bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature Distribution and Thermal Degradation of the Lubricant in EHL Line Contact Conjunctions
Authors: Hsiao Hsing-Sen S. , Hamrock Bernard J.
Published: June 2008

Temperatures of the lubricant in the inlet zone, the central contact zone, and the outlet zone of EHL line contact conjunctions were simultaneously calculated for a complete range of sliding conditions by using the new thermal circular non-Newtonian EHL model. The complete range of sliding conditions includes pure rolling, normal sliding, simple sliding, and opposite sliding. The result reveals the strong effect of sliding condition on the magnitude and distribution of the lubricant temperature. It also shows the diffusion time effect on the lubricant temperature, which builds up near the slower moving solid surface in the inlet zone. The effect of viscous normal stress on the temperature distribution was also investigated. Results indicate that the effect is not significant except for applications at very high entraining velocities. In addition, thermal degradation of the lubricant was studied. Results show that it may become a problem for applications at high speeds and high sliding conditions.
The Effects of Surface Roughness and Topography on the Contact Behavior of Elastic Bodies
Authors: Ren Ning , Lee Si C.
Published: June 2008

The real area of contact, average gap, and mean asperity contact pressure are important variables for understanding friction, wear, and lubrication in contacting systems. They are known as “performance variables.” Contact simulations were conducted for a wide range of surface topographies and an extensive amount of information was generated. Using this information, the performance variables were curve fitted and convenient mathematical relationships were formulated. The surfaces used in the simulations were numerically generated and they varied widely in statistical roughness properties, ranging from isotropic to strongly anisotropic. The effect of surface roughness on the performance variables were studied using three parameters—composite Root Mean Square roughness (σ), autocorrelation length (λx*), and asperity aspect ratio (γ).
A Fractal Theory of the Interfacial Temperature Distribution in the Slow Sliding Regime: Part I—Elastic Contact and Heat Transfer Analysis
Authors: Wang S. , Komvopoulos K.
Published: June 2008

The frictional temperature rises at the microcontacts of rough surfaces are analyzed by characterizing the surfaces as fractals and assuming Hertzian contacts of spherical asperity tips. The maximum temperature rise of a fractal surface domain in the slow sliding regime, where transient effects are negligible, is expressed as a function of thermomechanical properties, sliding speed, friction coefficient, real and apparent contact areas of the fractal domain, and fractal parameters. The distribution density function of the temperature rise at the real contact area is also determined based on the statistical temperature rise distributions of individual microcontacts and the maximum temperature rise of a fractal domain. This function characterizes the fractions of the real contact area subjected to different temperature rises, and can be used to analyze tribological interactions on dry and boundary-lubricated sliding surfaces.
A Fractal Theory of the Interfacial Temperature Distribution in the Slow Sliding Regime: Part II—Multiple Domains, Elastoplastic Contacts and Applications
Authors: Wang S. , Komvopoulos K.
Published: June 2008

The limitation of the fractal theory as applied to real surfaces is interpreted, and engineering surfaces are considered as a superimposition of fractal structures on macroscopic regular shapes by introducing the concepts of fractal-regular surfaces and multiple fractal domains. The effects of frictional heating at neighboring microcontacts are analyzed, and a simple solution of the temperature distribution is obtained for contact regions that are appreciably larger than a fractal domain. It is shown that the temperature rise at an elastoplastic microcontact does not differ significantly from that at an elastic microcontact of a similar geometry under the same load. The fractional real contact area subjected to temperature rises greater than any given value is represented by a complementary cumulative distribution function. The analysis yields that the average value and standard deviation of the temperature rise at the real contact area are 0.4 and 0.24 times the maximum temperature rise, respectively. The implications of the theory in boundary lubrication are demonstrated in light of results for ceramic materials.
A Deterministic Conformal/Counterformal Microcontact Model
Authors: Aronov V. , Nair S. , Wang J. M.
Published: June 2008

Microcontact models, describing contact of two rough surfaces, are fundamental for the modeling of friction and wear. This paper presents a critical analysis of existing models and discusses their limitations. A new deterministic microcontact model based on conformal/counterformal contact of asperities, as opposed to the combined surface statistics and counterformal asperity contact, is presented. Based on this model, a computer program has been developed. The input data are the digitized 3-D topographies, separately measured from the two contacting surfaces. The program first determines the original mating position and then calculates the surface contact parameters: contact radius, contact pressure, real contact area and the number of elastic, plastic, conformal, and counterformal contacts.
Effect of Three-Dimensional Near Surface Defects on Rolling and Sliding Contact Fatigue
Authors: Liu M. , Farris T. N.
Published: June 2008

