bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effect of Coatings on the Steady-State and Short Time Constriction Resistance for an Arbitrary Axisymmetric Flux
Authors: Dryden J. R. , Yovanovich M. M. , Deakin A. S.
Published: October 2009

The effect of a coating upon the short-time and steady-state constriction resistance is analyzed for an arbitrary axisymmetric contact spot flux. At very short times the expression obtained for R is identical to the expression for one-dimensional transient heat flow through a two-layer wall. At steady-state, the results of the analysis predict that the effect of the coating are mainly dependent on the relative thermal properties of the coating and substrate. The limiting cases, where the coating thickness approaches either zero or infinity, are discussed.
Heat Conduction in Heterogeneous Materials
Authors: Baker-Jarvis J. , Inguva R.
Published: October 2009

A new solution to the heat equation in composite media is derived using a variational principle developed by Ben-Amoz. The model microstructure is fed into the equations via a term for the polar moment of the inclusions in a representative volume. The general solution is presented as an integral in terms of sources and a Green function. The problem of uniqueness is studied to determine appropriate boundary conditions. The solution reduces to the solution of the normal heat equation in the limit of homogeneous media.
An Analytical Model of Thermal and Vapor Diffusion in Freezing of Wet Coal
Authors: Moog D. B. , Rubinsky B.
Published: October 2009

An analytical study is presented for the freezing of a semi-infinite domain of wet coal particles with special attention to the role of the airborne water vapor diffusion process. The results indicate that energy transport in this multiphase system can occur by means of thermal diffusion or mass diffusion. The magnitude of the Lewis number can be used to determine the mechanism. For a Lewis number smaller than 1, water vapor will evaporate from the coal particles, condense on the wall, and increase the freezing rate in the system.
Analysis of Laminar Flow and Heat Transfer in the Entrance Region of an Internally Finned Circular Duct
Authors: Prakash C. , Liu Ye-Di
Published: October 2009

Steady, laminar, forced convection flow and heat transfer in the entrance region of an internally finned circular duct has been analyzed by numerically integrating the governing partial differential equations. The fins are radial, continuous, and of zero thickness. Two types of thermal boundary conditions are considered: (i) a uniform heat input per unit axial length with peripherally uniform temperature at any cross-section, and (ii) uniform temperature both axially and peripherally. Results exhibit the expected high-pressure gradients and heat transfer coefficients in the entrance region, approaching, asymptotically, the fully developed values at large axial distance. Results are presented for 8, 16, and 24 fins, and for the fin height to duct radius ratio 0.3, 0.6, and 1. The Prandtl number is kept as .707 corresponding to air.
Heat Rejection to the Surface Layer of a Solar Pond
Authors: Jaluria Y. , Cha C. K.
Published: October 2009

An analytical and numerical study of the thermal and fluid flow effects of heat rejection to the surface layer of a salt-gradient solar pond, by means of a recirculating thermal discharge, is carried out. The use of solar ponds for power generation involves heat rejection, for which the surface zone may be employed. However, it is very important to determine the effect of the discharge of hot fluid on the temperature field in the surface zone and on the stability of the non-convective zone, which lies between the surface and storage zones. Of particular interest is the dependence of this flow on the inflow conditions, on heat loss at the surface and on the inflow-outflow configuration. The downward penetration of the flow is strongly governed by the buoyancy effects, and the study considers both the transient and the steady-state circumstances. The effect of the surface energy loss and of the conductive heat gained from below the surface zone is also studied. The flow is found to be strongly dependent on the inflow and outflow conditions and on the surface heat loss. The disturbance to the nonconvective zone is also studied. The basic physical processes involved are considered in detail, and the relevance of the results obtained in the design of the corresponding recirculating flow is outlined.
Adiabatic Film Cooling Effectiveness From Heat Transfer Measurements in Compressible, Variable-Property Flow
Authors: Ligrani P. M. , Camci C.
Published: October 2009

A variable property correction is given for turbulent boundary layers that are film-cooled using staggered rows of injection holes inclined at 35 deg. With the correction, a relation is provided between the adiabatic film cooling effectiveness for constant property flow and heat transfer coefficients for variable property flow, which are based on the difference between the freestream recovery temperature and wall temperature. The variable property correction was determined from heat transfer measurements for a range of injection parameters at different values of the nondimensional coolant temperature and from results in the literature. Because the flow is compressible, the importance of the injection mass flux ratio, momentum flux ratio, and velocity ratio are considered in the determination of effectiveness.
Internal, Shellside Heat Transfer and Pressure Drop Characteristics for a Shell and Tube Heat Exchanger
Authors: Sparrow E. M. , Perez J. A.
Published: October 2009

