bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Analysis of the Unsteady Flow and Heat Transfer to a Cylinder in Crossflow
Authors: Paolino M. A. , Kinney R. B. , Cerutti E. A.
Published: October 2009

The unsteady, two-dimensional viscous flow of an incompressible, constant-property fluid flowing over a cylinder is numerically analyzed by integrating the vorticity transport equation and the energy equation. Departing from the usual stream function approach, the velocity distribution is obtained from the vorticity distribution by integrating the velocity induction law. Calculations start with the impulsive motion of the free stream and a step change in the surface temperature of the cylinder. The solution is advanced in time until steady-state conditions are achieved. Results are obtained for a Prandtl number of 0.7 and Reynolds numbers of 3000 and 70,800. Local and average Nusselt numbers and force coefficients are presented and compared to available experimental data.
An Experimental and Analytical Study of Close-Contact Melting
Authors: Moallemi M. K. , Webb B. W. , Viskanta R.
Published: October 2009

Close-contact melting was investigated by performing a series of experiments in which blocks of solid n-octadecane (with circular or rectangular cross section) were melted by a horizontal planar heat source at constant surface temperature. Close contact between the source and the solid prevailed throughout the experiments by permitting the uncontained solid to descend under its own weight while squeezing the melt out of the gap separating it from the source. The velocity of the solid was measured and is reported as a function of the instantaneous weight of the solid. Effects of the surface temperature of the source and radius of the solid on its temporal velocity are also reported. A closed-form approximate solution is developed for the motion of solid and predictions are compared with the experimental data. The results for the solid velocity are correlated in terms of the governing parameters of the problem as revealed by the approximate solution. Compared with natural convection-dominated melting from below (solid confined and contained in a rectangular cavity) close contact gives rise to approximately a sevenfold increase in the melting rate of the solid.
An Approximate Three-Dimensional Solution for Melting or Freezing Around a Buried Pipe Beneath a Free Surface
Authors: Zhang G.-P. , Weinbaum S. , Jiji L. M.
Published: October 2009

This paper presents a quasi-steady-state approximate solution for small Stefan number for the three-dimensional melting or freezing around a fluid-carrying pipe buried in a semi-infinite phase change medium (PCM). The two-dimensional quasi-steady approximate solution method, the virtual free surface technique [18], has been extended to three dimensions where axial thermal interaction between the moving fluid and the PCM is considered. Of particular interest in the motion of the phase change interface and the time variation of the axial temperature distribution in the fluid. Due to the singularities of the differential equations along the pipe surface, an axisymmetric analytic solution is provided for the region near the pipe wall. Solutions are presented for several representative dimensionless pipe burial depths and initial conditions. The computational time to predict the three-dimensional interface location up to 10 years is several minutes on an IBM 4341 computer.
Subcooled-Boiling and Convective Heat Transfer for Heptane Flowing Inside an Annulus and Past a Coiled Wire: Part II—Correlation of Data
Authors: Mu¨ller-Steinhagen H. , Epstein N. , Watkinson A. P.
Published: October 2009

In Part I of this paper, the authors reported an extensive series of heat transfer data for subcooled boiling of heptane in turbulent flow in an annulus, and in laminar flow past a coiled wire. These data plus some new measurements for laminar flow in the annulus were compared to the predictions of some 12 correlations from the literature. The applicability of these correlations to the present data is determined and a combination of correlations proposed to predict heat transfer with satisfactory accuracy.
Structure of Bubbly Round Condensing Jets
Authors: Sun T.-Y. , Parthasarathy R. N. , Faeth G. M.
Published: October 2009

A theoretical and experimental study of turbulent bubbly condensing jets is reported. Tests involved initially monodisperse carbon dioxide bubbles in water (∼ 1 mm diameter bubbles with initial gas volume fractions of 2.4 and 4.8 percent) injected vertically upward in still water. Measurements were made of mean and fluctuating phase velocities, mean bubble diameters, mean bubble number intensities, and mean concentrations of dissolved carbon dioxide. Three theoretical methods were used to interpret the measurements: (1) locally homogeneous flow analysis, assuming infinitely fast interphase transport rates; (2) deterministic separated flow analysis, where finite interphase transport rates are considered but bubble/turbulence interactions are ignored; and (3) stochastic separated-flow analysis where both finite interphase transport rates and bubble/turbulence interactions are considered using random-walk methods. Both finite interphase transport rates and the turbulent dispersion of bubbles were important for present test conditions; therefore, only the stochastic separated flow analysis provided reasonable agreement with measurements.
Buoyancy Induced Flow in a Vertical Internally Finned Circular Duct
Authors: Prakash C. , Liu Ye-Di
Published: October 2009

Laminar buoyancy induced flow in a vertical internally finned circular tube has been analyzed by numerically integrating the governing partial differential equations. The temperature of the duct is greater than that of the surrounding fluid; hence the density of the fluid inside the duct is less than that of the fluid on the outside, which creates a hydrostatic pressure head across the duct that induces the flow (chimney effect). Results show that despite reduction in the induced flow rate due to increased friction, a finned tube can yield significant heat transfer enhancement compared to a finless tube of the same height. Depending on the parameters, the degree of enhancement can be comparable to the factor by which the heat transfer area increases due to the presence of the fins.
Laser-Induced Thermophoresis and Particulate Deposition Efficiency
Authors: Morse T. F. , Wang C. Y. , Cipolla J. W.
Published: October 2009

