bluuebunny/crossref_metadata_2025 huggingface dataset:
Transient Elastic and Viscoelastic Thermal Stresses During Laser Drilling of Ceramics
Authors: Modest M. F.
Published: February 2008

Lasers appear to be particularly well suited to drill and shape hard and brittle ceramics, which are almost impossible to netshape to tight tolerances, and are presently machined in industry only by diamond grinding. Unfortunately, the large, focussed heat fluxes that allow the ready melting and ablation of material, also result in large localized thermal stresses within the narrow heat-affected zone, which can lead to microcracks, significant decrease in bending strength, and even catastrophic failure. In order to assess the where, when, and what stresses occur during laser drilling, that are responsible for cracks and decrease in strength, elastic and viscoelastic stress models have been incorporated into our two-dimensional drilling code. The code is able to predict temporal temperature fields as well as the receding solid surface during CW or pulsed laser drilling. Using the resulting drill geometry and temperature fields as well as the receding solid surface during CW of pulsed laser drilling. Using the resulting drill geometry and temperature field, elastic stresses as well as viscoelastic stresses are calculated as they develop and decay during the drilling process. The viscosity of the ceramic is treated as temperature-dependent, limiting viscoelastic effects to a thin layer near the ablation front where the ceramic has softened.
Transient Thermal Response of a Rotating Cylindrical Silicon Nitride Workpiece Subjected to a Translating Laser Heat Source, Part I: Comparison of Surface Temperature Measurements With Theoretical Results
Authors: Rozzi J. C. , Pfefferkorn F. E. , Incropera F. P. , Shin Y. C.
Published: February 2008

Laser-assisted machining (LAM), in which the material is locally heated by an intense laser source prior to material removal, provides an alternative machining process with the potential to yield higher material removal rates, as well as improved control of workpiece properties and geometry, for difficult-to-machine materials such as structural ceramics. To assess the feasibility of the LAM process and to obtain an improved understanding of governing physical phenomena, a laser assisted machining facility was developed and used to experimentally investigate the thermal response of a rotating silicon nitride workpiece heated by a translating CO2 laser. Using a focused laser pyrometer, surface temperature history measurements were made to determine the effect of rotational and translational speed, as well as the laser beam diameter and power, on thermal conditions. The experimental results are in good agreement with predictions based on a transient three-dimensional numerical simulation of the heating process. With increasing workpiece rotational speed, temperatures in proximity to the laser spot decrease, while those at circumferential locations further removed from the laser increase. Near-laser temperatures decrease with increasing beam diameter, while energy deposition by the laser and, correspondingly, workpiece surface temperatures increase with decreasing laser translational speed and increasing laser power, In a companion paper (Rozzi et al., 1998), the detailed numerical model is used to further elucidate thermal conditions associated with laser heating and to assess the merit of a simple, analytical model which is better suited for online process control.
Transient Thermal Response of a Rotating Cylindrical Silicon Nitride Workpiece Subjected to a Translating Laser Heat Source, Part II: Parametric Effects and Assessment of a Simplified Model
Authors: Rozzi J. C. , Incropera F. P. , Shin Y. C.
Published: February 2008

In a companion paper (Rozzi et al., 1998), experimental validation was provided for a transient three-dimensional numerical model of the process by which a rotating workpiece is heated with a translating laser beam. In this paper, the model is used to elucidate the effect of operating parameters on thermal conditions within the workpiece and to assess the applicability of an approximate analysis which is better suited for on-line process control. From detailed numerical simulations, it was determined that the thickness of a surface thermal layer decreases with increasing workpiece rotational speed and that the influence of axial conduction on the workpiece temperature distribution increases with decreasing laser translational velocity. Temperatures increase throughout the workpiece with increasing laser power, while the influence of increasing beam diameter is confined to decreasing near-surface temperatures. Temperature-dependent thermophysical properties and forced convection heat transfer to the laser gas assist jet were found to significantly influence the maximum temperature beneath the laser spot, while radiation exchange with the surroundings and mixed convection to the ambient air were negligible. The approximate model yielded relations for calculating the radial temperature distribution within an r-φ plane corresponding to the center of the laser source, and predictions were in reasonable agreement with results of the numerical simulation, particularly in a near-surface region corresponding to the depth of cut expected for laser-assisted machining.
Thermal Transport and Flow in High-Speed Optical Fiber Drawing
Authors: Yin Zhilong , Jaluria Y.
Published: February 2008

The thermal transport associated with optical fiber drawing at relatively high drawing speeds, ranging up to around 15 m/s, has been numerically investigated. A conjugate problem involving the glass and the purge gas regions is solved. The transport in the preform/fiber is coupled, through the boundary conditions, with that in the purge gas, which is used to provide an inert environment in the furnace. The zonal method, which models radiative transport between finite zones in a participating medium, has been employed to compute the radiative heat transfer in the glass. The flow of glass due to the drawing process is modeled with a prescribed free-surface neck-down profile. The numerical results are compared with the few that are available in the literature. The effects of important physical variables such as draw speed, purge gas velocity and properties, furnace temperature, and preform diameter on the flow and the thermal field are investigated. It is found that the fiber drawing speed, the furnace temperature, and the preform diameter have significant effects on the temperature field in the preform/fiber, while the effects of the purge gas velocity and properties are relatively minor. The overall heating of the preform/fiber is largely due to radiative transport in the furnace and the changes needed in the furnace temperature distribution in order to heat the glass to its softening point at high speeds are determined.
A New Way of Solving Transient Radiative-Conductive Heat Transfer Problems
Authors: Andre S. , Degiovanni A.
Published: February 2008