The effect of three-dimensional defects such as voids and inclusions on the sliding contact subsurface stress distribution and fatigue limit is investigated. Three-dimensional finite-infinite boundary elements are utilized to model the body by discretizing its surface and the interface between the solid and the defect. The multidomain boundary element formulation is employed to accurately model the effect of the inclusion on the stress distribution. It is shown that the void has a greater near field effect on the stress distribution while the effect of a stiff inclusion applies over a larger distance. The critical stress points during a load passage are predicted based on a search of the maximum distortional strain energy. The stresses at the critical points are used for the fatigue limit pressure estimation by incorporating the equivalent stress concept with the Haigh diagram. The model predicts that a near surface spherical void of given size and depth reduces the maximum allowable fatigue limit design pressure by 75 percent while a stiff inclusion of the same size, shape, and location reduces it by 25 percent.
Contact Analysis of a Smooth Ball on an Anisotropic Rough Surface
Authors: Poon C. Y. , Sayles R. S.
Published: June 2008

The effects of surface roughness and waviness upon the real contact areas, gaps between contact spots, and asperity contact pressures were studied. The distribution of real areas, gaps, and contact pressures are presented for different surface roughness, σ and correlation lengths, β*. The load-area relationship is compared to Bush’s model of strongly anisotropic rough surface contact using a stochastic approach.
Fracture Mechanisms of Ceramic Coatings in Indentation
Authors: Diao D. F. , Kato K. , Hokkirigawa K.
Published: June 2008

Indentation tests on ceramic coatings are carried out in this paper and fracture pattern diagrams as a function of normal load and coating thickness are introduced for Al2O3 and TiN coatings deposited on cemented carbide substrates. Ring cracks are observed in indentation with a blunt indenter (1.5 mm radius), while lateral and radial cracks are observed in indentation with a sharp indenter (0.2 mm radius). The mechanisms for generating ring and lateral cracks are analyzed using elastic theory and equations predicting the critical loads for crack generation are introduced. The effective elastic modulus Eeff, is introduced for this analysis as a new elastic parameter for a coated surface. The results show that the theoretically calculated critical loads for crack generation agree well with the experimental ones.
The Effects of Low-Energy-Nitrogen-Ion Implantation on the Tribological and Microstructural Characteristics of AISI 304 Stainless Steel
Authors: Wei R. , Shogrin B. , Wilbur P. J. , Ozturk O. , Williamson D. L. , Ivanov I. , Metin E.
Published: June 2008

The effects of nitrogen implantation conditions (ion energy, dose rate, and processing time) on the thickness and wear behavior of N-rich layers produced on 304 stainless-steel surfaces are examined. Surfaces implanted at elevated temperatures (≈400°C) with 0.4 to 2 keV nitrogen ions at high dose rates (1.5 to 3.8 mA/cm2) are compared to surfaces implanted at higher energies (30 to 60 keV) and lower current densities (0.1 to 0.25 mA/cm2). The most wear-resistant surfaces are observed when the implanted-ion energy is near 1 keV and the dose is very large (> 2 × 1019 ions/cm2). Typically, surfaces implanted under these optimum conditions exhibit load-bearing capabilities at least 1000 times that of the untreated material. Some comparisons are also made to surfaces processed using conventional plasma-nitriding. Samples treated using either process have wear-resistant surface layers in which the nitrogen is in solid solution in the fcc phase. It is argued that the deep N migration (> 1 μm) that occurs under low-energy implantation conditions is due to thermal diffusion that is enhanced by a mechanism other than radiation-induced vacancy production.
Nonaxisymmetric Effects for Three-Dimensional Analysis of a Brake
Authors: Floquet A. , Dubourg M. C.
Published: June 2008

A three-dimensional model of axisymmetric moving solids submitted to nonaxisymmetric transient heat flux conditions is presented in this paper. Temperature fields are obtained using a new hybrid FFT-FEM method that combines Fourier transform techniques and finite element method. A fast Fourier form algorithm is used which leads to inexpensive computer time. Validation tests are presented. Efficiency of the method is demonstrated.
An Experimental Investigation of Frictionally-Excited Thermoelastic Instability in Automotive Disk Brakes Under a Drag Brake Application
Authors: Lee Kwangjin , Barber J. R.
Published: June 2008

Thermoelastic instability in an automotive disk brake system is investigated experimentally under drag braking conditions. The onset of instability is clearly identifiable through the observation of nonuniformities in temperature measured using embedded thermocouples. A stability boundary is established in temperature/speed space, the critical temperature being attributable to temperature-dependence of the brake pad material properties. It is also found that the form of the resulting unstable perturbations or eigenfunctions changes depending upon the sliding speed and temperature.
Frictional Heating of a Composite
Authors: Obara Shingo , Kato Takahisa
Published: June 2008

A numerical analysis has been carried out for a three-dimensional frictional heating problem of a composite material, in which an asperity contact (heat source) moves across the boundary of the constituents of a composite. The mathematical model adopted here is that a surface asperity on a semi-infinite body slides on a composite which consists of two semi-infinite bodies. Expressions of temperature distribution both in the composite and on the moving surface are derived. Then the temperature distribution and its change with time are obtained by a numerical procedure. It is shown that the temperature rise caused by frictional heating remarkably depends on the thermal properties of constituents of the composite, and temperature distributions in the vicinity of the asperity contact both on the moving surface and in the composite rapidly change when the asperity passes over the boundary of the constituents. The effect of the frictional heating on the worn surface profile of a composite is also discussed.
Chemically Assisted Machining of Ceramics
Authors: Wang J. C. , Hsu S. M.
Published: June 2008

