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A mathematical model describing the heat and mass transfer in the dryer section of a paper machine has been applied to the production data from four paper machines. Model predictions for the machine speed are compared to actual machine speeds for a total of 163 data sets. The mathematical model assumes that the temperature and moisture content remain homogeneous in the thickness direction of the sheet. For three paper machines producing paper with basis weights ranging from 0.056 to 0.159 kg d.s./m2 the model predictions are adequate. For the paper machine producing the heaviest grades with basis weights ranging from 0.189 to 0.390 kg d.s./m2 the model predictions are flawed by a systematic error. For low machine velocities/high basis weights the machine velocity is over-predicted and for high machine velocities/low basis weights the machine velocity is under-predicted. This systematic error is caused by the assumption of homogeneous moisture content and temperature within the sheet being severely in error for thick sheets. [Copyright &y& Elsevier]

The present study investigates two models—two limiting cases—for the internal heat and mass transfer in a multicylinder dryer: the first model assumes that complete redistribution of heat and moisture in the thickness direction occurs instantaneously. The second model assumes that moisture transfer occurs only by vapor diffusion and that heat transfer takes place by conduction and condensation. The two models are written in dimensionless form, and applied for the same set of standard paper machine data. For basis weights below approximately 0.05 kg d.s./m2 the results are so close that the first, simple model is sufficient. A higher limiting basis weight—approximately 0.16 kg d.s./m2—was previously indicated by comparison of model predictions, assuming the simple model, to extensive sets of machine data from four different paper machines. [Copyright &y& Elsevier]

Absorption machines are frequently used for cooling and heating. However, they must be improved to achieve high performance at a lower cost. Being a key component of these machines, this paper focuses on the absorber and specifically on falling film absorber configuration due to its compactness and simple geometry. This paper reviews the different analytical and numerical approaches used to analyze the thermal, mass and hydrodynamic phenomena occurring in falling films for both smooth and wavy-laminar regimes. It shows the significant progress made since 2000 in terms of mathematical models developed and coupling effect resolution. Research using thermodynamics to identify, locate and quantify the entropy generation sources in falling film absorbers is also presented. [ABSTRACT FROM AUTHOR]

Numerous metrics and indices have been proposed for the assessment of thermal perception and comfort. Commonly used indices based on the estimate of the heat flow between the human and its environment includes the well-known Fanger's method, also known as the PMV-PPD thermal comfort model. Recently this thermodynamic approach has been expanded with the application of the second law of thermodynamics. In this paper, an exergy-based analysis of the relation between the human and its environment is presented. Heat and mass flow rate due to convection, radiation, evaporation and respiration are calculated and expressed in terms of exergy loss. This value is used in the modified Fanger's thermal comfort model, where the heat balance equation is replaced by the exergy loss. The paper builds upon prior studies on determination of internal and external exergy losses by revised and newly defined formulas taking into account clothing and indoor conditions. The results quantitatively show that thermal energy load in the original Fanger's PMV model could be substituted by exergy loss and it corresponds to a certain level of subjective assessed thermal comfort level. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the performance and heat and mass transfer processes that occur in counter-flow indirect evaporative air coolers. Different types of counter-flow air coolers were compared: a typical counter-flow unit, the same unit operating as a heat recovery exchanger, a regenerative unit and a novel modified exchanger, proposed by authors. Additionally the paper investigated impact of location of the perforated holes in the channel plate of regenerative air cooler. The numerical model is based on a one-dimensional heat and mass transfer analysis that was validated against existing experimental data. The results obtained from the simulation reveal the high effectiveness of the presented units. It was established that selected arrangements of the presented exchangers are characterized by the different efficiency in different air-conditioning applications. [ABSTRACT FROM AUTHOR]

This paper discusses the effect of temperature polarization in Vacuum Membrane Distillation (VMD). The main motivation for using VMD in this work is that this module configuration is much simpler and more suitable for this kind of investigation than the other MD configurations such as Direct Contact Membrane Distillation (DCMD). The coupling between heat and mass transfer mechanisms at the feed-membrane interface is presented from a theoretical point of view. In addition, a new simple graphical method and a mathematical model for determining VMD flux are presented. The two methods used in evaluating the extent of temperature polarization effect on water vapor flux (flux sensitivity factors and temperature polarization coefficient (TPC)) are also analyzed and compared. The effect of integrating a heat recovery system in a large scale module on the TPC coefficient has also been studied and presented in this paper. [ABSTRACT FROM AUTHOR]

• A mathematical model based on non-equilibrium formulation was established. • The simulation results show better accordance with the experimental results compared with the model based on equilibrium formulation. • The heat and mass transfer during the drying stage were analyzed. • The effects of different operating parameters on the drying time were investigated. Spray freeze drying has been expanding its influence in many fields. However, the long duration dramatically hinders its further development. A model established based on non-equilibrium formulation was established in this paper. The accuracy of the model proposed with that established based on equilibrium formulation was compared. And the history of temperature, vapor concentration and ice saturation were then investigated. Lastly, the effects of product thickness, particle diameter on the drying time were investigated. The simulation results reproduce well with the experimental data, and show a better agreement with the increase rate of experimentally measured temperature curve than that based on equilibrium formulation. In addition, the temperature increases from the bottom to the top region while the vapor concentration distribution presents the opposite trend. The ice saturation first obtains a slight increase and then decreases. Lastly, the drying time decreases as: (i) the product thickness decreases, (ii) the packing porosity increases and (iii) the chamber temperature increases. The mathematical model proposed in this paper could offer better understanding of the process and guide the design and optimization for the process. [ABSTRACT FROM AUTHOR]

