bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat Pipe Heat Sinks With Matched Coefficient of Thermal Expansion Surfaces for Direct Silicon Attach
Authors: Hartenstine John R. , Toth Jerome E.
Published: April 2009

Die level power densities are hitting unprecedented fluxes of over 200W/cm2, in turn driving the junction-to-case and case-to-sink resistances higher than the sink to ambient resistance. The biggest potential for thermal improvement now lies within the heat sink to device interface. Reduction in this critical interface resistance can be realized through direct attach of the heat sink base to the silicon. Challenges to this approach include the near matching of the coefficient of thermal expansion (CTE) of these two surfaces. Design concepts for a heat pipe heat sink with a CTE similar to that of silicon are investigated. Design concepts take into account materials, heat sink design configurations, and material compatibility with heat pipe working fluids. Prototype test articles were designed and fabricated using a copper/molybdenum/copper laminate as the heat input surface. Test data indicating an evaporator thermal resistance of 0.080 cm2·°C/W at 290W/cm2 are presented.
Unsteady Discharging of Wet Vapor From a Vessel
Authors: Fridman Ella , Dorfman Abram
Published: April 2009

A saturated liquid-vapor mixture discharging from a pressurized thermal-insulated vessel through a nozzle or a valve is considered as an unsteady-flow problem. It is shown that in the case of relatively small energy losses a valve may be modeled as a converging nozzle. After losses exceed certain value a condition of constant enthalpy typical for throttling devices is applicable to a valve. On the basis of such consideration, the step by step calculation method for small time intervals or pressure drops is developed. The proposed approach allows obtaining all characteristics of the discharging process as functions of time. The expression defining the speed of sound in the case of discharging of wet vapor is derived. A special technique is proposed to calculate and correct the usually recommended value of critical pressure ratio. A numerical example of calculation is provided. To estimate the accuracy and limits of the applicability of the results presented in the paper a comparison with corresponding experimental data is needed.
Heat Transfer in Cavities Having a Fixed Amount of Solid Material
Authors: Braga Edimilson J. , de Lemos Marcelo J. S.
Published: April 2009

This work compares two different approaches for obtaining numerical solutions for laminar natural convection within a square cavity, which is filled by a fixed amount of a solid conducting material. The first model considered, namely, porous-continuum model, is based on the assumption that the solid and the fluid phases are seen as the same medium, over which volume-averaged transport equations apply. Secondly, a continuum model is considered to solve the momentum equations for the fluid phase that would resemble a conjugate heat transfer problem in both the solid and the void space. In the continuum model, the solid phase is composed of square obstacles, equally spaced within the cavity. In both models, governing equations are numerically solved using the finite volume method. The average Nusselt number at the hot wall, obtained from the porous-continuum model, for several Darcy numbers, are compared with those obtained with the second approach, namely the continuum model, with different number of obstacles. When comparing the two methodologies, this study shows that the average Nusselt number calculated for each approach for the same Ram differs between each other and that this discrepancy increases as the Darcy number decreases, in the porous-continuum model, or the number of blocks increases and their size decreases, in the continuum model. A correlation is suggested to modify the macroscopic thermal expansion coefficient in order to match the average Nusselt numbers calculated by the two models for Ram = const = 104 and Da ranging from 1.2060×10−4 to 1.
Stability of Thin Free Liquid Films and the Onset Conditions for Film Rupture: A MD Simulation Study
Authors: Gan Yu , Carey Van P.
Published: April 2009

Free liquid films bounded on two sides by vapor are of particular interest in nucleate boiling process because the merging of bubbles is ultimately associated with the rupture of a liquid film separating the vapor inside the bubbles. The stability and conditions for the free film rupture are therefore of central importance to the merging process because they dictate whether coalescence is likely to occur. Most previous studies of free film stability have focused on a continuum model of wave phenomena at the interfaces of the film. In the investigation summarized here, molecular dynamics (MD) simulation was used to explore the stability of free films of liquid argon surrounded by saturated argon vapor. The MD simulations predict that the wave perturbation or density fluctuation induced by the thermodynamic intrinsic instability can lead to rupture of the liquid film, as its thickness decreases below a critical value. It further predicts that the critical film thickness derived from density fluctuation increases with system temperature. These predictions were also compared with those of conventional interface wave perturbation analysis. The result of this investigation suggests that during bubble merging, thermodynamic instability of the liquid phase may play a role in the onset of film rupture as the liquid film separating the bubbles gets progressively thinner.
Numerical Simulation of an Acoustically Driven Resonator With Internal Thin Parallel Plates
Authors: Lin Yiqiang , Farouk Bakhtier
Published: April 2009

