Minimum Film Research Articles

Exploring the range of applicability of anisotropic optical detection in axially coordinated supramolecular structures

Ensuring the highest accuracy in determining the molecular assembling and the preservation of the chemical and physical properties of the molecules during the growth of organic layers requires a real-time monitoring of film formation. In this respect, optical techniques are preferred since they result in minimum organic film damage. Among these techniques, Reflectance Anisotropy Spectroscopy (RAS) proved to be the one with the highest sensitivity due to the development of intrinsic anisotropies in the optical response of molecular films, where molecular packing is driven by Van der Waals interactions. Recently, we proposed an original strategy to enable the growth of organic films via molecular self-assembly through highly directional coordination bonds. Herein, we consider a straightforward supramolecular structure employing axial bonds, based on the 1:1 assembly of a CoII porphyrin (CoTPP) and a linear ligand (DPNDI). This system is characterized by a rather isotropic structure that might, in principle, result in a weak RAS signal. Therefore, in the present work, we critically assess the range of applicability of such a spectroscopy. A number of other surface science techniques, including Low Energy Electron Diffraction (LEED), photoemission spectroscopies (PES and IPES) and Atomic Force Microscopy (AFM) is employed to fully characterize the axially coordinated molecular film.

Finite element modeling and simulation of mixed elastohydrodynamic lubrication for the finite line contact under tilting loads

Misalignment is a prevalent problem in rotor machinery. This paper proposes a numerical analysis for the mixed EHL of finite-line contacts considering tilting loads based on FEM. The Reynolds and linear elasticity equations, and Greenwood-Tripp contact model are coupled in an iterative process to solve the fluid-solid/solid-solid interaction under the compound external force and moment. Lubrication characteristics at both ends of the cylinder are analyzed and an asymmetric profile design scheme is presented to improve the lubrication performance at the cylinder's lower end. It is found that proper profile parameters can reduce EHL pressure by 40%, increasing the minimum oil film thickness by up to 100%, and thus reduce asperity contact pressure by up to 50%.

Simulation based analysis to study the effect of compression ring crown height in ring liner assembly on the performance of four stroke petrol engine

Among different piston rings of an Internal Combustion Engine (IC), the top compression ring-liner assembly shared a momentous amount of the total frictional losses, particularly at the Top Dead Center (TDC) and Bottom Dead Center (BDC) where boundary lubrication subsists. In the present study, the lubrication mechanism in piston ring cylinder liner conjunction for top compression ring using one dimensional Reynolds equation assuming axisymmetric contact was studied. The hydrodynamic and mixed lubrication regimes were taken during the study. Gumbel boundary conditions were used for Reynolds equation solution. For boundary pressure calculation, first, combustion chamber pressure variation was determined, then, interring pressure was calculated assuming orifice volume method by solving mass flow rate through crevice volumes using Runge-Kutta 4th order method. The Reynolds equation was solved using finite difference method. Whole solution process was repeated for different engine speed and crown height of top compression ring to study the effect of them in wear and power loss. First, the solution was found for 3000, 5000 and 7500 RPM keeping constant crown height of 10 µm. Average power loss was found to be increased with the increasing engine speed. There was increased in oil film thickness around mid-stroke with the increased engine speed, but there was no significant change in film thickness with the engine speed at the TDC and BDC positions. Then, the solution was again repeated for different RPM varying the crown height. The compression ring with lower crown height showed the better wear performance by increasing the minimum film thickness in critical zones of engine cycle for all RPM. The crown height of 5,7 and 9 µm gave the minimum power loss (77.89, 163.47 and 297.33 W) for 3000, 5000 and 7500 rpm. Moreover, the power loss in the range of 5-9 µm had no significant differences.

Improvement of lubrication performance of sliding pairs with multi-depth groove textures based on genetic algorithm

During the wear and tear process of bearings, the friction coefficient between the friction pairs can be effectively decreased by employing the suitable surface texture on the frition surface. In the study, the distribution and depth variation of the surface texture were used as variables, and the genetic algorithm was used for iterative optimization to obtain the optimal texture distribution and depth. The friction and wear performance of the rectangular texture bearing sliding blocks was optimized. The depth of the texture was represented by a 4-bit binary number, and different binary numbers were set to represent different texture depths. Finally, the genetic algorithm was used for continuous iteration and evolution to obtain the optimal texture combination. The study showed that, compared with the regular texture with a depth of 0.2 μm, the friction coefficient decreased by 15.0% under the optimal texture with a non-uniform depth. Simultaneously, compared with the regular 3 μm deep texture, texture with a optimized depth makes the friction coefficient decreased by 37.5%, and the minimum oil film thickness increased by 0.979 μm. The optimal texture and oil film thickness combination obtained from the study can effectively reduce solid contact force and alleviate mechanical wear.