One-dimensional transient energy transfer by conduction and radiation is solved for a finite medium. The semitransparent layer emits, absorbs, and scatters radiation (participating medium). The coupled transfer is solved analytically by considering the well-known two-flux approximation, assuming linear transfer and using the Laplace transform. The semitransparent layer can then be modeled by a matrix transfer function. The accuracy of the solution is verified in the case of sharp thermal excitation by a heat pulse on the front face. It is shown that this general model is very accurate for simulating both the limiting cases of purely scattering and purely absorbing media. In the latter case, the same modeling is derived using the kernel substitution technique, and very good agreement is achieved compared with numerical simulations. The resulting computation times are very small, and suggest that such a model can be used in the inverse approach of thermal problems involving semitransparent materials.
Effective Thermal Conductivity of a Thin, Randomly Oriented Composite Material
Authors: Phelan P. E. , Niemann R. C.
Published: February 2008

The thermal conductivity of a randomly oriented composite material is modeled using a probabilistic approach in order to determine if a size effect exists for the thermal conductivity at small composite thicknesses. The numerical scheme employs a random number generator to position the filler elements, which have a relatively high thermal conductivity, within a matrix having a relatively low thermal conductivity. The results indicate that, below some threshold thickness, the composite thermal conductivity increases with decreasing thickness, while above the threshold the thermal conductivity is independent of thickness. The threshold thickness increases for increasing filler fraction and increasing kf/km, the ratio between the filler and matrix thermal conductivities.
Constructal Three-Dimensional Trees for Conduction Between a Volume and One Point
Authors: Ledezma G. A. , Bejan A.
Published: February 2008

This paper extends to three-dimensional heat conduction the geometric “constructal” method of minimizing the overall thermal resistance between a finite-size volume and a small heat sink. The volume contains (i) low-conductivity material that generates heat at every point, and (ii) a small amount of high-conductivity material that must be distributed optimally in space. The given volume is covered in a sequence of building blocks (volume sizes) that starts with the smallest volume element, and continues toward larger assemblies. It is shown that the overall shape of each building block can be optimized for minimal volume-to-point resistance. The relative thicknesses of the high-conductivity paths can also be optimized. These optima are developed analytically and numerically for the smallest elemental volume and the first assembly. The high-conductivity paths form a tree network that is completely deterministic.
Fluid Flow and Heat Transfer Over a Three-Dimensional Spherical Object in a Pipe
Authors: Shahcheraghi N. , Dwyer H. A.
Published: February 2008

An incompressible viscous fluid flow with heat transfer over a spherical object inside a pipe is considered. The flow is made three-dimensional by an eccentric positioning of the sphere inside the pipe. The governing equations are solved by a numerical method which uses a finite volume formulation in a generalized body fitted coordinate system. An overset (Chimera) grid scheme is used to resolve the two geometries of the pipe and sphere. The results are compared to those of an external flow over a sphere, and the code is validated using such results in the intermediate Reynolds number range. The blockage effects are analyzed through evaluation of lift, drag, and heat transfer rate over the sphere. Also the change in the shear stress pattern is examined through evaluation of the local friction factor on a pipe wall and sphere surface.
The Centerline Pressure and Cavity Shape of Horizontal Plane Choked Vapor Jets With Low Condensation Potential
Authors: Eden T. J. , Miller T. F. , Jacobs H. R.
Published: February 2008

A study of plane, underexpanded, condensing vapor jets was undertaken using flash photography and a ventilated pressure probe. This study examined horizontal jets with much lower condensation driving potentials than have been previously studied. Photographic measurements of jet expansion angles, spread angles, cavity lengths, and cavity shapes were recorded and compared with numerical predictions using a parabolic, locally homogeneous flow model that had been modified to incorporate entrainment and condensation effects. When rendered dimensionless by the nozzle width rather than diameter, the plane condensation length agreed well with previously published round jet correlations for higher condensation driving potentials. At lower condensation driving potentials, the jets began to disperse, showing behavior similar to submerged air and energetic reacting vapor jets. Numerical predictions of condensation length were in good agreement over the entire range of measurement. Numerical predictions of vapor cavity shape were in reasonable agreement at higher condensation potentials but underpredicted the width of the vapor cavity at lower potentials. Pressure measurements showed the existence of periodic expansion/compression cells associated with underexpanded noncondensing gas jets. When these measurements were compared with similar measurements of air jets into quiescent water baths, the lengths of the initial steam vapor expansion/compression cells were substantially greater than those of the air jets, and the degree of pressure recovery over the cell length was substantially less.
Experimental Study of Turbulent Flow Heat Transfer and Pressure Drop in a Plate Heat Exchanger With Chevron Plates
Authors: Muley A. , Manglik R. M.
Published: February 2008

