bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of an Evaporating Extended Meniscus Based on the Augmented Young–Laplace Equation
Authors: DasGupta S. , Schonberg J. A. , Wayner P. C.
Published: May 2008

The microscopic details of fluid flow and heat transfer near the contact line of an evaporating extended meniscus of heptane formed between a horizontal substrate and a “washer” were studied at low heat fluxes. The film profile in the contact line region was measured using ellipsometry and microcomputer-enhanced video microscopy, which demonstrated the details of the transition between a nonevaporating superheated flat thin film and an evaporating curved film. Using the augmented Young-Laplace equation, the interfacial properties of the system were initially evaluated in situ and then used to describe the transport processes. New analytical procedures demonstrated the importance of two dimensionless parameters. Both fluid flow and evaporation depend on the intermolecular force field, which is a function of the film profile. The thickness and curvature profiles agreed with the predictions based on interfacial transport phenomena models. The heat flux distribution and the pressure field were obtained. Since there are significant resistances to heat transfer in this small system due to interfacial forces, viscous stresses, and thermal conduction, the “ideal constant heat flux” cannot be attained. The description of the pressure field gives the details of the coupling between the disjoining and capillary pressures.
On the Use of Monolayers to Reduce Evaporation From Stationary Water Pools
Authors: Tang T. D. , Pauken M. T. , Jeter S. M. , Abdel-Khalik S. I.
Published: May 2008

An experimental investigation has been conducted to quantify the extent by which monolayers of fatty alcohols can reduce evaporation from a deep stationary water pool within a controlled environment. Octadecanol (stearyl alcohol), C17H35–CH2–OH, was chosen as the surface film and ethanol was selected to be the spreading agent. Evaporation suppression of 60 percent was achieved at a water temperature of 25°C with an air temperature of 20°C and a relative humidity of 70 percent. The experimental techniques and data have been validated by comparing the measured evaporation rates for film-free water with earlier data published by other investigators. Data for the evaporation rates of water covered by octadecanol films were correlated as a function of vapor concentration differences between the water surface and air.
Nucleation Site Density in Forced Convection Boiling
Authors: Zeng L. Z. , Klausner J. F.
Published: May 2008

Measurements of nucleation site density, n/A, have been obtained for forced convection boiling of refrigerant R113 in a horizontal 25 × 25 mm i.d. square transparent test section with a nichrome heating strip. It has been found that the behavior of the nucleation site density is significantly different from that found in pool boiling. The mean vapor velocity, heat flux, and system pressure appear to exert a strong parametric influence. It is demonstrated that although the nucleation site density is dependent on the critical cavity radius, rc, it is not sufficient for correlating n/A.
Flashing Inception in Water During Rapid Decompression
Authors: Elias E. , Chambre´ P. L.
Published: May 2008

A phenomenological model is presented for predicting the thermodynamic conditions at the onset of flashing in liquid undergoing a static or flow depressurization transient. It is shown that at extremely high rates of decompression (pulse expansion), a liquid may reach the homogeneous nucleation limit of superheat before appreciable phase transition occurs. A criterion for pulse expansion is derived by an asymptotic solution of the mass and energy equations in conjunction with the equation for spontaneous bubble nucleation and growth near the flashing inception point. The effect of impurities and dissolved gases is considered by an empirically defined heterogeneous nucleation coefficient. The model predicts the minimum pressure at the flashing point with a probable error of less than 11 percent, using 83 experimental data points.
Microscale Heat Conduction in Dielectric Thin Films
Authors: Majumdar A.
Published: May 2008

Heat conduction in dielectric thin films is a critical issue in the design of electronic devices and packages. Depending on the material properties, there exists a range of film thickness where the Fourier law, used for macroscale heat conduction, cannot be applied. This paper shows that in this microscale regime, heat transport by lattice vibrations or phonons can be analyzed as a radiative transfer problem. Based on Boltzmann transport theory, an equation of phonon radiative transfer (EPRT) is developed. In the acoustically thick limit, ξL ≫ 1, or the macroscale regime, where the film thickness is much larger than the phonon-scattering mean free path, the EPRT reduces to the Fourier law. In the acoustically thin limit, ξL ≪ 1, the EPRT yields the blackbody radiation law q = σ (T14 − T24) at temperatures below the Debye temperature, where q is the heat flux and T1 and T2 are temperatures at the film boundaries. For transient heat conduction, the EPRT suggests that a heat pulse is transported as a wave, which becomes attenuated in the film due to phonon scattering. It is also shown that the hyperbolic heat equation can be derived from the EPRT only in the acoustically thick limit. The EPRT is then used to study heat transport in diamond thin films in wide range of acoustical thicknesses spanning the thin and the thick regimes. The heat flux follows the relation q = 4σT3ΔT/(3ξL/4 + 1) as derived in the modified diffusion approximation for photon radiative transfer. The thermal conductivity, as currently predicted by kinetic theory, causes the Fourier law to overpredict the heat flux by 33 percent when ξL ≪ 1, by 133 percent when ξL = 1, and by about 10 percent when ξL increases to 10. To use the Fourier law in both ballistic and diffusive transport regimes, a simple expression for an effective thermal conductivity is developed.
Stagnation-Point Heat Transfer During Impingement of Laminar Liquid Jets: Analysis Including Surface Tension
Authors: Liu Xin , Gabour L. A. , Lienhard J. H.
Published: May 2008