Non‐spherical shape optimization of artificial hip joint based on elastohydrodynamic lubrication analysis

AbstractStructural optimization design is a promising way to further promote the lubrication performance of the artificial hip joint (AHJ). In this work, a non‐spherical shape optimization design strategy for the femoral head is firstly proposed. The generatrix, the line rotated about an axis that generates the bearing surface of the femoral head, is characterized based on a third‐order Bezier curve. The performance of the AHJ under steady‐state condition is analyzed by solving the governing equations of the elastohydrodynamic lubrication problem, and the minimum lubricating film thickness of the AHJ is maximized by adjusting the positions of four control points of the Bezier curve based on the simulated annealing method. The results show that, when compared with the standard spherical femoral head, the optimized non‐spherical femoral head can significantly improve the minimum thickness and reduce the maximum pressure of the lubricating film of the AHJ. Through the analysis of the optimization process, the results further reveal that the shape of the femoral head has a direct impact on the thickness distribution of the lubricating film. This research will provide guidance and theoretical support for novel design of AHJs.

Analysis of Modified Finite Length Journal Bearing under Position Perturbation

The performance of journal bearings is significantly affected by the presence of misalignment, which is usually an accompanying problem for this type of bearing. This includes exceeding the design limits for the maximum pressure and the minimum film thickness levels, which affect, in other words, the load-carrying capacity of the system. In addition, it raises the possibility of increasing the wear rate at the bearing edges and increases the friction coefficient at high levels of misalignment. This paper deals with the problem of finite-length misaligned journal bearings, considering a novel comparison between two cases of misalignments: the general 3D misalignment and the vertical misalignment problems for modified bearings. The effect of introducing a variable axial bearing profile on the bearing characteristics and the time responses of the rotor bearing system under position perturbation has also been investigated. The numerical solution of this hydrodynamic problem is based on the finite difference method using Reynolds boundary conditions method. Results show that using a variable bearing profile improves bearing characteristics, such as increasing the minimum film thickness significantly and reducing the pressure levels in addition to reducing the friction coefficient. Furthermore, the modification enhances the rotor-bearing stability under position perturbation, extending the speed range for a safe operation.

Investigation and control of the ticking noise of the full floating piston pin

The focus of this work is to investigate the ticking noise produced by the vibro-impacts between piston pin and connecting rod small head bearing. Numerical calculation and experimental verification methods were adopted and a novel method to control this intermittent noise was proposed. The dynamic model of crankshaft connecting rod piston with full floating piston pin was established and the essence of piston pin ticking was analyzed. The occurrence time of the ticking corresponded to the crankshaft angle when the resultant force of piston pin passed through the 0 axis. At the moment of impact, the peak hydraulic pressure of lubricating oil was found to be proportional to the impact strength. In order to suppress this noise, the oil groove on the bearing was designed considering the minimum oil film. After adding the oil groove, the impact intensity was significantly reduced and the noise was even eliminated. The calculated results were in good agreement with the experimental tests, and the research of this work provided a reference for controlling the ticking noise of piston pin.

Dual experimental-numerical study of oil film thickness and friction in a wide elliptical TEHL contact: From pure rolling to opposite sliding

This study presents a dual experimental-numerical approach to simultaneously investigate oil film thickness and friction in a wide elliptical thermo-elastohydrodynamic (TEHL) contact over sliding conditions ranging from pure rolling to opposite sliding (slide-to-roll ratio SRR between 0 and 4). Experiments are performed on a barrel-on-disk tribometer instrumented for friction and optical oil film thickness measurements. In addition, a TEHL numerical model based on the full system approach is used to predict film thickness and friction response variation with sliding. The trends of film thickness variation with SRR are different for SRR lower and higher than 2 because of the genesis of the thermal viscosity wedge effect at opposite sliding. The friction coefficient measured experimentally increases with increasing SRR and then stabilizes at high SRR. The numerical model can accurately predict the film thickness over a wide range of sliding conditions. Also, the model predicts friction with a good accuracy for SRR>2. In contrast, for SRR<2, numerical results overestimate friction coefficient. In addition, a numerical parametric investigation is realized to understand the effect of varying ambient temperature, normal load, and entrainment velocity on film thickness and friction over a wide range of sliding conditions. Numerical results are then used to create a simple formula for an accurate estimation of the minimum film thickness based on classical dimensionless parameters and SRR for a wide range of sliding conditions (0≤SRR≤4).

