A Review of Tip Clearance in Propeller, Pump and Turbine
Abstract
:1. Introduction
- (1)
- Flow mechanism and vortex dynamics in the tip-clearance region;
- (2)
- Influence of tip clearance on the energy performance of pumps and turbines;
- (3)
- Operation stability of pumps and turbines with tip clearance;
- (4)
- Influence parameters of tip clearance and optimization strategies for propellers, pumps and turbines.
2. Experimental Study on Flow Mechanism near Tip-Clearance Region
2.1. Flow Pattern Near Hydrofoil
2.2. Two-Dimensional Structure and Evolution of Tip-Leakage Vortex (TLV)
2.3. Three Dimensional Structure and Evolution of TLV
2.4. Cavitation Behavior
3. Numerical Study on Flow Mechanism near Tip-Clearance Region
3.1. Comparison and Validation of Numerical Methods
3.2. Vortex Types and Corresponding Characteristics
3.3. TLV Trajectory and Related Factors
3.4. Cavitation Behavior
4. Energy Performance of Pumps and Turbines with Tip Clearance
4.1. Energy Characteristics in Pumps
4.2. Energy Characteristics in Hydroturbines
4.3. Energy Characteristics in Inducer of Turbopumps
4.4. Other Hydraulic Machinery Types
5. Operation Stability of Pumps and Turbines with Tip Clearance
5.1. Pressure Fluctuation
5.2. Radial Force
6. Key Parameters and Optimization
6.1. Tip-Clearance Size
6.2. Tip Shape
6.3. Other Parameters and Optimization Methods
7. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Tip-Clearance Range | Machinery Type | Performance Change |
---|---|---|
0–1 mm | Mixed-flow pump [5,6] | Head drops by 10% at design flow rate; efficiency drops by 4% at design flow rate; pressure fluctuation near blade-trailing edge increases by 20 times; unsymmetrical tip clearance deteriorates cavitation performance and increases radial-force fluctuation. |
0.5–1.5 mm | Mixed-flow pump [7] | Head drops by about 8% at design flow rate; tip-leakage reverse flow becomes stronger, and further thickens the casing boundary layer and intensifies vortexes in flow passage. |
0–1 mm | Axial-flow pump [8] | Significant pressure drop and cavitation inception in tip-clearance region. |
0–0.3 mm | Axial-flow pump [9] | Head drops by 3% at design flow rate; pressure fluctuation on blade increases by 34%. |
Authors | Techniques | Conclusions |
---|---|---|
Boulon et al. [12] | Laser-Doppler Velocimetry | The clearance size (confinement) influences the minimum pressure at vortex core and associated tip-vortex cavitation around a hydrofoil. Strong tip-vortex cavitation occurs in the range of 4–20 mm. |
Dreyer et al. [14,15] | Particle-Imaging Velocimetry | Distance between the TLV and the hydrofoil with decreasing tip-clearance size, and the changing tip clearance shifts the TLV trajectory after the trailing edge. |
Miorini, Wu, and Tan [16,17,18,19,20,21,22] | Particle-Imaging Velocimetry High-Speed Imaging | Two-dimensional and three-dimensional structure of TLV in an axial waterjet pump; five periods the evolution of TLV; vortex structures and corresponding flow characteristics along blade-chord direction. |
Tan et al. [24] | High-Speed Imaging | The separation angle between the perpendicular cavitation vortex (PCV) and the blade tip increases from 8° to 12° when the cavitation number varies from 0.39 to 0.45 in an axial waterjet pump. |
Zhang et al. [8] | High-Speed Imaging | TLV cloud cavitation keeps a quite-persistent triangular structure from the leading edge to 80% chord, but the cavitation pattern is quite unstable near the trailing edge in an axial pump. |
Authors | Method and Object | Conclusions |
---|---|---|
You and Moin [34,35,36,37,38] | LES model Hydraulic turbomachine | The tip-clearance flow can be divided into three parts: the tip-leakage vortex, the tip-separation vortex, and the induced vortex. The three parts vary in volume and swirling direction, and TLV is the dominant structure among them. |
