The study of self-lubricant coatings is a rapidly developing research field that is expected to a... more The study of self-lubricant coatings is a rapidly developing research field that is expected to address major issues arising from operation under high loads and thermal stresses of machine parts. It is of utmost importance that various critical tribological pairs such as crankshaft bearings, piston rings, etc. maintain their integrity, thus contributing to the engines efficient and reliable operation for long periods of time. Therefore, a choice of high-strength metals is required, with special treatments and/or coatings in combination with sufficient lubrication. In the present study, liquid lubricants encapsulated in poly(urea-formaldehyde) were incorporated in ceramic-metal matrices for the production of composite coatings by Atmospheric Plasma Spraying. Aspects concerning the survival of the capsules during their flight through the high temperature plasma flame, the maintenance of their integrity at impact with the substrate with high kinetic energy, their homogeneous dispersion within the coating mass, thermal spray deposition parameters adjustment and optimization are thoroughly discussed. Coating properties such as roughness, friction, adhesion strength, and wear behaviour were also investigated. Microstructural characteristics and friction-wear behaviour were found to be critical to the durability of coating.
Hydrophobic surfaces, enabling flow slip past a solid boundary, can potentially be effective for ... more Hydrophobic surfaces, enabling flow slip past a solid boundary, can potentially be effective for heat transfer enhancement in heat exchanger applications. The scope of the present work is the computational study of forced convection heat transfer in flow past a hydrophobic cylinder, maintained at constant surface temperature. Hydrophobic surfaces are applied in flow control applications, since they enable flow slip past a solid boundary; as a result, they can contribute to flow stabilization. At the same time, hydrophobic surfaces are a potential means for heat transfer enhancement. In the present study, a computational investigation of forced convection heat transfer in cross-flow past a circular hydrophobic cylinder is performed by means of Computational Fluid Dynamics; the effects of hydrophobicity on flow stability and forces are also quantified. Here, low Reynolds number values, Re, are considered, whereas the Prandtl number, Pr, is maintained constant, equal to unity. Hydropho...
The technique of Proper Orthogonal Decomposition (POD) has been used to develop an identification... more The technique of Proper Orthogonal Decomposition (POD) has been used to develop an identification tool to analyze steady state dynamics in acoustics. The information on the dynamics needed for the POD technique is obtained by solving numerically with the method of finite elements the wave equation inside a cavity. The POD technique identifies optimum spatial patterns it terms of Proper Orthogonal Modes. The steady state sound pressure field in a cavity excited by a single harmonic source responds with a very small number of POD modes. Under certain forcing conditions, the POD modes are identical to the Fourier acoustic modes. The POD technique of Proper Orthogonal Decomposition (POD) proves to be a very effective identification tool. INTRODUCTION Frequently, in sound control problems, a sound source is enclosed in a small box to prevent it from radiating noise to the exterior. If the frequency of the source is low enough so that the wavelength of the sound is long compared to the la...
Proceedings of the 17th LACCEI International Multi-Conference for Engineering, Education, and Technology: “Industry, Innovation, and Infrastructure for Sustainable Cities and Communities”, 2019
In the present study, a numerical investigation on the effect of different manufacturing errors o... more In the present study, a numerical investigation on the effect of different manufacturing errors on the performance of a parallel thrust bearing is reported. The calculations have been performed utilising a CFD-based TEHD computational approach. An initial investigation of the pressure build-up mechanism in parallel thrust bearings concluded that the two main reasons of load carrying capacity of parallel thrust bearings are (a) the thermal deformation of the pad upper surface and (b) the initial (non-deformed) “quasi-parallel” pad upper surface, being an outcome of the manufacturing process. The present work investigates the importance of the initial geometry of a “quasi-parallel” thrust bearing, and indicates manufacturing tolerance thresholds, below which, the defects of the pad-fluid interface are not the major factor of pressure generation in the lubricant domain of the bearings. Keywords– Manufacturing Errors, Thermal deformations, TEHD regime, CFD, Parallel thrust bearings.