Ceramics are hard and brittle. Machining such materials is time-consuming, difficult, and expensive. Current machining technology requires stiff machine, high hardness tools, and small material removal rates to minimize surface damage. This study demonstrates the feasibility of a novel ceramic machining concept that utilizes chemical reactions at the tool-workpiece interface to reduce the stress and minimize the surface damage. A series of cutting tests using a diamond wheel on silicon nitride with different chemical compounds has been performed. The results demonstrate that by using different chemistries, the material removal rate and the surface finish of the machined ceramic can be significantly altered. Some halogenated hydrocarbons show a significant improvement over some commercial machining fluids currently in use.
Effects of Friction on Contact of Transverse Ground Surfaces
Authors: Mann J. B. , Farris T. N. , Chandrasekar S.
Published: June 2008

The two-dimensional plane-strain sliding contact of a smooth rigid roller on a transverse ground rough surface is analyzed. The rough surface is idealized as an elastic half-space with periodic roughness modeled as cylindrical ridges oriented transverse to the sliding direction. The contact problem is solved using a numerical iterative method in which each asperity contact is treated as a micro-Hertz contact, and the exact treatment of asperity interaction is included. The subsurface stress field is calculated using Westergaard stress functions. The subsequent analysis compares the rough surface stress fields with the corresponding smooth Hertz contact to evaluate the influence of surface roughness and friction on the subsurface stress distributions. The results show that the real area of contact is less than the corresponding smooth surface Hertz contact area, and the magnitude of the actual localized maximum contact pressure is always greater than the corresponding smooth surface contact pressure. The asperity level subsurface effective stresses are greater in magnitude than the maximum subsurface stress due to the macro-Hertz contact for low coefficients of friction, and for high coefficients of friction the maximum effective stresses occur on the bulk material surface.
Observations of Wear of Abrasive-Waterjet Nozzle Materials
Authors: Hashish M.
Published: June 2008

This paper addresses the wear characteristics of the mixing tube of an abrasive-waterjet nozzle. An effective nozzle material should possess high values of both hardness and toughness. The mixing tube, which is where the abrasives are mixed, accelerated, and focused with the high-pressure waterjet, is the component in the abrasive-water jet nozzle that receives the greatest wear. Accelerated wear tests were conducted on relatively soft (steel) mixing tubes using a typical soft abrasive (garnet sand) and on harder (tungsten carbide) tubes using a harder abrasive material (aluminum oxide). A wide range of candidate tool materials, including several carbides and ceramics, was also tested using actual machining parameters. The tungsten carbide grades exhibited greater longevity than the harder ceramics, such as boron carbide, when garnet abrasives were used. The reverse trend was observed with aluminum oxide abrasives. Wear trends suggest that the wear mechanisms along the mixing tube change from erosion by particle impact at the upstream sections to abrasion at the downstream sections. Linear cutting tests were also conducted on several candidate nozzle materials to gain more information related to wear performance. It was found, for example, that the binder in tungsten carbide, which controls these properties, is a critical factor that also controls the lifetime of tungsten carbide mixing tubes.
Friction and Wear Properties of Chemomechanically Polished Diamond Films
Authors: Gupta B. K. , Malshe Ajay , Bhushan Bharat , Subramaniam Vish V.
Published: June 2008

The role of surface roughness on the coefficient of friction and wear of polished CVD diamond films has been investigated. Diamond films grown on single crystal silicon (001) substrates by Hot Filament Chemical Vapor Deposition (HFCVD) process were polished by a chemomechanical process in which a diamond film was polished against another diamond film in the presence of a fused alkaline oxidizer at 320°C. Friction and wear properties of these polished films were measured at elevated temperatures and in the presence of various gaseous environments. The coefficient of friction of the polished diamond films was found to be about 0.09, which is very close to that of natural diamond (0.07). The wear rate of the mating alumina ball slid against a polished diamond film was also found to be comparable, when slid natural diamond.
Tribological Behavior of Amorphous Hydrogenated Carbon Films on Silicon
Authors: Gangopadhyay A. K. , Vassell W. C. , Tamor M. A. , Willermet P. A.
Published: June 2008

Unlike polycrystalline diamond films, amorphous hydrogenated carbon (AHC) films can be deposited at room temperature, are amorphous in atomic structure, and form very smooth surfaces. Amorphous hydrogenated carbon film consists of very small 10–20 Å sp2 bonded (graphitic) clusters captured in a largely sp3 coordinated, partially hydrogenated random network of covalently bonded carbon. Because of the extreme stiffness of the carbon-carbon bond, this hydrocarbon composite, less dense even than graphite, exhibits hardness rivaling that of the hardest ceramics. We report a systematic study of the tribological characteristics of AHC films deposited on silicon substrates by radio frequency plasma assisted chemical vapor deposition. The friction and wear behavior of these films in sliding contact with a steel ball without any lubrication was evaluated as a function of film deposition conditions, contact stress, sliding speed, sliding distance, and relative humidity. The friction coefficient and the wear of both the contacting surfaces were found to increase with relative humidity. At low relative humidity, (a) the films exhibited friction coefficients in the range of 0.05–0.16 under a contact stress ranging from 0.83 to 1.66 GPa and a sliding speed ranging from 0.03 to 1 m/s and (b) the wear rates of the film and the steel ball were significantly lower than that of other hard coating, such as TiN or TiC, evaluated under similar conditions. These results are very encouraging for some tribological applications of AHC films.
Retainer-Dependent Wear of Silicon Nitride Bearings at High Temperatures
Authors: Mutoh Y. , Tanaka K. , Uenohara M.
Published: June 2008

