Leakage Clearance Research Articles

Wear modelling and sealing performance prediction of C/C composite finger seal

Due to the high speed and high pressure operating conditions, wear between the finger seal and the rotor is inevitable, and this leads to an increase of leakage clearance and becomes an important factor affecting the seal life. The theory of anisotropic elasticity was employed to describe the mechanical characteristics of the seal material. An improved Archard wear model was proposed to calculate the nodal wear rate, and the local wear direction was also determined. The wear simulation model and procedure were established based on the Arbitrary Lagrangian Eulerian adaptive mesh strategy. The effects of material density, fabric orientation, pressure differential and rotor speed on the wear characteristics of finger seal and the change of leakage clearance caused by wear loss were studied. The results show that the wear first occurs at the heel area of the finger foot bottom and gradually extends to the toe area. C/C (carbon fibre reinforced carbon matrix) composite finger seal with higher material density has smaller leakage clearance and wear depth. With the increase of fabric orientation angle, the wear depth in the heel area of finger foot is smaller, and the circumferential wear area is narrower. The average wear depth of finger seal decreases with the orientation angle. When the orientation angle is 0, the seal leakage clearance is smaller. The increase of pressure differential not only aggravates the seal wear, but also expands the wear area, and also increases the average contact pressure and leakage clearance. Increasing the rotor speed make the wear depth of the heel area of finger foot gradually increase, but the wear area becomes narrower. The average contact pressure decreases and the average leakage clearance increases. This study provides a theoretical method and basis for the prediction and evaluation of the wear life and performance of C/C composite finger seal.

Effect of thermal deformation on leakage clearance of claw hydrogen circulating pump for fuel cell system

The leakage clearance have a strong impact on the performance and reliability of hydrogen circulating pump in a fuel cell system, and the thermal deformantion is the most significant factor because of its high working temperature, small size. In this paper, the distribution of leakage clearance with different temperatures and rotors materials is obtained. The radial leakage clearance decreases with the increase of temperature in the working chamber. The axial clearance is increased by about 100 μm at rated working condition. When the material of rotors is aluminum alloy, the minimum radial clearance under rated working condition is about 8.5 μm, and the risk of interference is large. When the material of rotors is structural steel and titanium alloy, the radial clearance is maintained above 30 μm, and the risk of interference is small. The variation of leakage clearance leads to the variation of leakage area. The variation of total leakage clearance first increases and then decreases with the increase of angle. The minimum variation of total leakage area is 6.25 mm2 at 0° and the maximum is 7.578 mm2 at 84°. The total leakage area increased by 7.144 mm2 on average. The results can be used as guidelines for the structural optimization of hydrogen circulating pump.

Fault Modeling and Simulation of Engine Driven Pump in Aircraft Hydraulic System

Abstract The structure of engine driven pump in aircraft hydraulic system is complex, its fault forms are diverse, and the fault transmission path is complex and multi factor coupling. In view of the above problems, based on the in-depth analysis of the working principle of the engine driven pump, the engine driven pump model is established, and the leakage fault injection mechanism of the friction pair is added. The relationship between the flow fluctuation characteristics of engine driven pump and the changes of pump spindle speed, swashplate inclination and pump outlet cavity volume are obtained through model simulation analysis. The mechanism of internal leakage fault of plunger pair, port pair and slipper pair are analyzed, the influence of leakage clearance parameters on key performance indexes such as pump pressure and flow are simulated, and the simulation test data of internal leakage fault of pump are obtained. The simulation results show that the inclination angle of swashplate and spindle speed are positively correlated with the output flow of the pump. The increase of pump outlet volume can effectively alleviate the pressure and flow fluctuation behind the pump. In addition, when the radial clearance of the plunger pair of this type of pump exceeds 0.2mm, the pump fault characteristics begin to appear. At the same time, the load change behind the pump will affect the characterization of the fault of internal leakage of the pump.

