Simultaneous heat and mass transport in paper sheets during moisture sorption from humid air

A model of simultaneous heat and mass transfer through a porous material is presented to explain transient moisture sorption by paper sheets from humid air. There are three primary resistances to moisture transport: (1) diffusion through an external boundary layer; (2) diffusion through the pore system and, (3) diffusion from the pore system into the fibers. It is found that diffusion through the fiber phase perpendicular to the plane of the sheet is not significant for the moisture content range considered here. The mass transport model is able to predict the results of transverse moisture gradient experiments which show that moisture content gradients in paper are not as large as previously thought during transient periods. The model shows that the sigmoidal temperature response of paper to a linear change of relative humidity is due to non-linearities of the moisture content isotherm and heat of sorption.     

Performance study on heat and moisture transfer in soil heat charging

In some cold areas, the system performance of the soil source heat pump system is reduced by the decreasing underground soil temperature, which is caused by the thermal imbalance between the heating demand in winter and the cooling demand in summer. Soil heat charging with solar energy in non-heating seasons is proposed for solving the problem. It has been found from previous studies that the effect of the moisture transfer on the heat transfer within porous media could not be neglected especially under higher temperature difference. Therefore, this paper provides an investigation on the heat and moisture transfer in soil during soil heat charging at high temperature. A numerical model is developed for the study. The simulation results are compared with the testing data from the authors' previous study for the model verification. Based on the verified model, the performance of the heat and moisture transfer in soil during soil heat charging in a longer time and a larger area is investigated in the paper. The results show that the testing data match very well with the simulation results within a relative error of ±9% and the mathematical model is reliable for the performance prediction of heat and moisture transfer in soil heat charging. The soil volumetric water content (VWC) distribution tends to be stable after soil heat charging for 13 days and the heat source has an effective influence on soil VWC distribution within 2.4?m. The effect of the heat source temperature and initial VWC on the soil temperature and VWC distribution and heat power is proved to be obvious. Loam has a better performance in soil heat charging than sand.     

Identification of temperature and moisture content fields using a combined neural network and clustering method approach

Studies on the dynamics of temperature and moisture content distributions in porous soils have provided important insight on their effect on the building hygrothermal behavior, where the interaction between both building and soil can contribute to reduce building thermal gains or looses. Hygrothermal aspects can be related to many attributes such as energy consumption, occupants' thermal comfort and health, and material deterioration. Recently, a great variety of mathematical models to predict thermal and moisture content profiles in porous media have been presented in the literature. Most of those models are based on analysis of multilayer measurements or on Fourier analysis. The development and validation of such mathematical models facilitate the understanding of heat and moisture flows at different soil depths. In this research, a radial basis function neural network (RBF-NN) approach, combined with Gath–Geva clustering method in order to predict the temperature and moisture content profiles in soils, has been presented. A set of data obtained from the computation of the coupled heat and moisture transfer in porous soils for the Curitiba city (Paraná State, Brazil) weather data file has been used by the RBF-NN modeling method. Simulation results indicate the potentialities of the RBF-NNs to learn, for the one step ahead identification, the behavior of temperature and moisture content profiles in the media at various depths.     

Influence of soil moisture content on soil solarization efficiency

Soil temperature under plastic cover is a function of incoming radiation and thermal characteristics of the mulching material and the soil. A field experiment was conducted at Fudhiliyah Agrometeorological research station to investigate how soil temperature is influenced by different soil moisture content regimes during soil solarization. Two soils, sandy loam and silty clay loam, were brought to five volumetric moisture regimes (M1–M5). The moistened soils were mulched with 180-μm-thick transparent polyethylene. Measurements of hourly soil temperatures for depths of 0.00, 0.05, 0.10 and 0.30 m were recorded from 1 June to 30 September. The results showed that the maximum soil temperatures decreased with increasing soil moisture content. The relationship between the hourly rise in soil temperature and heat flux were significantly affected by soil moisture content. Moreover, soil solarization efficiency decreased with increasing moisture content. The higher temperatures in both soils under the M4 irrigation regime resulted in faster eradication of the pathogenic fungus, Fusarium oxysporum (sp. lycopersici).     

An analytical method to calculate the coupled heat and moisture transfer in building materials

A dynamic model for evaluating the transient thermal and moisture transfer behavior in porous building materials was presented. Both heat and moisture transfer were simultaneously considered and their interactions were modeled. An analytical method has been proposed to calculate the coupled heat and moisture transfer process in building materials. The coupled system was first subjected to Laplace transformation, and then the equations were solved by introducing the Transfer Function Method. The transient temperature and moisture content distribution across the material can thus be easily obtained form the solutions. The results were compared with the experimental data and other analytical solutions available in the literature; a good agreement was obtained.     

