bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermocapillary Driven Turbulent Heat Transfer
Authors: Arpacı V. S. , Kao S.-H.
Published: February 2008

A dimensionless number depending on the usual Prandtl and Marangoni numbers, Πs ∼ Ma/(1 + Pr1) = Ma Pr/(1 + Pr), is introduced for thermocapillary driven flows. Three heat transfer models are proposed in terms of Πs. The first model on laminar flow, using some dimensional arguments with a flow scale and the boundary layer concept, leads to Nu ∼ Πs1/4, Nu being the usual Nusselt number. The second model on transition flow, extending Landau’s original idea on the amplitude of disturbances past marginal stability of isothermal flow, leads to Nu − 1 ∼ (ΠS−ΠSc)1/2, ΠSc corresponding to the critical value of Πs for the marginal state. The third model on turbulent flow, introduces a thermal microscale ηθ ∼ (1 + Pr-1)1/4(να2/Ps)1/4 = (1 + Pr)1/4 (α3/Ps)1/4, with ν and α, respectively, being kinematic and thermal diffusivities, and Ps the production rate of thermocapillary energy. The first expression relating ηθ to Prandtl number explicitly includes its limit for Pr → ∞, ηθB ∼ (να2/ε)1/4, which is a Batchelor scale, and the second one explicitly includes its limit for Pr → 0, ηθC ∼ (α3/ε)1/4, which is an Oboukhov-Corrsin scale. In terms of ηθ and an integral scale l, the model leads to Nu ∼ l/ηθ ∼ Πs1/3. Recent experimental literature are interpreted by special cases of the foregoing models corresponding to Pr > 1.
Thermocapillary Effects on the Stability of a Heated, Curved Meniscus
Authors: Pratt D. M. , Brown J. R. , Hallinan K. P.
Published: February 2008

An investigation of thermocapillary effects on heated menisci formed by volatile liquids in capillary pumped heat transfer devices has been conducted. This research was motivated by the importance of the evaporation process from porous or grooved media integral to the operation of capillary pumped heat transport devices such as heat pipes and capillary pumped loops. From analysis, a criteria was established which predicts the thermal conditions at which the destablizing influences of thermocapillary stresses near the contact line of a heated and evaporating meniscus cause the meniscus to become unstable. Experimentally, two different idealized models of capillary pumped phase change loops were investigated to assess the suitability of the predictions. Correspondence between theory and experiment was observed. Given the observed dry-out of the evaporator at higher heat inputs after the meniscus becomes unstable, the importance of predicting the conditions at the instability onset is made clear.
Two-Phase Heat Dissipation Utilizing Porous-Channels of High-Conductivity Material
Authors: Peterson G. P. , Chang C. S.
Published: February 2008

The results of an experimental study of two-phase heat dissipation in high-conductivity porous channel heat sinks are presented. Porous channels of various sizes were fabricated using sintered copper particles inside rectangular copper channels with base dimensions of 25 mm by 25 mm, either 3 or 10 mm in height. The experiments were conducted using subcooled water as the working fluid and test conditions ranged from an inlet temperature of 85 to 95°C, inlet pressures of 1.062 to 1.219 bars, flow rates of 22.5 to 150 ml/min, and heat fluxes of 10 to 25 W/cm2. The experimental results were compared to the results predicted using a previously developed numerical model. For water with inlet subcooling in the range of 6.6 to 10.8°C, heat transfer coefficients for open channel flow were increased from 1.25 to 1.94 W/cm2°C to 1.79 to 3.33 W/cm2°C, or a 43 to 142 percent improvement through the use of porous channels with mean particle diameters of 0.97, 0.54, 0.39, or 0.33 mm. The results indicate that the high thermal conductivity of the porous material and the large solid-fluid contact area combine to create a highly effective, two-phase heat sink, which may provide an effective mechanism for cooling high heat flux microelectronics.
Temperature-Dependent Thermal Conductivity of Single-Crystal Silicon Layers in SOI Substrates
Authors: Asheghi M. , Touzelbaev M. N. , Goodson K. E. , Leung Y. K. , Wong S. S.
Published: February 2008

Self heating diminishes the reliability of silicon-on-insulator (SOI) transistors, particularly those that must withstand electrostatic discharge (ESD) pulses. This problem is alleviated by lateral thermal conduction in the silicon device layer, whose thermal conductivity is not known. The present work develops a technique for measuring this property and provides data for layers in wafers fabricated using bond-and-etch-back (BESOI) technology. The room-temperature thermal conductivity data decrease with decreasing layer thickness, ds, to a value nearly 40 percent less than that of bulk silicon for ds = 0.42 μm. The agreement of the data with the predictions of phonon transport analysis between 20 and 300 K strongly indicates that phonon scattering on layer boundaries is responsible for a large part of the reduction. The reduction is also due in part to concentrations of imperfections larger than those in bulk samples. The data show that the buried oxide in BESOI wafers has a thermal conductivity that is nearly equal to that of bulk fused quartz. The present work will lead to more accurate thermal simulations of SOI transistors and cantilever MEMS structures.
Application of Diffuse Mismatch Theory to the Prediction of Thermal Boundary Resistance in Thin-Film High-Tc Superconductors
Authors: Phelan P. E.
Published: February 2008