Heat transfer and flow characteristics in a two-dimensional resonator with inside thin plates due to acoustic excitations are investigated numerically. The effect of the presence of the internal thin plates in the resonators is then studied. Such parallel plates (stacks) have been used in acoustic resonators for developing thermoacoustic refrigerators and thermoacoustic engines. A fully compressible form of the Navier-Stokes equations is considered for the numerical model and an explicit time-marching algorithm is used to track the acoustic waves and energy flux. Numerical solutions are obtained by employing a highly accurate flux corrected transport (FCT) algorithm. In the present model, the acoustic waves are induced by vibrations of the left wall, and the right wall is stationary. By neglecting the effect of side walls, the top and bottom boundary conditions are assumed to be symmetric. No simplifying assumption is made regarding the existence of the acoustic field. The interaction of acoustic standing waves with the internal parallel plates produces a temperature difference between the two ends which can be used for refrigeration or to do work (as a heat engine). The temperature differences are found to be significantly dependent on the location, length and gap of the internal plates. The model developed can be used for the analysis of flow and temperature fields driven by acoustic transducers, as well as in the design of high-performance resonators for thermoacoustic refrigerators and engines.
Solutions for Transient Heat Conduction With Solid Body Motion and Convective Boundary Conditions
Authors: McMasters Robert L. , Beck James V.
Published: April 2009

The analytical solution for the problem of transient thermal conduction with solid body movement is developed for a parallelepiped with convective boundary conditions. An effective transformation scheme is used to eliminate the flow terms. The solution uses Green’s functions containing convolution-type integrals, which involve integration over a dummy time, referred to as “cotime.” Two types of Green’s functions are used: one for short cotimes comes from the Laplace transform and the other for long cotimes from the method of separation of variables. A primary advantage of this method is that it incorporates internal verification of the numerical results by varying the partition time between the short and long components. In some cases, the long time solution requires a zeroth term in the summation, which does not occur when solid body motion is not present. The existence of this zeroth term depends upon the magnitude of the heat transfer coefficient associated with the convective boundary condition. An example is given for a two-dimensional case involving both prescribed temperature and convective boundary conditions. Comprehensive tables are also provided for the nine possible combinations of boundary conditions in each dimension.
Energy Loss and Pumping Power Analysis of Fully Developed Laminar Convection in a Helical Coil
Authors: Javadinezhad Seeaidreza , Asadikapourchali Zahra
Published: April 2009

Energy loss characteristics of heat transfer and fluid flow due to forced convection of steady laminar flow of incompressible fluid in a helical coil is analysed. This paper analyses energy loss and pumping power in the fully developed laminar convection in a helical coil with constant wall heat flux. The important design parameters, including Reynolds number (Re), coil to tube radius (δ) and dimensional coil pitch (λ) are varied to investigate their influences on the energy loss and pumping power values. Amount of heat transfer is the primary concern in a heat exchanger design. The amount of energy loss during the heat exchange process has been investigated by introducing a new dimensionless number This dimensionless number has been defined as the energy loss to total heat transfer rate ratio. The energy loss dimensionless number behavior have been compared with entropy generation dimensionless number behavior for this problem.
Shower Head Film Cooling Effectiveness of a Symmetrical Turbine Blade Model: Effect of Lateral Injection
Authors: Nemdili Fade´la , Azzi Abbe`s , Jubran Bassam A.
Published: April 2009

The performance of the SSG Reynolds Stress Model for the prediction of film cooling at the leading edge of a symmetrical turbine blade model is investigated. The test case blade model is symmetric and has one injection row of discrete cylindrical holes on each side near the leading edge. In the present computation several blowing ratios of 45° lateral injection are tested and compared with previous streamwise computations. Further, film cooling effectiveness contours on the blade surface and lateral averaged adiabatic film cooling effectiveness are presented and compared with available measurements. In addition to validation data, several longitudinal and transversal contours and vector planes are reproduced and clearly found to underscore the anisotropic turbulent field occurring in the present shower head film cooling configuration. The advantage of lateral versus streamwise injection is highlighted by the destruction of the two contra rotating vortices which are responsible of cooling effectiveness decrease. In case of lateral injection only one vortex is found and the flow structure is radically different from that known by streamwise jet in cross flow.
The Effect of the Top Wall Temperature on the Laminar Natural Convection in Rectangular Cavities With Different Aspect Ratios
Authors: Wu Wenjiang , Ching Chan Y.
Published: April 2009

The effect of the top wall temperature on the laminar natural convection in air-filled rectangular cavities driven by a temperature difference across the vertical walls was investigated for three different aspect ratios of 0.5, 1.0 and 2.0. The temperature distributions along the heated vertical wall were measured and the flow patterns in the cavities were visualized. The experiments were performed for a global Grashof number of approximately 1.8×108 and non-dimensional top wall temperatures from 0.52 (insulated) to 1.42. As the top wall was heated, the flow separated from the top wall with an undulated flow region in the corner of the cavity, which resulted in a non-uniformity in the temperature profiles in this region. The location and extent of the undulation in the flow is primarily determined by the top wall temperature, and nearly independent of the aspect ratio of the cavity. The local Nusselt number was correlated to the local Rayleigh number for all three cavities in the form of Nu = C · Ran, but the values of the constants C and n changed with the aspect ratio.
Modeling Natural Convection in Obstructed Channel
Authors: Wang Xiuling , Pepper Darrell W.
Published: April 2009

