Numerical investigation of vorticity transport and energy dissipation in the runaway transition process following pump-trip in a pump-turbine

Once the runaway transition process following a pump-trip occurs in a pump-turbine, it will cause a sudden variation in flow rate and rotational speed within a short period of time, causing incalculable damage to the runner. This paper uses numerical simulation to systematically analyze the characteristics of rotational speed, flow rate, torque, energy dissipation, runner forces, and vorticity evolution during this transition process. The results show that in the pump condition after a power trip, the flow rate decreases significantly faster than the rotational speed, and due to changes in the inter-blade vortex and inflow angle, the radial force increases by about 2000 N, while the axial force rises by 2.1 times. After entering the turbine braking condition, the torque and the runner forces fluctuate significantly due to the combined action of the runner rotation and the local backflow at the inlet. The results of quantifying energy dissipation based on entropy production theory show that the total entropy production reaches a maximum value of 571 W/K during the pump braking condition, and the entropy production inside the runner contributes about 57%. After entering the runaway condition, the total entropy production stabilizes at about 280 W/K. In addition, the spatial distribution and vorticity evolution were revealed using the vorticity transport equation. The results show that the backflow at the runner inlet induces the formation of regions with high relative vorticity stretching, while the Coriolis force has little effect on the variation of the vorticity in the middle of the flow channel.