Per-tube heat transfer coefficients and per-compartment and intracompartment pressure drops were measured on the shell side of a shell and tube heat exchanger. The main focus of the work was to determine the response of these quantities to variations in the size of the baffle window; the Reynolds number was also varied parametrically. The pressure measurements showed that the fluid flow is fully developed downstream of the first compartment of the heat exchanger and that the per-compartment pressure drop is constant in the fully developed regime. Within a compartment, the pressure drop in the upstream half is much larger than that in the downstream half. The per-tube heat transfer coefficients vary substantially within a given compartment (on the order of a factor of two), giving rise to a nonuniform thermal loading of the tubes. Row-average and compartment-average heat transfer coefficients were also evaluated. The lowest row-average coefficients were those for the first and last rows in a compartment, while the highest coefficient is that for the row just upstream of the baffle edge. It was demonstrated that the per-tube heat transfer coefficients are streamwise periodic for a module consisting of two consecutive compartments.
Transient Heat Flux Measurements in a Divided-Chamber Diesel Engine
Authors: Alkidas A. C. , Cole R. M.
Published: October 2009

Transient surface heat flux measurements were performed at several locations on the cylinder head of a divided-chamber diesel engine. The local heat flux histories were found to be significantly different. These differences are attributed to the spatial nonuniformity of the fluid motion and combustion. Both local time-averaged and local peak heat fluxes decreased with decreasing speed and load. Retarding the combustion timing beyond TDC decreased the peak heat flux in the antechamber but increased the peak heat flux in the main chamber. This is attributed to the relative increase in the portion of fuel that burns in the main chamber with retarded combustion timing.
Inclination-Induced Direct-Contact Melting in a Circular Tube
Authors: Sparrow E. M. , Myrum T. A.
Published: October 2009

Melting of a phase-change medium encapsulated in a circular tube was investigated experimentally at a succession of tube inclinations relative to the vertical. It was found that repeatable and continuous direct contact of the melting solid with the tube wall was established at an inclination of about 15 deg and that further increases in inclination had virtually no effect on the melting results. Direct contact gave rise to substantial enhancements (up to a factor of two) in the amount of melted mass and in the energy transfer, relative to those for natural-convection-dominated melting (e.g., for the vertical tube case). The energy transfer was subdivided into two components, the latent heat supplied to the melting interface and the sensible heat stored in the liquid melt, and a general correlation of the two components was achieved. With regard to the pattern of melting, it was observed that the continuous-contact mode was preceded by a period of partial contact. During that period, the solid was tipped toward the wall while pivoting about its lower edge.
Analysis of Freeze Coating on a Nonisothermal Moving Plate by a Perturbation Method
Authors: Cheung F. B.
Published: October 2009

The behavior of a frozen layer (or “freeze coat”) on the surface of a chilled flat plate traveling with a constant velocity through a warm liquid bath is studied analytically. A perturbation technique is employed to solve the equations governing the shape and temperature of the freeze coat, taking full account of the axial variation of the plate temperature and heat convection from the warm liquid to the moving object. Unlike the case of an isothermal plate where the freeze-coat thickness increases monotonically along the axial direction, here the freeze coat is found to grow only within a limited distance from the inlet. Beyond this distance, the freeze coat begins to decay through remelting. The location at which remelting occurs and the maximum freeze-coat thickness are determined displaying the principal effects of three controlling dimensionless parameters. Criteria for selection of the optimum freeze-coating operation conditions are established and discussed.
Thermal Stability of a Vertical Fluid Layer With Volumetric Energy Source
Authors: Shaaban A. H. , O¨zis¸ik M. M.
Published: October 2009

The effects of nonuniform volumetric energy sources on the thermal stability of natural convection in a vertical fluid layer are investigated using the linear theory of stability. The neutral states of stability are obtained for four values of Prandtl number, ranging from 0.71 to 100, and for different values of β in the range 0.1 ≤ β ≤ 10. The critical parameters and the energetics of the critical disturbances are determined for each case. For all values of Prandtl number and β studied here, the critical instabilities set in as traveling waves, moving in the direction of gravity. In the low range of Prandtl number, 0.71 to 5, the critical curves approach the same asymptotic value for β ≥ 10. The critical disturbances are found to be buoyancy driven for all Prandtl numbers and β considered here, except for the low range of Prandtl number, for which they become shear driven disturbances as β increases.
Pool Boiling Heat Transfer in Narrow Horizontal Annular Crevices
Authors: Hung Ying-Huei , Yao Shi-Chune
Published: October 2009

Experimental results of the pool boiling in horizontal narrow annuli are reported. The effects of fluid properties, pool subcooling, crevice length, and gap size on the boiling behavior and the critical heat flux (CHF) are also studied. The CHF decreases with decreasing gap size or increasing length of the annuli. The lower CHF of narrow crevices may be explained by the thin film evaporation. A semi-empirical correlation is established for the CHF of pool boiling in horizontal confined spaces. This correlation is compared with the CHF data of the present experiment. Satisfactory agreement is obtained.
Axially Varying Vapor Superheats in Convective Film Boiling
Authors: Evans D. , Webb S. W. , Chen J. C.
Published: October 2009