Thermal boundary resistance (Rb) plays an important role in the design and performance of thin-film high-temperature superconducting devices, such as infrared detectors and optical switches, which rely upon the temperature rise of the film as the basis for their operation. Although there is general agreement on the magnitude of Rb from experimental data, there is at present no generally accepted theory capable of predicting Rb for these films, particularly at the intermediate cryogenic temperatures where they are likely to be used. Here, the Diffuse Mismatch Model (DMM), which considers that all phonons reaching the interface between the film and substrate scatter diffusely, is applied to the calculation of Rb. The results indicate that when employing the Debye model for the phonon density of states, the DMM yields results slightly more in agreement with data than the Acoustic Mismatch Model (AMM). Considering the measured phonon density of states, however, greatly increases Rb over that calculated assuming the Debye model, thus bringing the DMM results in relatively good agreement with the experimental data.
Prediction of Thermal Contact Conductance in Vacuum by Statistical Mechanics
Authors: Leung M. , Hsieh C. K. , Goswami D. Y.
Published: February 2008

Despite substantial theoretical studies of thermal contact conductance in the past, the application of statistical mechanics in this field has never been attempted. This paper addresses contact conductance from macroscopic and microscopic viewpoints in order to demonstrate the promise of the statistical mechanics approach. In the first part of the derivation, the Boltzmann statistical model is applied to determine the most probable distribution of asperity heights for a homogeneously, isotropically rough surface. The result found is equivalent to Gaussian distribution, which has only been assumed but not rigorously substantiated in the past. Subsequently, the Boltzmann statistical model is applied to predict the distribution of true contact spots when two such surfaces are pressed together, resulting in a relationship between the total thermal contact conductance and the relative interfacial pressure. The numerical results are compared to published empirical data, and a good order-of-magnitude agreement is found.
Heat Transfer From a Circular Cylinder at Low Reynolds Numbers
Authors: Kurdyumov V. N. , Ferna´ndez E.
Published: February 2008

A correlation formula, Nu = W0(Re)Pr1/3 + W1(Re), that is valid in a wide range of Reynolds and Prandtl numbers has been developed based on the asymptotic expansion for Pr → ∞ for the forced heat convection from a circular cylinder. For large Prandtl numbers, the boundary layer theory for the energy equation is applied and compared with the numerical solutions of the full Navier Stokes equations for the flow field and energy equation. It is shown that the two-terms asymptotic approximation can be used to calculate the Nusselt number even for Prandtl numbers of order unity to a high degree of accuracy. The formulas for coefficients W0 and W1, are provided.
Second Law Analysis of Laminar Viscous Flow Through a Duct Subjected to Constant Wall Temperature
Authors: S¸ahin A. Z.
Published: February 2008

Entropy generation for a fully developed laminar viscous flow in a duct subjected to constant wall temperature is investigated analytically. The temperature dependence on the viscosity is taken into consideration in the analysis. The ratio of the pumping power to the total heat flux decreases considerably and the entropy generation increases along the duct length for viscous fluids. The variation of total exergy loss due to both the entropy generation and the pumping process is studied along the duct length as well as varying the fluid inlet temperature for fixed duct length. For low heat transfer conditions the entropy generation due to viscous friction becomes dominant and the dependence of viscosity with the temperature becomes essentially important to be considered in order to determine the entropy generation accurately.
Spray Cooling Enhancement by Addition of a Surfactant
Authors: Qiao Y. M. , Chandra S.
Published: February 2008

An experimental study was done on the effect of dissolving a surfactant in water sprays used to cool a hot surface. A copper surface was heated to an initial temperature of 240°C and then rapidly cooled using a spray of either pure water or an aqueous solution containing 100 ppm by weight of sodium dodecyl sulfate. The variation of surface temperature was measured during cooling, and spray impact was photographed. Addition of the surfactant was found to enhance nucleate boiling heat flux by up to 300 percent. The surface temperature required to initiate vapor bubble nucleation was reduced from 118°C to 103°C. These effects were attributed to the surfactant promoting bubble nucleation and foaming in spray droplets. Nucleate boiling heat transfer enhancement was observed at all liquid mass fluxes and droplet velocities in the range of our experiments. The surfactant slightly reduced transition boiling heat transfer, and also reduced the temperature at which spray droplets started to wet the surface. Changing the orientation of the surface with respect to gravity had no effect on heat transfer.
Thermal Aspects of a Novel Viscous Pump
Authors: Sharatchandra M. C. , Sen M. , Gad-el-Hak M.
Published: February 2008

