Visualized observations of trajectory and dynamics of unsteady tip cloud cavitating vortices in axial flow pump

Abstract

A large-scale perpendicular cavitating vortices (PCVs), at the trailing edge of attached cavitation on the blade suction side near the tip region, has been found recently due to the great impact on performance breakdown in an axial waterjet pump. However, the trajectory and dynamics of this structure have been given scant attention. In this study, some visualized experiments were carried out to elucidate the PCVs for different conditions. The high-speed imaging coupled with numerical computations show that the vortical cloud cavitation is induced by the combination of tip leakage vortex (TLV) and radial re-entrant jets from the hub to blade tip. Moreover, the trajectory and intensity of PCVs depend on the operating conditions strongly, whether the other parameters, e.g. blade number and blade geometries, are modified. When taken the blade number into consideration, as a consequence of flow passage width and blade loading distributions, the dynamics and strength of PCVs vary considerably. Furthermore, an optimum clearance geometry is seen to eliminate corner vortex and clearance cavitation when the clearance edge is rounded on the pressure side. However, the more intensive tip leakage vortex cavitation is observed due to the increased amount of leakage flux. Additionally, in the original blade with sharp edges, the PCVs is relative weak and has a loose structure, resulting in the multiple interaction with the next blade. These phenomenon are responsible for the severe performance degradation and flow instabilities in the tip region of an axial-flow pump. Lei Shi, Zhang, Zhao, Weidong Shi and van Esch, Journal of Fluid Science and Technology, Vol.12, No.1 (2017) 7 low pressure in the vortex core (Arndt, 2002;Higashi et al., 2002;Murayama, 2006). Thus, cavitation inception is often coupled with vortex structures, such as tip leakage, trailing and hub vortices as well as turbulent vortex structures developing in regions of flow separation.In the aforementioned research, much attention has been paid to the tip leakage vortex cavitation around the fully-wetted hydrofoils (Boulon et al., 1999;Park et al., 2006;Dreyer et al., 2014). Although many progress has been made in regard to the vortex structures and self-induced cavitation, some important parameters are still missing in actual hydraulic machines, for instance, the body force in rotating frame which can alter the cavitation phenomenon. As a result, for the practical applications, the numerical and experimental investigations have been carried out in many rotating machines, both for single and multiphase flows. Farrell and Billet (1994) measured the important variables for the correlation which predict the vortex minimum pressure in a high Reynolds number axial-flow test rig. Moreover, an optimum tip clearance has been theoretically identified as experiments have shown. Afterwards, a combined study of tip clearance and tip vortex cavitation in an axial flow pump was carried out by Laborde et al. (1997). A conclusion is drawn that cavitation inception is...

Analysis of the Formation Mechanism and Evolution of the Perpendicular Cavitation Vortex of Tip Leakage Flow in an Axial-Flow Pump for Off-Design Conditions

“…The most significant vortex was defined as the perpendicular cavitation vortex (PCV). Zhang et al [26] simulated the formation of a three-dimensional tip leakage vortex cavitation cloud and the periodic collapse of TLV-induced suction-side perpendicular cavitating vortices (SSPCV), which were consistent with the observations reported by Tan et al Shi et al [27] used numerical calculations combined with visualization experiments to reveal the influence of the flow rate, number of blades, and blade tip geometry on the PCV, with particular emphasis on the trajectory and dynamics of the PCV. The SSPCV were generated from the trailing edge of the cloud, resulting in the collapse of the triangular area.…”

“…It is found through visualization experiments that the PCV becomes stronger under off-design conditions [27][28][29], and this is obviously beneficial to observation and research. Therefore, the axial flow pump under the off-design working condition was selected (Q = 0.8 QBEP).…”

“…where F vap and F cond are the empirical coefficients for the mass transfer process with the recommended values of 50 and 0.01, respectively, R b is the typical bubble radius with a value of 1 × 10 −6 m, p v is the saturation vapor pressure, with a value of 3169 Pa, and α nuc is the nucleation site volume fraction with a value of 5 × 10 −4 . These parameters were validated for the simulations of cavitating flows in pumps [26][27][28][29][30]38,42]. The commercial code ANSYS CFX 17.1 was used in this study since it is widely used in engineering applications.…”

“…Additionally, when cavitation occurs, it is difficult to obtain accurate flow information inside a cavity with experimental methods [30][31][32][33]. Numerical simulation can provide detailed and insightful information about the evolution of tip leakage vortex flow and unsteady cavitation flow based on the Reynolds averaged Navier-Stokes (RANS) equations [8][9][10]19,20,22,24,[26][27][28][29][30].…”

“…It is found through visualization experiments that the PCV becomes stronger under off-design conditions [27][28][29], and this is obviously beneficial to observation and research.…”