Several retainer materials for high-temperature silicon nitride bearings to be used in air were examined with a high-temperature rolling three-ball tester. The “soft” or “self-lubricating” retainer materials (metal-bonded MoS2-WS2 alloys, a carbon-carbon composite and graphite) tended to yield longer bearing life, lower silicon nitride wear rate and lower traction coefficients than the hard retainers (Fe-based sintered alloys, chromia and silicon nitride). The main problem in the use of the self-lubricating retainer materials was their rapid wear by tribo-oxidation at high temperatures.
Numerical Simulation of Piston Ring in Mixed Lubrication—A Nonaxisymmetrical Analysis
Authors: Hu Yuanzhong , Cheng Herbert S. , Arai Takayuki , Kobayashi Yoichi , Aoyama Shunichi
Published: June 2008

The assumption of axisymmetry, employed by most of studies on piston ring lubrication, probably gives a too idealistic model for the real situation. A theoretical model for a nonaxisymmetrical analysis of piston ring lubrication has been established in the present study. When a piston ring with an arbitrary free shape is fitted into the cylinder bore, the determination of ring deflection and contact load has been modeled mathematically as a Linear Complementary Problem (LCP). By combining LCP solution with lubrication analysis, the film thickness and contact load distribution over the circumference are obtained, leading to a more realistic simulation for piston ring lubrication. The friction force between piston ring and cylinder bore is predicted by the mixed lubrication model including the effects of surface roughness and asperity contact. The static distortion of cylinder bore, gas pressure variation, and lubricant starvation are also considered in the simulation. Results show that the contact pattern and film thickness between piston ring and cylinder bore are not exactly axisymmetrical. The main reason for the nonuniform contact is the asymmetry of ring elasticity, the static distortion and dynamic load created by the secondary movement of piston skirt.
Squeeze Film Dynamics of Two-Phase Seals: Part II—Turbulent Flow
Authors: Yasuna J. A. , Hughes W. F.
Published: June 2008

A previously developed dynamic model for two-phase face seals is extended to include turbulent flow. Thermal transients over the entire range of laminar and turbulent flow are considered for both parallel and tapered geometries. Inlet losses and choking are accounted for in the turbulent regime. Axial responses to perturbations from equilibrium are examined, and general criteria are discussed for predicting stable, unstable and bistable seal operation.
Forces and Moments Due to Combined Motion of Conical and Cylindrical Whirls for a Long Seal
Authors: Kanemori Yuji , Iwatsubo Takuzo
Published: June 2008

The mutual interaction effects of cylindrical and conical whirl on the dynamic fluid forces and moments, which act on a long annular seal, were studied experimentally. A whirling motion composed of cylindrical and conical whirls is actuated by intentionally giving the phase difference between the seal exit and inlet whirling movements. This whirling motion is believed to generate during actual pump running. The experiment was conducted by changing the phase difference, at various rotor speeds and with a pressure difference between the seal inlet and exit. The result of this study revealed that fluid forces and moments are greatly dependent on the phase difference of the whirl, namely the long seal has a significant coupling between displacements and rotations. Furthermore, dynamic fluid forces and moments were derived theoretically, assuming that total fluid force acting on the rotor could be determined by superposing fluid forces due to conical and cylindrical whirling movements. It was confirmed that the experimental results moderately agree with the theoretical values, if the rotor and seal are set in concentric alignment, the principle of superposition becomes applicable.
Harmonic Balance Analysis of General Squeeze Film Damped Multidegree-of-Freedom Rotor Bearing Systems
Authors: Hahn E. J. , Chen P. Y. P.
Published: June 2008

Squeeze film dampers introduce nonlinear motion dependent damper forces into otherwise linear rotor bearing systems, thereby considerably complicating their analysis. Noncircular orbit type dampers, such as unsupported or uncentralized dampers, have generally necessitated transient solutions, which are computationally prohibitive for design studies of large order systems, particularly for systems with low damping. By utilizing harmonic balance with appropriate condensation, it is possible to considerably reduce the number of simultaneous nonlinear equations inherent to this approach. The stability (linear) of the equilibrium solutions may be conveniently evaluated using Floquet theory, particularly if the damper force components are evaluated in fixed, rather than rotating, reference frames. The versatility of this technique is illustrated on systems of increasing complexity with and without damper centralizing springs. Of particular interest, is its applicability to unsupported systems illustrating how such systems can lift off and, with further increase in speed, the damper forces can be linearized about the orbit center.
Test Results for Rotordynamic Coefficients of Anti-Swirl Self-Injection Seals
Authors: Kim C. H. , Lee Y. B.
Published: June 2008

