Sensitivity analyses of convective and diffusive driving potentials on combined heat air and mass transfer in hygroscopic materials

ABSTRACT

The focus of this paper was to examine the contribution of two key mechanisms—moisture convection and diffusion–on heated air and moisture transfer in porous building envelopes and to define the validity of the sub-models. A numerical simulation was performed and is focused on the one-dimensional problem for drying test boundary conditions. Thereafter, a detailed parametric analysis was carried out in order to investigate the influence of typical nondimensional parameters. Results show that convection is a prominent driving potential with respect to the diffusion process when the hygric state is stable between the environment and the envelope.     

Thermally induced hygroscopic mass transfer in a fibrous medium

A fiberglass sample was used to study the hygroscopic mass transfer produced by a temperature difference across a moist, fibrous medium. Humidity probes and thermocouples were implanted in the sample and used to continuously monitor changes in the relative humidity and temperature as a result of the moisture migration. The effects of average temperature, thermal gradient magnitude and average moisture content were some of the parameters studied. The data was analyzed using a mechanistic analogy to the irreversible thermodynamic model. Vapor and liquid fluxes were evaluated along with vapor and liquid conductivities. The phenomenological coefficients associated with the liquid and vapor fluxes were calculated, and the flux contributions due to the thermal and concentration gradients were determined for steady-state conditions. Transient data for the humidity, temperature and moisture content were also either measured or calculated.     

Investigation on heat and mass transfer in 3D woven fibrous material

A 3D mathematical model is developed to describe coupled heat and mass transfer in woven fibrous materials with consideration of its geometrical characters. The liquid water diffusion tensor is derived by matrix transformation. The finite volume method is used to discrete the governing equations, and the obtained linear discrete equations are solved by iteratively utilizing the TDMA (Tri-Diagonal Matrix Algorithm). A high order and absolutely stable difference scheme for water vapor diffusing in fiber is developed. The processes of liquid water transfer in the yarns, water vapor in both inter-yarn and intra-yarn, and their interaction with heat transfer are illustrated by a series of 3D or 4D diagrams. The effects of porosity of the yarn ε and the fabric count on heat and mass transfer are also discussed. The predictions of temperature changes on the fabric surface are compared with experimental measurements, good agreement is observed between the two.     

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.     

Transient heat and mass transfer in soils

Transient heat and mass transfer in soil surrounding a buried heat source are considered. One dimensional (spherical) models are developed to predict the coupled heat and moisture migration phenomena. Numerical solutions of the exact formulation are given. In addition, two types of closed-form solutions are derived employing approximate models. Experimental measurements are presented for two disparate soil types — a well-graded loam (slow hydraulic response) and sand (more rapid hydraulic response). Predicted and measured temperature variations at the heat source surface are compared. In all cases the numerical solution of the exact formulation agrees with the experimental data reasonably well. The closed-form solutions deviate somewhat from the measurements, but are shown to be useful for obtaining approximate predictions in a simple manner.     

Coupled heat and mass transfer in a counter flow hollow fiber membrane module for air humidification

Hollow fiber membrane based air humidification offers great advantages over the traditional methods because the liquid water droplets are prevented from mixing with the process air, while water vapor can permeate through the membranes effectively. The novelty in this research is that the coupled heat and moisture transport in a hollow fiber membrane module for air humidification is investigated, both numerically and experimentally. The air stream and the water stream flow in a counter flow arrangement. It is found that the membranes play a key role in humidification performances. For sensible heat transfer, both the liquid side and the membrane side resistance can be neglected, while the total heat transfer coefficients are determined by the air side heat transfer coefficients. In contrast, in mass transfer, only the liquid side resistance can be neglected, while the total mass transfer coefficients are co-determined by membrane properties and the air side convective mass transfer coefficients.     

Coupled heat and mass transfer in an application-scale cross-flow hollow fiber membrane module for air humidification

This study is a step forward from previous researches involving hollow fiber membrane contactors for air humidification. A real application-scale cross-flow hollow fiber membrane module is investigated. The air stream flows transversely across the fiber bundle while being humidified. The novelty is that the shell-and-tube module is converted to a parallel-plates heat mass exchanger in the model set up. The equations governing the heat and moisture transfer from the water to the air, through the membranes, are described. The equations are then normalized with newly defined dimensionless parameters, which summarize the operating conditions and have clear physical meanings. Following this step, the two-variable two-dimensional partial differential equations are numerically solved. Tests are conducted to validate the model. Effects of varying operating conditions on system performance are discussed. It is found that the system is dominated by mass transfer in membranes with a total Lewis number larger than 10. The packing density has a direct influence on performances. In contrast, the geometry of fiber packing arrangement has a negligible effect. This is tremendously different from the traditional metal tube bundles for sensible-only heat transfer.     

Entropy generation in combined heat and mass transfer

Irreversible entropy generation for combined forced convection heat and mass transfer in a twodimensional channel is investigated. The heat and mass transfer rates are assumed to be constant on both channel walls. For the case of laminar flow, the entropy generation is obtained as a function of velocity, temperature, concentration gradients and the physical properties of the fluid. The analogy between heat and mass transfer is used to obtain the concentration profile for the diffusing species. The optimum plate spacing is determined, considering that either the mass flow rate or the channel length are fixed. For the turbulent flow regime, a control volume approach that uses heat and mass transfer correlations is developed to obtain the entropy generation and optimum plate spacing.     

