Resumen
A detailed numerical simulation of a transonic tandem fan stage was conducted, and the change rule of the flow structure inside the fan stage under the design and off-design conditions was discussed to determine the internal flow mechanisms. The results demonstrate that the total pressure ratio of the fan stage steadily increases with the rotating speed, exhibiting an approximately quadratic growth rate. The peak efficiency reaches the maximum at 80% design speed and rapidly declines under the overspeed condition. Furthermore, the peak efficiency point for different rotating speeds was investigated. The changes in the flow features, such as shock wave/boundary layer interaction and radial migration of low-energy fluids, mainly determine the isentropic efficiency at the higher span. At the middle-lower span, higher or lower inflow relative Mach number increases the flow loss. Moreover, the strength of the tip vortex and wake affect the flow loss at the lower span, while the radial motion of the former flow structure dominated by the equivalent inertial force is another essential factor. Under the high-speed condition, the gain of a high-throughflow fan on choke mass flow can be exhibited. However, the throat position causes an abnormal change under the overspeed condition.