Fernando Cesar De Lai

Numerical Investigation of Natural Convection in Heterogeneous Rectangular Enclosures

The aim of this study is to investigate numerically the steady natural convection resulting from horizontally heating a rectangular enclosure filled with a fluid and containing uniformly distributed, conducting, fixed and disconnected solid blocks (i.e., not touching each other). In particular, the effects of solid volume fraction, solid–fluid thermal conductivity ratio, and total number of blocks on the heat transfer process are determined by solving the mass, momentum, and energy conservation equations using the finite-volume method. The enclosure aspect ratio is varied from 0.25 to 4, the Rayleigh number from 105 to 108, and the Prandtl number is set as unity. Results of the numerical simulations are presented in terms of the surface-averaged Nusselt number, streamfunction, and streamline and isotherm distributions. The interference phenomenon caused by the solid blocks on the natural convection process is considered in detail and used to explain and predict the surprising and complex behavior of surface-averaged Nusselt number.

SIMULAÇÃO NUMÉRICA DE ESCOAMENTO PARTICULADO APLICADO AO PREENCHIMENTO DE CANAL FRATURADO

RESUMO No presente trabalho, uma analise numerica do preenchimento de um canal vertical com fratura transversal e apresentada. A abordagem escolhida para o preenchimento consiste em adicionar material particulado ao escoamento para promover a deposicao de particula no interior da fratura, reduzindo a fuga de fluido nessa regiao. A analise e realizada atraves de uma abordagem euleriana para o fluido e lagrangiana para as particulas. Para efetuar o acoplamento entre as fases e realizar o calculo das colisoes entre as particulas os modelos DDPM (Dense Discrete Phase Model) e DEM (Discrete Element Method) sao combinados. O resultado do processo de preenchimento, estudado atraves da variacao da quantidade de particulas injetadas no canal, e analisado atraves das caracteristicas geometricas do leito de particulas formado no interior da fratura, da vazao de fluido sendo perdido pela fratura e atraves do monitoramento da pressao na entrada do canal devido ao processo de injecao. Os resultados mostram que a influencia da quantidade de particulas injetadas exerce maior influencia sobre o tempo necessario para realizar o preenchimento. Assim, a concentracao reduz o tempo de preenchimento, porem implica no aumento da pressao na regiao de entrada do canal.

The Effects of Solid Thermal Conductivity and Volume-Fraction in the Natural Convection Inside a Heterogeneous Enclosure

In this study, the natural convection inside a fluid filled enclosure containing several solid obstructions and heated from the side is simulated numerically as to determine the effects of the solid thermal conductivity and volume-fraction. The solid obstructions are conducting, disconnected square blocks, uniformly distributed inside the enclosure. The mathematical model follows a continuum approach, with balance equations of mass, momentum and energy presented for each one of the constituents (i.e., fluid and solid) inside the enclosure. The equations are then solved numerically via the finite-volume method. The effects of varying the solid-fluid thermal conductivity ratio (K), the fluid volume-fraction or porosity (φ), the number of solid blocks (N) and the heating strength (represented by the Rayleigh number, Ra) on the natural convection process inside the enclosure are investigated parametrically. The Nusselt number based on the surface-averaged heat transfer coefficient along the heated wall is chosen to characterize the convection strength inside the enclosure. The results indicate a competing effect caused by the proximity of the solid blocks to the heated and cooled walls of the enclosures, vis-a-vis hindering the boundary layer growth, hence reducing the heat transfer effectiveness, and at the same time enhancing the heat transfer when K is large. An analytical estimate of the minimum number of blocks beyond which the convection hindrance becomes predominant is presented and validated by the numerical results.Copyright