Heat Transport Processes Research Articles

Numerical Study on Effect of Initial Concentration Difference on Onset of Rollover.

Momentum, heat and mass transport processes leading to rollover of density-stratified liquid layers in a vessel are investigated numerically. The main purpose of the study is to clarify the effect of initial concentration difference on the onset of rollover. We have calculated a number of cases in which the upper and lower fluids have initially different concentrations and different heating conditions. The result reveals that the initial concentration difference greatly influences the time elapsed before the onset of rollover. If the heating conditions are equal and the initial concentration difference is small, the rollover onset time is shortened. When the initial concentration difference is the same and the heating condition is changed, the rollover onset time is shortened with increasing of wall heat flux. The processes leading to rollover are as follows. Initially, two convection are induced in the upper and lower fluids separately, this situation being continued for a certain period, while the concentration is diffused with the convection. Finally, a large amount of the lower high-density fluid reaches the top surface, and then the fluids are suddenly mixed together.

Three-Dimensional Finite Element Analysis of Moisture Absorption in Expanding Alfalfa Cubes

A three-dimensional finite element analysis for a set of non-linear coupled heat and mass transfer equations is presented. Changes in dimensions of the elements during the heat and mass transport processes were incorporated in the finite element model. The model was used to simulate the transient temperature and moisture distribution in alfalfa cubes during moisture absorption. The models consider temperature and moisture-dependent material properties. Moisture absorption tests were conducted in an environmental chamber for different combinations of air relative humidity (70 to 90%) and temperature (10 to 40°C).Simulated temperature and moisture results were compared with experimental data, and related to stability of cubes during storage and transport. Results indicate that there was significant improvement in temperature predictions (by about 2·5°C) but only a minor improvement in moisture predictions (by 0·5%) when volume change was incorporated into the heat and mass transfer analysis. The alfalfa cubes reached the equilibrium temperature within 30 min of exposure to the humid air. The rate of moisture absorption was dependent upon the relative humidity and temperature of the surrounding air.

The molecular theory of structural relaxation and transport phenomena in liquids. II. Thermotransport dynamic process in classical liquids

Thermoelastic properties of classical liquids are investigated on the basis of generalized kinetic equations. Thermotransport in dynamic processes is shown to be characterized by a complex thermoconductivity coefficient. Its real part gives the thermoconductivity dynamic coefficient, and the imaginary part gives the dynamic thermal modulus of elasticity. The asymptotic behaviour of the thermoelastic properties is examined; the thermoconductivity of liquids is determined to play the principal part in the domain of low frequencies in the dynamic process of heat transport, whereas in the domain of high frequencies its thermal elasticity modulus plays the main part. The mechanism of heat transport is shown to change from the diffusion one into the wave one in the domain of high frequencies. An analytical expression of the high-frequency velocity of heat waves in liquids is obtained. The expressions obtained for liquid Ar in a wide range of thermodynamic changes are calculated and their results are compared to the available literature data.

A study of the effects of macrosegregation and buoyancy-driven flow in binary mixture solidification

A generalized anisotropic porous medium approach is developed for modelling the flow, heat and mass transport processes during binary mixture solidification. Transient predictions are obtained using FEM, coupled with an implicit time-marching scheme, for solidification inside a two-dimensional rectangular enclosure. A parametric study focusing attention on the effects of solutal buoyancy and thermal buoyancy is presented. It is observed that three parameters, namely the thermal Rayleigh number ( Ra v ), the solutal Rayleigh number ( Ra F ) and the relative density change parameter (σ 2), significantly alter the flow fields in the liquid and the mushy regions. Depending upon the nature of these flow fields, the solute enrichment caused by macrosegregation may occur on the top or the bottom region of the enclosure.

Experimental and Analytical Investigations of Simultaneous Heat and Moisture Transport through Glass Fiber Insulation

Glass fiber insulation is one of the most commonly used building materials in residential wall cavities. In the normal conditions under which buildings operate, the moisture content in this insulation may locally vary from that of a very dry to a fully saturated state. This gives rise to many mechanisms for moisture trans port in the insulation. This article looks at all the possible mechanisms through which moisture is transported. This is done through two series of experiments and detailed numerical analysis The experiments include investigations on water vapour transport in the presence of thermal gradients and drying of fully saturated speci mens through evaporation. Physical quantities measured include history of heat flux, boundary temperatures and relative humidities and transient moisture distributions. The simultaneous heat and moisture transport processes are mathematically described through a pair of conservation equations In a finite difference method, these equations are used for detailed numerical analysis of the processes. The numeri cal analysis makes use of the experimental data to derive consistent values for a set of moisture transport properties, representative of a sample of medium density glass fiber insulation. The properties include thermal conductivity as a function of mois turc content, vapour permeability as a function of temperature, moisture diffusivity as a function of moisture content and temperature, heat capacity and hydraulic con ductivity as functions of moisture content and sorption isotherm. A sensitivity anal ysis and an independent experiment on the drying of a randomly wet specimen sup port the results obtained from the numerical analysis