Experimental heat transfer and isothermal pressure drop data for single-phase water flows in a plate heat exchanger (PHE) with chevron plates are presented. In a single-pass U-type counterflow PHE, three different chevron plate arrangements are considered: two symmetric plate arrangements with β = 30 deg/30 deg and 60 deg/60 deg, and one mixed-plate arrangement with β = 30 deg/60 deg. For water (2 < Pr < 6) flow rates in the 600 < Re < 104 regime, data for Nu and f are presented. The results show significant effects of both the chevron angle β and surface area enlargement factor φ. As β increases, and compared to a flat-plate pack, up to two to five times higher Nu are obtained; the concomitant f, however, are 13 to 44 times higher. Increasing φ also has a similar, though smaller effect. Based on experimental data for Re a 7000 and 30 deg ≤ β ≤ 60 deg, predictive correlations of the form Nu = C1,(β) D1(φ) Rep1(β)Pr1/3(μ/μw)0.14 and f = C2(β) D2(φ) Rep2(β) are devised. Finally, at constant pumping power, and depending upon Re, β, and φ, the heat transfer is found to be enhanced by up to 2.8 times that in an equivalent flat-plate channel.
Performance Analysis and Optimization of Eccentric Annular Disk Fins
Authors: Kundu B. , Das P. K.
Published: February 2008

In the first part of the paper, a semi-analytical method has been described for solving the two-dimensional heat conduction equation in an eccentric annular disk fin circumscribing a circular tube, subjected to convective cooling. Analysis has been done considering both convective and insulated conditions at the fin tip. The effects of surface and tip heat transfer coefficients and eccentricity on the performance of the fin have been studied. Comparative studies have also been made between the performance of concentric and eccentric fins with same radius ratio. Next, the optimum dimensions for eccentric annular fins have been determined using Lagrange multiplier technique. In the scheme, either the fin volume or the heat transfer duty can be taken as the constraint. Finally, it has been shown that when space restriction is imposed on one side of the tube, eccentric annular fins can be designed to have lesser volumes compared to concentric annular fins above a certain heat transfer duty.
Accounting for Penetration of Laser Heating in Flash Thermal Diffusivity Experiments
Authors: McMasters R. L. , Beck J. V. , Dinwiddie R. B. , Wang H.
Published: February 2008

Since the early 1960s, the laser flash method of thermal diffusivity measurement has been used on a large variety of materials. Several parameter estimation methods have also been used in analyzing such experiments, employing various levels of sophistication. Estimation of thermal parameters, using the models developed as part of this research, is performed on experimental data from the Oak Ridge National Laboratory in Oak Ridge, TN. The material used is carbon bonded carbon fiber (CBCF) which is designed as an insulating material for atmosphere re-entry applications. Ambient temperatures in the experiments range from 800°C to 1200°C. The approximate thermal diffusivity of the material is 0.3 mm2/sec. This research investigates the penetration of the laser flash beyond the surface of the material being heated. Three heat transfer models are presented, each with different assumptions about the initial temperature distribution inside the material. An evaluation is made of the response of the methods to factors which may enter into the experimental process. This is done in quantitative terms so as to assess the adequacy of the models in comparison to one another.
A Conservative Iterative-Based Zonal Decomposition Scheme for Conduction Heat Transfer Problems
Authors: Ruiz O. E. , Black W. Z.
Published: February 2008

A new conservative iterative-based zonal decomposition technique for the solution of complex heat conduction problems is proposed. This numerical technique is based on dividing the domain into subdomains and ensuring that the heat flux and temperature are continuous at the boundary between subdomains. An example problem is used to illustrate the zonal decomposition technique for both steady and transient problems. This numerical technique results in accuracy which equals or exceeds traditional finite difference solutions and solution times which are significantly less than traditional finite difference solutions. A numerical relaxation parameter is introduced and its value is optimized to provide the most rapid convergence to an accurate solution.
Prediction of Heat Transfer in Turbulent, Transpired Boundary Layers
Authors: Sucec J.
Published: February 2008

Considered here is turbulent boundary layer flow with injection or suction and pressure gradient along the surface. The velocity and thermal inner laws for transpired turbulent boundary layers are represented by simple power law forms which are then used to solve the integral form of the thermal energy equation. Solving this equation leads to the variation of Stanton number with position, x, along the surface. Predicted Stanton numbers are compared with experimental data for a number of different cases. These include both blowing and suction with constant blowing fractions, F, in zero and non-zero pressure gradient and more complicated situations in which the blowing fraction, F, varies with position or where F or the surface temperature have step changes in value.
Numerical Solutions of Three-Dimensional Non-Grey Gas Radiative Transfer Using the Statistical Narrow-Band Model
Authors: Liu F.
Published: February 2008