Abstract: This paper aims at developing an analytical model for the coupled heat and mass transfer processes in a counterflow Reversibly Used Cooling Tower (RUCT) based on operating conditions, which is more realistic than most conventionally adopted Merkel approximations. Temperature and moisture content differences are chosen as the driving forces of heat and mass transfer correspondingly and a system of specific difference equations is developed to solve the model more efficiently. The model is investigated by using an iterative algorithm, which is validated with the experimental data reported. The analytical model also accommodates the direct and quick calculation of air and water temperature profiles, moisture content of air and the water mass flow rate change along the vertical length of the RUCT. With the aid of the developed model, the thermal behavior of the counterflow RUCT under various operating and environmental conditions is also studied in this paper. The results reveal that the proposed model can provide a theoretical foundation for practical design and performance evaluation of counterflow RUCT. [Copyright &y& Elsevier]

Abstract: This paper presents a quasi-steady state heat and mass transfer green roof model that can be incorporated in different energy simulation software or calculation procedures. The model considers heat and mass transfer processes between the sky, plants, and substrate. This paper also presents new equations to calculate (1) substrate thermal conductivity for green roofs, (2) substrate resistance to calculate green roof soil evaporation, (3) and set of compiled stomatal resistance functions to calculate plants’ transpiration. The model is validated with robust experimental data that consists of surface temperatures, conduction heat flux, convection heat flux, net radiation and evapotranspiration. The data was obtained from laboratory experiments using a new “Cold Plate” apparatus set in an environmental chamber. The validation shows that the model predicts the heat and mass transfer accurately, except that it tends to underestimate peak evapotranspiration rates. [Copyright &y& Elsevier]

Abstract: A model for predicting the flux and evaporation ratio in direct contact membrane distillation (DCMD) using a compressible membrane is presented. Polytetrafluoroethylene (PTFE) membranes, one of the most common types of membranes employed in MD, are characterised with high porosity (∼90%) and high hydrophobicity, and therefore have high water vapour permeability and high wetting resistance. However, the PTFE membrane is compressible due to its structure. Compression of the membrane will cause a change of its physical structure, such as porosity, thickness, and pore size. As a result, the thermal conductivity and vapour permeability of the membrane will be altered, causing a change in flux and energy efficiency. Such effects need to be accounted for when scaling up from laboratory data to full scale design, because there may be significant differences in the applied pressure. Therefore, in this paper, the influence of pressure on the flux of the compressible PTFE membrane was modelled. This paper also provides a mathematical method to correlate the applied pressures with physical structure changes based on the assumption of constant tortuosity. The modelling results were compared with experimental results over a range of variable process parameters, i.e., temperatures, velocities, membrane lengths, and pressure applied to the membrane. The errors between the model predictions and experimental results were less than 10% within the operating range used in this investigation. [Copyright &y& Elsevier]

Abstract: This paper presents a review of studies on mathematical modeling of solid oxide fuel cells (SOFCs) with respect to the tubular and planar configurations. In this work, both configurations are divided into five subsystems and the factors such as mass/energy/momentum transfer, diffusion through porous media, electrochemical reactions with and without CO oxidation, shift and reforming reactions, and polarization losses inside the subsystems are discussed. Using variety of fuels fed to SOFCs is issued and their effect on the system is compared briefly. A short review of solid oxide fuel cell configurations and different flow manifolding are also presented in this study. Novel models based on statistical data-driven approach existing in the literatures are considered shortly. Although many studies on solid oxide fuel cells modeling have been done, still more research needs to be done to improve the models in order to predict the fuel cell behaviors more accurately. At the end of this paper the works and studies that can be done for improving the fuel cell models is suggested and pointed by the authors. [Copyright &y& Elsevier]

Abstract: This paper describes the application of exergy analysis to human heat and mass exchange with the indoor environment. The basis for this paper is the existence of a formal similarity between the parameters defining human heat and mass transfer governed by physiological processes and the parameters defining exergy destruction. Basic heat and mass transfer is calculated using a standard two-node human physiological model. To enable an exergy analysis the model was divided according to heat and mass transfer over appropriate boundaries. The exergy analysis was performed for each process with regard to the type of exergy that participates in heat and mass exchange with respect to the indoor environment. The results provide insight into the complex interactions between the human body and environment as characterized by exergy destruction. Furthermore, the analysis indicates that at given physiological parameters only a certain combination of environmental conditions ensures minimal exergy destruction. These environmental and human conditions coincide with expected level of thermal comfort. [ABSTRACT FROM AUTHOR]

Abstract: Mathematical modeling of the convective processes caused by impact of various forces on the fluid and gas media is rather important nowadays. The increased interest to these problems is caused also by the preparation of experiments on the International Space Station in the frame of the scientific project “Convection and Interfacial Mass Exchange” (CIMEX) of the European Space Agency. They are the experiments to investigate the convective flows of the fluids with a thermocapillary interface between liquid and gas phases. In the case, when a gas flow generates the tangential stresses on a free boundary of liquid, the additional characteristics of the convective flows should be identified. In this paper a stationary coupled problem of gravitational, thermocapillary convection in a horizontal layer with free boundary under conditions of a co-current gas flow is studied. The kinematic and dynamic conditions are fulfilled exactly on the gas–liquid interface. The exact solutions for different types of thermal boundary conditions have been obtained. An evaporation effect through the gas–liquid interface is modeled qualitatively with the help of a heat transfer condition. It has been found that the direction of the velocity at the gas–liquid interface is determined by the governing parameters. The equal-zero condition for the interface velocity has been found, as well. The paper presents the velocity and temperature profiles in the conditions, which correspond qualitatively to the CIMEX experiments. [Copyright &y& Elsevier]

Abstract: The main objectives of this paper are to present a procedure of how to create and set up a model for the physical processes that take place within an infant radiant warmer and to validate that Computational Fluid Dynamics (CFD) can be used to resolve such problems. In this study, the results are obtained for a simplified model, both in terms of the geometry employed and the prescribed boundary conditions. The results were numerically verified in terms of the convergence history, monitor data and the physical correctness. This study shows that the physical situation is unsteady and the results tend to oscillate, almost periodically, around a mean value. The results presented in the paper are found to be in qualitative agreement with the experimental data. This gives us confidence that the techniques employed in this paper are appropriate and form the starting point for the inclusion of more realistic effects, e.g. real shape of the newborn and radiant lamp, heat generated inside the newborn, moisture transport, etc. [Copyright &y& Elsevier]

