bluuebunny/crossref_metadata_2025 huggingface dataset:
Natural Convection in an Inclined Rectangular Cavity With Different Thermal Boundary Conditions at the Top Plate
Authors: Karayiannis T. G. , Tarasuk J. D.
Published: October 2009

Natural convection inside a rectangular cavity with different temperature boundary conditions on the cold top plate was studied using a Mach-Zehnder interferometer for θ = 15, 45, and 60 deg to the horizontal. At θ = 60 deg coupling with external forced convection and non-coupled heat transfer from a cavity with an isothermal top plate was studied. In all experiments the bottom hot plate was isothermal. The Rayleigh number Ra was varied from subcritical to 6×105 and the cavity aspect ratio ARx, from 6.68 to 33.4. The Reynolds number of the external forced flow Redh was constant and approximately equal to 5.8×104. It was found that for Ra ≲ 3×104 the differing thermal boundary conditions at the top plate did not affect the local or average heat transfer rates from the cavity. For Ra ≳ 3×104 coupling at the top plate compared to the non-coupled case resulted not only in a reduction in the variation of the local heat transfer rates at the cold plate, but also in a significant reduction in the variation of the average transfer rates from hot and cold plates of the cavity. Forced convection at the top plate as compared to natural convection resulted only in a small reduction in the heat transfer coefficient at the cold plate. Correlation equations for coupled and noncoupled average heat transfer rates are presented.
An Experimental Study on Laser Annealing of Thin Silicon Layers
Authors: Grigoropoulos C. P. , Buckholz R. H. , Domoto G. A.
Published: October 2009

A laser annealing technique directed toward producing single crystalline silicon on substrates is studied. In this paper the laser-induced melting of thin silicon films is studied experimentally. Direct heating of thin silicon layers on substrates is shown to produce a variety of different silicon melting patterns. A systematic study of these phase change phenomena has been performed. The important parameters are: (1) the laser beam power, (2) the laser beam intensity distribution, and (3) the speed of the translating silicon layer. Unstable silicon phase boundaries break up to form regions where solid and melt silicon coexist. Complicated silicon phase boundary patterns are shown. The experimental results showed the occurrence of organized patterns of alternating solid and liquid silicon stripes for two-dimensional heating distributions. Finally, temperature fields for the experimental operating conditions are calculated using an enthalpy model.
Thermal Analysis of Laser-Assisted Thermoplastic-Matrix Composite Tape Consolidation
Authors: Beyeler E. P. , Gu¨c¸eri S. I.
Published: October 2009

A novel approach to producing composite parts using thermoplastic-matrix tapes is described. A thermal analysis is presented for the case of focused heat sources such as lasers for melting and consolidating the prepregs in a continuous manufacturing process. A numerical grid generation method is employed to account for the complex geometry of the solution domain. Heat transfer is modeled using an orthotropic domain made of two-dimensional, continuous fiber anisotropic laminates. Heat of crystallization melting/solidification are included in the form of a heat generation term. The temperature distributions and thermal histories in the laminated composites are presented for varying consolidation speeds. The effects of preheating the consolidated laminate are investigated and the overall feasibility of the proposed process is discussed.
An Integral Analysis of Two Simple Model Problems of Wind-Aided Flame Spread
Authors: Wichman I. S. , Baum H. R.
Published: October 2009

The method of integral analysis is applied to two simple model problems closely related to the general wind-aided flame spread problem. The analysis of the two models strongly suggests approximating the general problem as a steady-state process in the pyrolysis zone and as an unsteady process in the preheat zone ahead of it.
Determination of Convective Diffusion Heat/Mass Transfer Rates to Burner Rig Test Targets Comparable in Size to Cross-Stream Jet Diameter
Authors: Go¨kog˘lu S. A. , Santoro G. J.
Published: October 2009

Two sets of supplementary experiments have been performed to be able to predict the convective diffusion heat/mass transfer rates to a cylindrical target whose height and diameter are comparable to, but less than, the diameter of the circular cross-stream jet, thereby simulating the same geometric configuration as a typical burner rig test specimen located in the cross stream of the combustor exit nozzle. The first set exploits the naphthalene sublimation technique to determine the heat/mass transfer coefficient under isothermal conditions for various flow rates (Reynolds numbers). The second set, conducted at various combustion temperatures and Reynolds numbers, utilizes the temperature variation along the surface of the abovementioned target under steady-state conditions to estimate the effect of cooling (dilution) due to the entrainment of stagnant room temperature air. The experimental information, combined with an analytical treatment, is used to predict high-temperature, high-velocity corrosive salt vapor deposition rates in burner rigs on collectors that are geometrically the same. The agreement with preliminary data obtained from Na2SO4 vapor deposition experiments is found to be excellent.
Analysis of Combined Conductive-Radiative Heat Transfer in a Two-Dimensional Rectangular Enclosure With a Gray Medium
Authors: Yuen W. W. , Takara E. E.
Published: October 2009

