bluuebunny/crossref_metadata_2025 huggingface dataset:
A General Semicausal Stochastic Model for Turbulence/Radiation Interactions in Flames
Authors: Chan S. H. , Pan X. C.
Published: February 2008

This paper presents a general two-dimensional non-stationary semicausal model for the simulation of mixture fraction, which improves our previous causal model. The proposed model includes not only the pre-correlation predictors (both in time space and geometric space) as well as the cross-correlation predictors, as in the causal model, but also post-correlation predictors. The latter makes possible the consideration of interactions of a scalar, such as mixture fraction, at a physical location with that of all its adjacent locations. It has also been shown that the complicated second-and higher-order correlation predictors can be neglected in the semicausal simulation of mixture fraction. To show the validity of the model, the stochastic mean and variance of the spectral intensities at different wavelengths were predicted and compared with detailed experimental data for turbulent carbon monoxide/hydrogen/air diffusion flames having different Reynolds numbers. These comparisons showed excellent agreement with existing data and the improvement over the prior causal model.
Suppression of Flow Boiling Nucleation
Authors: Thorncroft G. E. , Klausner J. F. , Mei R.
Published: February 2008

A simple model is presented for estimating the ratio of the maximum to minimum cavity radius required for ebullition in two-phase flow with heat transfer. The resulting dimensionless parameter, rmax/rmin, is demonstrated to correlate flow boiling nucleation site density. As the convective heat transfer associated with bulk turbulence in two-phase flow is enhanced, rmax→rmin, and the probability of finding surface cavities whose radii lie between rmaxandrmin is reduced. Thus, active nucleation sites become deactivated. A vertical flow boiling facility was fabricated in which the nucleation suppression point can be measured. Experiments conducted for mass flux ranging from 183–315 kg/m2-s and inlet quality ranging from 0–0.151, along with data available from the literature, suggest that rmax/rmin is the leading order dimensionless parameter on which the complete suppression of nucleation sites depends. Although the suppression of nucleation sites also depends, to a certain extent, on the surface/fluid combination and heat flux, it is found that complete suppression occurs for rmax/rmin ranging from 40 to 120. This is proposed as a criterion to discriminate the purely convective regime from the nucleate boiling regime.
Numerical Simulation of Saturated Film Boiling on a Horizontal Surface
Authors: Son G. , Dhir V. K.
Published: February 2008

The past efforts in applying linear Taylor instability theory to the prediction of heat transfer during film boiling on a horizontal surface have suffered from the fact that empirical correlations must be used to define the shape of vapor-liquid interfaces and to determine the transport of mass and heat across these interfaces. The objective of this study is to clarify the physics of film boiling and to predict heat transfer coefficients through complete numerical simulation of the evolving interface between superposed layers of immiscible fluids. A coordinate transformation technique supplemented by a numerical grid generation method and a second-order projection method are combined to solve for the flow and temperature fields associated with an evolving interface. From the numerical simulation, the film thickness and, in turn, the heat transfer coefficient are found to vary both spatially and temporally. Increased wall superheat not only thickens the vapor film in the valley but also enlarges the vapor bulge. The effect of increased system pressure is to slow down the growth of the interface.
Modeling the Characteristics of Thermally Governed Transient Flow Surges in Multitube Two-Phase Condensing Flow Systems With Compressibility and Thermal and Flow Distribution Asymmetry
Authors: Wedekind G. L. , Kobus C. J. , Bhatt B. L
Published: February 2008

In a tube-type condenser involving complete condensation, small changes in the inlet vapor flow rate momentarily cause very large transient surges in the outlet mass flow rate. An Equivalent Single-Tube Model (ESTM), based on the System Mean Void Fraction Model, is developed that predicts these transient flow surges for a multitube system; including the effects of compressibility as well as thermal and flow distribution asymmetry. The model is verified theoretical and experimentally. From a design perspective, the significant value of the ESTM is that it includes the primary physical mechanisms involved in such complex flow transients, yet is simple enough to be solved on typical “spreadsheet” software.
Modeling of Transient Turbulent Natural Convection in a Melt Layer With Solidification
Authors: Fan T. H. , Cheung F. B.
Published: February 2008

The phenomenon of turbulent natural convection in a horizontal heat-generating melt layer with solidification taking place at the cooled upper and lower boundaries is investigated theoretically. The objective is to determine the transient behavior of the crust at the upper and lower surfaces and the effect of crust formation on the turbulent natural convection process in the melt layer. Various surface temperatures, latent heats, and the heat source strengths are considered along with the effects of the Stefan number and Rayleigh number. Special attention is given to the interaction between the melt pool heat transfer and the crust dynamics. Numerical results are presented for the transient crust thickness, transient temperature distribution, eddy heat transport, and the heat transfer characteristics at the solid-liquid interface during the freezing process. The present study provides basic information needed to predict the transient behavior of a melt pool in a reactor lower head following a severe core-meltdown accident.
Zonal Method to Model Radiative Transport in an Optical Fiber Drawing Furnace
Authors: Yin Z. , Jaluria Y.
Published: February 2008

