Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2001"

A Theoretical model to predict pool boiling CHF incorporating effects of contact angle and orientation

Abstract: A theoretical model is developed to describe the hydrodynamic behavior of the vapor-liquid interface of a bubble at the heater surface leading to the initiation of critical heat flux (CHF) condition. The momentum flux resulting from evaporation at the bubble base is identified to be an important parameter. A model based on theoretical considerations is developed for upward-facing surfaces with orientations of 0 deg (horizontal) to 90 deg (vertical). It includes the surface-liquid interaction effects through the dynamic receding contact angle. The CHF in pool boiling for water, refrigerants and cryogenic liquids is correctly predicted by the model, and the parametric trends of CHF with dynamic receding contact angle and subcooling are also well represented

Monte Carlo Study of Phonon Transport in Solid Thin Films Including Dispersion and Polarization

Abstract: The Boltzmann Transport Equation (BTE) for phonons best describes the heat flow in solid nonmetallic thin films. The BTE, in its most general form, however, is difficult to solve analytically or even numerically using deterministic approaches. Past research has enabled its solution by neglecting important effects such as dispersion and interactions between the longitudinal and transverse polarizations of phonon propagation. In this article, a comprehensive Monte Carlo solution technique of the BTE is presented. The method accounts for dual polarizations of phonon propagation, and non-linear dispersion relationships. Scattering by various mechanisms is treated individually. Transition between the two polarization branches, and creation and destruction of phonons due to scattering is taken into account. The code has been verified and evaluated by close examination of its ability or failure to capture various regimes of phonon transport ranging from diffusive to the ballistic limit. Validation results show close agreement with experimental data for silicon thin films with and without doping. Simulation results show that above 100 K, transverse acoustic phonons are the primary carriers of energy in silicon.

Thermal Modeling of Unlooped and Looped Pulsating Heat Pipes

Abstract: by experimental results. The visualization test showed that the oscillatory flow formed waves that traveled among the turns of PHPs. Their theoretical model could be used to estimate the pressure and displacement of oscillatory flow. Kiseev and Zolkin @7# experimentally investigated the effects of acceleration and vibration on the performance of the unlooped PHP. Acetone was used as the working fluid and the filling charge ratio was 60 percent. Their results indicated that the PHP operates successfully by various acceleration effects. There was an increase in evaporator temperature, about 30 percent by increase of the acceleration from 26g to 112g. Dobson and Harms @8# presented simple mathematical models in which the behavior of PHPs was simulated. The mathematical model was applied to the open-ended PHP. They showed that the oscillating behavior would be different for different initial values. Wong et al. @9# proposed a theoretical model of PHPs based on a Lagrangian approach in which the flow was modeled under adiabatic conditions for the entire PHP. A sudden pressure pulse was applied to simulate local heat input into a vapor plug. They were able to show the pressure and velocity variations with time for the vapor plugs. In the present study, a very detailed theoretical model will be developed to accurately simulate the behavior of liquid slugs and vapor plugs in both unlooped and looped PHPs. Heat transfer due to the phase change is also considered.

Using Porous Fins for Heat Transfer Enhancement

Abstract: This work introduces a novel method that enhances the heat transfer from a given surface by using porous fins. The thermal performance of porous fins is estimated and compared with that of the conventional solid fins. It is found that using porous fin of porosity ∈ may enhance the performance of an equal size conventional solid fin and, as a result, save 100 e percent of the fin material. The effect of different design and operating parameters on the porous fin thermal performance is investigated. Examples of these parameters are Ra number, Da number, and thermal conductivity ratio. It is found that more enhancement in the porous fin performance may be achieved as Ra increases especially at large Da numbers. Also, it is found that there is an optimum limit for the thermal conductivity ratio beyond which there is no further improvement in the fin performance.