Test results are presented for rotordynamic coefficients and leakage for three annular seals which use an anti-swirl self-injection concept to yield significant improvement in whirl frequency ratios as compared to smooth and damper seals. A new anti-swirl self-injection mechanism is achieved by deliberately machining self-injection holes inside the seal stator to partially divert inlet flow into the anti-swirl direction. The anti-swirl self-injection mechanism is used to achieve effective reduction of the tangential flow which is considered as a prime cause of rotor instability in high performance turbomachinery. Test results show that the self-injection mechanism significantly improves whirl frequency ratios; however, the leakage performance degrades due to the introduction of the self-injection mechanism. Through a series of tests, an optimum anti-swirl self-injection seal which uses a labyrinth stator surface with anti-axial flow injections is selected to obtain a significant improvement in the whirl frequency ratio as compared to a damper seal, while showing moderate leakage performance. The best whirl frequency ratio is achieved by an anti-swirl self-injection seal of 12 holes anti-swirl and 6 degree anti-leakage injection with a labyrinth surface configuration. When compared to a damper seal, the optimum configuration outperforms the whirl frequency ratio by a factor of 2.
The Effects of Baseplate Warpage and Skew on the Configuration of a Spinning Flexible Disk
Authors: Wu R. Y. , Adams G. G.
Published: June 2008

The behavior of a flexible disk, spinning in close proximity to a warped/skewed stationary baseplate, is investigated. The governing partial differential equation for the disk deflection is coupled to the Reynolds equation of the air film. Four warped/ skewed baseplate configurations are modeled. The effects of baseplate warpage and skew on the steady-state configuration of the disk are determined by investigating small deviations away from the axisymmetric configuration of the disk corresponding to a perfect baseplate. Exponential Fourier series expansions in the circumferential direction, along with finite differences in the radial direction, are used. Numerical results are determined and compared for various values of the angular velocity and initial thicknesses of the air film. Among the three warpages considered, the saddle warped baseplate provides the largest change in disk deflection whereas the spherically warped baseplate gives the smallest change. The total response of the disk is obtained by superposition of the deflection change caused by the warped/skewed baseplate and the deflection obtained from the axisymmetric solution.
Experimental Behavior of a Short Cylindrical Squeeze Film Damper Executing Circular Centered Orbits
Authors: Zhang J. , Roberts J. B. , Ellis J.
Published: June 2008

The experimentally determined behavior of a short radial squeeze-film damper with no end seals, executing circular centered orbits, is discussed. Accurate circular orbits were achieved, for ε values in the range 0.1 and 0.8, by using digitally generated signals to drive two electromagnetic shakers. Radial and tangential dynamic fluid force coefficients were estimated from measurements of the applied forces and the orbit radii, using a simple algebraic method. Cavitation was found to occur when ε exceeded 0.5, at large orbit frequencies, and was the cause of an observed jump-up phenomenon. The magnitude of an oil stiffness effect, previously reported by the authors and confirmed by the present results, was found to depend significantly on the oil supply pressure. Its contribution to the total fluid force was of the same order as that from fluid inertia, in the case of small orbits (ε ≪ 1).
Numerical Simulation of the Lubrication of the Head-Disk Interface Using a Non-Newtonian Fluid
Authors: De Bruyne Frank A. , Bogy D. B.
Published: June 2008

The prospect of contact and near-contact recording in magnetic hard disk files naturally leads to reduced flying heights of the read-write head over the rigid disk. To avoid dry contact at these low head-to-disk spacings, a lubricant should be used to minimize wear and maximize reliability. Since fluids generally have a much greater viscosity than air and very large shear rates develop under the slider, it is believed that a fully flooded interface can only be practically possible if the fluid possesses a non-Newtonian character with a significant amount of shear-thinning. In this paper, we present results from extensive numerical simulations of the fully flooded head-disk interface using the finite element technique. This approach has proven very successful in calculating a wide variation of slider geometries for various fluid nonlinearities.
A Transient EHL Analysis for Line Contacts With Measured Surface Roughness Using Multigrid Technique
Authors: Ai Xiaolan , Cheng Herbert S.
Published: June 2008

Transient EHL analysis for line contacts with measured surface roughness is performed by using the multigrid method. Results show that the transient effect, induced by surface roughness, has a remarkable influence on pressure distribution and the film thickness profile. Pressure fluctuation increases with the relative sliding speed between the contact surfaces. For simple sliding with stationary surface roughness, the roughness profile is almost flattened. When the rough surface moves, the elastically deformed surface roughness, in the contact zone, increases with the moving speed. As the moving speed of surface roughness equals to or exceeds the rolling speed, the roughness of the deformed surface profile in the contact zone is close to the roughness of the undeformed roughness profile.
Non-Newtonian and Thermal Effects on Film Generation and Traction Reduction in EHL Line Contact Conjunctions
Authors: Hsiao Hsing-Sen S. , Hamrock Bernard J.
Published: June 2008

