CO oxidation over structured carriers: A comparison of ceramic foams, honeycombs and beads

This work aims an experimental comparison of different packings on the basis of their pressure drop, mass and heat transfer properties. Ceramic foams, beads and a honeycomb monolith were used as carriers in the oxidation of carbon monoxide. The carriers were coated with active Pt/SnO₂. The CO oxidation rate was measured in the regime of external diffusion control at superficial gas velocities between 1 and 10 m/s. The volumetric rate coefficients and the pressure drop of packings with similar geometric surface area decreased in the sequence particles > foams > honeycomb. The magnitude of the temperature gradient along the catalytic bed decreased as going from honeycomb over larger particles to foams and small particles. Foams were superior over particle beds from the viewpoint of combined high mass transfer and low-pressure drop. The main advantage of foams as compared to honeycomb resided in the radial mixing enabling a better heat transfer to the reactor walls.     

Study of the influence of axial conduction in a boiling heated pipe

In this work a characterization of a horizontal heated pipe has been performed. This characterization consists in a steady state analysis of the thermohydraulic behavior of a boiling heated channel with subcooled liquid at the inlet. The temperature, velocities and pressure profiles along the heated section have been analyzed and the length of the single- and two-phase flow regions have been characterized. Without axial conduction and due to the big difference in the magnitude of the single- and two-phase heat transfer coefficient, steep temperature gradients were observed. By adding the effect of axial conduction, more heat is removed from the single phase region and added to the two-phase region through the wall. The net effect was a decrease in length of the two-phase flow region and consequent increase of the single-phase region. It was also observed that the axial conduction decreases the gradients in the wall temperature profile but it does not influence markedly its temperature differences with the fluid.     

Computational analysis of the flow of pseudoplastic power-law fluids in a microchannel plate

The flow of pseudoplastic power-law fluids with different flow indexes at a microchannel plate was studied using computational fluid dynamic simulation.The velocity distribution along the microchannel plate and especially in the microchannel slits,flow pattern along the outlet arc and the pressure drop through the whole of microchannel plate were investigated at different power-law flow indexes.The results showed that the velocity profile in the microchannel slits for low flow index fluids was similar to the plug flow and had uniform pattern.Also the power-law fluids with lower flow indexes had lower stagnation zones near the outlet of the microchannel plate.The pressure drop through the microchannel plate showed huge differences between the fluids.The most interesting result was that the pressure drops for power-law fluids were very smaller than that of Newtonian fluids.In addition,the heat transfer of the fluids through the microchannel with different channel numbers in a wide range of Reynolds number was investigated.For power-law fluid with flow index (n =0.4),the Nusselt number increases continuously as the number of channels increases.The results highlight the potential use of using pseudoplastic fluids in the microheat exchangers which can lower the pressure drop and increase the heat transfer efflciency.     

The Proper Use of Mass Diffusion Equations in Drying Modeling: Introducing the Drying Intensity Number

This article intends to clearly define the possibilities and limitations offered by a simple diffusion approach of drying. Actually, many works use a simple diffusion equation to model mass transfer during drying, probably because a simple analytical solution of this equation does exist in the case of simple boundary conditions. However, one has to be aware of the limitations of this approach. Using a comprehensive formulation and a relevant computational solution, the most frequent assumptions of the diffusion approach were rigorously tested. It is concluded that analytical solutions must be discarded for several reasons: analytical solutions, either using Dirichlet or third kind boundary conditions, are often misleading and should be avoided; in the drying process, the coupling between heat and mass transfer is mandatory; nonlinearity (variation of diffusivity with moisture content) can hardly be avoided for mass transfer. In order to reach a verdict, a dimensionless number, the Drying Intensity Number (N_DI), is introduced. It allows the level of coupling between heat and mass transfer to be easily assessed. Thanks to this number, a guide is proposed for choosing the right level of modeling, depending on the drying configuration.     