3D transient numerical simulation of a helical roots air compressor for FCVs

Oil-free helical Roots air compressors which have great application potential in air circulation systems for high-power fuel cell systems, such as commercial fuel cell vehicles (FCVs), have the advantages of active adaptation, low cost, large flow rate and high reliability. In this study, a three-dimensional transient numerical simulation model of a helical Roots air compressor was established by considering all leakage clearances. In this study, hexahedral structured dynamic grids were generated in the working chamber and the rotating angle was updated at an increment of 1° to ensure the mesh quality of the entire solving domain. The accuracy of the simulation model was validated using experimental data, and the maximum deviation was less than 4.0%. Based on the simulated results, the pressure field and variation of the pressure field with the rotation angle are presented. It shows that the pressure fluctuation at the discharge side was larger than that at the suction side. The influence of various leakage clearance on the volumetric efficiency was analyzed comparatively. Additionally, the flow field characteristic of clearance was revealed. It is found that the rotor tip clearance was the major factor for the reduction of volumetric efficiency when the size was larger than 0.12 mm instead of the interlobe clearance. It is suggested that more attention should be paid to control the clearance size to ensure the performance of helical Roots air compressors.

Numerical and experimental investigation on thermal-hydraulic characteristics of a scroll expander for organic Rankine cycle

A new numerical model of a scroll expander is developed applying the deterministic method to analyze its thermal-hydraulic characteristics for the application to the organic Rankine cycle. Several geometrical models are refined as a subsection function of the orbiting angle and area models regarding suction and tangential gas force are newly developed. The geometrical models match well with the actual values extracted from the 3D computer-aided design model. In addition, mathematical models that reflect the effects of mass leakage and heat transfer are integrated in a closed-loop iteration process. Two critical unknown parameters of overall leakage clearance and wall-to-gas temperature ratio are identified via the coupling of the simulation model and experimental data. Experiments were conducted on a 1 kW scale organic Rankine cycle test rig using R245fa as the working fluid. The identified parameters are expressed as functions of the operating conditions and the simulation model is validated within 5% error range. The best isentropic efficiency appeared to be 62.8% according to the test results of the scroll expander. The simulation model allows the analysis of various aspects of the internal expansion process that could not be examined through empirical or semi-empirical models. Mechanisms regarding under and over-expansion and heat transfer are thoroughly investigated for profound understanding of the scroll expander transient behavior and energy level degradation.

Analysis of total leakage of finger seal with side leakage flow

The flow through the finger interstices and the micro voids in rough contact interface between finger sticks is considered as the side leakage flow of finger seal. The porous approach that the side leakage flow of finger seal is transformed into the seepage flow through a porous medium was proposed. The CFD model of total leakage of finger seal was established. When the side leakage flow is considered, the relative increment of total leakage of finger seal ranges from 3 to 6% in most cases. The work also shows that the total leakage without side leakage flow is between the leakage flowing into the main leakage clearance and the leakage exiting from the main leakage clearance when considering the side leakage flow.

Leakage Model of Axial Clearance and Test of Scroll Compressors

Leakage clearance plays an important role in guaranteeing high efficiency of scroll compressors. In view of the shortcomings of the existing leakage models of axial clearance of scroll compressors, a modified Fano flow model and a turbulence model are presented based on the flow characteristics of fluid in the leakage passage under actual working conditions. Firstly, according to the Fano flow energy equation and the turbulence theory, two kinds of leakage models, Fano flow model and turbulence model, of axial clearance are established. Then, the established models are verified through an experimental platform established to measure the axial clearance leakage in the working process of a scroll compressor, and the measured values are compared with the values obtained from the two theoretical models. Finally, the effect of such factors as pressure difference, clearance amount, spindle speed on the axial clearance leakage is analyzed. Results show that the two established models can precisely reflect the variation law of axial clearance leakage of scroll compressor under different working conditions. In particular, the Fano flow model is more suitable for predicting the clearance leakage when the spindle speed is low (less than 3 500 r/min) and the clearance is small (less than 0.025 mm), whereas the turbulence model is suitable for high spindle speed (more than 3 500 r/min) and large clearance (more than 0.025 mm).