The stagnation-zone characteristics of an impinging liquid jet are of great interest because the maximum heat transfer coefficient occurs in that region. This paper is an analytical study of the fluid flow and heat transfer in the stagnation zone of an unsubmerged liquid jet. The role of surface tension is emphasized. Stagnation-zone transport is strongly dependent on the potential flow above the boundary layer. Only a few studies have examined the potential flow of an unsubmerged jet, each using approximate potential flow theory and neglecting surface tension. In this paper, numerical solutions for a laminar unsubmerged jet are obtained, using a simulation method for steady, inviscid, incompressible flow with surface tension. A series solution that satisfies the boundary conditions in an approximate manner is constructed in terms of Legendre functions. Numerical solution of the momentum equation shows that surface tension has an effect on the stagnation-point flow field when the Weber number is small. Solutions of the associated boundary layer problem are used to obtain predictions of the influence of Weber number on the stagnation-zone heat transfer. The results are validated by comparison to measurements at high Weber number.
Natural Convective Heat Transfer in a Divided Vertical Channel: Part I—Numerical Study
Authors: Naylor D. , Tarasuk J. D.
Published: May 2008

This is a two-part study of two-dimensional laminar natural convection heat transfer in a divided vertical channel. The divided channel consists of an isothermal dividing plate located on the center line of a vertical channel formed by two isothermal walls. The study examines the effect of Rayleigh number, plate-to-channel length ratio, vertical plate position, and plate thickness on the heat transfer rate from the channel walls, the dividing plate, and the channel as a whole. In Part I, solutions to both the full elliptic and parabolic forms of the Navier–Stokes and energy equations are obtained for Prandtl number Pr = 0.7 (air). Positioning the plate at the bottom of the channel was found to give the highest average Nusselt numbers for the plate and channel. Dividing plate average Nusselt numbers as much as two times higher than the isolated plate Nusselt number were predicted numerically. Experimental measurements and data correlations for the divided channel are presented in Part II of this paper.
Enhanced Heat Transfer, Missing Pin, and Optimization for Cylindrical Pin Fin Arrays
Authors: Jubran B. A. , Hamdan M. A. , Abdualh R. M.
Published: May 2008

This paper reports an experimental investigation on the effects of interfin spacing, shroud clearance, and missing pin on the heat transfer from cylindrical pin fins arranged in staggered and in-line arrays. The interfin spacing in the span wise direction was so small that the pins were almost touching each other. It was found that the optimum interfin spacing in both spanwise and streamwise directions is 2.5 D regardless of both type of array and shroud clearance used. The effect of missing pin for various interfin spacing arrays was found to be negligible for the in-line array but more significant for the staggered arrays.
Crystallization Fouling in Plate Heat Exchangers
Authors: Bansal B. , Mu¨ller-Steinhagen H.
Published: May 2008

Crystallization fouling of calcium sulfate was investigated in a plate and frame heat exchanger. The effects of flow velocity, wall temperature, and CaSO4, concentration on the fouling rates have been investigated and the distribution of scale along the heat transfer surface has been observed. The measured fouling curves are compared with predictions from a surface reaction controlled model.
Experimental and Numerical Studies of Natural Convection With Flow Separation in Upward-Facing Inclined Open Cavities
Authors: Showole R. A. , Tarasuk J. D.
Published: May 2008

Steady two-dimensional laminar natural convection heat transfer from isothermal horizontal and inclined open cavities of rectangular cross section has been investigated experimentally using a Mach-Zehnder interferometer and numerically by a finite difference technique. Experimental results are presented for Prandtl number, Pr = 0.7, Rayleigh numbers from 104 to 5 × 105, cavity aspect ratios, A (or h/w) = 0.25, 0.5, and 1.0, and inclination angles (or angles of rotation about the longitudinal axis), θ = 0, 30, 45, and 60 deg to the horizontal. The numerical model uses a relaxation technique to solve the governing elliptic, partial differential equations. Numerical results are presented for the range of Rayleigh number, 103 ≤ Raw ≤ 5 × 105, θ = 0 and 45 deg, and A = 1. Flow and temperature patterns, velocity and temperature profiles, and local and average heat transfer rates are presented. Flow recirculation with two counterrotating convective rolls developed in the cavity at Ra ≥ 105. The inclination of the cavity induced flow entrainment, causing flow separation at the lower corner and flow reattachment at the upper corner of the aperture opening except in shallow cavities, A < 0.5, where the flow reattachment occurred on the base of the inclined cavity. For all Ra numbers, the first inclination of the cavity from the horizontal caused a significant increase in the average heat transfer rate, but a further increase in the inclination angle caused very small increase in heat transfer rate. However, for every angle of inclination considered, the average heat transfer rate increased significantly as Ra was increased. The equation of the form Nu = mRan, where 0.018 ≤ m ≤ 0.088 and 0.325 ≤ n ≤ 0.484, correlates the experimental and numerical results satisfactorily for the range of Ra, 104 ≤ Ra ≤ 5 × 105 and of θ, 0 ≤ θ ≤ 60 deg. The present experimental and numerical results are in good agreement with the results reported in the literature.
Nongray Gas Analyses for Reflecting Walls Utilizing a Flux Technique
Authors: Menart J. A. , Lee HaeOk Skarda
Published: May 2008

