Effect of vane tip clearance on cavitation and gas–liquid two-phase flow characteristics of high-speed rotary vane pump Open Access

The engine lubricating oil system in modern aircraft poses increasing challenges and demands higher performance from its core component, the lubricating oil pump.¹ The lubricating oil pump serves as the “heart” of an eroengine lubricating oil system, and its reliability is critical to the proper functioning of the oil system and even safe operation of the entire eroengine. The rotary vane pump (RVP, also known as the sliding vane pump) shows significant potential as a lubricating oil pump owing to its simple geometric structure, convenient maintenance, and excellent fuel supply regulation performance.^2,3 Current research on this type of positive displacement pump focuses primarily on structural design, flow characteristics, and leakage studies.

Research on the structural design of RVPs concentrates on the stator profile, where the transition curve of the stator inner profile plays a crucial role in determining pump performance, longevity, and stability.^4,5 Chen et al.⁶ proposed a mathematical model for asymmetric cylindrical profiles and a general approach to their design, leading to the development of geometric structures for single-chamber, double-chamber, and three-chamber asymmetric sliding vane vacuum pumps. Their findings indicated that the asymmetric structure outperforms a symmetrical sliding vane pump in terms of suction performance and energy consumption. Rundo et al.⁷ analyzed the influence of various geometric parameters on vane pump filling through numerical simulation. They found that a pump with a low ratio between axial thickness and diameter has a higher volumetric efficiency if the chambers suction flow from one side only, while increasing the number of chambers and reducing the diameter of the rotor can improve the filling.

The flow characteristics of sliding vane pumps have been explored through experiments and numerical simulations. For instance, Suzuki et al.⁸ investigated the rotational speed and flow characteristics and pressure in the pump chamber of a vehicle-mounted sliding vane pump through computational fluid dynamics (CFD) analysis. They also established an experimental test platform to validate the numerical results. Their study highlighted that cavitation in the pump chamber hampers oil suction and discharge. They also demonstrated that implementing a cutting port structure in the stator enhances flow rates during gas–liquid two-phase operation, particularly as deeper cutting depths reduce cavitation zones. Ye et al.^9–11 introduced a grid generation technique based on user-defined nodal displacement. They employed this method in a CFD simulation of the cavitation extent and area within a rotating plate energy recovery device that was part of a seawater reverse osmosis system. Their investigation revealed a higher propensity for cavitation in the space between the blade tip and the stator inner wall, the downstream section of the rotating vane, and a specific low-pressure zone at the base of the vane trough. Wang et al.¹² analyzed the cavitation characteristics of an RVP and its impact on pump performance under different inlet pressure and oil temperature conditions. They concluded that the cavitation and its distribution in the pump chamber directly affected the average oil discharge flow rate and volumetric efficiency at the pump outlet. Lobsinger et al.¹³ employed the twin mesh method to simulate a simplified two-dimensional vane pump model under multiphase flow conditions. They discovered that inlet gas content significantly increases outlet pressure/flow pulsations while reducing volumetric efficiency and power demand.

During the operation of a high-speed RVP, the vanes rotate closely against the stator wall under drive shaft actuation, enabling continuous oil suction and discharge through cyclic chamber volume changes. Prolonged high-speed operation inevitably leads to vane tip wear, resulting in an increase in the clearance between the vane tip and stator wall. Studies have demonstrated that this radial clearance critically governs leakage rates and volumetric efficiency degradation in RVPs.^10,11,13 Bianchi et al.^14–16 conducted numerical simulations of the leakage of a blade expander for organic Rankine cycle (ORC) applications and experimentally verified the accuracy of the numerical simulation method. The results indicate that for every 10 µm increase in the vane tip clearance, the leakage of the mass flow rate increases by 1.66 g/s, and the filling factor increases by 6%. Owing to significant leakage at the vane tips, the mass flow rate and expansion ratio increase with increasing vane tip clearance, and the leakage flow rate at the vane tips is proportional to the tip clearance dimension. Mascuch et al.¹⁷ identified internal leakage as a primary limiting factor for achieving maximum design pressures, demonstrating its severe impact on expansion efficiency through systematic experimental analyses. These findings collectively establish vane tip clearance as a pivotal design parameter requiring precise control in high-speed rotary machinery applications.

