The prediction of mass transfer rates when equilibrium does not prevail at the phase interface

It is shown that departures from thermodynamic equilibrium at the interface call for only small modifications in standard procedures for calculating mass transfer rates. If enthalpy-composition diagrams are used for representing thermodynamic properties and evaluating mass transfer driving forces, the modifications comprise the replacement of the single curve, valid for equilibrium mixtures in contact with the neighbouring phase, by a family of curves; the parameter of this family is the ratio of the mass transfer conductance to a molecular flux.

These and other aspects of mass transfer theory are illustrated by reference to: the vaporization of water into air; the combustion of carbon in air; the pyrolysis of a solid or liquid without chemical reaction; and the combustion of a pellet of ammonium perchlorate in a stream of fuel gas. The examples discussed numerically relate to axi-symmetrical stagnation point flows with laminar boundary layers.     

Two-dimensional two-phase thermal model for passive direct methanol fuel cells

A two-dimensional two-phase thermal model is presented for direct methanol fuel cells (DMFC), in which the fuel and oxidant are fed in a passive manner. The inherently coupled heat and mass transport, along with the electrochemical reactions occurring in the passive DMFC is modeled based on the unsaturated flow theory in porous media. The model is solved numerically using a home-written computer code to investigate the effects of various operating and geometric design parameters, including methanol concentration as well as the open ratio and channel and rib width of the current collectors, on cell performance. The numerical results show that the cell performance increases with increasing methanol concentration from 1.0 to 4.0 M, due primarily to the increased operating temperature resulting from the exothermic reaction between the permeated methanol and oxygen on the cathode and the increased mass transfer rate of methanol. It is also shown that the cell performance upgrades with increasing the open ratio and with decreasing the rib width as the result of the increased mass transfer rate on both the anode and cathode.     

An easy-to-approach and comprehensive model for planar type SOFCs

An easy-to-approach and comprehensive mathematical model for planar type solid oxide fuel cells is presented in the current work. It provides a tool for researchers to conduct parametric studies with less-intensive computation in order to grasp the fundamentals of coupled mass transfer, electrochemical reaction, and current conduction in a fuel cell. In the model, the analysis for the mass transfer polarization at a known average fuel cell operating temperature is based on an average mass transfer model analogous to an average heat transfer process in a duct flow. The effect of the species' partial pressure at electrode/electrolyte interfaces is therefore included in the exchange current density for activation polarizations. An electrical circuit for the current and ion conduction is used to analyze the ohmic losses from anode current collector to cathode current collector. The three types of over-potentials caused by different polarizations in a planar type solid oxide fuel cell can be identified and compared. The effects of species concentrations, properties of fuel cell components to the voltage–current performance of a fuel cell at different operating conditions are studied. Optimization of the dimensions of flow channels and current-collecting ribs is also presented. The model is of significance to the design and optimization of solid oxide fuel cells for industrial application.     

Evaluation of the different definitions of the convective mass transfer coefficient for water evaporation into air

In literature different definitions of the convective mass transfer coefficient are used by different authors. The definitions differ in the driving force used to describe mass transfer. In this paper, the limitations to the use of convective mass transfer coefficients related to four commonly used driving forces (vapour density, mass fraction, vapour pressure and mole fraction) are studied for evaporation of water into air. A theoretical study based on the adiabatic saturation process and a numerical CFD study of an existing evaporation experiment show that the use of convective mass transfer coefficients related to vapour densities is only allowed under isothermal conditions while convective mass transfer coefficients related to vapour pressure show a dependence on the total gas pressure. The use of mole or mass fractions as driving force results in values for the transfer coefficient which are little affected by the thermodynamic properties such as temperature, relative humidity and total pressure and are hence better suited to describe convective mass transport.     

Note on the mechanism of interfacial mass transfer of absorption processes

A concept that the driving force of gas-liquid interphase mass transfer comes from the interfacial non-equilibrium is proposed in this paper. For the absorption process, based on the general chemical potential driving force equation, the concentration relation between two phases at interface is derived and solved under different conditions as mass transfer occurs, it shows that the interfacial concentration of absorbed component at liquid side is strongly affected by a Biot number Y₀ and is bulk concentration dependent. The CO₂ interfacial concentration of liquid side in stationary absorption by pure methanol, ethanol and n-propanol absorbent respectively are measured by the use of micro laser holographic interference technique, the experimental results are in good agreement with the computation.     

Effects of the gas diffusion-layer parameters on cell performance of PEM fuel cells

The characteristic parameters of the gas diffusion-layer (GDL) on cell performance and mass transfer of a proton exchange membrane fuel cell have been investigated numerically. A two-dimensional, isothermal and multi-phase numerical model has been established to investigate the influence of the GDL parameters on the transport phenomenon and cell performance of PEM fuel cells. The porosity and thickness of the GDL are employed in the analysis as the parameters. In addition, the effects of liquid water and the flow direction of the fuel and air on the performance are also considered in this paper. The results show that both the porosity and thickness of the GDL affect the fuel cell performance significantly, especially the water mass transfer. It is shown that the cell performance with consideration of a liquid water effect is always less than that without consideration of the liquid water effect. In addition, the cell performance with a co-flow pattern of fuel and air is better than that with a counter flow pattern.     