The interaction of laser radiation and an absorbing aerosol in a tube flow has been considered. The aerosol is produced by external heating of reactants as in the MCVD (Modified Chemical Vapor Deposition) process to produce submicron size particles in the manufacture of optical fiber preforms. These particles are deposited by thermophoretic forces on the inner wall of the tube as they are convected by a Poiseuille velocity profile. Axial laser radiation in the tube interacts with the absorbing particles, and the laser heating of the gas induces additional thermophoretic forces that markedly increase the efficiency of particulate deposition. A particle concentration dependent absorption coefficient that appears in the energy equation couples the energy equation to the equation of particle conservation, so that a nonlinear set of coupled partial integro-differential equations must be solved. Numerical solutions for aerosol particle trajectories, and thus deposition efficiencies, have been obtained. It is shown that laser enhanced thermophoresis markedly improves the deposition efficiency.
Experimental Study of Evaporation in the Contact Line Region of a Thin Film of Hexane
Authors: Wayner P. C. , Tung C. Y. , Tirumala M. , Yang J. H.
Published: October 2009

The transport processes in the contact line region (junction of evaporating thin liquid film, vapor, and substrate) of stationary steady-state evaporating thin films of hexane with various bulk compositions were studied experimentally. The substrate temperature distribution and liquid film thickness profile were measured, analyzed, and compared with previous results on other systems. The results demonstrate that small changes in the bulk composition significantly alter the characteristics of the transport processes in the contact line region. The curvature gradient at the liquid-vapor interface is a strong function of evaporation rate and composition. Concentration and temperature gradients give interfacial shear stresses and flow patterns that enhance contact line stability.
Natural Convection/Radiation Heat Transfer From Highly Populated Pin Fin Arrays
Authors: Sparrow E. M. , Vemuri S. B.
Published: October 2009

Experiments were performed to determine the combined-mode natural convection/radiation heat transfer characteristics of highly populated arrays of rodlike cylindrical fins (i.e., pin fins). The fins were oriented with their axes horizontal and were attached to a vertical heated baseplate. The investigated parameters included the number of fins in the array, the fin length and diameter, the baseplate-to-ambient temperature difference, and the presence or absence of adjacent shrouding surfaces. Finning was found to be highly enhancing (up to a sixfold increase in heat transfer), and even the longest fins were highly efficient. When the number of fins was increased for fixed values of the other parameters, the heat transfer increased at first, attained a maximum, and then decreased. Arrays having different diameter fins yielded about the same performance when the surface area of the fin-baseplate assembly was held fixed. Shrouding surfaces positioned close to the array decreased the rate of heat transfer. Calculations showed that the contribution of radiation was substantial and was greatest for more populous arrays, for longer fins, and at small temperature differences.
Effects of Upstream Geometry on Natural Convection of a Darcian Fluid About a Semi-Infinite Inclined Heated Surface
Authors: Hsu C. T. , Cheng P.
Published: October 2009

The method of matched asymptotic expansions is applied to the problem of steady natural convection of a Darcian fluid about a semi-infinite inclined heated surface with a power law variation of wall temperature, i.e., Tˆwaxˆλ for xˆ≥0 where 0≤λ<1. The leading edge of the inclined surface intercepts at an angle, Λ0, with another impermeable unheated surface extending upstream. The effects of the inclination angle α0 (0 ≤ α0 < < π/2) of the heated surface as well as the upstream geometry at xˆ<0 (as specified by Λ0) on heat transfer and fluid flow characteristics near the heated surface are investigated. At a given inclination angle α0, it is found that heat transfer from an upward-facing heated inclined surface is larger than that of a downward-facing heated surface, and that decreasing the intercepting angle (Λ0) tends to lower the heat transfer rate. These effects become increasingly pronounced as the Rayleigh number is decreased.
Analysis of Buoyancy Effect on Fully Developed Laminar Heat Transfer in a Rotating Tube
Authors: Siegel R.
Published: October 2009

Laminar heat transfer is analyzed in a tube rotating about an axis perpendicular to the tube axis. The solution applies for flow that is either radially outward from the axis of rotation, or radially inward toward the axis of rotation. The conditions are fully developed, and there is uniform heat addition at the tube wall. The analysis is performed by expanding velocities and temperature in power series using the Taylor number as a perturbation parameter. Coriolis and buoyancy forces caused by tube rotation are included, and the solution is calculated through second-order terms. The secondary flow induced by the Coriolis terms always tends to increase the heat transfer coefficient; this effect can dominate for small wall heating. For radial inflow, buoyancy also tends to improve heat transfer. For radial outflow, however, buoyancy tends to reduce heat transfer; for large wall heating this effect can dominate, and there is a net reduction in heat transfer coefficient.
Computation of Laminar Heat Transfer in Rotating Rectangular Ducts
Authors: Neti S. , Warnock A. S. , Levy E. K. , Kannan K. S.
Published: October 2009

