Search

Atmospheric stability can significantly influence the spreading of wind-turbine wakes. The previous studies often coupled atmospheric stability with the turbulence intensity and attributed the influence of atmospheric stability to the enhancement or suppression of turbulence due to the buoyancy effect. In this study, we decouple atmospheric stability with the ambient turbulence intensity, and the pure influence of atmospheric stability on the turbine wakes is investigated at a certain hub-height turbulence intensity via large-eddy simulation. We find that the spanwise turbulence transport plays a crucial role in wake recovery, and atmospheric stability influences this transport by redistributing the turbulence intensity between the three components and altering the spatial scales of the atmospheric motion. Under the convection condition, the spanwise turbulence intensity is greatly enhanced with enlarged flow scales. Hence, more Reynolds shear stress is generated under the shear effect between the ambient flow and the wake flow, which thus enhances spanwise turbulence transport, resulting in a faster recovery of turbine wakes. While for the stable condition, although the spanwise turbulence intensity is slightly enhanced, the flow scales are obviously reduced, resulting in a decrease in wake meandering, which leads to a decrease in turbulence transport in the wake region and a slower recovery of wind-turbine wakes. [ABSTRACT FROM AUTHOR]

The S-shaped characteristic curves in pump-turbines complicate synchronization with the electrical grid and affect system safety. Misaligned guide vanes (MGVs) are one of the most effective solutions to avoid S-shaped characteristics. The internal flow mechanism with the MGV for improving S-shaped characteristics was studied by numerical analysis. Six operating conditions were modeled in the S-shaped region. Four guide vanes were arranged as the MGVs to qualitatively and quantitatively analyze the flow behavior. The internal flow was quite complex at the four operating points without the MGV; here, the attack angle and the flow behavior had no obvious difference at each vane. For the similar conditions with MGVs, attack angles and internal flow fields varied clearly at each vane, especially in the vaneless region and in the runner blade passages. For the same discharge rates, total openings, and rotating speeds, the internal flows were quite different between with and without the MGVs. The MGVs disrupt the high-speed circumferential water ring (appreciably faster compared to the main flow) in the vaneless region and maintain operation with higher unit speeds. Consequently, the unit speed is larger at the same unit discharge in the S-shaped region. Therefore, the MGV method can reduce S-shaped characteristics. [ABSTRACT FROM AUTHOR]

The present paper aims to find out the influence of operating head on the rotating bucket free surface sheet flow and hydrodynamics performance for a Pelton turbine. Three-dimensional unsteady air-water two-phase flow simulations in the rotating buckets were performed by adopting the shear stress transport curvature correction turbulence model and homogenous model. The sensitivities of the unsteady simulation results to moving mesh resolution and computational fluid dynamics solver time-step have been evaluated to discuss the effect of Courant–Friedrichs–Lewy conditions on the two-phase flow simulation. The accuracy of the numerical predicted flow pattern and the hydrodynamic performance results are reasonable when compared with the experimental data. The simulation results indicate that the remaining kinetic energy carried by the outflow under the nondesigned water head is the main reason for the efficiency loss in the Pelton turbine. Under low water head conditions, jet distortion caused by the Coanda effect and flow interference will lead to more severe efficiency deterioration. [ABSTRACT FROM AUTHOR]

Purpose – The purpose of this paper is to numerically analyze the flow characteristic and explore the hydrodynamic mechanism of the pump mode hump district formation of a Francis pump-turbine. Design/methodology/approach – Numerical simulations were conducted of the entire pump-turbine flow passage under different discharge conditions by adopting the SST-CC turbulence model. The internal flow at hump district has been explained in detail combined with the model test in this paper. The unsteady flow and pressure fluctuation characteristics are analyzed under five different discharge conditions in the hump and nearby region. The reason of the hump district formation is explored combined with the flow components hydraulic loss. Findings – The large hydraulic loss, high relative peak-to-peak amplitudes and low dominant frequencies are on account of the disorganized internal flow condition. The formation of the hump district is concerned with the large hydraulic loss inside the draft tube, runner and guide vanes as there occurs secondary flow, backflow even vortex in the hump district. In addition, the low dominant frequencies at recording points inside the flow passage are always accompanied with the change of flow patterns and the spreading of the pressure fluctuations. Originality/value – The analysis method of each flow components hydraulic loss combined with internal flow structure is adopted to explore the mechanism of pump mode hump characteristic. The flow characteristic and pressure pulse characteristics all correspond to the flow components hydraulic loss. [ABSTRACT FROM AUTHOR]

Purpose – The purpose of this paper is to numerically analyze the flow characteristic and to explore the hydrodynamic mechanism of the S-shaped region formation of a Francis pump-turbine. Design/methodology/approach – Three-dimensional steady and unsteady simulations were performed for a number of operating conditions at the optimal guide vanes opening. The steady Reynolds averaged Navier-Stokes equations with the SST turbulence model were solved to model the internal flow within the entire flow passage. The predicted discharge-speed curve agrees well with the model test at generating mode. This paper compared the hydrodynamic characteristics of for off-design cases in S-shaped region with the optimal operating case, and more analysis focusses particularly on very low positive and negative discharge cases with the same unit speed. Findings – At runaway case toward smaller discharge, the relative circumferential velocity becomes stronger in the vaneless, which generates the “water ring” and blocks the flow between guide vane and runner. The runner inlet attack angle becomes larger, and the runner blade passages nearly filled with flow separation and vortexes. The deterioration of runner blade flow leads to the dramatic decrease of runner torque, which tends to reduce the runner rotation speed. In this situation, the internal flow cannot maintain the larger rotating speed at very low positive discharge cases, so the unit discharge-speed curves bend to S-shaped near runaway case. Originality/value – The analysis method of four off-design cases on S-shaped region with the comparison of optimal operation case and the calculated attack angles are adopted to explore the mechanism of S characteristic. The flow characteristic and quantitative analysis all explain the bending of the unit discharge-speed curves. [ABSTRACT FROM AUTHOR]

Unlike the reaction turbines, the hydraulic performance of Pelton turbines varies due to the unsteady flow of the rotating buckets in time and space. This study experimentally and numerically investigates the dynamic performance of a Pelton turbine at five different operating conditions. The unsteady numerical simulations were performed with the SST turbulence model. The three-dimensional unsteady free surface sheet flow behavior in the rotating buckets was modeled by the two-phase homogeneous model. The model was used to analyze the dynamic flow patterns and the torque variations as the buckets rotated. The predicted hydraulic performance at different unit discharge rates are compared with the field test while the free surface flow patterns are compared with previous experimental and numerical results. The predicted relative efficiency agrees well with the field test results. The calculations were used to clarify the hydraulic mechanisms with unsteady free surface sheet flow in the rotating bucket. [ABSTRACT FROM PUBLISHER]