Performance and optimum geometry of spines with simultaneous heat and mass transfer

An analysis was carried out to study the performance of spine fins of different configurations when subjected to simultaneous heat and mass transfer mechanisms. The temperature and humidity ratio differences are the driving forces for the heat and mass transfer, respectively. Analytical solutions are obtained for the efficiency and temperature distribution over the spine surface when the surface condition is fully wet. A correction chart is developed to correct the value of the dry fin parameter if the fin surface condition is fully wet. The effect of atmospheric pressure on the spine efficiency was also studied as well as the spine optimum geometries were obtained such that a maximum amount of heat transfer rate occurs. It is shown that the closed-form solution for a dry spine case discussed in text books is a special case for the solutions presented in this paper.     

Pore migration under high temperature and stress gradients

An analytical model is proposed for the evolution of ellipsoidal pores by surface diffusion under the influence of large temperature gradients and the associated thermoelastic stress field. It is found that both of these influences affect the migration velocity of a pore as well as its shape. The shape of a pore is determined by competition between the thermoelastic stress field, the interfacial energies of the pore and grain boundaries, and the kinetics of the mass transport process. The dramatic case of void migration in a uranium dioxide nuclear fuel rod is considered, in which very large temperature gradients of the order of 4 × 10⁵ K/m are predicted. It is found that crack-like pores are expected to form on the radial grain boundaries prevalent in the fuel rod microstructure, and that these crack-like pores lead to the eventual structural failure of the component. This agrees with experimental observations. The temperature gradient is the dominant driving force for the migration of near-spheroidal and prolate pores, whereas the stress field is the dominant driving force for oblate crack-like pores.     

Coupled heat and mass transfer characteristic in packed bed dehumidifier/regenerator using liquid desiccant

The dehumidifier and regenerator are two key components in liquid desiccant air conditioning systems. The heat transfer driving force and the mass transfer driving force influence each other, the air and desiccant outlet temperatures or humidity ratio may exceed the air and desiccant inlet parameters in the dehumidifier/regenerator. The uncoupled heat and mass transfer driving forces, enthalpy difference and relative humidity difference between the air and desiccant are derived based on the available heat and mass transfer model and validated by the experimental and numerical results. The air outlet parameter reachable region is composed of the air inlet isenthalpic line, the desiccant inlet equivalent relative humidity line and the linkage of the air and desiccant inlet statuses. Except the mass flow rate ratio and the heat and mass transfer coefficients, the air and desiccant inlet statuses and flow pattern have great effects on the dehumidifier/regenerator performance. The counter flow configuration expresses the best mass transfer performance in the dehumidifier and the hot desiccant driven regenerator, while the parallel flow configuration performs best in the hot air driven regenerator.     

Mass transfer in plane and square ducts

A numerical mass transfer analysis for plane and square duct geometries for developing and fully-developed scalar transport with laminar flow is described. A methodology for prescribing stream-wise periodic scalar boundary conditions under conditions of constant-transformed-substance state, is detailed. The solution to the fully-developed mass transfer problem is presented in terms of driving force, blowing parameter and normalised conductance. A suitably-defined polarisation factor is shown to be functionally equivalent to the former. The data compress onto a single curve with good correspondence to the 1-D convection-diffusion solution, except for high rates of wall injection or suction.     

Buoyancy-induced inclined boundary layer flow in a porous medium resulting from combined heat and mass buoyancy effects

An implicit finite difference method is used to analyze the natural convection boundary layer flow in a saturated porous medium resulting from combined heat and mass buoyancy effects adjacent to an inclined surface. Both the streamwise and normal components of the buoyancy force are retained in the momentum equations. The present formulation permits the angles of from the horizontal. Numerical results indicate that, as the buoyancy ratio or inclination parameter increase, the surface heat and mass transfer rates increase. These results are compared with the approximate similarity solutions that are obtained by neglecting the normal component of the buoyancy force in the momentum equations. It is shown that the approximate similarity solutions may significantly underpredict the heat and mass transfer rates for small values of inclination parameter.     

Transition from natural to mixed convection for steam–gas flow condensing along a vertical plate

An analysis method based on two-phase boundary layer analysis has been developed to study the effects of superimposed forced convection on natural convection steam–gas flow condensing along a vertical plate. The mechanism by which superimposed forced convection enhances heat transfer is evaluated: the bulk flow blows away non-condensable gases accumulating near the interface, resulting in an elevated condensation driving force. Further, this bulk flow blowing capability may be characterized by a conventional mass transfer driving potential. Results of the new model are shown to be consistent with experimental data. Finally, a simple criterion was developed to identify transition to mixed convection from natural convection steam–gas flow.     

