bluuebunny/crossref_metadata_2025 huggingface dataset:
A Dynamic Friction Model for Unlubricated Rough Planar Surfaces
Authors: Shi Xi , Polycarpou Andreas A.
Published: September 2003

Modeling dynamic or kinetic friction for realistic engineering surfaces continues to be a challenge, partly due to the coupling between system dynamics and interfacial forces. In this paper, a dynamic friction coefficient model for realistic rough surfaces under external normal vibrations is developed. From the system dynamic model, the instantaneous time varying normal separation at the interface is obtained under normal harmonic excitation. Subsequently, the instantaneous dynamic contact and tangential (friction) forces are calculated as a function of the instantaneous normal separation. The dynamic friction coefficient defined as the ratio of the time varying friction to the interfacial normal forces that explicitly includes interfacial damping, is also calculated. The results show that a mean increase in the instantaneous normal separation may or may not lead to a decrease of the mean friction force and the mean friction coefficient, which is supported by published data. For unlubricated elastic sliding contact conditions considered in this paper, the effect of damping on the dynamic friction coefficient is found to be negligible, whereas loss of contact causes significant apparent dynamic friction force and dynamic friction coefficient reductions. Several different interpretations of the time varying dynamic friction coefficient are presented and the implications of using a simple constant value to represent the time varying dynamic friction coefficient are discussed.
Possibility of Organic Monolayer Films as Lubricants for Disk Drives: Comparative Study of PFPE and Organosilane
Authors: Choi Junho , Kawaguchi Masahiro , Kato Takahisa
Published: September 2003

Organosilane monolayer of 1H,1H,2H,2H-perfluorodecyltrichlorosilane and PFPE lubricant films were deposited on magnetic hard disk surfaces protected with amorphous carbon overcoats, and their frictional properties were investigated by a ball-on-flat tribotester. The thickness of PFPE films having only an immobile layer or both immobile and mobile layers was varied from about 0.6 nm to 4 nm. The friction coefficient of the organosilane monolayer coated surface is comparable to that of the PFPE coated surface with a 1.0 nm thick immobile layer and lower than that of the PFPE coated surface with a 0.6 nm thick immobile layer. The thickness of the lubricant has to decrease to less than 1 nm to further increase recording density. In that case, the mobile portion of PFPE lubricant is negligible. This study suggests that organosilane monolayers (with shorter chain lengths) may be used as an alternate lubricant in hard disk drive systems.
Measurement of Velocity-Dependent Adhesive Force Exerted on Magnetic Disk Using Micro-Probe of SPM
Authors: Mitsuya Yasunaga , Ohshima Yasuji
Published: September 2003

A novel method for measuring the velocity-dependent adhesive force exerted on a magnetic disk has been developed using the micro-probe of a scanning probe microscope (SPM). The deformation of a cantilever having a diamond tip was measured based on the Michelson laser interferometry, wherein an interference fringe pattern was formed over the cantilever. The fringe images were captured by an ultra-high speed CCD camera, and then the deformation of the cantilever was accurately captured through image processing for extracting ridgelines from the fringe pattern. The high-speed camera enabled observation of the fringe image variation while the tip was being separated from a target surface, and identification of the transient response of the cantilever arising after separation by using the regression analysis. Then, the separation instant and the separation force were identified as the values of the initial condition of the transient response. Finally, this novel method was applied to the measurement of the separation force exerted on a magnetic disk coated with a molecularly thin lubricant film. As a result, the separation force was found to be approximately proportional to the square root of the retractive velocity.
A Scale-Dependent Model for Multi-Asperity Contact and Friction
Authors: Adams George G. , Mu¨ftu¨ Sinan , Azhar Nazif Mohd
Published: September 2003

As loading forces decrease in applications such as MEMS and NEMS devices, the size of the asperity contacts which comprise the real contact area tend to decrease into the nano scale regime. This reduction in size of the contacts is only partially offset by the nominally increased smoothness of these contacting surfaces. Because the friction force depends on the real area of contact, it is important to understand how the material and topographical properties of surfaces contribute to friction forces at this nano scale. In this investigation, the single asperity nano contact model of Hurtado and Kim is incorporated into a multi-asperity model for contact and friction which includes the effect of asperity adhesion forces using the Maugis-Dugdale model. The model spans the range from nano-scale to micro-scale to macro-scale contacts. Three key dimensionless parameters have been identified which represent combinations of surface roughness measures, Burgers vector length, surface energy, and elastic properties. Results are given for the friction coefficient versus normal force, the normal and friction forces versus separation, and the pull-off force for various values of these key parameters.
An Exact Explicit Analytical Solution of the Steady-State Temperature in a Half Space Subjected to a Moving Circular Heat Source
Authors: Laraqi Najib
Published: September 2003

