Showing papers on "Thermal diffusivity published in 1995"

The Science and Engineering of Thermal Spray Coatings

Abstract: Preface to the Second Edition. Preface to the First Edition. Acronyms, Abbreviations and Symbols. 1 Materials Used for Spraying. 1.1 Methods of Powders Production. 1.1.1 Atomization. 1.1.2 Sintering or Fusion. 1.1.3 Spray Drying (Agglomeration). 1.1.4 Cladding. 1.1.5 Mechanical Alloying (Mechanofusion). 1.1.6 Self-propagating High-temperature Synthesis (SHS). 1.1.7 Other Methods. 1.2 Methods of Powders Characterization. 1.2.1 Grain Size. 1.2.2 Chemical and Phase Composition. 1.2.3 Internal and External Morphology. 1.2.4 High-temperature Behaviour. 1.2.5 Apparent Density and Flowability. 1.3 Feeding, Transport and Injection of Powders. 1.3.1 Powder Feeders. 1.3.2 Transport of Powders. 1.3.3 Injection of Powders. References. 2 Pre-Spray Treatment. 2.1 Introduction. 2.2 Surface Cleaning. 2.3 Substrate Shaping. 2.4 Surface Activation. 2.5 Masking. References. 3 Thermal Spraying Techniques. 3.1 Introduction. 3.2 Flame Spraying (FS). 3.2.1 History. 3.2.2 Principles. 3.2.3 Process Parameters. 3.2.4 Coating Properties. 3.3 Atmospheric Plasma Spraying (APS). 3.3.1 History. 3.3.2 Principles. 3.3.3 Process Parameters. 3.3.4 Coating Properties. 3.4 Arc Spraying (AS). 3.4.1 Principles. 3.4.2 Process Parameters. 3.4.3 Coating Properties. 3.5 Detonation-Gun Spraying (D-GUN). 3.5.1 History. 3.5.2 Principles. 3.5.3 Process Parameters. 3.5.4 Coating Properties. 3.6 High-Velocity Oxy-Fuel (HVOF) Spraying. 3.6.1 History. 3.6.2 Principles. 3.6.3 Process Parameters. 3.6.4 Coating Properties. 3.7 Vacuum Plasma Spraying (VPS). 3.7.1 History. 3.7.2 Principles. 3.7.3 Process Parameters. 3.7.4 Coating Properties. 3.8 Controlled-Atmosphere Plasma Spraying (CAPS). 3.8.1 History. 3.8.2 Principles. 3.8.3 Process Parameters. 3.8.4 Coating Properties. 3.9 Cold-Gas Spraying Method (CGSM). 3.9.1 History. 3.9.2 Principles. 3.9.3 Process Parameters. 3.9.4 Coating Properties. 3.10 New Developments in Thermal Spray Techniques. References. 4 Post-Spray Treatment. 4.1 Heat Treatment. 4.1.1 Electromagnetic Treatment. 4.1.2 Furnace Treatment. 4.1.3 Hot Isostatic Pressing (HIP). 4.1.4 Combustion Flame Re-melting. 4.2 Impregnation. 4.2.1 Inorganic Sealants. 4.2.2 Organic Sealants. 4.3 Finishing. 4.3.1 Grinding. 4.3.2 Polishing and Lapping. References. 5 Physics and Chemistry of Thermal Spraying. 5.1 Jets and Flames. 5.1.1 Properties of Jets and Flames. 5.2 Momentum Transfer between Jets or Flames and Sprayed Particles. 5.2.1 Theoretical Description. 5.2.2 Experimental Determination of Sprayed Particles' Velocities. 5.2.3 Examples of Experimental Determination of Particles Velocities. 5.3 Heat Transfer between Jets or Flames and Sprayed Particles. 5.3.1 Theoretical Description. 5.3.2 Methods of Particles' Temperature Measurements. 5.4 Chemical Modification at Flight of Sprayed Particles. References. 6 Coating Build-Up. 6.1 Impact of Particles. 6.1.1 Particle Deformation. 6.1.2 Particle Temperature at Impact. 6.1.3 Nucleation, Solidification and Crystal Growth. 6.1.4 Mechanisms of Adhesion. 6.2 Coating Growth. 6.2.1 Mechanism of Coating Growth. 6.2.2 Temperature of Coatings at Spraying. 6.2.3 Generation of Thermal Stresses at Spraying. 6.2.4 Coatings Surfaces. 6.3 Microstructure of the Coatings. 6.3.1 Crystal Phase Composition. 6.3.2 Coatings' Inhomogeneity. 6.3.3 Final Microstructure of Sprayed Coatings. 6.4 Thermally Sprayed Composites. 6.4.1 Classification of Sprayed Composites. 6.4.2 Composite Coating Manufacturing. References. 7 Methods of Coatings' Characterization. 7.1 Methods of Microstructure Characterization. 