Cooling passages are often used in the rotor windings of large electrical generators, where the flow channel rotates about an axis parallel to but displaced from its axis of symmetry. In such a rotating passage, Coriolis accelerations and density gradients caused by the heated wall lead to the development of secondary flows which can have strong effects on heat transfer and pressure drop characteristics. Finite difference solutions were obtained for laminar flow of air in a rotating rectangular duct with aspect ratio 2/1, where the duct wall is subjected to a uniform heat flux. The solutions show the effect of rotation on the development of the flow patterns and velocity and temperature variations. Results for Nusselt number and friction factor are presented in both the inlet and fully developed regions over a range of Grashof and Reynolds numbers.
An Experimental Investigation of Heat Transfer in Variable Porosity Media
Authors: Vafai K. , Alkire R. L. , Tien C. L.
Published: October 2009

This paper presents an experimental investigation on the effects of a solid impermeable boundary and variable porosity on forced convection in porous media. Emphasis is placed on the channeling effects on heat transfer in packed beds. The local volume-averaging technique is used to establish the governing equations and a numerical scheme is developed which incorporates the boundary and variable porosity effects on heat transfer. The experimental results for the heat flux at the boundary are presented as a function of the pertinent variables in a packed bed. The Nusselt number is found to increase almost linearly with an increase in the Reynolds number based on the pore diameter. The experimental results are found to be in good agreement with the theoretical results which account for the variable porosity effects. A comparison between the numerical and the experimental results demonstrates the importance of boundary and variable porosity effects on heat transfer in variable porosity media.
Reflooding With Steady and Oscillatory Injection: Part I—Flow Regimes, Void Fraction, and Heat Transfer
Authors: Kawaji M. , Ng Y. S. , Banerjee S. , Yadigaroglu G.
Published: October 2009

Simultaneous void fraction and wall temperature measurements were made during bottom-reflooding of a vertical Inconel tube under both constant and oscillatory injection rates. To support interpretation of these data, flow regime visualization experiments were also conducted by reflooding a heated quartz tube. With constant, high reflooding rates, inverted annular, transition, and dispersed flow regimes exist above the quench front, with typical chordal-average void fractions in the ranges of 10–30 percent, 30–70 percent, and 70–90 percent, respectively. Each regime exhibits different heat transfer rates. With lower injection rates or higher heating rates, annular droplet and dispersed flow regimes appear with void fractions above 80 percent. For reflooding with oscillatory inlet flow and high injection rates, large oscillations are seen in void fraction and wall temperature, indicating periodic changes in the flow regime near the quench front: The regime alternated between inverted annular (during an upstroke) and annular droplet flow (during a downstroke). These flow regimes were observed in the flow visualization experiments to be qualitatively similar to those for the constant injection cases. Heat transfer rates are substantially affected by the flow regime and increase (or decrease) as the void fraction falls (or rises). Compared to the constant-injection tests, increased rates of entrainment were observed during the forced-oscillation tests.
Photographic Observations of Bubble Formation in Flashing Nozzle Flow
Authors: Miller R. S.
Published: October 2009

Visual observations have been made of bubble growth in the nucleation region of flashing flow of initially subcooled water in a converging-diverging nozzle. Experiments performed under various flow rates, saturation temperatures, turbulence levels, noncondensable gas content, and artificial nucleation sites failed to produce isolated spherical bubbles of the size or density predicted by common bubble nucleation and growth models. Heterogeneous nucleation in the bulk flow was never observed and it is concluded from bubble growth rates that the role of convection in the heat and mass transfer environment of the bubbles is an important consideration in the physics of flashing flows near the nucleation region.
Melting in Rectangular Enclosures: Experiments and Numerical Simulations
Authors: Be´nard C. , Gobin D. , Martinez F.
Published: October 2009

This paper presents a numerical and experimental analysis of the heat transfer process that takes place while melting a solid material, in a rectangular enclosure. Natural convection is present in the melt layer, and the solid phase is assumed to be isothermal. Very detailed and precise experimental results are given that are used to validate a particularly rapid numerical code. Some insights into the kinetics of the melting process lead to a deeper understanding of the coupling between convection and phase change and allow us to propose a simple algebraic correlation that predicts the time evolution of the melting front to within 5 percent.
Observations of Complex Oscillations in a Closed Thermosyphon
Authors: Hart J. E.
Published: October 2009

Observations have been made of thermal oscillations in a slightly inclined closed thermosyphon. The thermosyphon is made up of two isothermal tubes, capped at the outer ends, and joined along their axes by an insulating section. The tube assembly is filled with liquid and inclined slightly with respect to the vertical. The lower isothermal section is hotter than the upper one and convection is driven across the insulating region. Between the applied temperature difference at which simple steady convection occurs, and that required for persistent turbulent motions, there is a wide range over which thermal oscillations are observed. These oscillations reflect quasi-periodic flow as well as a type of periodic chaos.
Entrainment Effects on Impingement Heat Transfer: Part II—Local Heat Transfer Measurements
Authors: Hollworth B. R. , Gero L. R.
Published: October 2009