Axially varying vapor superheats in convective film boiling have been measured for water flowing in a vertical tube at low to moderate pressures and mass flow rates. Using a slow “reflood” process, measurements of wall temperature and nonequilibrium vapor temperature were obtained as functions of distance from the quench front. With the low quench front velocity, the time required to progapate the front a few millimeters corresponds to many fluid residence times, and the thermal hydraulic data thus obtained are quasi-steady state. These experimental results indicate a zone near the quench front where the vapor generation rate is relatively high, followed by a far zone where the generation rate drops off to a relatively low magnitude. The data obtained agree with the very limited previously reported steady-state data. Comparison with existing heat transfer models shows the models give poor predictions of vapor superheats but reasonable predictions of wall heat fluxes.
Choking and Optimization of Extended Surface Arrays
Authors: Kraus A. D. , Snider A. D.
Published: October 2009

When individual extended surface components such as fins or spines are assembled into an array, the performance of the array may become limited by the performance of the fin closest to the base of the array. This paper draws upon previous work of the authors and provides guidance for the selection of the width or diameter of the innermost fin or spine.
Transient Simultaneous Condensation and Melting of a Vertical Surface
Authors: Galamba D. , Dhir V. K.
Published: October 2009

Transient melting of a vertical wall as a result of condensation of saturated vapor is investigated analytically. Using a Nusselt-type analysis the equations governing the laminar melt and condensate films are solved by a combination of analytical and numerical finite-differencing techniques. The solutions in the axial distance-time plane fall into three regions: the piling region where the melt and condensate thicknesses are only functions of time, the Goursat region where the thicknesses are functions of both the axial distance and time, and the steady-state region where the film thicknesses are only functions of the axial distance. The time to achieve a steady-state condition is obtained analytically.
On the Oscillatory Instability of Closed-Loop Thermosyphons
Authors: Chen K.
Published: October 2009

The stability of natural convection flows in single-phase closed-loop thermosyphons is investigated. The thermosyphons considered in the present analysis are fluid-filled tubes bent into rectangular shapes. The fluid is heated over the lower horizontal segment and cooled over the upper horizontal segment. Analytical and numerical solutions are presented for a range of loop aspect ratios and radii for both laminar and turbulent flows. It is found that the steady-state results for thermosyphons with different aspect ratios and radii can be expressed in terms of a single dimensionless parameter. When this parameter is less than a critical value, the flow is always stable. Above this critical point, oscillatory instability exists for a narrow range of a friction parameter. The calculated neutral stability conditions show that the flow is least stable when the aspect ratio of the loop approaches unity. The frequency of the convection-induced oscillation is slightly higher than the angular frequency of a fluid particle traveling along the loop.
Augmentation of Laminar Flow Heat Transfer in Tubes by Means of Wire Coil Inserts
Authors: Uttarwar S. B. , Raja Rao M.
Published: October 2009

Experimental studies have been carried out on isothermal pressure drop and convective heat transfer to servotherm medium grade oil in laminar flow in one smooth tube and seven wire coil-inserted tubes of varying wire diameter and pitch of wire coil. The performance of these wire coil inserted tubes has been evaluated by two different criteria based on the objective of either maximizing heat transfer rate or minimizing exchanger size. The results indicated that as much as fourfold improvement can be obtained in laminar flow heat transfer coefficient using these tubes.
Effect of Nonuniform Heating on Laminar Mixed Convection in Ducts
Authors: Law Hin-Sum , Masliyah J. H. , Nandakumar K.
Published: October 2009

Combined convection flow in square, circular, and semicircular ducts was numerically studied for axially uniform heat flux. The peripheral condition is that of the top half of the duct is insulated while the bottom half is heated. For the case of a square duct, dual solutions are obtained where the secondary flow is characterized either with two or four vortices. Regions of dual solutions have been mapped in the parameter space of aspect ratio and Grashof number. For the case of a circular duct, only the solution characterized by two vortices is obtained.
Spectral and Total Radiation Properties of Turbulent Hydrogen/Air Diffusion Flames
Authors: Gore J. P. , Jeng S.-M. , Faeth G. M.
Published: October 2009

A study of the structure and radiation properties of round turbulent hydrogen/air diffusion flames is described. Measurements were made of mean and fluctuating streamwise velocity, mean temperatures, species concentrations, spectral radiation intensities, and radiant heat fluxes. The measurements were used to evaluate predictions based on the laminar flamelet concept and narrow-band radiation models both ignoring (using mean properties) and considering (using a stochastic method) effects of turbulence/radiation interactions. State relationships found by correlating auxiliary measurements in laminar flames proved to be almost equivalent to conditions for local thermodynamic equilibrium. Structure and radiation predictions were reasonably good for present test conditions. Effects of turbulence/radiation interactions were significant for these flames, causing almost a 100 percent increase in spectral radiation intensities, in comparison to mean property predictions, upstream of the flame tip.
Selective Radiative Preheating of Aluminum in Composite Solid Propellant Combustion
Authors: Brewster M. Q. , Patel R.
Published: October 2009

