Abstract
Partial admission is an essential method to ensure the effective operation of turbines in various operating conditions. The transition section of partial admission turbines is pivotal in governing the unsteadiness and non-uniformity, yet its flow dynamics remain elusive. To enhance the comprehension within supersonic partial admission impulse (SPAI) turbines, this study uses 3D unsteady simulations on a single-stage SPAI turbine with four nozzles to understand its dynamics, and compares the performance of centralized and symmetrical nozzle arrangements. The findings reveal that shock waves are produced in the admission area, followed by two flow separations, while the turbulent, stagnant fluid exists in the non-admission area. A strong shock wave is formed at the entrance of the transition section channel leaving the admission area. High- and low-energy gases mix in the subsequent channel and undergo an emptying process, resulting in significant energy loss. Furthermore, Blades undergo fluctuating aerodynamic forces and phenomena of under- and over-shoot are observed as the blades enter or exit the admission area, impeding turbine rotation or providing an additional torque. When adopting symmetrical nozzle arrangements, the mixing process is strengthened, resulting in a 0.29% decrease in the isentropic efficiency with the spacing of 40°, and a 1.30% decrease with the spacing of 60° compared to the centralized arrangement. However, the lateral force and overturning moment on the rotor are mitigated. Consequently, in practical engineering, adopting symmetrical arrangement is a feasible solution to ensure the stability and safety of SPAI turbine operation.