Investigation of heat and moisture migration in unsaturated soil under natural boundary conditions

Theoretical and experimental investigations were conducted to determine the heat and moisture migration in unsaturated soil under natural surface boundary conditions. Theoretically, a new model of heat and moisture migration in unsaturated porous media was developed, in which the gradients of volume water content, temperature, and partial vapor pressure were considered as the main driving forces which influence the process of heat and moisture migration in unsaturated soil. A set of coupled, nonlinear, partial differential equations were developed, which are related dynamically to the surface boundary conditions. Heat and moisture migration in sandy soil under solar radiation and air convection were studied experimentally. Temperature, volume water content, and water table evaporation were measured under unsteady conditions. The predictions are in good agreement with experimental data from a fairly sandy soil. © 1999 Scripta Technica, Heat Trans Asian Res, 28(1): 3–17, 1999     

Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying

The drying behavior of a moist object subjected to convective drying is analyzed numerically by solving heat and moisture transfer equations. A 3-D numerical model is developed for the prediction of transient temperature and moisture distribution in a rectangular shaped moist object during the convective drying process. The heat transfer coefficients at the surfaces of the moist object are calculated with an in-house computational fluid dynamics (CFD) code. The mass transfer coefficients are then obtained from the analogy between the thermal and concentration boundary layer. Both these transfer coefficients are used for the convective boundary conditions while solving the simultaneous heat and mass transfer governing equations for the moist object. The finite volume method (FVM) with fully implicit scheme is used for discretization of the transient heat and moisture transfer governing equations. The coupling between the CFD and simultaneous heat and moisture transfer model is assumed to be one way. The effect of velocity and temperature of the drying air on the moist object are analyzed. The optimized drying time is predicted for different air inlet velocity, temperature and moisture content. The drying rate can be increased by increasing the air flow velocity. Approximately, 40% of drying time is saved while increasing the air temperature from 313 to 353 K. The importance of the inclusion of variable surface transfer coefficients with the heat and mass transfer model is justified.     

热湿耦合作用下新建墙体保温层湿积累研究

墙体新建初期含湿量较大，在墙体干燥过程中可能产生严重的湿积累问题，导致保温层受潮严重，影响墙体保温性能。以夏热冬冷地区长沙市的典型建筑墙体为研究对象，采用WUFI@Pro建筑围护结构热湿传递模拟软件，对新建墙体内部保温层从初建成到湿稳定过程中的含湿量变化规律及湿积累进行研究。同时从饰面层的选择、隔汽层的设置、保温层的位置三方面对新建墙体湿积累的影响进行分析，为更好控制建筑墙体保温层中湿积累的产生提供参考。     

A new method for simulating the combustion of a large biomass particle—A combination of a volume reaction model and front reaction approximation

Combining a volume reaction model and front reaction approximation is proposed to simulate the combustion of a large biomass particle. Two intraparticle processes—drying and char oxidation—are simplified as front reaction because they are transport controlled. The other intraparticle process—pyrolysis—is described as the volume reaction because it is controlled by both heat transfer and kinetics. A new numerical method based on the basic mechanism of the process is applied to mitigate oscillations of the solution of the front reactions. To compare the calculation results with the experimental results presented in the literature, combustion of cubic wood particles between 5 and 25 mm is chosen to test the new method. Drying, pyrolysis, char oxidation, vapor condensation, shrinkage of the process, heat transfer via conduction, diffusion, convection, radiation and mass transfer via diffusion, and convection inside particle are taken into account. Finite volumes attached to solid materials are used to discretize the domain and explicit method with variable time step is used to calculate the process. A program was written and the calculation showed that the conversion of a particle is almost independent of computational mesh from 10 cells on. However there is significant instability in the mass loss rate curve when the number of cells is less than 20. Predictions for different particle sizes, furnace temperatures and moisture contents were compared with measurements and they agree reasonably well. The results highlight the significance of pyrolysis kinetics on prediction. Thus, the front reaction model of pyrolysis assuming a constant reaction temperature of 773 K is sometimes inadequate. The proposed method also showed that moisture content and pyrolysis reactivity significantly affect the thickness of devolatilizing fuel.     