Abstract Texture patterns implemented on thrust bearing surfaces have been proven very beneficial... more Abstract Texture patterns implemented on thrust bearing surfaces have been proven very beneficial in terms of load carrying capacity and friction coefficient. In the present study, the potential of thrust bearings with a curved pocket stator geometry is investigated for realistic operating conditions. Bearing performance is quantified by means of Computational Fluid Dynamics (CFD) simulations based on the numerical solution of the Navier-Stokes and energy equations for incompressible flow, taking into account conjugate heat transfer at the bearing stator and rotor. After defining a reference slider design, optimization problems are formulated and solved, to arrive at optimal curved pocket designs. Here, the slider convergence ratio, as well as the pocket size, shape and depth, are defined in terms of proper design variables, whereas the load carrying capacity and friction coefficient constitute the problem's objective functions. The present results illustrate a superior performance of the curved open pocket slider designs in comparison to corresponding open square pocket sliders. Optimal solutions are characterized by parallel slider geometries. The improved performance of optimal designs can be attributed to reduced side leakage, in comparison to open square pocket geometries. The good performance of a thrust bearing with a corresponding curved pocket geometry is verified. Overall, the present optimal geometries are rather easy to manufacture, which makes them an attractive choice for the design of thrust bearings operated in a wide range of rotor speeds.
Above a critical value of the Reynolds number, flows past bluff bodies are characterized by self-... more Above a critical value of the Reynolds number, flows past bluff bodies are characterized by self-sustained oscillations, due to the formation of the Karman vortex street. This results in the dynamic loading of the body, which, in practical applications, may lead to structural fatigue. In recent work, it was demonstrated that, in flow past a cylinder, implementing surface hydrophobicity results in decreased drag and lift forces, leading to vortex street cancellation at a critical level of control effort. In order to achieve vortex street suppression at lower levels of control effort, utilizing materials with controllable wettability properties appears as a promising possibility. Thus, in the present work, the effectiveness of a proportional feedback control scheme, utilizing the application of time-dependent slip conditions on the cylinder surface, is investigated by means of CFD simulations. Firstly, the feedback control scheme is tested for a problem definition characterized by the application of slip on the entire cylinder surface, for a wide range of the control scheme parameters; it is demonstrated that flow unsteadiness is reduced at substantially lower levels of control effort, in comparison to corresponding passive control schemes. Secondly, in the frame of feedback control, an optimization problem is formulated and solved, aiming at the partial or full suppression of the Karman vortex street at a minimal control effort, using time-dependent slip conditions on a part of the cylinder surface. The results demonstrate a further reduction in control effort. Overall, in comparison to passive flow control, a reduction in control effort by two orders of magnitude is attained with the present optimized feedback control scheme.
Volume 5: Manufacturing Materials and Metallurgy; Marine; Microturbines and Small Turbomachinery; Supercritical CO2 Power Cycles, 2012
ABSTRACT Recent research has demonstrated that proper use of texture geometries can improve the p... more ABSTRACT Recent research has demonstrated that proper use of texture geometries can improve the performance of journal and thrust bearings. In particular, for journal bearings, elliptical and egg-shaped texture patterns appear as promising candidates for improving bearing performance, in terms of load carrying capacity and friction coefficient. The expected advantages should also hold for partial-arc bearings, which, for small scale – high speed applications, are characterized by significant advantages, in comparison to full bearings. In the present paper, a tribological study of partial-arc journal bearings with periodic egg-shaped texture applied on the stator surface is presented. Computational Fluid Dynamics (CFD) simulations are performed and processed to yield the bearing performance indices. Here, the bearing geometry and the texture characteristics are defined parametrically; a wide range of bearing designs is thus accounted for. Flow simulations are based on the numerical solution of the Navier-Stokes equations for incompressible isothermal flow. The effects of dimple shape, bearing eccentricity, bearing arc angle, and slenderness ratio on the bearing performance are investigated. The present results demonstrate that a substantial improvement of journal bearing performance, especially in terms of the friction coefficient, in comparison to that of smooth bearings, is feasible.