Abstract: We present a two-phase multiscale solidification model, which accounts for coupled heat, mass, solute and momentum transfer and notably describes the motion of globular equiaxed grains. On the microscopic scale the model takes into account nucleation, finite diffusion in both solid and liquid phases, and resolves the diffusion-controlled phase change by a solute balance at the solid–liquid interface coupled with a thermodynamic equilibrium condition for the interface. The main idea of the paper is the solution algorithm of the multiscale model. We propose and present in detail an operator splitting method that resolves the global conservation equations in two stages. In the first solution stage we resolve and integrate globally only the convection terms. In the second solution stage we resolve and integrate locally the contributions of the grain growth kinetics, i.e. the phase change and the microscopic diffusion. We analyze this splitting method based on a scaling analysis that shows that the justification of the method stems from the separation of time scales of convection, diffusion and phase change. We discuss this scale separation in detail and define a test case that is presented and analyzed in a companion paper. [Copyright &y& Elsevier]

Abstract: Enthalpy exchangers have been used as an efficient means to recover both sensible heat and moisture from exhaust ventilation air. A cross-flow plate-fin structure is the most popular arrangement for the exchanger core due to its compactness and high mechanical strength even with very small channel wall thickness. Traditionally, hygroscopic paper is selected as the plate and fin materials. Though the sensible effectiveness with this material is satisfactorily high, the latent effectiveness is disappointingly low due to the low moisture diffusivity in paper. To solve this problem, in this study, a novel concept is proposed to augment moisture transfer in the exchanger. Plates and fins are made with different materials. A novel membrane – the composite supported liquid membrane (CSLM) is used as the plate material. Paper is still used as the fin material for its cheapness and high support strength. To make comparisons, two cores, one is paper-fin and paper-plate, and another one is paper-fin and membrane-plate, are constructed and tested for heat and moisture recovery. Simultaneous heat and moisture transfer in the plate-fin core is studied. Mathematical model governing the heat and moisture transfer in the cores is set up and numerically solved. Both the experimental data and numerical results indicate that the latent effectiveness of the paper-fin and membrane-plate core is 60% higher than the traditional paper-fin and paper-plate core, due to the high moisture diffusivity in the CSLM. [Copyright &y& Elsevier]

Abstract: The mathematical models of evaporative fluid coolers and evaporative condensers are studied in detail to perform a comprehensive design and rating analysis. The mathematical models are validated using experimental as well as numerical data reported in the literature. These models are integrated with the fouling model presented in an earlier paper, using the experimental data on tube fouling. In this paper, we use the fouling model to investigate the risk based thermal performance of these evaporative heat exchangers. It is demonstrated that thermal effectiveness of the evaporative heat exchangers degrades significantly with time indicating that, for a low risk level (p=0.01), there is about 66.7% decrease in effectiveness for the given fouling model. Furthermore, it is noted that there is about 4.7% increase in outlet process fluid temperature of the evaporative fluid cooler. Also, a parametric study is performed to evaluate the effect of elevation and mass flow rate ratio on typical performance parameters such as effectiveness for rating calculations while surface area for design calculations. [Copyright &y& Elsevier]

• The desirable operating conditions to achieve high operational efficiency are obtained. • The performance of the IEC under different operating conditions is investigated. • The mathematical model is validated against published data. • The optimal dimensions that give good efficiency in climates with moderate humidity are highlighted. [Display omitted] This paper presents a study investigating and analyzing the effect of heat and mass exchanger (HMX) dimensions and the flow parameters on the indirect evaporative cooler (IEC) system's performance. For this purpose, a mathematical model based on heat and mass transfer concepts was developed, and the model was verified using experimental and simulation results from published papers. Simulations were performed based on different operating and structural conditions affecting the outlet air temperature, cooling capacity, coefficient of performance, and wet-bulb efficiency. Results showed that the optimal dimensions that give good efficiency in climates with moderate humidity, the length of the duct should be between 0.6 to 1.0 m , the width of the channel between 0.3 to 0.5 m , and the channel gap between 0.004 to 0.008 m. It has also been observed that the increasing working air velocity has a positive effect on cooling performance, and its velocity should not be less than 1 m / s. As for increasing the product air velocity, it reduces the heat exchange period, which is undesirable, so it should not exceed 1 m / s. The prescribed flow rate ratio between product and working air should be 0.3 to 0.5. This analysis provides desirable operating conditions to achieve high operational efficiency as well as optimal dimensions for designing such a cooler. [ABSTRACT FROM AUTHOR]

Despite the fact that the conventional one-dimensional model for counter-flow spray separation tower is advanced in reasonable accuracy as well as time and computational resources saving, it fails to describe the physical processes in details. In this paper, the concept of supersaturated state of the moist air was introduced, then a new sectional one-dimensional model was developed with a detailed derivation. A prototypic apparatus was set up and the reliability of the model was validated by the experimental study. When predicting the outlet solution temperature and outlet moist air wet bulb temperature, the relative errors would be more pronounced with a bigger heat and mass transfer driving potential, but they would not exceed 3.59% and 9.54% respectively under the experimental conditions. The simplifying assumptions, the ignored heat and mass transfer at cyclone region and the measuring errors are mainly blamed for the predicted errors. An in-depth heat and mass transfer investigation was conducted with the help of the validated model by studying the profiles of humidity ratio and moist air dry bulb temperature. The proposed model succeeded in capturing the state transformation of the air flow along the axis of the tower and the effects of the operating parameters on the process profiles are discussed. [ABSTRACT FROM AUTHOR]