A two-phase model has been developed to study aluminum (Al) particle preheating through selective radiation absorption in composite solid propellants. The two phases considered are one strongly absorbing particle (Al) phase and another weakly absorbing matrix (ammonium perchlorate (AP), binder and catalyst particle) phase surrounding the Al phase. Separate energy balance equations for the Al and matrix phases are developed. Both the matrix and the Al phase are assumed to be nonemitting, anisotropically scattering, absorbing media. The parameters identified which strongly influence Al preheating and melting are Al size, mass fraction, burn rate, and level of incident radiant flux. It was found that large Al mass fractions and small Al particle sizes promote lower Al temperatures. The effect of adding submicron iron oxide burn rate catalyst particles on aluminum preheating was also investigated. It was found that the addition of small amounts of catalyst can theoretically reduce Al temperatures significantly by dominating the optical properties. These results should prove useful to propellant formulators in trying to reduce the problem of unwanted Al agglomeration.
Application of a Multifield Model to Reflooding of a Hot Vertical Tube: Part 1—Model Structure and Interfacial Phenomena
Authors: Kawaji M. , Banerjee S.
Published: October 2009

This is the first of a series of two papers reporting on a study of reflooding of a hot vertical tube. A mechanistic model of reflood is developed using a multifield modeling approach to analyze experimental data reported previously [1]. In Part 1 of this paper, the mathematical model for the thermohydraulic processes during reflood is derived from the general two-field conservation equations, and its structure is examined. Linear stability of the equation system is analyzed incorporating consideration of phase pressure differences. For inverted annular flow, the system is stable to short-wavelength perturbations and captures long-wavelength interfacial instabilities. The length of the most unstable waves is also derived in the analysis and agrees well with the available data. For dispersed flow, the system is predicted to become unstable if the Weber number exceeds a critical value of 8. In Part 2 [2], constitutive relations for reflood are fomulated and model is numerically solved for comparison with experimental data.
Exponential Wake Structure of Heated Turbulent Boundary Layers at Elevated Levels of Free-Stream Turbulence
Authors: Sepri P.
Published: October 2009

The wake region of a turbulent boundary layer is demonstrated to exhibit simple exponential behavior at elevated levels of free-stream turbulence. As a predictive tool, the computer code STANCOOL has been modified to include FST effects in heated turbulent boundary layers. Preliminary comparisons with experimental data indicate improvements in computational capability, although further development of the code is required. From these comparisons, three new results are offered: (1) At elevated levels of FST, several statistical profiles in the boundary layer wake region decay exponentially into the free stream; (2) v′T′ decays at half the rate of the mean velocity and temperature; (3) analytical expressions are provided for u′v′ and v′T′ in this case.
Natural Convection Flow and Heat Transfer Between a Fluid Layer and a Porous Layer Inside a Rectangular Enclosure
Authors: Beckermann C. , Ramadhyani S. , Viskanta R.
Published: October 2009

A numerical and experimental study is performed to analyze the steady-state natural convection fluid flow and heat transfer in a vertical rectangular enclosure that is partially filled with a vertical layer of a fluid-saturated porous medium. The flow in the porous layer is modeled utilizing the Brinkman–Forchheimer–extended Darcy equations. The numerical model is verified by conducting a number of experiments, with spherical glass beads as the porous medium and water and glycerin as the fluids, in rectangular test cells. The agreement between the flow visualization results and temperature measurements and the numerical model is, in general, good. It is found that the amount of fluid penetrating from the fluid region into the porous layer depends strongly on the Darcy (Da) and Rayleigh (Ra) numbers. For a relatively low product of Ra × Da, the flow takes place primarily in the fluid layer, and heat transfer in the porous layer is by conduction only. On other hand, fluid penetrating into a relatively highly permeable porous layer has a significant impact on the natural convection flow patterns in the entire enclosure.
An Improved Approach to Conductive Boundary Conditions for the Rayleigh-Be´nard Instability
Authors: Lienhard J. H.
Published: October 2009

A technique is developed for predicting the stability limit of conductively coupled horizontal fluid layers heated from below and cooled above. The approach presented gives exact solutions of the stability problem and is numerically much simpler than previous multilayer solutions. Critical Rayleigh numbers are obtained for the case of three and four fluid layers separated by equally spaced identical midlayers of various thicknesses and conductivities with isothermal outer walls and for the symmetric two-layer problem with outer walls of finite thermal conductivity. Other configurations are considered briefly.
Effect of Insulated/Uninsulated Channel Walls on Heat Transfer From a Horizontal Finned Tube in a Vertical Channel
Authors: Sparrow E. M. , Ansari M. A.
Published: October 2009

Measurements were made of the combined natural convection and radiation heat transfer from a horizontal finned tube situated in a vertical channel open at the top and bottom. In one set of experiments, both walls of the channel were heavily insulated, while in a second set of experiments, one of the insulated walls was replaced by an uninsulated metallic sheet. In general, the heat transfer coefficients were found to be lower with the metal wall in place, but only moderately. With the finned tube situated at the bottom of the channel, the differences in the heat transfer coefficients corresponding to the two types of walls were only a few percent. When the tube was positioned at the mid-height of the channel, larger differences were encountered, but in the practical range of Rayleigh numbers, the differences did not exceed 5 percent.
Experimental and Numerical Studies of Cold Inflow at the Exit of Buoyant Channel Flows
Authors: Modi Vijay , Torrance K. E.
Published: October 2009

