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.     

Conjugate heat and mass transfer in a hollow fiber membrane module for liquid desiccant air dehumidification: A free surface model approach

Conjugate heat and mass transfer in a hollow fiber membrane module used for liquid desiccant air dehumidification is investigated. The module is like a shell-and-tube heat exchanger where the liquid desiccant stream flows in the tube side, while the air stream flows in the shell side in a counter flow arrangement. Due to the numerous fibers in the shell, a direct modeling of the whole module is difficult. This research takes a new approach. A representative cell comprising of a single fiber, the liquid desiccant flowing inside the fiber and the air stream flowing outside the fiber, is considered. The air stream outside the fiber has an outer free surface (Happel’s free surface model). Further, the equations governing the fluid flow and heat and mass transfer in the two streams are combined together with the heat and mass diffusion equations in membranes. The conjugate problem is then solved to obtain the velocity, temperature and concentration distributions in the two fluids and in the membrane. The local and mean Nusselt and Sherwood numbers in the cell are then obtained and experimentally validated.     

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.     

Heat and mass transfer in a randomly packed hollow fiber membrane module: A fractal model approach

Hollow fiber membrane modules are widely used in various industries. The disordered nature of hollow fiber distributions in the module exhibits the existence of a fractal structure formed by the voids between the fibers. The area fractal dimension of the voids on the module cross section is obtained. Then the shell side flow distribution and convective heat and mass transfer are investigated based on the fractal theory developed. An experimental work where an air flow in the shell side is humidified by a water flow in the tube side is performed to validate the model. It is found that the model predicts the flow distribution and the heat and mass transfer deteriorations well with local data for a triangular array. With the model, friction factor and Sherwood number deteriorations which take into account of the degree of irregularity, in terms of fractal dimension, are analyzed. The results show that the higher the packing density is, the less the fractal dimension is, and the less the non-uniformity of the flow distribution is. The Sherwood and Nusselt numbers of a randomly distributed fiber module are only 1–5% of a uniformly spaced tube array. Correlations are proposed for the estimation of friction factor and Sherwood numbers considering the degree of irregularity. The predictions are also compared to the available mass transfer correlations in the literature.     

An experimental study on vapor transport of a hollow fiber membrane module for humidification in proton exchange membrane fuel cells

Water management is key in the optimization of proton exchange membrane fuel cell performance and durability. Humidifiers can be used to provide water vapor to cathode air, ensuring the proper operation of proton exchange membrane fuel cells. In this study, water vapor transport characteristics of hollow fiber membrane modules were investigated in shell-tube humidifiers under isothermal conditions, using two different test jig constructions: a convection jig and a diffusion jig. The mass transfer rate of water vapor was evaluated via the impact of various operating parameters, including temperature, flow rate, pressure, and relative humidity of inlet wet air, flow arrangements, and surface area of the tube side. The result was presented by the water vapor transport rate from wet air flow to dry air flow across the hollow fiber membrane. It was found that humidification performance could be improved with higher operating temperature, flow rate, and relative humidity of inlet wet air, lower pressure, larger membrane surface area, higher convection effect, and substituting co-current with counter-current flow configuration.     

Gas-liquid mass transfer in a cross-flow hollow fiber module: Analytical model and experimental validation

The cross-flow operation of hollow fiber membrane contactors offers many advantages and is preferred over the parallel-flow contactors for gas-liquid mass transfer operations. However, the analysis of such a cross-flow membrane gas-liquid contactor is complicated due to the change in concentrations of both phases in the direction of flow as well as in the direction perpendicular to flow. In addition, changes in the volumetric flow rate of compressible fluid can also occur over the volume of membrane contactor. These hollow fiber membrane contactors resemble to the more conventional shell and tube cross-flow heat exchanges where a similar variation in the local driving force within the module occurs. Hence heat transfer analogy can be applied to predict the performance of these contactors.Analytical expressions are derived in this work to describe the mass transfer in these hollow fiber cross-flow contactors analogously to heat transfer in cross-flow shell and tube heat exchangers. CO₂ absorption experiments were carried out in a commercial as well as in the lab-made single-pass cross-flow hollow fiber membrane contactors to check the applicability of this heat transfer analogy under different conditions. Experimental results show that the derived analytical expressions can be applied to the cross-flow membrane gas-liquid contactor under the asymptotic conditions of negligible or small volumetric flow changes. However, in the case of significant changes in the flow rate of compressible fluid, the application of heat transfer analogy results into slight under predictions of the module performance. A more rigorous model is then required for an accurate prediction of the performance.     

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.     