Sub-Continuum Simulations of Heat Conduction in Silicon-on-Insulator Transistors

Abstract: The temperature rise in sub-micrometer silicon devices is predicted at present by solving the heat diffusion equation based on the Fourier law. The accuracy of this approach needs to be carefully examined for semiconductor devices in which the channel length is comparable with or smaller than the phonon mean free path. The phonon mean free path in silicon at room temperature is near 300 nm and exceeds the channel length of contemporary transistors. This work numerically integrates the two-dimensional phonon Boltzmann transport equation (BTE) within the silicon region of a silicon-on-insulator (SOI) transistor. The BTE is solved together with the classical heat diffusion equation in the silicon dioxide layer beneath the transistor. The predicted peak temperature rise is nearly 160 percent larger than a prediction using the heat diffusion equation for the entire domain. The disparity results both from phonon-boundary scattering and from the small dimensions of the region of strongest electron-phonon energy transfer. This work clearly shows the importance of sub-continuum heat conduction in modern transistors and will facilitate the development of simpler calculation strategies, which are appropriate for commercial device simulators.

Extinction and Scattering Properties of Soot Emitted From Buoyant Turbulent Diffusion Flames

Abstract: Extinction and scattering properties at wavelengths of 250-5200 nm were studied for soot emitted from buoyant turbulent diffusion flames in the long residence time regime where soot properties are independent of position in the overfire region and characteristic flame residence times. Flames burning in still air and fueled with gas (acetylene, ethylene, propane, and propylene) and liquid (benzene, toluene, cyclohexane, and n-heptane) hydrocarbon fuels were considered. Measured scattering patterns and ratios of total scattering/absorption cross sections were in good agreement with predictions based on the Rayleigh-Debye-Gans (RDG) scattering approximation in the visible. Measured depolarization ratios were roughly correlated by primary particle size parameter, suggesting potential for completing RDG methodology needed to make soot scattering predictions as well as providing a nonintrusive way to measure primary soot particle diameters. Measurements of dimensionless extinction coefficients were in good agreement with earlier measurements for similar soot populations and were independent of fuel type and wavelength except for reduced values as the near ultraviolet was approached. The ratios of the scattering/absorption refractive index functions were independent of fuel type within experimental uncertainties and were in good agreement with earlier measurements. The refractive index junction for absorption was similarly independent of fuel type but was larger than earlier reflectometry measurements in the infrared. Ratios of total scattering/absorption cross sections were relatively large in the visible and near infrared, with maximum values as large as 0.9 and with values as large as 0.2 at 2000 nm, suggesting greater potential for scattering from soot particles to affect flame radiation properties than previously thought.

Surface Chemistry and Characteristics Based Model for the Thermal Contact Resistance of Fluidic Interstitial Thermal Interface Materials

Abstract: Microprocessor powers are increasing at a phenomenal rate, which requires very small thermal resistance between the die (chip) and the ambient, if the current economical methods of conduction and convection cooling are to be utilized. A typical thermal solution in flip chip technology utilizes two levels of thermal interface materials: between the die and the heat spreader, and between the heat spreader and the heat sink, Phase change materials and thermal greases are among the most prominent interstitial thermal interface materials (TIM) used in electronic packaging. These TIMs are typically polymeric matrix loaded with highly conducting filler particles. The dwindling thermal budget has neces-sitated a better understanding of the thermal resistance of each component of the thermal solution. Thermal conductivity of these particle-laden materials is better understood than their contact resistance. A careful review of the literature reveals the lack of analytical models for the prediction of contact resistance of these types of interstitial materials, which possess fluidic properties. This paper introduces an analytical model for the thermal contact resistance of these types of interstitial materials. This model is compared with the experimental data obtained on the contact resistance of these TIMs. The model, which depends on parameters such as, surface tension, contact angle, thermal conductivity, roughness and pressure matches very well with the experimental data at low pressures and is still within the error bars at higher pressures.

