| The enhancement in aero-turbine engine performance mainly depends on the performance improvement of its components, such as the compressor. As an effective way to improve the cascade aerodynamics and to reduce the end-wall losses that constitute the main part of energy loss in the cascades, bowed compressor blades have been widely investigated and used. The three-dimensional geometries of the through-flow passage have great effect on the aerodynamic performance of compressor. As the aero-turbine engines with larger thrust-weight ratio develop, it has been a hot spot to study how to use the design idea of bowed blade in the highly-loaded compressor cascade, thus to enhance its performance to the greatest extent.A mixed optimization algorithm based on the genetic algorithms was used in the optimum design of bowed compressor cascades. The optimization variables were camber angle, (positive) bowed angle, and bowed height. The optimization goal was to search for the optimum matching between the bowed angle and bowed height when the blade camber angles were varied. According to the optimization results, three sets of the bowed compressor cascades with the camber angles of 40°, 50°and 60°respectively were designed and investigated experimentally in a low speed, large scale wind tunnel. Each set of the bowed cascades included one conventional straight blade cascade (STR), and three positive bowed blade cascades with bowed angle of 15°(PB15), 20°(PB20), and 25°(PB25) respectively. At design and off-design conditions, the detailed cascade flow fields were measured by five-hole probe. Static pressure taps were installed on both the end-walls and blade surfaces to obtain the distribution of the static pressure. The ink-trace flow visualizations were performed to show the boundary layer development. A numerical simulation was also conducted to supplement the understanding of the mechanism of the bowed blade effects.Both numerical and experimental results showed that for the three sets of the cascades, the bowed blade was able to improve the flow behavior near the end-walls of the cascade, the end-wall losses therefore decreased at 0°incidence. The total loss of the cascade could be reduced to the maximum extent by adopting the reasonable matching between the bowed angle and bowed height. Experimental results also showed that C-shape static pressure distribution along the spanwise direction existed on the suction surface of the straight cascade with larger camber angles, which was beneficial to the movement of low-energy fluid from the end-wall region to the mid-span. When bowed blade was applied, the larger bowed angle and larger bowed height would further enhance the accumulation of the low-energy fluid at the mid-span, thus deteriorated the flow condition there. As the incidence deviated from the design condition (0°incidence), with the increase of the camber angle, the value of the optimum bowed angle and the optimum bowed height decreased due to the increased losses at the mid-span. Under positive incidences, flow behavior near the end-wall region of some bowed cascades even deteriorated instead of improving due to the blockage of the separated flow near the mid-span keeping the low-energy fluid near the end-walls from moving towards the mid-span region.Numerical investigation into the flow field of the bowed blade cascade was carried out to study the effects of the separated flow and vortex structure on the aerodynamic performance of the cascade. Inlet and exit boundary conditions from the experimental data were used in the CFD simulation. The results showed that within the bowed compressor cascades with larger camber angle, the trend of the vortex structure from the multi-vortices to the single-vortex was obvious. The spanwise movement and mixing of vortices on the blade suction surface towards the mid-span was stronger due to the effect of the increased positive bowed angle. The severe flow separation resulted in the breakdown of the shedding vortex, thus the large flow blockage near the mid-span, which was one of the main causes of the rapid loss increase. Therefore, when positive bowed blade was used in highly-loaded compressor cascade, the worsening of flow behavior near the mid-span must be suppressed effectively. The flow within the highly-loaded compressor cascade presented three-dimensional characteristics. Severe flow separation occurred as the blade loading increased, resulting in a rapid loss augmentation. It was therefore significant to delay the three-dimensional flow separation, hence to reduce the increased loss through the study on the mixing and interaction between the vortices. A thicker inlet boundary layer thickness was found under the current experimental conditions, which was proved to have a large influence on the cascade performance. The bowed blade cascades with a thinner inlet boundary layer thickness were studied numerically. The results showed that as the inlet boundary layer was getting thinner, the improvement in the aerodynamic performance of cascades increased under the most flow conditions. However, rapid increase in the total loss also occurred when the bowed angle was larger enough for the cascades with larger camber angle under positive incidences.To achieve the better control of the flow separation in the highly-loaded compressor cascade in which the flow structure were asymmetric between the two end-wall regions, the correlation between the cascade geometrical parameters and the cascade total loss were derived through the Uniform Design of Experiment method and the regression analysis. The geometrical parameters included the bowed angle and bowed height at the two end-walls, the solidity, and blade camber angles. The Genetic Algorithms was adopted to search for the optimum results. The results showed that the optimum matching ranges of the bowed angle and bowed height reduced as the blade camber angle increased. For the cascades with small solidity, a negative bowed angle at the upper end-wall region was found to be beneficial to the cascade performance improvement. Reasonably optimized bowed blade extended its application to modern advanced compressors, indicating the large potential of the design method of the bowed blade in the highly-loaded compressor cascade. |
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