Modelling of the observed particle and heat fluxes in the X-point region in JET

Particle and heat fluxes, deduced from experimental CCD camera and magnetic measurements in the upper X-point region during Ohmic and auxiliary heated discharges in JET have been analysed. Modelling of the particle and heat transport processes at the edge of the plasma with Monte Carlo orbit following calculations agrees well with the observations. The importance of the finite orbit effects in low collisionality plasmas is assessed. The convection by plasma ions is found to play a very important role in the heat transport from the plasma to the wall

A one-dimensional simulation of the interaction between land surface processes and the atmosphere

A one-dimensional soil-vegetation model is developed for future incorporation into a mesoscale model. The interaction of land surface processes with the overlying atmosphere is treated in terms of three coupled balance equations describing the energy and moisture transfer at the ground and the energy state of the vegetation layer. For a complete description of the interaction, the coupled processes of heat and moisture transport within the soil are included as a multilayer soil model. As model verification, successful reproductions of the observed energy fluxes over vegetated surfaces from the HAPEX-MOBILHY experiment in southwestern France and from the LOTREX-10E/HIBE88 field experiment in Germany are presented. Finally, some sensitivity studies are performed and discussed in order to investigate the influence of different soil and vegetation types on the energy state of the atmosphere.

Investigations of the electrodischarge machining process for various electrode kinematics

In electrodischarge machining, the material excess is removed as a result of complex mass, heat and electric charge transport processes, which have a random and often unstable character. Investigations on classical electrodischarge sinking have shown that within steady-state process parameters, the standard deviation of the workpiece dimensions (a measure of accuracy) decreases with the decrease of the machined area. It is possible to take advantage of this fact by using an electrode with a working area smaller than the machined area and different electrode kinematics than in classical sinking. This assumption was corroborated by investigations on electro discharge sinking machine by means of a flat and a segmental rotating electrode as well as by means an electrode moving above the machined surface.

Mechanism of substrate heating in diamond-forming low-pressure microwave discharges

Experimental results concerning basic heat transport processes to and from substrates in diamond-forming low-pressure microwave discharges are presented. Special properties of the used planar microwave plasma source allow to distinguish clearly between different transport processes. As a rule, at pressures of few kPa substrate heating is dominated by surface recombination of atomic hydrogen. Substrate cooling is dominated by thermal conductivity of neutral gas. Detailed neutral gas temperature measurements demonstrated the completely non-isothermal character of the plasma with gas temperatures down to ambient temperature. Therefore, a basic problem of achieving very low deposition temperatures is to get detailed information about the concentration of atomic hydrogen that is absolutely necessary for high-quality diamond growth and its effect on substrate temperature.

Numerical modeling of heat and mass transport processes in an evaporative thermal protection system

We propose a mathematical model of heat and mass transport processes in a moist, porous material subject to capillary action. The material is in contact with a heated surface, and the processes take place while the liquid is evaporating in a cavity with a drainage hole. A sample calculation based on the model is presented. 45 refs., 4 figs.

Role of heat and gas transport processes in thermal analysis

In the course of solid phase reactions heat and gas transport processes play a dominant role. Therefore, it would be necessary to standardize the experimental conditions which succeeded only in part with the elaboration of the simultaneous techniques. Likewise the two transport processes are responsible for the fact that on the basis of non-isothermal curves one cannot calculate kinetic parameters of the reactions. The quasi static-methods seem to be suitable for the elimination of the pernicious effect of these transport processes.

Well-Ordered Motions and Relevant Heat Transfer in Wall Turbulence.

In wall turbulence, the well-ordered motions determine the essential features of turbulent transport phenomena. In the present study, we have refined a previously developed detection algorithm for well-ordered motions (i.e., trajectory analysis method based on the (u, v)-quadrant splitting technique) and have investigated the relationship between well-ordered motions and heat transfer from structural points of view, i.e., trajectory analysis of the VITA heat transfer events, extraction of basic flow modules and the relevant heat transport, and the simulation of temperature fluctuations by means of an autoregressive (AR) model. As a result, it is shown that phase relationship of fluctuating velocity components dominates the essential characteristics of the transport processes of heat and momentum in wall turbulence and there exist the distinct differences in individual correspondence between well-ordered motions and heat transport processes which cannot be revealed by the widely used VITA technique.

Double-Diffusive Convection during Solidification of a Binary System.

An experimental study of solidification is performed for a super-eutectic sodium carbonate solution (Na 2 CO 3 -H 2 O) in a confined cavity with varying initial concentrations and temperatures. We show important features of transport processes of heat and species during horizontal solidification by quantitative measurements of temperature and concentration. The development of the vertical density stratification that forms as a result of solidification can effectively damp thermal convection in the liquid region, and leads to the establishment of either a stagnant fluid region or double-diffusive cells. Initial superheat is found to be responsible for the initiation of the double-diffusive cells

熱収支モデルによる東京圏の熱環境解析

An energy budget model is developed to quantify the effects of the energy consumption and land cover on the surface air temperature in Tokyo metropolitan area. The model has several factors such as albedo, relative humidity, roughness height, heat capacity, and thermal diffusivity which take the values depending on the land use. The distribution and diurnal variation of anthropogenic heat release rate are given based on the limited data available.The simulated surface air temperature by the present model agrees favorably with the two set of AMeDAS data observed in mid-summer and winter with weak general wind. The analysis of heat transport process for each land use has shown that the air temperature in urban areas increases due to the decrease in humidity, particularly in daytime. It is also shown that the increase of energy consumption enhances the air temperature level through the intensification of turbulent diffusion process.

