A gas-solid reaction model — Variable temperature,pressure and structural parameters

Diffusion and reactions in a porous pellet are treated as transient, nonisothermal, nonisobaric processes. Continuity and energy balance equations are solved simultaneously for temperature and concentration profiles in the pellet. Conductive, convective as well as radiative heat transfer are included. The ‘dusty gas’ flux model is used to describe the transport of diffusing gases. Viscous, bulk and concentration gradient terms have been included. Structural changes with reaction are accounted by considering the effect of changing grain radius on porosity and pore diameter. The model predictions match conversion trends for carbon gasification over a temperature range of 800 to 1100°C investigated experimentally.     

Influence of heat treatment on the activity and structure of CoTETA/C catalysts for oxygen reduction reaction

The influence of heat treatment on the improvement of the catalytic activity of CoTETA/C catalysts is investigated. These non-precious metal oxygen reduction catalysts are prepared from carbon-supported cobalt triethylenetetramine (CoTETA/C) and heat treated in the temperature interval from 500 to 1000 °C in Ar atmosphere. Electrochemical characteristics are demonstrated in oxygen-saturated acid electrolyte by rotating disk electrode, cyclic voltammetry, as well as single fuel cell tests. The results show that the effect of heat treatment is important on the catalytic activity of CoTETA/C catalysts for the ORR and a maximum catalytic activity is obtained after heat treatment at 800 °C. The ORR reaction mechanism on the catalysts heat treated at 700, 800 and 900 °C is mainly through a 4e reaction path, while a 2e reaction is dominant on the catalysts heat treated at 500, 600 and 1000 °C. Tafel slopes of the CoTETA/C catalysts are all around −200 mV/dec. X-ray absorption measurements reveal that the CoN₄ centers are no longer detected after heat treatment. XRD results clearly confirm the formation of nanometallic α-Co with different sizes aggregated. A possible interpretation of the catalytic active sites is also discussed.     

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.     

Application of different CFD multiphase models to investigate effects of baffles and nanoparticles on heat transfer enhancement

In this work, the effect of baffles in a pipe on heat transfer enhancement was studied using computational fluid dynamics (CFD) in the presence of Al₂O₃ nanoparticles which are dispersed into water. Fluid flow through the horizontal tube with uniform heat flux was simulated numerically and three dimensional governing partial differential equations were solved. To find an accurate model for CFD simulations, the results obtained by the single phase were compared with those obtained by three different multiphase models including Eulerian, mixture and volume of fluid (VOF) at Reynolds numbers in range of 600 to 3000, and two different nanoparticle concentrations (1% and 1.6%). It was found that multiphase models could better predict the heat transfer in nanofluids. The effect of baffles on heat transfer of nanofluid flow was also investigated through a baffled geometry. The numerical results show that at Reynolds numbers in the range of 600 to 2100, the heat transfer of nanofluid flowing in the geometry without baffle is greater than that of water flowing through a tube with baffle, whereas the difference between these effects (nanofluid and baffle) decreases with increasing the Reynolds number. At higher Reynolds numbers (2100–3000) the baffle has a greater effect on heat transfer enhancement than the nanofluid.     

A numerical study of pellet model consistency with respect to molar and mass average velocities,pressure gradients and porosity models for methanol synthesis process: Effects of flux models on reactor performance

The objective of this work is to compare mass- and mole based diffusion flux models, convection, fluid velocity and pore structure models for methanol synthesis process. Steady-state models have been derived and solved using least-squares spectral method (LSM) to describe the evolution of species composition, pressure, velocity, total concentration and diffusion fluxes in porous pellets for methanol synthesis. Mass diffusion fluxes are described according to the rigorous Maxwell Stefan model, dusty gas model and the more simple Wilke model. These fluxes are defined with respect to molar- and mass averaged velocities. The different effects of choosing the random- and parallel pore models have been investigated. The effects of Knudsen diffusion have been investigated. The result varies significantly in the dusty gas model. The effectiveness factors have been calculated for the methanol synthesis process for both mass- and mole based pellet models. The values of effectiveness factors for both mass- and mole based pellet models do not vary so much. The effect of Wilke-, Maxwell–Stefan- and dusty gas mass diffusion fluxes on the reactor performance have been studied. Steady-state heterogeneous fixed bed reactor model is derived where the intra-particle mass diffusion fluxes in the voids of the pellet are described by Wilke-, Maxwell–Stefan- and dusty gas models. Furthermore, the total computational efficiency of the heterogeneous fixed bed reactor model is calculated with several closures for the intra-particle mass diffusion fluxes. The model evaluations revealed that:

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The mass- and mole based pellet models are not completely consistent. However, the small deviation (less than 2%) between mass- and mole based pellet models is due to the model equations are not fully consistent. If one pellet model is to be chosen for the methanol synthesis process, the optimal diffusion flux model is the Maxwell–Stefan model.