DNS of Turbulent Heat Transfer in Channel Flow With Heat Conduction in the Solid Wall

Abstract: The Direct Numerical Simulation (DNS) of the fully developed velocity and temperature fields in the two-dimensional turbulent channel flow coupled with the unsteady conduction in the heated walls was carried out. Simulations were performed at constant friction Reynolds number 150 and Prandtl numbers between 0.71 and 7 considering the fluid temperature as a passive scalar. The obtained statistical quantities like root-mean-square temperature fluctuations and turbulent heat fluxes were verified with existing DNS studies obtained with ideal thermal boundary conditions. Results of the present study were compared to the findings of Polyakov (1974), who made a similar study with linearization of the fluid equations in the viscous sublayer that allowed analytical approach and results of Kasagi et al. (1989), who performed similar calculation with deterministic near-wall turbulence model and numerical approach. The present DNS results pointed to the main weakness of the previous studies, which underestimated the values of the wall temperature fluctuations for the limiting cases of the ideal-isoflux boundary conditions. With the results of the present DNS it can be decided, which behavior has to be expected in a real fluid-solid system and which one of the limiting boundary conditions is valid for calculation, or whether more expensive conjugate heat transfer calculation is required. @DOI: 10.1115/1.1389060#

A Scattering-Mediated Acoustic Mismatch Model for the Prediction of Thermal Boundary Resistance

Abstract: Solid-solid thermal boundary resistance (R{sub b}) plays an important role in determining heat flow, both in cryogenic and room-temperature applications, such as very large scale integrated circuitry, superlattices, and superconductors. The acoustic mismatch model (AMM) and the related diffuse mismatch model (DMM) describe the thermal transport at a solid-solid interface below a few Kelvin quite accurately. At moderate cryogenic temperatures and above, R{sub b} is dominated by scattering caused by various sources, such as damage in the dielectric substrates and formation of an imperfect boundary layer near the interface making R{sub b} larger than that predicted by AMM and DMM. From a careful review of the literature on R{sub b}, it seems that scattering near the interface plays a far more dominant role than any other mechanism. Though scattering near the interface has been considered in the past, these models are either far too complicated or are too simple (i.e., inaccurate) for engineering use. A new model, called the scattering-mediated acoustic mismatch model (SMAMM), is developed here that exploits the analogy between phonon and radiative transport by developing a damped wave equation to describe the phonon transport. Incorporating scattering into this equation and finding appropriate solutions for a solid-solid interfacemore » enable an accurate description of R{sub b} at high temperatures, while still reducing to the AMM at low temperatures, where the AMM is relatively successful in predicting R{sub b}.« less

An Integral Formulation of Transient Radiative Transfer

Abstract: A time-dependent integral formulation is developed for modeling transient radiative transfer The development is based on a rigorous analysis of the wave propagation process inside the participating media The physical significance of the present integral formulation is the consideration of the time-dependent domain of computation, which is different from the domain disturbed by radiation (ie, the wave front envelope) Numerical computations are performed for the medium that is an absorbing and isotropically scattering one-dimensional plane slab geometry The spatial and temporal incident radiation and radiative flux distributions are presented for different boundary conditions and for uniform and nonuniform property distribution

Jet Impingement Boiling From a Circular Free-Surface Jet During Quenching: Part 1—Single-Phase Jet

Abstract: A proposed technique for controlling jet impingement boiling heat transfer involves injection of gas into the liquid jet. This paper reports results from an experimental study of boiling heat transfer during quenching of a cylindrical copper specimen, initially at a uniform temperature exceeding the temperature corresponding to maximum heat flux, by a two-phase (water-air), circular free-surface jet. The second phase is introduced as small bubbles into the jet upstream of the nozzle exit. Data are presented for single-phase convective heat transfer at the stagnation point, as well as in the form of boiling curves, maximum heat fluxes, and minimum film boiling temperatures at locations extending from the stagnation point to a radius of ten nozzle diameters. For void fractions ranging from 0.0 to 0.4 and liquid-only velocities between 2.0 and 4.0 m/s (11,300≤Re d,fo ≤22,600), several significant effects are associated with introduction of the gas bubbles into the jet. As well as enhancing single-phase convective heat transfer by up to a factor of 2.1 in the stagnation region, addition of the bubbles increases the wall superheat in nucleate boiling and eliminates the temperature excursion associated with cessation of boiling. The maximum heat flux is unaffected by changes in the void fraction, while minimum film boiling temperatures increase and film boiling heat transfer decreases with increasing void fraction. A companion paper (Hall et al., 2001) details corresponding results from the single-phase jet.