Most research has focused on low-speed RVPs, leaving a gap in the investigation regarding high-speed RVPs (i.e., those with a rotational speed more than 3000 rev/min). An increased vane tip clearance results in leakage, reducing the oil delivery capacity of the RVP. In critical situations, this deficiency may lead to inadequate lubrication of eroengine components, consequently disrupting the normal operation of the eroengines. The influence of tip clearance on the cavitation and gas–liquid two-phase flow characteristics in high-speed RVPs remains ambiguous. The present study utilizes numerical simulations to analyze the internal flow patterns of an RVP with varying vane tip clearances under diverse inlet conditions. The investigation delves into the cavitation and gas–liquid two-phase flow characteristics of a high-speed RVP, with the objective of elucidating the influence of vane tip clearance on its internal flow dynamics and the oil suction and discharge performance. This research is great importance in providing a better understanding of the inner flow mechanisms of RVPs of this kind and in ensuring the safe and stable operation of eroengines.

The subject of this study is a new high-speed RVP designed to meet the lubrication requirements of an aircraft engine. The rated rotational speed of the RVP is 6910 rev/min. As shown in Fig. 1, its three-dimensional model consists of four vanes that divide the pump chamber into four varying-sized chambers. There are two oil suction and discharge windows on both the inlet and outlet sides of the RVP. The stator inner contour follows a high-order curve, which can enhance the force situation during the vane movement process and thus improve the pump suction performance, reduce noise, improve high-altitude performance, and extend the pump service life. During operation, the RVP rotor rotates with the drive shaft, and the four vanes work closely against the rotor surface, leading to continuous oil suction and discharge as the volumes of the four working chambers vary. The designed clearance between the vane tip and the stator inner wall (i.e., the tip clearance) is 0.025 mm, and the clearance between the pump body and the pump stator end face is 0.03 mm.

Cavitation transport models treat the combined motion of liquid and gas (vapor and other gases) as a variable-density single-phase flow. The set of fundamental governing equations for this mixture flow coincide with those for multicomponent flows, with a dedicated transport equation regulating the vapor mass fraction produced during cavitation. These governing equations are as follows:^18,19

The volume-of-fluid (VOF) multiphase flow model is employed in the present simulation. The governing equations of the VOF model²¹ are the volume fraction equation for each secondary phase, the momentum equation for the mixed fluid utilized by each phase, and the energy equation for the mixed fluid shared by each phase, together with additional scalar equations such as the turbulent kinetic energy equation.

This study investigates the influence of vane tip clearance on cavitation and gas–liquid two-phase flow characteristics of the RVP under single-phase oil and two-phase oil–gas inlet conditions. The simulation of two-phase flow utilizes BP Turbo Oil 2197 lubricating oil as the liquid phase and air as the gas phase, with an inlet gas volume fraction (IGVF) of 20% being examined. The IGVF refers to the percentage of gas volume in the total volume of the gas–liquid mixture when it enters the pump. Note that the selection of an IGVF of 20% for this study is determined by practical considerations specific to the lubricating oil system under investigation. Six different vane tip clearances ranging from 0.025 to 0.05 mm (0.025, 0.03, 0.035, 0.04, 0.045, and 0.05 mm) are considered for calculation under an inlet pressure of 101 kPa, an outlet pressure of 413 kPa, and an oil temperature of 120 °C. The fluid inside the pump is assumed to be a continuous incompressible fluid, the inlet and outlet are set as pressure boundary conditions, the walls are considered no-slip, and the velocity and pressure are solved iteratively using the SIMPLE algorithm. The specific numerical simulation method is shown in Table I. The total simulation duration is six revolutions of the RVP rotor, with 3° of rotation of the rotor considered as one time step.

The fluid domain of the RVP is divided into three distinct regions: inlet, outlet, and rotating domains. Each domain utilizes specialized grid configurations, with hexahedral Cartesian grids applied to the inlet and outlet regions and structured hexahedral grids implemented in the rotating domain. A grid-independence study was performed using five mesh resolutions: 306 364, 354 728, 394 731, 450 411, and 496 872 cells. As shown in Fig. 2, the outlet flow rate demonstrates stabilization when the resolution exceeds 450 411 cells, establishing this resolution as optimal for subsequent simulations. The selected configuration features a maximum cell size of 0.003 and a surface grid scaling of 0.0025. Figure 3 presents the final grid distribution employed in the RVP numerical model.

RVPs are employed for transporting and recovering lubricating oil in eroengines, delivering the necessary fluid and pressure to the transmission system. The adequacy of the oil flow rate in meeting operational requirements serves as a key performance metric for an RVP. Experimental validations were conducted in this study to ascertain the reliability of the numerical simulation method. These experiments focused on verifying the numerical simulation method for cavitation in the RVP under varying inlet pressures for the single-phase oil inlet. Furthermore, comparisons were made between the oil discharge flow rates of the RVP with the entry of oil–gas two-phase flow and experimental data to validate the numerical simulation method for gas–liquid two-phase flow. Figure 4 illustrates the architecture of the integrated experimental system for multiphase flow analysis, with details of the instrumentation and calibration methodologies, which are fully documented in Ref. 23, including measurement uncertainty quantification protocols for flow parameters.