So far in the literature, the distribution of stationary temperature over the surface of a half space subjected to a moving circular heat source has been reported in an integral or asymptotic form. In this paper, an exact explicit analytical solution is provided, which allows the determination of the temperatures over the contact area with a very short computational time, regardless of the value of the Peclet number. The solution is based on special functions (Bessel and hypergeometric functions) that are pre-programmed under a formal calculation software (e.g., Maple). The results of the proposed solution are in agreement with the asymptotic models available in the literature.
The Stress and Displacement Fields Produced in a Semi-Infinite Solid by a Uniform Heat Source Over a Rectangular Area on the Surface
Authors: Lin Yuan , Ovaert Timothy C.
Published: September 2003

An important problem in thermo-mechanical contacts is the determination of the stress and displacement fields caused by heat flow. Heat flow may come from a difference in temperature between the contacting solids, or from frictional heating at the sliding interface. Generally, the distribution of heat flow in a contact area is unknown. In many cases, however, it is approximately uniform or one may divide the contact area into small parts, and in each part the heat flow may be treated as approximately uniform. This work provides closed-form solutions of the stress and displacement fields in a semi-infinite solid caused by uniform steady-state heat flow over a rectangular area on the surface. The material is assumed to be homogeneous and isotropic.
Surface Normal Thermoelastic Displacement in Moving Rough Contacts
Authors: Liu Shuangbiao , Wang Qian , Harris Stephen J.
Published: September 2003

Computing the thermoelastic displacement of three-dimensional stationary or moving bodies subject to frictional heating is an essential numerical procedure for the complex modeling of the contact of tribological components. Surface Roughness inevitably causes the irregularity of the frictional heat distribution, and thus complicates the process of the numerical simulation of contact problems. The surface normal thermoelastic displacement has been studied in previous papers for either stationary bodies with irregularly distributed heat or moving bodies with regularly distributed heat. In this work, irregularly distributed frictional heat is applied on the surface of a moving body. Temperature and surface normal thermoelastic displacement are solved by using an efficient numerical procedure involving the discrete convolution and fast Fourier transform algorithm and frequency response functions. The thermoelastic displacement due to frictional heat that is proportional to the contact pressure is comparable to the elastic displacement caused by the contact pressure and is not sensitive to the roughness texture. The transient performance of multiple heat sources with different shapes, as well as the mutual influence, is also studied.
A New Mechanism of Asperity Flattening in Sliding Contact—The Role of Tool Elastic Microwedge
Authors: Lo Sy-Wei , Yang Tung-Sheng
Published: September 2003

Based on real-time observation of the workpiece surface in a series of Lo and Tsai’s (2002) compression-sliding experiment, it is found that the asperity contact area is much greater than that evaluated by the existing theorems such as the junction-growth theorem. With the aid of finite element analysis, it is verified that the tool sliding motion along with the minute elastic deformation (microwedge) of the tool surface around the asperity peaks increase the asperity contact area significantly even in a frictionless sliding. The microwedge induces a component of force along the sliding direction on the asperity. A combination of flattening and smearing effects can therefore aid in expanding the contact area. The greater the wedge angle, the stronger the propellent effect. An incremental model has also been developed to predict the evolution of contact area during sliding. The numerical simulation compares well with the experimental results. The new mechanism not only introduces an important tribological variable to forming processes, but also brings in a new concept of surface quality control for processes having a considerable sliding distance between workpiece and tool such as ironing, forging, and extrusion. New processes performing high relative sliding velocity can therefore be developed to ameliorate the brightness of products.
Mixed Lubrication of Coupled Journal-Thrust-Bearing Systems Including Mass Conserving Cavitation
Authors: Wang Yansong , Wang Q. Jane , Lin Chih
Published: September 2003

A mixed lubrication model is developed to investigate the lubrication of coupled journal-thrust-bearing systems. The governing equations are mapped into a computational domain with a conformal mapping technique developed in a previous study by Wang et al. (2002). In order to study the influence of the boundary conditions on the performance of the bearing system, either the Reynolds boundary conditions or the JFO’s mass conserving conditions are applied to the governing Reynolds equations. The performance of a typical coupled journal-thrust bearing system is numerically investigated and the effects of viscosity, misalignment angle and feeding scheme are discussed.
A Numerical Study of the Contact Mechanics and Sub-Surface Stress Effects Experienced Over a Range of Machined Surface Coatings in Rough Surface Contacts
Authors: Kadiric A. , Sayles R. S. , Zhou Xiao Bo , Ioannides E.
Published: September 2003

The paper employs a rough-surface numerical elastic contact method designed to analyze Hertzian elastic contact effects of surface coatings. In particular the paper explores the differences in the surface contact mechanics and the resulting sub-surface stresses experienced over a range of differing coating material-properties, thickness, and machined roughness levels in a quantitative manner. The effect of a range of surface roughness properties and in particular root mean square roughness (σ) and correlation length β*, on the magnitude and depth of maximum shear stresses in the layer under individual asperities is investigated. This is done for a hard and stiff, and also for a soft and compliant coating, and for two coating thicknesses in each case. The results suggest that the magnitude of the local shear stress increases with increasing ratio σ/β* approximately linearly. The depth of the maximum local shear stress is found to correlate best with β*, however a further clear trend is observed between this depth and the number of profile peaks. The depth also shows a relation to the ratio σ/β* but the correlation in this case is weaker with significant deviations. Neither the magnitude nor the depth of shear stresses shows any significant trend in relation to the roughness (σ) alone. The tensile stresses at the interface, and the subsequent potential for delamination, are also investigated and found to be significant. Approximate correlation between the magnitude of interface tensile stress and root mean square roughness is achieved, but no clear trend in relation to correlation length is evident.
Mixed EHL Analysis of the Variable Torque Slipping Clutch With Skewed Rollers
Authors: Feng Ming , Ono Kyosuke , Mimura Kenji
Published: September 2003