We have previously introduced a novel method for pumping fluids via a viscous mechanism. The device essentially consists of a cylindrical rotor eccentrically placed in a channel, and it is suited for hauling highly viscous polymers in macroducts, or more common fluids in microducts. Under certain operating conditions, viscous dissipation can be important, and a significant attendant temperature rise can have adverse effects on the pump operation. For this reason, we have conducted a numerical experiment to characterize the associated phenomena. The coupled system of the two-dimensional Navier-Stokes equations, with temperature-dependent viscosity, and the energy equation, with viscous dissipation terms retained, are solved using a finite-volume method. Different types of thermal boundary conditions at the rotor-fluid interface are explored in the numerical scheme. An approximate theoretical model is also developed to analyze flow in the region between the rotor and the nearest plate (for small gaps). The results indicate that although the bulk temperature rise is minimal for typical microscale situations, significantly steep temperature gradients are observed in the region between the rotor and the nearest channel wall where the most intense shear stress occurs. For certain combinations of Re, Ec, and Pr, temperature rises along the channel wall of the order of 30 K were calculated. Moreover, for very small values of this gap, large errors in the computed flowrates and pumping power estimates can arise for large Brinkman numbers, if the effects of viscous dissipation are ignored. Furthermore, the existence of an optimum value of rotor position, such that the bulk velocity is a maximum, is demonstrated. These findings are significant, as they are indicative of trends associated with the flow of highly viscous polymeric liquids, where much larger temperature rises and their attendant degradation in performance are likely to occur.
Review: CFD Applications in the Automotive Industry
Authors: Dhaubhadel M. N.
Published: February 2008

A general review of Computational Fluid Dynamics (CFD) applications in the automotive industry is presented. CFD has come a long way in influencing the design of automotive components due to continuing advances in computer hardware and software as well as advances in the numerical techniques to solve the equations of fluid flow. The automotive industry’s interest in CFD applications stems from its ability to improve automotive design and to reduce product cost and cycle time. We are able to utilize CFD more and more in day-to-day automotive design, and we can expect better conditions for CFD applications in the coming years. CFD applications in the automotive industry are as numerous as are the codes available for the purpose. Applications range from system level (e.g., exterior aerodynamics) to component level (e.g., disk brake cooling). The physics involved cover a wide range of flow regimes (i.e., incompressible, compressible, laminar, turbulent, unsteady, steady, subsonic and transonic flows). Most of the applications fall in the incompressible range and most are turbulent flows. Although most of the flows encountered are unsteady in nature, a majority of them can be approximated as steady cases. The challenge today is to be able to simulate accurately some very complex thermo-fluids phenomena, and to be able to get CFD results fast, in order to effectively apply them in the “dynamic” design environment of frequent design changes. The key is to utilize CFD in the early design phases so that design changes and fix-ups later are minimized. Proper use of CFD early, helps to significantly reduce prototyping needs and consequently, reduce cost and cycle time.
Turbulent Flow Induced by an Impeller in a Closed Toroidal Loop
Authors: Khalid A. , Legrand J. , Rosant J. M.
Published: February 2008

Swirl flow induced by an impeller was studied in a toroidal geometry. Both mean and root mean square axial velocities were measured using an electrochemical velocity probe. The effects on the flow of the rotational speed of the impeller and the circumferential position inside the torus reactor were investigated. The values of the mean axial velocity were compared with measurements obtained from residence time distribution method and from visualization experiments. The second part of this paper is devoted to the study of turbulent toroidal flow by determining power spectrum density of axial velocity fluctuations. The turbulent intensity in the torus reactor was shown to be homogeneous far downstream of the impeller; the turbulent intensity is strongly dependent on the radial coordinate near the agitator. The dissipation rate of energy, determined from the spectral analysis, is quasi-proportional to the Reynolds number. The dissipation rate of energy enabled the estimation of Kolmogorov’s microscales in the different studied axial and radial locations. The results are consistent with the available literature.
A First Analysis of Flow Field Hysteresis in a Pump Impeller
Authors: Kaupert K. A. , Holbein P. , Staubli T.
Published: February 2008

The measured pump pressure discharge characteristic for a high specific speed radial pump (ωs = 1.7) reveals distinct discontinuities in part load operation. These pressure discontinuities occur at different threshold volume fluxes when increasing or decreasing the pump discharge and make up a hysteresis loop. The pump impeller characteristic was evaluated experimentally and numerically by taking the difference between the integrated impeller outlet and impeller inlet total pressure. The experimental and numerical characteristics agree well including the volume flux location and magnitude of the pressure discontinuities in the hysteresis loop. For volume fluxes within the hysteresis loop two stable well converged flows were calculated numerically. The numerical calculations were made on coarse and fine grids using commercially available software with and without the impeller clearance leakage flow. Further experimental and numerical comparisons are made at the impeller inlet/outlet with emphasis on the changing flow field in the hysteresis loop flow regime and its coupling to the onset of reverse flow zones. This combined application of numerical and experimental tools provides insight for the hysteresis flow field of a pump impeller characteristic.
Computation of Three-Dimensional Viscous Flow in High Reynolds Number Pump Guide Vane
Authors: Yu W. S. , Lakshminarayana B. , Thompson D. E.
Published: February 2008

