Moisture Transfer Mechanisms Research Articles

Performance comparisons of desiccant wheels for air dehumidification and enthalpy recovery

Desiccant wheels have two major applications: air dehumidification and enthalpy recovery. Since the operating conditions are different, heat and mass transfer behaviors in the wheels are quite different. In this paper, the performances of desiccant wheels used in air dehumidification and enthalpy recovery are compared with each other. To accomplish this task, a two-dimensional, dual-diffusion transient heat and mass transfer model which takes into account the heat conduction, the surface and gaseous diffusion in both the axial and the thickness directions is presented. Effects of the rotary speed, the number of transfer units, and the specific area on the performance of the wheel are investigated and compared in the two situations. The cycles that the desiccant and air undergo in the wheel are plotted in psychrometric charts to demonstrate the different heat and moisture transfer mechanisms during the dehumidification and enthalpy recovery processes.

Mathematical Simulation of the Perception of Fabric Thermal and Moisture Sensations

A mathematical simulation of the perception of thermal and moisture sensations is developed using mathematical models that describe the physical mechanisms of heat and moisture transfer in fabrics, the neurophysiological responses of thermoreceptors. and the psychoneurophysiological relationships identified from experiments. The sensations of coolness and dampness to the touch of knitted fabrics are predicted and compared with the subjective perceptions measured in a psychological experiment. The experimental results show that, in general, the relationships between the neurophysiological responses of thermoreceptors and the psychological perceptions follow Stevens' power function. The good agreement found between the measured and simulated perceptions shows that this mathematical simulation, based on heat and moisture transfer in fabrics, neurophysiolog ical mechanisms of the thermoreceptors, psychophysical relationships, is able to predict the perception of coolness and dampness sensation to the touch with reasonable accuracy.

Membrane-based Enthalpy Exchanger: material considerations and clarification of moisture resistance

The fundamental dimensionless groups for coupled heat and moisture transfer in a cross flow air-to-air enthalpy exchanger with hydrophilic membrane cores are derived and validated with experimental data. The thermal and moisture transfer mechanisms in membranes are studied. The finite difference numerical solutions of the model are used to study heat and moisture transfer in enthalpy exchangers. The variations of sensible, latent, and enthalpy effectiveness with various operating parameters are calculated for different types of material. Studies show that the sensible effectiveness is mainly determined by number of transfer units (NTU) of the exchanger, while the latent effectiveness is influenced by both the material and the operating conditions. Unlike thermal diffusive resistance, the moisture diffusive resistance in membrane is not a constant. It is co-determined by the slopes of sorption curves and the operating conditions. To account for these influences, a new dimensionless factor named the coefficient of moisture diffusive resistance (CMDR) is defined. With this coefficient, the performance of an enthalpy exchanger can be more easily predicted and clearly understood. By comparing the performances with different membrane materials, it is revealed that the membrane material with a linear sorption curve performs better than other materials under common conditions.

Evaluation of Discretized Transport Properties for Numerical Modelling of Heat and Moisture Transfer in Building Structures

Over the past decade, a large number of numerical models have been developed to predict heat and moisture transfer within building envelopes. In these models, the moisture transfer mechanism has been described and correlated by reference to the various transport phenomena and corresponding theories, viz. heat transfer and fluid flow. However, predicting the coupled heat and moisture performance of a building construction has never been a straightforward task, since a steady state situation hardly ever occurs and the transport properties (heat and moisture) of a material vary with moisture content and temperature. This paper discusses the transport phenomenon and the various numerical algorithms used in the discretization equations and how different algorithms affect the modelled results. Computer simulations have been conducted for different building materials and material combinations and comparisons have been made to evaluate the selection of discretized transport properties. Discrepancies in results are demonstrated between different mathematical interpolations, namely the Resistance(R) type formula and Linear(L)interpolation. Recommen-dations are given as guidance towards applying the most appropriate formulations for a given modelling scenario.

Investigation of moisture transfer mechanism in porous media during a rapid drying process

In this work, moisture transfer mechanism in wet porous media during rapid drying process was investigated experimentally and analytically. By use of scanning electron microscopic device, the rapid drying processes for potato, carrot and radish species were observed and recorded. A new displacement mechanism by pressure gradient in porous material during rapid drying was suggested. To analyze this displacement flow in porous material, a single capillary tube model was set up. The velocity of the displacement flow and its flow rate as well as displacement time of moisture were calculated.