The thermal circular non-Newtonian model accompanied with three specialized models was used to study the mechanisms of film generation and traction reduction in EHL line contact conjunctions. Results revealed that the film generation capability is mainly controlled by the inlet zone pressure buildup and the inlet zone piezo-thickening. The diffusion time effect enhances the thermal thinning that reduces this capability. On the other hand, the piezoviscosity and the shear rate in the central contact zone are the main traction generation factors whereas shear skidding and thermal skidding are the traction reduction mechanisms. Results also showed that neglecting either viscous heating or the combination of shear thinning and shear stress reduction in formulating an EHL simulator is inaccurate for many cases.
Normal Instantaneous Squeeze Film Force for a Finite Length Cylinder
Authors: Lu Yong , Rogers Robert J.
Published: June 2008

A theoretical model for the normal instantaneous squeeze film force for a finite length cylinder is developed in this paper. The model assumes large unidirectional cylinder motion along a sleeve diameter. Based on the assumption of a parabolic flow field, a normal squeeze film model for an infinitely long cylinder is first obtained. Combining the infinitely long model with side-leakage factors, a finite length model is then obtained. The model shows that the instantaneous squeeze film force consists of three position-dependent nonlinear terms: namely a viscous term, an unsteady inertia term and a convective inertia term. From experimental measurements using water and a clearance to radius ratio of 0.032, the viscous term of the theoretical model should be corrected by a factor involving the instantaneous squeeze film Reynolds number and the absolute value of instantaneous eccentricity. The synthesized squeeze force waveforms obtained using the corrected equation with averaged weighting coefficients agree very well with the experimental waveforms for eccentricity ratios up to 0.9 and a wide frequency range. The corrected equation is suitable for the calculation of the normal instantaneous squeeze film force given the instantaneous position, velocity, and acceleration of the cylinder center.
A Numerical Model for Elastohydrodynamic Analysis of Plunger and Barrel Clearances in Fuel Injection Equipment
Authors: Gibson D. H. , Dionne P. J. , Singhal A. K.
Published: June 2008

This paper describes a numerical model developed to predict the elastohydrodynamic (coupled solid-fluid) response of unit injector fuel systems. These systems consist of a concentric barrel and plunger with a small annular clearance. During operating (axial movement of the plunger), highly nonuniform pressure and clearance fields are developed which are strongly coupled with each other. The model simultaneously solves for the transient response of the fluid film pressure distribution and three different structural deformation components in a two-dimensional (axial-circumferential) domain. These structural components are the transverse bending of the plunger, radial expansion of the barrel, and radial growth of the plunger from a Poisson effect. The fluid film pressure distribution is governed by the transient Reynolds equation (i.e., lubrication theory) and the structural deformation components governed by linear elastic theory. Full account is taken of these hydrostatic, hydrodynamic, and squeeze-film forces generated in the fluid. The model has been applied to several injector designs. Results have been compared with known performance characteristics and have been found to be qualitatively accurate, in that locations of plunger/barrel contact, and potential for failure, have been accurately predicted.
A Study of the Mechanism of Lubrication in Porous Journal Bearings: Effects of Dimensionless Oil-Feed Pressure on Static Characteristics Under Hydrodynamic Lubrication Conditions
Authors: Kaneko Satoru , Ohkawa Yasuhiro , Hashimoto Yuji
Published: June 2008

The static characteristics of porous journal bearings under hydrodynamic lubrication conditions are theoretically investigated assuming that the oil is fed through their outside diameters under a small pressure. The angular extent of the oil film formed in the bearing clearance is numerically solved on the basis of the following postulate: when the oil film extent reaches steady state, the inflow of oil into the bearing clearance through the porous matrix due to the oil-feed pressure must make up for the oil leakage from the ends through the clearance gap and that into the porous matrix due to the hydrodynamic pressure in the film. Numerical results show that the dimensionless oil-feed pressure significantly influences the static characteristics. Experiments are also conducted for confirmation after the theoretical examination.
Investigation of Parameters Affecting the Limiting Shear Stress-Pressure Coefficient: A New Model Incorporating Temperature
Authors: Wikstro¨m Victoria , Ho¨glund Erik
Published: June 2008

When calculating film thickness and friction in elastohydrodynamically lubricated contacts, assuming a non-Newtonian fluid, the lubricant limiting shear stress is an essential parameter. It influences minimum film thickness and determines traction in the contact. The limiting shear stress is pressure dependent according to the Johnson and Tevaarwerk equation: τL=τ0+γp The limiting shear stress-pressure coefficient γ has in a previous screening investigation been shown to depend on several parameters: oil type, oil viscosity at + 40°C, maximum contact pressure and temperature. In the present investigation, the preliminary data is used together with response surface methodology. With these results in mind, further experiments are made and an empirical model is built. This paper presents a new model for γ which is valid for two types of oil (a polyalphaolefine with diester and a naphthenic oil) with different viscosities at +40°C. The model incorporates the influence of maximum contact pressure and oil temperature on γ. The measurements on which the model is based were carried out at temperatures ranging from −20 to + 110°C. The pressure range was 5.8–7 GPa and the shear rate was about 106 s−1.
Dynamic Analysis of Tilting-Pad Journal Bearing—Influence of Pad Deformations
Authors: Desbordes H. , Fillon M. , Wai C. Chan Hew , Frene J.
Published: June 2008

