bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Investigation of a Flexible Bellows Heat Pipe for Cooling Discrete Heat Sources
Authors: Babin B. R. , Peterson G. P.
Published: May 2008

A computer model was developed to aid in the design of a flexible bellows heat pipe for cooling small discrete heat sources or arrays of small heat sources. This model was used to evaluate the operational characteristics and performance limitations of these heat pipes and to formulate and optimize a series of conceptual designs. Three flexible bellows heat pipes approximately 40 mm in length and 6 mm in diameter were constructed and tested using three different wick configurations. The test pipes were found to be boiling limited over most of the operating temperature range tested. Heat fluxes in excess of 200 W/cm2 were obtained and thermal resistance values of less than 0.7 °C/W were measured. Although the computer model slightly underestimated the experimentally determined transport limit for one of the wicking configurations, the remaining transport predictions were consistently within 8 percent of the experimental values.
Convection in the Cavity Formed Between Two Cylindrical Rollers
Authors: Litsek P. A. , Bejan A.
Published: May 2008

This paper describes the flow and temperature distribution in the hourglass-shaped cavity formed between two rollers and two parallel runner surfaces. The cavity fluid is heated uniformly over the roller surfaces, and cooled along the two runners, which are modeled as isothermal. The focus is on the characteristics of the flow (the Reynolds number effect) and the temperature distribution in the fluid, especially at the roller surface. These features are determined numerically. The results document the extent to which the Reynolds number, Prandtl number, geometric aspect ratio, and viscous dissipation number influence the steady-state temperature of the roller surface. It is shown that at Reynolds numbers greater than 100, the cooling effect that the cavity fluid provides to the roller approaches the behavior anticipated based on laminar forced-convection boundary layer theory.
Natural Convection From a Heat Source in a Top-Vented Enclosure
Authors: Myrum T. A.
Published: May 2008

Natural convection from a heated disk situated at the bottom of a top-vented enclosure was studied experimentally. The experiments were performed in water (Pr ≅ 5) for parametric variations of the vent opening size, inner enclosure height, and disk-to-enclosure-wall tempearture difference (Rayleigh number). For comparison purposes, baseline data were obtained for an unvented enclosure and for the “infinite” case (no enclosure). The heat transfer data were supplemented by cross-vent temperature measurements and by flow visualization using the thymol-blue electrochemical technique. The experiments demonstrated that, for the range of parameters considered, the average Nusselt numbers could be correlated using a single correlation to within 8 percent. It was also found that the presence of the enclosure (vented or unvented) acted to reduce the Nusselt number, especially at the lower Rayleigh numbers. Flow visualization experiments revealed an unstable flow pattern in the vicinity of the vent that fluctuated in a nonperiodic manner between four basic modes. Temperature measurements revealed asymmetric mean cross-vent temperature profiles, with the mean temperature level increasing with decreasing vent size. The intensity of the temperature fluctuations in the vent opening also increased with decreasing vent size.
Steady-State Natural Convection in Empty and Partitioned Enclosures at High Rayleigh Numbers
Authors: Olson D. A. , Glicksman L. R. , Ferm H. M.
Published: May 2008

Steady-state natural convection, which occurs in building enclosures (Rayleigh numbers of 1010), was studied experimentally in a full-scale room and in a 1:5.5 small-scale physical model containing R114 gas. The model was geometrically similar, had the same Rayleigh number, and had the same dimensionless end wall temperatures as the full-scale room. Configurations were tested with the enclosure empty, with a vertical partition extending from the floor to midheight, and with the vertical partition raised slightly off the floor. For isothermal opposing end walls at different temperatures, excellent agreement was found between the full-scale room and the scale model in flow patterns, velocity levels, temperature distributions, and heat transfer, even though the radiation heat transfer was not scaled between the two models.
Mixed Convection Transport From an Isolated Heat Source Module on a Horizontal Plate
Authors: Kang B. H. , Jaluria Y. , Tewari S. S.
Published: May 2008

An experimental study of the mixed convective heat transfer from an isolated source of finite thickness, located on a horizontal surface in an externally induced forced flow, has been carried out. This problem is of particular interest in the cooling of electronic components and also in the thermal transport associated with various manufacturing systems, such as ovens and furnaces. The temperature distribution in the flow as well as the surface temperature variation are studied in detail. The dependence of the heat transfer rate on the mixed convection parameter and on the thickness of the heated element or source, particularly in the vicinity of the source, is investigated. The results obtained indicate that the heat transfer rate and fluid flow characteristics vary strongly with the mixed convection variables. The transition from a natural convection dominated flow to a forced convection dominated flow is studied experimentally and the basic characteristics of the two regimes determined. This transition has a strong influence on the temperature of the surface and on the heat transfer rate. As expected, the forced convection dominated flow is seen to be significantly more effective in the cooling of a heat dissipating component than a natural convection dominated flow. The location of the maximum temperature on the module surface, which corresponds to the minimum local heat transfer coefficient, is determined and discussed in terms of the underlying physical mechanisms. The results obtained are also compared with these for an element of negligible thickness and the effect of a significant module thickness on the transport is determined. Several other important aspects of fundamental and applied interest are studied in this investigation.
Variation of Heat and Mass Transfer Coefficients During Drying of Granular Beds
Authors: Rogers J. A. , Kaviany M.
Published: May 2008