Convective heat transfer was measured for a heated axisymmetric air jet impinging on a flat surface. It was found that the local heat transfer coefficient does not depend explicitly upon the temperature mismatch between the jet fluid and the ambient fluid if the convection coefficient is defined in terms of the difference between the local recovery temperature and target surface temperature. In fact, profiles of local heat transfer coefficients defined in this manner were found to be identical to those measured for isothermal impinging jets.
Convective Heat Transfer in the Impingement Region of a Buoyant Plume
Authors: Alpert R. L.
Published: October 2009

Fires of hazardous scale generate turbulent plumes within which convective heat transfer to surfaces can be important. Relatively little work has been done on developing reliable convective heat transfer correlations applicable to such large-scale flows. The present study, confined to heat transfer rates within the plume impingement region on a ceiling, achieves plume Reynolds numbers an order of magnitude beyond those of previous work by performing laboratory-scale experiments at elevated ambient pressures. Flow disturbances which normally cause scatter in plume heat transfer data are reduced as a consequence of this technique. It is shown that impingement zone Nusselt number depends on the 0.61 power of plume Reynolds numbers in the range of 104 to 105. This result is between the 1/2 power dependence expected for strain rate control (forced jet impingement) and the 2/3 power expected for buoyancy control of turbulent heat transfer rates.
Criteria for the Onset of Flow Recirculation and Onset of Mixed Convection in Vertical Rod Bundles
Authors: Symolon P. D. , Todreas N. E. , Rohsenow W. M.
Published: October 2009

In a vertical rod bundle with forced upflow and a nonuniform radial power profile, the potential for flow reversal into the low power side of the bundle exists due to buoyancy forces. In this paper a criterion is developed to predict the onset of flow recirculation in rod bundles. The criterion is validated with experimental data for flow recirculation in water-cooled bare rod bundles and with the results of a standard multichannel rod bundle code. A criterion for the onset of mixed convection is also given. Thus, the boundaries between the forced convection, mixed convection, and recirculation regimes are identified. Results are expressed on a flow regime map in terms of the dominant dimensionless parameters (the Reynolds number and a buoyancy parameter B*) and as a closed-form equation.
Thermal Response of Unconfined Ceilings Above Growing Fires and the Importance of Convective Heat Transfer
Authors: Cooper L. Y. , Stroup D. W.
Published: October 2009

A procedure is developed to calculate the thermal response of unconfined nonburning ceilings above growing fires. The procedure uses an algorithm for conduction into the ceiling material. It takes account of heat transfer due to radiation from the combustion zone to the ceiling surface, and due to reradiation from the ceiling to the floor and furnishings. Finally, the procedure uses a previously developed algorithm for convective heat transfer to the ceiling from the fire-plume-driven ceiling jet. The procedure is used to predict radial-dependent surface temperature histories of typical ceiling materials under a variety of different realistic levels of hazardous fire energy generation rates and combustion zone-ceiling separation distances. The results give an indication of the influence of convective heat transfer on peak ceiling thermal response, losses from fire plume gases, and radial variations and peak values of ceiling-to-floor irradiation during enclosure fires.
Thermal Performance of Spray Cooling Ponds at Zero Wind Velocity
Authors: Moussiopoulos N. , Ernst G.
Published: October 2009

The present model allows predictions of the thermal performance of spray cooling ponds in the case of zero wind velocity. Droplet cooling is described by modeling of the heat and water vapor transfer to the air. Temperature and humidity of air are obtained by the numerical solution of transport differential equations. Detailed results are presented and discussed. Predictions are in good agreement with the observed thermal performance of spray cooling ponds. Results are utilized for a check of assumptions made by NTU models.
A Study on the Thermophysical Properties of a Soil
Authors: Sasaki A. , Aiba S. , Fukuda H.
Published: October 2009

The effect of water content on the apparent thermal diffusivity of a soil has been investigated in relation to time and position in the direction of heat flux. To evaluate the apparent thermal diffusivity the method of minimizing the sum of squares based on a forward-difference equation was used. The sample used for experiments was a sand whose mean grain size was 208 μm and mean dry bulk density about 1360 kg/m3. The measurements were conducted over a range of water content ratios φ (percent pore volume filled with water) from 0 to 93.9 percent and temperatures from 20 to 40°C. The result shows that the overall apparent thermal diffusivity in the case of φ = 9.6 percent and the local apparent thermal diffusivity change with time.
Effect of Yaw on Heat Transfer in a Cuplike Cavity Facing a Freestream Flow
Authors: Sparrow E. M. , Zhang X.
Published: October 2009