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2014
Different geometry patterns of the surface of thrust bearings have been proven very beneficial in... more Different geometry patterns of the surface of thrust bearings have been proven very beneficial in terms of bearing load-carrying capacity and friction coefficient. In this study, four different types of sector-pad thrust bearings have been cross-evaluated for operation under realistic operating conditions: (a) an open pocket bearing, (b) a closed pocket bearing, (c) a tapered-land bearing, and (d) a bearing partially textured with rectangular dimples. Bearing performance has been computed by means of computational fluid dynamics simulations based on the numerical solution of the Navier–Stokes and energy equations for incompressible flow. Conjugate heat transfer at the bearing pad and rotor has been taken into account. Initially, for a reference design of each bearing, the effects of varying rotational speed and minimum film thickness have been investigated. Further, characterization of each bearing for a constant level of thrust load has been performed. Finally, the effects of varyi...
Volume 3: Controls, Diagnostics and Instrumentation; Education; Electric Power; Microturbines and Small Turbomachinery; Solar Brayton and Rankine Cycle, 2011
ABSTRACT The paper presents an optimization study of the geometry of three-dimensional micro-thru... more ABSTRACT The paper presents an optimization study of the geometry of three-dimensional micro-thrust bearings, in a wide range of convergence ratios. The optimization goal is the maximization of the bearing load carrying capacity. The bearings are modeled as microchannels, consisting of a smooth moving wall (rotor), and a stationary wall (stator) with partial periodic rectangular texturing. The flow field is calculated from the numerical solution of the Navier-Stokes equations for incompressible isothermal flow; processing of the results yields the bearing load capacity and friction coefficient. The geometry of the textured channel is defined parametrically for several width-to-length ratios. Optimal texturing geometries are obtained by utilizing an optimization tool based on genetic algorithms, which is coupled to the CFD code. Here, the design variables define the bearing geometry and convergence ratio. To minimize the computational cost, a multi-objective approach is proposed, consisting in the simultaneous maximization of the load carrying capacity and minimization of the bearing convergence ratio. The optimal solutions, identified based on the concept of Pareto dominance, are equivalent to those of single-objective optimization problems at different convergence ratio values. The present results demonstrate that the characteristics of the optimal texturing patterns depend strongly on both the convergence ratio and the width-to-length ratio. Further, the optimal load carrying capacity increases at increasing convergence ratio, up to an optimal value, identified by the optimization procedure. Finally, proper surface texturing provides substantial load carrying capacity even for parallel or slightly diverging bearings. Based on the present results, we propose simple formulas for the design of textured micro-thrust bearings.
Journal of Engineering for Gas Turbines and Power, 2014
In the present work, a computational optimization study of thrust bearings lubricated with spatia... more In the present work, a computational optimization study of thrust bearings lubricated with spatially varying viscosity lubricants is presented, with the main goal of minimizing friction coefficient. In practice, spatial variation of viscosity could be achieved by utilizing electrorheological or magnetorheological fluids. The bearings are modeled as two-dimensional (2D) channels, consisting of a smooth moving wall (rotor) and a parallel or inclined stationary wall (stator), which can be (i) smooth, (ii) partially textured with rectangular dimples, and (iii) smooth and partially hydrophobic. The bearings are considered to be operated with an ideal lubricant that exhibits different values of viscosity in two distinct regions of the fluid domain: a high viscosity area is considered at the channel inflow, with the viscosity acquiring a reference (low) value farther downstream. The flow field is calculated from the numerical solution of the Navier–Stokes equations for 2D incompressible is...
