Showing papers on "Thermal published in 1988"

Processing apparatus and method

Abstract: A process module wherein radiant heating is combined with the capability for generating a plasma in proximity to the wafer face. It is useful to clamp the slice to a susceptor which is of the same material as the wafer, or at least has essentially the same thermal coefficient of expansion. The susceptor is directly heated by a radiant heater, but the rate of temperature change is slow enough that no large thermal gradients between the susceptor and the wafer develop.

Origin of glassy crystalline behavior in the thermal properties of clathrate hydrates: a thermal conductivity study of tetrahydrofuran hydrate

Abstract: The thermal conductivity of tetrahydroruran hydrate was measured from 15 to 100 K. The present data complement earlier measurements at higher temperatures and confirm the speculation on the glassy behavior in the tehermal conductivity of clarathrate hydrates. The experimental results can be interpreted by using a resonant scattering mechanism. The interactions between localized low-frequency vibrations of the guest molecules with the acoustic phonons of the host lattice are responsible for the thermal glassy behavior of the clathrate hydrates, despite their well-defined crystalline structures

Numerical simulations of mantle convection: time-dependent, three-dimensional, compressible, spherical shell

Abstract: We describe nonlinear time-dependent numerical simulations of whole mantle convection for a Newtonian, infinite Prandtl number, anelastic fluid in a three-dimensional spherical shell for conditions that approximate the Earth's mantle. Each dependent variable is expanded in a series of 4,096 spherical harmonics to resolve its horizontal structure and in 61 Chebyshev polynomials to resolve its radial structure. A semiimplicit time-integration scheme is used with a spectral transform method. In grid space there are 61 unequally-spaced Chebyshev radial levels, 96 Legendre colatitudinal levels, and 192 Fourier longitudinal levels. For this preliminary study we consider four scenarios, all having the same radially-dependent reference state and no internal heating. They differ by their radially-dependent linear viscous and thermal diffusivities and by the specified temperatures on their isothermal, impermeable, stress-free boundaries. We have found that the structure of convection changes dramatically a...

Three‐dimensional infinite Prandtl number convection in one and two layers with implications for the Earth's gravity field

Abstract: The three-dimensional planform of infinite Prandtl number convection is studied in one and two layers, with a numerical code which combines spectral and finite difference methods. The equations are solved in rectangular cavities supposing mirror symmetry on the sides. The one-layer experiments serve mainly for testing the code. These experiments reveal the outstanding importance of the initial and boundary conditions in the evolution of the flow pattern. The main emphasis of this paper is on two-layer convection models, with implications for the Earth's mantle and its gravity anomaly field. In this case, the interface between the two layers is fixed at a given depth, and coupling of the two circulations is ensured by the continuity of velocities and horizontal stress across the interface. The two-layer cases have been run with Rayleigh numbers up to 130 times the critical value, the thickness of the lower layer being twice that of the upper one. The ratio of the lower layer viscosity to the upper one is gradually increased. When this ratio is small (between 1 and 10), viscous coupling prevails, and bimodal or square cells formed in the lower layer induce similar, but counterrotating cells in the thin upper layer. When the viscosity ratio is high (several hundred), thermal coupling becomes dominant: again, the convective structure of the lower layer is more or less duplicated in the upper layer, but now the flows of both layers rotate in the same sense. In these situations the gravity anomalies are positively correlated with the topography of the upper surface. When the viscosity ratio is moderate (e.g., 25), the flow direction of the rolls in the thin upper layer becomes perpendicular to that observed in the lower layer, leading to a balance between the conflicting effects of the viscous and thermal couplings. This allows the upper rolls to have an aspect ratio close to 1. In this case, gravity is positevely correlated with deflection of the interface, but no more correlation can be seen with surface topography, which is determined by small-scale flow of the upper layer. These results suggest that the gravity and bathymetry anomalies of the Earth should be either positively correlated or not correlated at all. This has to be contrasted with earlier findings of two-dimensional modeling [Cserepes and Rabinowicz, 1985] which allow the possibility of having anticorrelation between gravity and topography.

Three-dimensional thermal cellular convection in rectangular boxes

Abstract: The extension of the classic Rayleigh–Benard problem of a horizontal layer heated from below to the three-dimensional convection in rectangular boxes is dealt with in this paper both numerically and experimentally. Also discussed is the influence of shear flows in tilted boxes and the transition to time-dependent oscillatory convection. Three-dimensional numerical simulations allow the calculation of stationary solutions and the direct simulation of oscillatory instabilities. We limited ourselves to laminar and transcritical flows. For studying the particular characteristics of three-dimensional convection in horizontal containers, we carefully selected two container geometries with aspect ratios of 10:4:1 and 4:2:1. The onset of steady cellular convection in tilted boxes is calculated by an iterative application of a combined finite-difference method and a Galerkin method. The appearance of longitudinal and transverse convection rolls is determined by means of inter-ferometrical measuring techniques and is compared with the results of the linear stability theory. The spatial flow structure and the transition to oscillatory convection is calculated for selected examples in the range of supercritical Rayleigh numbers. Experimental investigations concerning the stability behaviour of the steady solutions with regard to time-dependent disturbances show a distinct influence of the Prandtl number and confirm the importance of nonlinear effects.