Analysis of Nonlinear Time-Domain Lubrication Characteristics of the Hydrodynamic Journal Bearing System

The nonlinear time-domain lubrication characteristics of the hydrodynamic journal bearing system are studied in this paper. The motion equation of the hydrodynamic journal bearing system is established based on the balance of the relationship among the water film force, journal inertia force, and external load. The water film pressure distribution of the sliding bearing is calculated by the finite difference method. Firstly, the variation law of the water film pressure distribution with time under the external periodic load is calculated considering the inertial force of the journal. The influence of the initial eccentricity on the orbit of the journal center is studied. Secondly, the maximum water film pressure, the orbit of the journal center, eccentricity, water film pressure, and the minimum water film thickness of the bearing under the action of circumferential and unidirectional periodic external loads are calculated, and the effects of inertial force and rotational speed on the dynamic characteristics of the bearing are analyzed. Finally, the water film dynamic characteristics under low speed and heavy load are studied. The result shows that the pressure of the dimensionless water film caused by inertial force is reduced by 7 to 10 percent at the rotational speed between 200 r/min and 800 r/min, which means that the influence of inertia force cannot be ignored.

Transient analysis for offset-elliptical journal bearings under misalignment and dynamic loading

Although the offset-elliptical journal bearings (OEJBs) are widely used in industry, there are few researches on the OEJBs, especially under the conditions of journal misalignment and sudden load. To overcome such deficiency, the dynamic response of misaligned OEJBs subjected to the dynamic load has been studied in this paper. Based on the Reynolds equation, Euler equation, and kinetic equations, a hydrodynamic model is developed and validated to conduct the transient analysis for the OEJBs. Moreover, the modified film thickness of the misaligned OEJB is given. The partial derivative method and the finite difference method are adopted to solve the lubrication equations. The correlations of bearing characteristics such as the minimum film thickness, maximum film pressure, journal orbits, and time-varying dynamic coefficients with the misalignment and dynamic load are studied. In addition, the effects of the misalignment, rotating speed, and clearance ratio on the transient performance of OEJBs under the dynamic load are discussed in detail. The results indicate that the journal misalignment and dynamic load have a significant impact on the transient performance of OEJBs. The decrease of the film thickness caused by the journal misalignment can be aggravated by the suddenly applied load. The proposed model can provide theoretical fundament in the design and monitoring of the dynamically loaded bearing-rotor system.

Effect of boundary slip on elastohydrodynamic lubrication with arbitrary entrainment angle in elliptical contacts

PurposeThe purpose of this study is to describe and observe the influence of boundary slip associated with an arbitrary entrainment angle on the contact lubrication properties of ellipses.Design/methodology/approachBased on the modified Reynolds equation, the boundary slip of any angle is considered in the elliptic contact, and numerical simulation is carried out. In the above calculation, the progressive mesh densification method is used, which greatly reduces the computation time.FindingsThe results indicate that the variation of film thickness corresponding to different entrainment angles is distinct from those without considering boundary slip. In addition, boundary slip reduces the central film thickness and minimum film thickness, which makes the hydrodynamic pressure distribution smoother.Originality/valueThe present study focuses on the specific condition with the arbitrary direction of rolling and sliding velocity found in hypoid gears and worm, and some other components. The influence of boundary slip associated with arbitrary entrainment angle on the lubrication film thickness in elliptical contacts is first revealed, which improves a good understanding of elastohydrodynamic lubrication characteristics.

Droplet-particle collision dynamics: A molecular dynamics simulation

Molecular dynamics simulations are conducted to investigate the normal collision of the droplet-particle system. A wide range of influencing factors including the initial Weber number (30 ≤ We≤250), equilibrium contact angle (45° ≤ θ ≤ 145°), droplet-particle size ratio (0.5 ≤ Ω ≤ 2), temperature (293 K ≤ t ≤ 573 K) and composition are examined. Based on the atomic trajectories and kinetic energy dissipation during collision, temporal evolutions of evaporation molecules, dimensionless spreading diameter and film thickness are quantified and compared with available formulations. A total of seven collision outcomes are observed, which can be summarized in the regime map. Two types of disintegration, i.e. ligament-structured disintegration and conical-structured disintegration, exhibit distinct energy dissipation modes. An increasing We and a decreasing θ would lead to an increase in the maximum spreading diameter, while the minimum film thickness is less sensitive to θ. Additionally, evaporation during collisions is primarily influenced by the initial Weber number, because of the formation of satellite droplets.