In this paper, a mixed EHL analysis is presented for the variable torque slipping clutch with skewed rollers. It is characterized by a finite spatial curve contact and two dimensional rolling and slipping. A combination of the average roughness Reynolds equation developed by Patir and Cheng and the pressure-compliance relationship of roughness contact developed by Greenwood and Tripp is adopted to combine the effect of surface roughness in the lubrication. The average roughness Reynolds equation is extended to involve the two dimensional rolling and slipping while the pressure dependence of both the density and viscosity are considered. Based on the numerical techniques of the FEM discretization and relaxation iteration, the average roughness Reynolds equation is solved simultaneously with the GT contact pressure-compliance relationship under the equilibrium condition of the elements of the slipping clutch. The hydrodynamic and asperity contact pressures are then calculated and the interactions and mutual influences between these two kinds of pressures are shown. In addition, the influences of the rotation speed on the resistant torque are investigated both theoretically and experimentally. Concurring agreements are found between the theoretical and experimental data.
On the Frictional Damping Characterization of Compliant Bump Foils
Authors: Salehi Mohsen , Heshmat Hooshang , Walton James F.
Published: September 2003

High-speed rotor systems use either fluid film or rolling element bearing supports, depending upon their design and operating constraints. Regardless of bearing type used, these systems require specific bearing and support stiffness and damping characteristics to achieve the desired stable and low vibration operation. Building upon the technology of thin metallic corrugated bump foils presently used in a particular class of film riding hydrodynamic bearings, a novel corrugated bump foil damped mount is introduced which provides stiffness and damping for application with rolling element bearings. These damping elements are capable of operating at elevated temperatures where implementation of conventional squeeze film dampers is ruled out. The frictional damping results from micro-slip motions between the bump foils and the mating surfaces. A semi-empirical model, based on a one degree of freedom model was developed in which damping is replaced by an equivalent frictional force in order to gain insight into the dynamic friction coefficient of the individual damping element interfaces. Experimental results, obtained in the form of hysteresis loops were compared to the developed model with good agreement. The variation in damping and dynamic coefficient of friction was found to be dependent primarily upon three factors: vibration frequency, amplitude of motion and applied static load. These parameters were tested within the range of 50–1400 Hz, 2.54–12.7 micron and 45–135 N, respectively. The tests were conducted at room and 538°C ambient temperatures under both dry and vapor phase lubricated conditions. Using the resulting empirical data, several bearing dampers were designed, built and tested in a small, high-speed gas turbine engine simulator. The tested novel foil dampers were capable of operating reliably under extremely high levels of shaft imbalance (i.e., 320 times greater than the air bearing supported with specification of 0.0002 oz-in) even while operating at temperatures to 560°C. These results show the great potential for wide application of these dampers on gas turbine engines and high-speed rotating machinery.
A Preliminary Nonlinear Analysis of the Axial Transient Response of the Sector-Shaped Hydrodynamic Thrust Bearing-Rotor System
Authors: Zhu Q. , Zhang W. J.
Published: September 2003

This paper describes a nonlinear model and analysis of the axial transient response of the sector-shaped hydrodynamic thrust bearing-rotor system in a turbo-expander under a suddenly applied step load. The model is comprised of a time-dependent Reynolds Equation for oil film forces, and a vibration equation for the axial shaft system. The time-dependent form of the Reynolds Equation is solved by a finite difference method with a successive over-relaxation scheme, and the vibration equation is solved by the fourth-order Runge-Kutta method and the Adams method. In addition, a linear analysis is attempted in order to evaluate its suitability for the situation under consideration. The result of the analysis has shown that the linear model is unsuited, while the nonlinear analysis appears reasonable. Two system parameters, the initial oil film thickness and the angle of the inclination of the tapered land in a thrust bearing, are shown to have significant impacts on the transient response under consideration, and to be possibly optimized to achieve a minimum axial transient response.
Simplification of Dynamic Capacity and Fatigue Life Estimations for Oscillating Rolling Bearings
Authors: Rumbarger John H.
Published: September 2003