Comparison curves illustrating the average flow rate Q of the pump obtained through both experiments and numerical simulation are depicted in Fig. 5, where the ordinate represents the dimensionless average flow rate of the RVP during suction and discharge. For single-phase oil inlet conditions, the simulated flow rate closely aligns with the experimental results, with a maximum relative error of no more than ±6%. Under oil–gas two-phase inlet conditions, the relative error of the numerical simulation remains below ±11%. These findings confirm the reliability of the numerical simulation method employed in this study.

To investigate the impact of varying tip clearance on the cavitation characteristics of the flow field in the RVP, a representative cross-section of the internal flow field is chosen for analysis, as illustrated in Fig. 6. In this investigation, six representative pressure pulsation measurement points were initially selected to characterize the discharge pressure fluctuations. Comparative analysis revealed similar pulsation patterns across all monitored locations. Considering both the practical constraints of sensor installation in operational vane pump systems and the need to provide directly applicable guidance for industrial maintenance, the pump outlet was ultimately chosen as the optimal measurement position (Fig. 6). All subsequent pressure pulsation analyses in this study focus exclusively on the outlet measurement point.

Under the single-phase lubricating oil inlet condition, the distributions of cavitation gas in the cross-section for different vane tip clearances are shown in Fig. 7. Analysis reveals that cavitation gas predominantly accumulates within the inlet-side oil suction chamber, with pronounced localization along the vane suction side and adjacent rotor wall. This phenomenon arises from the significant pressure differential between the vane’s suction and discharge sides, compounded by high lubricating oil velocities near the rotor wall in the low-pressure chamber. Cavitation gas diminishes progressively within the discharge-side pressure chamber owing to elevated regional pressures, ultimately dissipating at the pump outlet except for minor residual gas near the vane tips. Notably, as the vane tip clearance increases from 0.025 to 0.05 mm, the distribution of cavitation gas in the pump chamber remains relatively consistent, with consistent mild cavitation across different clearances. Quantitative comparisons indicate moderate cavitation intensities at the discharge chamber wall for intermediate clearances (0.035 and 0.045 mm), although without significant deviation from baseline clearance performance.

As shown in Fig. 8, the instantaneous flow curve at the outlet of the RVP varies according to different vane tip clearances. The abscissa represents the rotor angle of the RVP, while the ordinate signifies the instantaneous flow Q′ at the outlet corresponding to the rotor angle, with the six curves in the figure corresponding to distinct vane tip clearances. Cavitation occurrence induces a complex gas–liquid two-phase state within the pump chamber, leading to increased flow pulsations. Notably, the instantaneous flow pulsations at the outlet for tip clearances of 0.035 and 0.045 mm in Fig. 8 are lower than those for other clearances. On correlating these data with the cavitation cloud diagrams shown in Fig. 7, it is evident that the cavitation intensity in the pump chamber for these two clearances is milder, resulting in relatively stable flow transportation.

The average flow rate and volumetric efficiency curves of the RVP outlet corresponding to different vane tip clearances at the single-phase oil inlet are displayed in Fig. 9. The abscissa represents the various vane tip clearances s, while the left ordinate indicates the standardized outlet average flow Q/Q₀ and the right ordinate the volumetric efficiency η of the RVP. The cavitation gas volume distribution cloud maps in the figure depict the axial middle section of the RVP chamber. Analysis of Fig. 9 reveals that as the vane tip clearance increases, both the outlet flow rate and volumetric efficiency of the RVP decrease. Specifically, increasing the vane tip clearance from 0.025 to 0.05 mm results in a 4% decrease in outlet flow rate and a drop in volumetric efficiency from 82% to 79%. The decreases in the outlet average flow rate and volumetric efficiency of the RVP are primarily attributable to leakage induced by the enlarged vane tip clearance. Excessive vane tip clearance induces amplified volumetric leakage flows and aggravated cavitation phenomena within the pump chamber, thereby generating detrimental flow pulsation amplitudes. Therefore, to ensure a steady flow output and prevent undue flow leakage, it is recommended to maintain an optimal vane tip clearance of ∼0.035 mm under the investigated operating conditions.

