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.     

Investigation of moisture transfer effectiveness through a hydrophilic polymer membrane with a field and laboratory emission cell

This research focuses studies of water permeation potential through a polymer membrane with the help of a standard field and laboratory emission cell. Special efforts are devoted to finding a correlation governing the relations between the number of transfer units (NTU) and the moisture exchange effectiveness. As a first step, moisture diffusivity in the hydrophilic polymer membrane is experimentally measured. In combination with mathematical modeling, the moisture concentration distributions in the cell, the water uptake gradients in the membrane, as well as the local vapor emission rate on membrane surface, are investigated. The results are that the emission rates show a non-uniform character and a polynomial equation governing the moisture exchange effectiveness and the dimensionless number of transfer units, could be used to inversely estimate the diffusivity of water vapor in hydrophilic membranes. The form and the value of constants in the equation are obtained.     

A method for predicting heat and moisture transfer through multilayered walls based on temperature and moisture content gradients

A mathematical formulation applied to a numerically robust solver is presented, showing that moisture content gradients can be used as driving forces for heat and moisture transport calculation through the interface between porous materials with different pore size distribution functions. For comparison purposes, several boundary conditions are tested—in order to gradually increase the discontinuity effects—and a detailed analysis is undertaken for the temperature and moisture content distributions and sensible and latent heat fluxes, when the discontinuity on the moisture content profile is taken or not into account.     

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.     

Transient model for coupled heat,air and moisture transfer through multilayered porous media

Most building materials are porous, composed of solid matrix and pores. The time varying indoor and outdoor climatic conditions result heat, air and moisture (HAM) transfer across building enclosures. In this paper, a transient model that solves the coupled heat, air and moisture transfer through multilayered porous media is developed and benchmarked using internationally published analytical, numerical and experimental test cases. The good agreements obtained with the respective test cases suggest that the model can be used to assess the hygrothermal performance of building envelope components as well as to simulate the dynamic moisture absorption and release of moisture buffering materials.     

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.     

Evaporation and condensation heat transfer in a heat pipe with a sintered-grooved composite wick

A mathematical model of evaporation and condensation heat transfer in a copper-water wicked heat pipe with a sintered-grooved composite wick is developed and compared with experiments. The wall temperatures are measured under different input power levels and working temperature conditions. The results show that the heat transfer in the condenser section was found to be only by conduction. In the evaporator, however, either conduction or boiling heat transfer can occur. The experimental data for the boiling heat transfer are well correlated by the theory of Stralen and Cole. Higher heat load drives the heat pipe to spend more time achieving the equilibrium state during the transient start-up process. The response curves of the evaporator thermal resistance are overlapped, and the condenser thermal resistance increases more sharply at the beginning. The total thermal resistance of the heat pipe ranges from 0.02 to 0.56 K/W.     

Conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor for liquid desiccant air dehumidification

The fluid flow and conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor are investigated. The shell-and-tube like contactor is used for liquid desiccant air dehumidification, where numerous fibers are packed into the shell and air flows across the fiber bank. To overcome the difficulties in the direct modeling of the whole contactor, a representative cell, which comprises of a single fiber, a liquid solution inside the fiber, and an air stream across the fiber, is selected as the calculation domain. The air stream in the cell is surrounded by an assumed outer free surface. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams are solved together with the heat and mass diffusion equations in the membrane. The friction factor and the Nusselt and Sherwood numbers on the air and stream sides are then calculated and experimentally validated.     

Calculating heat transfer through windows

To calculate the energy performance of buildings, one must know the heat-transfer characteristics of the windows as functions of environmental variables, such as temperature and wind speed. Window designs are becoming more complex in response to the need for energy conservation. In this paper, we develop a general procedure for calculating the net energy flux through the glazed area of a window composed of an arbitrary number of solid layers. These layers, which may have thin-film coatings, can have any specified solar and thermal radiation properties and enclosed spaces between solid layers can contain either air or other gases. We verified our results by comparing them with experimental measurements of heat flow using a calibrated hot-box.     