A Dynamic capacity for oscillating rolling bearings was published in 1968 and correlated with available laboratory fatigue life data. That development of the Dynamic Capacity extended the classic fatigue life theory of Lundberg and Palmgren (1947 and 1952) to oscillating rolling bearings. The calculation of the Dynamic Capacity is simplified as a modification of present ABMA and ISO load rating and life standards for continuously rotating rolling bearings. The simplified formulas agree with the Harris, 1991, text book formulation for oscillation amplitudes (greater than the critical amplitude) which cause an overlapping of stressed contact areas by adjacent rolling elements. Oscillation amplitudes less than the critical amplitude result in separate, discrete contact areas on each raceway. Use of the Harris equations will lead to overestimation of the fatigue for oscillation amplitudes which are less than the critical amplitude.
A Numerical Analysis for TEHL of Eccentric-Tappet Pair Subjected to Transient Load
Authors: Wang J. , Yang P.
Published: September 2003

A full numerical analysis is carried out to simulate the thermal elastohydrodynamic lubrication (TEHL) of an eccentric-tappet pair. Comparisons between thermal and isothermal results are given to reveal the role of the thermal effect. Under various eccentricities, the influences of two surfaces moving in opposite directions on pressure and film thickness profiles are analyzed and explained by the mechanism of the temperature-viscosity wedge. Pressure and film thickness profiles, the temperature and velocity distributions at zero entraining velocity are discussed fully. Particular analyses are given on the entrapped immobile surface layers that influence the velocity distribution at zero entraining velocity. Furthermore, in a working cycle, variations of central and minimum film thicknesses and frictional coefficients, under different eccentricities, are discussed.
Test Results for PTFE-Faced Thrust Pads, With Direct Comparison Against Babbitt-Faced Pads and Correlation With Analysis
Authors: Ettles C. M. , Knox R. T. , Ferguson J. H. , Horner D.
Published: September 2003

The use of PTFE-faced pads in large vertical axis hydro-generators was pioneered in Russia in the 1970s, prompted by a series of failures of conventional babbitt-faced bearings. Some advantages claimed include higher specific loading, lower power loss and the omission of oil-lift facilities. There is strong interest in the Industry concerning this material, but limited data are available on actual performance. Some results from extensive testing of PTFE-faced pads are given, for two sizes of pad. These are compared directly size-for-size with results for babbitt bearings of nominally the same area. The power losses for the two types of bearing were found to be almost identical. Some of the effects observed during testing are described and discussed, including the effect of creep. The test results are compared with predictions using the GENMAT analysis software. A method of allowing for creep in numerical modeling is discussed.
Some Effects of Start-Up and Shut-Down on Thrust Bearing Assemblies in Hydro-Generators
Authors: Ettles C. M. , Seyler J. , Bottenschein M.
Published: September 2003

The modernization of hydro-generators can involve the analysis of many different manufacturer’s designs of thrust bearings. Recent designs of bearing in common use are very reliable, but when failures do occur, it is often with older machines and within the first few minutes of start-up. This paper is a result of general design studies of various thrust bearing configurations subjected to transient operating conditions. It is shown that transient effects can induce an ‘overshoot’ of thermal deformation which can become unstable, leading to ‘thermal ratchetting.’ Examples are given of pads of various manufacturer’s bearings that have been subjected to this mechanism. Results from operating turbines, basic studies and measurements of the thermal bending of plates indicate that a peak deflection occurs well before thermal equilibrium is attained. The peaking phenomenon may be obscured in some designs or in cases where the run-up is gradual. The beneficial effects of using an oil-lift system during start-up are described. During shut-down it is important that the contact of hot, crowned pads against the runner be prevented. Minimum times for operation of the lift system are suggested, based on the thickness of the pads.
Variable Property and Temperature Ratio Effects on Nusselt Numbers in a Rectangular Channel With 45 Deg Angled Rib Turbulators
Authors: Mahmood G. I. , Ligrani P. M. , Chen K.
Published: September 2003

Measured local and spatially-averaged Nusselt numbers and friction factors (all time-averaged) are presented which show the effects of temperature ratio and variable properties in a rectangular channel with rib turbulators, and an aspect ratio of 4. The ratio of air inlet stagnation temperature to local surface temperature Toi/Tw varies from 0.66 to 0.95, and Reynolds numbers based on channel height range from 10,000 to 83,700. The square cross-section ribs are placed on two opposite surfaces, and are oriented at angles of +45 deg and −45 deg, respectively, with respect to the bulk flow direction. The ratio of rib height to channel hydraulic diameter is 0.078, the rib pitch-to-height ratio is 10, and the ribs block 25 percent of the channel cross-sectional area. Ratios of globally-averaged rib Nusselt numbers to baseline, constant property Nusselt numbers, Nu̿/Nuo,cp, increase from 2.69 to 3.10 as the temperature ratio Toi/Tw decreases from 0.95 to 0.66 (provided Reynolds number ReH is approximately constant). Friction factor ratios f/fo,cp then decrease as Toi/Tw decreases over this same range of values. In each case, a correlation equation is given which matches the measured global variations. Such global changes are a result of local Nusselt number ratio increases with temperature ratio, which are especially pronounced on the flat surfaces just upstream and just downstream of individual ribs. Thermal performance parameters are also given, which are somewhat lower in the ribbed channel than in channels with dimples and/or protrusions mostly because of higher rib form drag and friction factors.
Cooling Enhancements in Thin Films Supported by Flexible Complex Seals in the Presence of Ultrafine Suspensions
Authors: Khaled A.-R. A. , Vafai K.
Published: September 2003