To investigate the influence of different vane tip clearances on the outlet pressure fluctuation of the RVP, time-domain curves illustrating this effect are plotted in Fig. 10, which displays the running time of the pump on the abscissa and the instantaneous pressure value at pressure monitoring point (shown in Fig. 6) on the ordinate. This figure reveals distinctive pulsating periods in the outlet pressure corresponding to various vane tip clearances, demonstrating a similar pulsation pattern. Initially, the outlet pressure gradually rises to a peak upon pump initiation, followed by a decline. After several pulsating cycles transitioning from large to small amplitudes, the pressure reaches its maximum amplitude in the time domain. After four rotations, which amount to ∼0.035 s of running time, the pressure pulsations gradually decrease, stabilize, and fluctuate within a specific range. As the vane tip clearance increases from 0.025 to 0.05 mm, the maximum amplitude of outlet pressure fluctuation decreases. Analysis indicates that owing to the small range of vane tip clearances examined in this study, the reduction in outlet pressure is minimal, resulting in insignificant effects on the cavitation characteristics of the RVP.

The pressure fluctuation spectrum at the monitoring point for different vane tip clearances is depicted in Fig. 11, while Table II shows the dominant frequencies associated with varying vane tip clearances. Analysis of Fig. 11 and Table II suggests that pressure pulsations at the monitoring point predominantly occur in the low-frequency range, with energy distributed within 15f_n (where f_n is the rotation frequency, which is 115.17 Hz at a rotational speed of 6910 rev/min). The pressure pulsations exhibit characteristics of low-frequency and high-amplitude oscillations. As the vane tip clearance increases, the dominant frequency of pressure pulsation equals the rotational frequency 1f_n of the RVP, leading to a decrease in its pulsation amplitude A. An increase in vane tip clearance leads to increased pump leakage, reducing outlet flow rate and subsequently diminishing the amplitude at the dominant frequency of outlet pressure fluctuation.

As shown in Fig. 12, the phase distribution cloud maps illustrate oil–gas spatial configurations for varying vane tip clearances at a 20% inlet gas volume fraction (IGVF = 20%). Analysis reveals a gas-phase concentration predominantly localized along the rotor wall and vane suction side at the inlet region, exhibiting broader gas dispersion on the inlet side compared with the discharge side, particularly under enlarged tip clearance conditions. Elevated pressure and fluid velocity within the discharge chamber induce gas fragmentation into the liquid. Inlet free gas entrainment generates intricate multiphase flow patterns within the pump chambers, resulting in an uneven distribution of oil and gas phases influenced by tip clearance variations. The gas volume in the pump chambers is most extensive and widely spread for a vane tip clearance of 0.04 mm. Furthermore, a higher gas fraction is detected on the outlet side throughout the pump chamber.

Figure 13 presents the instantaneous flow rate curves of the RVP outlet for varying vane tip clearances. Analysis of the figure demonstrates periodic flow oscillations synchronized with vane rotation cycles at a 20% IGVF, exhibiting clearance-dependent fluctuation characteristics. These cyclic flow instabilities correlate directly with gas–liquid phase fraction and spatial distribution dynamics within the pump chamber. Comparative evaluation reveals progressive attenuation of instantaneous flow pulsation amplitudes at enlarged tip clearances relative to single-phase operational benchmarks. This attenuation suggests that entrained gas in the oil exerts a stabilizing effect on fluid delivery, effectively dampening flow irregularities while maintaining transport continuity.

Figure 14 illustrates the average flow rate and volumetric efficiency curves of the RVP outlet corresponding to various vane tip clearances at an IGVF of 20%. Q₀ is the average flow rate of the RVP for single-phase oil with a vane tip clearance of 0.025 mm. It is found that for a tip clearance of 0.025 mm, the average outlet flow rate is 8% lower than that at a single-phase oil inlet, while a tip clearance of 0.05 mm results in a 16% reduction compared with the single-phase oil inlet. With an increase in vane tip clearance from 0.025 to 0.05 mm at the two-phase inlet, the average outlet flow decreases by 12%, accompanied by a decline in volumetric efficiency from 76% to 66%. Analysis indicates that for a two-phase oil–gas inlet, heightened leakage due to increased vane tip clearance and the uneven distribution of oil and gas phases in the pump chamber lead to reduced oil suction and discharge efficiency of the RVP. Therefore, to maintain optimal performance, regular maintenance of the RVP is essential, ensuring operation under low gas content to prevent clearance-induced leakage that may impact pump performance over prolonged use.