Thermohydraulic Study of Laminar Swirl Flow Through a Circular Tube Fitted With Twisted Tapes

Abstract: Heat transfer and pressure drop characteristics in a circular tube fitted with twisted tapes have been investigated experimentally. Laminar swirl flow of a large Prandtl number (205

Phonon heat conduction in thin films : Impacts of thermal boundary resistance and internal heat generation

Abstract: The measured thermal resistance across a thin film deposited on a substrate often includes the internal thermal resistance within the film and the thermal boundary resistance (TBR) across the film-substrate interface. These two resistances are frequently lumped and reported as an equivalent thermal conductivity of the film. Two fundamental questions should be answered regarding the use of this equivalent thermal conductivity. One is whether it leads to the correct temperature distribution inside the film. The other one is whether it is applicable for thin films with internal heat generation. This paper presents a study based on the Boltzmann transport equation (BTE) to treat phonon heat conduction inside the film and across the film-substrate interface simultaneously, for the cases with and without internal heat generation inside the film. Material systems studied include SiO 2 and diamond films on Si substrates, representative of thin-film materials with low and high thermal conductivity. It is found that for a SiO 2 film on a Si substrate, the film thermal conductivity and TBR can be treated independently, while for a diamond film on a Si substrate, the two are related to each other by the interface scattering. When the free surface behaves as a black phonon emitter, the TBR for thin diamond films with internal heat generation is the same as that without the internal heat generation. When the free surface is adiabatic, however, the TBR increases and approaches the value of the corresponding black surface as the film thickness increases. Results of this study suggest that great care must be taken when extending the effective thermal conductivity measured for thin films under one experimental condition to other application situations.

A Numerical Study of Flow and Heat Transfer in a Smooth and Ribbed U-Duct With and Without Rotation

Abstract: Computations were performed to study the three-dimensional flow and heat transfer in a U-shaped duct of square cross section under rotating and non-rotating conditions. The parameters investigated were two rotation numbers (0, 0.24) and smooth versus ribbed walls at a Reynolds number of 25,000, a density ratio of 0. 13, and an inlet Mach number of 0.05. Results are presented for streamlines, velocity vector fields, and contours of Mach number, pressure, temperature, and Nusselt numbers

Clouds over Soot Evaporation: Errors in Modeling Laser-Induced Incandescence of Soot

Abstract: The ambiguity and incorrect treatment of the evaporation term among some Lll models in the literature are discussed. This study does not suggest that the correct formulation presented for the evaporation model is adequate, or that it reflects the soot evaporation process under intense evaporation. The emphasis is that the current evaporation model must be used correctly in the evaluation of the LIl model against experimental data. Numerical results are presented to demonstrate the significance of the molecular weight associated with the heat of evaporation and the thermal velocity of carbon vapor on the results obtained with the evaporation model. Other errors frequently repeated in the literature are also identified.

