Étude numérique du modèle de Boussinesq de convection naturelle laminaire transitoire dans un ellipsoide de révolution de grand axe vertical rempli d'air

The authors observed the transition of the system between two states of equilibrium by solving, with the help of a finite-difference method, the thermal unsteady laminar natural convection equations in an axisymmetric ellipsoid with vertical great axis filled with air. They showed that the number of convective cells varies from one to one by passing through a maximal value that depends on the Grashof number and on the aspect ratio. Simultaneously, the instantaneous average Nusselt number decreases in a monotonous way with time when the flow is unicellular and increases or decreases abruptly with each appearance or disappearance of cells.     

Étude numérique de l'influence du transfert radiatif sur la convection naturelle laminaire,bidimensionnelle, permanente,dans un espace annulaire d'axe horizontal,délimité par deux cylindres circulaires isothermes

Combined radiation and natural convection in a participating medium between concentric or vertically eccentric horizontal cylinders is investigated numerically. The annular medium is considered as a gray, emitting, absorbing, and isotropically scattering gas. The equations of steady, laminar, two-dimensional, thermal, natural convection are written by using a two-cylindrical coordinate system, the stream function, and the vorticity. The finite volumes method is used to discretize the coupled equations of momentum, energy, and radiative transfer. To solve the global nonlinear algebraic equations the successive-over-relaxation iterating scheme is applied. Numerical solutions are obtained for a Rayleigh number in the range 10³–10⁵ and radiation-conduction parameter ranging from 0 to ∞. The influences of radiation-conduction parameter, Rayleigh number and other parameters on flow and temperature distributions and heat transfer are discussed.     

Auto-inflammation de substances explosives. Comparaison entre calcul analytique et numérique en vue d'une optimisation dans le domaine de la pyrotechnie

Self-ignition of energetic material was investigated in order to optimize safety in the field of pyrotechnic applications. Two approaches were used; the first one is relative to Frank-Kamenetskii stationary thermal explosion theory. The second approach consists of a choice of some numerical solutions of heat conduction equations in a non-stationary state. Comparison between these results was carried out in order to find the numerical scheme which is the most compatible with Frank-Kamenetskii stationary thermal explosion theory. Numerical data were used for three explosive substances. One of them was studied by the author. In all cases, the numerical stationary state is in agreement with the Frank-Kamenetskii stationary thermal explosion theory, more or less accurately. From this comparison, it may be concluded that it is preferable, for this kind of problem, to use an implicit scheme with linearization of the heat source term. Explicit numerical methods, with or without the addition of the heat term with the Zinn and Mader scheme are revealed to be less accurate and to need a greater optimization of spatial and temporal meshing.     

Étude de deux cas d'écoulements thermoconvectifs le long d'une plaque verticale non uniformément chauffée dans une cavité

Study for two types of thermoconvective flows along a non-isothermal vertical wall in a cavity. For the study of the natural convection along a non-uniformly heated vertical wall in a cavity, we experimentally defined thermal curves. These curves were corrected by our semi-analytical corrective model. The two configurations, the first one instable, with a warm part lower to a cold part, and the other one, with warm and cold parts inverted are studied. We calculated convective flux densities and we showed a different comportment according to the configuration. We proposed some values for the convective exchange coefficient in the case of regular profiles with no extreme. We recommended some correlations for the stable areas of the flow close to the active wall.     

Etude du désenfumage par extraction mécanique dans un local siège d'un feu: expériences en grandeur et simulations numériques par modèles de zones et de champs

This study regards conditions of control of fire-induced smoke in a 7-m × 7-m rectangular cell, for an ‘intermediate configuration’ of a volume larger than that of an apartment room but smaller than that of an entrance hall, of a fire source a little less powerful than ‘design fires’ considered in standard approaches, of a fan-powered exhaust rate a little higher than rates typically required in regulations.From case to case were experimentally modified the following parameters: ceiling height (3 m, or 6.4 m), source type and power (between 100 and 500 kW), exhaust volume flow rate (from a few tenths of m³.s⁻¹ to a few m³.s⁻¹). Each experiment was supposed to reach a steady-state configuration, which has proved more or less true in practice. Smoke stratification within the cell has proved less sharp for a fire source non located in the centre of the cell, and the mean temperature of gases in the exhaust inlet has proved lower than under the cell ceiling, which suggested a phenomenon of ‘plugholing’ (air mixing upstream). In spite of the weaknesses of the computer models, and of a certain unaccuracy of measurements, plugholing phenomenon could be recognized in simulation results yielded by the zone model (which were in a better accordance with test results for higher exhaust rates), since the field model gave rise to a better agreement between computed and measured results for lower heat release rates and/or higher rates of smoke exhaust.