ABSTRACT The paper presents a detailed computational study of flow patterns and performance indic... more ABSTRACT The paper presents a detailed computational study of flow patterns and performance indices in a dimpled parallel thrust bearing. The bearing consists of eight pads; the stator surface of each pad is partially textured with rectangular dimples, aiming at maximizing the load carrying capacity. The bearing tribological performance is characterized by means of Computational Fluid Dynamics (CFD) simulations, based on the numerical solution of the Navier-Stokes and energy equations for incompressible flow. Realistic boundary conditions are implemented. The effects of operating conditions and texture design are studied for the case of isothermal flow. First, for a reference texture pattern, the effects of varying operating conditions, in particular minimum film thickness (thrust load), rotational speed and feeding oil pressure are investigated. Next, the effects of varying texture geometry characteristics, in particular texture zone circumferential/radial extent, dimple depth and texture density on the bearing performance indices (load carrying capacity, friction torque and friction coefficient) are studied, for a representative operating point. For the reference texture design, the effects of varying operating conditions are further investigated, by also taking into account thermal effects. In particular, adiabatic conditions and conjugate heat transfer at the bearing pad are considered. The results of the present study indicate that parallel thrust bearings textured by proper rectangular dimples are characterized by substantial load carrying capacity levels. Thermal effects may significantly reduce load capacity, especially in the range of high speeds and high loads. Based on the present results, favorable texture designs can be assessed.
The paper presents a detailed computational study of flow patterns and performance indices in a d... more The paper presents a detailed computational study of flow patterns and performance indices in a dimpled parallel thrust bearing. The bearing consists of eight pads; the stator surface of each pad is partially textured with rectangular dimples, aiming at maximizing the load carrying capacity. The bearing tribological performance is characterized by means of computational fluid dynamics (CFD) simulations, based on the numerical solution of the Navier–Stokes and energy equations for incompressible flow. Realistic boundary conditions are implemented. The effects of operating conditions and texture design are studied for the case of isothermal flow. First, for a reference texture pattern, the effects of varying operating conditions, in particular minimum film thickness (thrust load), rotational speed and feeding oil pressure are investigated. Next, the effects of varying texture geometry characteristics, in particular texture zone circumferential/radial extent, dimple depth, and texture ...
This paper presents an optimization study of the geometry of three-dimensional micro-thrust beari... more This paper presents an optimization study of the geometry of three-dimensional micro-thrust bearings in a wide range of convergence ratios. The optimization goal is the maximization of the bearing load carrying capacity. The bearings are modeled as micro-channels, consisting of a smooth moving wall (rotor), and a stationary wall (stator) with partial periodic rectangular texturing. The flow field is calculated from the numerical solution of the Navier-Stokes equations for incompressible isothermal flow; processing of the results yields the bearing load capacity and friction coefficient. The geometry of the textured channel is defined parametrically for several width-to-length ratios. Optimal texturing geometries are obtained by utilizing an optimization tool based on genetic algorithms, which is coupled to the CFD code. Here, the design variables define the bearing geometry and convergence ratio. To minimize the computational cost, a multi-objective approach is proposed, consisting ...
Journal of Engineering for Gas Turbines and Power, 2012
In the present paper, a study of stiffness and damping in sector-pad micro thrust bearings with a... more In the present paper, a study of stiffness and damping in sector-pad micro thrust bearings with artificial surface texturing is presented, based on computational fluid dynamics (CFD) simulations. The bearing pads are modeled as consecutive three-dimensional independent microchannels, each consisting of a smooth rotating wall (rotor) and a partially textured stationary wall (stator). CFD simulations are performed, consisting in the numerical solution of the Navier–Stokes equations for incompressible isothermal flow. The goal of the present study is to characterize the dynamic behavior of favorable designs, identified in previous optimization studies, comprising parallel and convergent thrust bearings with rectangular texture patterns. To this end, a translational degree of freedom (DOF) along the thrust direction and a rotational (tilting) DOF of the rotor are considered. By implementing appropriate small perturbations around the equilibrium (steady-state) position and processing the...