A flux formulation for a planar slab of molecular gas radiation bounded by diffuse reflecting walls is developed. While this formulation is limited to the planar geometry, it is useful for studying approximations necessary in modeling nongray radiative heat transfer. The governing equations are derived by considering the history of multiple reflections between the walls. Accurate solutions are obtained by explicitly accounting for a finite number of reflections and approximating the spectral effects of the remaining reflections. Four approximate methods are presented and compared using a single absorption band of H2O. All four methods reduce to an identical zeroth-order formulation, which accounts for all reflections approximately but does handle nonreflected radiation correctly. A single absorption band of CO2 is also considered using the best-behaved approximation for higher orders. A zeroth-order formulation is sufficient to predict the radiative transfer accurately for many cases considered. For highly reflecting walls, higher order solutions are necessary for better accuracy. Including all the important bands of H2O, the radiative source distributions are also obtained for two different temperature and concentration profiles.
A Study of In Situ Specific Absorption Coefficients of Soot Particles in Laminar Flat Flames
Authors: Sivathanu Y. R. , Gore J. P. , Janssen J. M. , Senser D. W.
Published: May 2008

Simultaneous measurements of absorption at 632 nm and emission at 800 nm and 900 nm along diametric paths are used to infer soot volume fractions based on both absorption and emission and temperatures based on emission in laminar, premixed flat flames. Agreement between soot volume fractions based on emission and absorption is used as an indicator of homogeneity of the optical path. Emission measurements at the gas band-free wavelengths of 2300 nm and 4000 nm for homogeneous paths are used to infer the specific absorption coefficients of soot particles. The results for methane, propane, and ethylene mixed with nitrogen and oxygen show insensitivity of the specific absorption coefficients to fuel type and weak dependence on temperature. An in situ check of optical homogeneity is crucial in obtaining reliable measurements of optical properties.
On the Gas Entrapment and Nucleation Site Density During Pool Boiling of Saturated Water
Authors: Wang C. H. , Dhir V. K.
Published: May 2008

A model to describe the effect of wettability on nucleation site density is presented. First, from Helmholtz free energy analysis, a criterion for the entrapment condition in a uniform temperature field is developed. Second, the stability condition of preexisting gas/vapor nuclei during the heating process and the minimum superheat required to initiate nucleation are determined. The prediction of the entrapment condition and the incipient temperature are consistent with the experimental observations made on surfaces having naturally existing cavities. Third, a naturally formed cavity on a heater surface is modeled as a spherical cavity. The cumulative active nucleation site density for a specified contact angle is expressed in terms of the cumulative density of cavities existing on the surface as Na = Pas · Nas where Nas is the heater surface cumulative cavity density with cavity mouth angles less than a specified value and Pas is a function of contact angle and cavity mouth angle. The model successfully predicts active site densities for different contact angles.
Evaporation of Multicomponent Drop Arrays
Authors: Annamalai K. , Ryan W. , Chandra S.
Published: May 2008

The conventional fuels that are used in the field of transportation are primarily composed of two or more components. Each component evaporates, mixes with hot oxidant gases, ignites, and burns. Since evaporation is the precursor of the sequence of events leading to combustion, the evaporation studies on the multi-component drops are essential for determining the governing parameters of spray evaporation. While single-component drop studies have been carried out extensively in the past, very limited literature exists on the multicomponent array evaporation. The present paper deals with the evaporation of multicomponent fuel droplets in an array using the recently developed point source method (PSM). First, the quasi-steady (QS) evaporation of an isolated, multicomponent droplet is briefly analyzed. The resultant governing equations, along with Raoult’s law and the Cox-Antoine relation, constitute the set of equations needed to arrive at the solutions for: (1) the droplet surface temperature, (2) the evaporation rate of each species, and (3) the vapor mass fraction of each species at the droplet surface. The PSM, which treats the droplet as a point mass source and heat sink, is then adopted to obtain an analytic expression for the evaporation rate of a multicomponent droplet in an array of liquid droplets. Defining the correction factor (η) as a ratio of the evaporation of a drop in an array to the evaporation rate of a similar isolated multi-component drop, an expression for the correction factor is obtained. The results of the point source method (PSM) are then compared with those obtained elsewhere for a three-drop array that uses the method of images (MOI). Excellent agreement is obtained. The treatment is then extended to a binary drop array to study the effect of interdrop spacing on vaporization. When the drops are close to each other, the evaporation rate of the droplet in the array containing the larger percentage of volatiles is higher than the rate under isolated conditions (η>1). The results qualitatively confirm the experimental data reported elsewhere. Parametric results were obtained for the effect of changing the composition on the correction factor and finally critical drop compositions in the binary array are given for which η>1. Even though the results for the average correction factor of the whole array of 2 to 9 drops obtained using PSM are almost the same as the results from MOI, the correction factor of the center drop under severe interaction may deviate from those results obtained with MOI.
Steam Injection Into a Slow Water Flow Through Porous Media
Authors: Chung M. , Catton I.
Published: May 2008