A theoretical nonlinear analysis of tilting-pad journal bearings is presented for small and large unbalance loads under isothermal conditions. The radial displacements of internal pad surface due to pressure field are determined by a two-dimensional finite element method in order to define the actual film thickness. The influence of pad deformations on the journal orbit, on the minimum film thickness and on the maximum pressure is studied. The effects of pad displacements are to decrease the minimum film thickness and to increase the maximum pressure. The orbit amplitude is also increased by 20 percent for the large unbalance load compared to the one obtained for rigid pad.
New Results in High Pressure and Low Temperature Rheology of Liquid Lubricants for Space Applications
Authors: Prat P. , Vergne P. , Sicre J.
Published: June 2008

This paper presents a rheological investigation conducted on three lubricants used in space applications under vacuum. Lubricant behavior was determined for representative pressures and temperatures. Relations between standardized methods and space requirements are discussed. Finally, the elastohydrodynamic load-carrying capacity of the three fluids in the Newtonian domain is computed over a wide temperature range and compared to those of two reference base oils.
Local Reduction in Thickness of Point Contact EHL Films Caused by a Transversely Oriented Moving Groove and Its Recovery
Authors: Kaneta M. , Nishikawa H.
Published: June 2008

Transient behavior of elastohydrodynamic (EHL) films caused by a transversely oriented groove passing through the EHL conjunction is directly observed using the optical interferometry technique. A steel ball on which the groove is formed is run against a smooth glass disk under conditions of rolling with sliding. It is shown that a local reduction in film thickness caused by the groove and its recovery depend strongly on shear flow dominating the EHL conjunction and side leakage along the groove. Surface kinematic conditions and the relative size of the groove with respect to the overall film thickness have also profound effects on the above phenomena.
A Study on Flow Phenomena in Mixed Lubrication Regime by Porous Medium Model
Authors: Lo Sy-Wei
Published: June 2008

A porous medium model is basically a macroscopic description of the fluid behavior and is considered more appropriate to the mixed lubrication regime. The present work has successfully combined the flow factor method, porous medium model and continuum percolation theory to investigate the flow phenomena in the mixed lubrication regime. Not only the lubricant flow can be solved but also the anisotropic percolation threshold can be obtained. Instead of the flow factor which is singular near h = 0, the lubricant flux itself is considered. A new definition of the pressure flow factor which is more appropriate to the mixed lubrication realm is suggested.
Generalized Reynolds Equation for Micropolar Fluid With Microrotations Near Surface and Its Application to Journal Bearings
Authors: Bessonov N. M.
Published: June 2008

The theory of micropolar liquid lubrication (see Prakash and Sinha, 1975; Tipei, 1979; Singh and Sinha, 1982) takes into account only the increasing of effective viscosity in thin layers. Modern experiments (see Derjaguin et al., 1985) show that effective viscosity can increase or decrease and approaches to a certain limit (boundary viscosity), depending on the type of liquid and nature of the solid surface. A new generalized Reynolds equation that takes into account both these effects and also all possible situations in microrotation near the friction surface is derived in this work. An example using the equation for calculation of the journal bearing performance is given. It is shown that the friction coefficient can be sufficiently decreased without a noticeable change in the load capacity by regulation of interaction between micropolar lubricant and surfaces.
Comments on Rolling Resistance
Authors: Benditkis R. , Necsulescu D. S.
Published: June 2008

Rolling resistance model, developed based on rigid body assumption, is further analyzed in this paper using a simplified analytical model and experimental results. The analysis leads to the conclusion that the effect of the reaction forces is equivalent to a resultant having the point of contact located outside the zone of contact.
Models for the Directional Distribution of the Diffuse Sky Radiance
Authors: Siala F. M. F. , Rosen M. A. , Hooper F. C.
Published: June 2008

Both traditional and recently proposed models for the angular distribution of diffuse sky radiance are reviewed and compared. The models considered include basic models, sky-type-specific models, empirical and semi-empirical continuous distribution models, models which account for time-dependent cloud effects and stochastic models.
Thermodynamics of Energy Storage by Melting Due to Conduction or Natural Convection
Authors: De Lucia M. , Bejan A.
Published: June 2008

This paper describes the most basic thermodynamic aspects of the process of energy storage by melting of a phase change material when the energy source is a stream of hot single-phase fluid. The first part of the paper considers the melting process ruled by pure conduction across the liquid phase, and the second part deals with the quasi-steady melting dominated by natural convection. The paper establishes the relationship between the total irreversibility of the melting process and design parameters such as the number of heat transfer units of the heat exchanger placed between the energy source and the phase change material, the duration of the melting process, and the position of the energy storage process on the absolute temperature scale. It is shown that the exergy transfer to the melting material is maximized when the melting temperature (Tm) equals the geometric average of the environment temperature (Te) and the temperature of the energy source (T∞), in other words when Tm=(TeT∞)1/2. This conclusion holds for both conduction-dominated melting and convection-dominated melting.
Application of Model Reference Adaptive Control to Solar Thermal Utilization Systems
Authors: Tanaka Tadayoshi
Published: June 2008

