Modeling of Diffusion in Capillary Porous Materials During the Drying Process

This paper presents a model of heterogenous diffusion in capillary porous materials during the process of drying. The governing heat and mass transfer equations have been established using the liquid as well as vapor flow. Two models have been presented. Model 1 does not consider the heat conduction while the model 2 has been established by considering the conduction. The developed models and the numerical solutions of the resulting differential equations can take into account the moisture and temperature dependent thermophysical properties of the product. All equations have been established in spherical coordinates but the programme written for the purpose of calculations can be used for other geometries also. Numerical calculations have been performed for gas concrete and tiles using model 1, while model 2 has been used for gas concrete only because of the lack of data for thermophysical properties of the tile. For gas concrete it was seen that conduction has only marginal effect on the drying process and the numerical predictions of the drying process were reasonably accurate.     

高初始含湿多孔介质喷雾干燥过程的热质耦合传递

多孔介质是大量干燥过程的主体,由于实际多孔介质干燥过程的复杂性,建立通用的干燥过程传热传质模型十分困难。通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响,根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况,反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性,为确定实际生产中喷雾干燥器的操作条件指明了新的出路。     

Drying of Porous Materials in the Presence of Solar Radiation

ABSTRACT

We present a combined heat and moisture transfer model for predicting the drying characteristics of porous building materials exposed to solar radiation. The model has been validated for convective drying using published data and for radiative drying using results of an experimental study carried out using a solar lamp to simulate solar radiation conditions. Actual and predicted moisture content profiles and the drying rates when compared give favourable results.     

Investigation on Moisture Transfer Mechanism in Porous Media During Rapid Drying Process

Abstract

In this work, moisture transfer mechanism in wet porous media during rapid drying process is 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. The microscopic drying experiments show that during high intense and rapid drying process, the mechanism of moisture migration in materials is mainly considered as a displacement flow driven by pressure gradient along a capillary passage. A simplified displacement flow model during rapid drying process is proposed and the time needed for moisture transfer in porous media is calculated. To examine this drying mechanism, one-dimensional displacement flow test device is built up and a set of experiments under different pressure gradients and temperatures are conducted. Glass beads of 0.8 mm in diameter are used as the porous material. The experimental results show that when pressure gradient is getting greater at constant temperature, the moisture removal time is getting smaller. On the other hand, under the same pressure gradient, when liquid temperature increases, the time for moisture transfer from the internal to the external surface decreases. The calculated moisture removal times are well agreed with the experimental data.     

Investigation on Moisture Transfer Mechanism in Porous Media During Rapid Drying Process

In this work, moisture transfer mechanism in wet porous media during rapid drying process is 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. The microscopic drying experiments show that during high intense and rapid drying process, the mechanism of moisture migration in materials is mainly considered as a displacement flow driven by pressure gradient along a capillary passage. A simplified displacement flow model during rapid drying process is proposed and the time needed for moisture transfer in porous media is calculated. To examine this drying mechanism, one-dimensional displacement flow test device is built up and a set of experiments under different pressure gradients and temperatures are conducted. Glass beads of 0.8 mm in diameter are used as the porous material. The experimental results show that when pressure gradient is getting greater at constant temperature, the moisture removal time is getting smaller. On the other hand, under the same pressure gradient, when liquid temperature increases, the time for moisture transfer from the internal to the external surface decreases. The calculated moisture removal times are well agreed with the experimental data.     

Numerical Simulation of the Drying of a Deformable Material : Evaluation of the Diffusion Coefficient

ABSTRACT

We are interested in the simulation of heat and mass transfer processes accompanying the drying of a deformable agricultural product. We have used an implicit method with classical finite differences to resolve the set of equations. The identification of the diffusion coefficient within a thin carrot layer is carried by trial and error on two analytical forms. These two forms take into account moisture content, temperature and shrinkage of the product.

The drying rates of thin layer of carrot pieces are measured in a laboratory tunnel dryer. These drying curves are established by varying some airflow parameters such as velocity, temperature and relative humidity. The internal and surface temperature of the slab were recorded by means of therrnosensors. The simulated solutions are in agreement with the experimental results.     

Modeling Superheated Steam Vacuum Drying of Wood

Abstract

A two-dimensional mathematical model developed for vacuum-contact drying of wood was adapted to simulate superheated steam vacuum drying. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady-state mass conservation of dry air. A drying test conducted on sugar maple sapwood in a laboratory vacuum kiln was used to infer the convective mass and heat transfer coefficients through a curve fitting technique. The average air velocity was 2.5 m s^?1 and the dry-bulb temperature varied between 60 and 66°C. The ambient pressure varied from 15 to 11 kPa. Simulation results indicate that heat and mass transfer coefficients are moisture content dependent. The simulated drying curve based on transfer coefficients calculated from boundary layer theory poorly fits experimental results. The functional relation for the relative permeability of wood to air is a key parameter in predicting the pressure evolution in wood in the course of drying. In the case of small vacuum kilns, radiant heat can contribute substantially to the total heat transfer to the evaporative surface at the early stages of drying. As for conventional drying, the air velocity could be reduced at the latter stage of drying with little or no change to the drying rate.     