Steam injection into a slow water flow (Darcy flow) was studied analytically for steady-state conditions. The continuity, momentum, and energy equations for both water flow and steam flow were solved subject to interface conditions. The governing equations were transformed using stream function and velocity potential coordinates to simplify the calculation domain. The resulting energy equation was converted to a simultaneous ordinary differential equation by an integral method. The unknown steam-water interface shape and location were determined through an optimization process. Temperature distribution on the water side, local condensation rate along the interface, water and steam flow fields and pressure distribution were found numerically. Comparison with experimental measurements showed that the average steam zone size could be fairly well predicted only when dispersion effects were incorporated into a modified thermal conductivity model.
Experiments on Convective Instability of Large Prandtl Number Fluids in a Vertical Slot
Authors: Wakitani S.
Published: May 2008

The stability of thermal convection of large Prandtl number fluids in a vertical slot (aspect ratio = 10, 15, or 20) was studied experimentally. Secondary cells began to appear from the center of the slot and then prevailed. The critical Rayleigh number showed an increasing trend as the aspect ratio was increased, and became higher than that obtained from a linear stability analysis owing to a possible traveling wave mode of instability. As the Rayleigh number was increased, either the onset of the tertiary cells or the traveling waves occurred depending on the Prandtl number. In the transition process from laminar to turbulent flow, the secondary cells around the end regions split into smaller cells and an unsteady flow structure appeared with an original secondary cell in the center region and some smaller cells along the sidewalls.
Scattering Tomography and Its Application to Sooting Diffusion Flames
Authors: Mengu¨c¸ M. P. , Dutta P.
Published: May 2008

A new analytical tomographic reconstruction technique was developed for the determination of the extinction and scattering coefficient distributions in axisymmetric media. This method, called “scattering tomography,” was tested for several particle concentration profiles corresponding to those for diffusion flames. After that, a series of experiments were performed on sooting acetylene flames using an argon-ion laser nephelometer. The experimental results were reduced using both the transmission and scattering tomography techniques to obtain the extinction coefficient profiles. It was shown that in the center of the flame, the results from these two approaches were in good agreement. Scattering tomography can be used to determine both the absorption and scattering coefficient distributions in the medium. In addition to that, it is preferable over the transmission tomography if the medium is optically very thin and particles are predominantly scattering.
Investigation of Radiative Transfer in Nongray Gases Using a Narrow Band Model and Monte Carlo Simulation
Authors: Liu J. , Tiwari S. N.
Published: May 2008

The Monte Carlo method (MCM) is applied to analyze radiative heat transfer in nongray gases. The nongray model employed is based on the statistical narrow band model with an exponential-tailed inverse intensity distribution. The amount and transfer of the emitted radiative energy in a finite volume element within a medium are considered in an exact manner. The spectral correlation between transmittances of two different segments of the same path in a medium makes the statistical relationship different from the conventional relationship that only provides the noncorrelated results for nongray analysis. Two features of the MCM that are different from other nongray numerical methods are discussed. The simplicity of the MCM is demonstrated by considering the case of radiative transfer between two reflecting walls. The results for the radiative dissipation distributions and the net radiative wall heat fluxes are obtained for uniform, parabolic, and boundary layer type temperature profiles, as well as for a parabolic concentration profile. They are compared with available results of other methods. Good agreements are found for all the cases considered.
Cooling of a Heated Surface by Mist Flow
Authors: Lee S. L. , Yang Z. H. , Hsyua Y.
Published: May 2008