A proportional plus integral plus derivative (PID) controller is used to obtain usable energy from the sun in almost all the solar systems in Japan. However, it is difficult to collect the heat continuously close to a prescribed temperature using a PID controller because the solar radiation is often interrupted by passing clouds. We investigated, therefore, a Model Reference Adaptive Control (MRAC) system. In order to demonstrate its effectiveness, we constructed a MRAC system and introduced it into the collector loop of a solar system. This paper gives an outline of the MRAC algorithm and describes the experimental results for the outlet fluid temperature response of the loop by the MRAC and PID. From these results, it is shown that the MRAC algorithm is suitable for controlling a system affected by irregular disturbances in the insolation.
Thermal Performance of a Heat Storage Module Using PCM’s With Different Melting Temperature: Experimental
Authors: Farid Mohammed M. , Kim Yongsik , Kansawa Atsushi
Published: June 2008

A latent heat storage module was constructed, consisting of 45 cylindrical capsules fixed vertically in 15 rows. The capsules, made of 0.335-m long copper tubes having external diameters of 31.8 mm, were fixed in an insulated rectangular duct. Three commercial waxes having melting temperatures of 44°C, 53°C, and 64°C were selected. Each of the three sets of 15 tubes was filled with different wax. For comparison purposes, experiments were also done with a single commercial wax, having a melting temperature of 53°C, in all the tubes. During heat charge, hot air flowed across the capsules such that the melting temperature of the waxes decreased in the flow direction. Air flow direction was reversed during heat discharge. Experimental measurements showed some improvement in the heat transfer rates during both heat charge and discharge when three types of PCM’s were used. There was no improvement in the heat transfer rate during the sensible heat storage period, while a maximum increase of 15 percent was observed during the latent heat period. Theoretical predictions for the performance of the storage module were in reasonable agreement with the experimental measurements.
The Measured Energy Impact of Infiltration in a Test Cell
Authors: Claridge David E. , Bhattacharyya Souvik
Published: June 2008

Infiltration is customarily assumed to increase the heating and cooling load of a building by an amount equal to the mass flow rate of the infiltration times the enthalpy difference between the inside and outside air—with the latent portion of the enthalpy difference sometimes neglected. Calorimetric measurements conducted on a small test cell with measured amounts of infiltration introduced under a variety of conditions show convincingly that infiltration can lead to a much smaller change in the energy load than is customarily calculated; changes as small as 20 percent of the calculated value have been measured in the cell. The data also suggest that the phenomenon occurs in full-sized houses as well. Infiltration Heat Exchange Effectiveness (IHEE), ε, is introduced as a measure of the effectiveness of a building in “recovering” heat otherwise lost (or gained) due to infiltration. Measurements show that ε increases as: (a) flow rate decreases; (b) flow path length increases; (c) hole/crack size decreases. There is a clear correlation between large values of ε and large values of the exponent, n, so fan pressurization results may be useful in predicting ε for buildings.
Thermodynamics of the Conversion of Solar Radiation
Authors: Shafey H. M. , Ismail I. M.
Published: June 2008

An important parameter in the analysis provided by the second law of thermodynamics for the evaluation of solar energy conversion systems is the radiation temperature. The solar radiation is diluted due to atmospheric scattering and absorption, and reflection at the absorber surface. This dilution reduces the temperature of the sun (as a blackbody source) to an effective radiation temperature. In the present study, spectral dilution functions were derived for the three components of the absorbed global terrestrial solar radiation (direct beam component + forward Mie component + diffuse component). The energy and entropy fluxes and effective temperature for each component were expressed in terms of these dilution functions. They were calculated for different atmospheric conditions using numerical integration over the wavelength. The effects of the air mass and the atmospheric parameters on these thermodynamic quantities and the maximum conversion efficiency of solar energy were investigated. An apparent temperature assigned for the sun as a high temperature reservoir for Carnot engine was calculated and found to vary between about 3600 K for clear sky and about 2000 K for highly turbid sky under the given atmospheric conditions.
A Liquid Sorption-Desorption System for Air Conditioning With Heat at Lower Temperature
Authors: Scalabrin G. , Scaltriti G.
Published: June 2008

An open process of summer air conditioning by dehumidification-humidification is presented and simulated. Air dehumidification is achieved in an absorption column through direct contact with a hygroscopic solution over a cooling tube bank inside which water flows, coming from a cooling tower. After heat exchange with the cooler outlet air, the processed air is adiabatically humidified. The diluted solution is reconcentrated in a desorber by direct contact with external air over a heating tube bank, inside which water flows, coming from an external heating or heat recovery system. A heat exchanger recovers sensible heat between inlet and outlet air. The independent variables controlling the process and their variation effects on its performance are defined through the study of three different modes of operation. The system performance for a certain number of cases are presented as well.