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In this paper, a novel slot–groove hybrid casing treatment is designed and optimized to improve the stall margin of a low-speed axial compressor. A combination of the axial slot and circumferential groove casing treatments is utilized to increase the stall margin without incurring efficiency loss. The slot meridional profile is described with 2 B-spline curves. Circumferential grooves are parametrically described with groove height and width. An in-house optimization design platform is constructed based on the nondominated sorting genetic algorithm II and Kriging surrogate model. The optimization objectives are the stall margin and the peak efficiency of the compressor at the design rotating speed. To avoid the large number of unsteady simulations that are required to predict the stall margin, a stall margin improvement indicator is proposed based on the axial momentum budget analysis at the rotor tip region. The performance of the optimal slot–groove hybrid casing treatment design is tested and simulated. The experimental data show that the optimal slot–groove hybrid casing treatment improves the stall margin by 8.42% without generating efficiency loss. The flow details are captured by unsteady simulations and analyzed in depth. The application of the optimal casing treatment enhances the blade tip axial momentum and the interface between tip leakage flow and incoming main flow is pushed downstream. Consequently, the stability of the compressor is improved. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of the variable cycle engine fan changes sharply during mode transition. Investigating the variations of flow structure and understanding the loss mechanism are helpful in providing guidance for the fan design. Three-dimensional models of single bypass and double bypass compression systems are established, and static pressure is applied at the bypass stream outlet to simulate the opening of the mode selection valve. The characteristic band of variable cycle engine fan is obtained by gradually increasing the bypass stream pressure while maintaining specific values for the core stream pressure. Results show that the overall performance of the double bypass configuration, without bypass recirculation, is almost identical to that of the conventional single bypass configuration during the throttling process. With the increase in bypass pressure, the shock wave and the trajectory of tip leakage vortex gradually move forward, thereby increasing the blockage region induced by the interaction between the shock and tip leakage vortex. In addition, the performance of fan with reverse flow is also calculated. The recirculation causes the operating point to move closer to the stability limit, reducing the isentropic efficiency. Additionally, the recirculation changes the radial distribution of axial velocity and total pressure, leading to inlet distortion in the core driven fan stage. Furthermore, the loss mechanism is clarified by modeling the splitter and conducting entropy generation analysis. The sharp expansion of bypass stream could cause severe flow separation, and reducing the curvature of casing can effectively suppress the viscous shear loss. [ABSTRACT FROM AUTHOR]

Casing treatment has been widely adopted to enhance the operating range of compressors by extending the stall margin. In this article, a high-precision experimental investigation was conducted in a transonic compressor. Unsteady pressure is measured in the casing wall using a cluster of time-resolved transducers with axial and circumferential spatial resolution. The experimental data show that the optimized axial slot casing treatment improves the stall margin without peak efficiency penalty for all measured speedlines. A comprehensive flow field analysis indicates that the surge boundary of transonic compressor is sensitive to the position of shock wave and tip leakage flow. After the application of optimized axial slot casing treatment, the flow structure can be significantly modified, which specifically manifests as both the mitigation in shock wave detachment and redistribution of tip leakage flow trajectory toward the trailing edge of the blade. Furthermore, power spectral density analysis is conducted to examine the unsteady effect. The amplitude of characteristic frequency band induced by the unsteady fluctuation of tip leakage flow and shock wave is considerably suppressed, which can also be regard as the stability enhancement mechanism of casing treatment. [ABSTRACT FROM AUTHOR]

In this paper, a novel axial slot casing treatment design is introduced via optimization to improve the stall margin of the compressor with minimal efficiency loss. Besides the conventional design parameters including the height, length, position, inclination angle and open area ratio of the slots, a series of new parameters are put forward to increase the freedom of the slot meridional profile. An in-house design optimization platform is constructed based on NSGA-II and Kriging surrogate model to perform the optimization. A Pareto Front is obtained through optimization, from which the optimal design is selected by comprehensively considering both stability and efficiency performances of the compressor, i.e. the stall margin is improved by 11.23% in conjunction with the peak efficiency reduced by 0.21%. The database is analyzed to investigate how different parameters affect the performance of the slots. Three non-dominated representative designs are selected from the Pareto Front and their influencing mechanisms on stability and efficiency are further studied. Results show that the application of slots would generate a circulated flow, which exerts suction and reinjection effects at the rotor tip region and alleviate the blockage in the blade passages. Entropy analysis indicates that the viscous shearing interaction between the reinjection flow and incoming main flow causes additional entropy generation, thus slightly reducing the peak efficiency of the compressor. [ABSTRACT FROM AUTHOR]