The Internal Structure of the Tip Leakage Vortex Within the Rotor of an Axial Waterjet Pump

Abstract

The complex flow field in the tip region of a turbomachine rotor, including the tip leakage flow and tip leakage vortex (TLV), has been studied for decades. Yet many associated phenomena are still not understood. This paper provides detailed data on the instantaneous and phase-averaged inner structures of the tip flow and evolution of the TLV. Observations are based on series of high resolution planar particle image velocimetry measurements performed in a transparent waterjet pump fitted into an optical refractive index-matched test facility. Velocity distributions and turbulence statistics are obtained in several meridional planes inside the rotor. We observe that the instantaneous TLV structure is composed of unsteady vortex filaments that propagate into the tip region of the blade passage. These filaments are first embedded into a vortex sheet, which is generated at the suction side of the blade tip, and then they wrap around each other and roll up into the TLV. We also find that the leakage vortex induces flow separation at the casing endwall and entrains the casing boundary layer with its counter-rotating vorticity. As it propagates in the rotor passage, the TLV migrates toward the pressure side of the neighboring blade. Unsteadiness associated with vortical structures is also investigated. We notice that, at early stages of the TLV evolution, turbulence is elevated in the vortex sheet, in the flow entrained from the endwall, and near the vortex core. Interestingly, the turbulence observed around the core is not consistent with the local distribution of turbulent kinetic energy production rate. This mismatch indicates that, given a TLV section, production likely occurs at preceding stages of the vortex evolution. Then, the turbulence is convected to the core of the TLV, and we suggest that this transport has substantial component along the vortex. We observe that the meandering of vortex filaments dominates the flow in the passage and we decompose the unsteadiness surrounding the TLV core to contributions from interlaced vortices and broadband turbulence. The two contributions are of the same order of magnitude. During late stages of its evolution, TLV breakdown occurs, causing rapid broadening of the phase-averaged core, with little change in overall circulation. Associated turbulence occupies almost half the width of the tip region of blade passage and turbulence production there is also broadly distributed. Proximity of the TLV to the pressure side of the neighboring blade also affects entrainment of flow into the incoming tip region.

Influence of Tip Clearance on Pressure Fluctuation in Low Specific Speed Mixed-Flow Pump Passage

“…Pressure fluctuation coefficients of points S5-S9 decrease gradually from inlet to outlet of the guide vane at all conditions, which illustrates that rotor-stator influence is weak; meanwhile, pressure fluctuation variation at corresponding points at the shroud region is small at different clearance conditions, which illustrates that clearance variation has less effect on fluctuation at the shroud region of the guide vane. Through MATLAB post processing, the distribution of in the guide vane passage was obtained according to Equation (10). Figure 15 shows the distribution of on the central hub-toshroud surface of the guide vane at the design condition.…”

“…In recent years, research has been conducted on the influence of tip clearance on performance [5,6], pressure fluctuation [7][8][9] and internal flow of pumps [10,11], but the subjects have mainly been centrifugal pumps and axial pumps. For mixed-flow pumps, related research has mainly concentrated on the influence of clearance size on performance parameters and tip-leakage.…”

Energy Performance and Radial Force of a Mixed-Flow Pump with Symmetrical and Unsymmetrical Tip Clearances

“…Wu et al [7][8][9][10] performed a series of experiments by using particle image velocimetry measurements to study the flow structures in the tip region of a water-jet pump rotor, including the tip clearance flow, tip leakage vortex and cavitation flow; they found that the tip leakage vortex started to form near the suction-side edge of a blade tip and appeared as a swirling jet, migrating to the middle of the blade passage. The backward leakage flow penetrated into the passage and created a shear layer in the interface with the main flow.…”

Unsteady Flow Numerical Simulations on Internal Energy Dissipation for a Low-Head Centrifugal Pump at Part-Load Operating Conditions

“…Asim et al [14] applied the Q-criterion to illustrate that a secondary flow generated near the volute tongue, with an impact on the pump head. Miorini et al [15] and Zhou et al…”

“…Asim et al [14] applied the Q-criterion to illustrate that a secondary flow generated near the volute tongue, with an impact on the pump head. Miorini et al [15] and Zhou et al[16] adopted a vorticity study to investigate tip leakage flow and rotating stall, respectively, to describe the development of these unsteady flow phenomena and to prove their effects on pump performance. These methods are able to make apparent specific flow structures inside the centrifugal pump, and could assist the design and optimization of centrifugal pumps [17][18][19].The quantity of energy loss related to these particular flow structures, needs to be investigated in details, at different operating conditions.…”