Chris Lautenberger

NUMERICAL MODELING OF CONVECTIVE EFFECTS ON PILOTED IGNITION OF COMPOSITE MATERIALS

ABSTRACT A combined solid and gas-phase numerical model has been developed and used to simulate the piloted ignition of a radiatively heated composite material in a boundary layer oxidizer flow. Condensed phase processes (oxidative and thermal pyrolysis, phase change, and in-depth radiation absorption) are simulated with a pyrolysis model developed by the authors. The solid-phase model is coupled to a modified version of the CFD code Fire Dynamics Simulator (FDS), which provides a transient solution to the low Mach number, reactive, buoyant, Navier Stokes Equations. The condensed phase and gaseous chemical kinetics are simplified with one-step Arrhenius reactions. Ignition occurs when a premixed flame propagates upstream from the igniter and forms a diffusion flame anchored at the surface of the solid fuel. Numerical simulations have been performed for a polypropylene/fiber glass blended composite slab with a glass concentration of 30% by mass and thickness of 3.2 mm. The influence of the incident radiant heat flux, oxidizer flow velocity, and gravitational acceleration on piloted ignition of this composite material has been investigated. Emphasis is given to low-velocity microgravity flows expected in spacecraft. Ignition delay time predictions are compared with experimental data for several external heat flux levels and two flow velocities. The ignition time, pyrolysis rate at ignition, and surface temperature at ignition are considerably lower in a 0.09 m/s microgravity flow than in a 1.0 m/s normal gravity flow. This has important fire safety implications because it indicates that piloted ignition of solid combustibles will occur more easily under the conditions expected in spacecraft.

The Role of Decomposition Kinetics in Pyrolysis Modeling - Application to a Fire Retardant Polyester Composite

This work assesses the effect of decomposition kinetics on overall pyrolysis behavior using experimental data from thermogravimetric analysis (TGA) and Fire Propagation Apparatus (FPA) experiments. TGA data are presented for an unsaturated brominated polyester resin (reinforcement free), and the FPA is used to investigate the pyrolysis behavior of a fiber reinforced polymer (FRP) composite slab with matrix comprised of the same resin tested via TGA. Three different kinetic models are fit to the TGA data: singlestep n th order, 3-step n th order, and 3-step n th order with one autocatalytic step. These kinetics models are then used to simulate the pyrolysis of a composite slab in the FPA, with thermophysical properties estimated by genetic algorithm optimization. It is shown that the two 3-step mechanisms provide nearly identical calculations of total mass loss rate (MLR) in the FPA, while the single-step mechanism provides similar, but quantitatively different, MLR predictions. Although no broad conclusions regarding the importance of multi-step thermal decomposition kinetics can be drawn on the basis of a single study, detailed reaction mechanisms may be superfluous unless TGA curves show multiple distinct reaction peaks and/or all thermophysical properties/model input parameters are precisely known.

Approximate Analytical Solutions for the Transient Mass Loss Rate and Piloted Ignition Time of a Radiatively Heated Solid in the High Heat Flux Limit

An approximate closed form solution is developed for the mass loss rate of a semiinfinite solid irradiated by a constant net heat flux. The solution is valid at high heat flux levels where surface losses and the endothermic heat sink due to pyrolysis are small in comparison to the applied heat flux. The expression obtained for the mass loss rate is used to develop an explicit closed form relation for the time to piloted ignition using a critical mass flux as the ignition criterion. The resultant formula is identical to that obtained from the classical thermal ignition theory, with the important difference that the surface temperature at ignition is not constant. Rather, it increases with applied heat flux, mass flux at ignition, and activation temperature, and decreases with increasing density, pre-exponential factor, and thermal conductivity. The model predictions are compared to recent high-heat flux ignition measurements for PMMA.

Spot Fire Ignition of Natural Fuel Beds by Hot Metal Particles, Embers, and Sparks

Wildland and wildland/urban interface fires are a serious problem in many areas of the world. It is expected that with global warming the wildfire and wildland/urban interface fire problem will only intensify. The ignition of natural combustible material by hot metal particles or embers is an important fire ignition pathway by which wildland and urban spot fires are started. There are numerous cases reported of wild fires started by hot metal particles from clashing power lines, or from sparks generated by machines or engines. Similarly there are many cases reported of industrial fires caused by grinding and welding sparks. Despite the importance of the subject, the topic remains relatively unstudied. The senior author of this article and his collaborators have been working for the past few years on this problem. In this article, we provide a comprehensive summary of that work to date. The work includes experimental and theoretical modeling of the ability of hot metal particles and embers to cause the ignition of cellulosic fuel beds. The metal particles studied are representative of clashing conductors (aluminum and copper) and those produced by machine friction and hot work such as welding (stainless steel and brass). In addition glowing and flaming wood embers are considered, as they represent an important source of fire spotting in wildfires. The overall results show a hyperbolic relationship between particle size and temperature, with the larger particles requiring lower temperature to ignite the fuel bed than the smaller particles. An important finding is that although particle energy is important in the capability of the particle to ignite the fuel, both energy and temperature are determining factors of the particle ignition capabilities. The thermal properties of the metal play a lesser role with the exception of the energy of melting if it occurs. It also appears that the controlling ignition mechanisms by large particles are different than those from the small particles. The former appear to be determined primarily by the particle surface temperature while the latter by the particle energy and surface temperature. Sparks are a specific type of particles with very small sizes and very high temperatures. Because of the small sizes, their energy is small and it is postulated that the sparks must accumulate for ignition of a fuel bed to occur. The results with embers indicate that the smoldering is the easier form of ignition, although flaming ignition can occur if the ember is flaming and the air velocities are moderate. To provide further information about the fire spot ignition process, both analytical and numerical modeling are used and compared with the experimental results. Although the models provide qualitative predictions further development is necessary to reach quantitative predictive capabilities.