Study of heat and moisture transfer in soil with a dry surface layer

Mathematical model for describing simultaneous heat and moisture transfer in the porous soil with a dry surface layer was developed by using the volume-averaging method. Numerical simulation was conducted to investigate water evaporation, transient distributions of temperature and moisture in the porous soil at environmental conditions, which might be useful for agricultural application. In order to validate the mathematical model and numerical method, an experiment was conducted under natural environmental conditions. An additional experiment was conducted in a closed-loop wind tunnel to investigate the temperature effect on soil moisture transport. Theoretical and experimental results indicate that the dry surface layer has an important effect on heat and moisture migration in soil and the influence of temperature on moisture transport in unsaturated soil is significant.     

地埋管换热器周围土壤热湿迁移的实验研究

文章搭建了土壤热湿迁移实验装置,并利用该装置研究了不同进口流体温度、土壤体积含水率条件下,地埋管换热器周围土壤温度场、湿度场的变化特性。分析结果表明:随着进口流体温度逐渐升高,土壤温度、湿度的最大值均逐渐升高,土壤温度、湿度的下降速率逐渐加快;土壤的初始体积含水率越大,热源对土壤温度的作用半径越大,当土壤的初始体积含水率分别为0%,35%时,热源对土壤温度的作用半径分别为280,380 mm;当土壤与地埋管之间的径向距离为0～225 mm时,土壤含水率主要受温度梯度的影响,随着该径向距离逐渐增加,土壤含水率逐渐升高,当土壤与地埋管之间的径向距离为225～380 mm时,土壤含水率会受到温度梯度与含水率梯度的共同作用,使得水分逐渐向热源方向逆向迁移,当土壤与地埋管之间的径向距离为225 mm时,土壤含水率出现峰值,当土壤与地埋管之间的径向距离大于380 mm时,温度梯度对土壤水分迁移的影响逐渐减弱,土壤含水率逐渐降低,当土壤与地埋管之间的径向距离为525 mm时,土壤含水率保持稳定。     

Investigation of the 3D model of coupled heat and liquid moisture transfer in hygroscopic porous fibrous media

This paper focuses on the investigation of the 3D mathematical model to simulate the coupled heat and liquid moisture transfer in hygroscopic porous fibrous media. The flow of the liquid moisture, the water vapor sorption/desorption by fibers and the diffusion of the water vapor are taken into account in this 3D model. Prediction-corrector method is used to solve the 3D governing equations. A series of computational results of the coupled heat and moisture transfer are obtained with the specific initial conditions and boundary conditions. The distribution of the water vapor concentration in the void spaces, the volume fraction of the liquid water in the void spaces, the distribution of the water content in fibers and the changes of the temperature in porous fibrous media are computed. It is shown that the effects of the gravity and capillary actions are significant in hygroscopic porous fibrous media. The comparison with the experimental measurements shows the reasonable agreement between the two. The results illustrate that the 3D model of the coupled heat and liquid moisture transfer in hygroscopic porous fibrous media is satisfactory.     

Simultaneous mass and heat transfer under mixed convection along a vertical cone

Steady laminar binary mixed convection flow along a vertical circular cone under the combined buoyancy effects of thermal and species diffusion is studied analytically. The analysis is confined to mass diffusion processes with low concentration levels. In the analysis the surface of the cone is assumed to be at a uniform temperature and uniform concentration. Numerical results for the local Sherwood number, local Nusselt number and local friction factor are presented. Representative temperature, concentration and velocity profiles are also shown. The analysis covers the diffusion of common gases and vapours into air. Considerations are given to the situations where the buoyancy forces assist and oppose the forced convection flow for various possible combinations of the thermal and species diffusion processes.     

Simultaneous heat and mass transfer with phase change in a porous slab

Simultaneous heat and mass transfer with phase change in a porous slab has been analytically investigated. Two spatially-steady regimes, corresponding to immobile and mobile condensate, are discovered. Closed-form analytical expressions for the temperature, vapor concentration, condensation rate and liquid-content distributions as well as the location of the condensation region for each of the two regimes is obtained.     

Numerical studies on flow and heat transfer in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger

The flow and heat transfer characteristics of synthesis gas (syngas) in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger under different operating pressures, inlet velocities and pitches are investigated numerically. The three-dimensional governing equations for mass, momentum and heat transfer are solved using a control volume finite difference method. The realizable k-ε model is adopted to simulate the turbulent flow and heat transfer in heat exchangers. There flows syngas in the channels consisting of the membrane helical coils or membrane serpentine tubes, where the operating pressure varies from 0.5 to 3.0 MPa. The numerically obtained heat transfer coefficients for heat exchangers are in good agreement with experimental values. The results show that the syngas tangential flow in the channel consisting of membrane helical coils is significant to the heat transfer enhancement to lead to the higher average heat transfer coefficient of membrane helical-coil heat exchanger compared to membrane serpentine-tube heat exchanger. The syngas tangential velocity in the membrane helical-coil heat exchanger increases along the axial direction, and it is independent of the gas pressure, increasing with the axial velocity and axial pitch rise and decreasing with the radial pitch rise.     