Combined conductive–radiative heat transfer in a two-dimensional enclosure is considered. The numerical procedure is based on a combination of two previous techniques that have been demonstrated to be successful for a two-dimensional pure radiation problem and a one-dimensional combined conductive–radiative heat transfer problem, respectively. Both temperature profile and heat transfer distributions are generated efficiently and accurately. Numerical data are presented to serve as benchmark solutions for two-dimensional combined conductive–radiative heat transfer. The accuracy of two commonly used approximation procedures for multidimensional combined conductive–radiative heat transfer is assessed. The additive solution, which is effective in generating approximation to one-dimensional combined conductive–radiative heat transfer, appears to be an acceptable empirical approach in estimating heat transfer in the present two-dimensional problem. The diffusion approximation, on the other hand, is shown to be generally inaccurate. For all optical thicknesses and conduction-radiation parameters considered (including the optically thick limit), the diffusion approximation is shown to yield significant errors in both the temperature and heat flux predictions.
An Investigation of the Propagation of Pressure Perturbations in Bubbly Air/Water Flows
Authors: Ruggles A. E. , Lahey R. T. , Drew D. A. , Scarton H. A.
Published: October 2009

Dispersion and attenuation was measured for standing waves in a vertical waveguide filled with a bubbly air/water mixture. The propagation speed of pressure pulses was also measured. The data were compared with a two-fluid model for a range of values of the virtual volume coefficient, CVM. The experimentally determined CVM was found to be a function of global void fraction (〈α〉). Moreover it was noted that this CVM was less strongly related to void fraction than those proposed by Zuber (1964) and Van Wijngaarden (1976).
On Numerical Methods Used in Mathematical Modeling of Phase Change in Liquid Metals
Authors: Poirier D. , Salcudean M.
Published: October 2009

The work presents an analysis and comparative evaluation of different methods used for the numerical solution of heat conduction with phase change problems. Both freezing (melting) water as well as solidifying liquid metal problems are examined. Emphasis is placed on weak formulations as they tend to be simple to program and easily implemented in existing single-phase codes. A new method based on the apparent capacity technique is proposed. In this technique an “effective capacity” is computed, based on the integration of temperature profiles over the nodal volumes. This method shows significantly better performance when compared with other methods for the numerical analysis of solidifying metals.
Flow Visualization Studies on Vortex Instability of Natural Convection Flow Over Horizontal and Slightly Inclined Constant-Temperature Plates
Authors: Cheng K. C. , Kim Y. W.
Published: October 2009

Flow visualization experiments were performed in a low-speed wind tunnel to study vortex instability of laminar natural convection flow along inclined isothermally heated plates having inclination angles from the horizontal of θ = 0, 5, 10, 15 and 20 deg. The temperature difference between plate surface and ambient air ranged from ΔT = 15.5 to 37.5°C and the local Grashof number range was Grx = 1.02×106 to 2.13×108. Three characteristic flow regimes were identified as follows: a two-dimensional laminar flow, a transition regime for developing longitudinal vortices, and a turbulent regime after the breakdown of the longitudinal vortices. Photographs are presented of side and top views of the flow and of cross-sectional views of the developing views of the developing secondary flow in the postcritical regime. Instability data of critical Grashof number and wavelength are presented and are compared with the theoretical predictions from the literature.
Natural Convection in Shallow Enclosures With Differentially Heated Endwalls
Authors: Paolucci S. , Chenoweth D. R.
Published: October 2009

We consider a low-aspect-ratio two-dimensional rectangular cavity, differentially heated with an arbitrarily large horizontal temperature difference. Steady-state results obtained from numerical solutions of the transient Navier-Stokes equations are given for air using the ideal gas law and Sutherland law transport properties. We clarify the different flow regimes possible by using numerical results for aspect ratios 0.025 ≤ A ≤ 1 and for Rayleigh numbers (based on height) 102 ≤ Ra ≤ 109. We present Nusselt numbers, and temperature and velocity distributions, and compare them with existing data. At high Ra in the Boussinesq limit we show the existence of weak secondary and tertiary flows in the core of the cavity. In addition we present novel solutions in the non-Boussinesq regime. We find that the classical parallel flow solution, accurate in the core of the cavity in the Boussinesq limit, does not exist when variable properties are introduced. For higher Rayleigh numbers, we generalize the well-known analytical boundary layer solution of Gill. The non-Boussinesq solutions show greatly reduced static pressure levels and lower critical Rayleigh numbers.
Fluid Flow and Mixed Convection Transport From a Moving Plate in Rolling and Extrusion Processes
Authors: Karwe M. V. , Jaluria Y.
Published: October 2009