Precise modeling of radiative heat exchange between the furnace and the glass preform is a very important part of the modeling of the fiber drawing process in a high temperature furnace. Most earlier studies on this process have used the optically thick approximation, i.e., the radiative heat exchange is assumed to depend only on the preform surface temperature while the transmission, emission, and absorption within the preform are approximated as a diffusion process. The validity of this approximation in the modeling of the fiber drawing process is dubious since the diameter of the preform undergoes a drastic reduction during the drawing process. The objectives of this research are to use a more accurate approach—the zonal method—to replace the optically thick approximation for computing the radiative heat exchange between the furnace and the preform, and to determine if the optically thick approximation is valid for this process. In applying the zonal method, the preform surface is assumed to be diffuse to both transmission and reflection. An enclosure analysis is performed for the radiative exchange between the furnace and the outer surface of the preform and the zonal method is employed to consider the radiative exchange within the glass preform. The emissivity for the glass preform has been calculated based on the diffuse surface assumption and applied to the computation of radiative heat flux with the optically thick approximation for the purpose of comparison with the present work. The results obtained by the zonal method show that the radiative heat flux is strongly influenced by the radial temperature variation within the preform, while those obtained by the optically thick approximation do not show this effect, as expected. Comparisons of the results obtained by these two approaches reveal that the optically thick approximation predicts the radiative heat flux satisfactorily for a range of axial temperature variations, but only when the radial temperature variation within the preform is small. The diameter change in the neck-down region has almost no effect on the validity of the optically thick approximation.
Theoretical and Experimental Study of Electrohydrodynamic Heat Transfer Enhancement Through Wire-Plate Corona Discharge
Authors: Owsenek B. L. , Seyed-Yagoobi J.
Published: February 2008

Heat and mass transfer between a surface and the surrounding gas can be enhanced by the application of electric body forces that induce jet or plume-like fluid motion. Such enhancement causes no noise or vibration, can be applied in complex, isolated geometries, and allows simple control of surface temperatures. This paper examines the potentially useful case of multiple fine-wire electrodes suspended in the open air above a grounded and heated horizontal surface. An infrared camera system was used to obtain a complete and accurate distribution of local heat transfer coefficients on the impingement surface. A numerical code was developed and verified by comparison with experimental data. This code was then used to investigate and compare the heat transfer generated by novel electrode geometries.
Heat Transfer in an Electromagnetic Bearing
Authors: Jones G. F. , Nataraj C.
Published: February 2008

An exact solution for two-dimensional, full transient, and steady periodic heat conduction in an electromagnetic bearing is obtained. Classical methods are used to obtain an analytical expression for the temperature distribution that arises from power dissipated in the pole windings. Among the key findings is the need for cooling in the immediate neighborhood of the bearing support due to the relatively large thermal resistance of the supporting structure. The results presented prove the existence of large temperature gradients in the bearing in both the radial and circumferential directions. This demands the need for a fine mesh when performing the commonly used nodal-network thermal analysis. Conditions are described under which the temperature distribution is independent of the frequency of the time-dependent current supplied to the poles. For these cases the problem reduces to steady state, and the solution is given. A peak circumferential temperature difference of about 55°C in the bearing is possible under certain conditions that are discussed. Attention to proper thermal design is critical to reduce the dimensional distortion of the bearing caused by thermal expansion. The effects of thermal expansion can range from catastrophic, should the shaft come in contact with the bearing, to an undesirable change in the force and dynamic control characteristics caused by a variation in the critical shaft-to-bearing clearance, which is of the order of a fraction of a millimeter.
Enhancement of Thermal Conductivity by Using Polymer-Zeolite in Solid Adsorption Heat Pumps
Authors: Hu E. J. , Zhu D.-S. , Sang X.-Y. , Wang L. , Tan Y.-K.
Published: February 2008

A thermal conductivity augmentation technology for zeolite beds has been studied in this paper. To enhance the effective thermal conductivity of zeolite, the zeolite particle was coated with a thermal polymer material in a new synthesis process. It was found that the effective thermal conductivity of the polymerzeolite was increased two to three times, while its adsorptive ability remains the same. The results of the experiment show that the polymer synthesis technique could be used as a heat transfer enhancement method to improve thermal conductivity among zeolite particles.
Thermal Convection in an Infinite Porous Medium Induced by a Heated Sphere
Authors: Ganapathy R.
Published: February 2008

This paper investigates the transient behavior of the free convection motion and heat transfer induced by a heated sphere with prescribed wall temperature embedded instantaneously in an infinite porous medium. Solutions for the velocity and temperature fields have been obtained in the form of series expansions in Rayleigh number which is based on the medium permeability and the temperature of the sphere. All discussions are based on the assumption that the flow is governed by Darcy's law and the thermal Rayleigh number is small.
A Global Time Treatment for Inverse Heat Conduction Problems
Authors: Frankel J. I. , Keyhani M.
Published: February 2008