Dynamic modeling of a finite volume of solid oxide fuel cell: The effect of transport dynamics

A dynamic model for a finite volume of cell based on physical principles is built in the form of a nonlinear state-space model to investigate dynamic behaviors of tubular solid oxide fuel cell (SOFC) and develop a control relevant model for further control studies. Dynamic effects induced by diffusions, intrinsic impedance, fluid dynamics, heat exchange and direct internal reforming/shifting (DIR) reactions are all considered. Cell temperature, ingredient mole fractions, etc. are the state variables and their dynamics are investigated. Dynamic responses of each variable when the external load changes are simulated. Simulation results show that fuel flow, inlet pressure and temperature have significant effects on the dynamic performance of SOFC. Further it is shown that, compared to other inlet flow properties, cathode side air inlet temperature has the most significant effect on SOFC solid phase temperature and performance. Compared with inlet pressures and temperatures, the effect of flow velocity is not significant. Simulation also indicates that the transient response of SOFC is controlled mainly by the dynamics of cell temperature owing to its large heat capacity.     

Microwave drying of spheres: Coupled electromagnetics-multiphase transport modeling with experimentation. Part I: Model development and experimental methodology

To understand the effects of shape, size and property changes in a spherical sample during microwave drying, a fundamentals-based coupled electromagnetics and multiphase porous media model is developed and associated experimental details are described. Microwave drying of different sized spheres is carried out in a domestic microwave oven operating at 10% power level. Maxwell's equations for electromagnetics are solved inside a three dimensional (3D) microwave oven to obtain the electric field distribution inside the oven cavity and the spheres. The drying samples are treated as a porous media consisting of three phases: solid (skeleton), liquid (water) and gas (water vapor and air). Modes of transport for the fluid phases include capillary flow, binary diffusion between vapor and air, gas pressure driven flow and phase change between liquid water and vapor which is spatially distributed. An elaborate experimental system comprising of infrared camera, optical fiber probe and digital balance is built to validate the model in terms of temperature distribution, point temperatures, gas pressure generation and moisture loss from the samples at different times during the drying process. Results, validation, sensitivity analysis and “what-if” scenarios are presented in the companion paper. The work together would provide tremendous benefits when designing and developing microwave drying processes and products through a novel synergy between physics-based modeling and detailed experimentation.     

Determination of the Mass Diffusion Coefficient Based on the Relative Humidity Measured at the Back Face of the Sample During Unsteady Regimes

This work proposes a new method for the determination of the mass diffusion coefficient in hygroscopic materials. The experiment consisted of submitting one face of the sample to a variation in time of the relative humidity (RH) and measuring the RH on its back face. The imposed RH and temperature were measured during the test and served as boundary conditions in a comprehensive computational code to solve heat and mass transfer in porous media. This model uses a physical engine embedded in the inverse procedure to determine the mass diffusion coefficient. Compared with classical methods, this new method has several advantages:

• It allows several samples to be measured simultaneously, simply by multiplexing the RH sensors.

• Accurate values can be obtained even when starting and ending out of equilibrium, which allows the characterization time to be drastically reduced.

• The external mass transfer coefficient has a negligible effect on the identified value.

• Nonstandard Fickian behaviors can be detected by the disagreement between the measured and simulated curves.

The results show that the diffusivity obtained for spruce wood is in good agreement with those found with classical methods. In contrast, the fiber board results differed between the experiment and model, or yielded unrealistic values, which confirms the dual-scale nature of mass transfer that occurs in this kind of material.     