The heat transfer arising due to the movement of a continuous heated plate in processes such as hot rolling and hot extrusion has been studied. Of particular interest were the resulting temperature distribution in the solid and the proper imposition of the boundary conditions at the location where the material emerges from a furnace or die. These considerations are important in the simulation and design of practical systems. A numerical study of the thermal transport process has been carried out, assuming a two-dimensional steady circumstance. The boundary layer equations, as well as full governing equations including buoyancy effects, are solved employing finite difference techniques. The effect of various physical parameters, which determine the temperature and flow fields, is studied in detail. The significance of these results in actual manufacturing processes is discussed.
Parametric Trends for Post-CHF Heat Transfer in Rod Bundles
Authors: Unal C. , Tuzla K. , Badr O. , Neti S. , Chen J.
Published: October 2009

A 3 × 3 rod bundle with a heated shroud was developed to study post-critical-heat-flux (post-CHF) dispersed-flow boiling. The hot-patch technique was applied to a rod bundle, which successfully arrested the quench front at the test section inlet. Measurements included mass flux, wall heat flux, inlet equilibrium quality, wall temperatures along the bundle axis, and actual vapor temperatures upstream and downstream of a spacer grid. The vapor superheat (up to 600°C) increased with increasing wall heat flux and decreasing mass flux and vapor quality. The heat partition ratio (fraction of total heat input that goes toward evaporation) was found to increase with increasing mass flux and decreasing inlet quality but remained essentially independent of heat flux. The results for the rod bundle were found to be in good agreement with trends previously reported for post-CHF heat transfer in single tubes.
Gas-Concentration Measurements and Analysis for Gas-Loaded Thermosyphons
Authors: Peterson P. F. , Tien C. L.
Published: October 2009

Using a miniature wet-bulb/dry-bulb probe, detailed measurements of noncondensable-gas-concentration profiles were made in a gas-loaded two-phase reflux thermosyphon to investigate the effects of orientation and gas/vapor molecular weight combinations. Natural convection and radial diffusion were found to have a large influence on the noncondensable-gas distribution, even when the gas and vapor molecular weights are equal. An axisymmetric two-dimensional diffusion model is presented to allow evaluation of the relative effects of natural convection versus diffusion. Gas recirculation, axial stratification, regions of unstable flow, and their effects on performance are discussed.
Predicting the Performance of an Evaporative Condenser
Authors: Peterson D. , Glasser D. , Williams D. , Ramsden R.
Published: October 2009

A simple analytical method, based on the method of Parker and Treybal (1961) for evaporative coolers, was developed to predict the performance of evaporative condensers. The transfer coefficients of the model were predicted from standard correlations in the literature or those determined by Parker and Treybal for evaporative coolers. Field tests were performed on an evaporative condenser and the measured and predicted heat loads and recirculating water temperature were compared. It was found that the heat load was underpredicted by about 30 percent and the water temperature by 3°C. A change in Uo of a factor of 1.9 adequately predicts both the heat load and sump temperature.
Similarity Analysis of Condensing Flow in a Fluid-Driven Fracture
Authors: Griffiths S. K. , Nilson R. H.
Published: October 2009

Similarity solutions are derived for some fundamental problems of condensing flow in a hydraulically driven fracture. The governing equations describe one-dimensional homogeneous turbulent flow along a wedge-shaped hydraulic fracture in an elastic medium. The instantaneous fracture speed is determined as an analytical function of fracture length, material properties, process parameters, and a single eigenvalue, which is calculated by solving a system of ordinary differential equations for the variation of pressure, energy, velocity, and opening displacement along the fracture. Results are presented for abrupt condensation of a pure substance and for gradual condensation of air/water mixtures. The rate of condensation is controlled by the rate of heat transfer to the fracture wall, which depends upon a single dimensionless parameter. For small and large values of this parameter the present multiphase solutions are in agreement with previous solutions for single-phase flows of vapors and liquids. Although most of the results are presented in dimensionless form, some numerical examples are given for steam-driven fractures emanating from the cavity resulting from an underground nuclear explosion.
Heat Losses From Underground Steam Pipelines
Authors: Beck J. V. , McLain H. A. , Karnitz M. A. , Shonder J. A. , Segan E. G.
Published: October 2009

A new transient solution describing the heat transfer around a single buried steam pipe was developed. The solution technique uses Green’s functions and is particularly appropriate at distances greater than two or three pipe radii from the pipe. The solution models the pipe as a line heat source and considers a convective boundary condition at the soil surface. It was shown that for long time periods, the solution reduces to a steady-state expression that is believed to describe heat loss rates better than some current, commonly used steady-state relations. The solution was used with parameter estimation techniques to design an experimental procedure for determining buried steam pipe heat loss rates and depths from near-surface soil temperature measurements. Sensitivity coefficients derived from the solution were used to identify the linearly independent parameters and the optimum locations for the temperature measurements. An example of the use of this procedure is presented.
Inverse Heat Conduction Problem of Periodically Contacting Surfaces
Authors: Flach G. P. , O¨zis¸ik M. N.
Published: October 2009