Experimental and numerical studies of the separation of a smooth attached buoyant flow from the inner wall of a duct, as the duct discharges into a quiescent environment, are reported. The associated penetration of neutrally buoyant ambient fluid into the duct is called cold inflow. The experimental study was carried out for air flows over ranges of Reynolds and Froude numbers, based on duct radius, of Re = 2400 to 3300 and Fr = 0.68 to 2.69. The experiments provide information on the onset and extent of cold inflow in a turbulent flow regime. Spatial profiles of fluctuating temperature reveal a wedge-shaped cold inflow region at the wall near the exit when Fr is decreased below a critical value. The numerical study examines the influence of Re and Fr on the structure of the cold inflow phenomenon at moderate Reynolds numbers (Re = 200 to 500 and Fr = 1 to 5). Steady-state, two-dimensional, laminar flow solutions reveal a region of downward-flowing cold air near the wall of the duct which leads to premature separation of the wall boundary layer. The separated boundary layer merges into the buoyant jet above the duct exit.
Steady, Two-Dimensional, Natural Convection in Rectangular Enclosures With Differently Heated Walls
Authors: Chen K. S. , Ho J. R. , Humphrey J. A. C.
Published: October 2009

Numerical results are presented for steady natural convection in two-dimensional rectangular enclosures in which the side walls, top wall, and bottom wall are at uniform temperatures θs, θt, and θb, respectively, and θs > θt > θb. Raylight numbers ranging from 104 to 107 and aspect ratios of 1 and 1.5 were investigated. The top wall was modeled as an impermeable rigid surface or an impermeable free-moving boundary. The calculations reveal two flow regions. In the upper part of the enclosure two large counterrotating cells appear, separated by a descending plume of fluid. Near the bottom wall the flow is almost motionless and stably stratified. The temperature in the central portion of the enclosure is almost uniform due to mixing by the recirculating cells. A temperature inversion occurs near the top wall and is particularly noticeable at high Rayleigh numbers. At high Rayleigh numbers the flow breaks up into smaller cells. The result is that each main recirculation region develops a secondary counterrotating eddy within it. The condition of a free surface as the top wall boundary condition significantly affects the circulation and heat transfer throughout the flow domain. Numerical experiments reveal the extent to which the flow field in the enclosure is affected by an asymmetric specification of side-wall temperature boundary conditions.
Scale Modeling of Radiation in Enclosures With Absorbing/Emitting and Isotropically Scattering Media
Authors: Liu Hsing-Pang , Howell J. R.
Published: October 2009

Exchange factor analysis has been shown to be an alternative to zonal analysis in enclosures with participating media. An experimental measurement of exchange factors for a cubic enclosure has been done, and practical problems associated with the measurements are discussed. The effects of isotropic scattering and absorption-isotropic re-emission processes are known to be the same for their contribution to energy exchange paths in systems with equal optical thickness. The measurement of these exchange factors is achieved by using an enclosure containing a near-isotropically scattering medium. The heat transfer results for the pure radiation case are demonstrated by using these experimentally determined exchange factors in the analysis, and are compared with the analytical results.
High-Pressure Transition Boiling in Internal Flows
Authors: France D. M. , Chan I. S. , Shin S. K.
Published: October 2009

Transition boiling heat transfer was studied for the case of internal flow of high-pressure water. Transition boiling results were determined from measurements from 138 steady-state tests conducted in a temperature-controlled facility. Water flowed vertically upward inside a tube with inside diameter = 10.1 mm and vertical length = 13.1 m, as it was heated by liquid sodium flowing countercurrent in a surrounding annulus. The water parameter range of the experiments was pressure 7–15.3 MPa and mass flux 0.7–3.2 Mg/m2•s. A mathematical model was developed that predicted the transition boiling heat transfer coefficient well over the entire parameter range.
Dehumidification: On the Correlation of Wet and Dry Transport Processes in Plate Finned-Tube Heat Exchangers
Authors: Eckels P. W. , Rabas T. J.
Published: October 2009

The transverse velocity of the condensing phase during dehumidification is analogous to the transverse velocity at the wall when exercising boundary layer control by fluid extraction through a permeable wall. Wet and dry pressure drop and heat transfer rates are analyzed for correlation using boundary layer suction theory. Data are presented for flat-plate finned-tube heat exchangers during air heating and dehumidification operations and the data show a significant effect of transverse velocity correlated by the boundary layer suction formulation. The condensate film is considered isothermal in this analysis and the results indicate that an improved modeling of the condensate film is required. We find that the transverse velocity of the condensing phase has an important effect on transport phenomena during dehumidification and that the validity of the Chilton–Colburn heat and mass transfer analogy in describing dehumidification is supported by these results. It should be noted that the dry data form the beginning of a plate fin heat exchanger data base. The present data show the effect of tube diameter and, independently, fin density variation on the Colburn and friction factors with all other geometric parameters held invariant.
On the Calculation of Flow and Heat Transfer Characteristics for CANDU-Type 19-Rod Fuel Bundles
Authors: Yueh Yuh-Shan , Chieng Ching-Chang
Published: October 2009