Flow and heat transfer inside thin films supported by flexible soft seals having voids of a stagnant fluid possessing a large coefficient of volumetric thermal expansion βT are studied in the presence of suspended ultrafine particles. The study is conducted under periodically varying thermal load conditions. The governing continuity, momentum and energy equations are non-dimensionalized and reduced to simpler forms. The deformation of the seal is related to the internal pressure and lower plate’s temperature based on the theory of linear elasticity and a linearized model for thermal expansion. It is found that enhancements in the cooling are achieved by an increase in the volumetric thermal expansion coefficient, thermal load, thermal dispersion effects, softness of the supporting seals and the thermal capacitance of the coolant fluid. Further, thermal dispersion effects are found to increase the stability of the thin film. The noise in the thermal load is found to affect the amplitude of the thin film thickness, Nusselt number and the lower plate temperature however it has a negligible effect on their mean values.
Flow and Heat Transfer Behavior for a Vortex-Enhanced Interrupted Fin
Authors: Smotrys M. L. , Ge H. , Jacobi A. M. , Dutton J. C.
Published: September 2003

The potential for enhanced air-side heat transfer in HVAC&R applications has been investigated using combined spanwise and streamwise vorticity. Spanwise vortices were created using an offset-strip fin array, and streamwise vortices were introduced into the flow using delta-wing vortex generators. Mass transfer measurements, obtained using the naphthalene sublimation technique, were used to quantify the heat transfer characteristics of a baseline and enhanced array, and these results were compared to flow visualization and particle image velocimetry measurements of the instantaneous velocity fields. Significant enhancement was observed over portions of the Reynolds number range tested (400⩽Re⩽3700), and a transitional Reynolds number range was identified within which the spanwise and streamwise vortices act to decrease the heat transfer performance of the array.
Maximum Attainable Performance of Stirling Engines and Refrigerators
Authors: de Boer P. C. T.
Published: September 2003

The flow through the regenerator of a Stirling engine is driven by differences of pressure in the compression and expansion spaces. These differences lead to power dissipation in the regenerator. Using linearized theory, it is shown that this dissipation severely limits the maximum attainable thermal efficiency and nondimensional power output. The maximum attainable values are independent of the value of the regenerator conductance. For optimized nondimensional power output, the thermal efficiency equals only half the Carnot value. The power dissipated in the regenerator is removed as part of the heat withdrawn at the regenerator’s cold side. Analogous results are presented for the Stirling refrigerator. At optimized nondimensional rate of refrigeration, its coefficient of performance is less than half the Carnot value.
Measuring Thermal and Thermoelectric Properties of One-Dimensional Nanostructures Using a Microfabricated Device
Authors: Shi Li , Li Deyu , Yu Choongho , Jang Wanyoung , Kim Dohyung , Yao Zhen , Kim Philip , Majumdar Arunava
Published: September 2003

We have batch-fabricated a microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon nitride beams for measuring thermophysical properties of one-dimensional nanostructures (nanotubes, nanowires, and nanobelts) bridging the two membranes. A platinum resistance heater/thermometer is fabricated on each membrane. One membrane can be Joule heated to cause heat conduction through the sample to the other membrane. Thermal conductance, electrical conductance, and Seebeck coefficient can be measured using this microdevice in the temperature range of 4–400 K of an evacuated Helium cryostat. Measurement sensitivity, errors, and uncertainty are discussed. Measurement results of a 148 nm and a 10 nm-diameter single wall carbon nanotube bundle are presented.
Self-Preserving Properties of Unsteady Round Buoyant Turbulent Plumes and Thermals in Still Fluids
Authors: Diez F. J. , Sangras R. , Faeth G. M. , Kwon O. C.
Published: September 2003

The self-preserving properties of round buoyant turbulent starting plumes and starting jets in unstratified environments. The experiments involved dye-containing salt water sources injected vertically downward into still fresh water within a windowed tank. Time-resolved images of the flows were obtained using a CCD camera. Experimental conditions were as follows: source diameters of 3.2 and 6.4 mm, source/ambient density ratios of 1.070 and 1.150, source Reynolds numbers of 4,000–11,000, source Froude numbers of 10–82, volume of source fluid for thermals comprising cylinders having the same cross-sectional areas as the source exit and lengths of 50–382 source diameters, and streamwise flow penetration lengths up to 110 source diameters and 5.05 Morton length scales from the source. Near-source flow properties varied significantly with source properties but the flows generally became turbulent and then became self-preserving within 5 and 20–30 source diameters from the source, respectively. Within the self-preserving region, both normalized streamwise penetration distances and normalized maximum radial penetration distances as functions of time were in agreement with the scaling relationships for the behavior of self-preserving round buoyant turbulent flows to the following powers: time to the 3/4 power for starting plumes and to the 1/2 power for thermals. Finally, the virtual origins of thermals were independent of source fluid volume for the present test conditions.
Importance of Turbulence-Radiation Interactions in Turbulent Diffusion Jet Flames
Authors: Li Genong , Modest Michael F.
Published: September 2003