Heat Rate Predictions in Humid Air-Water Heat Exchangers Using Correlations and Neural Networks

Abstract: We consider the flow of humid air over fin-tube multi-row multi-column compact heat exchangers with possible condensation. Previously published experimental data are used to show that a regression analysis for the best-fit correlation of a prescribed form does not provide an unique answer, and that there are small but significant differences between the predictions of the different correlations thus obtained. It is also shown that it is more accurate to predict the heat rate directly rather than through intermediate quantities like the j-factors. The artificial neural network technique is offered as an alternative technique. It is trained with experimental values of the humid-air flow rates, dry-bulb and wet-bulb inlet temperatures, fin spacing, and heat transfer rates. The trained network is then used to make predictions of the heat transfer. Comparison of the results demonstrates that the neural network is more accurate than conventional correlations. @DOI: 10.1115/1.1351167#

Interaction Effects Between Surface Radiation and Turbulent Natural Convection in Square and Rectangular Enclosures

Abstract: The interaction effects of surface radiation with turbulent natural convection of a transparent medium in rectangular enclosures have been numerically analyzed, covering a wide range of Rayleigh number from 10 9 to 10 12 and aspect ratio from 1 to 200. The vertical walls of the enclosure are isothermal and maintained at different temperatures. The adiabatic top and bottom walls of the enclosure have been modelled for the limiting cases of negligible or perfect conduction along their lengths. The interaction with surface radiation results in larger velocity magnitudes and turbulence levels in the vertical as well as horizontal boundary layers, leading to an increase in the convective heat transfer by ∼25 percent. Due to the asymmetrical coupling of radiation, the augmentation of convective Nusselt number of the cold wall is larger than that of the hot wall. In tall enclosures, the convective Nusselt number exhibits three distinct regimes with respect to aspect ratio, viz. the slow growth regime, the accelerated growth regime and the invariant (or saturated) regime

Effects of Rib Arrangements on Heat Transfer and Flow Behavior in a Rectangular Rib-Roughened Passage: Application to Cooling of Gas Turbine Blade Trailing Edge

Abstract: Experimentation was conducted to examine the heat transfer and pressure drop characteristics in a rib-roughened rectangular passage with aspect ratio 2:1 for four rib configurations: 90 deg, 75 deg, 60 deg and 45 deg oblique ribs. The ribs were attached to two opposing long side walls instead of short side walls. In this study the oblique ribs were intended to function as secondary flow inducers as well as turbulators to improve the heat transfer of the bottom wall (one of the short side walls)

An Efficient Method for Modeling Radiative Transfer in Multicomponent Gas Mixtures With Soot

Abstract: An efficient approach for predicting radiative transfer in high temperature multicomponent gas mixtures with soot particles is presented. The method draws on the previously published multiplication approach for handling gas mixtures in the spectral line weighted-sum-of-gray-gases (SLW) model. In this method, the gas mixture is treated as a single gas whose absorption blackbody distribution function is calculated through the distribution functions of the individual species in the mixture. The soot is, in effect, treated as another gas in the mixture. Validation of the method is performed by comparison with line-by-line solutions for radiative transfer with mixtures of water vapor, carbon dioxide, and carbon monoxide with a range of soot loadings (volume fractions). Comparison is performed also with previously published statistical narrow band and classical weighted-sum-of-gray-gases solutions.

An Experimental Study of Heat Transfer of a Porous Channel Subjected to Oscillating Flow

Abstract: Experiments have been conducted to study the heat transfer of a porous channel subjected to oscillating flow. The surface temperature distributions for both steady and oscillating flows were measured. The local and length-averaged Nusselt numbers were analyzed. The experimental results revealed that the surface temperature distribution for oscillating flow is more uniform than that for steady flow. Due to the reversing flow direction, there are two thermal entrance regions for oscillating flow. The length-averaged Nusselt number for oscillating flow is higher than that for steady flow. The length-averaged Nusselt number for both steady and oscillating flows increase linearly with a dimensionless grouping parameter k * /k f (D e /L) 1/2 Pe *1/2 . The porous channel heat sink subjected to oscillating flow can be considered as an effective method for cooling high-speed electronic devices

Photo-acoustic measurement of thermal conductivity of thin films and bulk materials