A numerical study is reported of flow and heat transfer in a CANDU-type 19 rod fuel bundle. The flow domain of interest includes combinations of triangular, square, and peripheral subchannels. The basic equations of momentum and energy are solved with the standard k–ε model of turbulence. Isotropic turbulent viscosity is assumed and no secondary flow is considered for this steady-state, fully developed flow. Detailed velocity and temperature distributions with wall shear stress and Nusselt number distributions are obtained for turbulent flow of Re = 4.35 × 104, 105, 2 × 105, and for laminar flow of Re ∼ 2400. Friction factor and heat transfer coefficients of various subchannels inside the full bundle are compared with those of infinite rod arrays of triangular or square arrangements. The calculated velocity contours of peripheral subchannel agreed reasonably with measured data.
Finite-Volume Solutions for Laminar Flow and Heat Transfer in a Corrugated Duct
Authors: Asako Y. , Faghri M.
Published: October 2009

A finite volume methodology was developed to predict fully developed heat transfer coefficients, friction factors, and streamlines for flow in a corrugated duct. The basis of the method is an algebraic coordinate transformation which maps the complex fluid domain onto a rectangle. The method can be adopted for other convection-diffusion problems in which two boundaries of the flow domain do not lie along the coordinate lines. Representative results were found for laminar flow, uniform wall temperature, and for a range of Reynolds number, Prandtl number, corrugation angle, and dimensionless interwall spacing. As seen from the streamlines, the flow patterns are highly complex including large recirculation zones. The pressure drops and friction factor results are higher than the corresponding values for a straight duct. Finally, the performance of the corrugated duct was compared with the straight duct under three different constraints—fixed pumping power, fixed pressure drop, and fixed mass flow rate. There are small differences in the heat transfer rate ratios under these constraints.
Vorticity-Velocity Method for the Graetz Problem and the Effect of Natural Convection in a Horizontal Rectangular Channel With Uniform Wall Heat Flux
Authors: Chou F. C. , Hwang G. J.
Published: October 2009

Numerical solutions given by a vorticity-velocity method are presented for combined free and forced laminar convection in the thermal entrance region of a horizontal rectangular channel without the assumptions of large Prandtl number and small Grashof number. The channel wall is heated with a uniform wall heat flux. Typical developments of temperature profile, secondary flow, and axial velocity at various axial positions in the entrance region are presented. Local friction factor and Nusselt number variations are shown for Rayleigh numbers Ra = 104, 3×104, 6×104, and 105 with the Prandtl number as a parameter. The solution for the limiting case of large Prandtl number and small Grashof number obtained from the present study confirms the data of existing literature. It is observed that the large Prandtl number assumption is valid for Pr = 10 when Ra ≤ 3×104 but for a larger Prandtl number when the Rayleigh number is higher.
Mixed Convection Along Vertical Cylinders and Needles With Uniform Surface Heat Flux
Authors: Lee S. L. , Chen T. S. , Armaly B. F.
Published: October 2009

Mixed convection along vertical cylinders and needles with uniform surface heat flux is investigated for the entire mixed convection regime. A single modified buoyancy parameter χ and a single curvature parameter Λ are employed in the analysis such that a smooth transition from pure forced convection (χ = 1) to pure free convection (χ = 0) can be accomplished. For large values of the curvature parameter and/or Prandtl number, the governing transformed equations become stiff. Thus, a numerically stable finite-difference method is employed in the numerical solution in conjunction with the cubic spline interpolation scheme to overcome the difficulties that arise from the stiffness of the equations. Local Nusselt numbers are presented for 0.1 ≤ Pr ≤ 100 that cover 0 ≤ χ ≤ 1 (∞ ≥ Ωχ ≥ 0) and 0 ≤ Λ ≤ 50. For needles (Λ ≥ 5), the local Nusselt numbers Nuχ/(Reχ1/2 + Grχ*1/5) are found to be nearly independent of the buoyancy parameter χ. Correlation equations for the local Nusselt numbers are also presented.
Condensation on Coolant Jets and Sheets Including the Effects of Noncondensible Gases
Authors: Jacobs H. R. , Nadig R.
Published: October 2009

The study of condensation of a pure vapor or a mixture of a vapor and a non-condensible gas on a laminar cylindrical jet or a falling sheet can provide the necessary information for designing cascade, curtain, or jet-type direct contact condensers. Prior to the present work the only theoretical solutions were based on the solution of the Graetz problem and were thus restricted to problems where the Jakob number is zero. Using an integral formulation of the governing equations, an extension is made which provides results for practical values of both the Jakob and Graetz numbers. For the case where noncondensibles are present, detailed results are presented for the steam-air system.
Perturbation Solutions for the Shape of a Solidification Interface Subjected to a Spatially Periodic Heat Flux
Authors: Sen A. K.
Published: October 2009

