No abstract

3D blading technique is an effective way to mitigate flow separation and improve the performance of turbomachinery. Non-axisymmetric endwall contouring technique is widely used to reduce transverse secondary flows near the endwall region in turbines, while the application of non-axisymmetric endwall contouring to compressors is rare. The investigation rig is the Northwestern Polytechnical University dual-stage counter-rotating compressor. In order to further improve the aerodynamic performance of the compressor, 3D blading optimization of the two rotors and outlet guide vane (OGV) was done in multistage environment at near stall condition. After 3D blading optimization, the radial secondary flows of the two rotors and the separation vortex at the tip of OGV are obviously reduced. However, the transverse secondary flows at the hub endwalls of the two rotors are scarcely improved. Based on the 3D blading optimization, non-axisymmetric hub endwall contouring optimization of the two rotors was carried out afterwards and the circumferential secondary flow losses of the two rotors are reduced effectively. The efficiency of the counter-rotating compressor on optimization point increases and the aerodynamic performance is improved largely after 3D blading and non-axisymmetric hub endwall contouring optimization.

In an axial flow turbine, almost one-third of the total losses are caused by secondary flows, and the non-axisymmetric endwall profiling has been a major tool for years to reduce the secondary flow loss. This paper presents the non-axisymmetric endwall profile construction and optimization both for the stator hub and shroud of a high pressure turbine in the presence of an axisymmetric rotor. The flow simulation in the turbine was conducted by using steady RANS. The perturbation law of non-axisymmetric endwall was based on Bezier curves, and the commercially available optimization software NUMECA Fine/Design 3D was used to design the non-axisymmetric endwall. A genetic algorithm based on the artificial neural network was used as the optimization method. The objective function was aimed at maximizing the stage isentropic efficiency. The change in mass flow rate was kept less than 0.5% (relative) so that efficiency might not be influenced by the mass flow through the variation of the throat area. From the design point of view, the stator hub endwall was optimized at design conditions firstly, but the shroud endwall was kept constant, which resulted in an increase of stage efficiency because flow angles at stator exit were changed. The flow structures in the passage of stator were compared pre and post optimization by using 3-D streamlines in the vicinity of the endwall. Subsequently, the shroud endwall was optimized using the optimized non-axisymmetric hub as initial design. Due to hub and shroud endwall perturbation, the cross passage gradient and entropy were reduced, and the turbine stage efficiency at design conditions was calculated and the improvement in the efficiency was noticed. In addition, the improved hub and shroud contour were considered for off-design conditions as well, and efficiency was even more increased over a considerable off-design regime than at the design point condition.

In this paper, an optimization system was applied to design the non-axisymmetric endwalls for the stator of a high pressure axial flow turbine. This optimization system combines the endwall parameterization, 3D Navier-Stokes flow field calculation and genetic algorithm based on artificial neural network, which has the advantages of flexible geometry representation and automatic design of the optimal non-axisymmetric endwalls. And, the 3D steady flow field calculation was carried out to analyze the detailed behavior of complex flow structures pre and post optimization and to examine the influences of the optimized endwalls on the stage performance as well. The results of investigation show that the optimized non-axisymmetric endwalls can significantly decrease the flow loss in the stator, but also affect other aerodynamic parameters at the stator exit, especially the flow angle, and then the flow loss at the rotor exit caused by both the passage vortex in the rotor passage and the tip leakage vortex were increased by changing the incidence angle of the rotor due to the non-axisymmetric endwalls. Finally, the stage performance of the HP turbine is not improved as expected.

Tip-leakage loss caused by tip-leakage flow is an important source of aerodynamic loss in turbine rotor. Squealer tips are often used to control the tip-leakge flow and loss. In this paper, a transonic single-stage high-pressure turbine is simulated numerically to investigate the effects of cavity width and height on tip-leakage flow and loss. Based on plenty of cases with various cavity widths and heights, it is found that the optimal value of cavity height is 2.5-3.0 times clearance height and the optimal value of cavity width is affected by cavity height. The decrease of cavity width and the increase of cavity height of cavity have similar effects on the evolution of the scraping vortex. The cavity width controls the tip-leakage loss by suppressing the breakdown of the tip leakage vortex and reducing the corresponding mixing loss. However, the cavity height mainly affects the loss inside the clearance.