Wind tunnel experiments were performed to determine the response of the heat transfer coefficient at the base of an open-ended cylindrical cavity to yawed (i.e., nonperpendicular) impingement of the freestream flow on the cavity opening. The experiments encompassed yaw angles from 0 (perpendicular impingement) to 45 deg, cavity depth–diameter ratios from 0 to 0.65, and Reynolds numbers from 4500 to 45,000. In the absence of yaw, very large reductions of the base surface transfer coefficient were brought about by increases of cavity depth. When the freestream flow is yawed relative to the cavity opening the coefficient rebounds from its low no-yaw values, with the rebound being accentuated for large yaw, deep cavities, and high Reynolds numbers. Notwithstanding the rebound, the resulting base surface heat transfer coefficients are lower than those for the no-yaw, zero-depth cavity. Operating conditions are identified for which reductions of the transfer coefficient of 50 percent or more are encountered. On this basis, cavities and recesses appear to be an effective means for reducing heat loss.
Effects of Several Roughness Elements on an Insulated Wall for Heat Transfer From the Opposite Smooth Heated Surface in a Parallel Plate Duct
Authors: Ichimiya K.
Published: October 2009

Experiments were carried out to examine the effects of several roughness elements on the insulated wall opposite the smooth heated plate for the heat transfer with air flowing through a parallel plate duct. The local heat transfer coefficient, the velocity distribution, the turbulence intensity, and the pressure drop were measured. An optimum pitch of roughness elements for the augmentation of heat transfer exists for each space between two parallel plates. Additionally, the correspondence between the heat transfer and the flow situation is also examined. Acceleration and turbulence produced by roughness elements contribute to the increase of the local heat transfer coefficients on the smooth heated plate. Thermal performance is evaluated at constant pumping power.
Effects of Crossflow Temperature on Heat Transfer Within an Array of Impinging Jets
Authors: Florschuetz L. W. , Su C. C.
Published: October 2009

Two-dimensional arrays of circular air jets impinging on a heat transfer surface parallel to the jet orifice plate are considered. In the experimental model the jet flow, after impingement, was constrained to exit in a single direction along the channel formed by the jet orifice plate and heat transfer surface. In addition to the crossflow that originated from the jets following impingement, an initial crossflow was present that approached the array through an upstream extension of the channel. By varying the initial crossflow temperature, the mixed-mean crossflow temperature approching each spanwise jet row n (Tm,n) was varied independently of the jet temperature (Tj). For each row, the effect of Tm,n relative to Tj on the heat flux opposite the row was determined. Results are formulated in terms of parameters defined for each individual spanwise row domain: a crossflow-to-jet temperature difference influence factor (ηr) and a Nusselt number (Nur) as functions of jet Reynolds number, crossflow-to-jet mass flux ratio, and geometric parameters. Effects of row position within the array are also considered. It was found that ηr and Nur are nominally independent of row position after the first two rows.
Natural Convection Heat Transfer Characteristics of Simulated Microelectronic Chips
Authors: Park K.-A. , Bergles A. E.
Published: October 2009

Microelectronic circuits were simulated with thin foil heaters supplied with d-c power. The heaters were arranged in two configurations: flush mounted on a circuit board substrate or protruding from the substrate about 1 mm. Heat transfer coefficients (midpoint) were obtained with two heater heights (5 mm, 10 mm) and varying width (2 mm ∼ 70 mm), in water and R-113. The height effect for single flush heaters agrees qualitatively with conventional theory; however, even the widest heaters have coefficients higher than predicted due to leading edge effects. The heat transfer coefficient increases with decreasing width, with the coefficient for 2 mm being about 150 percent above that for 20 mm ∼ 70 mm. This is attributed to three-dimensional boundary layer effects. The protruding heaters have a coefficient about 15 percent higher. Data were obtained for in-line and staggered arrays of flush heaters with varying distance between heaters. Coefficients for the upper heaters are below those for lower heaters, with the differences diminishing as the vertical or horizontal spacing increases. For the protruding heaters, the upper heaters have higher coefficients than the lower heaters.
A Model for Fluid Flow During Saturated Boiling on a Horizontal Cylinder
Authors: Kheyrandish K. , Dalton C. , Lienhard J. H.
Published: October 2009

A model has been developed to represent the vapor removal pattern in the vicinity of a cylinder during nucleate flow boiling across a horizontal cylinder. The model is based on a potential flow representation of the liquid and vapor regions and an estimate of the losses that should occur in the flow. Correlation of the losses shows a weak dependence on the Weber number and a slightly stronger dependence on the saturated liquid-to-vapor density ratio. The vapor jet thickness, which is crucial to the prediction of the burnout heat flux, and the shape of the vapor film are predicted. Both are verified by qualitative experimental observations.
Heat Transfer to Turbulent Swirling Flow Through a Sudden Axisymmetric Expansion
Authors: Dellenback P. A. , Metzger D. E. , Neitzel G. P.
Published: October 2009