Traditional modeling of radiative transfer in reacting flows has ignored turbulence-radiation interactions (TRI). Radiative fluxes, flux divergences and radiative properties have been based on mean temperature and concentration fields. However, both experimental and theoretical work have suggested that mean radiative quantities may differ significantly from those predictions based on the mean parameters because of their strongly nonlinear dependence on the temperature and concentration fields. The composition PDF method is able to consider many nonlinear interactions rigorously, and the method is used here to study turbulence-radiation interactions. This paper tries to answer two basic questions: (1) whether turbulence-radiation interactions are important in turbulent flames or not; and (2) if they are important, then what correlations need to be considered in the simulation to capture them. After conducting many flame simulations, it was observed that, on average, TRI effects account for about 1/3 of the total drop in flame peak temperature caused by radiative heat losses. In addition, this study shows that consideration of the temperature self correlation alone is not sufficient to capture TRI, but that the complete absorption coefficient-Planck function correlation must be considered.
A Study of Fin Effects in the Measurement of Temperature Using Surface-Mounted Thermocouples
Authors: Tszeng T. C. , Saraf V.
Published: September 2003

The present study addresses the effects of thermocouples on the measured temperature when such thermocouples are mounted directly on the surface of the object. A surface-mounted thermocouple is a very convenient way of measuring the surface temperature. However, the heat conduction into/from the thermocouple wire changes the local temperature at the thermocouple junctions along with the immediate vicinity of the thermocouple. As a consequence, the emf appearing at the thermocouple terminals does not correspond to the actual surface temperature. In this paper, we first discuss the general characteristics of the enhanced heating/cooling due to the so-called “fin effects” associated with the surface-mounted thermocouples. An embedded computational model is then developed so that the model can be used in conjunction with a regular FEM model for the multidimensional calculation of the heating or cooling of a part. The embedded computational model is shown to offer very accurate calculation of the temperature at the junction of thermocouple wire. The developed computational model is further used in the inverse heat transfer calculation for a Jominy end quench experiment.
Direct Numerical Simulation of Turbulent Separated Flow and Heat Transfer Over a Blunt Flat Plate
Authors: Yanaoka Hideki , Yoshikawa Hiroyuki , Ota Terukazu
Published: September 2003

Three-dimensional simulation of turbulent separated and reattached flow and heat transfer over a blunt flat plate is presented. The Reynolds number analyzed is 5000. The vortices shed from the reattachment flow region exhibit a hairpin-like structure. These large-scale vortex structures greatly influence the heat transfer in the reattachment region. Present results are compared with the previous three-dimensional calculations at low Reynolds number and it is found that there is no essential difference between two results with respect to the flow structure. The reattachment length is about five plate thicknesses, which is nearly equal to the previous experimental ones. The velocity distributions and turbulence intensities are in good agreement with the experimental data. Further, it is clarified that Nusselt number and temperature distributions greatly depend upon the Reynolds number, though their characteristic behaviors are qualitatively well simulated.
Fully-Developed Thermal Transport in Combined Pressure and Electro-Osmotically Driven Flow in Microchannels
Authors: Maynes D. , Webb B. W.
Published: September 2003

Thermally fully-developed heat transfer has been analyzed for combined electro-osmotic and pressure driven flow in a circular microtube. The two classical thermal boundary conditions of constant wall heat flux and constant wall temperature were considered. Such a flow is established by the combination of an imposed pressure gradient and voltage potential gradient along the length of the tube. The induced flow rate and velocity profile are functions of the imposed potential gradient, electro-osmotic mobility of the fluid, the ratio of the duct radius to the Debye length, the established streamwise pressure gradient, and the fluid viscosity. The imposed voltage gradient results in Joule heating in the fluid, with an associated distributed volumetric source of energy. For this scenario, the solution for the fully developed, dimensionless temperature profile and corresponding Nusselt number have been determined. The fully-developed Nusselt number is found to depend on the duct radius/Debye length ratio (termed the relative duct radius), the dimensionless volumetric source, and a dimensionless parameter that characterizes the relative strengths of the two driving mechanisms. This parameter can take on both positive and negative values, depending on the signs of the streamwise voltage and pressure gradients imposed. Analytical results are presented and discussed for a range of the governing dimensionless parameters.
A Vascular Model for Heat Transfer in an Isolated Pig Kidney During Water Bath Heating
Authors: Chen Cuiye , Xu Lisa X.
Published: September 2003

