NUMERICAL STUDY ON UNSTEADY FLOW OF A SUPERSONIC PARTIAL ADMISSION IMPULSE TURBINE

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 somewhat elusive. To enhance the comprehension of the flow characteristics within supersonic partial admission impulse (SPAI) turbines, this study employs a three-dimensional unsteady numerical simulation on a single-stage SPAI turbine equipped with four circular nozzles. The findings reveal that shock waves are produced at the leading edge of the admission area, and flow separation occurs twice on the suction surface. In the non-admission area, the fluid is in a turbulent state and predominantly static. Particularly in the transition section, processes of emptying and filling are evident. An intense shock wave forms at the entrance to the channel in the transition section leaving the admission area, as the high- and low-energy fluids mix in the subsequent channels, resulting in significant energy losses. During the rotor rotation, the blades experience fluctuating aerodynamic forces, which are pronounced in the admission area, while negligible in the non-admission area. These forces are mainly dominated by the intake passage frequency and other frequencies associated with nozzle spacing. Phenomena of under-shoot and over-shoot are observed as the blades enter or exit the admission area, potentially impeding turbine performance or providing additional torque.