CFD Modelling of Coupled Heat Transfer between Solid and Fluid in a Twin Screw Compressor

Temperature variation in a compressor is very critical for compression performance and efficiency. To accurately predict the thermal dynamic behaviour of a compressor, heat transfer between metal components and the working fluid needs to be properly evaluated in CFD simulation. Thermal expansion of solid components due to temperature increase may also cause excessive abrasion of metal parts and changing in leakage clearance. Solving conjugate heat transfer (CHT) of twin screw compressors not only needs to deal with complex moving rotor geometry, but also needs to consider huge difference of temperature propagation time scale inside different media. Typically, heat propagation is much slower in solid than in compressed gas. Therefore, it may take many compressor revolutions to reach a stable CHT solution if heat conduction inside solid and thermodynamic temperature change of compressed gas are directly coupled in a CFD simulation. The run time of simulation may become impractical.In this paper, an innovative approach solving conjugate heat transfer using the Mixed Timescale Coupling method between different media will be described in detail. This approach can address time scale difference in heat propagation. The proposed approach will be applied to a twin screw compressor. Simulation results will be compared with available test data. Effects of conjugate heat transfer will be evaluated by comparing the simulation results for the cases with and without considering conjugate heat transfer. Thermal expansion of the solid rotor will also be evaluated using predicted temperature distribution. Simulation will also demonstrate that the approaches used in the paper are robust, fast, and user friendly, and can be readily applied to industrial compressor systems.

Prediction on the lubrication and leakage performance of the piston–cylinder interface for axial piston pumps

Hydraulic piston pump has the advantages of high efficiency and low-pressure pulsation and has been widely used in the field of construction and agriculture machinery. However, the oil leakage problem, especially the internal leakage of the piston, seriously affects the efficiency of the pump and causes unstable state and high noise. First of all, to improve the performance of the hydraulic pump, a nonlinear mathematical model of the Reynolds equation and energy equation was established to predict the oil film thickness and pressure distribution. What's more, the influence of multiple parameters such as pressure, temperature, swash plate angle, leakage clearance, and the length of leakage flow path on the internal leakage of the piston–cylinder interface was explored. Last but not the least, a test rig was built to measure the corresponding values and to evaluate the effectiveness of the mathematic model. Simulations and experiments consider the variables such as pressure, temperature, and swash plate angle. Both results show that the nonlinear mathematical model could predict the piston pair leakage accurately, so it could be an effective way to improve the design of the piston–cylinder surface structure, gaining higher efficiency.

Leakage effect analysis on the performance of a cylindrical adjustable inertance tube

The inertance tube plays a significant role in improving the performance of the Stirling type pulse tube cryocooler by providing the desired phase angle between the mass flow and pressure wave. The phase angle is highly depended on the inertance tube geometry, such as diameter and length. A cylindrical threaded root device with variable thread depth on the outer screw and inner screw creates an adjustable inertance tube whose diameter and length can be adjusted in the real time. However, due to its geometry imperfectness, the performance of this threaded inertance tube is reduced by the leaks through the roots between the two screws. Its phase angle shift ability is decreased by 30% with the leakage clearance thickness of 15.5 μm according to both the theoretical prediction and the experimental verification.

A New Scroll-Type Air Motor With Magnetic Spirals

The scroll-type air motor, also named the scroll expander, has been widely used for different applications due to its characteristics of compact structure and high energy conversion efficiency. However, the leakage and the friction result in non-negligible energy losses. This paper presents the recent work on developing a new scroll-type air motor with mounted permanent magnetic spirals and investigates its potential in leakage reduction and efficiency improvement, especially at low-pressure air supply conditions. A method for the implementation of the magnetic scroll air motor is proposed. A prototype is manufactured, and initial experimental tests are conducted to study the generalized torque distribution. A mathematical model for the magnetic scroll air motor is developed, and a corresponding simulation study is presented. The study shows that the proposed magnetic scroll air motor structure is feasible in terms of manufacturing and has the potential to reduce the air leakage and, thus, to improve the energy efficiency by a maximum of around 15% at a supply pressure of $2 \times {10^5}$ Pa, with a flank leakage clearance reference of 0.06 mm.