Effects of Pretreated Phosphogypsum and Granulated Blast-Furnace Slag on the Rheological Properties of the Paste Excited by NaOH

The main component of phosphogypsum (PG) is CaSO4·2H2O. PG contains a few impurities, heavy metals, and radioisotopes, which limit the use of PG and pose a danger to the environment. In this study, under the excitation of a sodium hydroxide solution, the rheological properties of a paste with granulated blast-furnace slag (GGBS) and PG treated with ultrasonic water washing were investigated. Experimental results showed that the ratio of GGBS to PG and the amount of sodium hydroxide solution significantly affect the density and viscosity of the paste, but the effect patterns of both are different. The maximum viscosity was 498 mPa·s when the ratio of GGBS to PG was 4:1. When the ratio changed from 3:2 to 1:4, the viscosity of the paste gradually decreased by 15.5%, 32.1%, 36.1%, and 46.8%, respectively. In contrast, the ratio of GGBS to PG had a greater effect on the viscosity than the amount of sodium hydroxide solution in terms of the standard consistency water consumption, viscosity, and water release ratio. The larger the PG ratio, the smaller the density, viscosity, and water release ratio of the paste. The variation in the ratio of GGBS to PG had a significant effect on the water film thickness of the paste, demonstrating that the larger the PG mixture, the larger the water film thickness of the paste, which reached 1.122 μm, 2.31 times the minimum water film thickness of the paste. At the same time, the water film thickness of the paste was negatively correlated with the water consumption of the standard consistency, viscosity, and water release ratio, and was positively correlated with the fluidity.

Numerical Solution for Thermal Elastohydrodynamic Lubrication of Line Contact with Couple Stress Fluid as Lubricant

In this paper, the detail analysis of the influence of thermal and non-Newtonian aspects of lubricant (couple stress fluid) on EHL line contact as a function of slide-roll ratio is presented. The novel low complexity FAS(Full approximation scheme), of Multigrid scheme, with Jacobi dipole and Gauss Seidel relaxationis used for the solution of coupled equations viz.modified Reynolds equation, film thickness equation and energy equation satisfying appropriate boundary conditions. The analysis reveals the combined influence of non-Newtonian, thermal and slide-roll ratio (of bearing movingwith different speeds) on pressure, film thickness and pressure spike covering wide range of physical parameters of interest. Results show that pressure spike is strongly influenced by thermal, slide-roll ratio and non-Newtonian character of lubricant with negligible effect on overall pressure distribution. Also, minimum film thickness is slightly altered and it increases with increase in couple stress parameter. These findings confirm the importance of non-Newtonian and thermal effects in the study of EHL.

Lubrication analysis of stern bearing during ship severe turning maneuver

Stern shaft deformation caused by ship maneuvering will lead to stern bearing lubrication failure and wear. Based on the displacement superposition method, the deflection equation of the stern shaft under the action of the interface force and the propeller load was established. The fluid-solid coupling model of the stern bearing was established by using the averaged Reynolds equation, the Greenwood-Tripp model, and the influence coefficient method. The model was experimentally validated. The influence of the shaft bending deformation caused by the propeller load change on the lubrication characteristics is discussed. The results show that the shaft bending deformation leads to the changes in lubrication characteristics along the axial and circumferential directions. Compared with the vertical load alone, the existence of horizontal propeller load will significantly reduce the minimum nominal oil film thickness and friction coefficient. Under hydrodynamic lubrication, the direction of horizontal load has little effect on friction coefficient.

Study on transient performance of tilting-pad thrust bearings in nuclear pump considering fluid-structure interaction

To study the lubrication performance of tilting-pad thrust bearing (TPTBs) during start-up in nuclear pump, a hydrodynamic lubrication model of TPTBs was established based on the computational fluid dynamics (CFD) method and the fluid-structure interaction (FSI) technique. Further, a mesh motion algorithm for the transient calculation of thrust bearings was developed based on the user defined function (UDF).The result demonstrated that minimum film thickness increases first and then decreases with the rotational speed under start-up condition. The influence of pad tilt on minimum film thickness is greater than that of collar movement at low speed, and the establishment of dynamic pressure mainly depends on pad tilt and minimum film thickness increases. As the increase of rotational speed, the influence of pad tilt was abated, where the influence of the moving of the collar dominated gradually, and minimum film thickness decreases. For TPTBs, the circumferential angle of the pad is always greater than the radial angle. When the rotational speed is constant, the change rate of radial angle is greater than that of circumferential angle with the increase of loading forces. This study can provide reference for improving bearing wear resistance.