Simulation results for fluid and natural convective heat transfer within obstructed channels are presented. Analyses of obstructed regions are important in many engineering applications such as the design of heat exchangers, electrical devices, reactors and energy storage systems. In this study, an h-adaptive FEM algorithm is used to solve for fluid flow and heat transfer in channels with single and multiple obstructions. For comparison purposes, simulation results are also obtained for natural convective heat transfer in a smoothed channel. Results are compared with data in the literature and good agreement is observed. The model is especially attractive for accurately and efficiently assessing designs involving obstructed flow geometries.
Film-Cooling on a Gas Turbine Blade Pressure Side or Suction Side With Compound Angle Shaped Holes
Authors: Gao Zhihong , Narzary Diganta P. , Han Je-Chin
Published: April 2009

The film cooling effectiveness on the surface of a high pressure turbine blade is measured using the Pressure Sensitive Paint (PSP). Four rows of fan-shaped, laid-back compound angled cooling holes are distributed on the pressure side while two such rows are provided on the suction side of the blade. The coolant is only injected to either the pressure side or suction side of the blade at five average blowing ratios from 0.4 to 1.5. Presence of wake due to upstream vanes is simulated by placing a periodic set of rods upstream of the test blade. The wake rods can be clocked by changing their stationary positions to simulate a progressing wake. Effect of wake is recorded at four phase locations with equal intervals along the pitch-wise direction. The free stream Reynolds number, based on the axial chord length and the exit velocity, is 750,000 and the inlet and the exit Mach numbers are 0.27 and 0.44, respectively, resulting in a blade pressure ratio of 1.14. Results reveal that the tip leakage vortices and endwall vortices sweep the coolant film on the suction side to the midspan region. The fan-shaped, laid-back compound angled holes produce good coolant film coverage on the suction side except for those regions affected by the secondary vortices. Due to the concave surface, the coolant trace is short and effectiveness level is low on the pressure surface. However, the pressure side acquires relatively uniform film coverage with the design of multiple rows of cooling holes. The presence of stationary upstream wake results in lower film cooling effectiveness on the blade surface. Variation of blowing ratio from 0.4 to 1.5 shows steady increase in effectiveness on the pressure side or the suction side for a given wake rod phases locations. The compound angle shaped holes outperform the compound angle cylindrical holes by the elevated film cooling effectiveness particularly at higher blowing ratios.
Experiments on Microencapsulated Phase Change Material Suspensions Flow in Rectangular Minichannels
Authors: Rao Yu , Dammel Frank , Stephan Peter
Published: April 2009

Comparative experiments were conducted on the laminar flow frictional characteristics of suspensions with microencapsulated phase change material (MEPCM) in water flowing through rectangular copper minichannels. The MEPCM particles were mixed with distilled water to form suspensions with various mass concentrations ranging from 0% to 20%. The bulk viscosities of the suspensions were measured by using a cylindrical rotating rheometer. The experiments were performed to explore the effect of MEPCM concentration on the friction factors and pressure drops of the MEPCM suspensions flowing in minichannels. It was found out that based on the measured viscosities, the friction factors approximately agreed with those of continuous Newtonian fluid in laminar flow region. The measured pressure drops of the MEPCM suspensions flowing in the minichannels increase over the entire range of the velocity as the concentration increases.
Thin Film Phonon Heat Conduction by the Dispersion Lattice Boltzmann Method
Authors: Escobar Rodrigo A. , Amon Cristina H. , Guzma´n Amador M.
Published: April 2009

Numerical simulations of time-dependent thermal energy transport in semiconductor thin films are performed using the Lattice Boltzmann Method applied to phonon transport. The discrete Lattice Boltzmann Method is derived from the continuous Boltzmann transport equation assuming nonlinear, frequency-dependent phonon dispersion for acoustic and optical phonons. Results indicate that the heat conduction in silicon thin films displays a transition from diffusive to ballistic energy transport as the characteristic length of the system becomes comparable to the phonon mean free path, and that the thermal energy transport process is characterized by the propagation of multiple, superimposed phonon waves. The methodology is used to characterize the time-dependent temperature profiles inside films of decreasing thickness. Thickness-dependent thermal conductivity values are computed based on steady-state temperature distributions obtained from the numerical models. It is found that reducing feature size into the subcontinuum regime decreases the thermal conductivity when compared to bulk values, at a higher rate than what was displayed by the Debye-based gray Lattice Boltzmann Method.
Natural Convective Heat Transfer From a Narrow Isothermal Vertical Flat Plate With a Uniform Heat Flux at the Surface
Authors: Oosthuizen Patrick H. , Paul Jane T.
Published: April 2009