One-Dimensional Analytical Model of Flame Spread Over Solids

In this work, a simple one-dimensional analytical model of flame propagation over a solid fuel layer was developed. It was assumed, on the basis of the previous works, that the main mechanism controlling the flame propaga tion is the process of heat transport from the flame to the fuel layer by radiation and convection in the gas phase (just above the surface layer). Heat conduction processes in the layer were neglected but heat losses to the surrounding area were taken into account.

Thermal radiation and laminar forced convection in the entrace region of a pipe with axial conduction and radiation

Combined forced convection and thermal radiation in the thermal entrance region of a circular pipe with axial heat conduction and thermal radiation are numerically studied in this article. The fluid is treated as a gray, absorbing and emitting medium with a fully developed velocity entering an isothermal semi-infinite long pipe. The method of moments is applied to approximately model the radiative heat transport process. The governing equations are solved by applying the SIMPLE algorithm. The effects of the Peclet number and its interaction with the conduction-radiation parameter and optical thickness on heat transfer behavior in the thermal entrance region are also investigated. The results obtained indicate that radiation plays more important role at a low Peclet number than at a high Peclet number and that axial radition is negligible when the Peclet number N>20. The numerical results for limited cases of no axial heat conduction and radiation are also compared with the available data published in open literature. In all cases, good agreement with these studies was obtained.

2-D modelling procedures for ice cover thermal decay

The computational procedures of a two-dimensional numerical model for ice cover thermal decay are presented. Applications to side discharge thermal effluents are used to test the model capability of evaluating and predicting the extent of ice cover melting under varying hydrodynamic and thermal conditions. The theoretical formulation of the model is based on the application of the heat conservation law where thermal exchanges between the river, ice cover, and the atmosphere are accounted for. The 2-D time-dependent shallow water equations govern the river flow hydrodynamics, while the heat transport process is simulated using the unsteady 2-D energy equation. In order to properly account for the effects of river flow on the melting process, mass, momentum and heat conservation laws are fully coupled. The two-dimensional character of the equations is required by the nature of the physical phenoman involved in ice cover melting and thermal decay.

Experimental evidence of homoclinic chaos and type-II intermittency during the oxidation of methanol

The dynamic behavior of heterogeneous catalytic oxidation of methanol on zeolite supported palladium is studied. By varying the flow rate, the methanol concentration and the temperature, several regimes of periodic and toroidal oscillations are observed. Chaos and type-II intermittency are found near a homoclinic situation. It is shown, that in the case of homoclinic chaos the Grassberger-Procaccia algorithm for dimension estimation fails whereas a near-neighbor algorithm is applicable. For the intermittent behavior the histogram of the lengths of laminar episodes is compared with a distribution derived from a local model. It is suggested that the observed complex behavior is caused by periodic formation and reduction of palladium oxide in connection with heat and mass transport processes within the zeolite matrix.

On the subadiabatic magnetohydrodynamic overstability in background polytropes

The instability properties are studied of nonadiabatic MHD oscillations in background polytropes permeated by a uniform vertical magnetic field with the effects of thermal conduction, viscosity, and resistivity included. In particular, the very existence of subadiabatic MHD overstabilities for the quasi-fast modes over a wide range of field strengths are shown, and it is demonstrated that their presence originates from the subadiabatic acoustic overstability. The magnetic Lamb mode based on the previous results is identified. It is also shown that, in the strong-field limit, isothermal oscillations are neutral. Since the convective stability is enhanced by the presence of vertical magnetic field, MHD overstabilities can thus occur in a wide range of temperature gradients (subadiabatic as well as superadiabatic) while the system retains convective stability. The growth rates of oscillations are larger for steeper adverse temperature gradients. These results are important for understanding umbral oscillations and the heat transport process in sunspots, and for determining possible mechanisms to excite rapid oscillations of magnetic Ap stars.

Measurements of carbon dioxide diffusivity and thermal conductivity in muscle tissue.

The values of effective diffusivity for carbon dioxide and thermal conductivity have been measured in freshly excised respiring rat abdominal muscle in order to understand the heat and mass transport process occurring in the muscle tissue. The values of diffusivity increase with the decrease of the average partial pressure of CO2, indicating the existence of significant facilitated diffusion. The thermal conductivity of the muscle tissue membrane in which the paritial pressure gradient of carbon dioxide was imposed was measured by the self-heated thermistor method. There appears to be a slight increase of diffusivity at the lower partial pressure and this finding indicates that there may exist some mechanism of simultaneous heat and mass transport in the muscle tissue.