DIFFUSION OF MOISTURE IN DRYING OF SUGAR CANE FIBERS AND BUNDLES

ABSTRACT Sugar cane fibers and arrangements of fibers in cylindrical bundles were dried in a thermoanalyzer and their diffusive coefficients were calculated using the slope method. The effect of temperature, moisture content as well as structural changes were analyzed. Diffusion coefficients changed nanlineariy with moisture content and followed an Arrhenius-like functionality with temperature. The analysis of these effects suggested a liquid diffusion transport mechanism of moisture transfer inside sugar cane fibers and bundles.

Investigation of moisture transfer mechanism in porous media during a rapid drying process

In this work, the moisture transfer mechanism in wet porous media during a rapid drying process was investigated experimentally and analytically. By means of scanning electron microscopy, the rapid drying processes for potato, carrot, and radish species were observed and recorded. A new displacement model using the pressure gradient in a porous material during rapid drying was suggested. To analyze this displacement flow in a porous material, the variables of this flow in a single capillary tube, such as velocity, flow rate, as well as the displacement time of internal moisture, were calculated. © 2000 Scripta Technica, Heat Trans Asian Res, 30(1): 22–27, 2001

Experimental analysis of unsteady heat and moisture transfer around a heated cylinder buried into a porous medium

An experimental analysis of heat and moisture transfer around a heated cylinder surrounded by an unsaturated soil is performed. The main motivation for the work is its application for high-voltage electrical power distribution in urban areas which, in general, makes use of underground cables. In the presence of electrical current, those cables generate heat that has to be dissipated through the surrounding soil in order to keep the cable temperature at safety levels; the cable operating temperature depends upon the soil ability to dissipate this heat. This is a complex phenomenon that involves several mechanisms of heat and moisture transfer. Experiments were performed for both constant and cyclic heating ranging from 8 to 80 W m−1. Spatial as well as temporal temperature, heat flux and moisture content results are reported. Computations using a two-dimensional continuum model were also performed and a good agreement prevailed between numerical and experimental results. It was found that well-graded soils such as the one used in the present work have a great capacity for retaining moisture which is an important feature if high thermal conductivities around the heat source are desired.

Choice of optimum regimes for electric heating of chamotte-polystyrene mixtures on a liquid glass solution

It is shown that introduction of liquid glass affects not only the mechanism of filtration moisture transfer but also the rate of increase of the temperature of the mixture. By increasing the amount and density of the introduced additive it becomes possible to heat a chamotte-polystyrene mixture more rapidly. The parameters of two-stage electric heating with various contents of liquid glass are presented and the optimum density of the solution is shown to be 1.36 g/cm3. Special features of the green and recommendations for removing the mold after electric heating are presented with allowance for the volume stress state of the mixture compacted in the closed volume of the mold. Taking into account the high temperature of the green after demolding, the drying time of the green and the temperature of the heating agent are determined.

THE INFLUENCE OF AIR TEMPERATURE ON EFFECTIVE DIFFUSION COEFFICIENT OF MOISTURE IN THE FALLING RATE PERIOD

ABSTRACT The convection drying of spheres of Al-Ni catalyst and thin clay plate was investigated on a laboratory scale. The aim of the investigation was to determine the mechanism of moisture transfer within the firm sample skeleton. By measuring the mass loss and the surface temperature of the samples during the drying process, basic dependencies necessary to describe the process kinetics was obtained. Found functional dependencies show high level of correlation with experimental data. Exponential model parameters are dependent of air temperature and velocity, and the type of the examined material. The applicability of the chosen model to describe the drying kinetics is proven regardless of drying material origin and geometry, The comparison of different drying experiment and determination of the average diffusional coefficient at given experimental conditions is enabled.