入炉污泥含水率对污泥干化焚烧工艺影响研究

以上海某污水处理厂为例,通过理论计算和工程设计分析,研究了入炉污泥含水率(质量分数)对污泥干化焚烧工艺的影响.结果表明,随着干化程度的提高,对干燥机处理能力的要求提高,对干燥机型式的选择余地缩小,对焚烧炉、余热锅炉等设备的要求也将提高,对设备材质、系统安装、运行管理的要求也将相应提高.随着污泥干化程度的降低,进料量和烟气量增大,导致焚烧和烟气处理设备体积庞大.由于污泥泥质特性随时间变化大,在污泥热值整体偏低的地区,采用60%入炉污泥含水率存在一定的风险.污泥入炉含水率对污泥焚烧处理工程中的工艺选择及布置影响较大,工程设计中不应简单照搬国内外类似工程,而应根据当地污泥泥质特性、热值、辅助热源等实际情况,合理选择入炉污泥含水率.     

Estimation of the effect of biomass moisture content on the direct combustion of sugarcane bagasse in boilers

Many of the large-scale biomass combustion systems for producing heat, hot water, or steam accept biomass fuels containing relatively large amounts of moisture. Dry biomass burns at higher temperatures and thermal efficiencies than wet biomass. Flame temperature is directly related to the amount of heat necessary to evaporate the moisture contained in the biomass, the lower the moisture content, the lower the amount of energy needed to remove the water and the higher the boiler efficiency. In this article, a simple predictive tool is developed to estimate boiler efficiency as a function of stack gas temperature and sugarcane bagasse moisture content. The method quantitatively illustrates the effect of moisture content on the performance of a thermochemical process, for the direct combustion of sugarcane bagasse in a conventional boiler. The results are found to be in excellent agreement with reported data in the literature with average absolute deviation being around 1%. The tool developed in this study can be of immense practical value for engineers to have a quick check on biomass moisture content on the boiler performance at various conditions without opting for any experimental trials. In particular, engineers would find the approach to be user-friendly with transparent calculations involving no complex expressions.     

Experimental investigation on heat,moisture and salt transfer in soil

In this paper, experiments on coupled heat, moisture and salt transfer in yellow vegetable soil are conducted. A remarkable thermal permeant region in the soil column is found, along with moisture releasing, transition and absorbing zones in the soil column. The capillary flow is the dominant mechanism that governs the moisture transfer. With the moisture transfer, the salt moves from the groundwater and accumulates at the top of the column. However, with the salt accumulation, there will be a gradient of salt content and, as a result, the salt will also diffuse due to this gradient.     

Simultaneous heat and moisture transfer through a composite supported liquid membrane

Composite supported liquid membranes (SLM) are an efficient transfer media to recover heat and moisture from exhaust air due to the high moisture diffusivity in the liquid layer. However, heat transfer has adverse effects on moisture transfer since the water concentration in the LiCl solution decreases at higher temperatures. This study gives a detailed quantitative analysis of these effects. More specifically, simultaneous heat and moisture transfer through a composite supported liquid membrane is modeled. The SLM involved comprises three layers: two hydrophobic porous skin layers and a hydrophilic porous support layer where a layer of LiCl liquid solution is immobilized in the macro and micro pores as the permselective substance. The equations governing the heat mass transport in the microstructures, as well as the transfer of heat and moisture in the air streams adjacent to the membrane, are solved numerically in a coupled way. An experiment has been built to validate the model. The results found that though heat transfer has adverse effects on moisture transfer, in general, the effects on moisture effectiveness are quite limited. The high moisture permeation rates of SLM can be sustained when there is concomitant simultaneous heat transfer.     

Heat and moisture transport in durian fiber based lightweight construction materials

This paper presents result on heat and moisture transport in durian (Durio zibethinus) fiber based lightweight construction materials composed of cement, sand and waste fiber from durian peel and the performance of the material was simulated with the surface treatment by using a computational tool. The commercial research software (WUFI 2D) was used to calculate heat and moisture transfer through a durian fiber based lightweight construction material. The materials were exposed to a climate condition similar to the one in Bangkok and the hygrothermal characteristics of the materials were investigated. The investigation reveals that the weekly mean water content on the surface of material was quite low. The effect of moisture on the apparent thermal performance of the composite was found to be higher as water absorbed in the pore structure contributed to higher thermal conductivity than the air it replaced. However, the mean value of thermal conductivity in material is still rather low as the mean value of water content in material is low. Coating the surface reduced the flow of moisture to or from the structure considerably. The results of simulation confirmed that the manufactured composite satisfied the requirement of construction materials. It is then reasonable to conclude that the use of such materials in the design and construction of passive solar buildings is promising. Laboratory investigation is undergoing to validate the simulated performance.     