Natural convective flow over a vertical plate with a uniform heat flux over its surface has been numerically studied. When the plate is wide compared to its height the flow can be adequately modeled by assuming two-dimensional flow. However, when the width of the plate is relatively small compared to its height, the heat transfer coefficient can be considerably greater than that predicted by these two-dimensional flow results. The Nusselt number distribution over a narrow vertical plate, with a uniform heat flux at the plate surface, has been numerically determined. This heated plate is embedded in a plane adiabatic surface, the surface of the adiabatic surface being in the same plane as the heated plate. It has been assumed that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces, this having been treated by using the Boussinesq approach. It has also been assumed that the flow is symmetrical about the vertical centre-plane of the plate. The solution has been obtained by numerically solving the full three-dimensional form of the governing equations, these equations being written in dimensionless form. The solution has the Rayleigh number, the dimensionless plate width and the Prandtl number as parameters. Results have been numerically determined for a relatively wide range of Rayleigh numbers and dimensionless plate widths for a Prandtl number of 0.7. The dimensionless plate width has been found to have a significant influence on the mean Nusselt number for the plate when the plate is narrow and the Rayleigh number is low. The conditions under which three dimensional flow effects can be neglected have been deduced and an empirical equation for the mean Nusselt number for narrow plates with a uniform surface heat flux has been derived from the numerical results.
Natural Convective Heat Transfer From a Vertical Cylinder With an Exposed Upper Surface
Authors: Oosthuizen Patrick H.
Published: April 2009

Natural convective heat transfer from a vertical cylinder which has a uniform heat flux at its surface and which has an exposed horizontal top surface has been numerically studied. The cylinder is mounted on an adiabatic cylindrical base which has the same diameter as the heated cylinder. In some circumstances the mean Nusselt number for the curved surface of the cylinder can be adequately predicted using vertical flat plate equations, i.e., by ignoring curvature effects, and in some circumstances the overall mean Nusselt number for the system considered can be adequately predicted by ignoring the heat transfer from the exposed upper surface of the cylinder. The flow has been assumed to be axisymetric about the vertical cylinder axis and to be steady and laminar. It has also been assumed that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces, this having been treated by using the Boussinesq approach. The solution has been obtained by numerically solving the governing equations, these equations being written in terms of dimensionless variables and the solution being obtained using a commercial finite element method based code, FIDAP. Because of the applications that motivated this study, results have only been obtained for Pr = 0.7. A wide range of the other governing parameters have been considered. The conditions under which the heat transfer from the exposed upper surface can be neglected compared to that from the cylindrical wall in the evaluation of the mean Nusselt number has been deduced and the conditions under which curvature effects can be ignored in evaluating the mean Nusselt number for the curved surface of the cylinder have been investigated.
Quantum Molecular Dynamics Study on Energy Transfer to the Secondary Electron in Surface Collision Process of an Ion
Authors: Shibahara Masahiko , Satake Shin-Ichi , Taniguchi Jun
Published: April 2009

It is well known that an emission of secondary electrons is observed in an ion collision process to a surface, such as the focused ion beam (FIB) process. However, the physical effect of secondary electron emission to energy and mass transfer is seldom considered and there are few examples of analysis of the secondary electron emission. It is one of interesting problems as an extreme small scale energy transfer problem how energy is transferred to the electron emitted from the surface by ionic collisions. In the present study the quantum molecular dynamics method was applied to an energy transfer problem to an electron during ionic surface collision process in order to elucidate how energy of ionic collision transfers to the emitted electrons. The energy transfer paths to the electron was discussed during the collision process of an ion with changing the interaction between the electron and ions and that between the electron and surface molecules by the quantum molecular dynamics method. Effects of various physical parameters, such as the collision velocity and interaction strength between the observed electron and the classical particles to the energy transfer to the electron were investigated by the quantum molecular dynamics method when the potassium ion was collided with the surface so as to elucidate the energy path to the electron and the predominant factor of energy transfer to the electron. Effects of potential energy between the ion and the electron and that between the surface molecule and the electron to the electronic energy transfer were shown in the present paper. The energy transfer to the observed secondary electron through the potential energy term between the ion and the electron was much dependent on the ion collision energy although the energy increase to the observed secondary electron was not monotonous through the potential energy between the ion and surface molecules with the change of the ion collision energy.
Experiment on Flow Boiling Heat Transfer in Tube and Study of Performance of a Vacuum Solar Collector
Authors: Hirasawa Shigeki , Taniguchi Masahiro , Nakauchi Shunsaku , Tanaka Tadayoshi
Published: April 2009