A new global time treatment is proposed and demonstrated for inverse heat conduction problems. This exposition illustrates the methodology by carefully and meticulously investigating the classic Beck’s problem. It is shown that accurate and stable numerical results occur without resorting to any stabilizing scheme beyond the implementation of a global basis representation for the temperature distribution. As a global time method the entire space-time domain is resolved in a simultaneous fashion. The approach is also extendable to multidimensional and multiprobe situations without difficulty. In direct problems the method has been successively applied to initial value problems, Volterra integral equations, and parabolic and hyperbolic partial and integro-partial differential equations.
Laminar Natural Convection From an Elliptic Tube With Different Orientations
Authors: Badr H. M.
Published: February 2008

The problem of two-dimensional natural convection heat transfer from a straight tube of elliptic cross section is investigated. The tube, which has an isothermal surface, is placed with its axis horizontal in an initially quiescent fluid of infinite extent. The velocity and thermal fields are obtained by studying the time development of these fields following a sudden increase of the tube surface temperature until reaching steady state. The study is based on the solution of the full conservation equations of mass, momentum, and energy with no boundary layer simplifications. The paper focuses on the effects of the tube orientation, axis ratio, and Rayleigh number while keeping the Prandtl number unchanged (Pr = 0.7). The study revealed that the maximum average Nusselt number is obtained when the tube major axis is vertical. Within the range of axis ratios considered (Ar = 0.4 to 0.98), smaller Ar resulted in higher heat transfer rate in most cases. Higher Rayleigh number leads to higher velocities and also higher local and average Nusselt numbers in all cases considered. The details of the steady flow and thermal fields are presented in the form of local Nusselt number and surface vorticity distributions as well as streamline and isotherm patterns for some selected cases.
Wide Band Correlated-k Approach to Thermal Radiative Transport in Nonhomogeneous Media
Authors: Marin O. , Buckius R. O.
Published: February 2008

The correlated-k approach is compared with the exact line-by-line calculations for thermal radiative transport in highly nonhomogeneous media containing water vapor and carbon dioxide. It is shown that the magnitude of the spectral interval over which k-distribution functions are generated can be increased up to approximately 1000 cm−1 for water vapor and 500 cm−1 for carbon dioxide, with only a slight loss of accuracy. Different solution techniques of the radiative transfer equation in nonhomogeneous media are used and compared. The wide band correlated-k method and a simplified approach to the exponential wide band correlated-k method are shown to provide very good results for the cases considered. The calculations include the entire infrared spectrum of water vapor and carbon dioxide for temperatures up to 2500 K.
Schemes and Applications of First and Second-Order Discrete Ordinates Interpolation Methods to Irregular Two-Dimensional Geometries
Authors: Koo H.-M. , Cheong K.-B. , Song T.-H.
Published: February 2008

This paper presents numerical schemes and comparison of predictions of radiative heat transfer for the first and the second order discrete ordinates methods (DOM1 and DOM2) using an interpolation scheme. The formulations are followed by derivation of numerical schemes for two-dimensional body fitted grids. With varying the optical depths and the numbers of grids and ordinates, radiative wall heat fluxes by DOM1 and DOM2 are calculated to compare with the exact solutions for three kinds of two-dimensional enclosures (square, quadrilateral, and J-shaped) containing absorbing/emitting and nonscattering media of known temperature with cold black walls. Emissive power and radiative wall heat fluxes by DOM1 and DOM2 are calculated to compare with zonal results for two-dimensional square enclosure containing absorbing/emitting and isotropically scattering medium of known uniform heat source with cold black walls. The results of DOM1 and DOM2 are in good agreement with the exact solutions or the zonal results. DOM1 gives more accurate results than DOM2 for most of the tested optical depths and the numbers of grids and ordinates. These methods appear as powerful candidates of very versatile radiation analysis tool. Their grid and ordinate dependencies are also discussed in depth.
An Approximate Solution to Radiative Transfer in Two-Dimensional Rectangular Enclosures
Authors: Pessoa-Filho J. B. , Thynell S. T.
Published: February 2008

The application of a new approximate technique for treating radiative transfer in absorbing, emitting, anisotropically scattering media in two-dimensional rectangular enclosures is presented. In its development the discontinuous nature of the radiation intensity, stability of the iterative solution procedure, and selection of quadrature points have been addressed. As a result, false scattering is eliminated. The spatial discretization can be formed without considering the chosen discrete directions, permitting a complete compatibility with the discretization of the conservation equations of mass, momentum, and energy. The effects of anisotropic scattering, wall emission, and gray-diffuse surfaces are considered for comparison with results available in the literature. The computed numerical results are in excellent agreement with those obtained by other numerical approaches.
A Method for Modeling the Mitigation of Hazardous Fire Thermal Radiation by Water Spray Curtains
Authors: Dembele S. , Delmas A. , Sacadura J. F.
Published: February 2008