In the manufacture of certain high-technology materials (such as strengthened composites for turbine and compressor blades) from a melt, it is essential to control the heat transfer conditions in such a way that a planar solidification interface is maintained within a given tolerance. Spatial nonuniformities in the heat flux imposed along the solidification interface may cause deformation. In this paper we analyze the interfacial shapes resulting from a periodic heat flux variation along the interface. Two cases of practical interest are examined. In case (a) the amplitude of heat flux variation is assumed small compared to the mean value, and in case (b) the wavelength of the imposed heat flux variation is considered to be much larger than a characteristic length in the transverse direction. In both cases, sensitivity of the interface shape with respect to heat flux nonuniformities is also determined.
Forced Convective Heat Transfer in Curved Diffusers
Authors: Rojas J. , Whitelaw J. H. , Yianneskis M.
Published: October 2009

Measurements of the velocity characteristics of the flows in two curved diffusers of rectangular cross section with C and S-shaped centerlines are presented and related to measurements of wall heat transfer coefficients along the heated flat walls of the ducts. The velocity results were obtained by laser-Doppler anemometry in a water tunnel and the heat transfer results by liquid crystal thermography in a wind tunnel. The thermographic technique allowed the rapid and inexpensive measurement of wall heat transfer coefficents along flat walls of arbitrary boundary shapes with an accuracy of about 5 percent. The results show that an increase in secondary flow velocities near the heated wall causes an increase in the local wall heat transfer coefficient, and quantify the variation for maximum secondary-flow velocities in a range from 1.5 to 17 percent of the bulk flow velocity.
Forced Convection in a Channel Filled With Porous Medium, Including the Effects of Flow Inertia, Variable Porosity, and Brinkman Friction
Authors: Poulikakos D. , Renken K.
Published: October 2009

This paper presents a series of numerical simulations which aim to document the problem of forced convection in a channel filled with a fluid-saturated porous medium. In modeling the flow in the channel, the effects of flow inertia, variable porosity and Brinkman friction are taken into account. Two channel configurations are investigated: parallel plates and circular pipe. In both cases, the channel wall is maintained at constant temperature. It is found that the general flow model predicts an overall enhancement in heat transfer between the fluid/porous matrix composite and the walls, compared to the predictions of the widely used Darcy flow model. This enhancement is reflected in the increase of the value of the Nusselt number. Important results documenting the dependence of the temperature and flow fields in the channel as well as the dependence of the thermal entry length on the problem parameters are also reported in the course of the study.
Stability of Thermal Convection in a Vertical Porous Layer
Authors: Kwok L. P. , Chen C. F.
Published: October 2009

Experiments were carried out to study the stability of thermal convection generated in a vertical porous layer by lateral heating in a tall, narrow tank. The porous medium, consisting of glass beads, was saturated with distilled water. It was found that the flow became unstable at a critical ΔT of 29.2°C (critical Rayleigh number of 66.2). Linear stability analysis was applied to study the effects of the Brinkman term and of variable viscosity separately using a quadratic relationship between the density and temperature. It was found that with the Brinkman term, no instability could occur within the allowable temperature difference across the tank. With the effect of variable viscosity included, linear stability theory predicts a critical ΔT of 43.4°C (Rayleigh number of 98.3).
Natural Convection in a Vertical Annulus Containing Water Near the Density Maximum
Authors: Lin D. S. , Nansteel M. W.
Published: October 2009

Steady natural convection of water near the density extremum in a vertical annulus is studied numerically. Results for flow in annuli with aspect ratio 1≤A≤8 and varying degrees of curvature are given for 103≤Ra≤105. It is shown that both the density distribution parameter R and the annulus curvature K have a strong effect on the steady flow structure and heat transfer in the annulus. A closed-form solution for the vertical flow in a very tall annulus is compared with numerical results for finite-aspect-ratio annuli.
An Analytical and Experimental Study of Falling-Film Evaporation on a Horizontal Tube
Authors: Chyu M.-C. , Bergles A. E.
Published: October 2009

Heat transfer for saturated falling-film evaporation on a horizontal smooth tube has been studied through analysis and experimentation to investigate the effects of film flow rate, liquid feed height, and wall superheat. Two models have been proposed, both based upon three defined heat transfer regions: the jet impingement region, the thermal developing region, and the fully developed region. The model that uses an experimental correlation for heat transfer in the fully developed region compares favorably with experimental data and other predictions reported for the case where the liquid feed is in a sheet.
Critical Heat Flux in Saturated Forced Convection Boiling on a Heated Disk With an Impinging Jet
Authors: Monde M.
Published: October 2009

Critical heat flux during forced convection boiling on an open heated disk being supplied with saturated liquids through a small round jet which impinges at the center of the disk has been studied experimentally employing refrigerant R12 at comparatively high pressures from 0.6 to 2.8 MPa. Generalized correlations, predicting the CHF within an experimental range of liquid-to-vapor density ratio 5.3–41.25 and the reciprocal of Weber number 2 × 10−3–2 × 10−7, are given for three different characteristic regimes: V-regime where the CHF increases with an increase in the jet velocity, I-regime where the CHF is nearly constant with jet velocity, and HP-regime where the CHF appears only at high pressure and again rises with an increase in the jet velocity.
Transient Natural Convection Between Two Zones in an Insulated Enclosure
Authors: Litsek P. A. , Bejan A.
Published: October 2009