Experimental data are presented for local heat transfer rates in the tube downstream of an abrupt 2:1 expansion. Water, with a nominal inlet Prandtl number of 6, was used as the working fluid. In the upstream tube, the Reynolds number was varied from 30,000 to 100,000 and the swirl number was varied from zero to 1.2. A uniform wall heat flux boundary condition was employed, which resulted in wall-to-bulk fluid temperatures ranging from 14° C to 50°C. Plots of local Nusselt numbers show a sharply peaked behavior at the point of maximum heat transfer, with increasing swirl greatly exaggerating the peaking. As swirl incressed from zero to its maximum value, the location of peak Nusselt numbers was observed to shift from 8.0 to 1.5 step heights downstream of the expansion. This upstream movement of the maximum Nusselt number was accompanied by an increase in its magnitude from 3 to 9.5 times larger than fully developed tube flow values. For all cases, the location of maximum heat transfer occurred upstream of the flow reattachment point.
The Heat Transfer Problem for the Modified Chemical Vapor Deposition Process
Authors: Choi M. , Baum H. R. , Greif R.
Published: October 2009

The heat transfer problem related to the modified chemical vapor deposition process has been analyzed in the high Peclet number limit. Variations in the axial, angular, and radial directions are presented. Of particular interest are the effects of tube rotation and variable properties on the temperature profiles.
Forced Convection in a Duct Partially Filled With a Porous Material
Authors: Poulikakos D. , Kazmierczak M.
Published: October 2009

This paper presents a theoretical study of fully developed forced convection in a channel partially filled with a porous matrix. The matrix is attached at the channel wall and extends inward, toward the centerline. Two channel configurations are investigated, namely, parallel plates and circular pipe. For each channel configuration, both the case of constant wall heat flux and constant wall temperature were studied. The main novel feature of this study is that it takes into account the flow inside the porous region and determines the effect of this flow on the heat exchange between the wall and the fluid in the channel. The Brinkman flow model which has been proven appropriate for flows in sparsely packed porous media and for flows near solid boundaries was used to model the flow inside the porous region. Important results of engineering interest were obtained and are reported in this paper. These results thoroughly document the dependence of the Nusselt number on several parameters of the problem. Of particular importance is the finding that the dependence of Nu on the thickness of the porous layer is not monotonic. A critical thickness exists at which the value of Nu reaches a minimum.
Cooling of a Vertical Shrouded Fin Array by Natural Convection: A Numerical Study
Authors: Karki K. C. , Patankar S. V.
Published: October 2009

An analysis is made of the laminar natural convection in a vertical shrouded fin array, in which the flow is induced from the surroundings by the chimney effect. This involves the calculation of developing flow in a duct. The results are obtained numerically for representative values of the parameters describing the system geometry and ducts of different lengths. The axial development of various flow quantities, such as the Nusselt number and the bulk temperature, has been presented. In addition, the overall flow and heat transfer characteristics have been discussed.
Thermal Instability of a Fluid-Saturated Porous Medium Bounded by Thin Fluid Layers
Authors: Pillatsis G. , Taslim M. E. , Narusawa U.
Published: October 2009

A linear stability analysis is performed for a horizontal Darcy porous layer of depth 2dm sandwiched between two fluid layers of depth d (each) with the top and bottom boundaries being dynamically free and kept at fixed temperatures. The Beavers–Joseph condition is employed as one of the interfacial boundary conditions between the fluid and the porous layer. The critical Rayleigh number and the horizontal wave number for the onset of convective motion depend on the following four nondimensional parameters: dˆ ( = dm/d, the depth ratio), δ ( = K/dm with K being the permeability of the porous medium), α (the proportionality constant in the Beavers–Joseph condition), and k/km (the thermal conductivity ratio). In order to analyze the effect of these parameters on the stability condition, a set of numerical solutions is obtained in terms of a convergent series for the respective layers, for the case in which the thickness of the porous layer is much greater than that of the fluid layer. A comparison of this study with the previously obtained exact solution for the case of constant heat flux boundaries is made to illustrate quantitative effects of the interfacial and the top/bottom boundaries on the thermal instability of a combined system of porous and fluid layers.
Natural Convection in a Vertical Porous Cavity: A Numerical Study for Brinkman-Extended Darcy Formulation
Authors: Lauriat G. , Prasad V.
Published: October 2009

A dimensional analysis of the Brinkman-extended Darcy formulation, which includes the transport and viscous terms, leads to four governing parameters for steady-state natural convection in a vertical porous cavity. They are: Rayleigh number, Darcy number, diffusion parameter Ω, and aspect ratio. Numerical results for 0≤Da≤10−1, 10≤Ra*≤5 ×103, and A=1 and 5, indicate that the temperature and velocity fields are significantly modified, the flow regimes are delayed, and the heat transfer rate is decreased when the Darcy number is increased beyond 10−5 for fixed Ra* and A. The slope of the ln (Nu) versus ln (Ra*) curve in the boundary layer regime decreases from 0.53 at Da=0 to 0.264 at Da=10−1 when A=5. The contribution of the transport term increases with Ω, Da, and Ra*, but the effect on the overall heat transfer is insignificant. However, the problem becomes ill formulated at high values of these parameters and may require the consideration of Forchheimer modifications. A scale analysis is also presented to show that the inertia term is of a low order of magnitude in comparison with the viscous term at high Prandtl numbers.
Thermal Convection in a Rectangular Cavity Filled With a Heat-Generating, Darcy Porous Medium
Authors: Prasad V.
Published: October 2009