A three-dimensional Navier-Stokes solver which utilizes a pressure based method is used to compute the incompressible flow field through the inlet guide vane (IGV) of a high Reynolds number pump. The solver has precise control of numerical dissipation through the second and fourth order artificial dissipation terms added to the momentum equations. A low-Reynolds-number form of two-equation turbulence model is used to account for the turbulence effects. Predicted blade surface static pressure distributions are in good agreement with the measurement. The tangential and radial components of the IGV wake velocity, as well as the IGV secondary flow, are predicted well. However, the predicted maximum defect in wake is larger than the measured data. Possible causes for this discrepancy are discussed. The defect in velocity in wakes at midspan is found to decay faster than the perturbation in velocity due to the secondary flow and wake in the tip region.
Fluid Mechanical Considerations in the Design of an Improved Aerator
Authors: Ramanathan V. , Arndt Roger E. A.
Published: February 2008

An efficient diffused aeration device was developed using the “air lift” principle. With the aid of theoretical analysis and experimentation over a wide range of geometrical parameters, simple empirical correlations were developed that accurately predict water velocity and void fraction.
Effects of Solid Particle Properties on Cavitation Erosion in Solid-Water Mixtures
Authors: Huang Si , Ihara Akio , Watanabe Hideo , Hashimoto Hiroyuki
Published: February 2008

A systematic investigation has been carried out on cavitation erosion in solid-water mixtures using vibratory test facilities and some analytical devices. A mixture of distilled water and heterogeneous solid particles is used. It is shown in this work that solid particles play significant roles on acoustic cavitation erosion through two ways when present in liquids. They, aggravate the abrasive wear of materials owing to the effects of particle size, concentration, and hardness. In addition, they relieve the damage produced by the reduction of the collapse pressure of cavitation bubble because of the variation of the physical properties in mixtures. Total erosion is dependent on this governing factor.
Effects of Inlet Conditions on the Flow in a Fishtail Curved Diffuser With Strong Curvature
Authors: Yaras M. I.
Published: February 2008

The paper presents detailed measurements of the incompressible flow at the exit of a large-scale 90-degree curved diffuser with strong curvature and significant stream-wise variation in the cross-section aspect ratio. The diffuser flow path approximates the so-called fish-tail diffuser utilized on small gas turbine engines for the transition between the centrifugal impeller and the combustion chamber. Five variations of the inlet boundary layer are considered. The results provide insight into several aspects of the diffuser flow including: the effect of flow turning on diffusion performance; the dominant structures influencing the flow development in the diffuser; and the effect of the inlet boundary layer integral parameters on the diffusion performance and the exit velocity field. The three-dimensional velocity distribution at the diffuser exit is found to be sensitive to circumferentially uniform alterations to the inlet boundary layer. In contrast, circumferential variations in the inlet boundary layer are observed to have only secondary effects on the velocity field at the diffuser exit. The static pressure recovery is observed to be comparable to the published performance of conical diffusers with equivalent included angle and area ratios. Furthermore, both the static pressure recovery and the total pressure losses are observed to be relatively insensitive to variations in the inlet boundary layer. The physical mechanisms dominating the flow development in the diffuser are discussed in light of these observations.
Convex Turbulent Boundary Layers With Zero and Favorable Pressure Gradients
Authors: Schwarz A. C. , Plesniak M. W.
Published: February 2008

A turbulent boundary layer subjected to multiple, additional strain rates, namely convex curvature coupled with streamwise pressure gradients (zero and favorable, ZPG and FPG) was investigated experimentally using laser Doppler velocimetry. The inapplicability of the universal flat-plate log-law to curved flows is discussed. However, a logarithmic region is found in the curved and accelerated turbulent boundary layer examined here. Similarity of the mean velocity and Reynolds stress profiles was achieved by 45 deg of curvature even in the presence of the strongest FPG investigated (k = 1.01 × 10−6). The Reynolds stresses were suppressed (with respect to flat plate values) due primarily to the effects of strong convex curvature (δo/R ≈ 0.10). In curved boundary layers subjected to different favorable pressure gradients, the mean velocity and normal Reynolds stress profiles collapsed in the inner region, but deviated in the outer region (y+ ≥ 100). Thus, inner scaling accounted for the impact of the extra strain rates on these profiles in the near-wall region. Combined with curvature, the FPG reduced the strength of the wake component, resulted in a greater suppression of the fluctuating velocity components and a reduction of the primary Reynolds shear stress throughout almost the entire boundary layer relative to the ZPG curved case.
Mixed Convection in Micropolar Boundary-Layer Flow Over a Horizontal Semi-Infinite Plate
Authors: Hassanien I. A.
Published: February 2008

A boundary layer analysis is presented to study the effects of buoyancy-induced streamwise pressure gradients on laminar forced convection heat transfer to micropolar fluids from a horizontal semi-infinite flat plate. The transformed boundary-layer equations have been solved numerically. The effects of the buoyancy force, material parameters, and viscous dissipative heat on the friction factor, total heat transfer, displacement thickness, and wall couple stress, as well as the details of the velocity, microrotation, and temperature fields are discussed. A comparison has been made with the corresponding results for Newtonian fluids. Micropolar fluids display drag reduction and reduced heat transfer rate as compared with Newtonian fluids. Also the micropolar properties of the fluid are found to play an important role in controlling flow separation. Furthermore, it is observed that, for high values of the buoyancy and material parameters, the flow and thermal fields are significantly affected by the presence of viscous dissipation heat.
Volume Flow-Rate Measurements and Scaling Laws for a Transverse-Inlet Multiple-Disk Fan
Authors: Schlutt M. G. , Dowling D. R.
Published: February 2008

