Aerodynamic Characteristics Of Supersonic Expander With Different Geometric Configuration

Improving the performance of the turbine which is one of the three core components of gas turbine is an effective way to enhance the overall performance of gas turbine. In addition to constantly increase the total temperature at inlet, it is also an efficient way to develop a novel structure to improve the performance of the turbine. Not only could well designed structure improve and enhance the aerodynamic performance of the turbine, it will also benefit the turbine with appropriately simplifying structure, thus the total weight of gas turbine could be reduced and the power-to-weight ratio increased.Integrating with the supersonic nozzle design technology and the traditional axial and centrifugal flow turbine design technologies, a supersonic expander is a new structure of turbine. The structural trait of the supersonic expander is the use of the thinner strake walls instead of the turbine blades, and the change of S2 stream surface along the direction of airflow to achieve expansion and acceleration. By comparison with the conventional turbine, the supersonic expander will decrease the flow loss in the expansion system, and have the advantages of simpler and more compact structure, lighter, lower cost and easier maintenance. In small commercial ship power, small gas turbine generating unit, turbopump and micro gas turbine in distributed energy system in the future, the supersonic expander could have enormous developing and application prospect. There is theoretical significance and application value for us to carry out comprehensive and thorough research on the new expansion system.The overall design scheme of a supersonic expander is developed firstly in this dissertation. One kind of supersonic expander with spoke-type disk structure is presented and the overall structure parameters are given out. Three dimensional flow field of the supersonic expander is simulated numerically based on it. According to the structural features of the supersonic expander and internal flow characteristics of three dimensional flow passage, influence of geometric structure on the flow field and performance of the supersonic expander are investigated numerically. The key geometric parameters, the expansion surface shapes and the strake wall section shapes are considered seriously. The results show that the supersonic expander is implemented mainly by expansion of the bottom wall along outside direction to realize decompression and acceleration. Existing of expansion waves at the entrance of the expansion flow passage leads to non-uniform distribution of static pressure in the pitch direction, which influence the flow field of three dimensional flow passage. Boundary layer separation, back flow, low energy fluid mixing with the mainstream, oblique shock waves, oblique shock waves and boundary layer interaction at the outlet of the expansion flow passage are the main sources of loss in the three dimensional flow passage. Flow field and performance of the supersonic expander with three kinds of strake stagger angle and five kinds of the ratio of the outlet to inlet area has been simulated numerically under design condition, it can be indicated that with the increasing of the strake stagger angle, the expansion ratio of the supersonic expander decreases, and the efficiency of the supersonic expander increases. And with the increasing of the ratio of the outlet to inlet area, the expansion ratio of the expander increases, and the efficiency of the expander decreases. In order to optimize the overall performance of the supersonic expander, the reasonable compromises should be made between the smaller ratio of the outlet to inlet area and larger strake stagger angle. Supersonic expander with different expansion surface shapes has been simulated numerically under design condition, the results show that the expansion surface shape has a significant influence on the local flow field and performance of the three dimensional flow-path under design condition, the velocity distribution of linear expansion surface along the direction of pitch is especially uniform, and the averaged Mach numbers at the outlet and the expansion ratio are maximum. It states that the stream surface of S2 expansion along the airflow gradually is more beneficial to achieve expansion and acceleration. Supersonic expander with different strake wall section shapes has been simulated numerically under design condition, it can be found that the suction surface of supersonic expander with rectangle strake wall section shape has high air velocity, low flow loss after the oblique shock waves, and thus higher efficiency can be obtained; Supersonic expander with positive trapezoid strake wall section shape has high outlet averaged Mach number, static pressure ratio and expansion ratio, its comprehensive performance is relatively optimal, the comprehensive performance of supersonic expander could be further enhanced by optimizing the structure of strake wall in radial direction.Then numerical study is conducted to investigate the influence of tip clearance heights, incidences and the relative movement between strake wall and casing on the tip clearance flow of the supersonic expander. The results show that it does not exist complex three-dimensional flow inside the tip clearance of supersonic expander, the airflow near the leading edge through the clearance flow to the suction surface and the leakage fluid around the trailing edge across the gap back to the pressure surface is the main form of movement within the clearance. With the increasing of the tip clearance, the highest relative Mach number of airflow decreases, and the scope of high speed reduces. The leakage vortex strengthens, scale largens, transverse and radial movement significantly, and the leakage loss increases, the scope of clearance height should be selected between 0.9% and 1.5% throat height in this paper. Viscous shear force would weaken due to relative static between the strake wall and the casing, but the wall effect has a tremendous effect on the clearance flow. When relative movement of endwall exists, transverse motion of the leakage vortex is inhibited, separation line is close to the suction surface, unlike the conventional turbine, the tip leakage flow is increase, and tip leakage vortex enhances, and increasing the flow loss. Static pressure of pressure surface almost doesn’t vary with incidence, but static pressure of suction surface is much sensitive, especially the leading edge of strake wall, static pressure of the suction surface obviously decreases when the incidence is increase, and eventually lead to the load of the strake wall increases. Incidences have little influence on the wake, small incidence will rise the loss in the corner between suction surface and endwall, and large incidence will result in the leakage loss rise.At the end of this dissertation, the variable condition properties of the supersonic expander are studied. It is observed that as the increasing of the absolute total pressure at inlet, leakage flow decreases, intensity and size of the leakage vortex reduce, but the transverse movement improves, flow loss along the flow direction of suction surface is obviously decreased, loss of bottom wall caused by low energy fluid is also declined. Flow loss of the three dimensional flow passage is huge under low absolute total pressure at inlet. With the increasing of the speed, velocity of the airflow in the three dimensional passage decreases, velocity gradient in the pitch direction drops, the high speed zone near the bottom wall migrates toward the outlet, scope of low speed zone near the casing caused by leakage flow is decrease, the high speed zone near the suction surface and pressure surface at outlet are both significantly decrease, the oblique shock waves migrate to the outlet, and the loss caused by leakage flow and low energy fluid near the wall are also reduce.