During convective drying of initially fully saturated granular beds, the solid matrix is gradually exposed to the ambient air, resulting in heat transfer to both the liquid and solid. In an attempt to examine the heat and mass transfer processes occurring on the surface and to examine the influence of particle size and Bond number on the drying rate, experiments are performed in which granular beds constructed of spherical particles (which range from d = 0.2 mm to d = 25.4 mm) are convectively dried. For beds constructed of very small particles (d = 0.2 mm, Bo = 0.0035) the surface areas of the liquid and solid are difficult to estimate due to the random arrangement of the particles. The experimental results confirm existing knowledge that the evaporation rate is nearly constant during the funicular state of drying. For beds constructed of large particles (d = 25.4 mm, Bo = 21.7) an estimate of the surface areas of the liquid and solid reveals that the surface areas and the evaporation rate are highly dependent on surface saturation, contact angle, and surface tension. The results indicate that heat transfer from the solid to the liquid is significant and that the heat and mass transfer processes are not analogous.
Analysis of Energy and Momentum Transport for Fluid Flow Through a Porous Bed
Authors: Vafai Kambiz , Sozen Mehmet
Published: May 2008

This paper presents an analysis for the forced convective flow of a gas through a packed bed of spherical solid particles, and the associated heat transport processes. Ergun’s correlation was used as the vapor phase momentum equation in order to account for the inertia effects as well as the viscous effects. No local thermal equilibrium was assumed between the solid and the vapor phases. A thorough discussion of the thermal interactions between the solid and vapor phases and their effects on the fluid flow as well as the pressure and density fields is presented. The analysis shows that the local thermal equilibrium condition was very sensitive to the particle Reynolds number (Rep) and the Darcy number (Da) while thermophysical properties did not have a very significant effect on this condition. On the other hand, two-dimensional behavior of certain variables was found to be very sensitive to thermophysical parameters but insensitive to Rep and Da.
Analysis of Surface Enhancement by a Porous Substrate
Authors: Vafai Kambiz , Kim Sung-Jin
Published: May 2008

Convective flow and heat transfer through a composite porous/fluid system have been studied numerically. The composite medium consists of a fluid layer overlaying a porous substrate, which is attached to the surface of the plate. The numerical simulations focus primarily on flows that have the boundary layer characteristics. However, the boundary layer approximation was not used. A general flow model that accounts for the effects of the impermeable boundary and inertia is used to describe the flow inside the porous region. Several important characteristics of the flow and temperature fields in the composite layer are reported. The dependence of these characteristics on the governing parameters such as the Darcy number, the inertia parameter, the Prandtl number, and the ratio of the conductivity of the porous material to that of the fluid is also documented. The results of this investigation point out a number of interesting practical applications such as in frictional drag reduction, and heat transfer retardation or enhancement of an external boundary.
Ice Formation and Heat Transfer With Water Flow Around Isothermally Cooled Cylinders Arranged in a Line
Authors: Hirata T. , Matsui H.
Published: May 2008

The ice formation phenomenon and heat transfer around isothermally cooled cylinders that are lined up in the direction of flow in a crossflow of cold water were examined experimentally. Under the conditions of low Reynolds numbers and low cooling temperatures, the cylinders became linked by an ice layer. The amount of ice formed around the cylinders was much more than that for a single cylinder. A correlation equation was proposed for the amount of linked ice around the cooled cylinders. It was found that the local heat transfer coefficients on the linked ice surface showed an alternating decrease and increase in the flow direction. The mean value of the local Nusselt number showed the same characteristic as that on a flat plate in a turbulent flow. A dimensionless parameter that represents how efficiently the cylinders can work as a thermal energy storage system was introduced.
One-Dimensional Analysis of the Hydrodynamic and Thermal Characteristics of Thin Film Flows Including the Hydraulic Jump and Rotation
Authors: Thomas S. , Hankey W. , Faghri A. , Swanson T.
Published: May 2008

The flow of a thin liquid film with a free surface along a horizontal plate that emanates from a pressurized vessel is examined numerically. In one g, a hydraulic jump was predicted in both plane and radial flow, which could be forced away from the inlet by increasing the inlet Froude number or Reynolds number. In zero g, the hydraulic jump was not predicted. The effect of solid-body rotation for radial flow in one g was to “wash out” the hydraulic jump and to decrease the film height on the disk. The liquid film heights under one g and zero g were equal under solid-body rotation because the effect of centrifugal force was much greater than that of the gravitational force. The heat transfer to a film on a rotating disk was predicted to be greater than that of a stationary disk because the liquid film is extremely thin and is moving with a very high velocity.
Effects of Surface Tension on Film Condensation in a Porous Medium
Authors: Majumdar A. , Tien C. L.
Published: May 2008