Thermal Conductivity of Intumescent Chars

Abstract: A procedure is developed to estimate the effective thermal conductivity of chars of intumescent systems The procedure is based on a heat transfer analy sis of temperature-time data from one-dimensionally designed experiments of coated coupons exposed to a fire environment typical of aviation-type fuel fires Thermal conductivities computed from both a first-order and second-order pro cedure are given, and are found to be within a factor of 2 or 3 of each other for the binders tested Additionally, a simple theoretical approach, based on a one- dimensional equivalent thermal resistance model, yields estimates that are within a factor of three compared to the experimentally determined thermal conductivities The conclusion of the theoretical model is that the low thermal conductivity of intumescent chars results from the insulating abilities of pockets of trapped gas within a porous char material, even though the char of an intumescent coating is a complex structure which has convective currents due to pyr

The breakdown of steady convection

Abstract: Two-dimensional convection in a Boussinesq fluid with infinite Prandtl number, confined between rigid horizontal boundaries and stress-free lateral boundaries, has been investigated in a series of numerical experiments. In a layer heated from below steady convection becomes unstable to oscillatory modes caused by the formation of hot or cold blobs in thermal boundary layers. Convection driven by internal heating shows a transition from steady motion through periodic oscillations to a chaotic regime, owing to the formation of cold blobs which plunge downwards and eventually split the roll. The interesting feature of this idealized problem is the interaction between constraints imposed by nonlinear dynamics and the obvious spatial structures associated with the sinking sheets and changes in the preferred cell size. These spatial structures modify the bifurcation patterns that are familiar from transitions to chaos in low-order systems. On the other hand, even large-amplitude disturbances are constrained to show periodic or quasi-periodic behaviour, and the bifurcation sequences can be followed in considerable detail. There are examples of quasi-periodic behaviour followed by intermittency, of period-doubling cascades and of transitions from quasi-periodicity to chaos, associated with a preference for narrower rolls as the Rayleigh number is increased.

Thermal attractor in chaotic convection with high-Prandtl-number fluids.

Abstract: The onset of chaotic convection in large-Prandtl-number fluids has been found to occur in the form of elongated cells with aperiodic boundary-layer instabilities. The dynamics of this phenomenon has been monitored in the physical domain, and also in the spectral and phase spaces. In order to describe more precisely the underlying mechanisms of chaotic thermal convection, the notion of 'thermal attractor' is introduced in the physical domain and a systematic study is conducted of the evolution of the spectral energy surfaces. A relationship between this thermal attractor and the presence of a strange attractor in the phase space is proposed.

Solar heating of water bodies as influenced by their inherent optical properties

Abstract: A simple one-dimensional model has been developed for calculating the development of thermal structure in water bodies under specified meteorological conditions, as a function of their inherent optical properties (spectral absorption coefficients, scattering coefficient). Penetration of radiant solar energy is modeled with a previously derived equation relating attenuation of irradiance in narrow wavebands to absorption coefficient, scattering coefficient and solar angle. Evaporative and other heat exchange processes at the surface are taken into account. Vertical heat transfer is calculated by making use of a recently published parameterization of the eddy diffusion coefficient. The calculations show marked changes in thermal structure as absorption and scattering are varied over a range of optical water types from coastal seawater to highly colored and turbid inland water. In general terms, increasing the color and/or turbidity shifts the zone of shortwave energy absorption more toward the surface and leads to warmer but shallower mixed layers.

Apparatus for cooling an X-ray device

Abstract: An X-ray generating tube with improved thermal performance comprises a stationary disc within a hollow rotating target disc. The target operates at the target metal temperatures limit. The stationary disc is cooled by forced convection with a dielectric liquid. Heat is transferred from the target inwardly and outwardly.

Time-periodic convection in porous media: transition mechanism

Abstract: We resolve the disturbance structures that destabilize steady convection rolls in favour of a time-periodic pattern in two-dimensional containers of fluid-saturated porous material. Analysis of these structures shows that instability occurs as a travelling wave propagating in a closed loop outside the nearly motionless core. The travelling wave consists of five pairs of thermal cells and four pairs of vorticity disturbances in the case of a square container. The wave speed of the thermal disturbances is determined by an average base-state velocity and their structure by a balance between convection and thermal diffusion. Interpretation of the ‘exact’ solution is aided by a one-dimensional convection-loop model which correlates (i) point of transition, (ii) disturbance wavenumber, and (iii) oscillation frequency given the base-state temperature and velocity profiles. The resulting modified Mathieu-Hill equation clarifies the role of the vertical pressure gradient, induced by the impenetrable walls, and the role of the base-state thermal layer.

Thermal recording apparatus

Abstract: A thermal recording apparatus capable of regulating the pressure of thermal head and the energizing time or supply voltage of the thermal head according to the quality of recording sheet. A higher pressure and a higher energy or voltage are used for a coarse recording sheet to ensure clear image recording. Plural sets of these values can be stored in a memory and suitably selected by the operator or automatically according to the quality of sheet.