7.1.1 Methods of Chemical Analysis. 7.1.2 Crystallographic Analyses. 7.1.3 Microstructure Analyses. 7.1.4 Other Applied Methods. 7.2 Mechanical Properties of Coatings. 7.2.1 Adhesion Determination. 7.2.2 Hardness and Microhardness. 7.2.3 Elastic Moduli, Strength and Ductility. 7.2.4 Properties Related to Mechanics of Coating Fracture. 7.2.5 Friction and Wear. 7.2.6 Residual Stresses. 7.3 Physical Properties of Coatings. 7.3.1 Thickness, Porosity and Density. 7.3.2 Thermophysical Properties. 7.3.3 Thermal Shock Resistance. 7.4 Electrical Properties of Coatings. 7.4.1 Electrical Conductivity. 7.4.2 Properties of Dielectrics. 7.4.3 Electron Emission from Surfaces. 7.5 Magnetic Properties of Coatings. 7.6 Chemical Properties of Coatings. 7.6.1 Aqueous Corrosion. 7.6.2 Hot-gas Corrosion. 7.7 Characterization of Coatings' Quality. 7.7.1 Acoustical Methods. 7.7.2 Thermal Methods. References. 8 Properties of Coatings. 8.1 Design of Experiments. 8.2 Mechanical Properties. 8.2.1 Hardness and Microhardness. 8.2.2 Tensile Adhesion Strength. 8.2.3 Elastic Moduli, Strengths and Fracture Toughness. 8.2.4 Friction and Wear. 8.3 Thermophysical Properties. 8.3.1 Thermal Conductivity and Diffusivity. 8.3.2 Specific Heat. 8.3.3 Thermal Expansion. 8.3.4 Emissivity. 8.3.5 Thermal Shock Resistance. 8.4 Electric Properties. 8.4.1 Properties of Conductors. 8.4.2 Properties of Resistors. 8.4.3 Properties of Dielectrics. 8.4.4 Electric Field Emitters. 8.4.5 Properties of Superconductors. 8.5 Magnetic Properties. 8.5.1 Soft Magnets. 8.5.2 Hard Magnets. 8.6 Optical Properties. 8.6.1 Decorative Coatings. 8.6.2 Optically Functional Coatings. 8.7 Corrosion Resistance. 8.7.1 Aqueous Corrosion. 8.7.2 Hot-medium Corrosion. References. 9 Applications of Coatings. 9.1 Aeronautical and Space Industries. 9.1.1 Aero-engines. 9.1.2 Landing-gear Components. 9.1.3 Rocket Thrust-chamber Liners. 9.2 Agroalimentary Industry. 9.3 Automobile Industry. 9.4 Ceramics Industry. 9.4.1 Free-standing Samples. 9.4.2 Brick-Clay Extruders. 9.4.3 Crucibles to Melt Oxide Ceramics. 9.4.4 Ceramic Membranes. 9.5 Chemical Industry. 9.5.1 Photocatalytic Surfaces. 9.5.2 Tools in Petrol Search Installations. 9.5.3 Vessels in Chemical Refineries. 9.5.4 Gas-well Tubing. 9.5.5 Polymeric Coatings on Pipeline Components. 9.5.6 Ozonizer Tubes. 9.6 Civil Engineering. 9.7 Decorative Coatings. 9.8 Electronics Industry. 9.8.1 Heaters. 9.8.2 Sources for Sputtering. 9.8.3 Substrates for Hybrid Microelectronics. 9.8.4 Capacitor Electrodes. 9.8.5 Conductor Paths for Hybrid Electronics. 9.8.6 Microwave Integrated Circuits. 9.9 Energy Generation and Transport. 9.9.1 Solid-oxide Fuel Cell (SOFCs). 9.9.2 Thermopile Devices for Thermoelectric Generators. 9.9.3 Boilers in Power-generation Plants. 9.9.4 Stationary Gas Turbines. 9.9.5 Hydropower Stations. 9.9.6 MHD Generators. 9.10 Iron and Steel Industries. 9.10.1 Continuous Annealing Line (CAL). 9.10.2 Continuous Galvanizing Section. 9.10.3 Stave Cooling Pipes. 9.11 Machine Building Industry. 9.12 Medicine. 9.13 Mining Industry. 9.14 Non-ferrous Metal Industry. 9.14.1 Hot-extrusion Dies. 9.14.2 Protective Coatings against Liquid Copper. 9.14.3 Protective Coatings against Liquid Zirconium. 9.15 Nuclear Industry. 9.15.1 Components of Tokamak Device. 9.15.2 Magnetic-fusion Energy Device. 9.16 Paper Industry. 9.16.1 Dryers. 9.16.2 Gloss Calender Rolls. 9.16.3 Tubing in Boilers. 9.17 Printing and Packaging Industries. 9.17.1 Corona Rolls. 9.17.2 Anilox Rolls. 9.18 Shipbuiding and Naval Industries. 9.18.1 Marine Gas-turbine Engines. 9.18.2 Steam Valve Stems. 9.18.3 Non-skid Helicopter Flight Deck. References. Index.

