Showing papers on "Tip clearance published in 2022"

Effect of vorticity transport on flow structure in the tip region of axial compressors

Abstract: Numerical simulations are carried out to investigate the flow structure in the blade tip region of axial compressors. Various tip clearance heights and end wall motion conditions in a linear compressor cascade are studied to assess the effect of vorticity transport on the tip leakage flow (TLF). Moreover, the effect of vorticity transport on the TLF in a compressor rotor at different operating conditions is studied using delayed detached eddy simulation. The results show that the vorticity transport at both the blade tip and the end wall plays an important role in the roll-up and evolution of the tip leakage vortex (TLV), resulting in great impacts on the loss and stability of the TLV. It is found that the TLV is composed of a two-layer structure. The inner vortex core region formed by the vorticity transport from the blade tip shear layer to the TLV has a great effect on the strength and loss of the vortex, and the structure of the outer shear layer is altered by the secondary vortex formed by the vorticity transport from the end wall shear layer and thus affects the stability of the TLV. By the mechanism of the vorticity transport, the effects of the clearance height, the end wall motion, and the non-uniform clearance as a control method can be explained uniformly. The new understanding of the TLF structure and the vorticity transport mechanism helps to improve the performance of axial compressors by controlling the vorticity transport of the TLF.

Control strategies for tip leakage vortex using inclined squealer rim in axial turbines

Abstract: In a typical gas turbine, due to its complicated blade geometry, complex vortex structures appear and cause significant aerodynamic loss. Vortex systems dominated by a tip leakage vortex near the tip region are the primary source of this loss. In this paper, to improve the aerodynamic performance of the turbine, two novel control strategies for tip leakage vortex and the tip leakage flow of the cavity tip are proposed, and their coupling control effects are numerically investigated. The first control strategy is intended to control the loss caused by the breakdown of tip leakage vortex. By inclining the external wall of the suction side rim toward the passage, the emergence of a trailing edge pressure spike is delayed. This significantly reduces the adverse pressure gradient, suppressing the breakdown of tip leakage vortex and reducing tip leakage loss. The second control strategy controls tip leakage flow using the inclined inner wall of the suction side rim, which enhances the separation bubble on the top of the rim of the suction side and reduces the leakage rate by 7.7%. In this way, the formation and development of tip leakage vortex are indirectly manipulated, inhibiting the tip leakage loss. The coupling of the two strategies reduces the blocking effect on tip leakage flow slightly compared to the second strategy. However, the stage efficiency of the turbine is still improved by 0.24% because of the effective suppression of tip leakage vortex breakdown.

Large Eddy Simulation On the Cavitation Flow and Noise Characteristics of a NACA0009 Hydrofoil with Different Tip Clearance Sizes

Abstract: All hydraulic machinery has a tip clearance, which not only produces tip-leakage vortexes (TLVs), but also reduces the energy performance of the machinery. In addition, tip clearance leads to cavitation and attendant vibration and noise. Therefore, investigating tip-leakage cavitating flow and noise characteristics is of great practical importance. In this paper, the energy performance and noise characteristics of NACA0009 hydrofoils with different tip clearance sizes are studied. A large eddy simulation model and Schnerr-Sauer cavitation model are employed to simulate tip-leakage cavitating flow. Additionally, a broadband noise source model and the Ffowcs Williams-Hawkings (FW-H) equation are used to calculate the noise source and far-field radiated noise characteristics, respectively. Results show that the numerical simulation of cavitation vortexand velocity field is in good agreement with the experimental data, illuminating the characteristics of energy performance, flow pattern, cavitation flow, broadband noise source, and near-field and far-field radiated noise. Compared with the original NACA0009 hydrofoil, the tip clearance reduces the noise of the Curle dipole on the hydrofoil surface and Proudman noise around the hydrofoil. Moreover, study of the far-field noise shows that the directivity curve of the overall sound pressure level (SPL) is distributed in a butterfly shape, symmetrically. Evidently, the tip clearance size has a large impact on the energy performance of the hydrofoil, the intensity of the TLV, and the cavitation. This paper lays a solid foundation for further research on cavitation flow in large-scale hydraulic machinery.

Simulation of Internal Flow Characteristics of an Axial Flow Pump with Variable Tip Clearance

Abstract: This study investigated the influence of the change in blade tip clearance on the internal flow characteristics of a vertical axial flow pump. Taking the actual running vertical axial flow pump of a pumping station as the research object, based on the SST k-ω turbulent flow model, the numerical simulation technology was used to study the effects of different tip clearances on the pressure, turbulent kinetic energy, Z–X section pressure and flow state of the impeller at the middle section. Furthermore, the impact of clearance layer tip leakage was also analyzed. Unsteady calculations of flow characteristics under the design conditions were performed. The research results showed that the variation trend of the pressure in the impeller was basically the same under different tip clearance values. With the increase in the clearance value, the pressure gradient along the water inlet direction of the blade decreased and the leakage vorticity increased. Observing the leakage vorticity distribution of the gap layer under the flow condition of 0.6Q0, it was found that when the tip clearance was smaller than 1 mm, the leakage flow was small and easily assimilated by the mainstream, and the leakage flow and mainstream had a certain ability to compete, which caused adverse effects on the performance of the pump device. The pressure pulsation characteristics showed that the leakage flow caused by the tip clearance caused a high-frequency distribution, and the clearance obviously influenced the pressure pulsation characteristics.

