Thermohydrodynamic Lubrication Research Articles

Effect of Al2O3 nanoparticle additives on the performance characteristics of rough journal bearings

This paper investigates the influence of Al2O3 nanoparticle additives on static and dynamic characteristics of water-lubricated rough journal bearings under thermo-hydrodynamic lubrication (THL). The time-dependent modified Reynolds equation and the approximated adiabatic energy equation have been formulated and solved numerically with boundary conditions and initial conditions. The results are presented for dimensionless maximum film pressure, dimensionless maximum film temperature, dimensionless minimum film thickness, eccentricity ratio, attitude angle, dimensionless friction force, spring constants, damping coefficients and mass parameters for both journal bearings with transverse roughness patterns and with longitudinal roughness patterns under a dimensionless load of 7. The results show that journal bearings with Al2O3 nanoparticle additives tend to significantly increase load carrying capacity and minimum film thickness by decreasing both the eccentricity ratio and temperature rise for longitudinal roughness patterns. For journal bearings with longitudinal roughness patterns and lubricated with Al2O3 nanoparticle additives, the stability region tends to increase.

Dynamic analysis of turbochargers with thermo-hydrodynamic lubrication bearings: Abstract

The use of a turbocharger in a downsized engine has become the current practice in all vehicle categories, to reduce engine emissions. Typically, the turbocharger is supported by hydrodynamic bearings, namely, fully or semi-floating ring bearings in radial direction, and double-acting thrust bearing in the axial direction. Given the high rotational speeds a typical automotive turbocharger can achieve, description of the shaft motion is a computationally expensive task, as the lateral oscillations are highly nonlinear and the phenomenon of oil whirl/whip is particularly strong in this type of rotating system. Moreover, axial motion is also crucial to be considered, as the thrust bearing may affect the shaft lateral oscillations. This work presents the development of a turbocharger rotor dynamic model, accounting for lateral and axial vibrations and cross-coupling bearing effects, to analyze run down or run up dynamic response. Thermal variations of the oil films are included in the modeling, accounting for temperature increases in both the radial and axial bearings. Two experiments enable to measure the turbocharger nonlinear oscillations in both run down and run up transients. Numerical results are compared with experimental responses leading to a previous tuning of the unbalance levels of the turbine and compressor wheels, as well as the floating rings residual unbalance. Besides, axial-lateral coupling occurs from the runner tilt regarding the thrust bearing. Predictions show a fair agreement with test data in frequency domain and temperature estimates, especially for higher rotational speeds, wherein eventual foundation effects are negligible.

Influence of Injection Pressure on the Thermohydrodynamic Behavior of Tilting Pad Journal Bearings—Model and Experiment

In rotating machines, tilting pad journal bearings (TPJBs) are widely used because of their enhanced design stability in comparison with alternative bearing configurations. In the present research, a thermohydrodynamic (THD) lubrication model for a tilting pad bearing with injection is developed. Within the THD lubrication model, the lubricant is also fed into each pad through a central injection orifice. The effects on the lubricant oil film pressures and temperatures and subsequent influences on the system dynamic behavior were investigated. Another important factor considered within the THD model is associated with the flexibility of the pivot of each pad and its impact on the behavior of the bearing when subjected to direct lubricant injection. The numerical model results are compared with the measurements of the rotor equilibrium position within the bearing and the pad temperatures obtained from the test rig designed specifically for the TPJB geometry with and without central pad injection. Pad temperatures are measured before the injection orifice position and after it. Agreement between experimental and model-predicted results provides confidence in the model’s ability to evaluate the dynamic behavior of the bearing and its effect on the complete system through the stiffness and damping coefficients.