Three-dimensional non-grey gas radiation analyses were conducted using the statistical narrow-band model along with up-dated band parameters. The exact narrow-band averaged radiative transfer equation was solved using a ray-tracing method. Accurate numerical results were presented for non-grey real gas radiative transfer in a three-dimensional rectangular enclosure containing (i) an isothermal pure water vapor at 1000 K and 1 atm, (ii) an isothermal and inhomogeneous H2O/N2 mixture at 1000 K and 1 atm, and (iii) a nonisothermal and homogeneous mixture of CO2/H2O/N2 at 1 atm.
A Model of Bubble Nucleation on a Micro Line Heater
Authors: Oh S.-D. , Seung S. S. , Kwak H. Y.
Published: February 2008

The bubble nucleation mechanism on a cavity-free micro line heater surface was studied by using the molecular cluster model. A finite difference numerical scheme for the three-dimensional transient conduction equation for the liquid was employed to estimate the superheated volume where homogeneous bubble nucleation could occur due to heat diffusion from the heater to the liquid. Calculation results revealed that bubble formation on the heater is possible when the temperature at the hottest point in the heater is greater than the superheat limit of the liquid by 6°C–12°C, which is in agreement with the experimental results. Also it was found that the classical bubble nucleation theory breaks down near the critical point where the radius of the critical bubble is below 100 nm.
Experimental and Numerical Investigation of Forced Convective Characteristics of Arrays of Channel Mounted Obstacles
Authors: Young T. J. , Vafai K.
Published: February 2008

An experimental investigation of the forced convective heat transfer of individual and arrays of multiple two-dimensional obstacles is reported. The airflow rate was varied from 800 ≤ ReDh ≤ 13000. The effects upon the Nusselt numbers and obstacle temperature differences of parametric changes in the Reynolds number, channel height, array configuration, and input heat flux are established. The input heat fluxes to the obstacles ranged from 950 ≤ q″ ≤ 20200 W/m2, which significantly extends beyond that seen in the open literature for forced convective air cooling of simulated electronic components. Comparisons of the obstacle mean Nusselt numbers are made with a two-dimensional laminar numerical model employing the Navier-Stokes equations. A set of correlations characterizing the heat transfer from the protruding heat sources within the channel is obtained. It was found that the obstacle temperature, the critical measure for electronic device failure, must be shown along with the corresponding Nusselt number to fully characterize the thermal state of the heated obstacle as the ratio definition of the Nusselt number can obscure large temperature increases. The results find that the proper placement of geometrically dissimilar obstacles, such as a taller obstacle, can be used to passively enhance the heat transfer in its vicinity. This effect would be dependent upon the flow rate and geometries in order to control the reattachment zones and their subsequent convective augmentation. The experimental results are found to be in good agreement with the results from the numerical simulation. Finally, a set of pertinent correlations for the arrays of channel mounted obstacles is given.
Three-Dimensional Simulation of Laminar Rectangular Impinging Jets, Flow Structure, and Heat Transfer
Authors: Sezai I. , Mohamad A. A.
Published: February 2008

The flow and heat transfer characteristics of impinging laminar jets issuing from rectangular slots of different aspect ratios have been investigated numerically through the solution of three-dimensional Navier-Stokes and energy equations in steady state. The three-dimensional simulation reveals the existence of pronounced streamwise velocity off-center peaks near the impingement plate. Furthermore, the effect of these off-center velocity peaks on the Nusselt number distribution is also investigated. Interesting three-dimensional flow structures are detected which cannot be predicted by two-dimensional simulations.
The Effects of Property Variations on Natural Convection in a Square Enclosure
Authors: Emery A. F. , Lee J. W.
Published: February 2008

Natural convection in a square enclosure with heated vertical walls and temperature-dependent conductivity and viscosity was simulated for Prandtl numbers ranging from 0.01 to 1.0 and Ra ≤ 106. Although the variable properties produced observable changes in the the temperatures and velocities, the overall heat transfer, as represented by the Nusselt number, was found to be unaffected and was accurately correlated in terms of the Rayleigh and Prandtl numbers using Nu=0.185Ra00.278Pr00.089 with properties evaluated at the average temperature. This correlation applies to both isothermal and constant heat flux hot walls.
Developing Free Convection in Open-Ended Vertical Eccentric Annuli With Isothermal Boundaries
Authors: El-Shaarawi M. A. I. , Mokheimer E. M. A.
Published: February 2008

A model has been developed to numerically study the developing laminar free convection in vertical open-ended eccentric annuli with isothermal boundary conditions. For a fluid of Prandtl number 0.7 in an annulus of radius ratio 0.5, the developing velocity profiles in addition to the development of the pressure along the annulus are presented. The channel heights required to naturally induce different flow rates have also been obtained for three values of the dimensionless eccentricity, namely, 0.1, 0.5, and 0.7. Variations of the total heat absorbed by the fluid with the channel height are provided for the three values of dimensionless eccentricity.
Two Constructal Routes to Minimal Heat Flow Resistance via Greater Internal Complexity
Authors: Bejan A. , Dan N.
Published: February 2008

