Simultaneous Heat Transport Research Articles

Passive heat and moisture removal from a natural vented enclosure with a massive wall

Simultaneous transport of heat and moisture by conjugate natural convection in a partial enclosure with a solid wall is investigated numerically. Moist air motions are driven by the external temperature and concentration differences imposed across enclosures with different ambient moisture conditions. The Prandtl number and Schmidt number used are 0.7 and 0.6, respectively. The fluid, heat and moisture transports through the cavity and solid wall are, respectively, analyzed using the streamlines, heatlines and masslines, and the heat and mass transfer potentials are also explained by the variations of overall Nusselt and Sherwood numbers. The numerical simulations presented here span a wide range of the main parameters (heat and mass diffusion coefficient ratios, solid wall thickness and thermal Rayleigh numbers) in the domain of aiding and opposing buoyancy-driven flows. It is shown that the heat transfer potential, mass transfer potential, and volume flow rate can be promoted or inhibited, depending strongly on the wall materials and size, thermal and moisture Rayleigh numbers.

Effects of Material Symmetry on the Coefficients of Transport in Anisotropic Porous Media

The objective of this article is to highlight certain features of a number of coefficients that appear in models of phenomena of transport in anisotropic porous media, especially the coefficient of dispersion, the second-rank tensor Dij ,a nd thedispersivity coef- ficient, the fourth-rank tensor aijkl, that appear in models of solute transport. Although we shall focus on the transport of mass of a dissolved chemical species in a fluid phase that occupies the void space, or part of it, the same discussion is also applicable to transport coefficients that appear in models that describe the advective mass flux of a fluid and the diffusive transport of other extensive quantities, like heat. The case of coupled processes, e.g. the simultaneous transport of heat and mass of a chemical species, are also considered. The entire discussion will be at the macroscopic level, at which a porous medium domain is visualized as a homogenized continuum.

Effect of surface layer thickness on simultaneous transport of heat and water in a bare soil and its implications for land surface schemes

A physically‐based multi‐layer numerical model is developed to determine the coupled transport of heat and water in the soil and in the soil‐atmosphere boundary layer. Using inputs of standard weather data and initial soil conditions the model is capable of predicting the surface energy balance components as well as water content and temperature profiles in the soil. It is used to predict these variables for a bare silt loam soil under two tillage treatments, viz. culti‐packed and left loose after disc‐harrowing, and the predicted results are compared with measurements. Very good agreement between the model predictions and measured evaporation and heat fluxes and soil water and temperatures for a ten‐day period shows that the model is capable of simulating the coupled transport of soil heat and soil water and their transfer across the soil surface‐atmosphere interface adequately. Model predictions were compared with those of CLASS (Canadian Land Surface Scheme). It is shown that CLASS, version 2.6, provides good estimates of evaporation and hence the latent heat flux density, QE, under wetter soil conditions, but overestimates QE at moderately wet soil conditions and underestimates it under dry soil conditions. Under dry to moderately wet soil conditions the calculation of evaporation from bare soil is very sensitive to the thickness of the top layer particularly as the thickness approaches 10 cm.

Developments in soil–water physics since the mid 1960s

The theory for movement of water in unsaturated soils published by Richards 70 years ago is still an important starting point for the analysis of most soil physical problems. Over the last quarter century, the interest in finding new solutions of the Richards equation by either analytical or numerical methods was intense, particularly with regard to field situations. Experimental methods became more diverse and sophisticated: electromagnetic methods for measuring water content and salinity are now reliable and widely available, inverse methods for inferring the soil physical properties have matured. But during this period, we see also a widening of the scope of soil physics beyond the classical theory of Richards with further studies of various multiphase aspects—particularly, simultaneous movement of water and air, simultaneous transport of heat and moisture, flow of water and transport of solutes in structured soils—, of water movement in soils subject to swelling and shrinkage, and of transport of solutes in unsaturated soils. The latter two subjects became manageable by replacing the traditional spatial descriptions by material descriptions in which, respectively, the solid phase serves as the reference continuum for the water and the water serves as the reference continuum for the solutes. Progress was driven not only by a healthy theoretical, computational, and experimental basis, but also by productive interaction with adjoining disciplines and by challenging societal problems.

On flows of granular materials

This article reviews the behavior of materials made up of a large assemblage of solid particles under rapid and quasi static deformations. The focus is on flows at relatively high concentrations and for conditions when the interstitial fluid plays an insignificant role. The momentum and energy exchange processes are then primarily governed by interparticle collisions and Coulomb-type frictional contact. We first discuss some physical behavior —dilatancy, internal friction, fluidization and particle segregation — that are typical to the understanding of granular flows. Bagnold's seminal Couette flow experiments and his simple stress analysis are then used to motivate the first constitutive theories that use a microstructural variable — the fluctuation energy or granular temperature — governing the subscale fluctuating motion. The kinetic theories formalize the derivation of the field equations of bulk mass, momentum and energy, and permit derivation of constitutive relations for stress, flux of fluctuation energy and its dissipation rate for simple particle assemblages and when frictional rubbing contact can be ignored. These statistical considerations also show that formulation of boundary conditions needs special attention. The frictional-collisional constitutive behavior in which both Coulomb-type rubbing contact and collisional encounters are significant are discussed. There is as yet no rigorous formulation. We finally present a phenomenological approach that describes rapid flows of granular materials under simultaneous transport of heat and close with a summary of stability analyses of the basic flow down an inclined plane.