The study of self-lubricant coatings is a rapidly developing research field that is expected to a... more The study of self-lubricant coatings is a rapidly developing research field that is expected to address major issues arising from operation under high loads and thermal stresses of machine parts. It is of utmost importance that various critical tribological pairs such as crankshaft bearings, piston rings, etc. maintain their integrity, thus contributing to the engines efficient and reliable operation for long periods of time. Therefore, a choice of high-strength metals is required, with special treatments and/or coatings in combination with sufficient lubrication. In the present study, liquid lubricants encapsulated in poly(urea-formaldehyde) were incorporated in ceramic-metal matrices for the production of composite coatings by Atmospheric Plasma Spraying. Aspects concerning the survival of the capsules during their flight through the high temperature plasma flame, the maintenance of their integrity at impact with the substrate with high kinetic energy, their homogeneous dispersion within the coating mass, thermal spray deposition parameters adjustment and optimization are thoroughly discussed. Coating properties such as roughness, friction, adhesion strength, and wear behaviour were also investigated. Microstructural characteristics and friction-wear behaviour were found to be critical to the durability of coating.
Hydrophobic surfaces, enabling flow slip past a solid boundary, can potentially be effective for ... more Hydrophobic surfaces, enabling flow slip past a solid boundary, can potentially be effective for heat transfer enhancement in heat exchanger applications. The scope of the present work is the computational study of forced convection heat transfer in flow past a hydrophobic cylinder, maintained at constant surface temperature. Hydrophobic surfaces are applied in flow control applications, since they enable flow slip past a solid boundary; as a result, they can contribute to flow stabilization. At the same time, hydrophobic surfaces are a potential means for heat transfer enhancement. In the present study, a computational investigation of forced convection heat transfer in cross-flow past a circular hydrophobic cylinder is performed by means of Computational Fluid Dynamics; the effects of hydrophobicity on flow stability and forces are also quantified. Here, low Reynolds number values, Re, are considered, whereas the Prandtl number, Pr, is maintained constant, equal to unity. Hydropho...
The technique of Proper Orthogonal Decomposition (POD) has been used to develop an identification... more The technique of Proper Orthogonal Decomposition (POD) has been used to develop an identification tool to analyze steady state dynamics in acoustics. The information on the dynamics needed for the POD technique is obtained by solving numerically with the method of finite elements the wave equation inside a cavity. The POD technique identifies optimum spatial patterns it terms of Proper Orthogonal Modes. The steady state sound pressure field in a cavity excited by a single harmonic source responds with a very small number of POD modes. Under certain forcing conditions, the POD modes are identical to the Fourier acoustic modes. The POD technique of Proper Orthogonal Decomposition (POD) proves to be a very effective identification tool. INTRODUCTION Frequently, in sound control problems, a sound source is enclosed in a small box to prevent it from radiating noise to the exterior. If the frequency of the source is low enough so that the wavelength of the sound is long compared to the la...
Proceedings of the 17th LACCEI International Multi-Conference for Engineering, Education, and Technology: “Industry, Innovation, and Infrastructure for Sustainable Cities and Communities”, 2019
In the present study, a numerical investigation on the effect of different manufacturing errors o... more In the present study, a numerical investigation on the effect of different manufacturing errors on the performance of a parallel thrust bearing is reported. The calculations have been performed utilising a CFD-based TEHD computational approach. An initial investigation of the pressure build-up mechanism in parallel thrust bearings concluded that the two main reasons of load carrying capacity of parallel thrust bearings are (a) the thermal deformation of the pad upper surface and (b) the initial (non-deformed) “quasi-parallel” pad upper surface, being an outcome of the manufacturing process. The present work investigates the importance of the initial geometry of a “quasi-parallel” thrust bearing, and indicates manufacturing tolerance thresholds, below which, the defects of the pad-fluid interface are not the major factor of pressure generation in the lubricant domain of the bearings. Keywords– Manufacturing Errors, Thermal deformations, TEHD regime, CFD, Parallel thrust bearings.