In this paper, a numerical study of heat and mass transfer in counter-flow indirect evaporative air coolers is presented using an approach proposed by previous work. The mathematical model equations of continuity, momentum, energy and concentration are solved by the finite volume method to obtain the value of primary outlet dimensionless temperature. This mathematical model developed is validated against published data acquired from literature. The key target of presented analysis is to develop a performance correlation based on five dimensionless factors: ratio of channel length to half width of secondary air channel, ratio of water to secondary air mass flow rate, primary air inlet Reynolds number, secondary air inlet dimensionless temperature and water film inlet dimensionless temperature. Numerical simulations are performed by changing the value of each dimensionless factor under a constant value for other parameters. The simulation results are regressed to obtain the empirical constants of the correlation. The value of outlet dimensionless temperature is varied from 0.47 to 0.92. The developed correlation can predict the performance of the counter-flow indirect evaporative air coolers within a discrepancy of 12.6%. The correlation would be helpful to provide a simpler equation to improve the calculation speed within the engineering allowable accuracy range. [ABSTRACT FROM AUTHOR]

For the problem of drying biomass particles, based on gas-solid two phase flow theory, this paper establishes a mathematical model of straight tube flue gas flow drying, and compiles MATLAB language program to sovle the model. By changing the particle flow, flue gas flow, flue gas temperature and other single factor, it analyzes the impact of gas-solid relative velocity and heat transfer temperature difference in the drying process. Last it obtains the degree of influence on the drying efficiency: particle Flow> flue gas flow> flue gas temperature. [ABSTRACT FROM AUTHOR]

In summer, a large amount of heat is transfer from outdoor circumstance into indoor space through building external wall in tropical and subtropical zones and much energy was consumed to remove the surplus heat to maintain a comfortable indoor environment. In order to save energy, a concept of evaporative wall was presented in this paper. The structure and principle of evaporative wall was described. To investigate the energy saving potential of evaporative wall, a mathematical model was developed. With the climatic design information of Guangzhou, the cooling load temperature (CLT) and cooling load per unit wall area of evaporative wall were computed by using the model developed. It was found that the CLT of evaporative wall is 5~7℃ lower than that of dry wall. A cooling load reduction of 83%~93% can be achieved by evaporative wall compared with dry wall. In addition, evaporative wall has the ability to reduce the hourly swing of CLT and cooling load. [ABSTRACT FROM AUTHOR]

The theoretical study of the counter-flow dew point evaporative cooler is currently limited to the influence of different operating conditions and geometric parameters. Therefore, this paper presents an in-depth investigation on the governing factors of the dew point evaporative cooling process. A 2-D mathematical model has been developed based on the momentum, continuity, energy and species balance equations. The model was able to achieve good agreement with a maximum discrepancy of ±8.0% in predicting the transient and steady-state performance of the counter-flow dew point evaporative cooler. A new dimensionless model was produced from the general model via scaling analysis. The relevant dimensionless groups controlling the operating conditions, geometric parameters and supply air conditions were derived. We further discussed the relative importance of the physical mechanisms involved in the heat and mass transfer process. In addition, a dimensional analysis was carried out to investigate the quantitative expressions of the dimensionless time constant and product air temperature. The key findings that emerged from this study are: (1) the dominant dimensionless groups for the evaporative cooling process are Re , r , H L , δ H , T 0 ∗ and π ; and (2) the correlations for the dimensionless time constant and product air temperature have been developed and validated with data to demonstrate good accuracy. [ABSTRACT FROM AUTHOR]

In this paper, MHD nanofluid flow, heat and mass transfer between non-parallel walls is investigated. Brownian diffusion and thermophoresis effects which are two momentous slip mechanisms in nanofluids have been considered in the nanofluid model. Also, Joule heating effect is taken into account. The governing radial momentum, energy and mass equations are solved by Duan–Rach Approach (DRA). This method allows us to find a solution without using numerical methods to evaluate the undetermined coefficients. This method modifies the standard Adomian Decomposition Method by evaluating the inverse operators at the boundary conditions directly. The effects of active parameters such as the Reynolds number, the opening angle parameter, the Schmidt number, thermophoretic parameter and Brownian parameter are investigated on the velocity, temperature and concentration. Also, the value of the Nusselt number is calculated and presented through figures. The results indicate that the temperature and concentration profiles and Nusselt number increase with the increasing Schmidt number. In addition, the limiting cases are gained and found to be in an excellent agreement with the previous works. [ABSTRACT FROM AUTHOR]

In this paper, mixed convection of non-Newtonian nanofluid, using the Buongiorno’s mathematical model in a cavity has been analyzed by Finite Difference Lattice Boltzmann method (FDLBM). The cavity is filled with nanofluid which the mixture shows shear-thinning behavior. The Prandtl number is fixed at Pr = 1. This study has been performed for the certain pertinent parameters of Richardson number ( Ri = 0.001, 0.01, and 1), buoyancy ratio number ( Nr = 0.1, 1, and 20), power-law index ( n = 0.2–1), Lewis number ( Le = 1, 5, and 10), Thermophoresis parameter ( Nt = 0.1, 0.5, 1), and Brownian motion parameter ( Nb = 0.1, 1, 5). Results indicate that the augmentation of the power-law index causes heat and mass transfer to drop at Ri = 0.001 and 0.01. The drop of Richardson number from Ri = 1 to 0.001 enhances heat and mass transfer for various power-law indexes. The increase in the Lewis number augments mass transfer while it causes heat transfer to drop. The rise of the Thermophoresis and Brownian motion parameters ameliorates mass transfer and declines heat transfer significantly. The augmentation of buoyancy ratio number enhances heat and mass transfer. [ABSTRACT FROM AUTHOR]

Crystallization is generally considered as the most destructive factor of porous material, but few studies have reported the coupling effects among fluid flow, heat transfer, crystallization and deformation in porous media. In this paper, the driving forces of phase transition were studied initially and then the crystallization kinetics models were established. Moreover, constitutive relations related to saline frozen soil were discussed, and the expression of unfrozen water content in non-equilibrium state was improved. Finally, a thermo-hydro-salt-mechanical coupled model for fully saturated saline frozen soil with phase change was proposed. Validation of the model is illustrated by comparisons between the simulation and experimental results. The predicted values of frost heave, temperature and moisture distribution are consistent with the experimental data, and it is demonstrated that the presented crystallization kinetics approach is valid for studying heat and mass transfer coupled problem with phase change in porous media. [ABSTRACT FROM AUTHOR]