Mathematical modeling of the coupled transport and electrochemical reactions in solid oxide steam electrolyzer for hydrogen production

A mathematical model was developed to simulate the coupled transport/electrochemical reaction phenomena in a solid oxide steam electrolyzer (SOSE) at the micro-scale level. Ohm's law, dusty gas model (DGM), Darcy's law, and the generalized Butler Volmer equation were employed to determine the transport of electronic/ionic charges and gas species as well as the electrochemical reactions. Parametric analyses were performed to investigate the effects of operating parameters and micro-structural parameters on SOSE potential. The results substantiated the fact that SOSE potential could be effectively decreased by increasing the operating temperature. In addition, higher steam molar fraction would enhance the operation of SOSE with lower potential. The effect of particle sizes on SOSE potential was studied with due consideration on the SOSE activation and concentration overpotentials. Optimal particle sizes that could minimize the SOSE potential were obtained. It was also found that decreasing electrode porosity could monotonically decrease the SOSE potential. Besides, optimal values of volumetric fraction of electronic particles were found to minimize electrode total overpotentials. In order to optimize electrode microstructure to minimize SOSE electricity consumption, the concept of “functionally graded materials (FGM)” was introduced to lower the SOSE potential. The advanced design of particle size graded SOSE was found effective for minimizing electrical energy consumption resulting in efficient SOSE hydrogen production. The micro-scale model was capable of predicting SOSE hydrogen production performance and would be a useful tool for design optimization.     

Experimental study of the evaporation of a falling film in a closed cavity

This article concerns the experimental study of heat and mass transfer in a distillation cell. This latter is a parallelepiped, of large form factor, whose active walls are vertical. The cell is fed with salt water, and pure water is evaporated from a thin film that falls along a heated wall while the opposite wall is maintained at a lower temperature and is used as a condensation surface. The experimental results show that the heat transfer in the distillation cell is dominated by the latent heat transfer associated with evaporation. A parametric study of the behavior of the distillation cell has been performed. A convenient choice of the operating parameters is suggested to optimize the distillation yield.     

Mathematical modelling of the pre-oxidation of a uranium carbide fuel pellet

Uranium carbide is a candidate fuel for future nuclear reactors. However, for it to be implemented in a closed fuel cycle, an outline for its reprocessing is necessary. One proposed method is to oxidise the uranium carbide into uranium oxide which can then be reprocessed using current infrastructure. A mathematical model describing the heat and mass transfer processes involved in such an oxidation has been constructed. The available literature was consulted for reaction coefficients and information on reaction products. A stable and convergent numerical solution has been developed using a combination of finite-difference approximations of the differential equations. Completion times of approximately 3–30 h are predicted given a spherical pellet with a radius of 9.35 mm under varying initial conditions. The transient temperature distribution throughout the system is predicted, with a maximum temperature of 1458 °C observed from an initial temperature of 500 °C at an oxygen concentration of 3.15 mol m⁻³.     

Use of effective conductivities and unit cell-based supraelements in the numerical simulation of solid oxide fuel cell stacks

The numerical simulation of current and temperature distribution in monolithic solid oxide fuel cell (SOFC) stacks requires fast computers because of the large number of mesh points required in casting a complex solid geometry into a finite difference form and the necessity to solve coupled, nonlinear differential equations. By analogy with the modelling of radiative heat transfer in packed bed reactors, a significant degree of simplification is achieved by defining effective electric and thermal conductivities for the repeating unit cell elements, identified as the basic building blocks of the SOFC stack. The effective conductivities are approximated by closed form formulae derived from the principles of electrostatics and heat conduction. The effect of radiation across the gas channels is incorporated into the expressions for the effective thermal conductivity. Using this approach, the unit cell geometry, local mass transfer processes and reaction kinetics are expressed in terms of a supraelement model in a finite difference grid for the numerical calculation of temperature and potential distributions in a stack by an iterative process. The simplifications thus provided render simulations of three-dimensional SOFC stacks tractable for desktop processors. By using the foregoing approach to numerical simulation, a parametric study of a cross-flow type SOFC is presented, and some of the results are compared with the available experimental data     