Moisture in concrete subjected to different kinds of curing

This article outlines an experimental and numerical study of moisture in concrete subjected to air, water or sealed curing. For this purpose columns of eight qualities of concrete were studied at 3 different ages each over a period of 450 days. Parallel studies of hydration, internal relative humidity, self-desiccation and strength were carried out. Finally the article presents an analysis of the internal relative humidity and the mechanisms of moisture transfer by modelling the diffusivity and the capillary conditions of 4 of the concretes. The project was carried out between the years 1989 and 1992. Cet article commence par un apercu des problemes en rapport avec l’humidite dans le beton. Il a ete etabli un resume de la methode experimentale, des materiaux etudies, du procede de mesure, etc. Dans les resultats experimentaux originaux presentes dans cet article, le sechage et l’autodessiccation de 8 betons ont ete etudies pour des colonnes de beton simulees sur une periode de 450 jours. Le rapport eau-ciment variait entre 0.22 et 0.58. La moitie du beton contenait 10% de fumee de silice. Plus de 450 mesures de 22 heures chacune ont ete realisees. Les experiences ont ete completees par des etudes sur la capillarite, la diffusivite et la permeabilite de 4 betons sur une periode de 750 jours. En voici les principaux resultats: 1) L’humidite relative interne, O, due a un taux de resistance eleve a la penetration de l’eau du beton a faible rapport eau+ciment de la couche superficielle, est descendue a environ 0.8 pres de la surface, meme lorsque le beton etait immerge. 2) L’autodessiccation du beton etait fonction du temps, du rapport eau+ciment et de la teneur en fumee de silice. 3) Les effets de la fumee de silice sur l’autodessiccation du beton etaient independants du temps, pour un rapport eau+ciment = 0.39. Pour un rapport eau+ciment plus faible, les effets de la fumee de silice etaient plus prononces a un stade initial. Des effets contraires de la fumee de silice ont ete observes avec du beton ancien. 4) En raison de sa tres faible diffusivite a une distance de 50 mm de la surface de sechage, O (humidite relative interne) a diminue d’environ 0.20 seulement pour un rapport eau+ciment plus eleve (diffusivite plus elevee). 5) Des formules de O (humidite relative interne) ont ete enoncees pour decrire son developpement dans le beton, reposant essentiellement sur le type de conservation, l’âge, le rapport eau+ciment et la teneur en fumee de silice. (Less)

Moisture sorption and movement in Parallam® and Timber Strand®

The equilibrium moisture contents and moisture diffusion coefficients of two types of Timber Strand® and four types of Parallam® were experimentally determined in this study. The Hailwood-Horrobin sorption model was fitted to theM e and the regression parameters that define the sorption isotherm were found to vary with material type, sorption mode and ambient temperature. An optimization technique was implemented to obtain diffusion coefficients from the experimental data. The results showed that the diffusion values of LSL and PSL were strongly affected by temperature and sorption direction relative to flake or strand orientation. Moisture content also affected the diffusion values, but the trend depended on the moisturetransfer mechanism that was dominant in the material.

乾留初期におけるコークス炉内炉幅方向水分移動機構

In order to clarify the moisture transfer mechanism, moisture distribution across the oven width at the early stage of carbonization has been investigated using the test coke oven with the upper heating wall. There is a region having a maximum moisture content below 100°C because of moisture condensation. Subsequently, as temperature is elevated close to 100°C, remarkable moisture vaporization has taken place so that the moisture in coal decreases rapidly. However, there remains a little moisture in coal beyond 100°C.Two dimensional coke oven mathematical model was hence updated by introducing this moisture transfer mechanism.

The effects of transient temperature gradients on soil moisture dynamics

The remediation of non-mobile non-aqueous phase liquid (NAPL) from the unsaturated zone involves a number of physical processes that may be functions of temperature. The coupled mechanism of soil moisture (liquid and vapour) and thermal energy transfer in a partially saturated porous medium has been investigated through the use of mathematical modelling. The physical flow processes involved in this analysis are liquid, vapour and adsorbed liquid flow, heat conduction, heat convection, latent heat and heat of wetting transfer. A mixed form of both the moisture and the thermal energy equations is utilized, ensuring a mass and energy conservative formulation. The Galerkin finite element method is employed for the spatial integration of the governing equations while temporal integration is achieved by a fully implicit finite difference method with a variable time-stepping scheme. The two-dimensional coupled equations of moisture and thermal energy flow are assembled through the use of a simultaneous solution approach, resulting in a non-linear system of equations which is linearized at each time-step using a full Newton-Raphson iteration scheme. The proposed model was tested on a variety of problems which verified that it was formulated properly. Simulation results indicate that transient temperature gradients affect the moisture velocity and water pressure fields, due mainly to viscosity effects on the hydraulic properties of the medium. The effects of pressure gradients on the temperature distribution are minimal, indicating that heat conduction is the dominant mechanism associated with thermal energy transport. This present study is a step toward the non-isothermal analysis of the transport of contaminant gases in the unsaturated zone and in the dissolution kinetics of residual organics which may lead to a better understanding of remedial actions that involve thermal processes.

