As a new type of expansion system, compared with conventional axial turbine, the structural traits of the supersonic expander are the use of the thinner strake walls instead of the turbine blades, and the change of the expansion surface shape along the direction of airflow to achieve expansion and acceleration. The expander with three kinds of expansion surface shape has been designed and simulated numerically in this paper. It has been found that the maximum averaged Mach numbers at the exit and the expansion ratio of the expander occur with the linear expansion surface. Because the performance of the expander ties up to the geometric parameters, so taking the expander structure with the linear expansion surface as a foundation, the key geometric parameters which affect the performance of the expander are studied numerically in detail. With the increasing of the strake stagger angle, the expansion ratio of the expander decreases, and the efficiency of the 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. Under the design and off-design conditions, the flow field and performance of the expander with an optimum performance is further numerically simulated. It has been found that dilatation waves at the entrance of the expansion passage will reduce the static pressure, so as to realize expansion and acceleration of the airflow in the expander passage. Pressure gradient in the direction of the pitch causes the difference of the airflow velocity distribution. The oblique shock waves which near to the outlet brings the rise of the static pressure, thus reduces the airflow velocity of the outlet and increases the flow losses. With increase in the inlet total pressure and rotational speed of the supersonic expander, its expansion ratio and efficiency improves.

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