Abstract Texture patterns implemented on thrust bearing surfaces have been proven very beneficial... more Abstract Texture patterns implemented on thrust bearing surfaces have been proven very beneficial in terms of load carrying capacity and friction coefficient. In the present study, the potential of thrust bearings with a curved pocket stator geometry is investigated for realistic operating conditions. Bearing performance is quantified by means of Computational Fluid Dynamics (CFD) simulations based on the numerical solution of the Navier-Stokes and energy equations for incompressible flow, taking into account conjugate heat transfer at the bearing stator and rotor. After defining a reference slider design, optimization problems are formulated and solved, to arrive at optimal curved pocket designs. Here, the slider convergence ratio, as well as the pocket size, shape and depth, are defined in terms of proper design variables, whereas the load carrying capacity and friction coefficient constitute the problem's objective functions. The present results illustrate a superior performance of the curved open pocket slider designs in comparison to corresponding open square pocket sliders. Optimal solutions are characterized by parallel slider geometries. The improved performance of optimal designs can be attributed to reduced side leakage, in comparison to open square pocket geometries. The good performance of a thrust bearing with a corresponding curved pocket geometry is verified. Overall, the present optimal geometries are rather easy to manufacture, which makes them an attractive choice for the design of thrust bearings operated in a wide range of rotor speeds.
Above a critical value of the Reynolds number, flows past bluff bodies are characterized by self-... more Above a critical value of the Reynolds number, flows past bluff bodies are characterized by self-sustained oscillations, due to the formation of the Karman vortex street. This results in the dynamic loading of the body, which, in practical applications, may lead to structural fatigue. In recent work, it was demonstrated that, in flow past a cylinder, implementing surface hydrophobicity results in decreased drag and lift forces, leading to vortex street cancellation at a critical level of control effort. In order to achieve vortex street suppression at lower levels of control effort, utilizing materials with controllable wettability properties appears as a promising possibility. Thus, in the present work, the effectiveness of a proportional feedback control scheme, utilizing the application of time-dependent slip conditions on the cylinder surface, is investigated by means of CFD simulations. Firstly, the feedback control scheme is tested for a problem definition characterized by the application of slip on the entire cylinder surface, for a wide range of the control scheme parameters; it is demonstrated that flow unsteadiness is reduced at substantially lower levels of control effort, in comparison to corresponding passive control schemes. Secondly, in the frame of feedback control, an optimization problem is formulated and solved, aiming at the partial or full suppression of the Karman vortex street at a minimal control effort, using time-dependent slip conditions on a part of the cylinder surface. The results demonstrate a further reduction in control effort. Overall, in comparison to passive flow control, a reduction in control effort by two orders of magnitude is attained with the present optimized feedback control scheme.
Volume 5: Manufacturing Materials and Metallurgy; Marine; Microturbines and Small Turbomachinery; Supercritical CO2 Power Cycles, 2012
ABSTRACT Recent research has demonstrated that proper use of texture geometries can improve the p... more ABSTRACT Recent research has demonstrated that proper use of texture geometries can improve the performance of journal and thrust bearings. In particular, for journal bearings, elliptical and egg-shaped texture patterns appear as promising candidates for improving bearing performance, in terms of load carrying capacity and friction coefficient. The expected advantages should also hold for partial-arc bearings, which, for small scale – high speed applications, are characterized by significant advantages, in comparison to full bearings. In the present paper, a tribological study of partial-arc journal bearings with periodic egg-shaped texture applied on the stator surface is presented. Computational Fluid Dynamics (CFD) simulations are performed and processed to yield the bearing performance indices. Here, the bearing geometry and the texture characteristics are defined parametrically; a wide range of bearing designs is thus accounted for. Flow simulations are based on the numerical solution of the Navier-Stokes equations for incompressible isothermal flow. The effects of dimple shape, bearing eccentricity, bearing arc angle, and slenderness ratio on the bearing performance are investigated. The present results demonstrate that a substantial improvement of journal bearing performance, especially in terms of the friction coefficient, in comparison to that of smooth bearings, is feasible.