An inverse heat conduction method for determining the periodically time-varying contact conductance between two periodically contacting surfaces is presented. The technique is based on solving two single-region inverse problems for the contact surface temperature and heat flux of each solid. The time variation of contact surface temperature is represented with a versatile periodic B-spline basis. The dimension of the B-spline basis is statistically optimized and confidence bounds are derived for the estimated contact conductance. Typical results based on both simulated and actual measurements are given and a parametric study is made to illustrate the general effects of measurement location, number of measurements, etc., on the accuracy of the results.
Analytical Solution for Unsteady Heat Transfer in a Pipe
Authors: Sucec J.
Published: October 2009

Under consideration is transient, convective heat transfer to a fluid flow within a pipe due to a sudden change in the temperature of the ambient medium outside the pipe. A solution is developed by the Laplace transformation, for the fastest portion of the transient, which gives the pipe wall temperature, surface heat flux, and fluid bulk mean temperature. These analytical results are compared with available finite difference results and with quasi-steady solutions. A criterion is developed that indicates when the zero thermal capacity wall solution can be used with adequate accuracy.
An Experimental Study of Natural Convection in a Toroidal Loop
Authors: Stern C. H. , Greif R. , Humphrey J. A. C.
Published: October 2009

Velocity and temperature profiles were measured at the entrance and exit to the heating section of a toroidal thermosyphon loop operating under steady flow conditions for a range of heat inputs. Velocity measurements were made with a laser-Doppler velocimeter and temperature measurements with a small thermocouple probe. Detailed results are presented for the longitudinal and circumferential components of the velocity for four heat inputs. The data for cross-stream secondary flows and streamwise flow reversals emphasize the importance of including three-dimensional effects in analyses of these systems.
Natural Convection Heat Transfer of Cold Water Within an Eccentric Horizontal Cylindrical Annulus
Authors: Ho C. J. , Lin Y. H.
Published: October 2009

Natural convection heat transfer of cold water, encompassing a density inversion, within an eccentric horizontal annulus made of two isothermal cylinders, is numerically studied via a finite difference method. Numerical results have been obtained for an annular radius ratio 2.6 with Rayleigh number ranging from 103 to 106, the inversion parameter being 0.0 to 1.0, the eccentricity varying from 0 to 0.8, and the orientation angle of the inner cylinder between 0 and π. Results indicate that the flow patterns and heat transfer characteristics are strongly influenced by the combined effect induced by the density inversion of water and the position of the inner cylinder of the annulus. For the cases considered in the present study, a minimum in heat transfer arises with the inversion parameter between 0.4 and 0.5 depending primarily on the position of the inner cylinder.
Liquid and Gas-Phase Distributions in a Jet With Phase Change
Authors: Dobran Flavio
Published: October 2009

A two-phase flow high-velocity jet with phase change was studied numerically. The jet is assumed to be created by the two-phase critical flow discharge through a pipe of variable length and attached to a vessel containing the saturated liquid at different stagnation pressures. The jet flow is assumed to be axisymmetric and the modeling of the two-phase flow was accomplished by a nonequilibrium model that accounts for the relative velocity and temperature difference between the phases. The numerical solution of the governing set of balance and conservation equations revealed steep gradients of flow properties in both radial and axial directions. The liquid phase in the jet is shown to remain close to the jet axis, and its velocity increases until it reaches a maximum corresponding to the gas velocity, and thereafter decreases at the same rate as the gas velocity. The effect of decreasing the pipe length is shown to produce a larger disequilibrium in the jet and a double pressure peak in the total pressure distribution. A comparison of the predicted total pressure distribution in the jet with the experimental data of steam–water at different axial locations is demonstrated to be very reasonable.
Thermal Conductivity of Gaseous and Liquid Ammonia
Authors: Clifford A. A. , Tufeu R.
Published: October 2009

The thermal conductivity of gaseous and liquid ammonia has been measured in the range 300–300 K and at pressures up to 50 MPa. The measurements were a necessary preliminary to a fitting of the thermal conductivity surface in the density–temperature plane, which is part of the current program of the Transport Properties Subcommittee of the I.U.P.A.C. Results were obtained that are believed to be accurate to 2 percent. It is difficult to make a good comparison of these results with previous data until a full correlation of the thermal conductivity of ammonia is carried out. A preliminary assessment for the liquid phase indicates that agreement is reasonable over much of the range with differences up to around 5 percent under certain conditions. For the gas phase an approximate extrapolation to atmospheric pressure can be made and the results compared with some recent recommended values. Differences of ±3 percent are observed.
Heat Transfer in Fluid Flows Which Do Not Follow the Contour of Bounding Walls
Authors: Sparrow E. M.
Published: October 2009

This paper is dedicated to the generations of students who honored me by their willingness to join their hearts and minds with mine, and to my wife who taught all of us that it is heart, rather than mind, that makes the world go ’round. The paper showcases a highly personalized style of research characterized by intense human involvement and a minimum of material resources, and exemplifies the notion that “less is more.” The technical content of the paper is a collection of themewise-related pieces of research, all of which have to do with fluid flows which do not follow the contour of the bounding wall(s) and with the related heat transfer ramifications. Two classes of such flows are considered. In one, the fluid is acted on by forces directed normal to the wall, causing lift-off. The other non-wall-adhering class of flows to be considered is separated flows.
An Evaluation of Film Condensation on Horizontal Integral-Fin Tubes
Authors: Marto P. J.
Published: October 2009

