Abstract
Bypass diffusers are used to drain the excess steam generated in the steam generator in case of sudden load reduction or shutdown of the steam turbine. However, the steam at the orifice outlet with the high flow velocity may reverse into the space of last-stage blades and cause forced vibration of the turbine blades. For this study, a full-scale CFD calculation model which couples the last stage and the second-last stage with the bypass diffusers was constructed. The fluid dynamic characteristics of the high-speed steam discharged from the outlet of the bypass diffusers and the effect of steam on the last-stage rotating blades were analyzed comprehensively via both steady and transient numerical methods. The steady results show that the steam at the orifice outlet of the bypass diffusers presents a typical jet flow with some steam flowing back into the last-stage blades region through the exhaust of the cylinders. This results in a notable disturbance to the last-stage rotating blades, characterized by a non-uniform circumferential pressure distribution. The transient numerical simulation results reveal that the outlet mass flow rate of the steam from the second-last-stage stationary blades has a significant effect on both the Mach number distribution and the surface forces acting on the last-stage rotating blades. The higher outlet mass flow rate of the steam escalates the instability of the flow field. The difference in the Mach number at the same position can reach as high as 60% or more under different operating conditions. The forces acting on the last-stage rotating blades in different directions change periodically with time, and the magnitude and period of the variation correlate with the outlet mass flow rate of the steam.