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

Abstract

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.

Optimal Selection and Operation of Pumps as Turbines for Maximizing Energy Recovery

“…In throttle and bypass control, the recovered power (P rec ) is calculated by using the PAT's power characteristic curve, which can be derived experimentally (as performed in this paper) or by applying one of the methodologies available in the literature. To this purpose, different approaches can be exploited, e.g., physics-based approaches (e.g., [36,37]), empirical models (e.g., [23,38,39]), black box models (e.g., [40,41]), CFD models (e.g., [39,42]) and entropy production analysis (e.g., [43]).…”

Analysis of Cavitation-Induced Unsteady Flow Conditions in Francis Turbines under High-Load Conditions

“…Just as stated in Kumar's paper [35], under rated conditions, the inlet flow at the draft tube is nearly identical to the normal outflow; however, as the flow rate increases, the strong swirl flow is directed contrary to the runner's rotation, and eventually, a cavitation vortex rope shaped like torch forms. In addition, as shown in the black highlighted area of the figure, the motion of the vortex rope has a significant effect on the flow pattern at the inlet inside the draft tube, forming a high-speed zone and a large-scale vortex structure below the vortex rope [36]. As the load increases, the vortex distribution at the outlet of the draft tube is improved; nevertheless, in the red wireframe area of the figure, the vortex ropes are gradually serious and closer to the elbow section in the Case 3 condition, so the energy loss caused by the vortex ropes inside the draft tube will increase.…”

Positive and Negative Performance Analysis of the Bi-Directional Full-Flow Pump with an “S” Shaped Airfoil

“…The highest pressure value was found in the impeller outlet region. Yang et al [19,20] investigated the energy-loss characteristics of an axial flow pump. Research has shown that the pressure energy loss in the impeller is the largest.…”