Multi-component lattice-Boltzmann model with interparticle interaction

Abstract: A previously proposed [X. Shan and H. Chen, Phys. Rev. E {\bf 47}, 1815, (1993)] lattice Boltzmann model for simulating fluids with multiple components and interparticle forces is described in detail. Macroscopic equations governing the motion of each component are derived by using Chapman-Enskog method. The mutual diffusivity in a binary mixture is calculated analytically and confirmed by numerical simulation. The diffusivity is generally a function of the concentrations of the two components but independent of the fluid velocity so that the diffusion is Galilean invariant. The analytically calculated shear kinematic viscosity of this model is also confirmed numerically.

Multicomponent lattice-boltzmann model with interparticle interaction

Abstract: A lattice Boltzmann model for simulating fluids with multiple components and interparticle forces proposed by Shan and Chen is described in detail. Macroscopic equations governing the motion of each component are derived by using the Chapman-Enskog method. The mutual diffusivity in a binary mixture is calculated analytically and confirment by numerical simulation. The diffusivity is generally a function of the concentrations of the two components but independent of the fluid velocity, so that the diffusion is Galilean invariant. The analytically calculated shear kinematic viscosity of this model is also confiremoed numerically.

Transport coefficients of air, argon-air, nitrogen-air, and oxygen-air plasmas

Abstract: Calculated values of the viscosity, thermal conductivity and electrical conductivity of air and mixtures of air and argon, air and nitrogen, and air and oxygen at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. The calculations, which assione local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30,000 K. The results for air plasmas are compared with those of published theoretical and experimental studies. Significant discrepancies are found with the other theoretical studies; these are attributed to differences in the collision integrals used in calculating the transport coefficients. A number of the collision integrals used here are significantly more accurate than values used previously, resulting in more reliable values of the transport coefficients.

Transient plane source (TPS) technique for measuring thermal transport properties of building materials

Abstract: The electrical circuit for the recently developed transient plane source (TPS) technique for fast and precise measurements of thermal transport properties of solids has been modified for more convenient and more automated measurements. The technique has been tested for measurements of thermal conductivity and thermal diffusivity for a series of building materials ranging from thermally insulating materials (extruded polystyrene and PMMA) to good thermal conductors (stainless steel and aluminium). The results obtained in this work agree well with other techniques and international standard materials. This agreement indicates that the TPS method is accurate to within ±5% over a thermal conductivity range of four orders of magnitude (0.02 W m −1 K −1 to 200 W m −1 K −1 )