Evaluation of the Rotor Temperature Distribution of an Automotive Turbocharger Under Hot Gas Conditions Including Indirect Experimental Validation

Abstract While turbocharging is a key technology for improving the performance and efficiency of internal combustion engines, the operating behavior of the turbocharger is highly dependent on the rotor temperature distribution as it directly modifies viscosity and clearances of the fluid film bearings. Since a direct experimental identification of the rotor temperature of an automotive turbocharger is not feasible at an acceptable expense, a combination of numerical analysis and experimental identification is applied to investigate its temperature characteristic and level. On the one hand, a numerical conjugate heat transfer (CHT) model of the automotive turbocharger investigated is developed using a commercial CFD-tool and a bidirectional, thermal coupling of the CFD-model with thermohydrodynamic lubrication simulation codes is implemented. On the other hand, experimental investigations of the numerically modeled turbocharger are conducted on a hot gas turbocharger test rig for selected operating points. Here, rotor speeds range from 64.000 to 168.000 rpm. The turbine inlet temperature is set to 600 °C and the lubricant is supplied at a pressure of 300 kPa with 90 °C to ensure practically relevant boundary conditions. Comparisons of measured and numerically predicted local temperatures of the turbocharger components indicate a good agreement between the analyses. The calorimetrically determined frictional power loss of the bearings as well as the floating ring speed are used as additional validation parameters. Evaluation of heat flow of diabatic simulations indicates a high sensitivity of local temperatures to rotor speed and load. A cooling effect of the fluid film bearings is present. Consequently, results confirm the necessity of the diabatic approach to the heat flow analysis of turbocharger rotors.

Comparative Study on Static Characteristics of Double-Pad Inwardly and Outwardly Pumping Water-Lubricated Spiral-Groove Thrust Bearings

Abstract The purpose of this technical brief is to investigate comparatively the static characteristics of the double-pad inwardly pumping and outwardly pumping water-lubricated spiral-groove thrust bearings (SGTBs). Firstly, a thermo-hydrodynamic lubrication model including the cavitating, centrifugal, and turbulent effects is established for the double-pad water-lubricated SGTB operating at a high-speed condition. Subsequently, the film thickness and volume flowrate are verified by an experimental study for the high-speed water-lubricated SGTBs. Finally, the effects of the operating conditions and configuration parameters on the static characteristics of the double-pad inwardly pumping and outwardly pumping SGTBs are analyzed systematically. The simulated results show that when the radius ratio exceeds 0.7, the outwardly pumping SGTB can perform better than the inwardly pumping version, with good axial load capacity, higher resistance to tilting and always positive radial flowrate.

A New Design Method of Cylindrical Bearings Based on a Simplified Thermohydrodynamic Lubrication Model

A New Design Method of Cylindrical Bearings Based on a Simplified Thermohydrodynamic Lubrication Model

Thermohydrodynamic Characteristics and Stability Analysis for a Journal Hybrid Floating Ring Bearing Within Laminar and Turbulent Mixed Flow Regime

Abstract There exist laminar and turbulent flow in the inner and outer fluid film of the hybrid floating ring bearing at high speed. The finite element method (FEM) and finite difference method (FDM) are used to solve the thermohydrodynamic (THD) lubrication model within the laminar and turbulent mixed flow regime which governs the pressure, temperature, viscosity, and Reynolds number of the two-layer fluid films together for a type of the journal hybrid floating ring bearing with deep/shallow pockets. The static and dynamic characteristics are analyzed including the load capacity, the friction moment, the volume flowrate, the stiffness, and damping coefficients with consideration of the mixed flow and thermal effect based on the floating ring balance. The unitized kinematic model of the shaft and floating ring is proposed to deduce the instability criteria and calculate the threshold rotational speed of a rigid Jeffcott rotor system supported on two hybrid floating ring bearings by the Routh–Hurwitz method. The results present that there are non-uniform pressure and temperature profile within the fluid film field, and the laminar flow and turbulent flow appear at certain positions of film through theory and experiment, separately. The mixed flow effect promotes the bearing load capacity and the friction moment at high speed and eccentricity, and the system threshold speed drops within the mixed flow regime. Therefore, the thermal effect and mixed flow existing in the fluid film should be considered into the lubrication analysis for the floating ring bearing.