In the process of film condensation in a porous medium, the thermodynamics of phase equilibria requires the existence of a two-phase zone lying between the liquid and the vapor regions. In the two-phase zone, solutions of the conservation equations indicate a boundary-layer profile for the capillary pressure. The liquid zone is analyzed using three models, which assume either slip or no slip at the wall and Darcy velocity or no shear at the interface with the two-phase zone. The results show that the condition of no slip at the wall must be satisfied in all cases except where the thickness of the liquid zone is much larger than the characteristic boundary layer in the porous medium. At the interface with the two-phase zone, the assumption of no shear is more realistic than that of an imposed Darcy velocity, in conjunction with no-slip condition at the wall. Comparisons with experiments suggest that the drag on the liquid film due to surface tension is significant for permeabilities lower than 10−7 m2. A dimensionless group, characterizing viscous flow due to surface tension forces, is introduced in this study.
An Experimental Study of R-113 Film Condensation on Horizontal Integral-Fin Tubes
Authors: Marto P. J. , Zebrowski D. , Wanniarachchi A. S. , Rose J. W.
Published: May 2008

Heat transfer measurements were made at near-atmospheric pressure on a smooth tube, on 24 integral-fin tubes having machined, rectangular-shaped fins, and on a commercial integral-fin tube. All tubes were made of copper. The vapor flowed vertically downward with a nominal velocity of 0.4 m/s. Vapor-side heat transfer coefficients were determined with a typical uncertainty of ± 7 percent using a “modified Wilson plot” technique. The vapor-side heat transfer coefficient of the integral-fin tubes (based upon the outside surface area of the smooth tube) was enhanced considerably more than the surface area enhancement provided by the fins. Heat transfer enhancements (for the same vapor-to-wall temperature difference) up to around 7 were measured for a corresponding area enhancement of only 3.9. The optimum fin spacing was found to lie between 0.2 and 0.5 mm, depending upon fin thickness and height. The data were compared with those of other investigations and with several existing theoretical models. Visual observations of condensate drainage patterns from the finned tubes were also made.
Row Effect for R-11 Condensation on Enhanced Tubes
Authors: Webb R. L. , Murawski C. G.
Published: May 2008

Experimental results of a condensation row effect study on enhanced tubes are presented. A test cell was constructed to condense Refrigerant-11 on the shell side of a vertical bank of five horizontal tubes. Four distinctly different commercially available tubes were tested. The tubes are a 1024-fpm integral fin, the Wolverine Turbo-C, Wieland GEWA-SC, and the Tred-D. A modified Turbo-C tube was also tested. Experimental and visual observations are used to understand the row effect due to condensate loading. By plotting the data in the form of the local condensation coefficient versus condensate Reynolds number, the results may be interpreted for any number of tube rows, up to the maximum Reynolds numbers tested. Bundle average condensation coefficients may be established by integrating the h versus Re values over the number of tube rows.
A Skewed PDF Combustion Model for Jet Diffusion Flames
Authors: Abou-Ellail M. M. M. , Salem H.
Published: May 2008

A combustion model based on restricted chemical equilibrium is described. A transport equation for the skewness of the mixture fraction is derived. It contains two adjustable constants. The computed values of the mean mixture fraction (f) and its variance and skewness (g and s) for a jet diffusion methane flame are used to obtain the shape of a skewed pdf. The skewed pdf is split into a turbulent part (beta function) and a nonturbulent part (delta function) at f = 0. The contribution of each part is directly related to the values of f, g, and s. The inclusion of intermittency in the skewed pdf appreciably improves the numerical predictions obtained for a turbulent jet diffusion methane flame for which experimental data are available.
Temperature Measurements of Falling Droplets
Authors: Wells M. R. , Melton L. A.
Published: May 2008

The temperature of 225-μm-dia decane droplets, which have fallen 100 mm through a hot quiescent, oxygen-free environment, has been measured using exciplex fluorescence thermometry. The droplets were doped with pyrene, and the relative intensities of pyrene monomer and excimer emissions were used to determine the droplet temperatures. The droplet temperature increases approximately 0.4°C per °C increase in the ambient temperature up to an ambient temperature of 200°C. Less than 10 percent evaporation was observed for the droplets at the highest ambient temperatures.
An Investigation of a Latent Heat Storage Porous Bed and Condensing Flow Through It
Authors: Vafai K. , So¨zen M.
Published: May 2008

In this work the transient analysis of the behavior of a packed bed of encapsulated phase change material (PCM) and the condensing flow through it is presented. The rigorous model used assumes no local thermal equilibrium between the bed particles and the working fluid, and incorporates the inertia effects in the momentum transport by the use of the Ergun-Forchheimer equation. Condensation in the working fluid is investigated. Thermal charging of the packed bed is analyzed and compared for a sensible heat storage material as well as for different latent heat storage materials (PCMs).
Evaporation and Condensation Heat Transfer and Pressure Drop in Horizontal, 12.7-mm Microfin Tubes With Refrigerant 22
Authors: Schlager L. M. , Pate M. B. , Bergles A. E.
Published: May 2008