This paper reviews nearly fifty years of progress that has been made in our understanding of film condensation on horizontal, integral-fin tubes. Both experimental and theoretical developments pertaining to single tubes and tube bundles are discussed. Special attention is given to the importance of surface tension, and appropriate research directions are provided.
An Experimental Study of Transition to Turbulence in Vertical Mixed Convection Flows
Authors: Krishnamurthy R. , Gebhart B.
Published: October 2009

Results are reported from an experimental study of a mixed convection flow, that is, combined forced and natural convection, undergoing transition to turbulence, adjacent to a vertical, uniform-heat-flux surface. Small and aiding forced convection effects were studied. The measurements, in air, were made at pressure levels ranging from 4.4 bars to 8 bars, at flux levels q″ in the range 14–1300 W/m2. The imposed free-stream velocities U∞ were around 5 cm/s. One objective was to determine any quantitative parameters that would predict the bounds of the transition region. Another was to measure disturbance growth characteristics during transition. Results show that, at a given U∞, the beginning of transition is not correlated by the local Grashof number Grx* alone. An additional dependence on both the downstream location and pressure level was found. Thermal and velocity transitions were found to begin when the mixed convection parameter εM reached a value of 0.18. Transition was found to be complete when the nondimensional convected energy in the boundary region, βq″x/5k, reached a value of 7.10. These experimental results confirm the prediction of linear stability analysis, that aiding mixed convection stabilizes the flow, compared to pure natural convection flow. The data also support a physical explanation of these mechanisms.
Radiative Heat Transfer in Multidimensional Emitting, Absorbing, and Anisotropic Scattering Media—Mathematical Formulation and Numerical Method
Authors: Tan Zhiqiang
Published: October 2009

Thermal radiative transmission in multidimensional emitting, absorbing, and anisotropic scattering media is studied in this paper. In the first part, starting from basic formulae of radiative heat transfer, a set of integral equations for the problem is derived. Then the product-integration method is applied to discretize the integral equations. This method, while analogous to Hottel’s zonal method or Razzaque’s finite element method, requires evaluation of only three or two-dimensional integrals for three-dimensional systems. Finally the formulation and the numerical technique are applied to the problems of thermal radiation in emitting, absorbing, and linearly anisotropic scattering planar and square media with gray surfaces and with or without internal heat generations. Computed results are discussed and compared with available data.
Performance of a Three-Phase, Spray-Column, Direct-Contact Heat Exchanger
Authors: C¸oban T. , Boehm R.
Published: October 2009

A numerical model of a three-phase, direct-contact, spray-column heat exchanger has been developed. This model has been used to calculate performance information about this type of device and to compare, where possible, to experiments. General equations are defined for distance up the column using a physically based model for the local heat transfer. This model has been used to investigate a number of characteristics of these devices, such as temperature and holdup distributions through the column. A new formulation is given for a mixed, time-averaged temperature that may be representative of measurements taken with temperature transducers in direct-contact heat exchangers. Little has been given in the literature about quantitative variations of performance as a function of the key independent variables, and information on these aspects is presented here. Although the results presented are for a specific geometry (0.61 m diameter, 3 m active column height, evaporating pentane in 85°C water), the variations shown can give insights generally into the factors affecting performance in these devices. In virtually all cases examined here, extremely good comparisons are shown between predictions and measurements. Conclusions are drawn about the applicability of the model and the important effects demonstrated.
Free-Stream Turbulence Effects on Convex-Curved Turbulent Boundary Layers
Authors: You S. M. , Simon T. W. , Kim J.
Published: October 2009

Free-stream turbulence intensity effects on a convex-curved turbulent boundary layer are investigated. An attached fully turbulent boundary layer is grown on a flat plate and is then introduced to a downstream section where the test wall is convexly curved, having a constant radius of curvature. Two cases, with free-stream turbulence intensities of 1.85 and 0.65 percent, are discussed. They were taken in the same facility and with the same strength of curvature, δ/R = 0.03−0.045. The two cases have similar flow conditions upon entry to the curve, thus separating the free-stream turbulence effects under study from other effects. The higher turbulence case displayed stronger curvature effects on the skin friction coefficient Cf, and on streamwise-normal and shear stress profiles, than observed in the lower turbulence case. Observations of this are: (1) As expected, the higher turbulence case has a higher Cf value ( ∼ 5 percent) upstream of the curve than does the lower turbulence case, but this difference diminishes by the end of the curve. (2) Streamwise turbulence intensity profiles, differing upstream of the curve for the two cases, are found to be similar near the end of the curve, thus indicating that the effect of curvature is dominating over the effect of free-stream turbulence intensity. Many effects of curvature observed in the lower turbulence intensity case, and reported previously, e.g., a dramatic response to the introduction of curvature and the rapid assumption of an asymptotic shape within the curve, are also seen in the higher turbulence case.
Comprehensive Room Transfer Functions for Efficient Calculation of the Transient Heat Transfer Processes in Buildings
Authors: Seem J. E. , Klein S. A. , Beckman W. A. , Mitchell J. W.
Published: October 2009