The natural convection flow and heat transfer between two enclosures that communicate through a vertical opening is studied by considering the evolution of an enclosed fluid in which the left half is originally at a different temperature than the right half. Numerical experiments show that at sufficiently high Rayleigh numbers the ensuing flow is oscillatory. This and other features are anticipated on the basis of scale analysis. The time scales of the oscillation, the establishment of thermal stratification, and eventual thermal equilibrium are determined and tested numerically. At sufficiently high Rayleigh numbers the heat transfer between the communicating zones is by convection, in accordance with the constant-Stanton-number trend pointed out by Jones and Otis (1986). The range covered by the numerical experiments is 102 < Ra < 107, 0.71 < Pr < 100, and 0.25 < H/L < 1.
Rectangular Heat Source on a Semi-infinite Solid—An Analysis for a Thin Film Heat Flux Gage Calibration
Authors: Keltner N. R. , Bainbridge B. L. , Beck J. V.
Published: October 2009

For a thin film heat flux gage designed to provide both rapid response and long use time, initial calibrations using standard techniques indicated differences between the literature and the estimated properties. In order to estimate thermophysical properties and subsequently the gage sensitivity, an analytical model of the response to a step change in heating current was developed. Starting from a Green’s function description, the model is reduced to three algebraic expressions, which correspond to the early, middle, and late time regimes. These expressions provide a framework for least-squares estimates of gage parameters. This provides an in-situ, nondestructive measurement of the thermal impedance of the substrate. There is very good agreement between the model and the experimental data. The estimated parameter values demonstrated good to excellent repeatability and good agreement with both new literature data and results from destructive property measurements.
Experimental Investigation of Laminar Mixed Convection in Tubes With Longitudinal Internal Fins
Authors: Rustum I. M. , Soliman H. M.
Published: October 2009

Experiments were performed to study the pressure drop and heat transfer characteristics for laminar flow in a smooth tube and four tubes with internal longitudinal fins, with emphasis on showing how the experimental results relate to previous analytical predictions. Measured quantities include the fully developed friction factor, local and fully developed Nusselt numbers. Good agreements were obtained between the friction factor results and previous analytical predictions, and between Nusselt number results for the smooth tube and previous experiments. Free convection is shown to have a strong influence on heat transfer in finned tubes and the results approach the forced-convection predictions as Rayleigh number decreases. Internal fins appear to retard the onset of significant free convective currents; however, once initiated, a faster rate of heat transfer enhancement occurs compared to the smooth tube. An empirical correlation of fully developed Nusselt number is presented taking into account the influences of tube geometry.
Mixed Convection in Horizontal Porous Layers Heated From Below
Authors: Prasad V. , Lai F.-C. , Kulacki F. A.
Published: October 2009

Numerical studies are reported for steady, mixed convection in two-dimensional horizontal porous layers with localized heating from below. The interaction mechanism between the forced flow and the buoyant effects is examined for wide ranges of Rayleigh number Ra* and Peclet number Pe*. The external flow significantly perturbs the buoyancy-induced temperature and flow fields when Pe* is increased beyond unity. For a fixed Peclet number, an increase in Rayleigh number produces multicellular recirculating flows in a domain close to the heat source. This enhances heat transfer by free convection. However, for a fixed Ra*, an increase in forced flow or Peclet number does not necessarily increase the heat transfer rate. Hence, there exists a critical Peclet number as a function of Ra* for which the overall Nusselt number is minimum. The heat transfer is, generally, dominated by the buoyant flows for Pe* < 1 whereas the contribution of free convection is small for Pe* > 10 when Ra* ≤ 10.
Combined Heat Transfer in a Semitransparent Multilayer Packed Bed
Authors: Flamant G. , Menigault T. , Schwander D.
Published: October 2009

Combined radiative, conductive, and forced convective heat transfer through a packed bed composed of two spectrally dissimilar slabs of particles is analyzed on the basis of the two-flux model. Taking into account the variation of absorption and scattering of the layers in the visible and infrared spectral ranges, the selectivity of such a system is proved theoretically. Experiments are run using a bed composed of glass and silicon carbide particles as a solar absorber for gas heating. Good agreement is shown in the comparison of predictions with experimental data.
Rapid, Steady-State Measurement of the Effective Diffusion Coefficient of Gases in Closed-Cell Foams
Authors: Ostrogorsky A. G. , Glicksman L. R.
Published: October 2009

A rapid steady-state technique was developed to measure the effective permeability and diffusion coefficients of closed-cell foam insulation. To test the new technique, N2 data were first obtained by the long-term steady-state technique, and then reproduced ten times faster by the rapid steady-state technique. By using the new technique, reference values of effective diffusion coefficients of N2, O2, and Fluorocarbon 11 in closed-cell polyurethane foams were obtained at different temperatures. Data for Fluorocarbon 11 were obtained 30 times faster than data could be obtained by long-term steady-state tests. To estimate when steady-state has been achieved, the transient diffusion equation was solved, and the solution was given in the form of a chart. The time needed to achieve steady-state mass flux in a foam sample was found to depend strongly on the ratio of the partial pressures imposed on the surface of a tested sample. By use of the solution, the value of the foam effective diffusion coefficient can be obtained before steady-state conditions are achieved within the sample.