Abstract: Thermal property data are important for every material that is exposed to thermal loading. Due to the microstructures of thin films such as grain size, amorphousness, and concentration of foreign atoms and defects, and also due to the physical dimensions of thin films, thermal conductivity of thin films may differ significantly from the bulk value. Because of the complexity of the thin film microstructure, experiments are often needed to determine thermal conductivity of thin films. The photoacoustic technique is one of the many techniques for measuring thermal conductivity of thin films. Compared with other techniques for thermal conductivity measurement, the photoacoustic method is relatively simple, yet is able to provide accurate thermal conductivity data for many types of thin films and bulk materials. In this work, a general analytical expression is developed, which relates the photoacoustic signal with the thermal properties, optical properties, and thermal contact resistance of a multi-layer system. Thermal conductivity of SiO{sub 2} with thicknesses from 0.05 to 0.5 {micro}m on Si wafer, e-beam evaporated thin nickel film on Si wafer and thermal barrier coatings on alloys is measured using the photoacoustic method up to a frequency of 20 kHz. In addition to the commonly used phasemore » shift fitting method, an amplitude fitting method is also employed. Using amplitude fitting, thermal conductivity for both thin films and bulk materials with smooth or rough surface are obtained, while phase shift fitting can only be used for films which are not thermally thick. Applications of the photoacoustic technique are discussed based on the experimental results.« less

A New Two-Phase Flow Map and Transition Boundary Accounting for Surface Tension Effects in Horizontal Miniature and Micro Tubes

Abstract: A new flow map is proposed to emphasize the importance of surface tension in two-phase flow in horizontal miniature and micro tubes. A transition boundary based on a force balance including shear, buoyancy and surface tension forces is also proposed. The flow map is compared against a number of existing experimental data sets totaling 1589 data points. Comparison of the proposed map and model with previous models shows substantial improvement and accuracy in determining surface tension dominated regimes. Furthermore, the proposed flow map shows how each regime transition boundary is affected by surface tension.

An Improved Design and Rating Analyses of Counter Flow Wet Cooling Towers

Abstract: Cooling towers are one of the largest heat and mass transfer devices that are in common use. In this paper, we present a detail model of counter flow wet cooling towers. The authenticity of the model is checked by experimental data reported in the literature. The values of number of transfer units (NTU) and tower effectiveness (e) obtained from the model were compared with the commonly described models. Appreciable difference in NTU and e values is found if the resistance to heat transfer in the water film and non-unity of Lewis number is considered in the calculations. The results demonstrate that the errors in calculating the tower effectiveness could be as much as 15 percent when considering the effect of air-water interface temperature. A procedure for the use of the model in designing and rating analyses of cooling towers is demonstrated through example problems. The limiting performance of the cooling towers; that is effectiveness equal to one, is explained in terms of air-approach temperature. The model is also used for obtaining the maximum possible mass-flow rate ratio of water-to-air, for different operating conditions.