• Molecular distillation processes and equipment have been revised. • Molecular distillation models and the corresponding equations governing the mass, energy and momentum phenomena have been reviewed. • Thermodynamic and transport properties estimation methods of natural-based mixtures were reviewed. • Several molecular distillation applications on different mixtures have been presented and summarized including operation conditions. Molecular distillation is a separation technique used to separate thermosensible compounds. This process is often used to obtain concentrates of value-added products. For this reason, the research in this field has grown steadily during the past years. However, many efforts to simulate the transport phenomena occurring in this process and the thermodynamic properties of the molecules to be separated have fallen flat. Achieving a simulation that resembles what occurs in this process requires a thorough understanding of the molecular behavior of various substances under different conditions. This paper reviews several recently proposed methodologies to predict this behavior during the molecular distillation process. Thus, group contribution models and empirical correlations are used to estimate the value of some properties and coefficients which, in turn, determine the specific ways in which heat, mass, and momentum vary in a Molecular Distillation separation system. The transfer equations are discussed in mathematical models. Thin film, vapor phase, molecule collision, and mean free path are also important topics in molecular distillation, so they are also discussed in this review. Finally, some applications are summarized and a new case for the cannabis industry is supported. [Display omitted] [ABSTRACT FROM AUTHOR]

Through analysis the MIG groove welding process and the welding experimental result, a complete two-dimensional mathematical model of weld section molten pool characteristics was established. Use Gaussian heat source distribution to describe the weld heat source, introduced the welding speed to establish transient heat source model on the cross section. By introducing scalar of liquid fraction, mushy region dynamic character which was described using “Darcy” model proposed for porous media, and phase change which was taken into account using source-based method, establish a unified liquid and solid control equation, CFD model was established to simulate transient heat and mass transfer phenomena in welding process. The effect of surface tension was considered in the molten pool free surface stress model. The reliability of the model was verified by experiments. The effect of groove angle, groove depth and the welding speed on the molten pool were discussed in this paper. [ABSTRACT FROM AUTHOR]

Heat and mass transfer phenomena occurring in cold storage chambers for vegetables have been modelled in the paper on the example of the experimental Chinese cabbage cold store. Special attention has been given to the problem of modelling of interrelationship between phenomena occurring in the bulk of vegetables and in the heat exchanger of a cooling unit, accomplished through User Defined Functions UDF so that the cooling capacity and the transpiration and respiration in the bed of cabbage were closely related. The comparisons between simulation and experimental results were conducted in order to indicate further improvements to the model. [ABSTRACT FROM AUTHOR]

Vacuum pre-cooling is a multi-physics process coupling heat transfer, water transport, and shrinkage, the stresses caused by water evaporation and temperature variation in food interact with its deformation. However, the developed models usually assume that the shrinkage of food is insignificant so that it is ignored, which is inconsistent with the actual vacuum pre-cooling process and makes the simulation results inaccurate. Taking strawberry as an example, a multiphase porous media model with heat-water and mechanics (HWM) bi-directional coupling was developed for food vacuum pre-cooling. The accuracy of the HWM model was verified by comparing the heat-water (HW) model without considering shrinkage and the experiments. The results show that the development of the evaporation front predicted by the HWM model is more sensitive to the vacuum chamber pressure change compared with the HW model. The maximum average relative errors corresponding to the temperature and shrinkage of the HWM model are 0.27% and 6.95%, respectively, which are better than the HW model. In addition, the edge gradient effect that occurs in the middle stage of vacuum pre-cooling was observed. When the vacuum chamber pressure approached the lowest point, the evaporation region rapidly developed toward the core. The increase of evaporation driving force and the decrease of boundary moisture caused drastic temperature and shrinkage changes on the product surface. The model developed in this paper considering shrinkage provides technical support to optimize the vacuum pre-cooling process and improve product quality. • Shrinkage of strawberry during vacuum pre-cooling was modeled. • Heat and mass transfer model coupled with shrinkage was developed. • An improved method of predicting vacuum chamber pressure was proposed. • The edge gradient effect due to pressure drop was predicted. [ABSTRACT FROM AUTHOR]

A numerical model for heat and mass transfer in vacuum membrane distillation (VMD) under laminar flow was developed using Computational Fluid Dynamics (CFD). Three-dimensional (3D) simulations of temperature and concentration polarization in single and multiple fibre VMD modules were used to estimate permeate flux (kg m −2 h −1 ) over a range of temperatures (30–70 °C), vacuum pressures (10–100 mmHg), crossflow velocities (0.0072–0.72 m/s), and feed concentrations (0–0.4 kg/L NaCl). Simulated flux differed by less than 7% from experimental data for a module with comparable dimensions. Simulations indicate that a 56% increase in fibre packing density resulted in a 24% flux decline at high operating temperature (70 °C), and more than 50% flux decline at low crossflow velocity (0.0072 m/s). The effect of vacuum pressure on flux was found to be independent to the module packing density, while the effect of salt concentration was found to be 28% lower than estimates based on Raoult's law, due to lower spatial variations in membrane surface temperatures at higher salt concentrations. The approach developed in this paper may be used to evaluate performance of alternative configurations for VMD models to aid module design, scale-up and process optimization. [ABSTRACT FROM AUTHOR]