Thermal behaviour of riboflavin

Abstract: From the thermal decomposition curves of riboflavin at various heating rates in static air atmosphere, the stages of thermal decomposition of this compound were established. The following kinetic parameters were calculated for the first decomposition stage of this process: activation energy (Ea), reaction order (n), preexponential factor (A) and reaction rate constant (k). A mechanism of the changes is proposed.

Thermal Effects on Very Large Space Structures

Abstract: Radiation thermal effects are studied simultaneously on the orbital motion, attitude motion, and axial deformation of a very large, axially flexible space structure describing a planar pitch motion around the earth. The relative importance of the three known sources of radiation in space environment—direct solar, Earth's albedo, and direct Earth—are studied for low Earth orbit (LEO) and geosynchronous Earth orbit (GEO). Influences of the area‐to‐mass ratio of the structure on thermal effects have been investigated for elliptical orbit. Radiation thermal effects are found to be significant in causing structural deformation and in producing libration in the attitude angle of a large space structure. However, on the orbital parameters of the space structure, the thermal effects are negligible. In LEO, effects of the earth's albedo and its direct radiation on the structure are appreciable, whereas in higher altitude orbits such as GEO, these may be neglected. The area‐to‐mass ratio of the structure is realize...

Infrared Radiation Model For Aircraft And Reentry Vehicle

Abstract: NIR.ATAM ( NATO InfraRed Air T.Arget Model ) is a computer model which predicts IR radiation of an aircraft in its natural surroundings. A special version of NIRATAM enables to treat a hypersonic reentry vehicle. The model takes into account the IR radiation emitted by aerodynamically and internally heated surface, hot engine parts and combustion gas and particles in the exhaust plume. The reflected radiation of the sky and terrain background and the sun on the aircraft surface is calculated. The atmospheric transmission and emission between target and observer and the radiation of a homogenous background in the scene is determined. The spectral response of a sensor is included. The development took place in collaboration with other NATO-countries. The code is of modular structure. One of the major modules (IRMA) , for the hot gas emission calculation, was developed at This paper describes the modules, the major features and shows some typical results. For the "Reentry-Version" the exhaust plume flew field model is replaced by an aerodynamic flow field model which determines the hypersonic flow around the body. A thermal model computes the aerodynamic heating and heat balance of the body by considering the convection , the thermal conduction and the radiative heat loss. A dynamic thermal map of the surface is computed for the course of reentry. The principal model components and steps for &terming the IR-signature are discussed and results for a typical reentry vehicle are presented in the paper. The code is applicable in the spectral range from 2 to 25 μm with 5cm ' spectral resolution. Radiation of gas can be predicted within the temperature range from 100 -- 3000° K . The results are presented as emission spectra and their cumulated integrals. Thermal images are provided in radiance and equivalent blackbody temperature values. The contributions of the different radiation sources are analysed.

Loss-at-source thermal modelling in salient pole alternators using 3-dimensional finite difference techniques

Abstract: A method of calculating the temperature rise in the core, conductors, insulation, and cooling air of the stator and rotor of a salient pole alternator is described, with emphasis on the development of a finite-difference model of sufficiently reduced complexity to allow reasonably economic computation. The electrical losses arising in each metal element of the 3-D mesh are derived from loss-at-source computations described elsewhere, while thermal properties and convection coefficients are obtained by experiment or from standard characteristics. The results of temperature rise predictions are presented and compared with measured values. >

Influence of secondary flows on the stability of chemically reacting systems

Abstract: A model is presented for convection and chemical reacton in porous media. An irreversible chemical reaction of arbitrary order is considered. Reactant depletion allows for basic solutions in either the kinetic or the diffusion regime. The cases of forced flow parallel to the lateral walls of the cavity and a closed system are addressed. A linear stability analysis of the basic states is performed and critical values of the thermal Rayleigh number for the onset of natural convection are determined. A dispersion relation is derived and a graphical representation of the linear stability analysis results is provided for typical values of the system parameters. Analytical predictions are verified by results obtained by numerical integration of the complete set of nonlinear partial differential equations. The effect of natural convection is discussed when the basic state is either in the kinetic or in the diffusion regime. For large gradients, associated with the diffusion regime, chemical reaction can drive free convection even for low values of the Rayleigh number. In forced flow systems, natural convection can change substantially the flow pattern of the system.

An Interactive Computer Simulation of Heating and Cooling a Row of Silicon Wafers

Abstract: We present herein a computer simulation describing the heating or cooling of a row of silicon wafers in a quartz "boat" undergoing a prescribed thermal cycle. The simulation includes the effects of conduction, convection, and radiation. A novel part of the simulation is the introduction of an "effective viewfactor" that enables the radiative losses from the wafers to be easily calculated. Using this simulation we have predicted the rates at which wafers of a given radius and thickness can be cooled without producing unacceptable thermal stresses.

Bifurcation phenomena in thermal processes and convection

Abstract: This book contains 16 papers. Some of the titles are: Tutorial on elementary bifurcation theory; Stability of convection rolls in porous media; Successive transition of steady states in moderate size containers of air heated below and cooled above; and A bifurcation study of a convention in a two-dimensional saturated porous cavity.