Cooling requirements in modern industrial applications, such as the removal of heat from electronic equipments, often demand the simultaneous attainment of a high heat flux and a low and relatively uniform and steady temperature of the heated surface to be cooled. The conventional single-phase convection cooling obviously cannot be expected to function adequately, since the heat flux there is directly proportional to the temperature difference between the heated surface and the surrounding medium. To maintain a high heat flux, the temperature of the heated surface usually must be kept at a high level. An attractive alternative is cooling by a spray, which takes advantage of the significant latent heat of evaporation of the liquid. However, in conventional industrial spray coolings, such as in the case of the cooling tower of a power plant, the temperature of the heated surface usually remains relatively high and is nonuniform and unsteady containing numerous flashy hot spots. In order to optimize the performance of the spray cooling of a heated surface by a mist flow, a clear understanding is required of (1) the dynamic interaction between the droplets and the carrier fluid and (2) the thermal reception of the droplets at the heated surface. It is the dynamic interaction between the phases that is causing the droplets to deposit onto the heated surface. The thermal reception at the heated wall develops mass and heat transfer leading to the mode of cooling of the heated surface. In the present study, an experimental investigation was made of the combination of the dynamic depositional behavior of droplets in a water droplet-air mist flow with the use of a specially designed particle-sizing two-dimensional laser-Doppler anemometer. Also, the heat transfer characteristics at the heated surface were investigated in relation to droplet deposition on the heated surface for wide ranges of droplet size, droplet concentration, mist flow velocity, and heat flux. It was discovered that over a certain suitable range of combination of these parameters, a superbly effective cooling scheme could be established by the evaporation on the outside surface of an ultrathin liquid film. Such a film was formed on the heated surface by the continuous deposition of fine droplets from the mist flow. Under these conditions, the heat flux is primarily related to the evaporation of the ultrathin liquid film on the heated surface and thus depends less on the temperature difference between the heated surf ace and the ambient mist flow. The heated surface is quenched to a low, relatively uniform and steady temperature at a very high level of heat flux. Heat transfer enhancement as high as seven times has been found so far. This effective heat transfer scheme is here termed mist cooling.
Effective Thermal Conductivity in Multidimensional Bodies
Authors: Lee Y.-M. , Haji-Sheikh A. , Fletcher L. S. , Peterson G. P.
Published: May 2008

The effective thermal conductivity in three-dimensional bodies is studied analytically. The three-dimensional model considers a spherical inclusion centrally located in a cubical body. Later, the spherical inclusion is replaced by an elliptical inclusion to study the biased effect or directionality of heat flux. Two different aspect ratios for the elliptical inclusion are considered. It is shown that the effective thermal conductivity is influenced by surface conductance in addition to geometric factors. Also, the effective thermal conductivity is measured for different samples. Spherical inclusions are placed inside cylindrical bodies for convenience of the experiments. The data show that cracks induced by applied pressure and thermal stress during the experiment reduce the thermal conductance. Using the measured effective thermal conductivity data, an analytical procedure is used to calculate the average values of the apparent contact conductance.
Experimental Study of the Convective Cooling of a Heated Continuously Moving Material
Authors: Kang B. H. , Yoo J. , Jaluria Y.
Published: May 2008

An experimental investigation of the heat transfer associated with a continuously moving material has been carried out. This thermal transport circumstance is encountered in many manufacturing processes, such as hot rolling, fiber drawing, plastic extrusion, crystal growing, and continuous casting. The transport associated with a heated plate or a cylindrical rod being cooled due to its own movement at uniform velocity in a stationary extensive fluid is considered. Very little experimental work has been done on this problem and this study focuses on the resulting thermal fields. Time-dependent temperature distributions in the solid, as well as in the flow, are measured for the material moving vertically downward in water and moving vertically upward or downward in air. The effects of thermal buoyancy, material speed, and properties of the material and the fluid on the thermal field are studied. The results indicate that the temperature profiles obtained are similar to those obtained in earlier numerical and analytical studies. At low material speeds, the upstream penetration of the conductive transport due to temperature variation in the material was seen to be substantial. This effect decreased with an increase in the material speed. The thermal boundary layer is found to be thicker in air than in water, as expected. The effect of thermal buoyancy on the temperature distributions in air was found to be very significant. High-thermal-conductivity materials, such as aluminum, are cooled down more rapidly than low-conductivity materials, such as teflon. The experimental results obtained lead to a better understanding of the underlying transport mechanisms and add to the data base needed for the design and optimization of the relevant systems.
Thermal Conductivity of Thin Metallic Films
Authors: Kumar Sunil , Vradis George C.
Published: May 2008

This study examines the effect of transverse thickness on the in-plane thermal conductivity of single crystal, defect-free, thin metallic films. The imposed temperature gradient is along the film and the transport of thermal energy is predominantly due to free electron motion. The small size necessitates an evaluation of the Boltzmann equation of electron transport along with appropriate electron scattering boundary conditions. Simple expressions for the reduction of conductivity due to increased dominance of boundary scattering are presented and the results are compared with other simplified approaches and experimental data from the literature. Grain boundary scattering is also considered via simple arguments.
Conjugated Periodic Turbulent Forced Convection in a Parallel Plate Channel
Authors: Guedes R. O. C. , Ozisik M. N. , Cotta R. M.
Published: May 2008

The transient conjugated turbulent heat transfer with axial conduction in the wall and convection boundary conditions is solved with the generalized integral transform technique for the flow of a Newtonian fluid in a parallel-plate duct subjected to periodically varying inlet temperature. A lumped model that neglects transverse temperature gradients in the solid, but takes into account the axial heat conduction along the wall, is adopted. Accurate numerical results are presented for the fluid bulk temperature, wall temperature, and wall heat flux. The effects of the conjugation parameter, fluid-to-solid heat capacitance ratio, and Biot number on the behavior of the periodic responses are investigated.
Convective Heat Transfer Due to Acoustic Streaming Across the Ends of a Kundt Tube
Authors: Gopinath A. , Mills A. F.
Published: May 2008