A radiative transfer model describing the interactions between hazardous fire thermal radiation and water sprays is presented. Both the liquid (water droplets) and gaseous (mainly water vapor and carbon dioxide) phases of the spray are considered in the present work. Radiative properties of the polydisperse water droplets are derived from Mie theory. The gaseous phase behavior is handled by the correlated-k distribution method, where the k-distribution function is evaluated for Malkmus narrow-band statistical model. The radiative transfer equation, in its integral form, is solved by a discrete ordinates method. After a general description of the radiative model developed and the experimental task to validate it, some results are discussed on its accuracy and CPU time. A deeper analysis is also carried out to point out the influence of the main parameters involved in the problem.
Calcium Carbonate Scale Formation During Subcooled Flow Boiling
Authors: Najibi S. H. , Mu¨ller-Steinhagen H. , Jamialahmadi M.
Published: February 2008

Scale deposition on the heat transfer surfaces from water containing dissolved salts considerably reduces fuel economy and performance of the heat transfer equipment. In general, this problem is more serious during nucleate boiling due to the mechanisms of bubble formation and detachment. In this study, a large number of experiments were performed to determine the effect of fluid velocity, initial surface temperature, and bulk concentration on the rate of calcium carbonate deposition on heat transfer surfaces during subcooled flow boiling. A physically sound prediction model for the deposition process under these operating conditions has been developed which predicts the experimental data with good accuracy. Two previously published models are also discussed and used to predict the experimental data.
Experimental Validation of Continuum Mixture Model for Binary Alloy Solidification
Authors: Krane M. J. M. , Incropera F. P.
Published: February 2008

Experiments were performed with binary metal alloys to validate a continuum mixture model for alloy solidification. Ingots of two compositions, Pb-20%Sn and Pb-40%Sn, were cast in a permanent mold, and the solidification process was simulated. Temperature histories were measured during casting, and composition profiles were found in the solidified ingot. Dendritic arm spacings were found from optical micrographs of the alloy microstructure and used to determine a constant in the Blake-Kozeny submodel for the mushy zone permeability in the liquid-solid interaction term of the momentum equations. Scaling analysis from a previous work and a large uncertainty in the permeability constant suggested that predictions of the composition are extremely sensitive to the choice of a permeability model. Three simulations of each casting were performed using the permeability constant as a parameter, and measured temperatures and compositions were compared with predictions based on different model constants. In the region of the liquidus interface, where all of the significant advection of solute takes place, the results suggest that the Blake-Kozeny model based on measured dendritic arm spacings significantly underpredicts the resistance of the dendritic array to fluid flow.
Unsteady Non-Darcian Forced Convection Analysis in an Annulus Partially Filled With a Porous Material
Authors: Al-Nimr M. A. , Alkam M. K.
Published: February 2008

Numerical solutions are presented for the problem of transient, developing, forced-convection flow in concentric annuli partially filled with porous substrates. The porous substrate is attached either to the inner cylinder (case I), or to the outer cylinder (case O). In both cases, the boundary in contact with the porous substrate is exposed to a sudden change in its temperature while the other boundary is kept adiabatic. Including the macroscopic inertial term, the Brinkman-Forchheimer-extended Darcy model is used to model the flow inside the porous domain. The effects of different parameters regarding the geometry, the solid matrix, and the fluid on the hydrodynamic and thermal behavior are investigated. It is shown that porous substrates may improve Nusselt number by 1200 percent keeping other flow and geometrical parameters fixed. Also, it is found that there is an optimum thickness for the porous substrate beyond which there is no significant improvement in Nusselt number. In the present work, the dimensionless hydrodynamic entrance length Zen varies within the range 2–45 and it has significant effect on the fully developed Nusselt number at steady-state conditions. As a result, the macroscopic inertial term in the porous domain momentum equation should not be neglected.
EHD Enhanced Convective Boiling of R-134a in Grooved Channels—Application to Subcompact Heat Exchangers
Authors: Salehi M. , Ohadi M. M. , Dessiatoun S.
Published: February 2008

Electrohydrodynamically (EHD) enhanced flow boiling of refrigerant R-134a inside grooved channels of approximately 1-mm hydraulic diameter was investigated with the objective of addressing the applicability of the EHD technique in highly compact heat exchangers. Two sets of experiments were performed. The first set included experiments in a channel with a smooth heat transfer wall, whereas in the second set a corrugated (enhanced) surface was used. In each case experiments were conducted as a function of the applied electrical field strength, electric field polarity, flow Reynolds number, inlet test section vapor quality, and flow direction (upward, downward, or horizontal). It is demonstrated that in all cases the EHD effect can substantially increase the heat transfer coefficient particularly at low Reynolds numbers and when applied over the enhanced heat transfer wall.
Impingement Cooling Flow and Heat Transfer Under Acoustic Excitations
Authors: Gau C. , Sheu W. Y. , Shen C. H.
Published: February 2008