This paper describes a method in which the transfer functions describing heat flows in building elements can be combined into a single transfer function for an enclosure, referred to as a comprehensive room transfer function (CRTF). The method accurately models long-wave radiation exchange and convection in an enclosure through an approximate network, referred to as the “star” network. Resistances in the star network are determined from a network that uses view factors to model long-wave radiation exchange. The Pade´ approximation and bilinear transformation are used to reduce the number of coefficients in a CRTF.
Optimal Heat Exchangers
Authors: Kovarik M.
Published: October 2009

The design of optimal heat exchangers is formulated as the solution of five simultaneous equations. The analysis of these equations yields general properties of optimal crossflow heat exchangers; in particular, an upper bound of 1/3 is given for the fractional cost of maintaining the flow through the heat exchanger. Some of these general properties also apply in the presence of a simple constraint. It is shown that some technically feasible designs cannot be optimal under realistic costs and others under any costs.
Kelvin–Helmholtz Instability and Cooling Limitations of a Vertical Molten Sheet Confined by Alternating Magnetic Fields
Authors: Hull J. R.
Published: October 2009

Limitations imposed by a Kelvin–Helmholtz-type instability on the cooling of a vertical molten sheet that is confined by magnetic fields alternating in time are calculated. The molten sheet is cooled by a gas stream flowing on both sides of the sheet. The Kelvin–Helmholtz instability imposes a maximum velocity on the cooling gas, which limits the cooling rate. The maximum cooling rate is calculated with respect to the maximum velocity, gas physical properties, flow channel geometry, surface tension, and magnetic field properties.
Experimental and Numerical Studies of Natural Convection in Trapezoidal Cavities
Authors: Lam S. W. , Gani R. , Symons J. G.
Published: October 2009

Natural convection heat transfer has been studied experimentally and numerically for horizontal prismatic cavities of trapezoidal section having a hot horizontal base, a cool inclined top, and insulated vertical walls. Experimental results are presented for a cavity with width-to-mean height ratio of 4, Rayleigh numbers (based on the mean cavity height) from 103 to 107, and top surface inclinations from 0 to 25 deg to the horizontal. For a given top surface inclination, the Nusselt–Rayleigh relationship follows the usual trend, but with an interesting anomaly, in which higher Nusselt numbers than expected are obtained in the range 8 × 103 < Ra < 2 × 105 for inclinations of 0 and 5 deg. Overall, as the inclination of the top surface is increased, the Nusselt number decreases, an effect that becomes greater at higher angles. The proportions of convective heat flow rate into the high side and low side of the cavity were measured and show distinct maxima at particular Rayleigh numbers (which are independent of the top surface inclination angle). The equation Nu = 0.168 [Ra (1 + cos θ)/2]0.278 [(1 − cos θmax)/(cos θ − cos θmax)]−0.199 correlates the experimental results to within 6.9 percent for the ranges 4 × 103 < Ra < 107 and 0 deg ≤ θ ≤ 25 deg, apart from the anomalous region previously indicated. It is suggested that this correlation applies for A ≥ 4. The numerical model uses a false transient ADI finite difference scheme to solve the governing two-dimensional vorticity and energy transport equations. Nusselt numbers computed by the model are in good agreement with the experimental values. The convective flow patterns generated by the model exhibit changes in number and in size of cells for different Rayleigh numbers and different top surface inclinations.
General Analysis of Steady Laminar Mixed Convection Heat Transfer on Vertical Slender Cylinders
Authors: Wang T.-Y. , Kleinstreuer C.
Published: October 2009

A general analysis has been developed to study fluid flow and heat transfer characteristics for steady laminar mixed convection on vertical slender cylinders covering the entire range from pure forced to pure natural convection. Two uniquely transformed sets of axisymmetric boundary-layer equations for the constant wall heat flux case and the isothermal surface case are solved using a two-point finite difference method with Newton linearization. Of interest are the effects of the new mixed convection parameter, the cylinder heating/cooling mode, the transverse curvature parameter, and the Prandtl number on the velocity/temperature profiles and on the local skin friction parameter and the heat transfer parameter. The results of the validated computer simulation model are as follows. Depending upon the magnitude and direction of the buoyancy force, i.e., the value of the mixed convection parameter and the heating or cooling mode applied, natural convection can have a significant effect on the thermal flow field around vertical cylinders. Specifically, strong variations of the local skin friction parameter and reversing trends in the heat transfer parameter are produced as the buoyancy force becomes stronger in aiding flow. The skin friction parameter increases with higher curvature parameters and Prandtl numbers. Similarly, the modified Nusselt number is larger for higher transverse curvature parameters; however, this parameter may reverse the impact of the Prandtl number on the Nusselt number for predominantly forced convection.
Mixed Convection Heat Transfer to Power Law Fluids in Arbitrary Cross-Sectional Ducts
Authors: Lawal A.
Published: October 2009