Assessment of artificial neural network for thermohydrodynamic lubrication analysis

Purpose In this study, artificial neural networks (ANNs) are constructed and validated by using the bearing data generated numerically from a thermohydrodynamic (THD) lubrication model. In many tribological simulations, a surrogate model (meta-model) for obtaining a fast solution with sufficient accuracy is highly desired. Design/methodology/approach The THD model is represented by two coupled partial differential equations, a simplified generalized Reynolds equation, considering the viscosity variation across the film thickness direction and a transient energy equation for the 3-D film temperature distribution. The ANNs tested are having a single- or dual-hidden-layer with two inputs and one output. The root-mean-square error and maximum/minimum absolute errors of validation points, when comparing with the THD solutions, were used to evaluate the prediction accuracy of the ANNs. Findings It is demonstrated that a properly constructed ANN surrogate model can predict the THD lubrication performance almost instantly with accuracy adequately retained. Originality/value This study extends the use of ANNs to the applications other than the analyses dealing with experimental data. A similar procedure can be used to build a surrogate model for computationally intensive tribological models to have fast results. One of such applications is conducting extensive optimum design of tribological components or systems. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-03-2020-0109/

Thermohydrodynamic Model of Cavitating Flow and Dynamic Characteristic Calculation for High-Speed Water-Lubricated Pump-Out Spiral Groove Bearing

A complete thermohydrodynamic lubrication model for pump-out spiral groove thrust bearings (SGTB) with cavitating flow is established based on two-phase flow and transport theory. Various effects (cavitating flow interface effect, convective heat transfer effect, heat conduction effect, inertia effect, breakage and coalescence of bubbles) of water lubricant at high speed are included when modeling. The bubble size distributions, bubble number density of the cavitating flow, and the dynamic coefficients of the SGTB are predicted using this model. The probability distribution of the bubble size, bubble number density, and axial film stiffness coefficient agree with the experimental values. The results show that the number of small-sized bubbles is much larger than the number of large-sized bubbles in cavitating flow. When the SGTB has a specified spiral angle, the direct stiffness coefficients increase and the cross-stiffness coefficients decrease due to the cavitation effect, but the influence of the cavitation effect on the damping coefficient is not obvious.

Gap flow between two circular plates with temperature-controlled wall: application of thermohydrodynamic lubrication theory and comparison with an iso-viscous model

Gap flow between two circular plates with temperature-controlled wall: application of thermohydrodynamic lubrication theory and comparison with an iso-viscous model

Approximate Expression of Maximum Temperature of Journal Bearings Based on Simplified Thermohydrodynamic Lubrication Model

Approximate Expression of Maximum Temperature of Journal Bearings Based on Simplified Thermohydrodynamic Lubrication Model

A Thermo-hydrodynamic lubrication model of a mechanical seal modified by equivalent film thickness

An effective way to improve the combined performance of mechanical seals is to optimize their surface textures using multi-objective optimization method. For compatibility with the multi-objective optimization algorithm, the theoretical performance of a mechanical seal is often determined using the finite-difference method (FDM). However, compared with the finite-volume method (FVM) and finite-element method (FEM), FDM is weaker for dealing with the issue of discontinuous film thickness for a textured surface. In the present study, the thermo-hydrodynamic lubrication model of a mechanical seal is modified by means of an equivalent-thickness treatment, and the accuracy of the modified lubrication model is assessed by comparing its predictions for film pressure and temperature with published FVM and FEM results, showing that the equivalent-thickness lubrication model is effective for addressing the issue of discontinuous film thickness. The present work is important in that it improves the simulation accuracy of multi-objective optimization for textured mechanical seals.