Experiments are performed to study (a) slot air jet impingement cooling flow and (b) the heat transfer under acoustic excitations. Both flow visualization and spectral energy evolution measurements along the shear layer are made. The acoustic excitation at either inherent or noninherent frequencies can make the upstream shift for both the most unstable waves and the resulting vortex formation and its subsequent pairing processes. At inherent frequencies the most unstable wave can be amplified, which increases the turbulence intensity in both the shear layer and the core and enhances the heat transfer. Both the turbulence intensity and the heat transfer increase with increasing excitation pressure levels Spl until partial breakdown of the vortex occurs. At noninherent frequencies, however, the most unstable wave can be suppressed, which reduces the turbulence intensity and decreases the heat transfer. Both the turbulence intensity and the heat transfer decreases with increasing Spl, but increases with increasing Spl when the excitation frequency becomes dominant. For excitation at high Reynolds number with either inherent or noninherent frequency, a greater excitation pressure level is needed to cause the enhancement or the reduction in heat transfer. During the experiments, the inherent frequencies selected for excitation are Fo/2 and Fo/4, the noninherent frequencies are 0.71 Fo, 0.75 Fo, and 0.8 Fo, the acoustic pressure level varies from 70 dB to 100 dB, and the Reynolds number varies from 5500 to 22,000.
Containerless Thermal Diffusivity Determination of High-Temperature Levitated Spherical Specimen by Extended Flash Methods: Theory and Experimental Validation
Authors: Shen F. , Khodadadi J. M. , Woods M. C. , Weber J. K. R. , Li B. Q.
Published: February 2008

In order to determine the thermal diffusivity of materials, especially solids and liquids at high temperatures, two extended containerless flash techniques that are applicable to levitated spherical specimen are proposed. The extended flash methods are modeled as an axisymmetric transient conduction heat transfer problem within the sphere. For the “single-step” method, analytic expressions for the temperature history on the surface of the sphere are obtained that are independent of the incident energy and the absorption layer thickness. It is shown that by knowing the sample diameter and recording the temperature transient history at least at two different points on the surface simultaneously, the thermal diffusivity can be determined. A detailed discussion of the effects of the various parameters is presented. For the “two-step” analysis the problem of nonlinearity of the radiative heat transfer boundary condition is overcome by replacing it with the measured time-dependent surface temperature data. Upon obtaining the temperature field the determination of the thermal diffusivity turns into a minimization problem. In performing the proposed two-step procedure there is a need to undertake a cool-down experiment. Results of an experimental study directed at determining the thermal diffusivity of high-temperature solid samples of pure Nickel and Inconel 718 superalloy near their melting temperatures using the single-step method are discussed. Based on close agreement with reliable data available in the literature, it is concluded that the proposed techniques can provide reliable thermal diffusivity data for high-temperature materials.
Refractive Indices at Visible Wavelengths of Soot Emitted From Buoyant Turbulent Diffusion Flames
Authors: Wu J.-S. , Krishnan S. S. , Faeth G. M.
Published: February 2008

Measurements of the optical properties of soot, emphasizing refractive indices, are reported for visible wavelengths (350–800 nm). The experiments considered soot in the fuel-lean (overfire) region of buoyant turbulent diffusion flames in the long residence time regime where soot properties are relatively independent of position in the overfire region and residence time. Flames fueled with acetylene, propylene, ethylene, and propane that were burning in still air provided a range of soot physical and structure properties. Measurements included soot composition, density, structure, gravimetric volume fraction, and scattering and absorption properties. These data were analyzed to find soot fractal dimensions, refractive indices, refractive index functions, and dimensionless extinction coefficients assuming Rayleigh-Debye-Gans scattering for polydisperse mass fractal aggregates (RDG-PFA theory). RDG-PFA theory was successfully evaluated using measured scattering properties. Soot fractal dimensions were independent of both fuel type and wavelength, yielding a mean value of 1.77 with a standard deviation of 0.04. Refractive indices were independent of fuel type within experimental uncertainties and were in reasonably good agreement with earlier measurements for soot in the fuel-lean region of diffusion flames due to Dalzell and Sarofim (1969). Dimensionless extinction coefficients were independent of both fuel type and wavelength, yielding a mean value of 5.1 with a standard deviation of 0.5, which is lower than earlier measurements for reasons that still must be explained.
Effect of Unsteady Wake With Trailing Edge Coolant Ejection on Detailed Heat Transfer Coefficient Distributions for a Gas Turbine Blade
Authors: Du H. , Ekkad S. , Han J.-C.
Published: February 2008