Thermal Resistance Models for Non-Circular Moving Heat Sources on a Half Space

Abstract: The analysis of heat transfer from sliding and rolling contacts is important in many tribological applications such as ball bearing and gear design. In these applications heavily loaded contacts are typical and knowledge of the contact temperatures which result from frictional heat generation is required for minimizing thermal related problems such as scoring, lubricant breakdown, and adhesive wear due to flash welding. A review of typical tribology books such as the texts by Halling @1# and Williams @2#, and Handbook sections by Winer and Cheng @3# and Cowan and Winer @4# shows that the analysis of heat transfer from sliding or rolling contacts has not been extensively modelled. These reviews generally present equations and results for only one configuration, the circular contact. Although this contact geometry arises quite frequently in tribology applications, others such as the elliptic contact are also quite common in ball bearing and gear applications where non-conforming contacts prevail @5‐7#. The analysis for moving heat sources which is presented in a number of tribology references @1‐4#, is based upon the assumption that one of the contacts can be modelled as a stationary heat source and the other as a fast moving heat source. In many problems the assumption of a fast moving heat source may not be valid and the analysis will incorrectly predict the average or maximum contact temperature. With this in mind, Tian and Kennedy @8# developed accurate correlations for the circular and square heat source which predict the temperature for any speed. These correlations were then used to formulate models for predicting flash temperatures in sliding asperities. In a recent paper @9#, a hybrid computational method for noncircular heat sources was developed. For this method, a numerical approach based upon the superposition of point heat sources was employed for the stationary portion and a transient finite element method was employed for the moving portion. This new approach was then used to predict temperatures in a steel/bronze sliding contact problem, with sliding motion normal and parallel to the grinding direction. The primary motivation for the work of Neder et al. @9# was that the conventional approach adopted in most tribology references was not applicable to non-circular heat sources. The present work discusses various aspects of heat transfer in tribological applications involving stationary and sliding contacts. In all cases heat is either supplied to the contact or is generated through contact friction. This paper has four objectives. These are ~i! provide a comprehensive review of the literature related to stationary and moving heat sources on half space, ~ii! examine the effect that heat source shape and heat flux distribution have on the thermal resistance, ~iii! develop a model which is applicable to a heat source of arbitrary shape and flux distribution, and ~iv! use the proposed model to predict the flash temperature in a noncircular contact for real surfaces. In addressing these issues, a number of gaps in the literature have been filled. In addition, a clear and consistent approach to modeling arbitrary contacts has been developed. Presently, the field of tribology has only adopted a simplified approach in the prediction of contact temperatures due to sliding. The present approach does not allow for the effect of shape, aspect ratio, and flux distribution to be modelled easily. This was the primary motivation of the development of a hybrid numerical scheme by Neder et al. @9#. The expressions and method developed in the present work have been validated against a small set of numerical data for real and ideal contacts. The results of Neder et al. @9# are readily computed using the present approach with significantly less effort.

Inverse Boundary Design Combining Radiation and Convection Heat Transfer

Abstract: We investigate inverse boundary design for radiation, convection and conduction combined-mode heat transfer. The problem consists of finding the heat flux distribution on a heater that satisfies both the temperature and the heat flux prescribed on a design surface of an enclosure formed by two finite parallel plates. A gray participating medium flows in laminar regime between the walls, which are gray, diffuse emitters arid absorbers. All the thermal properties are uniform. This problem is described by an ill-conditioned system of non-linear equations. The solution is obtained by regularizing the system of equations by means of truncated singular value decomposition (TSVD)

Modeling Dynamic Electrical Resistance During Resistance Spot Welding

Abstract: Dynamic electrical resistance during resistance spot welding has been quantitatively modeled and analyzed in this work. A determination of dynamic resistance is necessary for predicting the transport processes and monitoring the weld quality during resistance spot welding. In this study, dynamic resistance is obtained by taking the sum of temperature-dependent bulk resistance of the workpieces and contact resistances at the faying surface and electrode-workpiece interface within an effective area corresponding to the electrode tip where welding current primarily flows. A contact resistance is composed of constriction and film resistances, which are functions of hardness, temperature, electrode force, and surface conditions. The temperature is determined from the previous study in predicting unsteady, axisymmetric mass, momentum, heat, species transport, and magnetic field intensity with a mushy-zone phase change in workpieces, and temperature and magnetic fields in the electrodes of different geometries. The predicted nugget thickness and dynamic resistance versus time show quite good agreement with available experimental data. Excluding expulsion, the dynamic resistance curve can be divided into four stages. A rapid decrease of dynamic resistance in stage I is attributed to decreases in contact resistances at the faying surface and electrode-workpiece interface. In stage 2, the increase in dynamic resistance results from the primary increase of bulk resistance in the workpieces and an increase of the sum of contact resistances at the faying surface and electrode-workpiece interface. Dynamic resistance in stage 3 decreases, because increasing rate of bulk resistance in the workpieces and contact resistances decrease. In stage 4 the decrease of dynamic resistance is mainly due to the formation of the molten nugget at the faying surface. The molten nugget is found to occur in stage 4 rather than stage 2 or 3 as qualitatively proposed in the literature. The effects of different parameters on the dynamic resistance curve are also presented.