Two-dimensional, steady natural convection in a rectangular cavity filled with a heat-generating, saturated porous medium has been studied numerically for the case when the vertical walls of the cavity are isothermal and the horizontal walls are either adiabatic or cold. Results are presented in terms of the streamlines and isotherms, the maximum temperature in the cavity, and the local and overall Nusselt numbers. The buoyant flow together with the uniform heat generation produces a highly stratified medium at high Rayleigh numbers. Although the maximum temperature in the cavity θmax invariably increases with the Rayleigh number Ra and aspect ratio A, the rate of increase diminishes with this enhancement in Ra and A. However, the change in the horizontal wall boundary condition from adiabatic to cold reduces θmax. The local heat flux on the bounding walls is a strong function of the Rayleigh number, the aspect ratio, and the wall boundary conditions. The variation in overall Nusselt number is qualitatively similar to that observed in the case of a differentially heated cavity, and the present heat transfer rates are close to that for the cavity heated by applying a uniform heat flux. Several correlations are presented for maximum temperature and overall Nusselt number.
Nonequilibrium Modeling of Two-Phase Critical Flows in Tubes
Authors: Dobran F.
Published: October 2009

A nonequilibrium two-phase flow model is described for the analysis of critical flows in variable diameter tubes. Modeling of the two-phase flow mixture in the tube is accomplished by utilizing a one-dimensional form of conservation and balance equations of two-phase flow which account for the relative velocity and temperature differences between the phases. Closure of the governing equations was performed with the constitutive equations which account for different flow regimes, and the solution of the nonlinear set of six differential equations was accomplished by a variable step numerical procedure. Computations were carried out for a steam-water mixture with varying degrees of liquid subcooling and stagnation pressures in the vessel upstream of the tube and for different tube lengths. The numerical results are compared with the experimental data involving critical flows with variable liquid subcoolings, stagnation pressures, and tube lengths, and it is shown that the nonequilibrium model predicts well the critical flow rate, pressure distribution along the tube, and the tube exit pressure.
Thermal and Hydrodynamic Phenomena in a Horizontal, Uniformly Heated Steam-Generating Pipe
Authors: Bar-Cohen A. , Ruder Z. , Griffith P.
Published: October 2009

This study is aimed at elucidating the relationship between the hydrodynamic and thermal phenomena in horizontal boiler tubes. It focuses on the empirical evaluation of the wall temperature distribution in a uniformly heated, 6-m-long, water-carrying pipe, operated at near-atmospheric pressure, over a range of imposed heat fluxes and water flow rates. The results reveal the existence of a complex temperature field on the pipe wall, incorporating circumferentially isothermal and anisothermal regions and a significant axial variation along the top of the pipe. For moderate flow rates these variations were found to give rise to two distinct anisothermal zones: a low-quality zone near the inlet and a moderate-quality zone approximately midway along the pipe. Physical models and semi-analytic relations for the liquid and vapor velocities, constituting the boundaries between isothermal and anisothermal operation, are presented.
Thermal Conductivity of Metal Cloth Heat Pipe Wicks
Authors: Phillips J. R. , Chow L. C. , Grosshandler W. L.
Published: October 2009

Heat conduction through a metal cloth wick saturated with a fluid has been investigated. An apparatus used to measure thermal conductivity, in which the condition of wick packing geometry is carefully controlled, and the basic experimental procedure are described. Experimental results are presented and compared to a new mean-gap-conductance model based upon the wick geometry, and to the simple series model. The mean-gap-conductance model evaluates the effects of the mesh geometry, and with the addition of a correction term to account for three-dimensional effects and layer-to-layer contact, the effective conductivity can be accurately predicted. In addition, a correlation of the mean gap which directly includes three-dimensional and contact conductance effects is presented. The correlation predicts the data within 10 percent whereas the series model may be more than 40 percent in error. From a parametric study using the new model, theoretical limits on the maximum and minimum conductivity enhancement have been determined as a function of geometric parameters. The implications of the research on heat pipe wick design are discussed.
Artificial Transformation of the Direct-Contact Condensation Pattern of Steam Bubbles in a Hydrophobic Liquid Medium
Authors: Mori Y. H.
Published: October 2009

When steam bubbles are released into a medium of a hydrophobic coolant, the condensate is scattered through the coolant in the form of tiny droplets. This condensation pattern is unfavorable for direct-contact condensers because it is difficult to separate the condensate and the coolant. This paper proposes a novel bubble-release device by which the condensation pattern is so transformed that the condensate remains within the confines of the bubble wall. An experimental examination with single steam bubbles released into a medium of liquid paraffin is demonstrated.
Optimal Positioning of Tubes in Arbitrary Two-Dimensional Regions Using a Special Boundary Integral Method
Authors: Parang M. , Arimilli R. V. , Ketkar S. P.
Published: October 2009