Coupled heat and mass transfer in laminar flow,tubular polymerizers

Thermal polymerization in a continuous flow, tubular reactor is largely determined by coupled heat and mass transfer processes. A detailed analysis is very important for the understanding of the transport phenomena involved and for the design and stable operation of such reactors. Computer experiments with this highly coupled, nonlinear system showed that for moderate flow rates in tubes larger than 4 cm in diameter, steep radial gradients in all principal variables and ultimately thermal runaway or flow channeling may occur. In this paper, the local interactions between strong temperature gradients, rapid polymerization, temperature/conversion-dependent transport properties and elongated velocity profiles have been analyzed for different thermal conditions.     

Thin carbon hollow fiber membrane with Knudsen diffusion for hydrogen/alkane separation: Effects of hollow fiber module design and gas flow mode

Recovery of heavier hydrocarbons, C₂~C₄ olefins and paraffins, from gas streams is of great importance economically. In this study, asymmetric carbon hollow fiber membranes (CHFMs) were prepared by a one-step vacuum-assisted dip coating and pyrolysis, and investigated for H₂/CO₂, H₂/C₂H₆, and H₂/C₃H₈ separations. To increase the mechanical strength of the CHFMs, a porous alumina hollow fiber with ID/OD = 2 mm/4 mm was used as the supporting material. A solution of polyetherimide in N-methyl-2-pyrrolidone was used as the casting solution. The effects of (1) membrane preparation parameters, (2) fiber packing densities, (3) fiber packing arrangement, and (4) gas flow configuration (inside-out or outside-in) on the gas-separation performance were also investigated. The results showed that decreasing the concentration of the casting dope and the number of coating cycles was found to be the most effective approach to increase the H₂ permeance, while maintaining the H₂/CO₂ selectivity. Further, as the fiber packing density was increased from 5.54% to 38.78% for the hexagonal packing configuration, the H₂ permeance increased from 362.04 GPU to 711.61 GPU, without any decrease in the gas selectivity. The as-prepared CHFM exhibited the maximum gas permeance of 711.61 GPU for H₂ and the following gas selectivity: 2.79, 4.65, and 5.34 towards H₂/CO₂, H₂/C₂H₆, and H₂/C₃H₈, respectively. The successful preparation and modularization of the CHFM is advantageous and industrially relevant for several gas-separation applications, such as H₂ energy production from CO₂, C₂H₆, and C₃H₈, and olefins/paraffins recovery.     

Analysis of heat and mass transfer between air and falling film in a cross flow configuration

Heat and mass transfer between air and falling solution film in a cross flow configuration is investigated. Effects of addition of Cu-ultrafine particles in enhancing heat and mass transfer process are also examined. A parametric study is employed to investigate the effects of pertinent controlling parameters on dehumidification and cooling processes and their subsequent optimization. It is found that low air Reynolds number enhances the dehumidification and cooling processes. An increase in the height and length of the channel and a decrease in the channel width enhance dehumidification and cooling processes. It is also found that an increase in the Cu-volume fraction increases dehumidification and cooling capabilities and produces more stable Cu-solutions.     

Effectiveness correlations for heat and mass transfer in membrane humidifiers

The latent effectiveness and the latent number of transfer units (NTUs) for mass transfer in membrane humidity exchangers were applied to proton exchange membrane fuel cell (PEMFC) membrane humidifiers. We report on two limitations that cause deviations in the theoretical outlet conditions reported previously: (1) using a constant enthalpy of vaporization derived from the reference temperature in the Clausius–Clapeyron equation; and (2) simplifying the relationship between relative humidity and absolute humidity as linear. These limitations are alleviated by using an effective mass transfer coefficient U_eff. The constitutive equations are solved iteratively to find the flux of water through the membrane. The new procedure was applied to three types of membrane and compared to the curves of ε_L and NTU_L found using Zhang and Niu’s method, which is normally applied to energy recovery ventilators (ERVs).     

逆流开式发生器内部的热、质传递规律

针对以太阳能加热的空气为携热介质，以LiBr溶液为工作介质的填料塔型开式发生器，建立热质传递数学模型。对2种系统结构形式进行比较，并分别研究太阳能集热板温度、液气比、环境相对湿度以及填料层高度对溶液再生的影响，以揭示此类发生器内热、质传递的规律，为产品开发、设计提供理论帮助。     

An analytical model for the heat and mass transfer processes in indirect evaporative cooling with parallel/counter flow configurations

This paper aims at developing an analytical model for the coupled heat and mass transfer processes in indirect evaporative cooling under real operating conditions with parallel/counterflow configurations. Conventionally, one-dimensional differential equations were used to describe the heat and mass transfer processes. In modeling, values of Lewis factor and surface wettability were not necessarily specified as unity. Effects of spray water evaporation, spray water temperature variation and spray water enthalpy change along the heat exchanger surface were also considered in model equations. Within relatively narrow range of operating conditions, humidity ratio of air in equilibrium with water surface was assumed to be a linear function of the surface temperature. The differential equations were rearranged and an analytical solution was developed for newly defined parameters. Also, performances with four different flow configurations were briefly discussed using the analytical model. Through comparison, results of analytical solutions were found to be in good agreement with those of numerical integrations.     

