| An axial-flow pump is widely used in the national strategic projects, such as large and medium-sized water transport, nuclear power, hydraulic jet propulsion and weapon launcher. The tip leakage flows and associated cavitation often occur in the tip region of the axial-flow pumps because of the presence of the tip clearance, resulting in the severe degradation of the pump performance.A fully-wetted hydrofoil with a tip clearance and an axial-flow pump model were selected as the investigated geometries. The numerical computations, coupled with the visualized experiments, were used to discuss the inherent three-dimensional structure of the tip leakage vortex and its evolution, as well as the unsteady cavitation. The main contributions can be concluded as follows:(1) It concludes that the roll-up process of TLV near the blade suction side occurs earlier in the narrow tip clearance, while the angle between the TLV trajectory and the blade tip increases. Corresponding to the distribution of vorticity, the mean static pressure in the TLV core is also lower in a wider tip clearance, which is most prone to generating cavitation. The mean flow structures in different tip clearances are highlighted by the statistics of the axial velocity and TKE distributions, as well as the complementing contours plots focus on the tip clearance. From the entrance to the exit of the blade tip, the axial velocity accelerates to a maximum and then slows down. Away from the blade tip towards the casing wall, the leakage jet accelerates gradually and reaches to a plateau. The in-plain TKE distribution has the similarity trend. With the increase of the tip-gap size, the backward flow is faster and the region where the tip separation vortex occurs has higher turbulence. The instantaneous vorticity fields with various tip clearances are shown via the LES computations. In the course of the migration towards the downstream, the TLV interacts with the endwall and the incoming flows. Consequently, the induced vortices are generated above the TLV core, and mixes with the wall detachment region. Eventually, it is entrained by TLV and wraps along the outer periphery of the TLV core. Similarity trend both emerges in two tip-gap sizes, but a bit difference is that the vortices attached on the blade suction side with high circumference vorticity also plays an important role in providing energy for TLV motion in a wider gap. According to the turbulence production rate and the Reynolds stresses distributions, it is found that turbulence production near the tip region mainly occurs near the endwall separation where the tip leakage jet starts to recirculate about the TLV core, and in the tip clearance where the tip separation vortex emerges. Near the endwall separation, the principle mechanism for turbulence production are those associated with the velocity gradient,Apart from a region of nearly isotropic turbulence within the TLV core, the normal stresses are quite non-isotropic. Generally, when one normal stress is high, another is usually low. The largest single contributor is the work of deformation of the secondary flow by the vwshear stresses. This mechanism,accounts for a lot. A secondary contribution comes from the normal stress term.(2) According to the streamlines near the tip region, due to the decrease of pressure difference near the leading edge, the origin of TLV would be delayed towards the mid-chord. Additionally, induced by the free stream, the TLV trajectory is close to the blade suction side. In general, the trend of the flow fields in the tip clearance is similar. From the tip to the casing wall, the mainstream with positive velocity turns into the leakage flows with negative velocity, and then the velocity reaches to a maximum. Afterwards, the velocity would decrease as a consequence of the boundary layer near the endwall. However, several distinctions were still obvious that can be given by: 1) the leakage flows enters into the tip clearance with a high velocity and separate from the pressure side. But near the exit of the tip clearance, it would re-attach on the blade tip, which can be changed for the different flow coefficients, especially for the large flow condition. 2) under small flow condition, the TLV trajectory is far away from the blade tip, due to the large axial velocity. On the basis of the TKE distributions in the tip clearance, it is evident that the TKE is low within the region of separate vortex with the decrease of the flow rate, indicating that the intensity of tip separate vortex is stronger. In the small flow condition, the TKE is uniform near the casing wall. However, with the increase of the flow rate, the trend is moving towards the middle passage. Compared with the flow fields in the tip clearance at different chord fractions, the velocity is large near the leading edge, regardless of the mainstream and leakage flows. Near the blade leading edge, the phenomenon of re-attachment would not happen. But near the aft part of the blade tip, the re-attached point would be more close to the entrance of the tip clearance. When it comes to the TKE distributions, more close to the blade trailing edge, weaker strength appears near the region where the separate vortex generates. Under the large flow condition, because of the delayed inception, the TLV strength is weaker at the same location. For a single-phase flow, using the minimum tension criteria, it can deduce that the separate vortex, TLV and blade suction side are most prone to generating cavitation. By the analysis of the various vorticity layers near the tip region clearly, the three-dimensional roll-up process by TLV is presented.(3) Based on the PIV data, it concludes that TLV emerges near the blade suction side and connects with the shear layer shedding from the blade tip. Simultaneously, the boundary layer near the endwall with high vorticity also appears. When migrates towards the next blade, TLV would absorb the vorticity from the shear layer and the endwall detachment region, resulting in the increase of vorticity in the vortex core. Afterwards, TLV moves downstream and its strength becomes weak, resulting from the dissipation itself and integration by the mainstream.(4) Compared with SST k-w turbulence model, the modified filter based model can greatly reduce the over-predicted viscosity near the blade trailing edge. In addition, the reduced viscosity and apparent vortex trip are obtained via the improved turbulence model by the comparison with RNG k-e and FBM models. Cavitation inception for the small flow rate occurs more early, and it mainly consists of TLV cavitation, corner vortex cavitation, tip clearance cavitation and shear layer cavitation. A combined effects of TLV trajectory and radial re-entrant jet from the hub to the blade tip induce the formation of vortical cloud cavitation at the trailing edge of the attached cavitation on the blade suction side near the tip region. Under the small rate condition, the PCVs trajectory is almost perpendicular to the blade suction side. When moves downstream, it would interacts with the incoming blade and be divided into two parts: an important proportion of the large-scale cloud cavitation becomes the nucleus for the cavitation formation on the next blade, while the remained directly impacts on the pressure side, leading to the blade loading realignment. With the increase of the flow coefficient, the PCVs migrates in the direction that is nearly parallel to the blade suction side, and collapses near the blade exit immediately. In the course of the presentations, an improved turbulence mode combined with a homogenous cavitation model were used to identify the existence of the re-entrant jet. An assumption is that its formation can be ascribed to the role of hub vortices and the velocity component of the mainstream. Compared with that in the rotor with three blades, the PCVs evolution is quite different due to the narrow flow passage and blade loading distributions. In the process of moving towards the pressure side of the next blade, both the main cloud cavitation and relative small-scale cavity shedding from the main structure would provide nucleus for the cavitation generation on the next blade. Under the large flow rate condition, the PCVs of one blade even interacts with another at the trailing edge of next blade. The blade with a rounded edge on the pressure side can effectively eliminate the corner vortex cavitation and tip clearance cavitation. Moreover, it shows that once the PCVs takes over, the cavitation at the aft part of the blade chord disappears, as a consequence of the substantial reduction in the flow. A compact and more intensive PCVs is generated in the rotor with a rounded edge on the pressure side. Conversely, in the original one, owing to some relative small-scale cavity separating from the main cloud, the phenomenon of multiple interactions with the adjacent blade would happen, trigging the flow disabilities in the blade tip region. |
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