Study of moisture transfer during the strand sintering process

Moisture transfer during the strand sintering operation was studied both experimentally and using a mathematical model. The drying of iron ore pellets was found to occur in two distinct periods: one at a constant drying rate and the other at a decreasing drying rate, whereas the drying of zinc ore pellets always occurs at a decreasing drying rate. Characteristic drying curves were determined for both materials. The moisture transfer mechanisms during the sintering process were demonstrated in detail, including the recondensation of water in the cold layers of the bed and the formation of an inert, overmoistened zone. The mathematical model presented simulates all of these phenomena and is used to calculate the variables related to moisture transfer. The model is adaptable to other processes where a hot gas passes through a moist packed bed.

A mathematical model of drying processes

A drying model is proposed which may be used to describe drying behavior of hygroscopic and non-hygroscopic materials. The constant rate, the first falling rate and the second falling rate periods in drying are addressed separately. The concept of ‘bound water conductivity’ is introduced. Movement of bound water and its contribution to moisture transfer within hygroscopic materials are discussed. The bound water conductivity is found to be affected by moisture content as well as desorption isotherms of the drying material. The major internal moisture transfer mechanisms are considered to be capillary flow of free water in the wet region and movement of bound water and vapor transfer in the sorption region. The convective heat and mass transfer coefficients are assumed to vary with the surface free water content in the first falling rate period. Three systems with different hygroscopic properties, wool, brick and corn kernels, are chosen to evaluate the validity of this model. The moving finite element method is used to solve the differential equations numerically. The predicted drying curves and the temperature and moisture distributions compare favorably with reported experimental results.

Drying of concrete: An application of the 3L-approach

Abstract Concrete is a composite material with dense aggregates embedded in a porous matrix. Hence moisture transfer is limited to water and vapour movement through the porous system of hardened cement paste and along the interface. It is shown that it is reasonable to subdivide the structure of concrete into three different levels. This concept is now called the 3L-approach. The micro-level serves to describe moisture transfer mechanisms. The meso-level has been introduced to take the composite structure of concrete into consideration and finally the macrolevel is used to formulate realistic material laws. The reliability of this approach is underlined by an arbitrarely chosen example, i.e. drying of a concrete element. It is shown that the time-dependent moisture distribution can be obtained for variable relative humidities of the environment.

Moisture diffusion within shredded tobacco leaves.

The mechanism of moisture transfer into shredded tobacco was investigated in connection with moistening process control in cigarette manufacturing. For shredded tobaccos of less than about 1.0 mm shred width, the moisture was sorbed mainly through the cut faces rather than through the leaf epidermis, indicating that the moisture diffusion within the leaf was the rate limiting step of moisture transfer. The moisture diffusion coefficient values within the cured tobacco leaf ranged from 8.80 × 10−7 to 64.2 × 10−7cm2-s−1 depending on the type of tobacco, the shred width of tobacco and the temperature at sorption. But these values were not affected by the relative humidity at sorption. The entire moisture transfer into the shredded tobacco leaves was predictable with the diffusion theory.

Moisture Permeability of Cured Tobacco Epidermis

The mechanism of moisture transfer into the cured tobacco leaf was investigated in connection with the moistening processes during cigarette manufacturing. The moisture was sorbed through three pathways: through the upper epidermis, lower epidermis and the cut face. For laminar tobacco larger than about 5.0 mm in width, most moisture was sorbed through the epidermes, with nearly equal flux through the upper and lower epidermes. The permeability of the culticular layer was the limiting factor to moisture transfer rate into the cured laminar tobacco through the epidermis, and the moisture transfer through the stomata was negligible as compared with that through the cuticular layer. An empirical kinetic model is presented to describe the entire moisture sorption process through cured tobacco epidermis, together with the values of the parameters involved. Rational moisture control of the cured laminar tobacco will be possible on the basis of this analysis.

Sorption and desorption phenomena in tobacco. Part III. Moisture permeability of cured tobacco epidermis.

The mechanism of moisture transfer into the cured tobacco leaf was investigated in connection with the moistening processes during cigarette manufacturing. The moisture was sorbed through three pathways: through the upper epidermis, lower epidermis and the cut face. For laminar tobacco larger than about 5.0mmin width, most moisture was sorbed through the epidermes, with nearly equal flux through the upper and lower epidermes. The permeability of the culticular layer was the limiting factor to moisture transfer rate into the cured laminar tobacco through the epidermis, and the moisture transfer through the stomata was negligible as comparedwith that through the cuticular layer. An empirical kinetic model is presented to describe the entire moisture sorption process through cured tobacco epidermis, together with the values of the parameters involved. Rational moisture control of the cured laminar tobacco will be possible on the basis of this analysis.