(in English) Abstract

This paper presents the results from the numerical study of the forward smoldering combustion process. The study is based on the transient model developed at University of Texas at Austin but extended with some modifications. In the model, the equations of conservation of energy and mass are solved. The chemistry is represented by a simplified scheme which consists of three reactions. Equations are discretized in space and solved in time. Neither thermal nor chemical equilibrium between solid and gas phases is assumed. The more important extensions made to the original model are: radiation heat transfer is included, a new ignition process is implemented and the porous structure of the solid is reconsidered. The extensions to the original model were implemented in order to predict the experimental data. The model with its extensions has been calibrated at a higher inlet air velocity and validated. The results show that the model predicts reasonably well the velocity of propagation of the smoldering front.

(in Spanish) Estudio numerico de combustion latente en flujo directo

Resumen

Este articulo presenta los resultados del estudio numerico del proceso de combustion latente en flujo directo. La combustion latente es una reaccion exotermica sin llama que se propaga en combustibles porosos. El estudio se basa en el modelo transitorio desarrollado en la Universidad de Texas en Austin, pero ampliado con varias modificaciones. En el modelo se resuelven las ecuaciones de conservacion de la energia y la masa. La cinetica quimica se modela con un esquema simplificado de tres reacciones. Las ecuaciones diferenciales se discretizan en el espacio y se resuelven en funcion del tiempo. El modelo no fuerza el equilibrio ni termico ni quimico entre las fases solida y gaseosa. Las modificaciones mas importantes realizadas son la inclusion de la transmision del calor por radiacion, la incorporacion de un nuevo proceso de ignicion y la reconsideración de la microestructura de los poros del solido. El objetivo de las ampliaciones es completar y adaptar el modelo a los experimentos publicados para comparar resultados. El modelo se calibra de nuevo a una velocidad mayor del aire de entrada. Los resultados muestran que se estima adecuadamente la velocidad de propagacion del frente de combustion.

In this work, the kinetic parameters governing the thermal and oxidative degradation of flexible polyurethane foam are determined using thermogravimetric data and a genetic algorithm. These kinetic parameters are needed in the theoretical modeling of the foam’s smoldering behavior. Experimental thermogravimetric mass-loss data are used to explore the kinetics of polyurethane foam and to propose a mechanism consisting of five reactions. A lumped model of solid mass-loss based on Arrhenius-type reaction rates and the five-step mechanism is developed to predict the polyurethane thermal degradation. The predictions are compared to the thermogravimetric measurements, and using a genetic algorithm, the method finds the kinetic and stoichiometric parameters that provide the best agreement between the lumped model and the experiments. To date, no study has attempted to describe both forward and opposed smolder-propagation with the same kinetic mechanism. Thus, in order to verify that the polyurethane kinetics determined from thermogravimetric experiments can be used to describe the reactions involved in polyurethane smoldering combustion, the five-step mechanism and its kinetic parameters are incorporated into a simple species model of smoldering combustion. It is shown that the species model agrees with experimental observations and that it captures phenomenologically the spatial distribution of the different species and the reactions in the vicinity of the front, for both forward and opposed propagation. The results indicate that the kinetic scheme proposed here is the first one to describe smoldering combustion of polyurethane in both propagation modes.

Results are presented from a model of forward smoldering combustion of polyurethane foam in microgravity. The transient one-dimensional numerical-model is based on that developed at the University of Texas at Austin. The conservation equations of energy, species and mass in the porous solid and in the gas phases are numerically solved. The solid and the gas phase are not assumed to be in thermal or in chemical equilibrium. The chemical reactions modeled consist of foam oxidation and pyrolysis reactions, as well as char oxidation. The model has been modified to account for new polyurethane kinetics parameters and radial heat losses to the surrounding environment. The kinetics parameters are extracted from thermogravimetric analyses published in the literature and using Genetic Algorithms as the optimization technique. The model results are compared with previous tests of forward smoldering combustion in microgravity conducted aboard the NASA Space Shuttle. The model calculates well the propagation velocities and the overall smoldering characteristics. Direct comparison of the solution with the experimental temperature profiles shows that the model predicts well these profiles at high temperature, but not as well at lower temperatures. The effect of inlet gas velocity is examined and the minimum airflow for ignition identified. It is remarkable that this one-dimensional model with simplified kinetics is capable of predicting cases of smolder ignition but with no self-propagation away from the igniter region. The model is used for better understanding of the controlling mechanisms of smolder combustion for the purpose of fire safety, both in microgravity and normal gravity, and to extend the unique microgravity data to wider conditions avoiding the high cost of space-based experiments.

An experimental investigation on the effects of buoyancy on opposed-flow smolder is presented. Tests were conducted on cylindrical samples of open-cell, unretarded polyurethane foams at a range of ambient pressures using the Microgravity Smoldering Combustion (MSC) experimental apparatus. The samples were tested in the opposed configuration, in which the flow of oxidizer is induced in the opposite direction of the propagation of the Smolder front. These data were compared with opposed-forced-flow tests conducted aboard STS-69, STS-77, and STS-105 and their ground-based simulations. Thermal measurements were made of the smolder reaction to obtain peak reaction temperatures and smolder velocities as a function of the ambient pressure in the MSC chamber. The smolder reaction was also observed using high-frequency ultrasound pulses as part of the ultrasound imaging system (UIS). The UIS measurements were used Lis a second means of providing smolder propagation velocities Lis well as to obtain permeabilities of the reacting samples. Results of forced-flow testing in normal gravity were compared to results in microgravity at a range of ambient pressures and forced flows. Results indicate that a critical oxidizer mass flux of roughly 0.5 to 0.8 g/m(2)s is required in normal gravity for a self-sustaining propagation in this configuration. In microgravity tests, self-sustained smolder propagation Was observed at a significantly lower oxidizer mass flux of 0.30g/m(2)s. Analysis Suggests that the removal of buoyancy-induced heat losses in microgravity allows for self-sustained propagation at an oxidizer mass flux below file critical value observed in normal-gravity testing. Normal-gravity tests also show that the smolder propagation velocity is linearly dependent oil the total oxidizer mass flux in an oxidizer-limited regime. Pressure effects on the chemical kinetics of a smolder reaction are interred by comparison of normal-gravity and microgravity tests and believed to be only weakly dependent oil Pressure (similar top(1/3)).