Using R-22 as the working fluid, a series of tests was performed to determine the evaporation and condensation performance of three 12.7-mm o.d. tubes having many small, spiral inner fins. The tubes, referred to as microfin tubes, had a 11.7-mm maximum i.d., 60 or 70 fins with heights ranging from 0.15 to 0.30 mm, and spiral angles from 15 to 25 deg. A smooth tube was also tested to establish a basis of comparison. The test apparatus had a straight, horizontal test section with a length of 3.67 m and was heated or cooled by water circulated in a surrounding annulus. Nominal evaporation conditions were 0 to 5°C (0.5 to 0.6 MPa) with inlet and outlet qualities of 15 and 85 percent, respectively; condensation conditions were 39 to 42°C (1.5 to 1.6 MPa) with inlet and outlet qualities of 85 and 10 percent, respectively. Mass flux varied from 75 to 400 kg/m2·s. The average heat transfer coefficients in the microfin tubes, based on a nominal equivalent smooth tube area, were 1.6 to 2.2 times larger for evaporation and 1.5 to 2.0 times larger for condensation than those in the smooth tube. The pressure drop increased, but by a smaller factor than the heat transfer coefficient.
Critical Heat Flux Modeling in Forced Convection Boiling During Power Transients
Authors: Pasamehmetoglu K. O. , Nelson R. A. , Gunnerson F. S.
Published: May 2008

In this paper, a theoretical prediction of critical heat flux (CHF) during power transients in forced convective boiling is presented. The analysis is restricted to departure from nucleate boiling (DNB) type of CHF at low qualities. The developed theory is compared with the experimental data available in the literature. The agreement is exceptionally good. The new model also is compared with the semi-empirical transient CHF model in the literature.
Analysis of Buoyancy and Tube Rotation Relative to the Modified Chemical Vapor Deposition Process
Authors: Choi M. , Lin Y. T. , Greif R.
Published: May 2008

The secondary flows resulting from buoyancy effects in respect to the MCVD process have been studied in a rotating horizontal tube using a perturbation analysis. The three-dimensional secondary flow fields have been determined at several axial locations in a tube whose temperature varies in both the axial and circumferential directions for different rotational speeds. For small rotational speeds, buoyancy and axial convection are dominant and the secondary flow patterns are different in the regions near and far from the torch. For moderate rotational speeds, the effects of buoyancy, axial and angular convection are all important in the region far from the torch where there is a spiraling secondary flow. For large rotational speeds, only buoyancy and angular convection effects are important and no spiraling secondary motion occurs far downstream. Compared with thermophoresis, the important role of buoyancy in determining particle trajectories in MCVD is presented. As the rotational speed increases, the importance of the secondary flow decreases and the thermophoretic contribution becomes more important. It is noted that thermophoresis is considered to be the main cause of particle deposition in the MCVD process.
Applying Heat Transfer Coefficient Data to Electronics Cooling
Authors: Moffat R. J. , Anderson A. M.
Published: May 2008

Many electronics cooling applications involve direct air cooling of the components and their heat transfer performance is often estimated, in the design phase, using heat transfer coefficient data from either in-house data or published sources. The agreement between predicted and observed system performance is quite often not acceptable, which means that the cooling system must be developed empirically—an expensive and time-consuming process. The authors believe they have identified one important reason for this state of affairs: the heat transfer coefficient values presented in the literature are being used improperly. Almost all of the published heat transfer coefficient data are from single-active-component experiments, which implicitly define h based on the adiabatic temperature of the component (had), while most users assume h to be defined on the basis of the mean fluid temperature (hm). This misunderstanding leads to underpredictions of the temperature rise of the components by 20–30 percent or more. This paper reviews the options for defining the heat transfer coefficient, shows how the problem arises, and then describes the steps necessary to properly use the existing heat transfer coefficient data base. There are two options for applying the existing data base to a fully powered array: One can either calculate the adiabatic temperatures of the components from the known heat release distribution in the array or calculate the values of hm that can be used with Tm, again using the known heat release distribution. The two options represent different ways to apply the same general method, superposition. The superposition method of Sellars, et al., (1956) is adapted to discrete systems and used as a guide to the forms recommended.
Heat Transfer for Forced Convection Past Coiled Wires
Authors: Dietrich M. , Blo¨chl R. , Mu¨ller-Steinhagen H.
Published: May 2008

Heat transfer coefficients were measured for forced convection of isobutanol in crossflow past coiled wires with different coil geometries. Flow rate and heat flux have been varied over a wide range to include laminar and turbulent flow for convective sensible and subcooled boiling heat transfer. To investigate the effect of coil geometry on heat transfer, the wire diameter, coil diameter, and coil pitch were varied systematically. The measured data are compared with the predictions of four correlations from the literature.
Heat Transfer and Pressure Drop for Short Pin-Fin Arrays With Pin-Endwall Fillet
Authors: Chyu M. K.
Published: May 2008