Isolated pig kidney has been widely used as a perfused organ phantom in the studies of hyperthermia treatments, as blood perfusion plays an essential role in thermoregulation of living tissues. In this research, a vascular model was built to describe heat transfer in the kidney phantom during water bath heating. The model accounts for conjugate heat transfer between the paired artery and vein, and their surrounding tissue in the renal medulla. Tissue temperature distribution in the cortex was predicted using the Pennes bioheat transfer equation. An analytical solution was obtained and validated experimentally for predicting the steady state temperature distribution in the pig kidney when its surface kept at a uniform constant temperature. Results showed that local perfusion rate significantly affected tissue temperature distributions. Since blood flow is the driving force of tissue temperature oscillations during hyperthermia, the newly developed vascular model provides a useful tool for hyperthermia treatment optimization using the kidney phantom model.
Determination of the Thermal Dispersion Coefficient During Radial Filling of a Porous Medium
Authors: Deleglise Mylene , Simacek Pavel , Binetruy Christophe , Advani Suresh
Published: September 2003

Resin Transfer Molding is one of the Liquid Composite Molding processes in which a thermoset resin is infiltrated into a fibrous porous media in a closed mold. To reduce the curing time of the resin, the mold may be heated, influencing other filling parameters such as the resin viscosity. Analysis of the non-isothermal effects during filling will help to understand the manufacturing process. One of the issues of non-isothermal filling in porous media is the variation of the velocity profile at the micro scale level, which as it is averaged, cannot be included in the convective term. To account for it, the thermal conductivity tensor is modified and a thermal dispersion coefficient Kd is introduced to model the micro convection effects. In this paper, we explore the temperature profile under non-isothermal conditions for radial injection during Resin Transfer Molding in order to determine the thermal dispersion coefficient. An approximate solution is derived from the series solution and validated with a numerical method. Experiments using carbon fibers and polyester resin were conducted. The thermal dispersion coefficient is determined by comparing experimental results with the steady state analytical solution. The comparison between radial and linear injection results shows that the same degree of dispersion is present in isotropic fibrous porous media.
Short-Time-Transient Surfactant Dynamics and Marangoni Convection Around Boiling Nuclei
Authors: Wasekar Vivek M. , Manglik Raj M.
Published: September 2003

The effects of surfactant concentration on the initial short-time-scale Marangoni convection around boiling nuclei in aqueous solutions have been computationally investigated. The model consists of a hemispherical bubble (1–100 μm radius) on a downward-facing constant-temperature heated wall in a fluid pool with an initial uniform temperature gradient. Time-dependent transport of liquid mass, momentum, energy, and surfactant bulk and surface convection along with the adsorption kinetics are considered. Conditions for bubble sizes, surfactant bulk concentrations, and wall heat flux levels are represented by a range of thermocapillary and diffusocapillary Marangoni numbers (6⩽MaT⩽103,0⩽MaS⩽8.6×105) over a micro-scale time period (1 μs–1 ms). With a surfactant in solution, a surface concentration gradient develops at the bubble interface that tends to oppose the temperature gradient and reduce the overall Marangoni convection. The maximum circulation strength, which is dependent on the bubble size, corresponds to a characteristic surfactant adsorption time. This, when scaled by a ratio of bubble radius, is found to depend solely on the surfactant bulk concentration. Moreover, the interfacial surfactant adsorption does not display a stagnant cap behavior for the range of parameters and time scales covered in this study.
Calculation of Direct Exchange Areas for Nonuniform Zones Using a Reduced Integration Scheme
Authors: Tian Weixue , Chiu Wilson K. S.
Published: September 2003

In the zonal method, considerable computational resources are needed to calculate the direct exchange areas (DEA) among the isothermal zones due to integrals with up to six dimensions, while strong singularities occur in the integrands when two zones are adjacent or overlaping (self-irradiation). A special transformation of variables to reduce a double integral into several single integrals is discussed in this paper. This technique was originally presented by Erkku (1959) for calculation of DEA using a uniform zone system in a cylindrical enclosure. However, nonuniform zones are needed for applications with large thermal gradients. Thus we extended this technique to calculate the DEA for non-uniform zones in an axisymmetrical cylinder system. A six-fold reduction in computational time was observed in calculating DEA compared with cases without a variable transformation. It is shown that accuracy and efficiency of estimation of radiation heat flux is improved when using a nonuniform zone system. Reasonable accuracy of all DEA are calculated without resorting to the conservative equations. Results compared well with analytical solutions and numerical results of previous researchers. This technique can be readily extended to rectangular enclosures with similar reduction in computation time expected.
Geometric Optimization of Radiant Enclosures Containing Specular Surfaces
Authors: Daun K. J. , Morton D. P. , Howell J. R.
Published: September 2003

This paper presents an optimization methodology for designing radiant enclosures containing specularly-reflecting surfaces. The optimization process works by making intelligent perturbations to the enclosure geometry at each design iteration using specialized numerical algorithms. This procedure requires far less time than the forward “trial-and-error” design methodology, and the final solution is near optimal. The radiant enclosure is analyzed using a Monte Carlo technique based on exchange factors, and the design is optimized using the Kiefer-Wolfowitz method. The optimization design methodology is demonstrated by solving two industrially-relevant design problems involving two-dimensional enclosures that contain specular surfaces.
Transition in Homogeneously Heated Inclined Plane Parallel Shear Flows
Authors: Generalis S. , Nagata M.
Published: September 2003