Condensate Retention Effects on the Performance of Plain-Fin-and-Tube Heat Exchangers: Retention Data and Modeling

Abstract: A study of condensate retention is presented for plain-fin-and-tube heat exchangers typical to those used in air-cooling applications. An experiment in which the retained mass of air-side condensate was measured under dynamic conditions is described, and the results are analyzed using conventional thermal-hydraulic measurements of j and f. With the coupling between condensate retention and thermal performance established, a new model for predicting the mass of retained condensate is described and compared to the steady-state retention data. The model is successful in predicting retained condensate under relatively restricted conditions. The promise of this new approach, and possible refinements that will add engineering value are discussed.

Spray Cooling Under Reduced Gravity Condition

Abstract: We report on the results of a series of authors' parabolic flight experiments on spray cooling in addition to ground-based experiments in which the influence of heater orientation and the behavior of rebounded droplets were especially studied in detail. Water and FC-72 (perfluorocarbon) were employed alternatively as a test liquid sprayed from a single full-cone nozzle onto a Cr-plated surface of an electrically heated copper block for transient cooling experiments or onto a transparent ITO (indium tin oxide) coated surface of a glass block for steady state experiments in a relatively low superheat region. Each experimental run was accomplished within some 15 sec through which a stable reduced gravity condition (0.01 times the terrestrial gravity) was maintained in the aircraft. Cooling curves were obtained over a wide range of each of the following parameters: the wall superheat, the spray volume flux and the Weber number for the spray droplets. It is demonstrated that the gravity dependency of the spray cooling characteristics varies with the spray volume flux and the droplet Weber number. Qualitative interpretations of the observed gravity dependency are provided

Flow Boiling Heat Transfer From Plain and Microporous Coated Surfaces in Subcooled FC-72

Abstract: The present research is an experimental study of subcooled flow boiling behavior using flat, microporous-enhanced square heater surfaces in pure FC-72. Two 1-cm 2 copper surfaces, one highly polished (plain) and one microporous coated, were flush-mounted into a 12.7 mm square, horizontal flow channel. Testing was performed for fluid velocities ranging from 0.5 to 4 m/s (Reynolds numbers from 18,700 to 174,500) and pure subcooling levels from 4 to 20 K. Both surfaces' nucleate flow boiling curves collapsed to one line showing insensitivity to fluid velocity and subcooling. The log-log slope of the microporous surface nucleate boiling curves was lower than the plain surface due to the conductive thermal resistance of the microporous coating layer. Both, increased fluid velocity and subcooling, increase the CHF values for both surfaces, however, the already enhanced boiling characteristics of the microporous coating appear dominant and require higher fluid velocities to provide additional enhancement of CHF to the microporous surface

Heat Transfer Enhancement to the Drag-Reducing Flow of Surfactant Solution in Two-Dimensional Channel With Mesh-Screen Inserts at the Inlet

Abstract: The heat transfer enhancement of drag-reducing flow of high Reynolds number in a two-dimensional channel by utilizing the characteristic of fluid was studied. As the networks of rod-like micelles in surfactant solution are responsible for suppressing the turbulence in drag-reducing flow, destruction of the structure of networks was considered to eliminate the drag reduction and prevent heat transfer deterioration. By inserting wire mesh in the channel against the flow, the drag-reducing function of the micellar structure in surfactant aqueous solution was successfully switched off. With the Reynolds number close to the first critical Reynolds number, the heat transfer coefficient in the region downstream of the mesh can be improved significantly, reaching the same level as that of water. The region with turbulent heat transfer downstream of the mesh becomes smaller as the concentration of surfactant in the solution increases. Three types of mesh of different wire diameter and opening space were evaluated for their effect in promoting heat transfer and the corresponding pressure loss due to blockage of the mesh. The turbulent intensities were measured downstream from the mesh by using a Laser Doppler Velocimetry (LDV) system