Ventilated-solar roof air flow and heat transfer investigation

The governing parameters for flows generated by heat transfer from solar cell modules to air gaps are discussed. Experimental results are presented from measurements in mock-ups of ventilated facades and roofs. The heat transmitted from the solar cells to the air have been mimicked by the use of heating foils. The inclination angle of the roof, position of solar cell module and the height to width ratio (aspect ratio) have been varied. The bulk properties as the air flow rate in the air gap, local temperatures and velocities have been measured. Results of importance for design of hybrid systems and cooling of solar cells have been obtained.     

Heat and mass transfer in a quasi-counter flow membrane-based total heat exchanger

Membrane-based total heat exchangers (or energy recovery ventilators) are the key equipments to fresh air ventilation, which is helpful for the control of respiratory diseases like Swine flu and SARs. Cross flow has been the predominant flow arrangement for these equipments. However performances are limited with this arrangement. A counter flow arrangement is the best. In this research, a quasi-counter flow parallel-plates total heat exchanger is constructed and investigated. A detailed mathematical modeling is conducted and the model is experimentally verified. The temperature and humidity values on membrane surfaces, and in the fluids are solved as a conjugate problem. The fluid flow, heat and mass transport equations in the entry regions are solved directly. The mean Nusselt and Sherwood numbers, and the sensible and latent effectiveness of the exchanger are calculated. It is found that the effectiveness of the current arrangement lie between those for cross flow and those for counter flow arrangements. The results also found that the flow can be divided distinctly into three zones: two cross-like zones and a pure-counter flow zone. The less the cross-like zones are, the larger the pure-counter flow zone is, and the greater the effectiveness is. The study also provides a solution of modeling mass transfer with FLUENT software from heat mass analogy.     

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.     

Buoyancy-induced flow,heat, and mass transfer in a porous annulus

This paper reports on double-diffusive natural convection heat transfer in a porous annulus between concentric horizontal circular and square cylinders. A pressure-based segregated finite volume method is used to solve the problem numerically. The diffusion fluxes are discretized using the MIND fully implicit scheme. Furthermore, a modified pressure correction equation is derived that implicitly accounts for the nonorthogonal diffusion terms, which are usually neglected in the standard SIMPLE algorithm. Results indicate that convection effects increase with an increase in Rayleigh number, Darcy number, porosity, and enclosure aspect ratio. Further, at low Darcy values, porosity has no effect on the flow, temperature, and concentration fields.     

Three-dimensional flow of a viscous fluid with heat and mass transfer

Three-dimensional unsteady free convection and mass transfer flow of an incompressible, viscous liquid through a porous medium past an infinite vertical flat plate subjected to a time-dependent suction velocity normal to the plate is studied. The equations encountered into the problem are solved using perturbation technique to obtain the velocity, temperature and concentration fields considering as reference parameter. Expressions for the skin-friction, rate of heat and mass transfers are also obtained. Two cases of most common interest viz. cooling case (G_r > 0) and heating case (G_r < 0) are discussed.     

Numerical analysis for flow,heat and mass transfer in film flow along a vertical fluted tube

We conducted a three-dimensional numerical investigation of the flow, heat and mass transfer characteristics of the fluted evaporating tube with films flowing down on both the inside and outside tube walls. Condensation occurs along the outside wall while evaporation takes place on the free surface of the inside film. The three-dimensional transport equations for momentum and energy were solved by using the finite volume method (FVM). The free-surface shape is tracked by using the moving-grid technique that satisfies the space conservation law (SCL). Because of the secondary motion of the fluid, the film becomes thin at the crest whereas it thickens at the valley. The velocity and temperature fields were successfully predicted for various flute shapes.     

Dynamic model of counter flow air to air heat exchanger for comfort ventilation with condensation and frost formation

In cold climates heat recovery in the ventilation system is essential to reduce heating energy demand. Condensation and freezing occur often in efficient heat exchangers used in cold climates. To develop efficient heat exchangers and defrosting strategies for cold climates, heat and mass transfer must be calculated under conditions with condensation and freezing. This article presents a dynamic model of a counter flow air to air heat exchanger taking into account condensation and freezing and melting of ice. The model is implemented in Simulink and results are compared to measurements on a prototype heat exchanger for cold climates.