The Influence of Air Gap Thickness between the Stator and Rotor on Nuclear Main Pump

The canned motor pumps are commonly used as main umps in nuclear primary circuit systems, because the typically outward leakage clearance in general centrifugal pumps does not exist in the canned motor pumps. The longer air gap thickness between the motor stator and the rotor, the more cooling liquid flowing through the air gap, which results the heat exchange becomes better between the motor and the cooling liquid and the temperature rise of the motor is lower. These are all good for improving the motor efficiency at the angle of the view, the temperature rise. From another angle of the view, the electromagnetism, the longer thickness between the motor stator and the rotor, the weaker interaction between the stator and rotor magnetic fields, which results the Lorentz force becomes weaker and the torque becomes smaller, and also the motor efficiency becomes lower. Therefore, there is an optimum value of the air gap thickness between the stator and the rotor, to balance the temperature rise and the electromagnetism interaction. In this paper with the case of 30kW canned motor, the coupled analysis of the heat exchange and the electromagnetic performance was studied. The influence of the air gap thickness on the motor temperature rise and on the electromagnetic performance was discussed respectively, and then the optimum value of the air gap thickness was given.

Investigation on the influence of leakage clearance on the flow field and performance of scroll hydraulic pump

In the present paper, the computer fluid dynamics(CFD) with dynamic mesh model had been applied in scroll hydraulic pump to obtain its flow field at different leakage clearance. The fluid force on the orbiting scroll, the mass flow rate and the hydraulic efficiency at different leakage clearance were calculated based on the flow field data. The results indicated that when the leakage clearance increased from 0.5mm to 1.5mm, the average pressure, maximum of pressure fluctuation, leakage jet flow velocity, shaft power, cavitation degree decreased and the leakage flow rate increased. If the leakage clearance was 2.0mm, the high pressure discharge fluid flowed through the clearance and led to the increase of the average pressure and fluid force. When the leakage clearance is 1.0mm, the average pressure is far lower than that at the 0.5mm clearance, and the hydraulic efficiency is the highest.

Progressive wear prediction of C/C composite finger seal based on ALE adaptive meshing strategy

Leakage clearance between the finger seal and the rotor is influenced by the seal wear, and a large leakage clearance may cause sealing failure. The aim of this study is to develop a predictive method to simulate the progressive wear of C/C composite finger seal. A finite element model is presented for the finger seal dynamic motion. A wear model is built by calculating the local wear rate for the nodes on finger foot bottom and by introducing an Arbitrary Lagrangian Eulerian adaptive meshing strategy. Progressive wear analysis is implemented by integrating the finger dynamic motion and the wear model. The results show that the contact pressure of finger seal is decreased and the leakage clearance is increased with operating time due to progressive wear. Compared to the case that the non-woven cloth is parallel to rotor surface (O1) and the case that the non-woven cloth is perpendicular to rotor surface but the laminated direction is perpendicular to sliding direction (O2), low leakage and low wear are obtained when the non-woven cloth is perpendicular to rotor surface and the laminated direction is the same as sliding direction (O3). In addition, the wear depth of finger foot and the leakage clearance between the finger seal and rotor are both increased with pressure differential and rotor speed.

Effect of honeycomb seals on loss characteristics in shroud cavities of an axial turbine

The loss in efficiency due to shroud leakage or tip clearance flow accounts for a substantial part of the overall losses in turbomachinery. It is important to identify the leakage loss characteristics in order to optimize turbomachinery. At present, little information is available in the open literature concerning the effect of honeycomb seals on the loss characteristics in shroud cavities of an axial turbine, despite of the widespread use of the honeycomb seals. Therefore, interaction between rotor labyrinth seal leakage flow with and without honeycomb facings and main flow is investigated to provide the loss characteristics of the mixing process of the re-entering leakage flow into the main flow. The effects of honeycomb seals on the flow in shroud cavities and interaction with the main flow are analyzed. An additional study on the impact of subtle shroud cavity exit geometry is also presented. The investigation results indicate that the honeycomb seal affects the over tip leakage flow and reduces mixing losses when compared to the solid labyrinth seal. The leakage flow interactions with the main flow have considerably changed the flow fields in the endwall regions. The proposed research reveals the effects of honeycomb seals on the loss characteristics in shroud cavities and the impact of subtle shroud cavity exit geometry, and it is helpful for the design optimization of turbomachinery.