A high-vacuum solar collector system with flow boiling in tube has high collector efficiency of solar energy. In this paper flow boiling heat transfer coefficient in tube was measured by changing mass flow rate (30–80 kg/m2s), heat flux (5–30 kW/m2) and inclined angle of collector plate. Inside diameter of tube is 4.4 mm, and saturation temperature is 100°C. Flow boiling heat transfer coefficient is about 8000 W/m2K and decreases at low flow rate. Effect of the inclined angle of collector plate is small. Experimental results of boiling heat transfer coefficients are similar to Sani’s correlation equation. The collector efficiency of vacuum solar collector systems with flow boiling in tube is analyzed and the efficiency is 69% at a standard calculation condition. There is 50°C temperature difference in the collector plate. Effects of the mass flow rate and the vacuum pressure on the efficiency are large. The efficiency decreases at high saturation temperature and at low solar radiation.
Experimental Study of Non-Condensable Effect on Passive Condenser System
Authors: Zhou Wenzhong , Revankar Shripad T.
Published: April 2009

One of the engineered safety systems in the advanced boiling water reactor is a passive containment cooling system (PCCS) which is composed of a number of vertical heat exchanger. A set of steam condensation experiments is conducted to evaluate the heat removal capacity of a PCCS condenser. A condensing tube is submerged in a water pool where condensation heat is transferred by secondary boiling heat transfer. The specific design of condensing tube is based on scaling analysis from the PCCS design of ESBWR. The two condensing tubes have same height (0.9m) but different inside diameters, 26.6mm and 52.5mm, respectively. Condensation heat transfer coefficients (HTC) are obtained under various test conditions, such as different primary pressure (150 – 450 kPa), inlet steam flow rate (1 – 5 g/s), air mass fraction (0 – 20%) and tube size (26.6 mm and 52.5 mm ID). The effects of these parameters to condensation performance are evaluated.
Comparison of Computational and Quasi-Static Solutions of Phase Change With Heat Generation
Authors: Crepeau John , Siahpush Ali , Spotten Blaine
Published: April 2009

In this paper, we investigate the effects that the volumetric heat generation has on the movement and steady-state location of a solid-liquid phase change front in melting and freezing processes. Volumetric heat generation enhances melting and impedes freezing. This phenomenon occurs in nuclear, geologic, cryogenic and material processing applications. We compare the results from a FLUENT computational model with analytical results of a quasi-static solution of the governing equations. These models are applied for constant surface temperature boundary conditions and various volumetric heat generation values in cylindrical plane wall and spherical geometries. The quasi-static method results in an exact steady-state solution which shows that the location of the phase change front is inversely proportional to the square root of the volumetric heat generation. This method is valid for Stefan numbers less than one and the computational results for this regime give excellent agreement with the analytical model, thereby validating the technique and solutions. For the sake of comparison, we also plot the analytical model and computational results for Stefan numbers of one and greater. The quasi-static analytical solution converges more rapidly to the steady-state value than the computational solution. As expected, at longer time intervals, both the analytical and computational solutions converge to the exact steady-state solution.
Two-Fluid Model Simulation of Thermophoretic Deposition for Fine Particles in a Turbulent Boundary Layer
Authors: Shahriari Sh. , Basirat Tabrizi H.
Published: April 2009

In this present paper, thermophoretic depositions of fine particles are used in a heated turbulent boundary layer over very small plate via two-fluid model, or Eulerian-Eulerian approach. The Prandtl’s mixing length model of turbulence is used for the closure problem. The governing equations of gas phase are coupled with the governing equations of particle phase in two-way model, while uses the particle diffusion term as another coupling term. The equations are solved numerically by using finite difference method. One can obtain the convergence by numerical calculations much easier than with no diffusion term. A vast amount of information can be extracted for this kind of modeling. The effect of important parameters such as diffusion factor, gravity and thermophoretic force are considered. The cooler temperature of plate results higher particles deposition or concentration on the flat plate. Also, the larger particle size diameters delay the maximum particles deposition further distance away from the plate front edge. The results give the correct physical prediction overall.
Natural Convection in a Trapezoidal Cavity With Linearly Heated Side Wall(s)
Authors: Natarajan E. , Basak Tanmay , Roy S.
Published: April 2009

The present numerical study deals with natural convection flow in a trapezoidal cavity when the bottom wall is uniformly heated and the vertical wall(s) are linearly heated and cooled whereas the top wall is well insulated. Nonlinear coupled partial differential equations governing the flow have been solved by penalty finite element method with bi-quadratic rectangular elements. Parametric study for the wide range of Rayleigh number (Ra), 103 ≤ Ra ≤ 105 and Prandtl number (Pr), 0.07 ≤ Pr ≤ 100 shows consistent performance of the present numerical approach to obtain the solutions in terms of stream functions and the temperature profiles. For linearly heated side walls symmetry is observed while representing the flow patterns in terms of stream functions whereas secondary circulation is observed for the linearly heated left wall and cooled right wall. Local Nusselt number becomes negative at the side wall for linearly heated side walls and at the left wall for linearly heated left wall and cooled right wall indicating the reversal of heat flow. The effect of Prandtl number in the variation of average Nusselt numbers is more significant for Prandtl numbers in the range 0.07 to 0.7 than 10 to 100.
Transient Leading to Periodic Fluid Flow and Heat Transfer in a Differentially-Heated Cavity Due to an Insulated Rotating Object
Authors: Shih Y.-C. , Khodadadi J. M. , Weng K.-H. , Oztop H. F.
Published: April 2009