You and Moin [39,40,41] | LES model hydraulic turbomachine | Tip-clearance flow leads to violent turbulence intensity, which increases the viscous losses near the tip clearance; Optimizing the direction of leakage flow by changing the tip shape is proposed; Cavitation is mostly prone to occur at TLV core in inception stage, and local optimization is recommended. |
Zhang et al. [42] | SST model Axial pump | When the flow rate increases from 0.85Qd to 1.2Qd, the formation point of TLV shifts from 5% blade chord to 40% blade chord. The separation angle between the TLV and blade tip shows an increasing trend with increasing flow rate. |
Liu et al. [5] | RNG model Mixed-flow pump | The separation angle between the TLV and the blade tip remained 10° for different tip-clearance sizes at the design flow rate. The separation angle is likely dependent on the flow rate [39], not the tip-clearance size [2]. |
Liu et al. [43] | LES model Mixed-flow pump | the TLV is classified into four categories, namely, primary TLV, secondary TLV, entangled TLV, and dispersed TLV; the corresponding vortex derivation mechanism is analyzed by the relative vorticity transport equation. |
Zhang et al. [44] | SST model Axial pump | The evolution of cavitation patterns related to tip-clearance flow, including TLV cavitation cloud, perpendicular cavitation vortex, and sheet cavitation. There are interactions among these cavitation patterns. |
Okita et al. [45] | DES model Three-dimensional inducer | A horseshoe-shaped cavitation structure is observed near the casing wall at the leading edge under the impact of tip-clearance flow. This structure makes more streamlines blocked at the leading edge and flow interaction more intensive. |
Tip-Clearance Size (mm) | Head (m) | NPSHC (m) |
---|---|---|
0.0 | 17.34 | 13.58 |
0.2 | 16.73 | 13.67 |
0.65 | 16.00 | 13.95 |
1.0 | 15.46 | 14.23 |
Optimization Scheme | Machinery Type | Conclusion |
---|---|---|
Round tip and sharp tip | Hydrofoil [78] | Sharp tip decreases TLV and round tip decreases TSV |
Blade-tip rounding | Axial-flow pump [79] | twice tip rounding is optimal to the pump performance |
Blade-tip thickening | Ducted propeller [80] | Both the strength of TLV and pressure drop in the TLV core on the middle chord section are reduced |
T-shape tip | mixed-flow pump [81] | This tip shape can improve pump efficiency by 1.86% and reduce leakage flow rate by 15.95% |
C groove | Hydrofoil [82] | C groove maximally suppresses tip-leakage vortex by 67.94%, and improves energy performance by 2.79%. |
Anticavitation lip | Kaplan turbine [83] | Large lip can suppress cavitation on the blade, but may intensify cavitation in the closing-gap region |
Gap configuration | Underwater propulsors [84] | Convergent gap shows the best cavitation performance |
Mass injection | Hydrofoil [86] | Flow instability in the tip-vortex core region is decreased and cavitation inception is suppressed. |
Pressurized-air injection | Kaplan turbine [87] | The fluid force driven by the guide vanes is decreased and the operation vibration is weakened |
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Liu, Y.; Tan, L.; Wang, B. A Review of Tip Clearance in Propeller, Pump and Turbine. Energies 2018, 11, 2202. https://doi.org/10.3390/en11092202
Liu Y, Tan L, Wang B. A Review of Tip Clearance in Propeller, Pump and Turbine. Energies. 2018; 11(9):2202. https://doi.org/10.3390/en11092202
Chicago/Turabian StyleLiu, Yabin, Lei Tan, and Binbin Wang. 2018. "A Review of Tip Clearance in Propeller, Pump and Turbine" Energies 11, no. 9: 2202. https://doi.org/10.3390/en11092202