Effects of pore size distribution and fiber diameter on the coupled heat and liquid moisture transfer in porous textiles

The pore size distribution and the fiber diameter on the coupled heat and liquid moisture transfer in porous textiles are investigated to reveal the mechanisms of the coupling effects. This paper focuses on a theoretical investigation of the coupling mechanism of heat transfer and liquid moisture diffusion in porous textiles by using an improved mathematical model. In this model, the pore size distribution is assumed to be a cubic-polynomial distribution, which is close to the experimental measurements [Text. Res. J. 56 (1) (1986) 35]. The liquid diffusion behavior in porous textiles can be described as a diffusion equation. The improved diffusion coefficient can be expressed as:

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. For comparison, two types of pore distribution and the fiber diameter in the porous textiles are discussed. With specification of initial and boundary conditions, the distributions of the temperature, moisture concentration, and liquid water content in the porous textiles can be numerically computed. The comparison with the experimental measurements shows the superiority of this new model in resolving the coupled heat and liquid moisture transfer in porous textiles. The results illustrate that the heat transfer process is influenced by the pore size distribution and fiber diameter of the porous textiles.     

逆流形式下废液与空气热湿传递驱动力的解耦分析及实验验证

为了深入研究逆流形式下废液与非饱和空气热湿传递过程中传热驱动力与传质驱动力之间的关系,建立了逆流形式下废水和非饱和空气的热质交换耦合模型,利用数学方法对该模型的热湿传递驱动力进行解耦分析。将利用根据文献中逆流除湿/再生搭建的实验装置和本文搭建的废液再生实验装置得到的实验数据进行比较、验证。结果发现:相互耦合的温度差驱动力Δt和含湿量差驱动力Δω可以由相互独立的焓差驱动力Δh和相对湿度差驱动力Δφ表示,这两个相互独立的驱动力可以用来独立表征废液和非饱和空气的传热传质过程;相关文献中的和该装置中的实验结果与解耦分析的数值模拟结果一致:非饱和空气出口的所有参数在相互独立的驱动力所界定的范围内变化。     

Exergy transfer characteristics of forced convective heat transfer through a duct with constant wall heat flux

The examination of exergy transfer characteristics caused by forced convective heat transfer through a duct with constant wall heat flux for thermally and hydrodynamic fully developed laminar and turbulent flows has been presented. The exergy transfer Nusselt number is put forward and the dependence relationships of the exergy transfer Nusselt number on the heat transfer Nusselt number, Reynolds number and Prandtl number are obtained. Expressions involving relevant variables for the local and mean convective exergy transfer coefficient, non-dimensional exergy flux and exergy transfer rate, etc. have been derived. By reference to a smooth duct, the numerical results of exergy transfer characteristics for fluids with different Prandtl number are obtained and the effect of the Reynolds number and non-dimensional cross-sectional position on exergy transfer characteristics is analyzed. In addition, the results corresponding to the exergy transfer and energy transfer are compared.     

Fluid flow and heat mass transfer in membrane parallel-plates channels used for liquid desiccant air dehumidification

Fluid flow and convective heat mass transfer in membrane-formed parallel-plates channels are investigated. The membrane-formed channels are used for liquid desiccant air dehumidification. The liquid desiccant and the air stream are separated by the semi-permeable membrane to prevent liquid droplets from crossing over. The two streams, in a cross-flow arrangement, exchange heat and moisture through the membrane, which only selectively permits the transport of water vapor and heat. The two flows are assumed hydrodynamically fully developed while developing thermally and in concentration. Different from traditional method of assuming a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, the real boundary conditions on membrane surfaces are numerically obtained by simultaneous solution of momentum, energy and concentration equations for the two fluids. Equations are then coupled on membrane surfaces. The naturally formed boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the channels. Experimental work is performed to validate the results. The different features of the channels in comparison to traditional metal-formed parallel-plates channels are disclosed.