Numerical Simulation of the Unsteady Flow in a High-Head Pump Turbine and the Runner Improvement

Abstract

The unsteady flow in a high-head pump-turbine whose head-discharge curve has the positive slopes at high-partial-load operation condition was investigated. It is noted that the numerical methods is very important for predicting this kind of head-discharge curve with positive slopes, and better agreement between calculation results and experimental data was achieved by using Spalart-Allmaras turbulence model and mesh strategy with y+ controlling for numerical simulation. From the analysis of hydraulic losses at different parts in the pump turbine, it is found that the head loss at the flow passage of the guide vane and stay vane was not small at pump mode. In order to make clear the reason why the positive slopes at head-discharge curve occur, the flow between the impeller exit and the inlet of spiral casing was checked carefully. Much intensive vortex was observed near the impeller shroud, and there was strong rotor stator interaction for those operation conditions with positive slope. It is suspected the instability such as positive slope at head-discharge curve was resulted from the vortex formation near the flow channel wall. Based on the flow analysis, the runner optimization was conducted so as to mitigate the intensive rotor stator interaction. It is noted that the pressure fluctuation as well as the flow pattern was improved by applying the optimized impeller.

Performance prediction and flow analysis in the vaned distributor of a pump turbine under low flow rate in pump mode

“…In Figure 8(a), the jet intensity is not distributed evenly in the circumference direction, leading to complex vortex structures in the spiral casing. But when Q/Q D is equal to or greater than 0.23, non-symmetric distribution of "jet-wake" flow can be seen and has more and more effects on the flow in spiral casing in the sense of the jet length, which may be the possible reason for the large radial forces under off-design conditions [9,10]. …”

“…As to pump turbine, numerical studies mainly have concentrated on the operating conditions near the design point [7,8], with few relevant numerical simulations on the off-design prediction. Hongjun Ran [9] studied the applicability of different turbulence models for off-design prediction in pump mode and concluded that the S-A model is capable of predicting the hump zone of the head-flow profile. In a word, although the stalled flow was observed in pump turbine, the flow mechanism is not clear so far.…”

Research on Groove Method to Suppress Stall in Pump Turbine

“…Ullum et al [11] measured the pressure and velocity data in a rotating stall flow field in a centrifugal pump for frequency domain analysis. Ran et al [12] improved the pulsation characteristics of the pump-turbine and the positive slope of the lift curve by impeller optimization, and achieved certain results. Moghaddam et al [13] used a decoupled sliding-mode neural network variable-bound control system to drive the system back to the optimal operating point to quickly get out of the stall state, but not to suppress the stall…”

Numerical simulation of the influence of distributor pitch diameter on performance and pressure fluctuations in a pump-turbine

“…He showed the capability of CFD to predict accurately the pressure fluctuations at the blade passing frequency. Ran [8] investigated the unsteady flow in a highhead pump-turbine using S-A turbulence model, for the runner improvement.…”