Moisture Content Field Research Articles

Approche experimentale des mecanismes de transfert au cours du sechage convectif a haute temperature d'un bois resineux

This paper presents an experimental approach of heat and mass transfer phenomena inside the wood and their connection with the external conditions during high temperature convective drying. The incipient flow velocity and temperature are in the range 6–19 m s −1and 150– 190°C. The drying curves, the time evolutions of temperature and moisture content field, and in wood gaseous phase pressure evolutions are analysed. The proposed moisture migration mechanism is justified by an approximate calculation. It is described by the classical three-phase model for convective drying of hygroscopic porous bodies, with particular aspects due to the material and the temperature level. We point out especially during the second phase of the drying a longitudinal free water movement caused by the total pressure gradient in the gaseous phase.

On the computation of Darcian velocity and mass balance in the finite element modeling of groundwater flow

Darcian velocity has been conventionally calculated in the finite element modeling of groundwater flow by taking the derivatives of the computed pressure field. This results in discontinuities in the velocity field at nodal points and element boundaries. Discontinuities become enormous when the computed pressure field is far from a linear distribution. It is proposed in this paper that the finite element procedure that is used to simulate the pressure field or the moisture content field also be applied to Darcy's law with the derivatives of the computed pressure field as the load function. The problem of discontinuity is then eliminated, and the error of mass balance over the region of interest is much reduced. The reduction is from 23.8% to 2.2% by one numerical scheme and from 29.7% to –3.6% by another for a transient problem.

Analytic calculation of deep vacuum drying of multilayer insulation elements of high-voltage transformers

The isothermal deep drying of electrically insulating elements of transformers in vacuum conditions is considered: A mathematical description of the unsteady moisture-content fields is given and a method of calculating the kinetics of the process is outlined.

Critical discharge of saturated and subcooled water through channels of different shapes

A complex experimental investigation of the outflow of saturated and subcooled water through channels of different forms has been carried out in the pressure range of 5–20 bar; the investigation included the measurements of the discharge, static pressure curves, moisture content field of the two-phase flow, and the photographic recording of the evaporation process. Empirical equations are proposed for determining the critical discharge.

Fire climates in the southwest

The climates of Arizona and New Mexico were analyzed from the standpoint of forest and range fire behavior. Climatological elements of temperature and humidity were used to define the mean equilibrium moisture content during the season of high fire danger. Phenological stages of the lesser herbaceous vegetation were taken as integrators of available moisture, and were used to evaluate the moisture content of the fine fuel complex, which influences fire behavior. Climatological boundaries were primarily defined by marked transition in the equilibrium moisture content field. The timespace domain of the fine fuel moisture complex was used as a secondary delineator of boundaries. These applied climatological techniques are now operationally used by fire control and weather agencies in the western United States.

OPTIMIZATION OF DRYING BY ELECTROMAGNETIC RADIATION: ANALYTICAL STUDY OF THE PROBLEM

The process of drying a flat sample with electromagnetic radiation is considered. As initial relations, the equations of the theory of heat and mass transfer are used. Lykov. To take into account the nonlinear nature of the process of mass transfer of the sample surface with the air medium, the boundary conditions for the moisture fluxes were adopted in the form of Dalton's evaporation law. A time-asymptotic analytic solution of the initial-boundary value problem is constructed, the characteristic feature of which is the stationarity of the temperature field T, the quasistationary nature of the moisture content field U, and the constancy of the drying intensity J. The presence of such features allows us to say that here we have, by analogy with convective drying, or a period of constant speed. The central notion in the relations obtained is the steady-state temperature of the material surface T∞, which is a generalization of the concept of the temperature of a wet thermometer to the case of electromagnetic drying. The problem of drying optimization has been solved and solved. The aim of optimization is to organize regimes in which the temperature field and/and the moisture content field are close to homogeneous. This corresponds to the minimization of the objective functions, which are the absolute values of the temperature and moisture content differences between the plate boundaries ç∆Т½ and ç∆U½. As optimization parameters, by varying the target functions, the intensity of radiation S and its penetration depth Δ are chosen. It is shown that the optimum regime should be chosen in the soft range, in which the ΔT and ΔU differences have the same signs, and the hard range in which these differences have opposite signs should be excluded from consideration. One of the limits of the soft range corresponds to the regime with ΔT=0, the other boundary to the regime with ΔU=0. An algorithm for calculating the optimization parameters S and Δ, corresponding to these modes, is developed, which makes it possible to organize drying within a soft range. As an example of using the developed algorithm, optimization of electromagnetic drying of a material with characteristics of quartz sand has been carried out.

Approximate computation method for unsteady moisture-content fields of material being dried

The approximate computational method presented can be used for unsteady moisture-content and moisture-diffusion fields, as well as for local and layer drying rates.

Mathematical model of heat and mass transfer in a moving bed of dispersed material

A moving bed of moist dispersed material is treated as a continuous medium in investigation of the temperature field and as a discrete medium in the determination of the moisture content field. A simplified combined transport system is presented.

Non-stationary temperature and moisture content fields of porous bodies in the convection heat transfer process

This paper presents the results of the experimental investigation of the moist capillary-porous bodies heating by means of convective heat supply. It is shown that the temperature and moisture content field reflect the nature of a moisture bond character with a capillary porous structure. The more precise definition of a mechanism of heating and removal of moisture from porous bodies is given.