Mathematical Modeling of a Continuous Vibrating Fluidized Bed Dryer for Grain

A mathematical model for the drying of grain in a continuous vibrating fluidized bed dryer was developed. Simple equipment and material models were applied to describe the process. In the plug-flow equipment model, a thin layer of particles moving forward and well mixed in the direction of the gas flow was examined. Mass and heat transfer within a single wet particle was described by effective transport coefficients. Assuming constant effective mass transport coefficient and thermal conductivity, analytical solutions of the mass and energy balances were obtained. The variation in both transport coefficients along the dryer was taken into account by a stepwise application of the analytical solution in space intervals with averaged coefficients from previous locations in the dryer. Calculation results were in fairly good agreement with experimental data from the literature. However, the results depend strongly on relationships used to determine the heat and mass transfer coefficients; because the results from correlations found in the literature vary considerably, the correlations should be adapted to the specific equipment in order to obtain reliable results.     

Effect of the condensing cover''s slope on internal heat and mass transfer in distillation: an indoor simulation

The objective of the study was to determine a relation for predicting convective and evaporative heat transfer coefficients for all three condensing surfaces inclined at 15°, 30° and 45° under indoor simulation. The condensing covers were made of the same flat transparent glass as found in any solar distillation unit. The operating temperature range for the experiment was maintained at steady state from 40°C to 80°C by using a constant temperature bath. The temperatures and yields obtained for a 10-min interval were used to determine the values of constants C and n and consequently convective and evaporative heat transfer coefficients. It was found that a higher yield was obtained with an increase in temperature for a 30° slope compared to 15° and 45° slopes of the condensing cover.     

Experimental optimization of a solar still: application to alcohol distillation

This paper presents an experimental study of two solar stills, a single-compartment model and a two compartment type. The single compartment still is optimized under local climatic conditions. An experimental study of the cover slope shows that a cover angle of 16° ensures a very good transmission of solar radiation within the still while preventing the drops of the distillate to fall into the basin. In the prototype using two compartments, the glass of the illuminated compartment is a transparent cover with a part of the condensation happening on it, while the other compartment, also made of glass, is covered with a non-transparent material, which shades the sun; it is used only as a condenser. It has been observed that the distillation of a 38% alcohol initial solution yields a product containing 48% of alcohol when using the single-cover model, while under the same climatic conditions the two compartments still gives a 71% alcohol distillate.     

Design and analysis of the commercialized drier processing using a combined unsymmetrical double-feed microwave and vacuum system (case study: tea leaves)

Combined microwave (MW) and vacuum drying of biomaterials has a promising potential for high-quality dehydrated products. A better knowledge of the drying kinetics of biomaterial products could improve the design and operation of efficient dehydration systems. The experiments were carried out on commercialized biomaterials drier using a combined unsymmetrical double-feed microwave and vacuum system. Three kilograms of tea leaves were applied with the microwave power of 800 (single-feed magnetron) and 1600 W (unsymmetrical double-feed magnetrons) operating at 2450 MHz frequency. Rotation rates of the rotary drum were held constant at 10 rpm. Vacuum pressure was controlled at the constant pressure of 385 Torr and 535 Torr, respectively. In this study, the system can be operated either in continuous or pulse mode in each experiments. Experiments show that in the case of high power level and continuous operating mode causes greater damage to the structure of tea leaves sample. Microwave drying with pulse operating mode at 385 Torr ensured the shortest drying time and the best overall quality of dried tea leaves, and thus was chosen as the most appropriate technique for tea leaves drying.     

Simulation of the High Temperature Drying of a Pasty Product: On the Influence of the Local Air Flow and the Thermal Radiation

This article deals with modeling and simulation of the industrial drying and calcination of a previously centrifuged pasty product. An experimental approach of this product is first carried out in terms of both product solvent content and product temperature during drying; using a previously designed devoted drying pilot system. An empirical model describing the product drying kinetics as a function of the industrial relevant controlled parameters is then derived. This model is based on the distinction of the convective power and radiative power supplies influence on the isenthalpic drying mass flux value; for this reason, it is suitable to up-scale the experimental product drying behavior from the pilot geometry to the industrial drier-calcinator geometry. This up-scaling approach, when combined with a classic mass and energy drying model, leads to derive a predictive tool for the product evolution in this industrial system.     