An analytical investigation of three-dimensional mixed convection flow and heat transfer to power-law fluids in horizontal arbitrary cross-sectional ducts is undertaken. The continuity equation and parabolic forms of the energy and momentum equations in rectangular coordinates are transformed into new orthogonal coordinates with the boundaries of the duct coinciding with the coordinate surfaces. The transformed equations are solved by the finite difference technique. The fluid enters the duct with constant velocity and temperature profiles with the wall of the duct subjected to constant temperature. Local heat transfer coefficients and pressure drop for several values of Gr/Re and power-law index n are computed for the triangular, square, trapezoidal, pentagonal, and circular ducts. The buoyancy force is found to increase both the Nusselt number and the pressure drop.
Freezing of Water-Saturated Porous Media in the Presence of Natural Convection: Experiments and Analysis
Authors: Chellaiah S. , Viskanta R.
Published: October 2009

Freezing of superheated water-porous media (glass beads) contained in a rectangular test cell has been studied both experimentally and numerically. The effects of liquid superheat and imposed temperature difference were investigated. When the superheat across the liquid region was small the flow in the porous media was weak, and the interface was almost planar. For larger superheats, natural convection flow and the solidification front shape and velocity were found to depend on the imposed temperature and the permeability of the porous medium. Due to the density inversion of water, the rate of freezing was higher, either at the top or at the bottom of the cell, depending on the amount of superheat. The measured temperature distributions were compared with predictions of numerical model that considered both conduction in the solid and natural convection in the liquid region. This model is based on volumetric averaging of the macroscopic transport equations, with phase change assumed to occur volumetrically over a small temperature range. Both Brinkman and Forchheimer extensions were added to the Darcy equations. The effect of density inversion of water on the fluid flow and heat transfer has been modeled. Good agreement has been found between the experimental data and numerical predictions.
The Relationship Between Standing Waves, Pressure Pulse Propagation, and Critical Flow Rate in Two-Phase Mixtures
Authors: Ruggles A. E. , Lahey R. T. , Drew D. A. , Scarton H. A.
Published: October 2009

A two-fluid model is presented that can be used to predict the celerity and attenuation of small-amplitude harmonic disturbances in bubbly two-phase flow. This frequency-dependent relationship is then used to predict the propagation of small-amplitude pressure perturbations through the use of Fourier decomposition techniques. Predictions of both standing waves and propagating pressure perturbations agree well with existing data. The low and high-frequency limits of the celerities predicted by the model are examined and their relationship to critical flow rate is demonstrated. Some limitations of the interfacial pressure model employed in conventional critical flow analysis are exposed and the implications to the prediction of critical flow rate are discussed.
Axisymmetric Unsteady Droplet Vaporization and Gas Temperature Distribution
Authors: Lee S. K. , Chung T. J.
Published: October 2009

Droplet vaporization and temperature distributions of axisymmetric unsteady sprays are investigated. The so-called discrete droplet model of two-phase flows, often known as the Eulerian–Lagrangian method, is used. Calculations are carried out with Eulerian coordinates using finite elements for the gas phase and the method of characteristics using the second-order Runge–Kutta scheme for the droplet liquid phase. The sensitivity of the numerical results to changes in time step, injection pulse time, grid spacing, and number of droplet characteristics is examined. Through a simple example, it is shown that applications of the finite element method to more complicated problems appear to be promising.
An Experimental and Theoretical Study of Transient Pressure Drop in Two-Phase Condensing Flows
Authors: Wedekind G. L. , Bhatt B. L. , Roslund G. L.
Published: October 2009

This paper presents the results of an experimental and theoretical investigation of the pressure drop associated with transient two-phase condensing flows involving complete condensation. Utilizing the system mean void fraction model, and the similarity relationships associated with it, an analytical prediction of the transient pressure drop is possible, including a simplified closed-form version. The capability of the proposed theory is demonstrated by comparison with experimental measurements of the transient pressure drop in a horizontal tube condenser following an exponential-type change in the inlet mass flow rate. Good agreement is shown to exist between the predicted and experimental results.
Design and Calibration of a Local Heat-Flux Measurement System for Unsteady Flows
Authors: Campbell D. S. , Gundappa M. , Diller T. E.
Published: October 2009