Effect of Environmental Variables on Flame Spread Rates in Microgravity

This paper reports 2D CFD-based computer modeling of opposed flow flame spread over thick samples of polymethylmethacrylate (PMMA). Model predictions are compared with experimental data from normal-gravity experiments at multiple forced flow velocities and KC-135 parabolic flight microgravity experiments. For the normal gravity experiments, good agreement between the model predictions and experimental data is obtained at one oxygen level, but flame spread rates at other oxygen levels are not well predicted. Of the four microgravity data points, the model underpredicts the spread rate of two of the data points by 35% or less. However, the model overpredicts the other two data points by almost a factor of two. Potential reasons for the discrepancies between the model predictions and the experimental data are discussed.

1 – Understanding materials flammability

Publisher Summary Flammability is the ease with which a material is ignited, the intensity with which it burns and releases heat once ignited, its propensity to spread fire, and the rate at which it generates smoke and toxic combustion products during gasification and burning. A comprehensive evaluation of a materials overall flammability may require data from several laboratory tests, perhaps combined with some form of analysis or modeling to interpret the results properly. Several of the fire properties like ignitability can be determined from bench-scale flammability tests. It is useful for establishing relative rankings or for developing input data for predictions of large-scale fire behavior. The cost of small-scale fire testing is considerably less than that of large-scale fire tests because relatively small quantities of sample material are required, and the setup and breakdown time is much shorter. These factors make bench-scale flammability testing a cost-effective screening tool and can reduce a new materials time-to-market. Due to potential time and cost savings, combined with an increased recognition of the importance of material fire properties, there is considerable interest in using data obtained from small-scale flammability tests in conjunction with correlations or models to predict large-scale fire behavior.

Spotting Ignition Of Fuel Beds By Firebrands

Wind can carry fire-lofted embers or molten/burning metal particles generated by powerline interactions long distances, where they may land on and ignite fuel beds remote from the source. This process, known as spotting, is a common mechanism of wildland and wildland urban interface fire propagation. The physical processes leading to spot fire initiation after an ember or heated particle has landed are not yet quantitatively understood. To provide insight into spot fire initiation, this paper presents a comprehensive 2D numerical model for the potential ignition of a porous fuel bed by an ember or hot metal particle. The model consists of a computational fluid dynamics (CFDs) representation of the gas-phase coupled to a heat transfer and pyrolysis model that simulates condensed-phase phenomena. The coupled model is used to simulate ignition of a powdered cellulose porous fuel bed by glowing pine embers in a laboratory experiment. The model provides qualitative information regarding the mechanisms that lead to ignition, smolder, or flame propagation on a porous fuel bed that agree qualitatively with experimental observations. This work provides the foundation for a more complete study of the problem where the effects of different factors (moisture content, humidity, temperature, porosity, particle size/heat content, etc.) are quantified.

A multi-component dataset framework for validation of CFD flame spread models

A review of the literature has shown the need for a comprehensive flame spread dataset framework for computational fluid dynamics model validation purposes. To develop this framework, the flame spread process was viewed as having four key components: turbulent fluid dynamics, gas phase kinetics, flame heat transfer, and condensed-phase pyrolysis. A series of extensively instrumented inter-related experiments based on the four components was conducted under different source fire permutations. This series of three progressively more complex experiments, from free plume, to inert wall fires, to combustible wall flame spread were carried out to enable collection of data relevant to each component of flame spread. Measurements made include heat release rate, plume centerline temperature and velocity, heat flux to wall, near-wall temperature, flame height, flame spread progression, mass loss, and burn pattern. The combustible wall test data in the current research may not be enough to validate a complex real-wo...

Modeling Microgravity and Normal Gravity Opposed Flame Spread over Polymer/Glass Composites

Experimental data of opposed -flow flame spread obtained both in microgravity and normal gravity is used to validate the predictions of an analytical model for opposed -flow flame spread r ate. The model is based on a balance between heat -transfer controlled flame spread and kinetically controlled flame spread. The model predicts that the flame spread rate increases with opposed flow velocity, reaches a maximum, then decreases until extincti on. The fuels examined are thermally thick polymethylmethacrylate (PMMA) and polypropylene glass fiber composite (PPG). The microgravity flame spread experiments were conducted on NASA’s KC -135 research aircraft in the Forced Ignition and Flame Spread Test (FIST) at opposed -flow velocities lower than can be achieved in ground -based tests due to buoyancy -induced flow. In order to apply the analytical model to predict the experimental data, four model parameters need to be calibrated. Once this is done, excel lent agreement between the model and the data is achieved. The model predicts that for some conditions and fuels, flame spread in microgravity occurs in the heat transfer dominated regime subject to low velocity flow while that in normal gravity in the kin etically controlled regime. Consequently, there is a maximum in the flame spread rate at velocities lower than those generated by buoyancy in normal gravity. These results, which are supported by the experimental measurements, are relevant for fire safety in spacecraft since ground bas ed tests may under predict microgravity flame spread rates of materials intended for use on spacecraft.