The air moving capacity of a transverse-inlet multiple-disk fan has been measured experimentally and the results successfully collapsed with a simple scaling law. The disk fan studied was an array of several hundred closely spaced plastic disks that rotated about a common axis inside an approximately semicylindrical housing. A splitter plate lying parallel to the axis of rotation and tangent to the disks separated the inflow and outflow streams. The volume flow rate of air per unit length along the axis of rotation was measured using standard LDV techniques. For disk spacings greater than 1 mm, the volume flow rate was found to be essentially proportional to ωhR3, where ω = radian rotation rate of the disks, h = disk spacing, and R = disk radius. This scaling law was derived from a simple force balance for a small fluid element moving between rotating disks. The Reynolds number of the experiments, ωh3R/ν, was varied from 2 × 102 to 103. In addition, the air-moving capability of the disk fan was found to be nearly independent of the geometrical placement of the rotating disks within the fan housing.
Convective Heat Transfer Enhancement Due to Gas Injection Into an Impinging Liquid Jet
Authors: Zumbrunnen D. A. , Balasubramanian M.
Published: February 2008

An experimental parametric investigation has been performed to assess the effect of gas injection on convective heat transfer from a constant heat flux surface to an impinging liquid jet in the absence of boiling. Capillary tubes were inserted into a planar nozzle and extended to a point near the nozzle opening along the nozzle midplane. Air was used as the gas phase and water was used as the liquid phase. The air was injected into the capillary tubes and combined with the water to form a two-phase jet. Convective heat transfer coefficients were enhanced by factors as high as 2.2 near the stagnation line and 1.6 at a distance of five jet widths with no discernible change in pressure drop in the water supply system. Results are generalized and referenced to conditions for single-phase, liquid jet correlations to facilitate heat transfer estimations in applications.
Cooling of High-Temperature Cylindrical Surfaces Using a Water–Air Spray
Authors: Buckingham F. P. , Haji-Sheikh A.
Published: February 2008

In materials processing such as heat treating, metal extrusion, forging, etc., control of the surface heat flux rate contributes to better products. Spray cooling using a mixture of water and air is studied for these applications. It is shown that the heat transfer rate depends on the mass fraction of liquid and that it can be adjusted to a desired value. Experimental heat flux data are presented for different liquid mass fractions, and at surface temperatures up to 1000°C. Two distinct heat transfer regions above the dry wall temperature are recognized: radiation-dominated and convection-dominated, where droplets evaporate in the boundary layer. In between these two regions is a transition region. The inverse thermal conduction technique based on the Monte Carlo simulation is used to provide surface temperature and subsequently to compute heat flux.
An Experimental Study of Heat Transfer and Particle Deposition for the Modified Chemical Vapor Deposition
Authors: Cho J. , Choi M.
Published: February 2008

An experimental study has been made of heat transfer and particle deposition for the Modified Chemical Vapor Deposition process. The tube wall temperature distributions and the rates and efficiencies of particle deposition were measured. Results indicate that the axial variation of the tube wall temperature is quasi-steady; i.e., the distributions fit onto one curve if the relative distance from the moving torch is used as the axial coordinate. Due to the repeated heating from the traversing torch, the wall temperature is shown to reach a minimum ahead of the torch. It is shown that the two-torch formulation suggested by Park and Choi (1994) is valid for predicting this minimum temperature. Comparison of the measurements of the wall temperature, the particle deposition efficiency, and the tapered entry length with calculations is in good agreement. Due to chemical reactions, the tube wall temperature increases as the flow rate of the carrier gas O2 is increased. The rate of particle deposition also increases as the flow rate of the carrier gas O2 is increased, but the efficiency decreases. The effect of torch speed on the tube wall temperature and on the particle deposition have also been determined.
Effects of Gravity on Rewetting of Capillary Groove Surface at Elevated Temperatures—Experimental and Theoretical Studies
Authors: Chan S. H. , Blake J. D. , Shen T. R. , Zhao Y. G.
Published: February 2008

Theoretical and experimental investigations of the rewetting characteristics of thin liquid films over heated and unheated capillary grooved plates were performed. To investigate the effect of gravity on rewetting, the grooved surface was placed in upward and downward-facing positions. Profound gravitational effects were observed as the rewetting velocity was found to be higher in the upward than in the downward-facing orientation. The difference was even greater with higher initial plate temperatures. With either orientation, it was found that the rewetting velocity increased with the initial plate temperature. But when the temperature was raised above a rewetting temperature, the rewetting velocity decreased with the initial plate temperature. Hydrodynamically controlled and heat conduction controlled rewetting models were then presented to explain and to predict the rewetting characteristics in these two distinct regions. The predicted rewetting velocities were found to be in good agreement with experimental data with elevated plate temperatures.
Thermal Characteristics of Conventional and Flat Miniature Axially Grooved Heat Pipes
Authors: Khrustalev D. , Faghri A.
Published: February 2008