Convective heat transfer due to acoustic streaming across the ends of a tube supporting a standing sound wave is investigated. Analytical techniques are used along with the numerical solver PHOENICS for the solution of the complete elliptic form of the equations governing the steady transport due to the streaming motion. A parametric study of the effects of the controlling acoustic and geometric variables is conducted, and Nusselt number correlations are developed for air. The results obtained can be used in the thermal analysis of test cells for containerless processing of materials in space using acoustic levitation.
Visualization of Flow Phenomena Near Enhanced Surfaces
Authors: Ravigururajan T. S. , Bergles A. E.
Published: May 2008

Passive augmentation techniques such as surface disruptions are being increasingly used in heat exchangers. Although many working correlations have been suggested to predict their thermal-hydraulic characteristics, the physical phenomena governing the heat transfer enhancement have not been clearly understood. The paper describes a qualitative study on the flow phenomena near an enchanced surface. Water was used as the working fluid. Experiments were conducted for different coil wire diameters and for a Reynolds number of 150-2600. The results show the simultaneous existence of different flow patterns in enhanced flow. Also, the study confirmed that the developing length is very much smaller than that of a smooth tube, even for laminar flow.
Local Heat Transfer in Enclosed Co-rotating Disks With Axial Throughflow
Authors: Kim S. Y. , Han J. C. , Morrison G. L. , Elovic E.
Published: May 2008

Local heat transfer in enclosed co-rotating disks with axial through flow is investigated. The rotating cavity has two plane disks and a cylindrical rim (shroud). The ratio of the rim span to the disk outer radius is 0.4 and the ratio of the disk inner radius to outer radius is 0.25. The objectives of this study are to investigate the effects of axial coolant flow rate, rotation speed, and disk surface temperature on the local heat transfer coefficients inside the disk cavity. Both uniform disk surface heat flux and uniform disk surface temperatures are tested for axial flow Reynolds numbers between 2500 and 25,000 rotational Reynolds numbers between 0 and 5.11 × 105, and rotational Grashof numbers between 5 × 106 and 1.3 × 1010. The results show that the local heat transfer coefficients for the nonrotating cavity increase with increasing axial flow Reynolds number. In general, the local Nusselt numbers at large radii of the disks and rim increase with increasing rotational Reynolds number. However, the local Nusselt numbers at small radii of the disks initially decrease and then increase with increasing rotational Reynolds number. The uniform heat flux condition provides slightly higher heat transfer coefficients than those for the uniform wall temperature condition.
Natural Convection Heat Transfer From Long Horizontal Isothermal Cylinders
Authors: Clemes S. B. , Hollands K. G. T. , Brunger A. P.
Published: May 2008

A new set of measurements is reported on natural convection heat transfer in air from isothermal long horizontal cylinders of noncircular cross section at various orientations, covering the Rayleigh number (Ra) range from about 103 to about 109. The data are correlated reasonably well by a conduction layer model with a constant value (i.e., the same for all body shapes and orientations) of 5.42 for the Churchill-Usagi coefficient blending the laminar and turbulent asymptotes. The resulting correlation equation normally requires only the geometric specification of the body height and perimeter. This model is also tested against data in the literature on the subject problem, and found to be generally predictive, to within about ±10 percent. A new set of data covering the same Ra range is also reported for the circular cross-section case, i.e., the long horizontal isothermal circular cylinder. Comparison of this data with the several existing correlations for this well-known problems shows that the Kuehn and Goldstein equation predicts the data best, although the Raithby and Hollands equation also predicts the data very well, but only after a revision to the blending coefficient.
Analysis of Heat Transfer Regulation and Modification Employing Intermittently Emplaced Porous Cavities
Authors: Vafai K. , Huang P. C.
Published: May 2008

The present work forms a fundamental investigation on the effects of using intermittently porous cavities for regulating and modifying the flow and temperature fields and therefore changing the skin friction and heat transfer characteristics of an external surface. A general flow model that accounts for the effects of the impermeable boundary and inertial effects is used to describe the flow inside the porous region. Solutions of the problem have been carried out using a finite-difference method through the use of a stream function-vorticity transformation. Various interesting characteristics of the flow and temperature fields in the composite layer are analyzed and discussed in detail. The effects of various governing dimensionless parameters, such as the Darcy number, Reynolds number, Prandtl number, the inertia parameter as well as the effects of pertinent geometric parameters are thoroughly explored. Furthermore, the interactive effects of the embedded porous substrates on skin friction and heat transfer characteristics of an external surface are analyzed. The configuration analyzed in this work provides an innovative approach in altering the frictional and heat transfer characteristics of an external surface.
The Effect of Temperature Modulation on Natural Convection in a Horizontal Layer Heated From Below: High-Rayleigh-Number Experiments
Authors: Mantle J. , Kazmierczak M. , Hiawy B.
Published: May 2008