The effects of array configuration and pin-endwall fillet on the heat transfer and pressure drop of short pin-fin arrays are investigated experimentally. The pin-fin element with endwall fillet, typical in actual turbine cooling applications, is modeled by a spool-like cylinder. The arrays studied include an in-line and a staggered array, each having seven rows of five pins. These arrays have the same geometric parameters, i.e., H/D = 1, S/D = X/D = 2.5, and the Reynolds number ranging from 5 × 103 to 3 × 104. One of the present results shows that the staggered array always has a higher array-averaged heat transfer coefficient than its in-line counterpart. However, the pressure drop for the staggered array is higher compared to the in-line configuration. These trends are unaffected by the existence of the pin-endwall fillet. Another significant finding is that an array with pin-endwall fillet generally produces lower heat transfer coefficient and higher pressure drop than that without endwall fillet. This leads to the conclusion that pin-endwall fillet is undesirable for heat transfer augmentation. In addition, nai¨ve use of the heat transfer results obtained with perfectly circular cylinders tends to overestimate the pin-fin cooling capability in the actual turbine. The effects of endwall fillet on the array heat transfer and pressure drop are much more pronounced for the staggered array than for the inline array; however, they diminish as the Reynolds number increases.
Heat Transfer Around a Tube in In-Line Tube Banks Near a Plane Wall
Authors: Aiba S.
Published: May 2008

Heat transfer in the third cylinder of four circular cylinders above a plane wall has been investigated in a crossflow of air. The turbulent boundary layer thickness along the wall with no cylinder present was about 21 mm. The cylinder diameter (d) was 15 mm, the clearance (c) between the cylinders and the wall was 0.75 ~ 60 mm (c/ d = 0.05 ~ 4.0), and the pitch (p) (which denotes the longitudinal spacing between cylinder centers) was 18 ~ 66 mm (p/d = 1.2 ~ 4.4). The Reynolds number (Re) based on the undisturbed uniform flow velocity above the wall ranged from 0.8 × 104 ~ 4 × 104. Variations in the characteristic features of local and mean Nusselt number Num are discussed in relation to c/d, p/d, and Re. Num yielded the compact expression Num = 0.103 (p/d)−0.12(c/d)0.23Re0.74, with a spread of ±5 percent in the ranges c/d = 0.18 ~ 0.61, p/d = 1.2 ~ 3.2, and Re = 0.8 × 104 ~ 4 × 104, except for the case with c/d = 0.18, p/d = 3.2.
Similarity Solution of Combined Convection Heat Transfer From a Rotating Cone or Disk to Non-Newtonian Fluids
Authors: Wang T.-Y. , Kleinstreuer C.
Published: May 2008

A powerful similarity solution of the highly nonlinear, coupled boundary-layer equations has been developed for steady laminar mixed convection heat transfer between a rotating cone/disk and power-law fluids. Of special interest are the effects of the power-law viscosity index, a generalized local Prandtl number, the buoyancy parameter, and the type of thermal wall condition on the velocity and temperature fields and hence the skin friction coefficient and the local Nusselt number. While the momentum boundary-layer thickness increases measurably with decreasing viscosity index n, the thermal boundary-layer thickness is less affected by changes in n. The magnitude and direction of the buoyancy force influence the upward velocity profile near the wall and the temperature profiles significantly. Both Prandtl number and buoyancy parameter have a more pronounced effect on the skin friction group, SFG ~ cf, than on the heat transfer group, HTG ~ Nu.
Mixed Convection Heat Transfer From Thermal Sources Mounted on Horizontal and Vertical Surfaces
Authors: Tewari S. S. , Jaluria Y.
Published: May 2008

An experimental study is carried out on the fundamental aspects of the conjugate, mixed convective heat transfer from two finite width heat sources, which are of negligible thickness, have a uniform heat flux input at the surface, and are located on a flat plate in the horizontal or the vertical orientation. The heat sources are wide in the transverse direction and, therefore, a two-dimensional flow circumstance is simulated. The mixed convection parameter is varied over a fairly wide range to include the buoyancy-dominated and the mixed convection regimes. The circumstances of pure natural convection are also investigated. The convective mechanisms have been studied in detail by measuring the surface temperatures and determining the heat transfer coefficients for the two heated strips, which represent isolated thermal sources. Experimental results indicate that a stronger upstream heat source causes an increase in the surface temperature of a relatively weaker heat source, located downstream, by reducing its convective heat transfer coefficient. The influence of the upstream source is found to be strongly dependent on the surface orientation, especially in the pure natural convection and the buoyancy dominated regimes. The two heat sources are found to be essentially independent of each other, in terms of thermal effects, at a separation distance of more than about three strip widths for both the orientations. The results obtained are relevant to many engineering applications, such as the cooling of electronic systems, positioning of heating elements in furnaces, and safety considerations in enclosure fires.
Modeling of the Convective Thermal Ignition Process of Solid Fuel Particles
Authors: Yang Jing-Tang , Wang Gwo-Guang , Li Hung-Yi
Published: May 2008