This paper shows that the geometry of the heat flow path between a volume and one point can be optimized in two fundamentally different ways. In the “growth” method of the original constructal theory the structure is optimized starting from the smallest volume element of fixed size. Growth, or optimal numbers of constituents assembled into larger volumes, is one route to resistance minimization. In the “design” method the overall volume is fixed, and the designer works “inward” by optimizing the internal features of the heat flow path. The design method is new. It is shown analytically that the two methods produce comparable geometric results in which the high-conductivity channels form constructal tree networks, and where the low-conductivity material fills the interstices. For simplicity, it is assumed that the high-conductivity channels and their tributaries make 90-deg angles. In both approaches, the overall resistance decreases as the internal complexity of the conductive composite increases. In the growth method the number of constituents in each assembly can be optimized. In the design method, some of the constituent numbers cannot be optimized: these numbers assume the roles of weak parameters. The growth method is the simplest, and provides a useful approximation of the design and performance that can be achieved using the design method. Numerical solutions of the volume-to-point optimization problem confirm the results obtained analytically, and show that the geometric features of the optimal design are robust.
Effects of Surface Orientation on the Pool Boiling Heat Transfer in Water/2-Propanol Mixtures
Authors: Ahmed S. , Carey V. P.
Published: February 2008

To explore the role of Marangoni effects in the pool boiling heat transfer of binary mixtures, experiments have been conducted with water/2-propanol mixtures at three different concentrations under normal gravity with different orientations of the heater surface. The system pressure was subatmospheric (∼5.5 kPa) and the bulk liquid temperature was near the saturation temperature of the fluids tested. The molar concentrations of2-propanol tested were 0.015, 0.025, and 0.1. These concentrations of 2-propanol are selected because their strong variation of surface tension with concentration gives rise to high surface tension gradients near the heater surface during nucleate boiling. Boiling curves were obtained both for an upward-facing and a downward-facing heater surface. For each concentration of 2-propanol, the critical heat flux has been reached in both orientations of the heater surface. Models of pool boiling heat transfer and the critical heat flux condition for binary mixtures are tested to correlate the data. Comparison of boiling curves and CHF obtained at different orientations of the heater surface indicates that there is strong gravity independent mechanism of boiling heat transfer in these mixtures.
Numerical Modeling of Conjugate Heat Transfer on Complex Geometries With Diagonal Cartesian Method, Part I: Methods
Authors: Lin W. L , Carlson K. D. , Chen C.-J.
Published: February 2008

In this study, a diagonal Cartesian method for thermal analysis is developed for simulation of conjugate heat transfer over complex boundaries. This method uses diagonal line segments in addition to Cartesian line segments to approximate complex boundaries in Cartesian coordinates. The velocity fields are also modeled using the diagonal Cartesian method. The transport equations are discretized with the finite analytic (FA) method. The current work is validated by simulating a rotated lid-driven cavity flow with conjugate heat transfer, and accurate results are obtained.
Numerical Modeling of Conjugate Heat Transfer on Complex Geometries With Diagonal Cartesian Method, Part II: Applications
Authors: Carlson K. D. , Lin W. L. , Chen C.-J.
Published: February 2008

Part I of this study discusses the diagonal Cartesian method for temperature analysis. The application of this method to the analysis of flow and conjugate heat transfer in a compact heat exchanger is given in Part II. In addition to a regular (i.e., Cartesian-oriented) fin arrangement, two complex fin arrangements are modeled using the diagonal Cartesian method. The pressure drop and heat transfer characteristics of the different configurations are compared. It is found that enhanced heat transfer and reduced pressure drop can be obtained with the modified fin arrangements for this compact heat exchanger.
Recovery of Temperature and Species Concentration Profiles in Flames Using Low-Resolution Infrared Spectroscopy
Authors: Yousefian F. , Sakami M. , Lallemand M.
Published: February 2008

The temperature and species distributions in semi-transparent gaseous axisymmetric objects are retrieved by carrying out the inversion of their directional low-resolution spectral transmission and/or spectral emission in the infrared by using methods based on the solution of the radiative transfer equation. The validity domains of the hypothesis on which the low resolution inversion methods are based are discussed by comparison with the corresponding exact radiative properties generated by line-by-line descriptions. A propane-air laminar premixed flame is experimentally studied. The infrared data are collected for the (ν3) band of CO2, either by a directional scanning (at a fixed frequency) or by a frequency scanning (in a fixed direction). Depending on the data acquisition methods, two different reconstruction techniques are used. For spatial scanning, a generalized Abel transformation or the regularized Murio’s method, is utilized. For spectral scanning the recovered results are carried out by the Chahine-Smith method. For both techniques, profile reconstructions are worked out accounting for a spectral data bank based on the narrow statistical band model. Finally, thermal and CO2 concentration mappings of the premixed flame are presented.
Measuring Thermal Conductivity of Fluids Containing Oxide Nanoparticles
Authors: Lee S. , Choi S. U.-S. , Li S. , Eastman J. A.
Published: February 2008