Detailed heat transfer coefficient distributions on a turbine blade under the combined effects of trailing edge jets and unsteady wakes at various free-stream conditions are presented using a transient liquid crystal image method. The exit Reynolds number based on the blade axial chord is varied from 5.3 × 105 to 7.6 × 105 for a five blade linear cascade in a low speed wind tunnel. Unsteady wakes are produced using a spoked wheel-type wake generator upstream of the linear cascade. Upstream trailing edge jets are simulated by air ejection from holes located on the hollow spokes of the wake generator. The mass flux ratio of the jets to free-stream is varied from 0.0 to 1.0. Results show that the surface heat transfer coefficient increases with an increase in Reynolds number and also increases with the addition of unsteady wakes. Adding grid generated turbulence to the unsteady wake further enhances the blade surface heat transfer coefficients. The trailing edge jets compensate the defect in the velocity profile caused by the unsteady passing wakes and give an increase in free-stream velocity and produce a more uniformly disturbed turbulence intensity profile. The net effect is to increase both the front parts of blade suction and pressure surface heat transfer. However, the jet effect diminishes in and after the transition regions on suction surface, or far away from the leading edge on pressure surface.
Enhancement of Heat Transfer Rate by Application of a Static Magnetic Field During Natural Convection of Liquid Metal in a Cube
Authors: Tagawa T. , Ozoe H.
Published: February 2008

Natural convection of liquid metal in a cubical enclosure under an external magnetic field was investigated by three-dimensional numerical analyses. The system parameters were Ra = 105 and 106, Pr = 0.025, and Ha = 0–1000. One vertical wall of the cubical enclosure was heated, and the opposing vertical wall was cooled, both isothermally; the other four walls were thermally insulated. A uniform horizontal magnetic field was applied parallel to the heated and cooled walls. At Ra = 105 and Ha = 50, the average Nusselt number on the heated wall attained almost the maximum value and was greater than that at Ha = 0. The velocity vectors along the vertical walls, and those along the horizontal planes, were rectified in a two-dimensional way at Ha = 50 or over, and the average Nusselt number decreased gradually for higher values of the Hartmann number. Similar characteristics were obtained at Ra = 106. The agreement with our earlier experiments was moderately good.
Thick-Film Phenomenon in High-Heat-Flux Evaporation From Cylindrical Pores
Authors: Khrustalev D. , Faghri A.
Published: February 2008

A physical and mathematical model of the evaporating thick liquid film, attached to the liquid-vapor meniscus in a circular micropore, has been developed. The liquid flow has been coupled with the vapor flow along the liquid-vapor interface. The model includes quasi-one-dimensional compressible steady-state momentum conservation for the vapor and also a simplified description of the microfilm at the end of the thick film. The numerical results, obtained for water, demonstrate that formation of extended thick liquid films in micropores can take place due to high-velocity vapor flow under high rates of vaporization. The model has also predicted that the available capillary pressure significantly changes with the wall-vapor superheat and other operational conditions.
Study of Droplet Sprays Prior to Impact on a Heated Horizontal Surface
Authors: Gonza´lez J. E. , Black W. Z.
Published: February 2008

This paper concerns a quantitative assessment of the heat and mass transfer behavior of spray droplets, downward oriented prior to their impact on a heated horizontal surface. An experimental and theoretical investigation of the coupling effects between a downward oriented spray and a rising saturated buoyant jet that results from evaporation of the spray on a heated surface has been successfully completed. A model describing the coupled thermal and hydrodynamic behavior of both the spray and the saturated buoyant jet has been developed. An experimental set-up involving a high speed photographic apparatus has been used to observe in-flight monodispersed sprays and to measure the diameter and the velocity of droplets as they approach the heated surface. The theoretical and experimental results indicate that the temperature of the saturated buoyant jet is highly affected by the presence of a subcooled spray and small droplet sprays, vertically projected, experience high condensation rates as they pass through the saturated buoyant jet, reaching the saturation temperature before impacting on the heated surface, as well as experience acceleration as a consequence of an increase in mass due to the condensation.
Evaporation and Growth of Multicomponent Droplets in Random Dense Clusters
Authors: Elperin T. , Krasovitov B.
Published: February 2008

Interaction between evaporating (growing) droplets in binary arrays and large random clusters of droplets of different sizes is analyzed in a quasi-steady approximation using the modified method of expansion into irreducible multipoles. Evaporation and condensation of binary arrays and clusters of droplets (i) composed of volatile components and (ii) composed of a volatile and a nonvolatile component was studied. The analytical and numerical results of the investigation are presented in terms of heat and mass correction factors. Solution of the transient problem is obtained, and the evaporation rate is determined. It is shown that for droplets of ideal solution with different compositions, Nusselt and Sherwood numbers depend on the concentration of components inside each droplet. When dense cloud contains both pure droplets and droplets containing soluble nuclei, interactions between temperature and concentration fields causes growth of some droplets in a cluster and evaporation of the other droplets. It is shown that inside an evaporating large cluster of multicomponent droplets composed of volatile components, recondensation occurs whereby evaporating droplets act as mass sources and heat sinks and growing droplets act as mass sink and heat sources. The results of this study are of relevance in the analysis of dynamics of droplet size distribution in clouds, artificial modification of clouds and precipitation, in-cloud pollutants scavenging, and in the analysis of evaporation and combustion of multicomponent (blended) liquid fuels.
Optimization of Pulsating Heating in Pool Boiling
Authors: Vargas J. V. C. , Bejan A.
Published: February 2008