Computational analysis of transient phenomenon followed by the periodic state of laminar flow and heat transfer due to an insulated rotating object in a square cavity is investigated. A finite-volume-based computational methodology utilizing primitive variables is used. Various rotating objects (circle, square and equilateral triangle) with different sizes are placed in the middle of the cavity. A combination of a fixed computational grid with a sliding mesh was utilized for the square and triangle shapes. The cavity is maintained as a differentially-heated enclosure and the motionless insulated object is set in rotation at time t = 0. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr = 5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta < 1750). The evolving flow field and the interaction of the rotating objects with the recirculating vortices at the four corners are elucidated. The corresponding thermal fields in relation to the evolving flow patterns and the skewness of the temperature contours in comparison to conduction-only case were discussed. The skewness is observed to become more marked as the Reynolds number is lowered. At the same time, similarity of the thermal fields for various shapes for the same Reynolds number varifies the appropriate selection of the hydraulic diameter. Transient variations of the average Nusselt numbers on the two walls show that for high Re numbers, a quasi-periodic behavior due to the onset of the Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt number of the cavity is correlated to the rotational Reynolds number and shape of the object. The triangle object clearly gives rise to high heat transfer followed by the square and circle objects.
Transient Leading to Periodic Fluid Flow and Heat Transfer in a Cavity With Constant Temperature Walls Due to an Isothermal Rotating Object
Authors: Shih Y.-C. , Khodadadi J. M. , Weng K.-H.
Published: April 2009

Computational analysis of transient phenomenon followed by the periodic state of laminar flow and heat transfer due to a rotating object in a square cavity is investigated. A finite-volume-based computational methodology utilizing primitive variables is used. Various isothermal rotating objects (circle, square and equilateral triangle) with different sizes are placed in the middle of the cavity. A combination of a fixed computational grid with a sliding mesh was utilized for the square and triangle shapes. The motionless object is set in rotation at time t = 0 and its temperature is maintained constant but different from the temperature of the walls of the cavity. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr = 5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta < 1750). The evolving flow field and the interaction of the rotating objects with the recirculating vortices at the four corners are elucidated. Similarities and differences of the flow and thermal fields for various shapes is discussed. Transient variations of the average Nusselt numbers on the surface of the rotating object and cavity walls show that for high Re numbers, a quasi-periodic behavior due to the onset of Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt number of the cavity is correlated to the rotational Reynolds number and shape of the object. The triangle object clearly gives rise to high heat transfer followed by the square and circle objects.
Application of Meshless Pore-Level Model to Pyrolysis and Synthesis of Nuclear Fuels
Authors: Wang Xiaolin , Zhang Hui , Zheng Lili
Published: April 2009

Uranium-ceramic nuclear fuels can be fabricated through pyrolysis-based materials processing technique. This technique requires lower energy compared to sintering route. During the fabrication process, the source material changes composition continuously and chemical reactions and transport phenomena vary accordingly. Therefore, to obtain such nuclear fuel materials with high uniformity of microstructure/species without crack, transport phenomena in the material processing needs to be better understood. A system-scale model has been developed to account for the pyrolysis-based uranium-ceramic nuclear material processing in our prior work. In this study, a pore-scale numerical model based on Smoothed Particle Hydrodynamics (SPH) will be described for modeling the synthesis of SiC matrix and U3O8. The system-level model provides thermal boundary conditions to the pore-level model. The microstructure and compositions of the produced composites will be studied. Since the control of process temperature plays an important role in the material quality, the effects of heating rate and U3O8 particle size and volume on species uniformity and microstructure are investigated.
Effects of Surface Roughness on the Pool Boiling of Water
Authors: Jones Benjamin J. , Garimella Suresh V.
Published: April 2009

The effect of surface roughness on the pool boiling of water is studied. Five aluminum surfaces of varying roughness were prepared: one polished (0.062 μm RMS) and four roughened by electrical discharge machining (EDM) with surface roughness of 1.37, 2.81, 7.37, and 12.53 μm RMS. All experiments were performed in water at atmospheric pressure and saturation temperature. The roughest EDM surface showed up to a 100% improvement in the heat transfer coefficient compared to the polished surface. The other EDM surfaces (1.37, 2.81, and 7.37 μm) showed up to a 60% enhancement in the heat transfer coefficient compared to the polished surface. Constants are proposed for prediction of pool boiling in water from the polished and rough aluminum surfaces studied using the Rohsenow pool boiling correlation.
Freezing of Cells: Role of Ice and Solutes in Cell Damage
Authors: Takamatsu Hiroshi
Published: April 2009