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2014
Different geometry patterns of the surface of thrust bearings have been proven very beneficial in... more Different geometry patterns of the surface of thrust bearings have been proven very beneficial in terms of bearing load-carrying capacity and friction coefficient. In this study, four different types of sector-pad thrust bearings have been cross-evaluated for operation under realistic operating conditions: (a) an open pocket bearing, (b) a closed pocket bearing, (c) a tapered-land bearing, and (d) a bearing partially textured with rectangular dimples. Bearing performance has been computed by means of computational fluid dynamics simulations based on the numerical solution of the Navier–Stokes and energy equations for incompressible flow. Conjugate heat transfer at the bearing pad and rotor has been taken into account. Initially, for a reference design of each bearing, the effects of varying rotational speed and minimum film thickness have been investigated. Further, characterization of each bearing for a constant level of thrust load has been performed. Finally, the effects of varyi...
Volume 3: Controls, Diagnostics and Instrumentation; Education; Electric Power; Microturbines and Small Turbomachinery; Solar Brayton and Rankine Cycle, 2011
ABSTRACT The paper presents an optimization study of the geometry of three-dimensional micro-thru... more ABSTRACT The paper presents an optimization study of the geometry of three-dimensional micro-thrust bearings, in a wide range of convergence ratios. The optimization goal is the maximization of the bearing load carrying capacity. The bearings are modeled as microchannels, consisting of a smooth moving wall (rotor), and a stationary wall (stator) with partial periodic rectangular texturing. The flow field is calculated from the numerical solution of the Navier-Stokes equations for incompressible isothermal flow; processing of the results yields the bearing load capacity and friction coefficient. The geometry of the textured channel is defined parametrically for several width-to-length ratios. Optimal texturing geometries are obtained by utilizing an optimization tool based on genetic algorithms, which is coupled to the CFD code. Here, the design variables define the bearing geometry and convergence ratio. To minimize the computational cost, a multi-objective approach is proposed, consisting in the simultaneous maximization of the load carrying capacity and minimization of the bearing convergence ratio. The optimal solutions, identified based on the concept of Pareto dominance, are equivalent to those of single-objective optimization problems at different convergence ratio values. The present results demonstrate that the characteristics of the optimal texturing patterns depend strongly on both the convergence ratio and the width-to-length ratio. Further, the optimal load carrying capacity increases at increasing convergence ratio, up to an optimal value, identified by the optimization procedure. Finally, proper surface texturing provides substantial load carrying capacity even for parallel or slightly diverging bearings. Based on the present results, we propose simple formulas for the design of textured micro-thrust bearings.
Journal of Engineering for Gas Turbines and Power, 2014
In the present work, a computational optimization study of thrust bearings lubricated with spatia... more In the present work, a computational optimization study of thrust bearings lubricated with spatially varying viscosity lubricants is presented, with the main goal of minimizing friction coefficient. In practice, spatial variation of viscosity could be achieved by utilizing electrorheological or magnetorheological fluids. The bearings are modeled as two-dimensional (2D) channels, consisting of a smooth moving wall (rotor) and a parallel or inclined stationary wall (stator), which can be (i) smooth, (ii) partially textured with rectangular dimples, and (iii) smooth and partially hydrophobic. The bearings are considered to be operated with an ideal lubricant that exhibits different values of viscosity in two distinct regions of the fluid domain: a high viscosity area is considered at the channel inflow, with the viscosity acquiring a reference (low) value farther downstream. The flow field is calculated from the numerical solution of the Navier–Stokes equations for 2D incompressible is...