Experimental study of the performance of a cooling tower used in a solar distiller

In this work, we investigated experimentally the thermal performance of a forced cooling tower used in a solar desalination system based on humidification-dehumidification of air. The cooling tower is a counter flow wet one filled with film packing materials.The measured variables were obtained for wide ranges of mass flow rates of air and water as well as for several inlet water temperatures; the tower characteristic and efficiency were then evaluated and expressed in terms of water to air mass flow rate ratio.     

Simultaneous effects of chemical reaction and Ohmic heating with heat and mass transfer over a stretching surface: A numerical study

In this article,we have considered the simultaneous influence of ohmic heating and chemical reaction on heat and mass transfer over a stretching sheet.The effects of applied magnetic field are also taken into consideration while the induced magnetic field is not considered due to very small magnetics Reynolds number.The governing flow problem comprises of momentum,continuity,thermal energy and concentration equation which are transformed into highly nonlinear coupled ordinary differential equations by means of similarity transforms,which are then,solved numerically with the help of Successive Linearization method (SLM) and Chebyshev Spectral collocation method.Numerical values of skin friction coefficient,local Nusselt number,and Sherwood number are also taken into account with the help of tables.The physical influence of the involved parameters of flow velocity,temperature and concentration distribution is discussed and demonstrated graphically.The numerical comparison is also presented with the existing published results and found that the present results are in excellent agreement which also confirms the validity of the present methodology.     

Four stages of the simultaneous mass and heat transfer during bubble formation and rise in a bubbly absorber

We studied nonisothermal absorption of a solvable gas from growing at an orifice and rising bubble when the concentration level of the absorbate in the absorbent is finite (finite dilution of absorbate approximation). It is shown that simultaneous heat and mass transfer at all stages of bubble growth and rise in a bubbly absorber can be described by a system of generalized equations of nonstationary convective diffusion and energy balance. Solutions of diffusion and energy balance equations are obtained in the exact analytical form. Coupled thermal effects during absorption and absorbate concentration level effect on the rate of mass transfer are investigated. It is found that the rate of mass transfer between a bubble and a fluid increases with the increase of the absorbate concentration level. The suggested approach is valid for high Peclet, Prandtl and Schmidt numbers. It is shown that for the positive dimensionless heat of absorption K thermal effects cause the increase of the mass transfer rate in comparison with the isothermal case. On the contrary, for negative K thermal effects cause the decrease of the mass transfer rate in comparison with the isothermal case. The latter effect becomes more pronounced with the increase of the concentration level of the absorbate in the absorbent. Theoretical results are consistent with the experiments of Kang et al. (Int. J. Refrigeration 25 (2002) 127) for absorption from ammonia gas bubbles rising in water and aqueous ammonia solutions.     

Combined Convective and Microwave Drying of Agglomerated Sand: Internal Transfer Modeling with the Gas Pressure Effect

In order to improve the industrial drying (hot air and microwave) of inserts made of agglomerated sand, a comprehensive internal heat and water transfer model has been proposed. In this model, the internal gas phase pressure effect was made perfectly explicit, especially the phenomena of liquid and vapour transfer by filtration and of liquid expulsion at the surface. This model was validated on the basis of the experimental mean water content and core temperature curves for drying trials at different microwave powers. Then, it was used for comparing the drying time and the internal pressure level calculated for four particular processes: a standard process with high temperature air applied all over the time, a strong process with high power microwaves applied all over, and two processes which alternate the two heating modes. It was demonstrated that the combined and alternative processes provide a real possibility for faster drying with less internal pressure and thus with less cracking risk. The microwaves should be applied only in the first hour of the process and with decreasing power. The decrease of the drying time was around 30% with regard to the hot air standard process.