Analysis and Numerical Simulation of Leakage Flow of Low Speed High Torque Water Hydraulic Motor’s Plain Flow Distribution Pair

In order to ensure volumetric efficiency of low-speed and heavy-torque water hydraulic motor, the relationship between leakage flow and clearance of motor’s plain flow distribution pair in which water is used as medium is analyzed, and it’s concluded that the clearance of port plate and rotor end should be controlled below 5μm. In addition, the port plate flow field was simulated by using CFD software, and got that the leakage flow of numerical simulation is greater than theoretical calculation, and the numerical simulation values are more credible. It is also found that the maximum flow velocity is 16 m/s when outer ring is working, and when inner ring is working the maximum flow velocity is 20 m/s.

Leakage research on supercritical carbon dioxide fluid in rolling piston expander

The leakage research on supercritical carbon dioxide fluid in rolling piston expander was investigated experimentally. Using expander instead of throttle valve is an important way to improve the efficiency of carbon dioxide refrigeration system. However, the supercritical fluid leakage in expander is serious and is the main factor affecting the expander’s efficiency. This paper presented and compared four classic leakage models. The analysis indicated that laminar leakage model is suitable in leakage simulation of expander. A leakage test system, including the leakage test part which has two types of leakage specimens with different gaps ranging from 5 to 15 μm, was established. The experimental results indicated that lubricant film played an important role. When the leakage clearance of cylindrical specimen was 5 μm, the mass flow rate of leakage was about 0.88 g s−1. The data was 3.638 g s−1 with leakage clearance being 10 μm and 7.11g s−1 with leakage clearance being 15 μm. A modified leakage model was developed, whose average deviation was within 10% compared with the experimental data. At last, this paper simulated the leakage in rolling piston expander presented by Tian et al. (2010). The leakage between rolling piston and cylinder was the most serious part with the value up to 0.04 kg s−1.

Moment Whirl due to Leakage Flow in the Back Shroud Clearance of a Rotor

Recent studies on the moment whirl due to leakage flow in the back shroud clearance of hydro-turbine runners or centrifugal pump impellers are summarized. First, destabilizing effect of leakage flow is discussed for lateral vibrations using simplified models. Then it is extended to the case of whirling motion of an overhung rotor and the criterion for the instability is obtained. The fluid moment caused by a leakage clearance flow between a rotating disk and a stationary casing was obtained by model tests under whirling and precession motion of the disk. It is shown that the whirl moment always destabilizes the whirl motion of the overhung rotor while the precession moment destabilizes the precession only when the precession speed is less than half the rotor speed. Then vibration analyses considering both whirl and precession are made by using the hydrodynamic moments determined by the model tests. For larger overhung rotors, the whirl moment is more important and cause whirl instability at all rotor speed. On the other hand, for smaller overhung rotors, the precession moment is more important and cancels the destabilizing effect of the whirl moment.

Simulation research on scroll refrigeration compressor with external cooling

The compression process can approach the isothermal one when compressors are cooled in the compression process, and the compression work can be reduced. The paper presents a special external cooling structure for scroll refrigeration compressor. A mathematical model is established to simulate the compressor. All of boundary conditions in heat transfer model are set to coincide with the real operation conditions as much as possible. The simulation results indicate that the isentropic efficiency is improved by 7.4% and the discharge temperature are reduced by 23 °C when the water of 30 °C flows through the cooling structure. Furthermore, with the simulation results, the details of heat transfer between refrigerant and work chamber are analyzed and figured out. The influence of water temperature, dimension of cooling structure, leakage clearance, rotational speed and pressure ratio on the performances of compressor is discussed. And some useful results can be used to enhance the effect of water cooling and the performance of compressors.

오비터 진공펌프 성능해석

Orbiter mechanism has been applied to vacuum pump design for small oxygen generator where low vacuum of about 200 ㎜Hg is required. Performance of the designed vacuum pump has been numerically investigated: calculated volumetric and adiabatic efficiencies were 69.7% and 83.9%, respectively for leakage clearance of 10㎛. Total efficiency of the orbiter vacuum pump was 77.5%. At the shaft speed of 1700 rpm suction displacement volume of 6.3㏄ provided discharge flow at the rate of 2.3 liter/min with power consumption of 10.1Watt. Torque variation of the orbiter pump was only about 20% of that of diaphragm pump.