Fluid-induced vibration evolution mechanism of multiphase free sink vortex and the multi-source vibration sensing method

Mixing mass transfer mechanism and dynamic control of gas-liquid-solid multiphase flow based on VOF-DEM coupling

As a low-cost scheme for small-scale hydropower generation, pump as turbines (PATs) are used at different hydrosites around the world. Nevertheless, a big number of recently conducted studies on PAT performance have mainly focused on the centrifugal type, despite the fact that the axial-flow type has a comparatively large flow capacity, thus disposing of higher power density. Therefore, this article seeks to investigate the flow dynamics of an axial-flow PAT and associated energy loss characteristics, under both pump and turbine operating modes. It adopts the numerical simulation method and uses entropy production theory to propose an energy loss intensity model in the cylindrical coordinate system, which quantitatively gives the spatial variation pattern for energy losses in pump and turbine operating modes. In addition, the correlation between energy loss and flow instability is deeply analyzed, where the energy characteristics in pump and turbine modes are quantitatively evaluated. It is shown that the energy loss within the impeller and the guide vane flow fields, for both operating modes, is mainly because of the turbulent entropy production. The proportion of direct entropy production and wall entropy production is found to be relatively small. The velocity gradient, flow vorticity, turbulence intensity, and energy losses within the flow passages of the axial-flow PAT have been closely related. However, owing to the difference in PAT operating modes, there is a significant difference in the location of energy losses. The unstable flow phenomena, such as the impact at the blade inlet, flow deviation at the blade outlet, flow separation, back-flow, and vortex, are the main reasons for entropy production. This study serves as a reference for the design, optimization, and application of axial-flow PATs. 

Energy performance evaluation of an axial-flow pump as turbine under conventional and reverse operating modes based on an energy loss intensity model

The pump mode of the low-head pumped hydro storage unit (pump-turbine) may operate in the hump region under extreme conditions due to the influence of water level variation, and the resulting energy conversion instability will seriously threaten the safety of the unit. However, the generation mechanism of the hump region is still not sufficiently understood, which is mainly due to two reasons: the dominant unstable flow structures that induce the formation of the hump region have not been uniformly recognized, and the influence of the dominant unstable flow structures on the impeller's working capacity has not been effectively revealed. In this study, experiments and numerical simulations were carried out on the low-head pumped hydro storage unit in pump mode, and the following results were obtained. It is found that the dominant unstable flow structures that induce the formation of the hump region are the leading edge backflow on the blade inlet shroud side and the horn-like vortex on the blade outlet hub side. The leading edge backflow reduces the blade loading and limits the impeller's working capacity, and the horn-like vortex increases the blade loading and increases the impeller's working capacity. The analysis revealed that the formation of the hump region is the result of the mutual restriction of the horn-like vortex and the leading edge backflow.

Investigation on the hump region generation mechanism of pump mode in low-head pumped hydro storage unit

Low-head vertical axial-flow pump as turbine (PAT) devices play a vital part in the development of clean energy for hydropower in plain areas. The traditional method of evaluating the flow loss in hydraulic machinery is calculated by the pressure drop method, the limitation of which is that the location of the occurrence of large losses cannot be accurately determined. In this paper, entropy production theory is introduced to evaluate the irreversible losses in the axial-flow PAT from the perspective of the second law of thermodynamics. A three-dimensional model of the axial-flow PAT is established and solved numerically using the Reynolds time-averaged equation, and the turbulence model is adopted as Shear Stress Transport–Curvature Correction (SST-CC) model. The validity of the entropy production theory to evaluate the energy loss distribution of the axial-flow PAT is illustrated by comparing the flow loss calculated by the pressure drop and the entropy production theory, respectively. The entropy production by turbulent dissipative dominates the total entropy production in the whole flow conduit, and the turbulent dissipative entropy accounts for the smallest percentage of the whole conduit entropy production at the optimal working condition Qbep, which is 51%. The impeller and the dustpan-shaped conduit are the essential sources of hydraulic loss in the entire flow conduit of the axial-flow PAT, and most of the energy loss of the impeller occurs at the blade leading edge, the trailing edge, and the flow separation zone near the suction surface. The energy loss of the dustpan-shaped conduit results from the high-speed flow from the impeller outlet to dustpan-shaped conduit to form a vortex, backflow and other chaotic flow patterns. Flow impact, flow separation, vortex and backflow are the main causes of high entropy production and energy loss.