For a 20% inlet gas volume fraction (IGVF), Fig. 15 displays the temporal pressure fluctuation characteristics at the pump outlet for varying vane tip clearances. Analysis of the figure reveals that upon pump initiation, the outlet instantaneous pressure reaches its peak within the time domain, gradually diminishing as the pump stabilizes during operation after four rotations, which amount to ∼0.035 s of running time. As the vane tip clearance increases from 0.025 to 0.05 mm, the maximum outlet pressure amplitude in the time domain decreases, resulting in reduced pressure fluctuations. This indicates that under two-phase conditions at the pump inlet, an increase in vane tip clearance leads to a decrease in the maximum amplitude of instantaneous pressure at the outlet. Furthermore, the uneven distribution of oil and gas phases within the pump causes irregular pressure pulsations at the outlet.

Figure 16 displays the pressure fluctuation spectra at the monitoring point for an IGVF of 20% and varying vane tip clearances. Table III presents the dominant frequencies for different vane tip clearances and an IGVF of 20%. It is found that in the case of a two-phase oil–gas inlet, pressure fluctuations occur primarily in the low-frequency range, with spectral energy concentrated within 10f_n. Pressure pulsations with dominant frequencies of 1f_n and secondary dominant frequencies of 4f_n (equal to the vane frequency) are observed for various vane tip clearances (Table III). This investigation demonstrates that these dominant and secondary dominant frequencies represent the fundamental and harmonic frequencies of the rotation frequency. This differs from the pressure fluctuation spectrum seen with a single-phase oil inlet, where the vane frequency is not prominently observed. The dominant frequencies persist at different vane tip clearances. This indicates that these fluctuations stem from typical dynamic and static fault features induced by rotation rather than vane tip alterations. These findings collectively establish that under two-phase operating conditions, pump outlet pressure fluctuations exhibit minimal sensitivity to vane tip clearance variations, being fundamentally governed by rotational kinematics and phase distribution asymmetries.

This study has investigated the effect of increasing vane tip clearance on the flow characteristics of a rotary vane pump (RVP) through numerical simulations conducted under single-phase oil and two-phase oil–gas inlet conditions. Variations in the flow field, oil suction and discharge performance, and outlet pressure fluctuations of the RVP with different vane tip clearances have been analyzed. The main conclusions are as follows:

1. Under the single-phase lubricating oil inlet condition, cavitation occurs within the pump chamber for each clearance value, with reduced severity for vane tip clearances of 0.035 and 0.045 mm. An increase in the vane tip clearance from 0.025 to 0.05 mm induces a 4% reduction in outlet flow rate and a 3% decline in efficiency (from 82% to 79%). Amplitude attenuation of pressure fluctuations correlates with clearance enlargement, dominated by fundamental frequency components in spectral analysis. Elevated leakage and cavitation intensity are identified as primary mechanisms driving performance degradation.

2. The oil and gas phases exhibit uneven distributions within the pump chamber in the case of a two-phase oil–gas inlet, with a higher gas fraction observed at a vane tip clearance of 0.04 mm. Increasing the vane tip clearance from 0.025 to 0.05 mm causes a 12% reduction in average outlet flow and a decline in volumetric efficiency from 76% to 66%. An increase in vane tip clearance leads to a reduction in maximum outlet pressure amplitude and pressure pulsation in the time domain. The pressure fluctuation frequencies for different vane tip clearances are the fundamental frequency and high-order harmonic frequencies of the rotational frequency. Increased vane tip clearance and uneven oil- and gas-phase distributions result in reduced oil suction and discharge performance of the RVP. In a two-phase scenario, the pump outlet pressure fluctuation is less affected by the vane tip clearance.

3. Excessive vane tip clearance increases pump leakage, and cavitation with uneven oil- and gas-phase distributions in the pump chamber leads to excessive output flow pulsation. Hence, maintaining an optimal vane tip clearance of ∼0.035 mm is suggested to ensure a smooth flow output and prevent excessive flow leakage under the specified operational conditions. In addition, timely maintenance of the RVP is recommended, and operating the pump under conditions of high air inlet should be avoided whenever possible.

This research was supported by the National Natural Science Foundation of China (Grant No. 52376031) and the School–Enterprise Collaborative Innovation Fund for graduate students of Xi’an University of Technology (Grant No. 252062402). The authors extend their sincere gratitude to Professor Bofeng Bai, Dr. Ming Cao, and Dr. Lina Yang for their invaluable support throughout this study.

The authors have no conflicts to disclose.

Denghui : Conceptualization (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Supervision (lead); Writing – original draft (equal); Writing – review & editing (equal). Wanru Lei: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Yijin Wang: Data curation (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.