The paper is focuses on the numerical simulation and analysis of coupled heat and mass transfer under ice formation conditions in the cross-flow plate heat exchanger. The simulation is performed using the original mathematical model. The presented model was validated against experimental data. The obtained results showed satisfactory agreement with the data obtained from the experimental measurements. Numerical simulation reveals many unique features of considered heat exchanger connected with the heat and mass transfer occurring in the return air channel under ice formation conditions. The results of computer simulation showed that the efficiency gains are sensitive to various inlet conditions, and allow for estimation of the safe operating conditions for different inlet return airflow parameters, based not only on the exhaust air temperature but also on its relative humidity and different thermal efficiency of the exchanger. Several goals were achieved, including: establishing the most unfavorable operating conditions (from the standpoint of ice formation on the plate surface of the return air channels) for cross-flow plate heat exchanger, which correspond to the inlet value of the return airflow dew point temperature equaled to 0 °C, impact of the ice formation on temperature effectiveness, impact of inlet relative humidity on temperature effectiveness and safe operating conditions and safe operating conditions for variety of inlet exhaust air parameters. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of heat and mass transfer in indirect evaporative air coolers with four air flow patterns: parallel-flow, counter-flow, cross-flow and regenerative. The numerical simulation was performed on the basis of original two-dimensional heat and mass transfer models (in the case of cross-flow heat exchanger the model was 3D). The mathematical models developed were validated against published experimental data presented in Appendix A . It was established, that heat and mass transfer processes in the wet channels of counter-flow, cross-flow and regenerative indirect evaporative coolers are characterized by creation of two particular heat and mass transfer zones. Detail analysis of the temperature and humidity ratio distributions and boundary conditions, characterizing the coupled heat and mass transfer process in each of these determined zones, revels the violation of the Lewis relation unity under a certain inlet and operation conditions. A theoretical method for estimating the Lewis factor was proposed. Using the developed models the thermal performances of the conventional designs of indirect evaporative air coolers were analyzed numerically and preferable climatic zones for considered heat exchanger were established. Four novel heat exchangers utilizing the advantages of the basic cycles heat exchangers were presented. [ABSTRACT FROM AUTHOR]

This paper investigates a mathematical simulation of the heat and mass transfer in the two different Maisotsenko Cycle (M-Cycle) heat and mass exchangers used for the indirect evaporative cooling in different air-conditioning systems. A two-dimensional heat and mass transfer model is developed to perform the thermal calculations of the indirect evaporative cooling process, thus quantifying the overall heat exchangers’ performance. The mathematical model was validated against the experimental data. Numerical simulations reveal many unique features of the considered units, enabling an accurate prediction of their performance. Results of the model allow for comparison of the two types of heat exchangers in different applications for air conditioning systems in order to obtain optimal efficiency. [ABSTRACT FROM AUTHOR]

This paper presents a transient model of internal catalytic combustion suitable for isolated channels and monolith reactors. Due to time-scales in the problem, the model considers a quasi-steady gas phase with a transient solid. The gas is described by axially varying bulk temperature and species. The gas includes lateral diffusion via transfer coefficients and the specification of a gas-phase species concentration at the wall; axial diffusion is neglected. The solid phase is a thermally thin shell with axially varying temperature, surface species, and surface species concentrations. The solid includes axial heat conduction and external heat loss by convection and radiation. The combustion process utilizes detailed gas and surface reaction models. The gas-phase model becomes a system of stiff ordinary differential equations with respect to axial position; the upstream (inlet) boundary conditions are specified and the axially varying solid properties are parameters in integration. The solid phase discretizes into a system of stiff ordinary differential-algebraic equations with respect to time. The time evolution of the system comes from alternating integrations of the quasi-steady gas phase and transient solid. The model is compared to two experimental cases using CO fuel: (1) steady-state conversion in an isothermal platinum tube and (2) transient propagation of a catalytic reaction inside a small platinum tube and includes external tube temperature measurements. This work presents sensitivity studies on important parameters including internal transfer coefficients, catalytic surface site density, external heat-loss, and others. Under mass-transfer limited conditions, global transfer coefficients are adequate to predict fuel conversion. Near light-off, the model predictions improve for the first case after adjusting the surface kinetics such that the net rate of CO adsorption increases compared to O 2 . For the second case, predictions of transient propagation speeds are good for equivalence ratios near unity and greater but require adjustment of external heat loss or kinetics to match under lean conditions. [ABSTRACT FROM AUTHOR]

Liquid-to air membrane energy exchangers (LAMEEs) are novel membrane based energy exchangers that allow simultaneous heat and moisture transfer between salt solutions and air streams. The similarity between a small-scale (2.5 L/s) LAMEE and a full-scale (100 L/s) LAMEE is investigated, both experimentally and numerically, in this paper. Dimensional analysis of the coupled heat and mass transfer equations in the air-side, solution side and membrane of the LAMEE shows that five important dimensionless parameters affect the performance of LAMEEs: (1) number of heat transfer units, NTU, (2) thermal capacity ratio, Cr*, (3) number of mass transfer units, NTUm, (4) mass flow rate ratio, m*, and (5) operating factor, H*. The differences in the small-scale and full-scale LAMEEs effectivenesses are numerically investigated for these five important dimensionless parameters. The results show that in most cases the differences between the small-scale and full-scale LAMEE effectivenesses are less than 2%. The comparison between the experimental and numerical sensible, latent and total effectivenesses of the small-scale LAMEE and the full-scale LAMEE shows that there is a similarity between the small-scale and full-scale LAMEEs, and that the small-scale LAMEE effectiveness results can be used to predict the performance of the full-scale LAMEE. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the advanced object-oriented design of finite element representations in a complex multi-physics finite element environment OOFEM [1–3]. The focus is on reuse of existing single-physics capabilities when implementing elements for coupled simulations. This has been achieved by decoupling the description of element geometry, element problem specific capabilities, element interpolation, and integration schemes. The individual problem specific capabilities, represented by a hierarchy of classes derived form ElementEvaluator, can be assembled together to define an evaluator for coupled analysis. The presented design leads to an extremely flexible implementation, with clean modular design. The application is demonstrated on an implicit gradient formulation of a damage-plastic model and on coupled heat and mass transfer. [Copyright &y& Elsevier]