Research on the Influence of Tip Clearance of Axial-Flow Pump on Energy Characteristics under Pump and Turbine Conditions

Abstract: In order to study the influence of tip clearance on the performance and energy dissipation of the axial-flow pump and the axial-flow pump as a turbine, and find the location of high dissipation rate, this study took an axial-flow pump model as its research object and designed four tip radial clearance schemes (0, 0.2, 1 and 2 mm). The unsteady calculation simulation of each tip clearance scheme was carried out based on CFD technology. The calculated results were compared with the experimental results, and the simulation results were analyzed using entropy production analysis theory. The results showed that, under both an axial-flow pump and axial-flow pump as turbine operating conditions, increasing the blade tip clearance led to a decrease in hydraulic performance. Compared with the 0 mm clearance, the maximum decreases in pump efficiency, head and shaft power under 2 mm tip clearance were 15.3%, 25.7% and 12.3% under the pump condition, and 12.7%, 18.5% and 28.8% under the turbine condition, respectively. Under the axial-flow pump operating condition, the change in blade tip clearance had a great influence on the total dissipation of the impeller, guide vane and outlet passage, and the maximum variation under the flow rate of 1.0Qdes was 53.9%, 32.1% and 54.2%, respectively. Under the axial-flow pump as a turbine operating condition, the change in blade tip clearance had a great influence on the total dissipation of the impeller and outlet passage, the maximum variation under the flow rate of 1.0Qdes was 22.7% and 17.4%, respectively. Under the design flow rate condition, with the increase in tip clearance, the dissipation rate of the blade surface showed an increasing trend under both the axial-flow pump and axial-flow pump as turbine operating conditions, and areas of high dissipation rate were generated at the rim and clearance.

Effect of Differential Tip Clearance on the Performance of a Tandem Rotor

Abstract: Abstract Tandem blade is an interesting concept that promises a higher total pressure rise per stage. Owing to two separate tip leakage vortices and their interaction, losses are likely to increase particularly near the tip region. Although rotors are designed with optimum tip clearance, the clearance changes during engine operation as well as during its service life. In the case of tandem rotors, the forward and the aft rotors can have different tip clearances. This will also impact the performance of the stage. Six different tip clearances have been investigated. ANSYS CFX is used for steady RANS computational analysis. The results suggest that the performance of the tandem rotor is highly sensitive to the forward rotor tip clearance. Higher tip clearance adversely affects the total pressure rise and operation stability of the tandem rotor. At design mass flowrate, the performance degradation for tandem configuration with the higher tip clearance (Case2, Case 3, Case 5, and Case 6) is attributed to the vortex breakdown of TLV1, which leads to the sudden expansion of the blockage region near the rotor tip. Vortex breakdown primarily depends upon the swirling strength of TLV1 and TLV2 as well as on the adverse pressure gradient. Near the stall point, the role of the adverse pressure gradient becomes more dominant in the vortex breakdown.

Extending Highly Loaded Axial Fan Operability Range Through Novel Blade Design

Abstract: The tip clearance size has historically been considered to be the main factor affecting stability range in axial fan and compressors. This paper reveals that the stall characteristics are defined by the axial momentum flux of the tip leakage flow and that tip clearance is primarily a strong driver for this metric. A bespoke methodology for carefully tailoing the axial momentum via three-dimensional design is presented, which enables a higher degree of control over the stability range for cases where the tip clearance responds to other considerations and cannot be defined for this purpose. The effect of the axial momentum on efficiency is also addressed and the trade-off between operability range and design point performance derived. The results show that that the conditions for optimal stability differ from those for optimal efficiency and that control over the axial momentum enables tuning the design for a desired exchange. Numerical simulations have been employed to drive the analysis through a high-fidelity computational model whose behavior is supported by rich set of experimental data. Contrary to current belief, results further indicate that an accurate characterization of stall, including onset mechanism, can be achieved through steady-state simulations, minimising the need for expensive time-accurate computations during the design phase.

Prediction of the Influence of Runner Tip Clearance on the Performance of Tubular Turbine

Abstract: Tubular turbine is a type of turbine with low-head. Due to the fact that the runner of a tubular turbine is of axial-flow type, there will be a certain width of blade tip between the blade and the chamber. In order to explore the influence of tip clearance width on the flow inside the turbine, taking the model tubular turbine as the research object, six different tip clearance widths were compared and analyzed. The research shows that the increase in blade tip clearance width affects the performance of the turbine, reduces the minimum pressure at blade tip and causes cavitation in advance. Larger tip clearance width significantly increases pressure pulsation intensity inside the turbine, especially in the vaneless region between the runner and guide vane and the area of the runner tip. However, the increase in tip clearance width can greatly reduce the axial force for about 100 N and radial excitation force for about 50% of rotating parts. Therefore, during the design and processing of tubular turbines, the blade tip clearance width should be carefully selected to ensure safe and stable operation of the unit.

Comparison of Turbulence Modeling for a Compressor Rotor at Different Tip Clearances

Abstract: Due to the high Reynolds numbers together with the complex turbulent motions in turbomachinery, selecting a proper turbulence-modeling method is of vital significance in interpreting the real flow physics. In this paper, shear stress transport (SST) as a Reynolds-averaged Navier–Stokes model and scale-adaptive simulation (SAS) and zonal large eddy simulation (ZLES) as two scale-resolving simulation approaches were chosen to simulate the flow in a low-speed axial compressor rotor at three different blade tip clearances (1.3, 2.6, and 4.3% of chord length). Results of unsteady simulations at the design point were compared to the experimental data. The results prove that the ZLES model performs best in characterizing the experimental data regardless of the blade tip clearance size. The performance of the SST model in capturing the tip flow is close to the measurement data when the blade tip clearance is small, while the deviation from the experimental results increases with the clearance size enlarged. In contrast, the use of the SAS model does not result in any benefits over the SST model for the small tip clearance but demonstrates similar performance with the ZLES model for the configurations with middle and large tip clearance sizes.