Two-dimensional steady conduction heat transfer from a set of parallel tubes located in a finite two-dimensional region enclosed by an arbitrarily shaped boundary is considered. A special boundary integral method is used in an optimization scheme where the tube sizes, positions, and surface temperatures can be determined in an iterative procedure with the objective of minimizing the variation of temperature over a specified segment of the boundary. Previous studies of this type were limited not only to rectangular regions but also to uniform heat flux results on the surface of each tube. However, the optimization scheme developed in this study is applicable to any arbitrarily shaped two-dimensional region and considers angular variation of heat flux on the surface of each tube. Results for three sample geometries are presented and discussed.
Inversion Method for Determining Effective Thermal Conductivities of Porous Materials
Authors: Kamiuto K. , Iwamoto M.
Published: October 2009

An inversion method for determining the effective thermal conductivies of porous materials from observed mean effective thermal conductivities is presented. Its validity is confirmed by numerical simulations. The effective thermal conductivities of glass beads are determined by the proposed method successfully used to predict the temperature profiles within the glass beads.
Non-Darcy Natural Convection in a Saturated Horizontal Porous Annulus
Authors: Muralidhar K. , Kulacki F. A.
Published: October 2009

A computational study of free convective flow and heat transfer in a saturated porous horizontal annulus is reported. Both isothermal and constant heat flux boundary conditions have been considered on the inner walls while the outer wall is held at a constant temperature. The calculation of the flow field involves consideration of non-Darcy effects, such as inertial and viscous forces, and also the variation of porosity near the walls. While the literature shows that Darcy flow model is inadequate in predicting average Nusselt numbers, the present study examines whether non-Darcy effects, and in particular the presence of the boundary, could play a significant role in explaining this discrepancy. Average Nusselt numbers have been obtained for Rayleigh–Darcy numbers from 20 to 4000 for the case of isothermal boundaries, and 20 to 20,000 for the case of constant heat flux on the inner wall. Radius ratio has been varied from 1.1 to 3. Over this range of parameters, inertia and viscous friction in the fluid phase have been found to produce a small effect on the Darcy flow. The effect of including variable porosity near a boundary is seen to produce channeling near the wall which in turn substantially increases the heat transfer coefficient.
Effect of Maximum Density of Water on Freezing of a Water-Saturated Horizontal Porous Layer
Authors: Sugawara M. , Inaba H. , Seki N.
Published: October 2009

This experiment simulates the freezing of water in a layer of earth by the use of a porous bead layer. The layer is cooled from above. The beads are 1, 5, and 11-mm-dia glass and 11-mm-dia steel. When the predominant heat transfer mode in an unfrozen layer is conduction, the freezing rate is not affected by the bead diameter. However, in natural convection the freezing rate is greatly affected by the maximum density of water at 4°C. An approximate numerical analysis which does not treat the maximum density effects predicts well the freezing rate in the range of 0°C < T2 < 4°C and 26°C < T2 (T2 = the lower plate temperature).
Theoretical Analysis of Weld Pool Behavior in the Pulsed Current GTAW Process
Authors: Tsai C. L. , Hou C. A.
Published: October 2009

A general, three-dimensional, closed-form welding heat-flow solution, which is capable of analyzing thermal behavior of the weldment in its transient state and/or under time-dependent power change during welding, is presented. The analytical model utilizes the finite heat source theory with a Gaussian distribution and also considers the effects of finite plate thickness. The numerical values of the solution are calculated using the computational schemes on a minicomputer. In this paper the welding parameters of the pulsed current GTAW were studied using the solution. Two sets of pulsation parameters were analyzed and their sensitivity to the heat input control were evaluated.
Structure and Radiation Properties of Luminous Turbulent Acetylene/Air Diffusion Flames
Authors: Gore J. P. , Faeth G. M.
Published: October 2009

An experimental and theoretical study of the structure and radiation properties of luminous, round, turbulent acetylene/air diffusion flames is described. Measurements were made of mean and fluctuating velocities, mean concentrations, laser extinction (514 and 632.8 nm), spectral radiation intensities (1200–5500 nm), and radiative heat fluxes. The measurements were used to evaluate structure predictions based on the laminar flamelet concept, and radiation predictions based on a narrow-band model both ignoring and considering turbulence/radiation interactions. State relationships needed for the laminar flamelet concept were found from auxiliary measurements in laminar flames. Predictions were encouraging; however, quantitative accuracy was inferior to earlier findings for luminous flames. This is attributed to the large radiative heat loss fractions of acetylene/air flames (approaching 60 percent of the heat release rate); coupled structure and radiation analysis should be considered for improved results. The findings suggest significant turbulence/radiation interactions (increasing spectral intensities 40–100 percent from estimates based on mean properties); and that soot volume fractions may approximate universal fractions of mixture fraction in turbulent acetylene/air diffusion flames.
Holographic Interferometry Investigation of Enhanced Tube Meniscus Behavior
Authors: Mirzamoghadam A. V. , Catton I.
Published: October 2009

Transport phenomena associated with the heating of stationary saturated fluid approximately one degree above saturation by an inclined partially submerged copper plate were studied using laser holographic interferometry. Interpretation of the hologram yielded the general characteristics of the meniscus region where the combined influence of natural convection, conduction, and surface tension gradients are important. Water and tri-chloro-fluoro-methane (Freon-11) were used as the working fluids.