Dynamics of aluminum combustion

Abstract: A study has been performed modeling both steady and unsteady combustion of aluminum. Law's steadystate aluminum combustion model has been expanded to include the effects of multiple oxidizers and their products, oxide accumulation on the surface of the burning aluminum particle, and convection. Both transport and thermodynamic properties are calculated internally for varying temperatures, relaxing the normal assumption of unity Lewis number. The aluminum combustion model has been compared to experimental data from burners, laser-ignited particles, and propellant under a variety of conditions, showing a reasonable degree of agreement. Calculations with the model show that O2 is a stronger oxidizer than H2O, which in turn is stronger than CO2. The aluminum combustion model was incorporated into a computer model for predicting acoustic effects in a Rijke burner. Calculations have shown that a significant part of the increase in acousticgrowth due to the addition of aluminum is due strictly to the change in the gas temperature profile. The change in temperature profile apparently causes the location of the velocity antinode to shift relative to the Rijke burner flame and thereby cause an increase in the flame response. The acoustic model agrees reasonably well with available acoustic growth rates for data where aluminum particles have been added to a propane Rijke burner. Nomenclature ak = sum of the mole fractions of the oxidizers in Eq. (1) Cp = heat capacity, J/kg D = diffusivity, irr/s d = particle diameter, m F = ratio of total mass flux to mass flux aluminum H = total flux of energy in aluminum combustion model, W/m2; heat of reaction, J/mol j = mass flux due to diffusion, kg/m2/s k =. arbitrary constant in Eq. (1) M = nondimensional mass flux used in Law's aluminum combustion model /?2 = mass flux, kg/m2/s Nu = Nusselt number p = pressure, N/m2 Qr = heat production due to a reaction, W/m3; heat caused by radiation, W/m2 <92 = enthalpy in the region from the flame to infinity q = heat flux, W/m2 Re = Reynolds number r = radial distance, m; reaction rate, kg/mVs T = temperature, K t = time, s v — velocity, m/s; volume, m3 \v = transport property weighting factor A- = mole fraction 77 = fraction of vaporized oxide that moves toward the particle surface 0 = fraction of metal oxide that vaporizes at the flame A = fraction of metal that reacts with the particular oxidizing species v = stoichiometric mass ratio of oxide or oxidizer to metal f = fraction of condensed products with move inward or outward p = density, kg/m3 T = characteristic time lag, s

Stabilization of thermal lattice Boltzmann models

Abstract: A three-dimensional thermal lattice-Boltzmann model with two relaxation times to separately control viscosity and thermal diffusion is developed. Numerical stability of the model is significantly improved using Lax-Wendroff advection to provide and adjustable time step. Good agreement with a conventional fiitedifference Navier-Stokes solver is obtained in modeling compressible Rayleigh-Benard convestion when boundary conditions are treated similarly.

Effect of artificial stress diffusivity on the stability of numerical calculations and the flow dynamics of time-dependent viscoelastic flows

Abstract: In this work, we investigate the effect of the introduction of a stress diffusive term into the classical Oldroyd-B constitutive equation on the numerical stability of time-dependent viscoelastic flow calculations. The channel Poiseuille flow at Re ⪢ 1 and O(1) We is chosen as a test problem. Through a linear stability analysis, we demonstrate that the introduction of a small amount of (dimensionless) diffusivity, typically of the order 10−3, does not affect the critical eigenmodes of the viscoelastic Orr-Sommerfeld problem appreciably. However, a diffusive term of that magnitude is shown to have a significant influence on the singular eigenmodes of the classical Oldroyd-B model, associated with the continuum spectra. A finite amplitude perturbation is constructed as a linear superposition of the eigenvectors corresponding to the most unstable eigenvalues of the problem. This is superimposed on the steady Poiseuille flow solution to provide the initial conditions for time-dependent simulations. The numerical algorithm involves a fully spectral spatial discretization and a semi-implicit second order integration in time. For the Oldroyd-B fluid, depending on the magnitude of the initial perturbation, numerical instabilities set in at relatively short times while the components of the conformation tensor increase monotonically in magnitude with time. Introduction of a diffusive term into this model is shown to stabilize the calculations remarkably, and for a three-dimensional simulation with Re = 5000 and We = 1, no instabilities were observed even at very large times. The effect of the magnitude of the diffusivity on the stability and the flow dynamics is addressed through a direct comparison of the results with those obtained for the Oldroyd-B model.