Oxide nanofluids were produced and their thermal conductivities were measured by a transient hot-wire method. The experimental results show that these nanofluids, containing a small amount of nanoparticles, have substantially higher thermal conductivities than the same liquids without nanoparticles. Comparisons between experiments and the Hamilton and Crosser model show that the model can predict the thermal conductivity of nanofluids containing large agglomerated Al2O3 particles. However, the model appears to be inadequate for nanofluids containing CuO particles. This suggests that not only particle shape but size is considered to be dominant in enhancing the thermal conductivity of nanofluids.
Higher-Order Sensitivity of Heat Conduction Problems to Random Data Using the Spectral Stochastic Finite Element Method
Authors: Ghanem R.
Published: February 2008

The spectral formulation of the stochastic finite element method is applied to the problem of heat conduction in a random medium. Specifically, the conductivity of the medium, as well as its heat capacity are treated as uncorrelated random processes with spatial random fluctuations. This paper introduces the basic concepts of the spectral stochastic finite element method using a simple one-dimensional heat conduction examples. The implementation of the method is demonstrated for both Gaussian and log-normal material properties. Moreover, the case of the material properties being modeled as random variables is presented as a simple digression of the formulation for the stochastic process case. Both Gaussian and log-normal models for the material properties are treated.
Swirling Effects on Laminarization of Gas Flow in a Strongly Heated Tube
Authors: Torii S. , Yang W.-J.
Published: February 2008

A numerical study is performed to investigate thermal transport phenomena in a process of laminarization from a turbulent flow in a strongly heated circular tube in coaxial rotation. The k-ε turbulence and t2-εt, heat transfer models are employed to determine the turbulent viscosity and eddy diffusivity for heat, respectively. The governing boundary layer equations are discretized by means of a control-volume finite difference technique and numerically solved using a marching procedure. When the tube is at rest, it is disclosed that: (i) when laminarization occurs, the streamwise velocity gradient at the wall is diminished along the flow, resulting in a substantial reduction in the turbulent kinetic energy over the whole tube cross section, (ii) the attenuation causes a deterioration in heat transfer performance, and (iii) simultaneously, both the turbulent heat flux and temperature variance diminish over the whole tube cross section in the flow direction. However, the presence of tube rotation contributes to the promotion of laminarization of gas flow. The mechanism is that a reduction in the velocity gradient induced by tube rotation suppresses the production of turbulent kinetic energy, resulting in an amplification in laminarizing the flow process.
Numerical Investigation of Flow Field and Heat Transfer in Cross-Corrugated Ducts
Authors: Blomerius H. , Ho¨lsken C. , Mitra N. K.
Published: February 2008

Flow field and heat transfer in sine-wave crossed-corrugated ducts have been investigated by numerical solution of the Navier-Stokes and energy equations in the laminar and transitional flow regime between Re = 170 and 2000. The ratio of the corrugation wave length λ* to amplitude a* has been varied between 7 and 10. The angle of the corrugation of the neighboring plates has been kept fixed at 45 deg. Results show that the critical Reynolds number for self-sustained flow oscillations is about 240. For Reynolds numbers larger than 1000, the Nusselt number and the friction factor are nearly independent of the dimensionless wavelength. Computational results compare well with available experimental results.
Turbulent Heat Transport in a Perturbed Channel Flow
Authors: Buice C. U. , Eaton J. K.
Published: February 2008

The recovering boundary layer downstream of a separation bubble is known to have a highly perturbed turbulence structure which creates difficulty for turbulence models. The present experiment addressed the effect of this perturbed structure on turbulent heat transport. The turbulent diffusion of heat downstream of a heated wire was measured in a perturbed channel flow and compared to that in a simple, fully developed channel flow. The turbulent diffusivity of heat was found to be more than 20 times larger in the perturbed flow. The turbulent Prandtl number increased to 1.7, showing that the turbulent eddy viscosity was affected even more strongly than the eddy thermal diffusivity. This result corroborates previous work showing that boundary layer disturbances generally have a stronger effect on the eddy viscosity, rendering prescribed turbulent Prandtl number models ineffective in perturbed flows.
Local and Instantaneous Heat Transfer Characteristics of Arrays of Pulsating Jets
Authors: Sheriff H. S. , Zumbrunnen D. A.
Published: February 2008

Recent investigations have revealed that pulsations in an incident jet flow can be an effective technique for modifying convective heat transfer characteristics. While these studies focused on single impinging jets, industrial applications of impinging jets usually involve arrays of jets. To explore the effects of flow pulsations on the heat transfer performance of jet arrays, an experimental investigation has been performed of instantaneous and time-averaged convective heat transfer to a square, in-line array of circular air jets within an unit cell of the array. Hot-film anemometry was used to document the jet flow field. Instantaneous and time-averaged convective heat transfer rates were measured using a heat flux microsensor. An ensemble averaging technique was used to separate the pulsating component of flow velocity and heat transfer from the turbulent components and thereby assess the effect of flow pulsation on turbulence intensity and heat transfer. For the ranges of parameters considered, results indicate convective heat transfer distributions become more uniform in response to pulsations but heat transfer is not enhanced. Improved uniformity can be a useful aspect in many jet applications.
Radiative Heat Transfer in Periodic Geometries Using a Finite Volume Scheme
Authors: Mathur S. R. , Murthy J. Y.
Published: February 2008