Fires in large scale ventilation systems

This paper summarizes the experience gained simulating fires in large scale ventilation systems patterned after ventilation systems found in nuclear fuel cycle facilities. The series of experiements discussed includes: (1) combustion aerosol loading of 0.61×0.61m HEPA filters with the combustion products of two organic fuels, polysterene and polymethylemethacrylate; (2) gas dynamic and heat transport through a large scale ventilation system consisting of a 0.61 m duct 90 m in length, with dampers, HEPA filters, blowers, etc.; (3) gas dynamic and simultaneous transport of heat and solid particulate (consisting of glass beads with a mean aerodynamic diameter of 10μ) through the large scale ventilation system; and (4) the transport of heat and soot, generated by kerosene pool fires, through the large scale ventilation system. The FIRAC computer code, designed to predict fire-induced transients in nuclear fuel cycle facility ventilation systems, was used to predict the results of experiments (2) through (4). In general, the results of the predictions were satisfactory. The code predictions for the gas dynamics, heat transport, and particulate transport and deposition were within 10% of the experimentally measured values. However, the code was less successful in predicting the amount of soot generation from kerosene pool fires, probably due to the fire module of the code being a one-dimensional zone model. The experiments revealed a complicated three-dimensional combustion pattern within the fire room of the ventilation system. Further refinement of the fire module within FIRAC is needed.

On thermohydrologic conditions near high‐level nuclear wastes emplaced in partially saturated fractured tuff: 1. Simulation studies with explicit consideration of fracture effects

We have performed modeling studies on the simultaneous transport of heat, liquid water, vapor, and air in partially saturated, fractured porous rock. Formation parameters were chosen as representative of the potential nuclear waste repository site in the Topopah Spring unit of the Yucca Mountain tuffs. The presence of fractures makes the transport problem very complex, both in terms of flow geometry and physics. The numerical simulator used for our flow calculations takes into account most of the physical effects believed to be important in multiphase fluid and heat flow. It has provisions for handling the extreme nonlinearities that arise in phase transitions, component disappearances, and capillary discontinuities at fracture faces. We model a region around an infinite linear string of nuclear waste canisters, taking into account both the discrete fractures and the porous matrix. Thermohydrologic conditions in the vicinity of the waste packages are found to depend strongly on relative permeability and capillary pressure characteristics of the fractures, which are unknown at the present time. If liquid held on the rough walls of drained fractures is assumed to be mobile, strong heat pipe effects are predicted. Under these conditions the host rock will remain in two‐phase conditions right up to the emplacement hole, and formation temperatures will peak near 100°C. If it is assumed that liquid cannot move along drained fractures, the region surrounding the waste packages is predicted to dry up, and formation temperatures will rise beyond 200°C. A substantial fraction of waste heat can be removed if emplacement holes are left open and ventilated, as opposed to backfilled and sealed emplacement conditions. Comparing our model predictions with observations from in situ heater experiments reported by Zimmerman and coworkers, some intriguing similarities are noted. However, for a quantitative evaluation, additional carefully controlled laboratory and field experiments will be needed.

Simultaneous transport of heat and moisture in a partially saturated porous media

The equations governing the transfer of heat and mass in a partially saturated soil are presented and solved numerically. A parametric study showing the effect of various terms on the distributions of temperature and moisture within the soil are given.

Coarsening in binary solid-liquid mixtures

A theory of Ostwald ripening has been developed for a solid-liquid mixture consisting of a low volume fraction array of spherical solid particles in a liquid wherein the coarsening process proceedsvia the transport of both heat and mass. We find that the simultaneous transport of heat and mass during ripening does not alter the exponents of the temporal power laws governing the ripening process from their classical values but does alter the amplitudes of these power laws. The growth rate of the cube of the average particle radius, the rate constant, is found to depend both on the alloy solute concentration and the ratio of the thermal to solutal diffusivities. In most metallic systems, a large decrease in the rate constant can be expected with small additions of solute to a pure metal. The mean-field temperature and concentration in the liquid will vary with time during ripening and will approach their equilibrium values along a unique path which is dependent only on the materials parameters of an alloy. Possible extensions of this theory to the analogous problem of ripening in isothermal ternary alloys are also discussed.

An analysis of temperature and moisture variations in the ground under natural climatic conditions

The earth/ground is one of the main components of the thermal environment and contains water, more or less, in liquid and gas phases. The water in the ground has a great influence on the surroundings, especially on the surface temperature. It is needed to predict the moisture content profile in the ground soil and its surface, for exact consideration of the heat transfer in and on ground soil. The moisture content in the ground soil is strongly influenced by precipitation and the depth of the water table in the ground. The heat and moisture (vapor and liquid water) transfer considerably affect each other. For prediction of the temperature and the moisture content profile in the ground, simultaneous transport of heat and moisture can be considered. This paper presents the results of transient analysis of the heat and moisture fields in the earth/ground under precipitation with non-linear simultaneous heat and moisture transfer equations using the water chemical potential as the potential of the surroundings. Using the weather data of Osaka, Japan, variations of moisture content and temperature in and on the earth/ground are numerically analyzed. An evaporation/condensation process is analyzed quantitatively. The transient process of surface moisture content and water chemical potential are shown. These values vary strongly but never reach saturation in the case of the sandy soil. Average moisture content in the earth/ground is strongly affected by the annual average precipitation but little by the depth of the water table. Using these results, thermal behavior of the thermal well and method of analysis are discussed. For analysis, approximation of the side boundary condition by uniform moisture content is applicable in two-dimensional analysis within the medium moisture content. And for practical calculation, approximation by a linear simple heat diffusion equation is applicable.

On simultaneous transport of heat and mass in natural convection

On simultaneous transport of heat and mass in natural convection