Farid Chejne

An experimental study on gasification of Colombian coal in fluidised bed

The main results of an experimental work on gasification of Colombian coal in a fluidised bed are reported in this paper. Experiments were carried out at different steam/coal (Fs/Fc) and air/coal (Fa/Fc) ratios and temperatures of gasifying agent. In addition, the influence of bed temperature on coal conversion was analysed. Results show a maximum value in the curve of high heating value versus Fa/Fc. From the environmental standpoint, low concentrations of sulphur compounds were obtained but more work should be done in order to decrease particulate matter.

Modelling and simulation of coal gasification process in fluidised bed

Abstract A one-dimensional steady state mathematical model and a numerical algorithm have been developed to simulate the coal gasification process in fluidised bed. The model incorporates two phases, the solid and the gas. The gaseous phase participates in the emulsion (with the solid phase) and forms the bubble. The solid phase is composed of carbonaceous material, limestone and/or inert bed material. The model can predict temperature, converted fraction, and particle size distribution for the solid phase. For the gaseous phase, in both emulsion and bubble, it can predict profiles of temperature, gas composition, velocities, and other fluid-dynamic parameters. In the feed zone, a Gaussian distribution for the solid particle size is considered. This distribution changes due to attrition, elutriation, consumption and drag inside the reactor. A system of 29 differential and 10 non-linear equations, derived from the mass, energy and momentum balances for each phase, at any point along the bed height, are solved by the Gear and Adams Method. Experimental data from the Universidad de Antioquia and Universidad Nacional-Medellin have been used to validate the model. Finally, the model can be used to optimise the gasification process by varying several parameters, such as excess of air, particle size distribution, coal type, and geometry of the reactor.

Multi-domain dual reciprocity BEM approach for the Navier–Stokes system of equations

This work presents a subdomain decomposition method as an alternative to improve the performance of the dual reciprocity boundary element method (DRBEM) in the BEM numerical solution of the Navier–Stokes equations. In the traditional DRBEM, the domain integrals that arise from the non-linear terms in the Navier–Stokes equations are approximated by a series of particular solutions and a set of collocation nodes distributed over the integration domain. In the present approach a subdomain technique is used in which the integration domain is divided into small quadrilateral elements whose four edges are linear discontinous boundary elements. The domain integrals in each subdomain are transformed into boundary integrals by dual reciprocity with augmented thin-plate splines i.e. r2 log r, plus three additional linear terms from a Pascal triangle expansion. It will be shown that this multi-domain technique is efficient and promising for the solution of high Reynolds number problems. Copyright

A rapid and novel approach for predicting water sorption isotherms and isosteric heats of different meat types

A rapid and novel approach for predicting sorption isotherms based on the Polanyi theory is proposed. This approach allows the prediction of the sorption isotherms at different temperatures from one experimental isotherm. The theoretical predictions of isotherms and isosteric heats were validated successfully using data from the literature for different meat types. This method allows total experimental time and operation costs to be reduced.

Modelling and simulation of time-dependent coal combustion processes in stacks

Abstract A time-dependent mathematical model and a numerical algorithm have been developed to simulate the combustion of piled coal particles. The model can predict the evolution profiles of unburned solid fraction along the bed height, the gas composition, heat of reaction, gas temperature and the coal (solid phase) temperature. Also, it predicts the radial temperature profile inside the particles. The model includes a system of six differential equations derived from the mass and energy balances for all phases at any point along the bed height and within each particle. For the numerical solution, implicit collocation and relaxation techniques were used with finite differences for the time advance. Additionally, the model can be used to optimise the combustion process varying the excess of air, particle size distribution, coal type, geometry of the reactor, different types of ignition mechanisms and the velocity of the grate.

A New Model for Predicting Sorption Isotherm of Water in Foods

A new model for predicting sorption isotherms of type II and III based on the Polanyi theory is proposed. This model allows the prediction of the sorption isotherms at different temperatures from one experimental isotherm. The theoretical predictions of isotherms and isosteric heat were validated successfully using data from literature for twenty one foods. This method allows total experimental time and operation costs to be reduced.

Mathematical Model and Simulation of a Thermal Diffusion Column

A two-dimensional mathematical model, in stationary state was developed for the separation of species in a thermal diffusion column working with total reflux. The model was applied to a gas mixture of CO 2 and N 2 . The existence of convective currents along the column was verified, the separation of species due to the temperature gradient was predicted and the effect of the thermal gradient and of the operation pressures on the composition profiles was analysed.

Non-linear phenomena in thermoacoustic engines

Abstract A non-linear simplified model for a thermoacoustic engine is presented in this work, which assumes the form for the spatial dependence of the state variables and considers only effects inside the stack. For such a model the limit cycle solution is found numerically using a shooting technique in the ∇T > ∇T crit region, where the stationary, fixed-point, trivial solution is unstable. With this solution a bifurcation diagram for thermoacoustic phenomena is built. Through such diagram an analysis of the non-linear nature of the generated oscillations is conducted, focusing on the presence of harmonics and the higher-order time-average pressure.

Multi‐domain DRM boundary element method for non‐isothermal non‐Newtonian Stokes flow with viscous dissipation

This paper presents a boundary element method (BEM) based on a subdomain approach for the solution of non‐Newtonian fluid flow problems which include thermal effects and viscous dissipation. The volume integral arising from non‐linear terms is converted into equivalent boundary integrals by the multi‐domain dual reciprocity method (MD‐DRM) in each subdomain. Augmented thin plate splines interpolation functions are used for the approximation of field variables. The iterative numerical formulation is achieved by viewing the material as divided into small elements and on each of them the integral representation formulae for the velocity and temperature are applied and discretised using linear boundary elements. The final system of non‐linear algebraic equations is solved by a modified Newtons method. The numerical examples include non‐Newtonian problems with viscous dissipation, temperature‐dependent viscosity and natural convection due to bouyancy forces.

Nanoscale hydrodynamics near solids

Density Functional Theory (DFT) is a successful and well-established theory for the study of the structure of simple and complex fluids at equilibrium. The theory has been generalized to dynamical situations when the underlying dynamics is diffusive as in, for example, colloidal systems. However, there is no such a clear foundation for Dynamic DFT (DDFT) for the case of simple fluids in contact with solid walls. In this work, we derive DDFT for simple fluids by including not only the mass density field but also the momentum density field of the fluid. The standard projection operator method based on the Kawasaki-Gunton operator is used for deriving the equations for the average value of these fields. The solid is described as featureless under the assumption that all the internal degrees of freedom of the solid relax much faster than those of the fluid (solid elasticity is irrelevant). The fluid moves according to a set of non-local hydrodynamic equations that include explicitly the forces due to the solid. These forces are of two types, reversible forces emerging from the free energy density functional, and accounting for impenetrability of the solid, and irreversible forces that involve the velocity of both the fluid and the solid. These forces are localized in the vicinity of the solid surface. The resulting hydrodynamic equations should allow one to study dynamical regimes of simple fluids in contact with solid objects in isothermal situations.