This work uses the boundary layer theory to study the thermal ignition process of the solid particle. The theoretical model explores the two-dimensional boundary layer equations in the gaseous phase. The governing equations of mass, momentum, energy, and species in gas phase are first transformed to ordinary differential equations through series expansion with respect to the azimuthal angle. The equations are then quasi-linearized to be the initial value problem and solved using Runge-Kutta method. The minimum heat flux from the gas phase to the fuel is evaluated as the most suitable criterion for the convective thermal ignition. The influences of the heat transfer rate, fluid dynamics, and the gas phase chemical reaction rate on the ignition delay and ignition position are discussed in detail.
Free Convection Between Series of Vertical Parallel Plates With Embedded Line Heat Sources
Authors: Kim S. H. , Anand N. K. , Fletcher L. S.
Published: May 2008

Laminar free convective heat transfer in channels formed between series of vertical parallel plates with an embedded line heat source was studied numerically. These channels resemble cooling passages in electronic equipment. The effect of a repeated boundary condition and wall conduction on mass flow rate (M), maximum surface temperature (θh,max and θc,max), and average surface Nusselt number (Nuh and Nuc) is discussed. Calculations were made for Gr*=10 to 106, K=0.1, 1, 10, and 100, and t/B=0.1 and 0.3. The effect of a repeated boundary condition decreases the maximum hot surface temperature and increases the maximum cold surface temperature. The effect of a repeated boundary condition with wall conduction increases the mass flow rate. The maximum increase in mass flow rate due to wall conduction is found to be 155 percent. The maximum decrease in average hot surface Nusselt number due to wall conduction (t/B and K) occurs at Gr*=106 and is 18 percent. Channels subjected to a repeated boundary condition approach that of a symmetrically heated channel subjected to uniform wall temperature conditions at K≥100.
Convection in Magnetic Fluids With Internal Heat Generation
Authors: Rudraiah N. , Sekhar G. N.
Published: May 2008

The effect of a uniform distribution of heat source on the onset of stationary convection in a horizontal Boussinesq magnetic fluid layer bounded by isothermal nonmagnetic boundaries is investigated. Solutions are obtained using a higher order Galerkin expansion technique, considering different isothermal boundary combinations (rigid-rigid, rigid-free, and free-free). It is found that the effect of internal magnetic number, due to a heat source, is to make the system more unstable. The results obtained, in the limiting cases, compare well with the existing literature.
Laser-Induced Heating of a Multilayered Medium Resting on a Half-Space: Part II—Moving Source
Authors: Kant R. , Deckert K. L.
Published: May 2008

Direct access storage devices (DASDs) are widely used in the computer industry to store and manage data. In conventional magnetic recording, an induction head flying very close to the disk surface alters the polarization of the magnetic field of the disk surface to erase and/or write the information on the disk. However, a new technology known as magneto-optical recording or optical recording has considerable promise to increase data densities and reliability of data storage. In magneto-optical storage, magnetic fields are altered by a laser source, which heats the magnetic medium beyond its Curie point, a temperature at which the magnetic medium loses its magnetization. This domain with zero magnetization is subsequently reversed by using an induction magnet. All these processes take place when the disk is rotating at a very high speed with respect to the laser source. An optical disk is a multilayered medium consisting of a thick glass disk on which many layers of different materials are sputtered, only one layer of which serves as a magnetic medium. Therefore, in this paper, a problem of laser-induced heating of a multilayered medium resting on a half-space is considered when the laser is translating with respect to it. The transient heat conduction equation is solved by employing the Laplace transform in the time domain and the Fourier Transform in the x, y dimensions. The resulting ordinary differential equation is solved and the inversion of the Laplace transform is obtained by a technique developed by Crump. The Fourier inversion is obtained by using a Fast Fourier Transform. The technique developed here is then applied to calculate domain size for recorded bits for a given disk, laser power, source characteristics, and rotational velocity.
Unsteady Thermosolutal Transport Phenomena Due to Opposed Buoyancy Forces in Shallow Enclosures
Authors: Jiang H. D. , Ostrach S. , Kamotani Y.
Published: May 2008

Thermosolutal convection driven by opposed thermal and solutal buoyancy forces is experimentally investigated, by employing an electrochemical system in a low aspect ratio enclosure. In certain parametric ranges, it is observed that both the current and the temperature fluctuate. Criteria associated with the solutal Grashof number, and buoyancy and aspect ratios have been correlated to classify the characteristics of mass and heat transfer processes and various flow regimes, and a new length scale is suggested. The unsteady transport phenomena are found to depend on the Lewis number, Grashof number, and buoyancy and aspect ratios.
Transient Double Diffusion in a Fluid Layer Extending Over a Permeable Substrate
Authors: Kazmierczak M. , Poulikakos D.
Published: May 2008