The transition of internally heated inclined plane parallel shear flows is examined numerically for the case of finite values of the Prandtl number Pr. We show that as the strength of the homogeneously distributed heat source is increased the basic flow loses stability to two-dimensional perturbations of the transverse roll type in a Hopf bifurcation for the vertical orientation of the fluid layer, whereas perturbations of the longitudinal roll type are most dangerous for a wide range of the value of the angle of inclination. In the case of the horizontal inclination transverse roll and longitudinal roll perturbations share the responsibility for the prime instability. Following the linear stability analysis for the general inclination of the fluid layer our attention is focused on a numerical study of the finite amplitude secondary travelling-wave solutions (TW) that develop from the perturbations of the transverse roll type for the vertical inclination of the fluid layer. The stability of the secondary TW against three-dimensional perturbations is also examined and our study shows that for Pr=0.71 the secondary instability sets in as a quasi-periodic mode, while for Pr=7 it is phase-locked to the secondary TW. The present study complements and extends the recent study by Nagata and Generalis (2002) in the case of vertical inclination for Pr=0.
Enhanced Forced Convection Heat Transfer From a Cylinder Using Permeable Fins
Authors: A/K Abu-Hijleh Bassam
Published: September 2003

The problem of cross-flow forced convection heat transfer from a horizontal cylinder with multiple, equally spaced, high conductivity permeable fins on its outer surface was investigated numerically. The heat transfer characteristics of a cylinder with permeable versus solid fins were studied for several combinations of number of fins and fin height over the range of Reynolds number (5–200). Permeable fins provided much higher heat transfer rates compared to the more traditional solid fins for a similar cylinder configuration. The ratio between the permeable to solid Nusselt numbers increased with Reynolds number and fin height but tended to decrease with number of fins. This ratio was as high as 4.35 at Reynolds number of 150 and a single fin with a nondimensional height of 3.0. The use of 1–2 permeable fins resulted in much higher Nusselt number values than when using up to 18 solid fins. Such an arrangement has other benefits such as a considerable reduction in weight and cost.
Binary Fluid Mixture and Thermocapillary Effects on the Wetting Characteristics of a Heated Curved Meniscus
Authors: Pratt David M. , Kihm Kenneth D.
Published: September 2003

An investigation has been conducted into the interactions of binary fluid mixtures (pentane [C5H12] coolant and decane [C10H22] additive) and thermocapillary effects on a heated, evaporating meniscus formed in a vertical capillary pore system. The experimental results show that adding decane, the secondary fluid that creates the concentration gradient, actually decreases the meniscus height to a certain level, but did increase the sustainable temperature gradient for the liquid-vapor interface, so did the heat transfer rate, delaying the onset of meniscus instability. The results have demonstrated that interfacial thermocapillary stresses arising from liquid-vapor interfacial temperature gradients, which is known to degrade the ability of the liquid to wet the pore, can be counteracted by introducing naturally occurring concentration gradients associated with distillation in binary fluid mixtures. Also theoretical predictions are presented to determine the magnitudes of both the thermocapillary stresses and the distillation-driven capillary stresses, and to estimate the concentration gradients established as a result of the distillation in the heated pore.
Simulation of Unsteady Small Heat Source Effects in Sub-Micron Heat Conduction
Authors: Narumanchi Sreekant V. J. , Murthy Jayathi Y. , Amon Cristina H.
Published: September 2003

In compact transistors, large electric fields near the drain side create hot spots whose dimensions are smaller than the phonon mean free path in the medium. In this paper, we present a study of unsteady hot spot behavior. The unsteady gray phonon Boltzmann transport equation (BTE) is solved in the relaxation time approximation using a finite volume method. Electron-phonon interaction is represented as a heat source term in the phonon BTE. The evolution of the temperature profile is governed by the interaction of four competing time scales: the phonon residence time in the hot spot and in the domain, the duration of the energy source, and the phonon relaxation time. The influence of these time scales on the temperature is investigated. Both boundary scattering and heat source localization effects are observed to have considerable impact on the thermal predictions. Comparison of BTE solutions with conventional Fourier diffusion analysis reveals significant discrepancies.
An Improved Computational Procedure for Sub-Micron Heat Conduction
Authors: Murthy J. Y. , Mathur S. R.
Published: September 2003

Popular numerical techniques for solving the Boltzmann transport equation (BTE) for sub-micron thermal conduction include the discrete ordinates method and the finite volume method. However, the finite wave speed associated with the BTE can cause large errors in the prediction of the equivalent temperature unless fine angular discretizations are used, particularly at low acoustic thicknesses. In this paper, we combine a ray-tracing technique with the finite volume method to substantially improve the predictive accuracy of the finite volume method. The phonon intensity is decomposed into ballistic and in-scattering components. The former is solved using a ray tracing scheme, accounting for finite wave speed; the latter is solved using an unstructured finite volume method. Comparisons between this new technique and traditional finite volume formulations are presented for a range of acoustic thicknesses, and substantial improvement is demonstrated.