Temperature distribution and heat generating/transfer mechanism of the circular bilayer porous bearing for thermo-hydrodynamic problem

Circular porous bearing is a widely used self-lubricating bearing structure but it exhibits higher temperature rise in bearing system — heat produced cannot be dissipated in time — which leads to poorer lubricating performance and makes it difficult to use in complex working conditions. We establish the thermo-hydrodynamic lubrication model of circular bilayer porous bearing in polar coordinates coupling with the fluid pressure and heat transfer equations, and reveal the fundamental origin of the heat generating and its transfer mechanism. The results indicate that the circumferential velocity is the major contributor to determine the temperature distribution, and the oil film shearing causes temperature rise of the bearing system which is gradually transferred from oil film to porous medium through the heat conduction of porous metal and the convection of fluid. Thus, the temperature in the oil film region is higher than that in the porous bearing. In the vertical direction, the temperature rises first and then decreases from the bearing bottom face to the counterpart surface, While in the radial direction, the temperature increases gradually from the inner radius to the outer radius. We find that the highest temperature of the bearing system occurs at the minimum film thickness of the external ring face in the oil film region. The numerical model is verified by comparing the experimental and numerical results, and the numerical accuracy of the lubrication performance has been improved after considering the thermal effect. Our results can be employed to control the oil film temperature and exert a better lubricating performance.

A 3D Approach for THD Lubrication in Tilting Pad Journal Bearing—Theory and Experiment

The study of rotating machines stands out in the context of systems and structures due to the significant number of typical phenomena that may be present during the operation of such equipment. Rotating systems play an important role in industrial environments, with a wide range of application, namely, pumps, turbines, generators and compressors, and turbochargers, among others. Given this context, tilting pad journal bearings (TPJBs) can offer considerable stability to the rotor system due to its damping and stiffness characteristics, especially under high rotation speeds, whereas journal bearings with a fixed geometry may be subjected to fluid-induced instability. Consequently, thermal effects are significant in rotating machines to evaluate the behavior of hydrodynamic bearings due to the increase in shear effects in the lubricant caused by the rotation speed, leading to excessive temperature increase under specific operational conditions and, therefore, significant changes in lubricant viscosity. These effects may influence the dynamic characteristics of bearings, affecting the dynamic response of the entire system. From this perspective, a specific test rig was designed to perform the characteristic uncoupled motion of the shaft when supported by this kind of bearing. Experimental tests were directly compared with numerical models, showing promising results. In addition, the thermohydrodynamic (THD) lubrication was numerically solved by a finite volume method, considering an approach for a 3D THD model and the pivot flexibility.

Solidification of the Lennard-Jones fluid near a wall in thermohydrodynamic lubrication.

We investigate the thermohydrodynamic lubrication of the Lennard-Jones (LJ) fluid in plain wall channels by using a molecular-dynamics simulation. It is found that the LJ fluid solidifies near the wall when the viscous heating of the LJ fluid in the bulk regime is sufficiently large. The thickness of the solidified layer increases with the channel width. Thus, a long-range-ordered crystal-like structure forms near the wall in high-speed lubrication when the channel width is large. The mechanism of this counterintuitive solidification is investigated from both macroscopic and microscopic points of view. It is elucidated that the LJ molecules are densely confined in the vicinity of the wall due to the macroscopic mass and heat transport in the bulk regime. In this densely confined regime, the fluid molecules form a crystal-like structure, which is similar to that of the wall molecules, via direct molecular interaction. Band formation is also observed in the solidified region when the channel width is sufficiently large.

Thermohydrodynamic lubrication analysis of misaligned journal bearing considering the axial movement of journal

Abstract In the journal-bearing system, the journal inevitably moves along the bearing axis, and the axial movement of journal may affect the performance of misaligned journal bearing. In this article, a numerical model for the thermohydrodynamic lubrication of bearing considering the axial movement of journal is established, and based on which a comprehensive analysis on the performance of misaligned journal bearing is conducted by solving the generalized Reynolds equation, heat conduction equation and energy equation. The results show that axial movement of misaligned journal has more obvious influence on the bearing lubrication characteristics at the lower speed, larger inclination angle and eccentricity. The impact of axial movement on the bearing lubrication performance is greatly affected by the thermal effect and surface roughness.