The mechanism of cellular damage associated with freezing of biological cells is discussed by summarizing the author’s recent studies that consists of four different types of experiments. The “solution effects” that designate the influence of elevated concentration of electrolytes during freezing is examined first by a nonfreezing experiment that exposes cells to hypertonic solutions using a perfusion microscope. The cell damage due to the solution effect is evaluated directly from a pseudo-freezing experiment, where cells were subjected to the milieu that simulated a freeze-thaw process in the absence of ice. Contribution of ice formed in the extracellular solution is then estimated from the difference in cell survival between the pseudo-freezing experiment and a corresponding freezing experiment. The cellular injury by the mechanical stress is also examined independently by a cell deformation experiment, which mimicked the situation that cells are compressed and deformed between ice crystals. This experiment was designed to examine a complex effect of mechanical stress from ice and elevated concentration of electrolytes. Based on all these experiments, the role of concentrated solutes and ice is revealed as a function of freezing conditions.
Effect on Natural Convection Heat Transfer of Nanofluid in an Enclosure Due to Uncertainties of Viscosity and Thermal Conductivity
Authors: Ho C. J. , Chen M. W. , Li Z. W.
Published: April 2009

In this study we aim to identify effects due to uncertainties in effective dynamic viscosity and thermal conductivity of nanofluid on laminar natural convection heat transfer in a square enclosure. Numerical simulations have been undertaken incorporating a homogeneous solid-liquid mixture formulation for the two-dimensional buoyancy-driven convection in the enclosure filled with alumina-water nanofluid. Two different formulas from the literature are each considered for the effective viscosity and thermal conductivity of the nanofluid. Simulations have been carried out for the pertinent parameters in the following ranges: the Rayleigh number, Raf = 103 ∼ 106 and the volumetric fraction of alumina nanoparticles, φ = 0 ∼ 4%. Significant difference in the effective dynamic viscosity enhancement of the nanofluid calculated from the two adopted formulas, other than that in the thermal conductivity enhancement, was found to play as a major factor, thereby leading to contradictory results concerning the heat transfer efficacy of using nanofluid in the enclosure.
Effects of Confinement and Surface Reconstruction on the Lattice Dynamics and Thermal Transport Properties of Thin Films
Authors: Turney Joseph E. , McGaughey A. J. H. , Amon C. H.
Published: April 2009

Phonon transport in argon and silicon thin films is examined using harmonic lattice dynamics theory and the Lennard-Jones and Stillinger-Weber potentials. Film thicknesses ranging from 0.8 to 33.5 nm for argon and 0.4 to 8.6 nm for silicon are examined at a temperature of 0 K. Both reconstructed films and films built using the bulk unit cell are considered. Phonon dispersion curves for the in-plane direction and the density of states are computed from lattice dynamics and compared to predictions for a bulk system. The results from the lattice dynamics calculations are used to predict the thermal conductivities of the bulk and thin film structures.
Calibration Methods for the Extended Prout-Tompkins Chemical Kinetics Model and Derived Cookoff Parameters for RDX, HMX, LX-10 and PBXN-109
Authors: Wemhoff Aaron P. , Burnham Alan K. , Nichols Albert L. , Knap Jaroslaw
Published: April 2009

Thermal explosions result when local temperature-dependent heat generation exceeds heat loss via conduction. The temperature dependence of the heat source term is directly related to the material’s chemical kinetics, and hence the chemical kinetics has a direct impact on the thermal explosion times of a material. Much success has been gained in past work to accurately model thermal explosions in various explosives using multi-step Arrhenius chemical kinetics models. However, the generation of these kinetics models is time consuming and complex. Therefore, a methodology has been developed that allows for calibration of a single-reaction global kinetics model using One Dimensional Time to Explosion (ODTX) experimental data, which combines an iterative approach with a steepest descents optimization. This methodology has been applied to calibrate kinetic parameters for the widely-used explosives RDX (1, 3, 5-trinitrohexahydro-striazine), HMX (octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine), LX-10 (95% HMX, 5% Viton binder), and PBXN-109 (64% RDX, 20% Al, 16% binders). The average error between experimental and simulated ODTX and STEX data using this technique is approximately equivalent to that using the traditional multi-step models, and the time required for calibration of the global kinetics model has been reduced from months to hours.
Control of Flow Pattern and Solidification Interface Shape in an Induction Heated Czochralski Crystal Growth System
Authors: Fang Haisheng , Zheng Lili , Zhang Hui
Published: April 2009