This paper describes an experimental and theoretical study of the periodic on and off heating of water on a horizontal surface. The heat transfer is effected by natural convection and isolated bubbles. The experiments cover the heat flux range 33–154 kW/m2 and the wall excess temperature range 7–13°C. It is shown experimentally that the cycle-averaged thermal conductance between the surface and the pool can be maximized by properly selecting the time intervals of the on and off heating cycle. The maximum relative augmentation of the thermal conductance is approximately 15 percent. In the second part of the study, an order of magnitude analysis shows that the cycle-averaged thermal conductance can be maximized analytically by considering only the single-phase natural convection effect, and that the optimal time interval when heating is “on” agrees with the experimental results.
Quasi-Homogeneous Nucleation in Microgravity at Low Heat Flux: Experiments and Theory
Authors: Merte H. , Lee H. S.
Published: February 2008

Experiments were conducted in the microgravity of space in which a pool of liquid (R-113), initially at a defined pressure and temperature, was subjected to a step imposed heat flux from a semitransparent thin-film heater forming part of one wall of the container such that boiling is initiated and maintained for a defined period of time at a constant pressure level. Measurements of the transient heater surface and fluid temperatures near the surface were made, noting in particular the conditions at the onset of boiling, along with motion photography of the boiling process in two simultaneous views, from beneath the heating surface and from the side. A total of nine tests were conducted at three levels of heat flux and three levels of subcooling. They were repeated under essentially identical circumstances in each of three space experiments. The absence of buoyancy resulted in the onset of boiling at low heat flux levels, with what is defined as quasi-homogeneous nucleation taking place. The influence of these low levels of heat flux and the pressure effect used to produce the bulk liquid subcooling are accounted for by a modification of classical homogeneous nucleation theory.
EHD-Enhanced Boiling Coefficients and Visualization of R-134a Over Enhanced Tubes
Authors: Cheung K. , Ohadi M. M. , Dessiatoun S. , Singh A.
Published: February 2008

In an earlier study by the authors, the applicability of the EHD technique for augmentation of pool boiling heat transfer of R-134a in a tube bundle was demonstrated. This paper reports additional experiments involving optimization of the electrode/heat transfer surface geometry as well as flow visualization studies that provide improved understanding of the EHD-enhanced pool boiling heat transfer in a tube bundle. Utilizing the flow visualization studies, it is demonstrated that combined electroconvection and improved nucleate boiling dynamics give rise to the augmented heat transfer coefficients.
Electrohydrodynamic Enhancement of Falling Film Evaporation Heat Transfer and its Long-Term Effect on Heat Exchangers
Authors: Yamashita K. , Yabe A.
Published: February 2008

Electrohydrodynamic (EHD) enhancement of vertical falling film evaporation heat transfer has been experimentally studied using HCFC123, which is an alternative working fluid for CFCs. This research verified that EHD enhancement of HCFC123 condensation was as effective as that of conventional working fluids. The electrodes used for EHD enhancement of vertical falling film evaporation utilized the following two EHD phenomena: extracting the liquid by a nonuniform electric field, and surface granulation by a nearly uniform electric field. As a result, an electrode with vertically arrayed offset-slits that is suitable for industrial applications has been developed that showed a six-fold enhancement of evaporation heat transfer over that for a smooth tube. The long-term effects of the high voltages associated with EHD on heat exchanger performance have been evaluated. Operation of EHD heat exchangers for 1000 hours indicates they do not sustain serious damage, and are, therefore, suitable for industrial applications.
Temperature Measurements of a Transient Thermal Plume in a Confined Space
Authors: Hsu C.-H. , Hsieh C.-F. , Teng J.-T.
Published: February 2008

Experiments are performed by measuring a developing thermal plume in an enclosure, that is generated by a constant heat flux annular cylinder heater, with six T-type thermocouples. An empirical correlation obtained among local Nusselt number, Fourier number, and modified Rayleigh number is Nux/Rax*1/4 = 0.00422FoL−0.893.
Nanoscale Temperature Distributions Measured by Scanning Joule Expansion Microscopy
Authors: Majumdar A. , Varesi J.
Published: February 2008

This paper introduces scanning Joule expansion microscopy (SJEM), which is a new thermal imaging technique with lateral resolution in the range of 10–50 nm. Based on the atomic force microscope (AFM), SJEM measures the thermal expansion of Joule-heated elements with a vertical resolution of 1 pm, and provides an expansion map of the scanned sample. Sunmicron metal interconnect lines as well as 50-nm-sized single grains of an indium tin oxide resistor were images using SJEM. Since the local expansion signal is a convolution of local material properties, sample height, and as temperature rise, extraction of the thermal image requires deconvolution. This was experimentally achieved by coating the sample with a uniformly thick polymer film, resulting in direct measurement of the sample temperature distribution. A detailed thermal analysis of the metal wire and the substrate showed that the predicted temperature distribution was in good agreement with the measurements of the polymer-coated sample. However, the frequency response of the expansion signal agreed with theoretical predictions only below 30 KHZ, suggesting that contilever dynamics may play a significant role at higher frequencies. The major advantage of SJEM over previously developed submicron thermal imaging techniques is that it eliminates the need to nanofabricate specialized probes and requires only a standard AFM and simple electronics.
Inverse Determination of Steady Heat Convection Coefficient Distributions
Authors: Martin T. J. , Dulikravich G. S.
Published: February 2008