A local heat–flux measurement system was built, calibrated, and tested for use in unsteady flows. The system was designed to maintain constant-wall-temperature boundary conditions. The measuring element is a thin-film heat flux gage made by sputter-coating gold onto a substrate. A constant-temperature anemometer is used to maintain the thin-film gage at a specified temperature under fluctuating conditions. A separate temperature control system maintains the surrounding boundary at the gage temperature. The system was calibrated for both steady and unsteady flows using a specially designed calibrator for local heat flux gages. The steady calibration was done with predominantly convective heat transfer. The unsteady calibration was achieved by adding oscillating radiant energy to the surface. Consequently, quantitative results can be obtained for both the mean and fluctuating components of the heat transfer. The frequency response was good to over 90 Hz. Sample results are presented of the unsteady heat transfer over a circular cylinder caused by natural vortex shedding at 70 to 80 Hz.
Suppression of the Sonic Heat Transfer Limit in High-Temperature Heat Pipes
Authors: Dobran Flavio
Published: October 2009

The design of high-performance heat pipes requires optimization of heat transfer surfaces and liquid and vapor flow channels to suppress the heat transfer operating limits. In the paper an analytical model of the vapor flow in high-temperature heat pipes is presented, showing that the axial heat transport capacity limited by the sonic heat transfer limit depends on the working fluid, vapor flow area, manner of liquid evaporation into the vapor core of the evaporator, and lengths of the evaporator and adiabatic regions. Limited comparisons of the model predictions with data of the sonic heat transfer limits are shown to be very reasonable, giving credibility to the proposed analytical approach to determine the effect of various parameters on the axial heat transport capacity. Large axial heat transfer rates can be achieved with large vapor flow cross-sectional areas, small lengths of evaporator and adiabatic regions or a vapor flow area increase in these regions, and liquid evaporation in the evaporator normal to the main flow.
Prediction of Conjugate Heat Transfer in a Solid–Liquid System: Inclusion of Buoyancy and Surface Tension Forces in the Liquid Phase
Authors: Keller J. R. , Bergman T. L.
Published: October 2009

Numerical predictions have been obtained for steady-state conjugate heat transfer in an open rectangular cavity. For the geometry considered, fluid motion is driven by augmenting buoyancy and surface tension forces. Predictions of the steady-state solid volume fraction and various solid thicknesses were obtained for a high Prandtl number fluid characterized by various Rayleigh and Marangoni (Ma) numbers. Due to numerical difficulties associated with large surface tension effects, a limited range of Ma was investigated (Ma≤250). The predictions show that surface tension induced flow can affect the solid geometry and, ultimately, freezing or melting rates. Specifically, the solid–liquid interface shape is altered, the steady-state solid volume fraction is decreased, and the solid thickness at the top surface is smaller, compared to the pure buoyancy-driven case. The dimensionless solid volume fraction and solid thicknesses are related to the governing dimensionless parameters of the problem. Finally, predictions are made for high Marangoni number flows (Ma>>250) to demonstrate the potential governing influence of surface tension effects in phase-change systems.
Void Fraction Measurements During Saturated Pool Boiling of Water on Partially Wetted Vertical Surfaces
Authors: Liaw S. P. , Dhir V. K.
Published: October 2009

Void fraction profiles adjacent to a vertical wall 6.3 cm wide and 10.3 cm high were measured during nucleate boiling. The experiments were conducted in saturated water at 1 atm pressure. In the experiments, the wettability of the surface was varied by controlling the degree of oxidation of the surface. Static contact angle was used as an indicator of the surface wettability. The void fraction was measured with a gamma densitometer. The experimental results show that the maximum void fraction occurs about 1–1.5 mm away from the heater surface. The wall void fraction, the maximum void fraction, and the thickness of the void layer increase with wall heat flux. It is found that for a given heat flux, the wall void fraction increases as the surface becomes less wettable, whereas the maximum heat flux decreases with increase in contact angle.
The Influence of Subcooling on the Frequency of Bubble Emission in Nucleate Boiling
Authors: Judd R. L.
Published: October 2009

The experimental results reported in Ibrahim and Judd (1985) in which bubble period first increased and then decreased as subcooling varied over the range 0 ≤ θsat ≤ 15° C is interpreted by means of a comprehensive boiling heat transfer model incorporating the contributions of nucleate boiling, natural convection, and micro-layer evaporation components. It is shown that bubble emission frequency varies to accommodate the impressed heat load in the manner described above because changes in subcooling cause the heat flux within the area influenced by the formation and departure of bubbles to change in the opposite sense to the fraction of the surface area within which bubble formation and departure occurs. The mechanism responsible for the nucleation of bubbles at exactly the frequency required at each level of subcooling is the object of continuing research.
Direct Contact Evaporation Between Two Immiscible Liquids in a Spray Column
Authors: Seetharamu K. N. , Battya P.
Published: October 2009

The present investigation deals with the direct contact evaporation of refrigerant 113 and n-pentane in a stagnant column of distilled water. The operational parameters investigated in the experimental study are the operating column height, the temperature difference, the dispersed phase flow rate, and the diameter and number of orifices in the distributor. The effects of these parameters on volumetric heat transfer coefficient, holdup, and agglomeration are investigated. A modified relation, based on the theoretical analysis of Smith et al. (1982), is also developed for predicting the theoretical volumetric heat transfer coefficient. Comparison with related works available in the literature shows reasonable agreement.