Abstract: In this paper, we investigate the time-dependent heat and mass transfer in a molten carbonate fuel cell at start-up. Thus, a three-dimensional, transient mathematical model is presented through a comprehensive inclusion of various physical, chemical and electrochemical processes that occur within the different components of molten carbonate fuel cells. The model is proposed as a predictive tool to provide a three-dimensional demonstration of variable variations at system start-up. The local distribution of field variables and quantities are showcased. It reveals that the electrochemical reaction rate is dominated by the over-potential, not by the reactants' molar fraction. Reversible heat generation and consumption mechanisms of the cathode and anode are dominant in the first 10 s while the heat conduction from the solid materials to the gas phase is negligible. Meanwhile, activation and ohmic heating have nearly the same impact within the anode and cathode. Based on these findings, the importance of heat conduction and its main features are finally assessed. [Copyright &y& Elsevier]

Abstract: The paper aims to overview various mathematical models for modeling the simultaneous heat and mass transfer process in the liquid desiccant dehumidifier. Firstly, the dehumidification principle is introduced briefly. Then the models are interpreted in terms of two classes of dehumidifiers. For the adiabatic dehumidifier, the models are mainly classified into three types: finite difference model, effectiveness NTU (ε–NTU) model, and simplified models. For the internally cooled dehumidifier, there are also three kinds of models: models without considering liquid film thickness, models considering uniform liquid film thickness, and models considering variable liquid film thickness. This review is meaningful for comprehending the development process and research status of the models and choosing suitable models for prediction. In addition, some suggestions are proposed for the model improvement. [Copyright &y& Elsevier]

Abstract: Combustion of pulverized coal in oxy-combustion technology is one of the effective ways to reduce the emission of greenhouse gases into the atmosphere. The process of transition from conventional combustion in air to the oxy-combustion technology, however, requires a thorough investigations of the phenomena occurring during the combustion process, that can be greatly supported by numerical modeling. The paper presents the results of numerical simulations of pulverized coal combustion process in swirl burner using RANS (Reynolds-averaged Navier–Stokes equations) and LES (large Eddy simulation) methods for turbulent flow. Numerical simulations have been performed for the oxyfuel test facility located at the Institute of Heat and Mass Transfer at RWTH Aachen University. Detailed analysis of the flow field inside the combustion chamber for cold flow and for the flow with combustion using different numerical methods for turbulent flows have been done. Comparison of the air and oxy-coal combustion process for pulverized coal shows significant differences in temperature, especially close to the burner exit. Additionally the influence of the combustion model on the results has been shown for oxy-combustion test case. [Copyright &y& Elsevier]

Abstract: Small scale particles/pores from micrometers and down to nanometers often occur in multi-functional porous electrodes in fuel cells, to enhance the catalytic reaction activities and accordingly the cell performance. Multi-component and -phase mass transport phenomena of reactants and products are strongly coupled with other transport processes as well as various reactions. All these processes form inter-linked circuits for the mass, heat and electricity, which determine electrode design, cell structure/configuration and operation, hence overall performance. Understanding of gas diffusion mechanisms and accurate estimating of the overall diffusion coefficient are essential for the operation and design of fuel cells, especially at high current density conditions. Several intensive research and investigations have appeared in recent years involving both experimental and modeling approaches for porous structure reconstruction and evaluation of effective diffusion coefficients. In this paper, the mass transfer equations commonly used for continuum models at porous-average level are outlined and highlighted, with the purpose to provide a general overview of the validity and the limitation of these approaches. The most often used models in the open literature are reviewed and discussed focusing on the effective gas diffusion coefficients and tortuosity factors. It is revealed that the effects of both small scale (Knudsen number) and tortuous pathways (tortuosity factor) on the effective diffusion coefficients are significant for the specific layers in the electrodes. Summary and suggestions are also provided for better understanding of gas diffusion phenomena and implementation of the effective gas diffusion coefficient models for fuel cell electrodes. [Copyright &y& Elsevier]

Abstract: Continuous frying systems are very energy intensive. They also involve the interaction of many variables and complex heat and mass transfer processes. Better control of these processes can lead not only to improved product quality but also energy reduction. This paper presents a model that can simulate the dynamic behaviour of a continuous frying system in sufficient detail that enables the influence of controls on the important product attributes of moisture and oil content to be investigated. A conjugate 2-D transient model of the fryer was used to develop correlations for the oil and moisture content based on the important fryer control variables, namely supply oil temperature, potato mass throughput, fryer paddle velocity and crisp takeout velocity. The correlations were validated against data for an industrial continuous fryer system and used within the overall frying system model which was implemented in MATLAB. Proportional Integral (PI) controls implemented within the MATLAB/Simulink environment have shown that closer control of the important parameters can lead to 10% energy savings compared to the actual industrial frying system. The modelling approaches developed can also be used to investigate further improvements in the design and control of continuous frying systems. [Copyright &y& Elsevier]

Abstract: The present study investigates the dynamic functioning of a new solar desalination process based on the humidification/dehumidification (HD) principle. The design and the principle of working of the desalination system are described. The paper deals with the modeling, simulation aspects, and experimental validation of the main components of the system. Mathematical models were established including the mechanism of heat and mass transfer in the various components. These models use real meteorological data to predict the performance of a thermal solar driven system. Series of experiments were conducted and experimental results were then compared with the simulation results. Our findings showed that the developed models could be used to design and help in predicting the behavior of the installation in various meteorological conditions. [Copyright &y& Elsevier]

Abstract: The development of two analytical models that describe the thermal behaviour of solar air heaters of double-parallel flow and double-pass counter flow is presented. Based on the local energy balances, algebraic expressions for the efficiency factor F′ and the overall heat loss coefficient, U L , as well as air temperature distributions along the collectors were obtained. In addition, the expressions of the mean temperatures of the two air streams and of the absorber plate were determined. The model corresponding to the solar collector with double-parallel flow predicts that the mass flow of air flowing between the absorber plate and the transparent cover is greater than the mass flow circulating through the other channel. The model corresponding to the solar collector of double-pass counter flow indicates that increasing the airflow proportionally increases the percentage temperature rise in the inlet channel. The analytical expressions developed are valuable tools that can be included in computational codes for the design and thermal analysis of solar air heating facilities or to improve current facilities. The paper include a graphic to estimate the heat transfer coefficients involved in energy balances. [Copyright &y& Elsevier]