An inverse Boundary Element Method (BEM) procedure has been used to determine unknown heat transfer coefficients on surfaces of arbitrarily shaped solids. The procedure is noniterative and cost effective, involving only a simple modification to any existing steady-state heat conduction BEM algorithm. Its main advantage is that this method does not require any knowledge of, or solution to, the fluid flow field. Thermal boundary conditions can be prescribed on only part of the boundary of the solid object, while the heat transfer coefficients on boundaries exposed to a moving fluid can be partially or entirely unknown. Over-specified boundary conditions or internal temperature measurements on other, more accessible boundaries are required in order to compensate for the unknown conditions. An ill-conditioned matrix results from the inverse BEM formulation, which must be properly inverted to obtain the solution to the ill-posed problem. Accuracy of numerical results has been demonstrated for several steady two-dimensional heat conduction problems including sensitivity of the algorithm to errors in the measurement data of surface temperatures and heat fluxes.
Comparison of Heat Transfer Characteristics of Radial Jet Reattachment Nozzle to In-Line Impinging Jet Nozzle
Authors: Seyed-Yagoobi J. , Narayanan V. , Page R. H.
Published: February 2008

The heat transfer characteristics of three submerged radial jet reattachment (RJR) nozzles with exit angles of +45, 0, and −10 deg are compared to the heat transfer characteristics of a conventional submerged in-line jet (ILJ) nozzle. The nozzles are compared at their favorable spacing from the impingement surface. The comparisons are based on two criteria: (1) identical fluid flow power, and (2) identical peak pressure exerted on the impingement surface. The local and area-averaged Nusselt numbers are presented. Experiments were conducted for two different flow power conditions. Comparison under identical flow power indicates that significant enhancements in local and comparable enhancements in area-averaged Nusselt numbers can be achieved with the RJR nozzles over the conventional ILJ nozzle while being able to control the net force exerted on the impingement surface. The comparison between the ILJ and RJR nozzles on the basis of the same peak pressure exerted on the impingement surface indicates that the zero degree exit angle RJR nozzle heat transfer characteristics are superior to the ILJ nozzle.
Competing Effects of Dielectrophoresis and Buoyancy on Nucleate Boiling and an Analogy With Variable Gravity Boiling Results
Authors: Snyder T. J. , Chung J. N. , Schneider J. B.
Published: February 2008

An experiment was performed that produced a controlled electrical body force (dielectrophoretic force or DEP force) over the length of a horizontal platinum wire heater during boiling. The DEP force was generated such that it either aided or opposed terrestrial gravity or acted nearly alone in microgravity. The net effect can be thought of as simply either “reducing,” or “increasing” buoyancy. To account for this change in force an “effective gravity ratio,” g′(b, e) was defined that represents the ratio of the total DEP and buoyancy body forces to the constant terrestrial-gravity buoyancy force. Based on our own experimentation and on the published literature, it was argued that the nucleate boiling will be enhanced if the effective gravity acts to hold the vapor bubbles near the heater surface, while at the same time permitting access of the liquid to the surface in order to prevent dryout. However, a large electroconvection effect can dominate and reverse the trend. The nonboiling portion of the heat transfer coefficient was shown to be significant with an applied electric field, especially at high subcoolings. It was found that for 1 < g′(b, e) < 3 a quarter power dependence appears to be a reasonable engineering approximation for the increase in critical heat flux with effective gravity.
Boiling Heat Transfer With Binary Mixtures: Part I—A Theoretical Model for Pool Boiling
Authors: Kandlikar S. G.
Published: February 2008

Experimental evidence available in the literature indicates that the pool boiling heat transfer with binary mixtures is lower than the respective mole- or mass-fraction-averaged value. Although a few investigators have presented analytical work to model this phenomenon, empirical methods and correlations are used extensively. In the present work, a theoretical analysis is presented to estimate the mixture effects on heat transfer. The ideal heat transfer coefficient used currently in the literature to represent the pool boiling heat transfer in the absence of mass diffusion effects is based on empirical considerations, and has no theoretical basis. In the present work, a new pseudo-single component heat transfer coefficient is introduced to account for the mixture property effects more accurately. The liquid composition and the interface temperature at the interface of a growing bubble are predicted analytically and their effect on the heat transfer is estimated. The present model is compared with the theoretical model of Calus and Leonidopoulos (1974), and two empirical models, Calus and Rice (1972) and Fujita et al. (1996). The present model is able to predict the heat transfer coefficients and their trends in azeotrope forming mixtures (benzene/methanol, R-23/R-13 and R-22/R-12) as well as mixtures with widely varying boiling points (water/ethylene glycol and methanol/water).