Modeling Superheated Steam Vacuum Drying of Wood

A two-dimensional mathematical model developed for vacuum-contact drying of wood was adapted to simulate superheated steam vacuum drying. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady-state mass conservation of dry air. A drying test conducted on sugar maple sapwood in a laboratory vacuum kiln was used to infer the convective mass and heat transfer coefficients through a curve fitting technique. The average air velocity was 2.5 m s^-1 and the dry-bulb temperature varied between 60 and 66°C. The ambient pressure varied from 15 to 11 kPa. Simulation results indicate that heat and mass transfer coefficients are moisture content dependent. The simulated drying curve based on transfer coefficients calculated from boundary layer theory poorly fits experimental results. The functional relation for the relative permeability of wood to air is a key parameter in predicting the pressure evolution in wood in the course of drying. In the case of small vacuum kilns, radiant heat can contribute substantially to the total heat transfer to the evaporative surface at the early stages of drying. As for conventional drying, the air velocity could be reduced at the latter stage of drying with little or no change to the drying rate.     

气体检测器中扩散过程的模拟计算

从费克和二定律出发,推导了气体在装有固体颗粒的小直径圆形管中有限扩散距离内扩散－反应过程的浓度分布表达式以及气体深度与扩散时间和扩散距离的关系。得出在一定的扩散时间内,气体浓度与扩散深度之间为抛物线关系;一定浓度的气体通过固体颗粒层扩散－反应时,其扩散深化度的平方与扩散时间的比值为常数。理论分析与实验结果完全符合。     

具有电介质核心多孔介质微波冷冻干燥过程的耦合传热传质的数值研究

利用变时间步长的有限体积法对具有电介质核心多孔介质微波冷冻干燥的耦合热质传递过程进行了数值模拟。计算结果表明：在有电介质核的多孔介质内部存在着两个升华界面,该双升华界面模型成功地模拟了该干燥过程：合理的选用电介质核心可以大大缩短干燥时间,对于几个大小分别为1．0,1.5,2．0和2．5mm的电介质核,其单位体积所需干燥时间同无核相比分别减少了8％,19％,33％,48％：在相同的电场强度下,电介质核的损耗系数越大,所需干燥时间越短。     

Drying in capillary-porous cylindrical media

Analytical solutions are presented for temperature and moisture distributions in a cylindrical capillary-porous body using Luikov's model and employing a solution technique based on the decoupling of the simultaneous heat and mass diffusion problems. The relative importance of the dimensionless parameters governing drying is presented and a comparison of temperature and moisture profiles for the cylinder and the slab is given.     

Diffusion During the Supercritical Drying of Silica Gels

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO₂ drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO₂. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.     

无机纤维板干燥过程的数学模拟研究

本文通过对无机纤维板干燥过程的分析,建立了干燥过程中热量传递和质量传递的数学模型,模拟结果与实测结果有较好的一致性.     

Microwave Drying of Porous Materials

Abstract

Experimental results on microwave drying of the porous particles exposed to air stream at 40°C are presented. The temperature and moisture distribution inside a particle were measured for gypsum spheres of 9, 18, 28, and 38 mm. The mass reduction was monitored during the drying process. The rate of drying and changes in temperature and moisture profiles for different drying conditions were analyzed and compared with the ones for convective drying.     

Microwave Drying of Porous Materials

Experimental results on microwave drying of the porous particles exposed to air stream at 40°C are presented. The temperature and moisture distribution inside a particle were measured for gypsum spheres of 9, 18, 28, and 38 mm. The mass reduction was monitored during the drying process. The rate of drying and changes in temperature and moisture profiles for different drying conditions were analyzed and compared with the ones for convective drying.     

Modeling of Moisture Diffusion in Microwave Drying of Hardwood

A one-dimensional mathematical model was developed to predict temperature and moisture content profiles in red maple (Acer rubrum L.) and white oak (Quercus alba) during microwave drying. The model was solved using the finite element analysis with MATLAB software. The predictions for temperature and moisture content agreed favorably well with the experimental data. The diffusion coefficients of the red maple and the white oak in microwave drying conditions were calculated and analyzed. Equations of the diffusion coefficient in longitudinal and transverse directions based on input microwave power level are presented in this article. In microwave drying of hardwood, the red maple was heated more efficiently than the white oak because of higher absorbing efficiency of the microwave power.     

Three-Dimensional Numerical Modeling of Convective Heat Transfer During Shallow-Depth Forced-Air Drying of Brown Rice Grains

We describe here a three-dimensional (3D) numerical study of the convective thermal transfer during forced-air drying of brown rice grains with the inclusion of moisture evaporation. Three levels of temperature were tested across a range from 40 to 60°C. The objective of the study was to determine the temperature distributions in the drying chamber as well as the surface and intra-kernel temperature gradients during convective drying of rice grains. The numerical model was based on the Navier-Stokes equation for fluid flow and the Fourier's equation for heat transfer. Results of the numerical solution showed that temperature distribution in the air and brown rice grains during the early minutes of drying were significantly influenced by the direction of flow of the heated air. Air gaps between grains and at the periphery of the drying chamber affected the rate and direction of airflow. The temperature history of the rice grains showed variations at different levels within the layers of grains. Moreover, the stream of heated air showed regions with airflows eight times greater than the inlet air velocity.