Tip clearance on pressure fluctuation intensity and vortex characteristic of a mixed flow pump as turbine at pump mode

Tip clearance in pump induces tip leakage vortex (TLV), which interacts with the main flow and leads to instability of flow pattern and decrease of pump performance. In this work, the characteristics of TLV in a mixed-flow pump are investigated by the numerical simulation using shear stress transport (SST) k–ω turbulence model with experimental validation. The trajectory of the primary tip leakage vortex (PTLV) is determined, and a power function law is proposed to describe the intensity of the PTLV core along the trajectory. Spatial–temporal evolution of the TLV in an impeller revolution period T can be classified into three stages: splitting stage, developing stage, and merging stage. The TLV oscillation period TT is found as 19/160 T, corresponding to the frequency 8.4 fi (fi is impeller rotating frequency). Results reveal that the TLV oscillation is intensified by the sudden pressure variation at the junction of two adjacent blades. On analysis of the relative vorticity transport equation, it is revealed that the relative vortex stretching item in Z direction is the major source of the splitting and shedding of the PTLV. The dominant frequency of pressure and vorticity fluctuations on the PTLV trajectory is 8.4 fi, same as the TLV oscillation frequency. This result reveals that the flow instability in the PTLV trajectory is dominated by the oscillation of the TLV. The blade number has significant effect on pressure fluctuation in tip clearance and on blade pressure side, because the TLV oscillation period varies with the circumferential length of flow passage.

Spatial-temporal evolution of tip leakage vortex in a mixed flow pump with tip clearance

Tip clearance between blade end and shroud is inevitable in pump operation and of great importance on pump energy performance and pressure fluctuation. As the tip clearance size increases, the head and efficiency of the mixed flow pump drop accordingly. The simulation results show that the development of a leakage vortex is observed as the tip clearance increases, and the trajectory of this leakage vortex remains in the same direction along the blade suction side for different tip clearances. With the increase in tip clearance size, the intensity of the leakage vortex is enhanced, and the separation between the main leakage vortex and the secondary leakage vortex is also strengthened. The leakage separation angle near the blade tip remains at the same value of 10° for different tip clearance sizes. As for the spectrum analysis, the maximum amplitudes of pressure fluctuations dramatically increase in the impeller when the tip clearance increases from 0.0 mm to 1.0 mm, and the dominant frequencies go from 145 Hz to 184 Hz due to the considerable leakage flow.

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Energy Performance and Flow Patterns of a Mixed-Flow Pump with Different Tip Clearance Sizes


 Tip clearance results in the leakage flow from blade pressure side to suction side, which will further cause the tip leakage vortex (TLV). Moreover, the flow pattern in an impeller is seriously deteriorated due to the TLV and its interaction with the main stream. In this work, the TLV in a mixed flow pump is investigated by numerical simulation validated by experiment measurement. The primary tip leakage vortex (PTLV) trajectory is specially studied with consideration of the tip clearance size δ, the impeller blade number Zi, and the impeller rotational speed n. The results show that δ slightly shifts the separation point (SP) of the PTLV but rarely affects the separation angle α. The increase in Zi and the decrease in n both lead to the shift of the SP toward the blade trailing edge and the decrease in α. Furthermore, a theoretical prediction model is proposed to predict the PTLV trajectory, by which the axial position and radial position of PTLV trajectory versus the rotation angle can be predicted. The proposed model is verified to be accurate to predict the PTLV trajectory, especially for the PTLV trajectory in the main flow passage. The dynamic evolution of TLV under different tip clearance sizes can all be classified into the same three stages: splitting stage, developing stage, and merging stage. Meanwhile, the dynamic evolution frequency fe is the same as the impeller rotational frequency fi.

Theoretical Prediction Model of Tip Leakage Vortex in a Mixed Flow Pump With Tip Clearance

Method of C groove on vortex suppression and energy performance improvement for a NACA0009 hydrofoil with tip clearance in tidal energy

Rotor-stator interaction in axial pumps can produce pressure fluctuations and further vibrations even damage to the pump system in some extreme case. In this paper, the influence of tip clearance on pressure fluctuations in an axial flow water pump has been investigated by numerical method. Three-dimensional unsteady flow in the axial flow water pump has been simulated with different tip clearances between the impeller blade tip and the casing wall. In addition to monitoring pressure fluctuations at some typical points, a new method based on pressure statistics was proposed to determine pressure fluctuations at all grid nodes inside the whole pump. The comparison shows that the existence of impeller tip clearance magnifies the pressure fluctuations in the impeller region, from the hub to shroud. However, the effect on pressure fluctuation in the diffuser region is not evident. Furthermore, the tip clearance vortex has also been examined under different tip clearances.

Influence of tip clearance on pressure fluctuations in an axial flow pump

The tip leakage flow formed by the tip clearance of a semi-open centrifugal pump adversely affects the energy performance and the energy waste. A T-shaped blade is proposed to be used to improve the energy characteristics. The internal flow field is simulated via the shear stress transport turbulence model to analyze the influence of the T-shaped blade on the performance of a centrifugal pump. It is shown that with the T-shaped blade, the hydraulic loss can be reduced, the Euler head can be increased, and the external characteristics can be improved. The maximum head and efficiency improvements are 3% and 1.6%, respectively. The relative flow angle near the tip clearance of the T-shaped blade decreases, and the strength of the backflow region is remarkably weakened. The area of the high entropy production region decreases, and the trend of the upstream diffusion is restrained under the design condition. The T-shaped blade can help to inhibit the diffusion of the high entropy production region to the hub under the low flow rate condition, but the entropy production and the mixing losses at the tip clearance are slightly increased. This research provides a new method for improving the energy performance of the semi-open centrifugal pump.

Numerical investigation of the effect of T-shaped blade on the energy performance improvement of a semi-open centrifugal pump

The invention discloses a Francis turbine runner multi-condition optimization method capable of giving consideration to hydraulic and strength performances. The method specifically comprises the following steps: obtaining three-dimensional blade aerofoil data of runner blades according to an optimization design variable of the Francis turbine runner and an inflow vortex runner full three-dimensional inverse problem calculation method, thereby obtaining the runner blade aerofoil; obtaining hydraulic efficiency eta i of the runner blades under each condition, the lowest pressure coefficient Cpion the runner blades, and the maximum equivalent stress sigma i on the blades; and processing the obtained hydraulic efficiency eta i of the runner blades under each condition, the lowest pressure coefficient Cpi on the runner blades and the maximum equivalent stress sigma i on the blades, obtaining the final objective functions d eta (X), d Cp (X) and d sigma (X) of the optimized problem, and establishing a mathematical model to optimize the Francis turbine runner blades according to the obtained objective functions. According to the method disclosed by the invention, the problem that the hydraulic and strength performances under different conditions cannot be simultaneously taken into account in the existing Francis turbine runner design technology is solved.

Peng-cheng Guo

Papers

Influence of tip clearance on pressure fluctuations in an axial flow pump

Numerical investigation of the effect of T-shaped blade on the energy performance improvement of a semi-open centrifugal pump

Francis turbine runner multi-condition optimization method capable of giving consideration to hydraulic and strength performances