The problem of transient double diffusion in a composite layer is studied numerically. The composite layer consists of a fluid region extending over a fluid-saturated permeable substrate. Initially, the fluid in the system is motionless, isothermal, and stably stratified with a linear salt distribution. A constant uniform heat flux is then suddenly applied to the bottom wall of the system. The resulting coupled flow, temperature, and concentration fields as they evolve in time are obtained numerically. The flow in the fluid region was determined by solving the complete form of the two-dimensional laminar governing equations subjected to the usual Boussinesq approximations. The flow in the porous region was modeled using the general flow model, which includes both the effects of macroscopic shear (Brinkman effect) and flow inertia (Forchheimer effect). Interesting results were obtained and are presented in a systematic manner so as to document the effect of changing the important system parameters, which include the height of the permeable substrate, its permeability, and the ratio of the thermal to the solutal Rayleigh number. It was found that these parameters had a major impact on the system behavior and their effects are thoroughly discussed.
Emittance of Boehmite and Alumina Films on 6061 Aluminum Alloy Between 295 and 773 K
Authors: Kollie T. G. , Radcliff T. D. , Weaver F. J.
Published: May 2008

The total hemispherical emittance of an oxide film that formed on 6061-T6 aluminum alloy parts in the Tower Shielding Reactor-II at Oak Ridge National Laboratory was measured from 295 to 773 K using an emissometer and/or a calorimeter. The emittance of this film was critically needed for heat transfer calculations in a simulated loss-of-coolant accident of the reactor. X-ray diffraction analysis identified the film as boehmite (Al2O3·H2O), which dehydrated to alumina (Al2O3) upon heating above 473 K. The measured emittances for the alumina film are in excellent agreement with published values for anodized aluminum films and for bulk alumina. Published values of the emittance of boehmite could not be found for comparison, but evidence is presented that some anodization processes for aluminum yield boehmite and not alumina films.
Development of a Flow Boiling Map for Subcooled and Saturated Flow Boiling of Different Fluids Inside Circular Tubes
Authors: Kandlikar S. G.
Published: May 2008

The thermal behavior of a flow boiling system is represented by a flow boiling map to illustrate visually the relationships among various system parameters. An earlier flow boiling map by Collier (1981) does not include the effect of mass flux and is specific to water at low pressures. For other fluids, significant departures from the parametric trends displayed in Collier’s map have been reported in the literature (e.g., Kandlikar, 1988b). In the present paper, a new flow boiling map is developed to depict the relationships among the heat transfer coefficient, quality, heat flux, and mass flux for different fluids in the subcooled and the saturated flow boiling regions. The trends observed in the experimental data and correlations for water and refrigerants are used in deriving the present map. The particular areas where further investigation is needed to validate the trends are also indicated. In the subcooled boiling region, hTP/hlo is plotted against x with Bo as a parameter, while in the saturated boiling region, hTP/hlo is plotted against x with ρl/ρg and a modified boiling number Bo* as parameters. It is hoped that the map would prove to be helpful in explaining the role of different heat transfer mechanisms in flow boiling of different fluids.
Variation of Superheat With Subcooling in Nucleate Pool Boiling
Authors: Judd R. L. , Merte H. , Ulucakli M. E.
Published: May 2008

An analysis is presented that explains the variation of superheat with subcooling that has been observed by a number of researchers investigating nucleate boiling heat transfer at constant heat flux. It is shown that superheat initially increases with increasing subcooling near saturated conditions because of the way in which changes in active site density and average bubble frequency with increasing subcooling affect the rate of heat removal from the heater surface by enthalpy transport and microlayer evaporation. As subcooling increases further, natural convection begins to play an increasingly important role in the heat transfer process. Ultimately, natural convection is able to accommodate the entire imposed heat flux, after which superheat decreases as subcooling increases. The success of the analysis in explaining the variation of superheat with subcooling suggests that the rate of the heat removal from the heater surface is completely determined by the mechanisms of enthalpy transport, natural convection, and microlayer evaporation.
An Experimental Study of the Relative Effects of Transverse and Longitudinal Ribbing of the Heat Transfer Surface in Forced Convective Boiling
Authors: Akhanda M. A. R. , James D. D.
Published: May 2008

This work describes an experimental investigation into the effects of surface geometry variations on heat transfer performance in forced flow boiling. Ribs of rectangular cross section were machined on flat stainless steel test specimens of 1.60 mm thickness. The height of the ribs was kept constant at 0.50 mm and a number of test specimens with rib width varying from 0.50 mm to 2.0 mm and rib spacing ranging from 0.50 mm to 3.5 mm were produced. In order to maintain a constant microsurface roughness of 0.7 μm over the entire test specimen, all test surfaces were subjected to vequa-blasting after machining. Precautions were taken to ensure that aging of the heat transfer surface had been established so that the data recorded were reproducible. Experimental data are reported for water at atmospheric pressure with flow velocities of 0.20 m/s to 1.40 m/s. Inlet subcoolings were varied from 5°C to 30°C. The results for the longitudinal ribbed surfaces (LRS) were compared with those for the transverse rectangular ribbed surfaces (TRS) and both sets of data were compared with the heat transfer performance of an optimum flat roughened surface.
Effect of Insoluble Surfactants in Condensation on a Moving Drop: Solutions for Intermediate Reynolds Numbers
Authors: Huang L. J. , Ayyaswamy P. S.
Published: May 2008