A detailed mathematical model of low-temperature axially grooved heat pipes (AGHP) is developed in which the fluid circulation is considered along with the heat and mass transfer processes during evaporation and condensation. The results obtained are compared to existing experimental data. Both capillary and boiling limitations are found to be important for the flat miniature copper-water heat pipe, which is capable of withstanding heat fluxes on the order of 40 W/cm2 applied to the evaporator wall in the vertical position. The influence of the geometry of the grooved surface on the maximum heat transfer capacity of the miniature AGHP is demonstrated.
Onset and Development of Natural Convection Above a Suddenly Heated Horizontal Surface
Authors: Goldstein R. J. , Volino R. J.
Published: February 2008

The onset and development of flow in a thick horizontal layer subject to a near-constant flux heating from below has been studied experimentally. The overall heat-flux-based Rayleigh number, Ra*, ranges from 2 × 108 to 7 × 1010. Flow visualization shows the growth and breakdown of a conduction layer adjacent to the heated surface. Convection is characterized by the release of warm meandering plumes and thermals from a boundary layer. The planform of convection at the heated surface begins with a pattern of small spots suggestive of Be´nard cells. Some of these cells expand, forming a larger cell pattern. This continues until a quasi-steady state is reached in which the former cell boundaries form a slowly moving pattern of warm lines on the heated surface. The lines are believed to be the source of the plumes and thermals. Quantitatively, the onset of convection occurs at a constant (critical) Rayleigh number based on the conduction layer thickness, Raδ. Based on the first observation of fluid motion, this critical Rayleigh number is approximately 1300. Based on the heated surface temperature the critical Rayleigh number is 2700. The nondimensional wavenumber associated with the observed instabilities at the onset of convection is about 2.2.
Dynamic Behavior of Plate Heat Exchangers—Experiments and Modeling
Authors: Das S. K. , Spang B. , Roetzel W.
Published: February 2008

Experiments on the transient behavior of two welded plate heat exchangers with identical construction but different numbers of plates have been carried out under different operating conditions. The temperature response on both sides following a step change in inlet temperature on one side has been compared to a theoretical model. The model takes the effects of flow maldistribution within the channels and between channels into account by introducing a dispersion term in the energy equation. The phase lag due to different flow path lengths between inlet or outlet of the heat exchanger and inlet or outlet of the individual channels are also taken into account. Heat conduction through the plates in the main flow direction of the fluids can be neglected for the exchangers under consideration. The model is validated by the experiments. It is found that the dispersion model considered gives a better simulation than the conventional plug flow model. From the experiments the effects of NTU, heat capacity rate ratio, and number of plates were also determined. This demonstrates the whole spectrum of dynamic behavior of plate heat exchangers. To suggest a proper control strategy for such heat exchangers, the parameters of conventional first and second-order systems with delay period have been determined from the results of the experiments and the theoretical model.
Readsorption Processes in a Sorption Bed Heat Exchanger During Pressurization and Depressurization
Authors: Zheng W. , Worek W. M.
Published: February 2008

Readsorption occurs inside a sorbent-bed heat exchanger during the pressurization and depressurization processes, causing the adsorbate to be relocated from one end of the sorption bed heat exchanger to the other. In this paper, the readsorption processes are quantitatively determined for both the pressurization and depressurization processes in a sorbent-bed heat exchanger with a uniform initial temperature, loading, and pressure. Also, the readsorption that occurs during a typical operation of a two-bed sorption heat pump is quantified. The results show that the readsorption can cause as much as 30 percent of the ammonia in a sorbent-bed heat exchanger to be reallocated.
Heat Transfer Characteristics of Arrays of Free-Surface Liquid Jets
Authors: Pan Y. , Webb B. W.
Published: February 2008

In this study, local heat transfer data under arrays of free-surface liquid jets are measured with a two-dimensional infrared radiometer. Experimental measurements were made for three nozzle diameters using a seven-jet staggered and a nine-jet inline geometric array configuration. Nozzle-to-plate spacings of two and five nozzle diameters were investigated for four jet center-to-center spacings ranging from two to eight diameters in the jet Reynolds number range of 5000 to 20,000. Results show that the stagnation Nusselt number under the central jet is independent of array configuration and jet-to-jet spacing. The different inter jet flow interaction, as represented by different jet array configurations (the in-line array and the staggered array with different nozzle-to-nozzle spacings), shows negligible influence on local heat transfer under the central jet. Differences in the heat transfer characteristics for the two nozzle-to-plate spacings investigated were the result of an observed transition from confined submerged central jet flow to free-surface jet flow as the nozzle-to-plate spacing was increased. Secondary maxima in the Nusselt number were observed between the adjacent jets, being a direct consequence of the radial flow interaction between jets. A correlation for average heat transfer is presented.
Velocity/Mixture Fraction Statistics of Round, Self-Preserving, Buoyant Turbulent Plumes
Authors: Dai Z. , Tseng L. K. , Faeth G. M.
Published: February 2008