Abstract: In the present paper, a computational fluid dynamics (CFD) modeling study was performed for the combustion of the brown coal in a large-scale tangentially-fired furnace (550MW) under different operating conditions. The AVL Fire CFD code has been used to model the combustion processes. The mathematical models of coal combustion with the appropriate kinetic parameters were written and incorporated to the code as user defined functions. These models consist of pulverised coal (PC) devolatilization, char burnout, and heat and mass transfer. The simulation of the PC combustion was carried out using multi-step reaction chemistry mechanisms. The level of confidence of this numerical model was based on the previous validations of the lignite combustion in a lab-scale furnace, as well as the validation parameters of the present furnace at the standard existing conditions in terms of temperature values and species concentrations. Performance of the boiler under ten different operating conditions was investigated. The strategy of operation schemes for the first six combustion scenarios were based on the change of the out-of-service (turned off) burners under full load operation, while the rest cases were carried out at 20% lower and 20% higher loads than the standard operating conditions. The validated model was used to perform the following investigation parameters: furnace gas temperatures, species concentrations (O2, CO and CO2), velocity distributions, and char consumption. The predictions demonstrated that there are good temperature distributions in the furnace when the turned off burners are set in the opposite direction under full load operation. For higher aerodynamic effect, the numerical results showed improvements on the combustion characteristics in terms of species concentrations and char burnout rates in comparison with the standard operating case. The findings of this study provide good information to optimize the operations of the utility tangentially coal-fired boiler with less emission. [Copyright &y& Elsevier]

Abstract: The paper addresses results for the case of convective drying of particulate solids in a packed and in a fluid bed, analyzing agreement between the numerical results and the results of corresponding experimental investigation, as well as the differences between packed and fluid bed operation. In the fluid bed simulation model of unsteady simultaneous one-dimensional heat and mass transfer between solids, gas phase and bubble phase during drying process, based on two-phase bubbling model, it is assumed that the gas–solid interface is at thermodynamic equilibrium. The basic idea is to calculate heat and mass transfer between gas and particles (i.e., the drying process) in suspension phase as for a packed bed of particles, where the drying rate (evaporated moisture flux) of the specific product is calculated by applying the concept of a “drying coefficient”. Mixing of the particles (i.e., the impact onto the heat and mass transfer coefficients) in the case of fluid bed is taken into account by means of the diffusion term in the differential equations, using an effective particle diffusion coefficient. Model validation was done on the basis of the experimental data obtained with narrow fraction of poppy seeds characterized by mean equivalent particle diameter (dS , d =0.75mm), re-wetted with required (calculated) amount of water up to the initial moisture content (X 0 =0.54) for all experiments. Comparison of the drying kinetics, both experimental and numerical, has shown that higher gas (drying agent) temperatures, as well as velocities (flow-rates), induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity. Bed temperature differences along the bed height are significant inside the packed bed, while in the fluid bed, for the same drying conditions, are almost negligible due to mixing of particles. Residence time is shorter in the case of a fluid bed drying compared to a packed bed drying. [Copyright &y& Elsevier]

Abstract: The paper deals with modeling the convective drying process. A relevant and reliable mathematical model that captures the history and distribution of temperature is presented. The attention is focused on the simultaneous heat and mass transfer occurring during drying where dry and hot air flows about the food. In the present study, external resistance to mass transfer is considered negligible. As a result, the drying curve is almost independent of the boundary conditions, which means that drying is diffusion-controlled. The main connotation of present study regards to undertake analytical procedure to establish the novel model for practical applications. The results show that the temperature evolution can be evaluated from an advanced analytical solution in a quick and efficient manner. The model is validated with the literature experimental data obtained for carrot and mango slabs. A good agreement is obtained between the model predictions and the available experimental results. [Copyright &y& Elsevier]

Abstract: This paper will show that so-called “Newton’s law of cooling” that is often used for calculating of heat transfer by convection is actually not a law, but a model of heat exchange (heat transfer). Limits of validity of this law were discovered and will be shown. Moreover, an existing practice of modernizing this law for the purpose of calculating convective heat transfer will be questioned. [Copyright &y& Elsevier]

Abstract: Propagation of uncertainty through the physical model has been investigated in the present paper by solving two specific simple stochastic problems using the Non-Intrusive Spectral Projection method. The uncertain parameters are described by either a Gaussian or a LogNormal probability distribution function. For each of the problems, the stochastic and the deterministic mean solutions have been compared and the respective confidence intervals have been obtained. For the deterministic problems, the confidence intervals have been estimated using both one-dimensional and multi-dimensional bound methods. From the results it has been observed that the differences between the stochastic and the deterministic mean solutions are apparent only when large uncertainties are introduced in the random variables. For both the specific problems, considered in the present study, the confidence intervals for the stochastic problems have been exactly predicted by the deterministic limits when uncertainty is introduced only in one of the parameters. For more than one uncertain parameters, the multi-dimensional bound method produces better agreement with the stochastic confidence intervals than the one-dimensional bound method. The findings are expected to be applicable to problems in heat and mass transfer with similar characteristics or input–output relations. [Copyright &y& Elsevier]

Abstract: The present paper proposes a sub-model for two-layer zone models to describe heat and mass transfer across the interface between the upper and lower layers as a result of the temperature difference between the two layers. The sub-model is integrated into a two-layer zone fire growth and smoke movement model, which now can predict both pre- and post-flashover fires. The comparison of the numerical predictions of the improved model and experimental data shows that the improved model produces better temperature and heat release rate predictions for both pre- and post-flashover fires than the previous model. Compared with hybrid models consisting of a two-layer zone model and a single-layer zone model, the improved model has no transitional difficulties in switching from the pre- to the post-flashover state and it produces more reasonable temperature gradients for post-flashover fires. [Copyright &y& Elsevier]