Modification of Growth Kinetics in Surfactant-mediated Epitaxy

Abstract: The influence of Sb, As, Ga, and In as surfactants on the Si/Si(111) homoepitaxy is studied using scanning tunneling microscopy. The nucleation of two-dimensional (2D) Si islands on the surfactant-terminated surface was studied as a function of temperature. The island densities and depleted zones show Arrhenius behavior. Surfactants modify the Si epitaxy in quite a different way. Indium as surfactant increases the diffusivity of the Si atoms, whereas Sb and As drastically decrease the diffusion length of Si. When we apply these results to Ge epitaxy on Si the reduction of the diffusion length is shown to be essential for the suppression of 3D islanding in surfactant-mediated Ge/Si heteroepitaxy. Generally, elements of group III and IV as surfactants that enhance the diffusivity in Si homoepitaxy lead to 3D islanding in Ge/Si heteroepitaxy. Elements of group V and VI reduce the diffusion length in Si homoepitaxy and give rise to a suppression of 3D islanding in Ge/Si heteroepitaxy. The temperature dependence and rate dependence of the nucleation of 2D islands in Si/Si(111) homoepitaxy can be described in the framework of classical nucleation theories yielding a critical nucleus size of 6 and an activation energy of diffusion of 0.75 eV. Experimentally no indication for a reflective potential barrier for step-down motion of diffusing Si atoms (Ehrlich-Schwoebel barrier) was found.

Thermal‐wave resonator cavity

Abstract: A thermal‐wave resonant cavity was constructed using a thin aluminum foil wall as the intensity‐modulated‐laser‐beam induced oscillator source opposite a pyroelectric polyvilidene fluoride wall acting as a signal transducer and cavity standing‐wave‐equivalent generator. It was shown that scanning the frequency of oscillation produces the fundamental and higher overtone resonant extrema albeit with increasingly attenuated amplitude—a characteristic of thermal‐wave behavior. Experimentally, scanning the cavity length produced a sharp lock‐in in‐phase resonance with simple linewidth dependencies on oscillation (chopping) frequency and intracavity gas thermal diffusivity. The thermal diffusivity of air at 294 K was measured with three significant figure accuracy: 0.211±0.004 cm2/s. The novel resonator can be used as a high‐resolution thermophysical property sensor of gaseous ambients.

Thermal conductivity and diffusivity of free‐standing silicon nitride thin films

Abstract: The thermal conductivity and diffusivity of free‐standing silicon nitride (Si‐N) films of 0.6 and 1.4 μm in thickness are measured. A new experimental technique, the amplitude method, is proposed and applied to measurement of the thin‐film thermal diffusivity. The thermal diffusivity is determined by three independent experimental approaches: the phase‐shift method, the amplitude method, and the heat‐pulse method. Good agreement among the measured thermal diffusivities obtained by the three methods indicates the validity of the amplitude method. High‐resolution electron microscopy studies show a large quantity of voids in the 1.4 μm Si‐N films. In contrast, very few voids are found in the 0.6 μm films. This difference may be responsible for the measured lower conductivity of the 1.4 μm Si‐N films as compared to the 0.6 μm thin films.

Gas sensing properties of porous silicon

Abstract: Conductivity of porous silicon layers (p-type) has been investigated for organic vapor sensing. A many orders of magnitude increase in conductivity in response to a vapor pressure change from 0 to 100% has been measured for some compounds. The conductivity (at a constant pressure) varies exponentially with the compound's dipole moment The temporal response of the porous silicon layers is in the seconds range, and the recovery is much slower (minutes). However, due to the tremendous conductivity changes and the low background noise, a complete recovery is not needed for sensing purposes. The mechanism of conductivity enhancement has been studied using several methods. It is attributed to an increase in the density of charge carriers. An additional mechanism based on increased diffusivity may take place in microporous silicon. The observed characteristics suggest the application of porous silicon to future chemical sensors. The sensors have the potential to be integrated monolithically with other silicon devices using current technologies.

Temperature dependence of thermophysical properties of GaAs/AlAs periodic structure

Abstract: This work investigates the temperature dependence of GaAs/AlAs thin‐film structures. Based on an ac calorimetric method, the thermal diffusivity of a 700 A/700 A GaAs/AlAs periodic structure is measured from 190 to 450 K. Thermal conductivity of the structure is derived from the experiment. The results demonstrate that the thermal conductivity/diffusivity of the structure are lower than its corresponding bulk values. The temperature dependence of its thermophysical properties is weaker than that of typical bulk III–V materials. Interface scattering is believed as the major cause of the observed reduction in thermal conductivity.