An examination of the hydrodynamics and heat transfer associated with condensation on a moving drop in the intermediate Reynolds number regime (Re = 0(100)) has been carried out. The droplet is taken to be initially contaminated with an insoluble monolayer surfactant material. The drop environment is taken to consist of its own vapor and air. The ambient pressure is taken to be one atmosphere. The formulation entails a simultaneous solution of the quasi-steady elliptic partial differential equations that describe the flow field and transport in the gaseous phase, and the motion inside the liquid drop. The heat transport inside the drop is treated as a transient process. A numerical procedure based on the hybrid difference scheme has been employed. The surface tension gradient force induced by the surfactant and the shear stress from the relative motion between the droplet and its outside velocity field are evaluated. Results have been provided for the interface velocity, drag, surface vorticity, external and internal flow structure, surfactant concentration along the droplet surface, and the Nusselt and Sherwood numbers.
Direct Air Cooling of Electronic Components: Reducing Component Temperatures by Controlled Thermal Mixing
Authors: Anderson A. M. , Moffat R. J.
Published: May 2008

This paper discusses forced convection heat transfer in a channel populated with discrete components similar to those found in electronics cooling situations. The temperature rise of each component is expressed as the sum of two parts; its adiabatic temperature rise, Tad–Tin, due to the thermal wakes of upstream components; and its self-heating temperature rise, Te–Tad due to its own power dissipation. A component’s temperature can be reduced either by reducing the adiabatic temperature rise or the self-heating temperature rise, or both. This investigation concentrates on the former: reducing the adiabatic temperature rise through increased thermal mixing in the coolant flow. The temperature of the air near the components exceeds the mean temperature of the cooling fluid, often by a large amount. In the present work, small scoops were installed in regions of high temperature but low velocity fluid (i.e., in between rows of components rather than in the free stream) to augment thermal mixing and reduce the nonuniformity. This approach does not induce as large a pressure drop as some conventional “turbulators” for a given decrease in operating temperature. In the present work, the scoops reduced the adiabatic temperature rises by 10 to 55 percent, resulting in up to 19 percent reduction in the overall temperature rise. The test section pressure drop increased 11 percent.
Laminar Natural Convection Heat Transfer From a Horizontal Circular Cylinder to Liquid Metals
Authors: Sugiyama K. , Ma Y. , Ishiguro R.
Published: May 2008

The objective of the present study is to clarify the heat transfer characteristics of natural convection around a horizontal circular cylinder immersed in liquid metals. Experimental work concerning liquid metals sometimes involves such a degree of error that it is impossible to understand the observed characteristics in a measurement. Numerical analysis is a powerful means to overcome this experimental disadvantage. In the present paper we first show that the Boussinesq approximation is more applicable to liquid metals than to ordinary fluids and that the present analysis gives accurate heat transfer rates, even for a cylinder with a relatively large temperature difference (>100 K) between the heat transfer surface and fluid. It is found from a comparison of the present results with previous work that the correlation equations that have already been proposed predict values lower than the present ones.
Shape Sensitivities and Optimal Configurations for Heat Diffusion Problems: a BEM Approach
Authors: Saigal S. , Chandra A.
Published: May 2008

Design sensitivity analysis, along with the shape optimization of heat diffusion problems using the boundary element method (BEM), is presented in this paper. The present approach utilizes the implicit differentiation of discretized boundary integral equations with respect to the design variables to yield the sensitivity equations. A technique based on the response of an object to a constant boundary temperature is presented for the evaluation of singular terms in the thermal sensitivity kernels. A procedure for the design sensitivity analysis of a reduced system of equations obtained via substructuring and condensation is also presented. The BEM formulations are implemented for both two-dimensional and axisymmetric objects. A number of sample problems are solved to demonstrate the accuracy of the present sensitivity formulation and to obtain optimal configurations of some mechanical components of practical interest, which are subjected to different thermal environments.
Optimization of Cost Subject to Uncertainty Constraints in Experimental Fluid Flow and Heat Transfer
Authors: Peterson J. , Bayazitoglu Y.
Published: May 2008

Uncertainty analysis in the initial stages of any experimental work is essential in obtaining high-quality data. It insures that the proposed experiment has been thoroughly planned, and that the quantities to be calculated from the experimental measurements will be known with reasonable accuracy and precision. While an uncertainty analysis helps insure reliable results, there is another equally significant aspect in the experimental planning stage: minimization of experimental equipment expenses. A method is presented here in which these two essential experimental elements are combined and viewed as an optimization problem for systematic examination. The analysis allows a systematic search for the least expensive combination of experimental equipment that will give the desired accuracy of results. For the numerical solution the Sequential Gradient Restoration Algorithm (SGRA) is selected. A typical experimental fluid flow and heat transfer problem is given, demonstrating the analysis and numerical solution.