I am developing a gas transfer model for the heating of the stone wool barrier. The system consists of a composite slab (1 mm thin steel plate + l meter thick stone wool). The stone wool material is a fibrous structure with high porosity (\Phi>0.95). one side of the stone wool consists of a steel plate and the other side is fully exposed to the atmosphere. Thus is more close to an open atmosphere scenario. When the system is heated by emitting radiative fluxes on the steel plate, the stone wool material heats up and the organic material (resin used for binding the fibers) decomposes and produces the gas. The gas produces diffuses to the open boundary. The decomposition reaction occurs as long as the air is available in the porous region. The question is whether the gas transfer is purely diffusive or not? Could there be pressure difference in such a highly porous material having an open boundary to the atmosphere? If there is pressure difference then there is velocity and the transfer is not diffusive alone. However for the one-dimensional model, if there is any, what could be the cause for the pressure difference? Or is there any factor that causes the gas convection, or causes the velocity of the gas?
At least for room temperatures the rock wool is assumed to behave isothermal. The soundspeed is much lower than in the air, about a tenth. There are impecance differences between the mineral wool and the sorrounnding air that can create reflections, and thus modal response. The sound absorption of mineral wool is well described by Mechels model. But I do not know the anwer to your real question if the transport mechanism is purely diffusive or also convective. I would believe convection can happen, and that the airflow resistivity in the mineral wool is essential in solving your problem, as it is for sound absorption.
First the thermal decomposition of resin is due to pyrolysis, the presence of oxygen is not required. The system is convective in the sense that volume of gas is created and transported to free surface, carrying oxygen outside material. If the entering heat flux is not too important one can suppose the convection quasi-stationary and the created gas in thermal equilibrium with fibers at any position in the material. Then the resulting problem is a thermal problem with source term. This source term is a result of a coupled equation for pyrolysis. This kind of problem is treated in my book "Ablative Thermal Protection Systems Modeling".
Dear Georges,
For high heat flux, there is resin thermal decomposition (Pyrolysis). how can there be pyrolysis without oxygen? The heat-flux is very important and the thermal energy of fibers heavily depends upon the porous media concentration of product gas. There is pyrolysis only if there is air (oxygen+nitrogen) and if the product gas is too much the air is limited for the exothermic reaction do not occur.
The pyrolysis is a pure thermal decomposition and don't need oxygen. When I say not too important fluxes that means less than 100 kW/m^2, typically.
Dear Georges,
Thank you, Just acknowledge that the pyrolysis is without oxygen. The experiment shows that there is bell-shaped temperature peaks on the cold side. I.e., Apart from a parabolic temperature curve, there are peaks due to heat release from the chemical reaction. The decomposition is exothermic. For exothermic reactions oxidation is necessary. The hot side is exposed to ISO standard fire, i.e, ISO834 curve. Rising from 20 to up to 1000 C.
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