ABSTRACT The paper presents a detailed computational study of flow patterns and performance indic... more ABSTRACT The paper presents a detailed computational study of flow patterns and performance indices in a dimpled parallel thrust bearing. The bearing consists of eight pads; the stator surface of each pad is partially textured with rectangular dimples, aiming at maximizing the load carrying capacity. The bearing tribological performance is characterized by means of Computational Fluid Dynamics (CFD) simulations, based on the numerical solution of the Navier-Stokes and energy equations for incompressible flow. Realistic boundary conditions are implemented. The effects of operating conditions and texture design are studied for the case of isothermal flow. First, for a reference texture pattern, the effects of varying operating conditions, in particular minimum film thickness (thrust load), rotational speed and feeding oil pressure are investigated. Next, the effects of varying texture geometry characteristics, in particular texture zone circumferential/radial extent, dimple depth and texture density on the bearing performance indices (load carrying capacity, friction torque and friction coefficient) are studied, for a representative operating point. For the reference texture design, the effects of varying operating conditions are further investigated, by also taking into account thermal effects. In particular, adiabatic conditions and conjugate heat transfer at the bearing pad are considered. The results of the present study indicate that parallel thrust bearings textured by proper rectangular dimples are characterized by substantial load carrying capacity levels. Thermal effects may significantly reduce load capacity, especially in the range of high speeds and high loads. Based on the present results, favorable texture designs can be assessed.
The paper presents a detailed computational study of flow patterns and performance indices in a d... more The paper presents a detailed computational study of flow patterns and performance indices in a dimpled parallel thrust bearing. The bearing consists of eight pads; the stator surface of each pad is partially textured with rectangular dimples, aiming at maximizing the load carrying capacity. The bearing tribological performance is characterized by means of computational fluid dynamics (CFD) simulations, based on the numerical solution of the Navier–Stokes and energy equations for incompressible flow. Realistic boundary conditions are implemented. The effects of operating conditions and texture design are studied for the case of isothermal flow. First, for a reference texture pattern, the effects of varying operating conditions, in particular minimum film thickness (thrust load), rotational speed and feeding oil pressure are investigated. Next, the effects of varying texture geometry characteristics, in particular texture zone circumferential/radial extent, dimple depth, and texture ...
This paper presents an optimization study of the geometry of three-dimensional micro-thrust beari... more This paper presents an optimization study of the geometry of three-dimensional micro-thrust bearings in a wide range of convergence ratios. The optimization goal is the maximization of the bearing load carrying capacity. The bearings are modeled as micro-channels, consisting of a smooth moving wall (rotor), and a stationary wall (stator) with partial periodic rectangular texturing. The flow field is calculated from the numerical solution of the Navier-Stokes equations for incompressible isothermal flow; processing of the results yields the bearing load capacity and friction coefficient. The geometry of the textured channel is defined parametrically for several width-to-length ratios. Optimal texturing geometries are obtained by utilizing an optimization tool based on genetic algorithms, which is coupled to the CFD code. Here, the design variables define the bearing geometry and convergence ratio. To minimize the computational cost, a multi-objective approach is proposed, consisting ...
Journal of Engineering for Gas Turbines and Power, 2012
In the present paper, a study of stiffness and damping in sector-pad micro thrust bearings with a... more In the present paper, a study of stiffness and damping in sector-pad micro thrust bearings with artificial surface texturing is presented, based on computational fluid dynamics (CFD) simulations. The bearing pads are modeled as consecutive three-dimensional independent microchannels, each consisting of a smooth rotating wall (rotor) and a partially textured stationary wall (stator). CFD simulations are performed, consisting in the numerical solution of the Navier–Stokes equations for incompressible isothermal flow. The goal of the present study is to characterize the dynamic behavior of favorable designs, identified in previous optimization studies, comprising parallel and convergent thrust bearings with rectangular texture patterns. To this end, a translational degree of freedom (DOF) along the thrust direction and a rotational (tilting) DOF of the rotor are considered. By implementing appropriate small perturbations around the equilibrium (steady-state) position and processing the...
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