Study on Film Characteristics of Piston-Cylinder Interface of High Pressure Common Rail Radial Piston Pump with Viscosity-Temperature-Pressure Effect

Aiming at the inaccuracy of equivalent viscosity method in solving the film characteristics of piston-cylinder interface of high-pressure common rail radial piston pump, the film characteristics equation of piston-cylinder interface was established based on the theory of thermo-hydrodynamic lubrication. Through the solution, the thermal properties of the piston-cylinder interface film, which accounted for viscosity-temperature-pressure effect, were studied. The effects of cam speed and film inlet pressure on the characteristics of the piston-cylinder interface film were discussed. The conclusion has certain theoretical and engineering application value for the design and basic research of piston-cylinder interface.

Investigations of the three-dimensional temperature field of journal bearings considering conjugate heat transfer and cavitation

PurposeThe growing demand of efficiency and economy has led to a dramatic increase of the operating speed of the journal bearing, with a higher temperature distribution. This paper aims to investigate the three-dimensional temperature distribution of journal bearings.Design/methodology/approachA thermo-hydrodynamic lubrication model of a journal bearing was established based on the full 3D CFD method. A two-sided wall was used to include the conjugate heat transfer effect. The temperature-dependent characteristics of lubrication and cavitation impact were also included. The simulation results well agreed with the experimental results. Based on this method, the three-dimensional temperature distribution was analyzed under different operating conditions.FindingsThe temperature distribution in the radial direction had a difference. An increase of speed and de-crease of inlet temperature promoted temperature differences in the higher temperature zone and the increasing temperature zone, respectively. However, the inlet pressure had less influence on these differences. The temperature distribution was basically the same at a lower bearing conductivity. As the conductivity increased, the radial temperature difference was increased.Originality/valueThe temperature distribution in the radial direction was found under different operating conditions, and the present research provides references to understand the three-dimensional temperature distribution of journal bearings.

A Model for Oil Flow and Fluid Temperature Inlet Mixing in Hydrodynamic Journal Bearings

The quality of predictions for the operating behavior of high-speed journal bearings strongly depends on realistic boundary conditions within the inlet region supplying a mixture of hot oil from the upstream pad and fresh lubricant from the inlet device to the downstream located pad. Therefore, an appropriate modeling of fundamental phenomena within the inlet region is essential for a reliable simulation of fluid and heat flow in the entire bearing. A theoretical model including hydraulic, mechanical, and energetic effects and the procedure of its numerical implementation in typical bearing codes for thermo-hydrodynamic lubrication is described and validated. Convective and conductive heat transfer as well as dissipation due to internal friction in the lubricant is considered for the space between pads or the pocket where the inlet is located. In contrast to most other models, the region between the physical inlet and the lubricant film is part of the solution domain and not only represented by boundary conditions. The model provides flow rate and temperature boundary conditions for extended Reynolds equation and a three-dimensional (3D) energy equation of film and inlet region, respectively. The impact of backflow from the inlet region to the outer supply channel possibly occurring in sealed pockets is taken into account. Moreover, the model considers the influence of turbulent flow in the inlet region.

Thermohydrodynamic Analysis of High-Speed Water-Lubricated Spiral Groove Thrust Bearing Using Cavitating Flow Model

A thermohydrodynamic lubrication model of turbulent cavitating flow for high-speed spiral groove thrust bearing was developed considering the effects of cavitation, turbulence, inertia, breakage, and coalescence of bubbles. Comparing with the classical thermohydrodynamic model, this model can predict not only the distributions of pressure and temperature rise but also the distribution of bubble volume and bubble number density. Static characteristics of the water-lubricated spiral groove thrust bearing in the state of turbulent cavitating flow were analyzed, and the influences of multiple effects on the static characteristics of the bearing were researched. The numerical calculation result shows that the bubbles are mainly distributed in inlet and outlet of the spiral groove, the distribution of bubble volume is skewed under the equilibrium state, and small bubbles account for a large proportion of the cavitating flow under high-speed condition. Furthermore, the load carrying capacity and the leakage flow of the bearing decrease due to the effect of cavitation under high-speed. The maximum temperature rise of the bearing decreases due to the effect of cavitation effect.