimental results showed that the low-speed model compressor produced comparable overall performance and flow field characteristics of the high-speed target.Axial-slot casing treatments were selected to study the effects of the number, depth, and the width of the slots on the endwall flow loss of the low-speed compressor. Entropy generation was taken as the quantity to measure the flow loss. By using the control volume method, the flow loss in the rotor tip region, as well as the flow parameters on the opening surfaces of the slots, was studied with simulation results. It is shown that the distribution of the flow loss in the tip region was altered by the variation in the flow parameters on the slot opening surfaces, when the geometry of the slots was changed. Moreover, the efficiency penalty with casing treatment was closely related to the total loss in the tip region. Three axial-slot configurations were chosen to validate the simulation predictions experimentally. It was indicated that the calculations showed good agreement with the measured efficiency.Control volume method was utilized to quantify the axial momentum along the rotor tip to reflect the competing consequence between the main flow and tip leakage flow. It was shown that the peak of the cumulative axial momentum curve can represent for the time-and spatial-averaged position of the interface between the main flow and tip leakage flow. The similarity analysis on the tip flows between the low and high-speed compressors indicated that the instability mechanisms of both compressors are similar. By comparing the peak values and positions of the cumulative axial momentum curves, the stall margin improvements of the chosen casing treatments were predicted numerically and then validated experimentally. Further, it is found that the ability in stability enhancement of the casing treatments has a close relationship with the radial transport of the axial momentum on the slot opening surfaces.Based on the parametric analysis on the effects of the casing treatments on the flow loss and stability of the low-speed compressor, the configuration of the casing treatment was refined to produce a good balance between the efficiency and stall margin. The effect of the optimized scheme was verified by both numerical and experimental results. Lastly, the geometry of the optimized scheme was transformed based on geometric similarity principle for the high-speed compressor. Numerical results showed that the high-speed version of the optimized casing treatment produced similar effects on the peak efficiency and stall margin of the high-speed compressor.