Estudo da fluidodinâmica em leito fixo utilizando-se métodos numéricos de solução: escoamento monofásico de ar.

AUTOR(ES)

Rodrigo Béttega

DATA DE PUBLICAÇÃO

2006

RESUMO

Studies accomplished in the structure of tubular fixed beds proved that the porosity in these systems presents an oscillatory behavior along the radial direction. The non-uniformity of the porosity generates a behavior of the velocity profile, inside the bed, characterized by oscillations in the central region and a peak of larger width in the region close to the wall. This velocity profile determines the residence time of the reactants, for example, and presents a strong influence in the heat and mass transfer coefficients of these systems. In spite of the progress obtained in that subject, it is still difficult to obtain the measure of flow velocity inside the bed without causing disturbances in its structure. Due to this restriction, indirect forms of experimental measure of the velocity distribution in these systems have been proposed to evaluate the fluid dynamics inside the bed. The most common way to obtain, experimentally, the velocity distribution inside the bed is to measure it outside the bed, at a certain distance from its exit, and then infer it inside the bed by considering that the flow measured at this point maintains the fluid dynamics characteristics at the interface. Looking for an alternative way to infer the velocity profile inside the fixed bed, this work proposes a measure of the change in flow characteristics from the exit of the fixed bed to the point of experimental measurement. The measure proposed before takes into account the change in the radial mean velocity evaluated over an area close to the central part of the flow. This measure is based on simulated results accomplished between these two points, located outside the fixed bed, from an appropriated boundary condition at the interface. This boundary condition at interface is the result of other simulations performed inside the fixed bed, from a convenient model proposed for this region. Both simulations were accomplished by using a simulator developed in FORTRAN language, based on a CFD numeric technique for outside and a standard finite difference method for inside the fixed bed. The validity of the proposed procedure was verified by comparing the velocity distribution simulated at a position pointed out in an experimental measurement of the velocity distribution obtained in the literature, and with similar operational conditions. By doing this validation, it was established a relationship between the change in flow characteristics and the axial position of measurement. As a result, it was possible to infer how much of the flow characteristics in the interface are observed from the position chose for measurement along the axial direction of the flow. Better results were observed in systems with a high dimensionless D/Dp relation and low values of Reynolds number for the flow simulations accomplished in the exit area of the bed. Deviations from experimental data, varying between 2,97% and 30,23%, were verified for different operational conditions of the system. Through the central velocity simulation along in the empty tube axial position, were possible to map the empty tube influences on the flow in the fixed bed exit area, evaluating the difference of that central medium velocity with the calculated for the bed exit.

ASSUNTO(S)

leito fixo escoamento monofásico métodos numéricos simulaçõa sistemas particulados engenharia quimica

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