Investigation of three‐dimensional heat and mass transfer in a metal hydride reactor

A mathematical model for three‐dimensional heat and mass transfer in metal–hydrogen reactor is presented. The model considers three‐dimensional complex heat, and mass transfer and chemical reaction in the reactor. The main parameter in hydriding processes is found to be the equilibrium pressure, which strongly depends on temperature. Hydride formation enhanced at regions with lower equilibrium pressure. Hydriding processes are shown to be two dimensional for the system considered in this study. Effects of heat transfer rate and R/H (radius to height) ratio on hydride formation are investigated. Hydride formation increases significantly with larger heat transfer rate from the boundary walls, however after a certain heat transfer rate, the increase in formation rate is found to be not significant, due to the low thermal conductivity of the metal‐hydride systems. The estimated results agree satisfactorily with the experimental data in the literature. Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

Numerical heat transfer predictions and mass/heat transfer measurements in a linear turbine cascade

The effect of secondary flows on mass transfer from a simulated gas turbine blade and hubwall is investigated. Measurements performed using naphthalene sublimation provide non-dimensional mass transfer coefficients, in the form of Sherwood numbers, that can be converted to heat transfer coefficients through the use of an analogy. Tests are conducted in a linear cascade composed of five blades having the profile of a first stage rotor blade of a high-pressure turbine aircraft engine. Detailed mass transfer maps on the airfoil and endwall surfaces allow the identification of significant flow features that are in good agreement with existing secondary flow models. These results are well suited for validation of numerical codes, as they are obtained with an accurate technique that does not suffer from conduction or radiation errors and allows the imposition of precise boundary conditions. The performance of a RANS (Reynolds-Averaged Navier–Stokes) numerical code that simulates the flow and heat/mass transfer in the cascade using the SST (Shear Stress Transport) k–ω model is evaluated through a comparison with the experimental results.     

Investigation of heat transfer characteristics in plate-fin heat sink

The present study applies the inverse method in conjunction with the experimental temperature data to investigate the accuracy of the heat transfer coefficient on the fin in the plate-fin heat sink for various fin spacings. The commercial software is applied to solve the governing differential equations with the RNG k–? model in order to obtain the heat transfer and fluid flow characteristics. Under the assumption of the non-uniform heat transfer coefficient, the entire fin is divided into several sub-fin regions before performing the inverse scheme. The average heat transfer coefficient in each sub-fin region is assumed to be unknown. Later, the present inverse scheme in conjunction with the experimental temperature data is applied to determine the heat transfer coefficient and fin efficiency. In order to determine a more reliable heat transfer coefficient, a comparison between the present inverse and numerical results and those obtained from the existing correlations will be made. The numerical fin temperatures will also be compared with the experimental data.     

热质渗透壁面饱和多孔介质通道流动与热质传递的数值模拟

利用数值模拟方法研究了多孔介质中存在温度梯度、浓度梯度并具有热质渗透壁面时的受迫对流对传热传质的影响。采用有限容积法在同位网格上离散控制多孔介质内流体流动与热质传递方程守恒方程(即N-S)，对流项采用二阶精度的QUICK格式，扩散项采用中心差分格式。利用SIMPLE算法求解压力和速度耦合问题。利用所发展的程序研究了在不同孔隙率，不同的温度、浓度边界条件下，流场、温度场和浓度场以及Nu和Sh的变化规律。     

Analysis of heat and mass transfer in asymmetric system

This study aims to investigate theoretically the effect of water evaporation from a wetted channel wall, on natural convection heat transfer and the effect of channel width. Major nondimensional groups identified are Gr_T, Gr_M, Pr, Sc and φ. Results are presented for an air–water system under various heating conditions. The influence of heated wall temperatures, wetted wall temperatures, channel width and the relative humidity of the moist air in the ambient on the heat and mass transfer are examined in great detail.     

Investigation of heat transfer in rectangular microchannels

An experimental investigation was conducted to explore the validity of classical correlations based on conventional-sized channels for predicting the thermal behavior in single-phase flow through rectangular microchannels. The microchannels considered ranged in width from 194 μm to 534 μm, with the channel depth being nominally five times the width in each case. Each test piece was made of copper and contained ten microchannels in parallel. The experiments were conducted with deionized water, with the Reynolds number ranging from approximately 300 to 3500. Numerical predictions obtained based on a classical, continuum approach were found to be in good agreement with the experimental data (showing an average deviation of 5%), suggesting that a conventional analysis approach can be employed in predicting heat transfer behavior in microchannels of the dimensions considered in this study. However, the entrance and boundary conditions imposed in the experiment need to be carefully matched in the predictive approaches.     

Investigation of heat transfer in square porous-annulus

The current study is focused to analyze the heat transfer characteristics in a porous duct. The mathematical model of heat transfer in a porous duct was solved by converting the governing partial differential equations into a set of algebraic equations with the help of finite element method. A simple three noded triangular element is used to mesh the duct domain. The current problem consists of a square duct with outer walls being exposed to hot temperature T_h, and inner walls subjected to cool temperature T_c. Emphasis is given to investigate the effect of width ratio of cavity on heat and fluid flow characteristics inside the porous medium. The results are reported for various duct width ratios, Rayleigh number etc. It is found that the Nusselt number increases with increase in height of cavity along the vertical walls of duct; however the Nusselt number for certain values of duct ratio oscillates along the width of the porous medium at bottom wall of the cavity.