A procedure for computing radiative heat transfer in translationally and rotationally periodic geometries is presented. The finite volume scheme is applied to meshes composed of arbitrary polyhedral control volumes. The angular domain is discretized into a finite number of control angles over which radiant energy is conserved. At periodic boundaries, control angle overhang occurs because of the misalignment of the arbitrary periodic face with the global angular discretization and due to the arbitrary rotation of adjacent modules with respect to each other. A discretization scheme using control angle pixelation is developed to conservatively transfer radiant energy between adjacent modules. The method is tested for a variety of radiation problems and shown to perform satisfactorily.
Nucleate Boiling Heat Transfer of Binary Mixtures at Low to Moderate Heat Fluxes
Authors: Benjamin R. J. , Balakrishnan A. R.
Published: February 2008

A model for nucleate pool boiling heat transfer of binary mixtures has been proposed based on an additive mechanism. The contributing modes of heat transfer are (i) the heat transferred by microlayer evaporation, (ii) the heat transferred by transient conduction during the reformation of the thermal boundary layer, and (iii) the heat transferred by turbulent natural convection. The model takes into account the microroughness of the heating surface which has been defined quantitatively. The model compares satisfactorily with data obtained in the present study and in the literature. These data were obtained on a variety of heating surfaces such as a vertical platinum wire, a horizontal stainless steel tube and flat horizontal aluminium, and stainless steel surfaces (with various surface finishes) thereby demonstrating the validity of the model.
Heat Transfer Behavior on Small Horizontal Heaters During Pool Boiling of FC-72
Authors: Rule T. D. , Kim J.
Published: February 2008

Detailed local measurements of wall heat flux during saturated pool boiling of FC-72 on a small heated area without sidewalls were made using an array of 96 temperature-controlled heaters. Data were obtained in the nucleate boiling, critical heat flux, and transition boiling regimes. The space and time resolved data were used to conditionally sample the heat flux according to whether or not boiling occurred on the surface, enabling the separation of the heat flux due to boiling from that due to natural convection or vapor contact. The heat transfer from the edge heaters was observed to be much higher than that for the inner heaters above the critical temperature. The heat transfer during liquid contact in transition boiling was constant for a given wall superheat for the inner heaters, and was observed to decrease with increasing wall superheat.
Assessment of CHF Enhancement Mechanisms in a Curved, Rectangular Channel Subjected to Concave Heating
Authors: Sturgis J. C. , Mudawar I.
Published: February 2008

An experimental study was undertaken to examine the enhancement in critical heat flux (CHF) provided by streamwise curvature. Curved and straight rectangular flow channels were fabricated with identical 5.0 × 2.5 mm cross sections and heated lengths of 101.6 mm in which the heat was applied to only one wall—the concave wall (32.3 mm radius) in the curved channel and a side wall in the straight. Tests were conducted using FC-72 liquid with mean inlet velocity and outlet subcooling of 0.25 to 10 m s−1 and 3 to 29°C, respectively. Centripetal acceleration for curved flow reached 315 times earth’s gravitational acceleration. Critical heat flux was enhanced due to flow curvature at all conditions but the enhancement decreased with increasing subcooling. For near-saturated conditions, the enhancement was approximately 60 percent while for highly subcooled flow it was only 20 percent. The causes for the enhancement were identified as (1) increased pressure on the liquid-vapor interface at wetting fronts, (2) buoyancy forces and (3) increased subcooling at the concave wall. Flow visualization tests were conducted in transparent channels to explore the role of buoyancy forces in enhancing the critical heat flux. These forces were observed to remove vapor from the concave wall and distribute it throughout the cross section. Vapor removal was only effective at near-saturated conditions, yielding the observed substantial enhancement in CHF relative to the straight channel.
The Measurement of Transient Two-Dimensional Profiles of Velocity and Fuel Concentration Over Liquids
Authors: Ito A. , Narumi A. , Konishi T. , Tashtoush G. , Saito K. , Cremers C. J.
Published: February 2008

We recently developed two different optical techniques in order to simultaneously measure transient two-dimensional profiles of velocity, temperature, and fuel concentration that were generated by a spreading flame over liquid fuels. One technique employs a particle-track system combined with a laser-sheet system (LSPT) and a high-speed camera, while the other technique employs dual wavelength holographic interferometry (DWHI). The LSPT system revealed transient two dimensional profiles of flame-induced flow, while DWHI revealed two-dimensional profiles of fuel concentration over liquids. In this paper we present a series of velocity profiles for a pulsating flame spread over 1-propanol and concentration profiles for gaseous 1-propanol determined with LSPT and DWHI, respectively.
An Experimental Investigation Into the Effects of Grain Transport on Columnar to Equiaxed Transition During Dendritic Alloy Solidification
Authors: Gao J. W. , Wang C. Y.
Published: February 2008