An experimental study of the structure of round buoyant turbulent plumes was carried out, limited to conditions in the self-preserving portion of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still and unstratified air environment. Velocity/mixture-fraction statistics, and other higher-order turbulence quantities, were measured using laser velocimetry and laser-induced fluorescence. Similar to earlier observations of these plumes, self-preserving behavior of all properties was observed for the present test range, which involved streamwise distances of 87–151 source diameters and 12–43 Morton length scales from the source. Streamwise turbulent fluxes of mass and momentum exhibited countergradient diffusion near the edge of the flow, although the much more significant radial fluxes of these properties satisfied gradient diffusion in the normal manner. The turbulent Prandtl/Schmidt number, the ratio of time scales characterizing velocity and mixture function fluctuations and the coefficient of the radial gradient diffusion approximation for Reynolds stress, all exhibited significant variations across the flow rather than remaining constant as prescribed by simple turbulence models. Fourth moments of velocity and velocity/mixture fraction fluctuations generally satisfied the quasi-Gaussian approximation. Consideration of budgets of turbulence quantities provided information about kinetic energy and scalar variance dissipation rates, and also indicated that the source of large mixture fraction fluctuations near the axis of these flows involves interactions between large streamwise turbulent mass fluxes and the rapid decay of mean mixture fractions in the streamwise direction.
The Shape of an Enclosure With Uniform-Flux, Isothermal, Nonhorizontal Walls
Authors: Kuyper R. A. , Hoogendoorn C. J.
Published: February 2008

This paper presents a computational procedure and results of a numerical quest for the shape of a special kind of differentially heated air-filled enclosure. The enclosure shape is such that the natural-convection flow in the enclosure gives rise to a uniform horizontal-heat-flux distribution at the isothermal hot and cold walls. By using an evolution algorithm, the isothermal walls are “consumed” according to the heat-flux distribution until a steady-state shape is reached. For both adiabatic and perfectly conducting horizontal walls, results of calculations are presented for Rayleigh numbers up to 8 × 104. The resulting flow and temperature distributions in the steady-state enclosures are presented and discussed.
Thermal Radiation Absorption in Doped Semiconductors Due to Direct Intersubband Transitions
Authors: Hebb J. P. , Cravalho E. G. , Flik M. I.
Published: February 2008

This work proposes an engineering model for thermal radiation absorption due to direct intersubband transitions in doped semiconductors, which are excitations of bound electrons in the infrared spectral region. An existing quantum-mechanical approach is to model these transitions as a continuum of damped harmonic oscillators. This study modifies this approach to yield a more effective method for determining the optical constants of doped semiconductors. The room-temperature absorptance spectra of p-type GaAs samples with dopant concentrations from 8 × 1017 to 1 × 1020 cm-3 are measured using a Fourier transform–infrared spectrometer in the spectral region from 1.5 to 25 μm. The fitting of these spectra using the proposed model combined with the Drude and Lorentz models provides the optical constants and the dependence of the adjustable parameters on dopant concentration. The measured and fitted spectra agree closely. Potential applications of the model are semiconductor process control and infrared detector design.
Radiative Heat Transfer in Circulating Fluidized Beds
Authors: Fang Z. H. , Grace J. R. , Lim C. J.
Published: February 2008

The radiation contributions to heat transfer in circulating fluidized beds are investigated based on a simple model involving clusters and dilute suspension. Local and length-averaged cluster transfer coefficients are derived based on a cluster renewal model with combined transient conduction and radiation. A three-component network is analyzed leading to a concise relation for the suspension-to-wall radiative transfer. Previous experimental data for heat transfer to a membrane wall with bed temperatures of 407 and 860°C (Wu et al., 1989) are in good agreement with model predictions.
A Simplified Model for Predicting Vapor Bubble Growth Rates in Heterogeneous Boiling
Authors: Chen W. C. , Klausner J. F. , Mei R.
Published: February 2008

A simplified model, based on heat transfer through a wedge-shaped liquid microlayer and a lumped thermal analysis for a solid heater, is developed for predicting the vapor bubble growth rate in heterogeneous pool boiling. A first-order ordinary differential equation is obtained for the bubble growth rate. An empirical parameter, C2, which characterizes the region of the heating surface influenced by microlayer evaporation, is determined by matching the existing experimental data with the predicted growth rate. The present bubble growth model compares well with the available experimental data for intermediate and moderately wetting fluids in which Jacob number ranges from 0.52 to 1974.
Elastic Fields Resulting From Concentrated Loading on a Three-Dimensional Incompressible Wedge
Authors: Hanson M. T.
Published: February 2008

This paper considers point force or point moment loading applied to the surface of a three-dimensional wedge. The wedge is two-dimensional in geometry but the loading may vary in a direction parallel to the wedge apex, thus creating a three-dimensional problem within the realm of linear elasticity. The wedge is homogeneous, isotropic, and the assumption of incompressibility is taken in order for solutions to be obtained. The loading cases considered presently are as follows: point normal loading on the wedge face, point moment loading on the wedge face, and an arbitrarily directed force or moment applied at a point on the apex of the wedge. The solutions given here are closed-form expressions. For point force or point moment loading on the wedge face, the elastic field is given in terms of a single integral containing associated Legendre functions. When the point force or moment is at the wedge tip, closed-form (nonintegral) expressions are obtained in terms of elementary functions. An interesting result of the present research indicates that the wedge paradox in two-dimensional elasticity also exists in the three-dimensional case for a concentrated moment at the wedge apex applied in one direction, but that it does not exist for a moment applied in the other two directions.
The Elastic Field Caused by a Circular Cylindrical Inclusion—Part I: Inside the Region x12+x22<a2,−∞<x3<∞ Where the Circular Cylindrical Inclusion is Expressed by x12+x22≤a2,−h≤x3≤h
Authors: Wu Linzhi , Du Shanyi
Published: February 2008