An adsorption air conditioning system to integrate with the recent development of emission control for heavy-duty vehicles

The recent development to control the emissions of large diesel engines has provided opportunities for heat-driven cooling methods in vehicles. An adsorption air conditioning system is therefore proposed in this work for heavy-duty truck application. This system is powered by engine waste heat when the engine of a truck is running. When the engine is off, it can be operated by fuel fired heaters, a newly implemented technology to reduce truck idling. Hence, this system can not only reduce engine emissions but also improve the overall energy efficiency. A lumped parameter model of the system using zeolite-water as its working pair is developed, and the adsorption capacity of zeolite is simulated with the linear driving force model. The dynamic performance of the system and a parametric study on adsorbent mass transfer, operating temperatures and cycle operating periods are presented. Alternative working pairs and the potential to commercialize the system are also discussed. This system may be designed to satisfy the cooling requirement for idle reduction of long-haul trucks.     

The effect of absorbate concentration level in hygroscopic condensation

The effect of absorbate concentration level on the transfer rates has been elucidated with reference to film hygroscopic condensation (of cold low pressure vapor on concentrated hot brine) characterized by relatively high absorbate concentration level.It is shown that, in distinction to gas absorption (infinite dilution), in the case of comparable concentrations of absorbate and absorbent, the lateral convective term is to be accounted for. The solution obtained is in form of Voltera integral equation.The resulting transfer rates are shown to depend on both the absorbate concentration level and driving force and are sifnificantly augmented compared to Higbie's theory prediction.     

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。     

Design of Flat-Plate Dehumidifiers for Humidification–Dehumidification Desalination Systems

Flat-plate heat exchangers are examined for use as dehumidifiers in humidification–dehumidification (HDH) desalination systems. The temperature and humidity ratio differences that drive mass transfer are considerably higher than in air-conditioning systems, making current air-conditioning dehumidifier designs and design software ill-suited to HDH desalination applications. In this work a numerical dehumidifier model is developed and validated against experimental data. The model uses a logarithmic mass transfer driving force and an accurate Lewis number. The heat exchanger is subdivided into many cells for high accuracy. The Ackermann correction takes into account the effect of noncondensable gases on heat transfer during condensation. The influence of various heat exchanger design parameters is thoroughly investigated and suitable geometries are identified. Among others, the relationship between heat flow, pressure drop, and heat transfer area is shown. The thermal resistance of the condensate layer is negligible for the investigated geometries and operating point. A particle-embedded polymer as a flat-plate heat exchanger material for seawater operation substantially improves the heat flux relative to pure polymers and approaches the performance of titanium alloys. Thus, the use of particle-embedded polymers is recommended. The dehumidifier model can be applied in design and optimization of HDH desalination systems.     

A new analytical approach to model and evaluate the performance of a class of irreversible fuel cells

An irreversible model of a class of hydrogen–oxygen fuel cells working at steady-state is established, in which the irreversibilities resulting from electrochemical reaction, electrical resistance, and heat transfer to the environment are taken into account. The entropy production analysis is introduced and applied to investigate the physical and chemical performances of the fuel cell by using the theory of electrochemistry and non-equilibrium thermodynamics. Expressions for the power output and efficiency of the fuel cell are derived by introducing the equivalent internal and leakage resistances. With the help of the model being applied to high temperature solid oxide fuel cells, the performance characteristic curves of the fuel cell are presented and the influence of some design and operating parameters on the performance of the fuel cell are discussed in detail. Moreover, the optimum criteria of some important parameters such as the power output, efficiency, and current density are given. The results obtained may provide a theoretical basis for both the optimal design and operation of real fuel cells. This new method can also be used in the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Prediction of mass transfer rates in solar stills

We present a modification of a mass transfer theory developed by Spalding to predict mass transfer rates in solar stills of different inclinations. In previous applications to solar stills, the authors considered the mass fractions at the evaporating surface and the bulk state to evaluate the driving force for mass transfer. In this study, the mass fraction at the condensing surface on the cover is employed instead of the bulk mass fraction on the moist air in the still, since the former can be more accurately determined. Predicted results from the modified theory and results from relation of Dunkle¹ are compared with experimental values. The results of the modified theory agree very well with the experimental results, whereas Dunkle's relation is accurate only in horizontal stills and stills with small inclinations.     

Novel gas distributors and optimization for high power density in fuel cells

A novel gas distributor for fuel cells is proposed. It has three-dimensional current-collecting elements distributed in gas-delivery fields for effective current collection and heat/mass transfer enhancement. An analysis model has been developed in order to understand the performance of the output power density when the dimensions and distributive arrangement of the current collectors are different. Optimization analysis for a planar-type SOFC was conducted in order to outline the approach in optimizing a gas-delivery field when adopting three-dimensional current-collecting elements in a fuel cell. Experimental test of a proton exchange membrane (PEM) fuel cell adopting the novel gas distributor was conducted for verification of the new approach. Significant improvement of power output was obtained for the proposed new PEM fuel cells compared to the conventional ones under the same conditions except for the different gas distributors. Both the experimental results and modeling analysis are of great significance to the design of fuel cells of high power density.