Resumen de Numerical study of a radial turbine of variable geometry at off-design conditions reaching choked flow
In turbochargers with variable geometry turbine (VGT), the stator vanes move to a closed position to drive high exhaust back pressure during the engine braking mode. Thus, shock waves are generated at the stator. Furthermore, depending on the operational conditions in the use of radial turbines in other applications like reverse Brayton cycle for refrigeration, Organic Rankine Cycles, and gas turbine auxiliary power unit (GTAPU), sonic flow and shock waves can appear. The current work focuses on studying the flow behavior of a commercial turbocharger turbine of variable geometry at off-design conditions reaching choked flow. A detailed examination of the flow patterns within the turbine has been carried out using CFD simulations, identifying and quantifying the most important phenomena under different operational points. Reynolds Averaged Navier Stokes (RANS) and unsteady RANS simulations have been performed to obtain the flow structures in stator and rotor as well as the turbine map. The CFD results show that the region of the computational domain where the sonic conditions appear depends on the stator vanes position and the pressure ratio. When the stator vanes are in the closed position (10% VGT) the flow through the stator accelerates and, depending on pressure ratio, the static pressure on the suction side decreases until a certain point where a sudden increase reveals the presence of a shock wave that expands through the vaneless space. The intensity of the shock wave at higher pressure ratio varies with the rotational speed. To analyze the rotor-stator interaction, numerical simulations were carried out with the stator vanes at the closed position, 10% VGT, and at wider position, 30% VGT. The number of shocks a fluid particle experiences upstream of the rotor is correlated with the fluid shock losses. Close to the stator vanes, the pressure losses are high; toward the center of the vaneless space, they start to decrease, and close to the rotor they start to increase. The rotor-stator interaction creates shock waves, whose intensity depends on the position of the rotor leading edge and the blade speed. At higher rotational speed, load fluctuation occurs close to the leading edge, which may compromise the blade's integrity. When the turbine has the stator vanes open (80% VGT) and operates at the selected higher pressure ratio, the choking condition appears in a plane at the rotor trailing edge. Furthermore, the development of the choked area depends on the rotational speed and tip leakage. Thus, the effect of the tip leakage flow on the main flow under sonic conditions was investigated decreasing and increasing the tip gap up to 50% of the original geometry given by the manufacturer. The flow through the gap accelerates and then mixes with the main flow, generating a vortex. The effects of the vortex on the flow at the rotor trailing edge plane when the tip gap varies are more significant at higher speed than at lower speed. The vortex stays closer to the tip suction side at higher speed, generating a subsonic region that increases with the tip gap height. At higher and lower rotational speeds, the tip leakage flow does not affect the main flow close to the hub.