/pdf/numerical-study-for-flow-loss-characteristic-of-an-axial-1g606x85.pdf

Numerical Study for Flow Loss Characteristic of an Axial-Flow Pump as Turbine via Entropy Production Analysis

Study on vortex flow and pressure fluctuation in dustpan-shaped conduit of a low head axial-flow pump as turbine

The energy loss of the vertical axial flow pump device increases due to the unstable internal flow, which reduces the efficiency of the pump device and increases its energy consumption of the pump device. The research results of the flow loss characteristics of the total internal conduit are still unclear. Therefore, to show the internal energy loss mechanism of the axial flow pump, this paper used the entropy production method to calculate the energy loss of the total conduit of the pump device to clarify the internal energy loss mechanism of the pump device. The results show that the energy loss of the impeller is the largest under various flow conditions, accounting for more than 40% of the total energy loss of the pump device. The variation trend of the volume average entropy production and the energy loss is similar under various flow coefficients (KQ). The volume average entropy production rate (EPR) and the energy loss decrease first and then increase with the increase of flow, the minimum volume average entropy production is 378,000 W/m3 at KQ = 0.52, and the area average EPR of the impeller increases gradually with the increase of flow. Under various flow coefficient KQ, the energy loss of campaniform inlet conduit is the smallest, accounting for less than 1% of the total energy loss. Its maximum value is 63.58 W. The energy loss of the guide vane and elbow increases with the increase of flow coefficient KQ, and the maximum ratio of energy loss to the total energy loss of the pump device is 29% and 21%, respectively, at small flow condition KQ = 0.38. The energy loss of straight outlet conduit reduces first and then increases with the increase of flow coefficient KQ. When flow coefficient KQ = 0.62, it accounts for 27% of the total energy loss of the pump device, but its area average entropy production rate (EPR) and volume average entropy production rate (EPR) are small. The main entropy production loss in the pump device is dominated by entropy production by turbulent dissipation (EPTD), and the proportion of entropy production by direct dissipation (EPDD) is the smallest.

/pdf/analysis-of-energy-loss-characteristics-of-vertical-axial-35jjmu1h.pdf

Analysis of Energy Loss Characteristics of Vertical Axial Flow Pump Based on Entropy Production Method under Partial Conditions

Mathematical Modeling of Mold Flow Field and Mold Flux Entrainment Assisted With High-Temperature Quantitative Velocity Measurement and Water Modeling

In order to investigate the internal flow characteristics and hydraulic stability of vertical submersible pump station in forward and reverse operation, the steady and unsteady three-dimensional numerical simulation of vertical submersible axial-flow pump device was carried out based on CFD and the energy performance of the pump device was predicted. In this paper, the internal flow characteristics of the vertical submersible axial-flow pump device under forward and reverse operation were analyzed, and the optimization of the structure passed by water was carried out to improve the efficiency of the pump device in forward and reverse operation. The results show that the performance of the pump device is good when it is running in the forward direction, the peak efficiency of the pump device is 72.22% and the corresponding head is 4.01 m. When it is running in the reverse direction, the performance of the pump device decreases significantly with the peak efficiency is only 36.03% and the corresponding head is 3.34 m. The hydraulic loss in the dust-pan-shaped conduit is very small in the forward operation, the wellbore conduit is the main factor which affects the efficiency of the submersible axial flow pump device, the loss ratio is 12%–44%. When the pump device is in reverse operation, the hydraulic loss is similar but the hydraulic performance is poor and the hydraulic loss is obviously larger than that in forward operation. A guide plate is added to the dust-pan-shaped conduit at 0.27D (D is the nominal diameter of impeller) along the axis direction of the conduit, and a elbow flow conduit with the corner radius R of 0.83D is selected to replace the wellbore conduit, the bad flow state disappeared and the efficiency of the pump device increased by 5.19% in the forward operation while the efficiency of the pump device increased by 9.62% in the reverse operation.

https://journals.sagepub.com/doi/pdf/10.1177/16878132221100641

Zhikang Lin

Papers

Study on vortex flow and pressure fluctuation in dustpan-shaped conduit of a low head axial-flow pump as turbine

Analysis of Energy Loss Characteristics of Vertical Axial Flow Pump Based on Entropy Production Method under Partial Conditions

Mathematical Modeling of Mold Flow Field and Mold Flux Entrainment Assisted With High-Temperature Quantitative Velocity Measurement and Water Modeling

Analysis of internal flow characteristics and structure optimization of vertical submersible axial flow pump device

Three-dimensional simulation of fuel relocation during loss-of-coolant accidents based on the coupled FEM-DEM approach