Optical crystals grown by Czochralski technique from a solute-rich melt usually suffer defects of melt inclusion or bubble core defects, which severely affect the optical, thermal and mechanical properties of the material. It is well known that the formation of melt inclusion or bubble core is highly related to species distribution in the growth system especially at the solidification interface and the shape of the growth interface. This paper has examined the flow pattern and solidification interface changes by changing the forced convection, e.g., crystal rotation and by changing the natural convection, e.g., inserting a horizontal disk plate. The relative effect of fluid-flow convection modes in the melt associated with crystal rotation rate is represented by a dimensionless parameter, Gr/Re2. Increasing the rotation rate will cause the solid-liquid interface change from the convex shape to concave. When the crystal rotation rate is relatively low and natural convection is strong, Gr/Re2 is large. In this case, the concentration of species pertinent to melt inclusion moves down along the axis of rotation. When the crystal rotation rate is increased, the value of Gr/Re2 decreases. The precipitated composition spreads over the growing interface may then be swiped away from the growth interface by increased crystal rotation. Melt inclusion-free crystals can thus be obtained. The relationship between Gr/Re2 and growth interface shape change is achieved by numerical simulations. The stagnant point location as a function of crystal rotation is also presented, which shows that the stagnant point moves outward by increasing Reynolds number and/or reducing Grashof number. From such understanding, the interface shape and melt inclusion position can then be controlled through control of Gr/Re2 in the growth system. Many times, it is, however, not practical in the experiments to use a high rotation rate for optical crystal growth since high rotation rate will introduce the striation defects. A new design to reduce natural convection is then proposed to improve the effect of crystal rotation and to control the solidification interface shape. Numerical simulations have been performed to demonstrate the possibility of the new design. Results show that such design is very effective and practical to control the melt inclusion and the solidification interface shape.
Thermo-Fluidic Characteristics in a Cross-Linked Silicon Microchannel Heat Sink
Authors: Muwanga R. , Hassan I.
Published: April 2009

This paper presents the flow and heat transfer characteristics in a cross-linked silicon microchannel heat sink. The heat sink is composed of 45 channels, 270 μm wide × 285 μm tall in a silicon substrate formed via deep reactive ion etching. A detailed discussion of the pressure drop data reduction is described, including characterization of the channel cross-sections and methods to account for inlet and exit loss coefficients. No significant difference is observed in the pressure drop measurements between the cross-linked and standard heat sinks flowing air and water. The use of un-encapsulated liquid crystal thermography was successfully utilized to obtain local heat transfer data with FC-72 as the working fluid. The heat transfer results show inflections in the thermal profile due to the cross-links.
Thermionic Emission From Potassium-Intercalated Carbon Nanotube Arrays
Authors: Westover Tyler L. , Powell Glen , Fisher Timothy S.
Published: April 2009

Vacuum and solid-state thermionic emission have long been proposed as a means of converting heat or solar energy directly into electrical power. However, low work function materials must be developed before a reasonably efficient power generation device can be realized. In this work, thermionic emission energy distributions were measured for four samples including a single-crystal tungsten (100) sample, a pristine CNT mat, and two potassium-intercalated CNT mats. One of the potassium-intercalated CNT mats was composed largely of randomly oriented CNTs while the other CNT sample was grown in templated anodized alumina to align the growth pattern. Thermionic emission data obtained from the tungsten sample validated the experimental apparatus and method. The pristine CNT mat exhibited an emission distribution with a work function of 4.7 eV, while the potassium-intercalated samples exhibited work functions of approximately 3.1 and 3.4 eV for the randomly oriented and the templated meshes, respectively. The differences in the measured work function values for intercalated samples may be due to emitter tip differences. Both intercalated CNT samples showed some degradation after cooling from 510°C and reheating to the same temperature.
Understanding High Heat Transfer in Spray Cooling for Different Droplet Velocities and Wall Superheats by 3D Multiphase Flow Modeling
Authors: Selvam R. Panneer , Sarkar Suranjan
Published: April 2009

Spray cooling with phase change has the advantage of relatively large amount of heat transfer from the hot surface of many power electronics system. In our previous works in 2-D model of spray cooling, the importance of moving the cooler liquid quickly to heated dry surface which causes the high heat flux due to transient conduction is recognized to be the main reason for high heat transfer. In reality the phenomena of spray cooling are three dimensional in nature. The major draw back in extending the 2-D model to 3-D model is huge computing time in serial computer. Here the 3-D model is developed in parallel computing environment to reduce the turn around time. The 3-D multiphase model used here considers the effect of surface tension between liquid and vapor, gravity, phase change and viscosity. The level set method is used to capture the movement of the liquid vapor interface. The governing equations of multiphase flow are solved using the finite difference method. In this work the spray cooling phenomena is studied in 3-D multiphase model where a vapor bubble is growing in a thin liquid film on a hot surface and a droplet is impacting on the thin liquid film. This study has been done for different droplet velocities and for different wall superheats with our 3-D multiphase model to understand the high heat removal mechanism in spray cooling for different velocities and wall superheat situations.