An experimental investigation was conducted to study the effects of wall temperature modulation in a horizontal fluid layer heated from below. A series of 45 transient experiments was performed in which the bottom wall temperature changed periodically with time in a “sawtoothlike” fashion. The amplitude of the bottom wall temperature oscillation varied from 3 to 70 percent of the enclosure’s mean temperature difference, and the period of the temperature swings ranged from 43 seconds to 93 minutes. With water as the fluid in the test cell, the flow was fully turbulent at all times. The Rayleigh number of the experiments (based on the enclosure’s height and on the mean temperature difference) was 0.4 × 108 < Ra < 1.2 × 109. It was found that for small changes in the bottom wall temperature, the cycle-averaged heat transfer through the layer was unchanged, independent of the period, and was equal in magnitude to the well-established steady-state value when the hot wall is evaluated at the mean temperature. However, this study shows that the cycle-averaged heat transfer increases notably, up to 12 percent as compared to the steady-state value, for the experiments with large temperature modulations. Futhermore, it was observed that the enchancement was a function of the amplitude and period of the oscillation.
Surface Radiation Effects on Flame Spread Over Thermally Thick Fuels in an Opposing Flow
Authors: West J. , Bhattacharjee S. , Altenkirch R. A.
Published: May 2008

A computational model of flame spread over a thermally thick solid fuel in an opposing-flow environment is presented. Unlike thermally thin fuels, for which the effect of fuel surface radiation is negligible for high levels of opposing flow, fuel surface radiation is important for thermally thick fuels for all flow levels. This result is shown to derive from the fact that the ratio of the rate of heat transfer by re-radiation from the surface to that by conduction from the gas to the solid is proportional to the length over which heat can be conducted forward of the flame to sustain spreading. For thin fuels, this length decreases with increasing flow velocity such that while radiation is important at low flow velocities it is not at the higher velocities. For thick fuels at low flow velocities, the conduction length is determined by gas-phase processes and decreases with increasing flow velocity. But at higher flow velocities, the conduction length is determined by solid-phase processes and is rather independent of the gas-phase flow. The result is that over a wide range of flow velocities, the conduction length of importance does not change substantially as it switches from one phase to another so that the ratio of radiation to conduction is of unit order throughout that wide range of flow.
Local Measurements in the Two-Phase Region of Turbulent Subcooled Boiling Flow
Authors: Roy R. P. , Velidandla V. , Kalra S. P. , Peturaud P.
Published: May 2008

Local measurements of vapor phase residence time fraction, vapor bubble size distribution, bubble axial velocity, and vapor and liquid temperatures were performed in turbulent boiling flow of Refrigerant-113. The dissolved air content of the experimental fluid was minimized. Data are reported for three wall heat fluxes, two fluid mass velocities, and three subcoolings at test section inlet. Local time-averaged interfacial area concentrations were estimated. The measuring devices, viz., dual-sensor fiber-optic probe for the vapor bubble measurements and phase-compensated chromel-alumel microthermocouple for the fluid temperature measurement, provided more complete and accurate data compared with our earlier work. The data should be helpful in the development and validation of multidimensional turbulent boiling flow models. Further work is needed, however, before the local interfacial area concentration can be determined with confidence.
Nucleate Pool Boiling of a TURBO-B Bundle in R-113
Authors: Memory S. B. , Chilman S. V. , Marto P. J.
Published: May 2008

Heat transfer measurements were made during nucleate boiling of R-113 from a bundle of 15 electrically heated, copper TURBO-B tubes arranged in an equilateral triangular pitch, designed to simulate a portion of a flooded evaporator. Five of the tubes that were oriented in a vertical array on the centerline of the bundle were each instrumented with six wall thermocouples. For increasing heat flux, the incipient boiling wall superheat of upper tubes decreased as lower tubes were activated. In the boiling region at low heat fluxes (≈ 1 kW/m2), the average bundle heat transfer coefficient was 4.6 times that obtained for a smooth tube bundle (under identical conditions) and 1.6 times greater than that obtained for a single TURBO-B tube; a similar bundle factor has been reported for a smooth tube bundle. At high heat fluxes (100 kW/m2), the average bundle heat transfer coefficient was 3.6 times that of a smooth tube bundle. Furthermore, there was still a significant bundle factor (1.22), contrary to a smooth tube bundle, where all effect of lower tubes was eliminated at high heat fluxes.
Transient Cooling of Hot Porous and Nonporous Ceramic Solids by Droplet Evaporation
Authors: Abu-Zaid M. , Atreya A.
Published: May 2008