The displacement and stress fields caused by uniform eigenstrains in a circular cylindrical inclusion are analyzed inside the region x12+x22<a2,−∞<x3<∞ and are given in terms of nonsingular surface integrals. Analytical solutions can be expressed as functions of the complete elliptic integrals of the first, second and third kind. The corresponding elastic fields in the region x12+x22>a2,−∞<x3<∞ are solved by using the same technique (by Green’s functions) in the companion paper (Part II).
The Elastic Field Caused by a Circular Cylindrical Inclusion—Part II: Inside the Region x12+x22>a2,−∞<x3<∞ Where the Circular Cylindrical Inclusion is Expressed by x12+x22≤a2,−h≤x3≤h
Authors: Wu Linzhi , Du Shanyi Y.
Published: February 2008

Analytical solutions are presented for the displacement and stress fields caused by a circular cylindrical inclusion with arbitrary uniform eigenstrains in an infinite elastic medium. The expressions obtained and those presented in Part I constitute the solutions of the whole elastic field, −∞<x1,x2,x3<∞. In the present paper, it is found that the analytical solutions within the region x12+x22>a2,−∞<x3<∞ can also be expressed as functions of the complete elliptic integrals of the first, second, and third kind. When the length of inclusion tends towards the limit (infinity), the present solutions agree with Eshelby’s results. Finally, numerical results are shown for the stress field.
Development of Generalized Plane-Strain Tensors for the Concentric Cylinder
Authors: Chandra N. , Xie Zhiyum
Published: February 2008

A pair of two new tensors called GPS tensors S and D is proposed for the concentric cylindrical inclusion problem. GPS tensor S relates the strain in the inclusion constrained by the matrix of finite radius to the uniform transformation strain (eigenstrain), whereas tensor D relates the strain in the matrix to the same eigenstrain. When the cylindrical matrix is of infinite radius, tensor S reduces to the appropriate Eshelby’s tensor. Explicit expressions to evaluate thermal residual stresses σr, σθ and σz in the matrix and the fiber using tensor D and tensor S, respectively, are developed. Since the geometry of the present problem is of finite radius, the effect of fiber volume fraction on the stress distribution can be easily studied. Results for the thermal residual stress distributions are compared with Eshelby’s infinite domain solution and finite element results for a specified fiber volume fraction.
Crack Propagation in Homogeneous and Bimaterial Sheets Under General In-Plane Loading: Nonlinear Analysis
Authors: Geubelle P. H. , Knauss W. G.
Published: February 2008

The problem of non-coplanar crack propagation in homogeneous and bimaterial sheets is investigated within the framework of the nonlinear theory of plane stress and for the Generalized Neo-Hookean class of hyperelastic solids. The analysis is performed numerically using a boundary layer approach and the maximum energy release rate criterion. The influence of the large deformation effect on the limiting process associated with the concept of “infinitesimal virtual crack extension” is examined, together with the possible relation between the size of the nonlinear zone and the additional length parameter appearing in the linearized analysis of the interfacial crack propagation problem. As the virtual crack extension is gradually shortened to a size comparable to that of the nonlinear zone, a transition is observed between the nonunique value of the kink angle predicted by the linearized theory and a single “nonlinear” value, which is independent of the crack extension length but also independent of the far-field loading conditions. In the limit of homogeneous properties this angle is zero and is corroborated by experiments on natural rubber undergoing large deformations.
Strip Element Method to Analyze Wave Scattering by Cracks in Anisotropic Laminated Plates
Authors: Liu G. R. , Achenbach J. D.
Published: February 2008

The strip element method (SEM) has been extended to investigate wave scattering by cracks in anisotropic laminated plates. The cracked plates are divided into domains in which the extended SEM is applied. For each domain a set of SEM equations is obtained which gives a relationship between the traction and displacement vectors on the vertical boundaries. These equations are solved by using the conditions at the junctures of the domains. To obtain the time domain response a Fourier transform technique is used, and an exponential window method is introduced to avoid singularities in the Fourier integration. For composite plates with horizontal and vertical cracks, scattered wave fields in both the time and frequency domains are computed, and discussed in comparison with results for uncracked plates. A technique for determining the length of a crack in a plate is also presented. It is shown that the SEM is an efficient technique for the calculation of elastodynamic fields in cracked anisotropic laminated plates.
Buckling Instability of Straight Edge Cracks
Authors: Jensen H. M. , Thouless M. D.
Published: February 2008

A buckling instability in a system where a straight edge crack lies at the interface between a thin elastic film and a substrate is analysed theoretically and experimentally. The buckling, which can occur also under remote tensile loads, may result in crack growth before the conventional criterion for fracture is met on the straight crack front by enhancing the mode adjusted crack driving force. If crack growth occurs, buckling will cause a wavy crack front to develop.