An experimental study has been conducted to investigate the effects of grain transport on the columnar to equiaxed transition (CET) in dendritic alloy solidification. Using the aqueous ammonium chloride solution as a transparent model alloy, experiments were performed in a vertical test cell with cooling from the top, resulting in unidirectional columnar crystals growing downwards. Ahead of the columnar front, equiaxed nuclei were observed to originate mostly by fragmentation of the columnar dendrites in the presence of a thermally driven flow in the melt beneath the columnar mushy zone. Being heavier than the liquid, these fragments fall into the bulk melt where they may grow or remelt. The survived equiaxed crystals finally settle towards the floor and pile up to form an equiaxed bed. The CET occurs when the bottom equiaxed packed bed rises and eventually obstructs the columnar mushy zone growing from the upper surface. Therefore, the CET in the present configuration was predominantly controlled by the sedimentation of equiaxed crystals. A parametric study by varying initial concentration, cooling rate, and superheat was performed.
Temperature Response of Heat Transport in a Micro Heat Pipe
Authors: Peterson G. P. , Ma H. B.
Published: February 2008

A detailed mathematical model for predicting the heat transport capability and temperature gradients that contribute to the overall axial temperature drop as a function of heat transfer in a micro heat pipe has been developed. The model utilizes a third-order ordinary differential equation, which governs the fluid flow and heat transfer in the evaporating thin film region; an analytical solution for the two-dimension heat conduction equation, which governs the macro evaporating film region in the triangular corners; the effects of the vapor flow on the liquid flow in the micro heat pipe; the flow and condensation of the thin film caused by the surface tension in the condenser; and the capillary flow along the axial direction of the micro heat pipe. With this model, the temperature distribution along the axial direction of the heat pipe and the effect on the heat transfer can be predicted. In order to verify the model presented here, an experimental investigation was also conducted and a comparison with experimental data made. This comparison indicated excellent correlation between the analytical model and experimental results, and as a result, the analysis provides a better understanding of the heat transfer capability and temperature variations occurring in micro heat pipes.
Heat Transfer of Sprays of Large Water Drops Impacting on High Temperature Surfaces
Authors: Cox T. L. , Yao S. C.
Published: February 2008

Experiments were performed to evaluate the heat transfer of monodisperse sprays of large droplet diameters, ranging from 3 to 25 mm, on high temperature surfaces. This range of drop sizes has not previously been studied, and it was of interest to determine their heat transfer characteristics and how they relate to sprays of smaller drops. Parametric tests showed that the spray heat flux depends on mass flux with a power-law relationship, and that spray effectiveness, which relates the actual spray heat transfer to the maximum heat transfer potential, varies with d−1/2. There was no discernible relationship between the heat transfer and droplet velocity. These results agreed favorably with published results for smaller droplets. The current data was compared to previous tests with smaller droplets using the droplet Reynolds and Weber numbers. This analysis showed some grouping, with a marked separation at We = 80, where the dynamic behavior of droplets has been shown to change for nonwetting impaction.
Effects of Surface Tension and Binary Diffusion on Pool Boiling of Dilute Solutions: An Experimental Assessment
Authors: Kandlikar S. G. , Alves L.
Published: February 2008

Pool boiling heat transfer with dilute binary mixtures introduces two additional effects due to binary diffusion, and due to change in the surface tension. The secondary effects due to changes in contact angle and wetting characteristics may also play a role. The present study focuses on identifying these effects for dilute aqueous solutions of ethylene glycol. Pool boiling experiments are conducted to generate data in the range of one to ten percent mass fraction. It is found that in the low concentration region, the binary diffusion effects are insignificant for aqueous solutions of ethylene glycol, and a slight improvement in heat transfer coefficient is observed over the pure water value. The binary diffusion effects are related to a volatility parameter, V1. The heat transfer coefficient does not degrade in the region where V1 < 0.03, and the surface tension does not change appreciably compared to pure water value. This points to the possibility that the changes in contact angle and wetting characteristics play an important role in the pool boiling heat transfer.
A Phase-Sensitive Technique for the Thermal Characterization of Dielectric Thin Films
Authors: Indermuehle S. W. , Peterson R. B.
Published: February 2008

A phase-sensitive measurement technique for determining two independent thermal properties of a thin dielectric film is presented. The technique involves measuring a specimen’s front surface temperature response to a periodic heating signal over a range of frequencies. The phase shift of the temperature response is fit to an analytical model using thermal diffusivity and effusivity as fitting parameters, from which the thermal conductivity and specific heat can be calculated. The method has been applied to 1.72-μm thick films of SiO2 thermally grown on a silicon substrate. Thermal properties were obtained through a temperature range from 25°C to 300°C. One interesting outcome stemming from analysis of the experimental data is the ability to extract both thermal conductivity and specific heat of a thin film from phase information alone. The properties obtained with this method are slightly below the bulk values for fused silica with a measured room temperature (25°C) thermal conductivity of 1.28 ± 0.12 W/m-K.