Error Analysis of Heat Pulse Method for Measuring Soil Heat Capacity, Diffusivity, and Conductivity

Abstract: A dual-probe heat pulse (DPHP) method was developed recently that allows for the simultaneous, automated measurement of soil thermal diffusivity (κ), volumetric heat capacity (ρc), and thermal conductivity (λ). Estimation of thermal properties is based on theory for the conduction of heat away from an infinite line source (ILS) that is heated for a short period of time. In this study, we examined possible sources of error in the use of the ILS theory by comparing it with other models that explicitly account for finite length and cylindrical shape of the actual heater. For probe geometry and heating times typical of our experimental work, the analysis of model error showed that assuming an infinite length for a heat source of finite length caused errors <2% in the estimated thermal properties. Assuming the cylindrically shaped heater to be a line heat source caused errors of <0.6% in the estimated thermal properties. Thus, the ILS theory appears to be appropriate for use in the DPHP method if probe geometry is considered carefully. However, small changes in probe geometry can lead to large model errors. First-order error analysis also was used to predict how thermal property estimates will be affected by experimental errors in the measured inputs to the ILS model. The analysis shows that κ and ρc estimates are sensitive to measurement error in probe spacing (r), but λ is unaffected by error in r. Estimates of κ and λ were shown to be sensitive to measurement error in the time to the temperature maximum (t m ), whereas ρc was affected only slightly by such error.

Transport properties of gas in silica aerogel

Abstract: The motion of gas molecules in silica aerogel is restricted by the solid silica matrix. As a result, the mean free path, velocity distribution function, diffusivity, viscosity and thermal conductivity are changed. In this work, relations for these quantities are derived for a gas in silica aerogel using an approach similar to that used for a gas in free space. Results for the mean free path predict that, for p ≥ 10 bar, the mean free path of the gas molecules in aerogel will be almost the same as in free space. However, as the pressure is reduced, the mean free path reaches a constant finite value instead of increasing as in free space. The thermal conductivity of a gas in aerogel starts to decrease at p = 10 bar and is almost negligible at 0.01 bar, while the thermal conductivity of a gas between parallel walls 1 cm apart starts to decrease at p = 10 −4 bar and is almost negligible at p = 10 −7 bar. The predicted thermal conductivity of a gas in aerogel is in good agreement with experimental results.

Transient thermal gratings at surfaces for thermal characterization of bulk materials and thin films

Abstract: Transient Thermal Gratings (TTGs) at surfaces of absorbing materials have been utilized for investigating heat diffusion in bulk materials and thin films. In this report, we describe the theoretical background of the technique and present experimental data. TTGs were excited in the surface plane by interference of two pulsed laser beams and monitored by a cw probe beam, either via temperature dependence of the reflectivity or by deflection from the displacement pattern. A theoretical model describing the thermal and thermoelastic surface response was developed, both for a homogeneous material and a multilayer structure. The potential of the technique will be demonstrated by experimental results on (i) thermal diffusivities of bulk materials, (ii) anisotropic lateral heat transport, and (iii) thermal diffusivities of metal and diamond films. Furthermore, we will show that TTGs allow thermal depth profiling of inhomogeneous materials whenever there is a vertical gradient in thermal conductivity.

Specific heat and thermal diffusivity of hardening concrete

Abstract: This paper focuses on the evolution of the thermal characteristics of early-age concrete as a function of the state of the hardening process. A literature review is presented, together with new test results obtained by means of improved test methods: For the determination of the specific heat of cement paste during hardening and of the thermal diffusivity of hardening concrete, test methods are developed based on existing methods. The specific heat is determined for hardening cement paste samples made with blastfurnace slag cement. It is concluded that the specific heat and thermal diffusivity decreases linearly with the degree of hydration.

Nucleation and growth in microcellular materials: Supercritical CO2 as foaming agent

Abstract: Bubble growth is a phenomenon encountered in several commercially important processes. A mathematical model presented here describes the growth of bubbles during phase separation of an initially homogeneous polymer-supercritical fluid mixture, triggered by a sudden pressure drop at constant temperature. It is a modification of the viscoelastic model of Arefmanesh and Advani (1991) in which the polymer is treated as a single relaxation-time Maxwell fluid. Since properties of the polymer-fluid mixture vary with the amount of fluid absorbed in the polymer (as a function of fluid pressure), the model needs to be used evaluating system properties as functions of temperature and pressure. The viscosity of polymer/fluid mixture, density of the mixture, diffusivity of CO[sub 2] in the mixture, and relaxation time for poly(methyl methacrylate) swollen by supercritical carbon dioxide are, therefore, predicted as functions of CO[sub 2] pressure and temperature using appropriate model equations at each step of the bubble growth simulation. The model predicts well the trends in equilibrium cell size vs. saturation pressure and temperature. Depending on the characteristics of the cellular structure, polymeric foams can be used for applications ranging from simple lightweight insulation to biomedical and nuclear applications.