Modeling heat and moisture transfer through fibrous insulation with phase change and mobile condensates

This paper reports on a transient model of coupled heat and moisture transfer through fibrous insulation, which for the first time takes into account of evaporation and mobile condensates. The model successfully explained the experimental observations of Farnworth [Tex. Res. J. 56 (1986) 653], and the numerical results of the model were found to be in good agreement with the experimental results of a drying test. Based on this model, numerical simulation was carried out to better understand the effect of various material and environmental parameters on the heat and moisture transfer. It was found that the initial water content and thickness of the fibrous insulation together with the environmental temperature are the three most important factors influencing the heat flux.     

Measurement of effective thermal conductivity of wheat as a function of moisture content

The effective thermal conductivity of two varieties of Triticum durum wheat and a wheat product, bulgur, is determined at different moisture contents and at ambient temperature by the transient line heat source method. The moisture contents of the samples ranged from 9.17 to 38.65 percent wet basis and the bulk densities ranged from 675 to 827 kg/m³. Under those conditions, the measured effective thermal conductivities ranged from 0.159 to 0.201 W/m.K. The effective thermal conductivity is found to be linearly increasing with moisture content. The results are also in good agreement with literature values.     

Numerical simulation of the transient heat and liquid moisture transfer through porous textiles with consideration of electric double layer

The liquid water transport coupled with moisture and heat transfer through porous textiles is a complicated process involving simultaneous, coupled heat and mass transfers. The flows in porous textiles are different from the traditional flows transfer in porous media due to the adsorption of moisture by fibers. Based on the Poisson-Boltzmann equation for electric double layers and Navier-Stokes equation for liquid flows, a mathematical model for describing resistance effects of electric double layer (EDL) on the coupled heat and liquid moisture transfer in porous textiles is developed. The resistance effect of the EDL in porous textiles can be measured by a dimensionless number, which is called electric resistance number. It is proportional to the square of the liquid dielectric constant, the solid surface zeta potential and inversely proportional to the liquid dynamic viscosity, electric conductivity and the square of the effective pore size. With specification of initial and boundary conditions, the distributions of the temperature, moisture concentration, and liquid water content in porous textiles have been obtained. The theoretical predictions are compared with experimental data, and good agreement is observed between the two, indicating that the heat and mass transfer process are influenced by the EDL in porous textiles.     

Heat,air and moisture transfer through hollow porous blocks

The combined heat, air and moisture transfer in building hollow elements is of paramount importance in the construction area for accurate energy consumption prediction, thermal comfort evaluation, moisture growth risk assessment and material deterioration analysis. In this way, a mathematical model considering the combined two-dimensional heat, air and moisture transport through unsaturated building hollow bricks is presented. In the brick porous domain, the differential governing equations are based on driving potentials of temperature, moist air pressure and water vapor pressure gradients, while, in the air domain, a lumped approach is considered for modeling the heat and mass transfer through the brick cavity. The discretized algebraic equations are solved using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three driving potentials. Comparisons in terms of heat and vapor fluxes at the internal boundary are presented for hollow, massive and insulating brick blocks. Despite most of building energy simulation codes disregard the moisture effect and the transport multidimensional nature, results show those hypotheses may cause great discrepancy on the prediction of hygrothermal building performance.     

Experimental investigation of thermal and moisture behaviors of wet and dry soils with buried capillary heating system

An experimental study of thermal and moisture behaviors of dry and wet soils heated by buried capillary plaits was done. This study was carried out on a prototype similar to an agricultural tunnel greenhouse. The experimental procedure consisted on three different measuring phases distinguished by three different operational conditions of the capillary plaits: heating at 70 °C, heating at 40 °C and without heating in summer. During an experimental run, quantities measured are soil temperature, soil water content at various depths, soil surface heat flux, solar radiation under the plastic cover, internal relative humidity, internal and external air temperature. In unsaturated moist soils, the transport of heat is complicated by the fact that heat and mass transfer is a coupled process. During the daily soil temperature variation, it was found that the surface temperature amplitude was higher in wet soil than in dry soil. The water content increased during daytime and decreased during nighttime. The diurnal variation amplitude of water content was higher without underground heating and decreased with the buried heat source temperature.     

Analysis of two‐dimensional heat and moisture transfer during drying of spherical objects

This paper deals with the numerical and analytical modelling of two‐dimensional heat and moisture transfer during drying of a spherical object. Drying is considered to be a process of simultaneous heat and moisture transfer whereby moisture is vapourized by means of a drying fluid (e.g. air), as it passes over a moist object. Numerical modelling of two‐dimensional heat and moisture transfer during drying of a spherical object is carried out using an explicit finite‐difference approach. Temperature and moisture distributions inside the object are determined by using the developed computer code. Moreover, the results predicted from the present model are compared with the experimental data available in the literature and a considerably high agreement is found. Copyright © 2003 John Wiley & Sons, Ltd.