Lubrication impact of ferrofluid on cylinder‐plate system: A comparative study using two flow models

AbstractSqueeze‐film characteristics between a long cylinder and an infinite flat plate system are analysed. Ferrofluid (FF) lubricant controlled by transverse uniformly strong magnetic field is used for reducing wear and prolonging life of the system. The modified combined Reynolds equation is derived using two different FF flow models (Shliomis and Jenkins), and continuity equation. The expression for dimensionless film‐pressure, load‐carrying capacity and squeeze time are derived and calculated numerically for different control parameters. The results show significant improvement in the performance of the system when smaller value of minimum film thickness is chosen. Moreover, the phenomenon can be simulated by both the models equally with proper choice of the parameters.

Performance Analysis of the Cylinder Gas Film Seal between Dual-Rotor Reverse Shafts Considering Lubrication and Centrifugal Expansion Effects

The cylinder gas film seal between dual-rotor reverse shafts is a new method for sealing aircraft engines that has excellent developmental prospects. We propose a novel method for determining if the sealing ring is fully fluid-lubricated using the average Reynolds equation. Considering surface roughness and centrifugal expansion, we calculated the dynamic pressure, gas film force and friction power consumption with and without inlet pressure differences for each of the four rotation modes. The gas film’s dynamic pressure effects were weakest when the dual rotors rotated in a reverse direction (Mode 3), with or without differences in inlet pressure, since solid contact is most likely to occur under counter-rotation. When the dual rotors were in counter-rotation with an inlet differential pressure of 80 kPa and fixed inner and outer rotor speeds (e.g., 3000 rpm), we calculated the minimum sealing ring film thickness, leakage rate, friction power consumption and gas film force. Changes in outer rotor speed greatly influenced the minimum film thickness and the sealing ring’s performance. Meanwhile, changes in the inner rotor speed had little influence on the minimum film thickness and sealing ring’s lubrication performance. When the speed difference was fixed and the sealing ring was fully fluid-lubricated, lubrication performance changes were mainly affected by changes in centrifugal expansion caused by the outer rotor speed. Given a fixed rotor speed and under dual-rotor counter-rotation, we calculated the effects of inlet differential pressure changes on minimum gas film thickness, leakage rate, friction power consumption, gas film force and other sealing performance variables. Leakage rate and the friction power consumption increase as inlet differential pressure increases. Our results provide a theoretical reference for avoiding solid contact of the cylinder seal between dual-rotor reverse shafts.

Modelling and experimental validation of heat transfer behavior during trans-critical transients

Though designed to be operated in supercritical conditions, supercritical power systems could experience trans-critical transients in which the pressure transfers between supercritical and subcritical condition during startup, shutdown, or accident condition. In the present work the thermal–hydraulic code ATHLET-SC was upgraded to predict the heat transfer behavior during trans-critical procedures. Prediction model of dispersed droplet flow film boiling (DFFB) heat transfer correlation, critical heat flux (CHF) correlation, and the correlation of minimum film boiling temperature TMFB has been upgraded. It is demonstrated by the validation work that the prediction error is large in the inverted annular film boiling (IAFB) regime. However, steady-state IAFB heat transfer experiment at high-pressure condition is unavailable. Hence, a calibration approach to IAFB heat transfer correlation is developed based on trans-critical transient experiment. As validated, the change of the wall temperature during the trans-critical transient gets well predicted by the corrected Bromley correlation.

Automatic EHL film measurement via a long short-term memory neural network and neural network forest

Optical interferometry is a ubiquitous and accurate approach among EHL investigations. Despite the classical analysis of optics delineates a general principle for EHL film measurement, implementation of such method involves time-costing and artificial transformations from the optical interferometry image to an EHL film profile. Herein, we design a long short-term memory neural network to achieve completely automatic and immediate EHL film profile evaluation. In addition, we provide a neural network forest model that outputs central and minimum EHL film thickness from optical interferometry images in less than 1 s, enabling the fast evaluation of EHL film thickness. Hence, we envision this novel and neural network-driven method will have spectacular implications in EHL film evaluation.