The sintering of two particles by surface and grain boundary diffusion -- A two-dimensional numerical study

Abstract: We investigate the sintering of two touching circular particles by surface and grain boundary diffusion. Typical examples for the evolution of the shape of the particles, their surface curvatures, and their surface fluxes are given. The sintering kinetics are evaluated as a function of the dihedral angle at the grain boundary-surface junctions and the grain boundary to surface diffusivity ratio. In particular, the growth rates of the neck between the two particles, the growth rate exponents, and the changes in the lengths of the particle pairs are monitored. The times needed to reach certain fractions of the final equilibrium neck sizes are tabulated for typical experimental dihedral angles and diffusivity ratios. Our simulation is based on a rigorous mathematical system modeling the sintering of the two particles, and a rigorous numerical method for solving this system is adopted.

Analysis of Forced Convection Heat Transfer in Microencapsulated Phase Change Material Suspensions

Abstract: B = Bi, = C = C* = C = E= = e = Fo = h, = K, = k = L = rn = Nu, = Pe = 4 = R = Rd = Re = r = r* = r,. = r,, = Is1. = Ste = S = T= t = u = 47, = X = X = a = A numerical solution of laminar forced convection heat transfer of a microencapsulated phase change material suspension in a circular tuhe with constant heat flux has been presented in this article. Melting in the microcapsule was solved by a temperature transforming model instead of a quasisteady model. The effects of the microcapsules crust, the initial subcooling, and the width of the phase change temperature range on the variation of the dimensionless tuhe wall temperatures, along the axial direction, were also considered in the present model. The agreement between the present numerical results and the experimental results is very good.

Measurement of thermal diffusivities through processing of infrared images

Abstract: The measurement of the thermal diffusivity of a thin layer in the direction of its plane is usually a difficult operation. The standard ‘‘flash technique’’ is very appropriate for diffusivity measurement in the direction of the thickness of the sample but adaptations of this method to in‐plane measurements remain very sensitive to the position and form of heat excitation and temperature sensors. The new procedure proposed here consists of applying any geometrically nonuniform heat impulse on the front face of the sample and recording the entire transient temperature image on the rear face thanks to an infrared camera. The influence of axial diffusion can be avoided for periods much longer than the axial diffusion characteristic time. Integral transforms on the radial space variables (Fourier transform) are very suitable for treating the temperature field and to estimate radial diffusivity. The main advantage of this method is to avoid any experimental precaution (no knowledge of the geometrical form of the excitation ‐ replacement of the sensor positioning by an image calibration). Furthermore, the considerable number of data produced by the camera is processed using a statistical approach. The validation of the method is made on a homogeneous sample by comparison between the in‐plane direction measurements (obtained with the present procedure) and the thickness direction measurements (obtained by the classical flash technique).

Diffusion in HPMC gels. I. Determination of drug and water diffusivity by pulsed-field-gradient spin-echo NMR.

Abstract: Purpose. This work describes diffusivity measurements of drug (adinazolam mesylate) and water in a variety of solutions including polymer gels.

Nondimensional transport scaling in DIII‐D: Bohm versus gyro‐Bohm resolved

Abstract: The scaling of cross‐field heat transport with relative gyroradius ρ* was measured in low (L) and high (H) mode tokamak plasmas using the technique of dimensionally similar discharges. The relative gyroradius scalings of the electron and ion thermal diffusivities were determined separately using a two‐fluid transport analysis. For L‐mode plasmas, the electron diffusivity scaled as χe∝χBρ1.1±0.3* (gyro‐Bohm‐like) while the ion diffusivity scaled as χi∝χBρ−0.5±0.3* (worse than Bohm‐like). The results were independent of the method of auxiliary heating (radio frequency or neutral beam). Since the electron and ion fluids had different gyroradius scalings, the effective diffusivity and global confinement time scalings were found to vary from gyro‐Bohm‐like to Bohm‐like depending upon whether the electron or ion channel dominated the heat flux. This last property can explain the previously disparate results with dimensionally similar discharges on different fusion experiments that have been published. Experimen...