This paper presents the results of an experimental investigation into transient cooling of low-thermal-conductivity porous and nonporous ceramic solids by individual water droplets. The initial surface temperature (Ts) of both solids ranged from 75 to 200°C. Both solids were instrumented with several surface and in-depth thermocouples and had the same thermal properties. This enabled investigation into the similarities and differences in the thermal behavior of porous and nonporous solids during droplet evaporation. The measured and theoretical contact temperatures, for both solids, were found to be in good agreement until they became equal to the boiling point of water (which occurs at an initial solid surface temperature of 164°C). Further increase in the initial solid surface temperature did not change the measured contact temperature. Instead, it became roughly constant at a value slightly greater than the boiling point of water. During the droplet evaporation process, surface and in-depth temperatures for the nonporous solid remain nearly constant, whereas for the porous solid there was a continuous decrease in these temperatures. A thermocouple in the porous matrix at the same location as that of the nonporous matrix cools faster under identical conditions, indicating an energy sink in the vicinity of the thermocouple. Also, evaporation time for the nonporous solid was found to be larger than that of the porous solid for the same droplet size and under the same conditions. These observations confirm that there is both in-depth and lateral penetration of water in the porous solid. The transient temperature measurements were used to determine the following quantities: (i) the recovery time (time required by the surface to recover to its initial temperature), and (ii) the size of surface and in-depth zones affected by the droplet. The instantaneous evaporation rate, and the instantaneous average evaporative heat flux for the nonporous solid, were also determined from video measurements of the droplet diameter on the solid surface and the transient temperature measurements. It was found that the average evaporative heat flux is higher for smaller droplets because of their smaller thickness on the hot surface.
Numerical Predictions of Local Entropy Generation in an Impinging Jet
Authors: Drost M. K. , White M. D.
Published: May 2008

Local entropy generation rates related to viscous dissipation and heat transfer across finite temperature differences can be calculated for isotropic and Newtonian fluids from the temperature and velocity fields in a thermal process. This study consisted of the development of a numerical procedure for the prediction of local entropy generation rates and the application of that procedure to convective heat transfer associated with a fluid jet impinging on a heated wall. The procedure involved expanding an existing computation fluid dynamics computer code to include the numerical calculation of local entropy generation. The modified code was bench-marked against analytical solutions and was then used to simulate a cold fluid jet impinging on a hot wall. The results show that the calculation of local entropy generation is feasible and can provide useful information.
Simultaneous Heat and Mass Transfer From a Two-Dimensional, Partially Liquid-Covered Surface
Authors: Tao Y.-X. , Kaviany M.
Published: May 2008

Simultaneous heat and mass transfer from partially liquid-covered surfaces is examined experimentally using a surface made of cylinders with the voids filled with liquid. The steady-state evaporation rate, surface temperature of the liquid and exposed solid, and location of meniscus are measured for various ambient air velocities and temperatures. Using these, we examine the effect of the extent to which the liquid covers the surface on the evaporation mass transfer rate resulting from the convective heat transfer from the ambient gas to this surface. The results show strong Bond and Reynolds number effects. For small Bond and Reynolds numbers, the presence of dry (exposed solid) surface does not influence the mass transfer rate. As the Bond or Reynolds number increases, a critical liquid coverage is found below which the mass transfer begins to decrease. Heat transfer from the exposed solid to the liquid is also examined using the measured surface temperature, a conduction model, and an estimate of the liquid and solid surface areas (using a static formation for the liquid meniscus). The results show that at the liquid surface an analogy between heat and mass transfer does not exist.
Three-Dimensional Laminar Natural Convection in an Inclined Air Slot With Hexagonal Honeycomb Core
Authors: Asako Y. , Nakamura H. , Chen Z. , Faghri M.
Published: May 2008

Numerical solutions are obtained for a three-dimensional natural convection heat transfer problem in an inclined air slot with a hexagonal honeycomb core. The air slot is assumed to be long and wide such that the velocity and temperature fields repeat themselves in successive enclosures. The numerical methodology is based on an algebraic coordinate transformation technique, which maps the complex cross section onto a rectangle, coupled with a calculation procedure for fully elliptic three-dimensional flows. The calculations are performed for Rayleigh numbers in the range of 103 to 105, inclination angles in the range of −90 to 80 deg, Prandtl number of 0.7, and for five values of the aspect ratio. Three types of thermal boundary condition for the honeycomb side walls are considered. The average Nusselt number results are compared with those for a rectangular two-dimensional enclosure.
Natural Convection in Vertically Vented Enclosures
Authors: Sefcik D. M. , Webb B. W. , Heaton H. S.
Published: May 2008

Steady, laminar natural convection in vertically vented two-dimensional enclosures has been investigated both experimentally and analytically. A vertically vented enclosure is one in which the buoyancy-driven flow and heat transfer are restricted by vents in the top and bottom bounding walls of the enclosure. The local heat transfer along the heated wall was determined using Mach-Zehnder interferometry, and the flow structure was determined using a smoke generation flow visualization technique. Analytically, the governing conservation equations were solved numerically using a control volume-based finite difference technique. The results reveal strongly nonuniform local heat transfer along the isothermal wall as a result of the blockage at the inlet. A local maximum and minimum occur in the lower half of the enclosure. The flow visualization and analytical predictions for the flow field reveal that these heat transfer extrema are attributed to separated flow effects near the inlet gap with the associated primary inlet flow impingement and bifurcation at the heated wall. The analysis predicts well the flow structure and local and average heat transfer data. The results show asymptotic behavior to the classical vertical parallel plate result in the limit as the vent gap approaches the enclosure width.