Combustion Processes Laboratories

This dissertation presents the derivation, numerical implementation, and verification/validation of a generalized model that can be used to simulate the pyrolysis, gasification, and burning of a wide range of solid fuels encountered in fires. The model can be applied to noncharring and charring solids, composites, intumescent coatings, and smolder in porous media. Care is taken to make the model as general as possible, allowing the user to determine the appropriate level of complexity to include in a simulation. The model considers a user–specified number of gas phase and condensed phase species, each having its own temperature–dependent thermophysical properties. Any number of heterogeneous (gas–solid) or homogeneous (solid–solid or gas-gas) reactions can be specified. Both in–depth radiation transfer through semi–transparent media and radiation transport across pores are considered. Volume change (surface regression or swelling/intumescence) is handled by allowing the size of grid points to change as dictated by mass conservation. All volatiles generated inside the solid escape to the ambient with no resistance to mass transfer unless a pressure solver is invoked; the resultant flow of volatiles is then calculated according to Darcy’s law. A gas phase convective–diffusive solver can be invoked to determine the composition of the volatiles. Oxidative pyrolysis is simulated by modeling diffusion of oxygen from the ambient into the pyrolyzing solid where it may participate in reactions. Consequently, the mass flux and composition of volatiles escaping from the solid can be calculated. To aid in determining the required input parameters, the model is coupled to a genetic algorithm that can be used to estimate the required input parameters from bench–scale fire tests or thermogravimetric analysis.

Standalone model predictions are compared to experimental data for the thermo– oxidative decomposition of non–charring and charring solids, as well as the gasification and swelling of an intumescent coating and forward smolder propagation in polyurethane foam. Genetic algorithm optimization is used to extract the required input parameters from the experimental data, and the optimized model calculations agree well with the experimental data. Blind simulations indicate that the predictive capabilities of the model are generally good, particularly considering the complexity of the problems simulated.

This paper presents a generalized pyrolysis model that can simulate the gasification of noncharring, charring, and intumescent materials, as well as smoldering in porous media. Separate conservation equations are solved for gaseous and condensed phase mass and species, solid phase energy, and gas-phase momentum. An arbitrary number of gas-phase and condensed-phase species can be accommodated, each having its own temperature-dependent thermophysical properties. The user may specify any number of solid to gas, solid to solid, or solid + gas to solid + gas reactions of any order. Both in-depth radiation transfer through a semi-transparent medium as well as radiation transport across pores are considered, and melting is modeled using an apparent specific heat. All volatiles generated inside the solid escape to the ambient with no resistance to flow unless the pressure solver is invoked to solve for the pressure distribution in the solid, with the resultant flow of volatiles calculated according to Darcy’s law. Similarly, the user may invoke a gas-phase convective-diffusive solver that determines the composition of the volatiles, including diffusion of species from the ambient into the solid. Thus, in addition to calculating the mass-flux of volatiles escaping from the solid, the actual composition of the vapors can be predicted. To aid in determining the required material properties, the pyrolysis model is coupled to a genetic algorithm that can be used to estimate the required input parameters from bench-scale fire tests, thermogravimetric analysis, or a combination thereof. Model predictions are compared to experimental data for the thermo-oxidative decomposition of a non-charring solid (PMMA) and the thermal pyrolysis of a charring solid (white pine), as well as the gasification and swelling of an intumescent coating, and finally smoldering in polyurethane foam. The predictive capabilities of the model are shown to be generally quite good.

A computational study has been carried out to investigate smoldering ignition and propagation in polyurethane foam. The one-dimensional, transient, governing equations for smoldering combustion in a porous fuel are solved accounting for improved solid-phase chemical kinetics. A systematic methodology for the determination of solid-phase kinetics suitable for numerical models has been developed and applied to the simulation of smoldering combustion. This methodology consists in the correlation of a mathematical representation of a reaction mechanism with data from previous thermogravimetric experiments. Genetic-algorithm and trail-and-error techniques are used as the optimization procedure. The corresponding kinetic parameters for two different mechanisms of polyurethane foam smoldering kinetics are quantified: a previously proposed 3-step mechanism and a new 5-step mechanism. These kinetic mechanisms are used to model one-dimensional smoldering combustion, numerically solving for the solid-phase and gas-phase conservation equations in microgravity with a forced flow of oxidizer gas. The results from previously conducted microgravity experiments with flexible polyurethane foam are used for calibration and testing of the model predictive capabilities. Both forward and opposed smoldering configurations are examined. The model describes well both opposed and forward propagation. Specifically, the model predicts the reaction-front thermal and species structure, the onset of smoldering ignition, and the propagation rate. The model results reproduce the most important features of the smolder process and represent a significant step forward in smoldering combustion modeling.

Results are presented from an experimental study on the ignition of the combustion modified (fire retarded) polyurethane foam Pyrell® (35.3 kg/m3 and 64.0 kg/m3) in elevated oxygen concentrations, ranging from 30% to 60%. The samples are exposed to an external flow and variable radiant heat flux on one face, and insulated on the other faces. The experiments show that Pyrell undergoes a weak smoldering reaction that requires significant assistance in the form of external heat input in order to propagate. The results also show that given sufficient oxygen and radiant heat flux, the smoldering reaction can produce enough volatile fuel and heat to trigger a gas phase ignition, i.e. a transition from smoldering to flaming, in pores in the char region. The experiments also indicate that high-density Pyrell is more ignitable than low-density Pyrell, which could be explained by the greater solid surface area for smoldering reactions to take place.

Smoldering combustion is described as an exothermic superficial heterogeneous-reaction that can propagate in the interior of porous fuels. Smoldering is generally an incomplete combustion reaction, which leaves behind a porous char that contains significant amounts of unburned fuel. If compared to flaming combustion, the heat release and the temperature characteristics of smoldering are low and its propagation is a slow process. Besides its characteristics of a weak combustion process, smoldering poses serious risk to fire safety; it is a common fire initiation scenario, because it is difficult to detect as, it can go unnoticed for long periods of time, It yields a high conversion of fuel to toxic products, and it can suddenly switch to flaming combustion. The propagation of the smoldering front is usually controlled by two factors: oxygen availability and heat losses. However it’s the result of several interacting mechanisms, such as chemical reactions (pyrolysis and oxidation), convection and diffusion of heat and oxygen, heat transfer between the gas and the solid phases, heat losses to the surrounding and flow in a porous media. The developed axisymmetric two-dimensional model solves the governing equations for forward propagation of smoldering in a porous fuel bed. The conservation equations for energy in the solid and the gas phases are considered separately but interact through an interfacial heat exchange. Species conservation in the solid and the gas phases, and overall mass conservation are numerically solved. The heterogeneous chemical kinetics include pyrolysis of the fuel, and oxidation of the fuel and of the carbonaceous residual. The second oxidation is accompanied by the formation of solid ash and gas products, which pose the potential risk of transition from smoldering to flaming combustion. The obtained numerical results feature the gas and solid temperature evolutions, the char mass fraction evolution, and the smoldering propagation velocity. The finite volume method was used for the spatial discretization, and the implicit one for time. The obtained equations were solved by the Bi-Conjugate Gradient Stabilized (BI-CGSTAB) technique