Flame spread over solid fuel presents distinctive characteristics in reduced gravity, especially when the forced flow velocity is low. The lack of buoyancy allows a blue, dim flame to sustain where the induced velocity would otherwise blow it off. At such low velocities, a quenching limit exists where the soot content is low and the effect of radiative heat loss becomes important. The objective of this study is to establish a high fidelity numerical model to simulate the growth and extinction of flame on solid fuels in a reduced gravity environment. The great importance of the spectral dependency of the gas phase absorption and emission were discovered through the model development and therefore, Statistical Narrow-Band Correlated-k (SNB-CK) spectral model was implemented. The model is applied to an experimental configuration from the recent space experiment, Burning And Suppression of Solids (BASS) project conducted aboard the International Space Station. A poly(methyl methacrylate) (PMMA) sphere (initial diameter of 2cm) was placed in a small wind tunnel (7.6cm x 7.6cm x 17cm) within the Microgravity Science Glovebox where flow speed and oxygen concentration were varied. Data analysis of the BASS experiment is also an important aspect of this research, especially because this is the first space experiment that used thermally thick spherical samples. In addition to the parameters influencing the flammability of thin solids, the degree of interior heat-up becomes an important parameter for thick solids. For spherical samples, not only is the degree of internal heating constantly changing, but also the existence of stagnation point, shoulder, and wake regions resulting in a different local flow pattern, hence a different flame-solid interaction. Parametric studies using the numerical model were performed against (1) chemical reaction parameters, (2) forced flow velocity, (3) oxygen concentration and (4) amount of preheating (bulk temperature of the solid fuel). Flame Spread Rate (FSR) was used to evaluate the transient effect and maximum flame temperature, standoff distance and radiative loss ratio were used to evaluate the spontaneous response of the gas phase to understand the overall response of the burning solid fuel. After evaluating the individual effect of each parameter, the efficacy of each parameter was compared. Selected results of this research are:[1]Experimental data from BASS and numerical simulation both showed that within the time periodbetween ignition until the flame tip reaches the shoulder of the sample, the flame length and timehave almost a linear relation.[2]Decreasing forced flow velocity increases the radiative loss ratio whereas decreasing oxygen molefraction decreases the radiative loss ratio. This finding must be considered in the effort to replicatethe behavior of flame spread over thick solid fuels in microgravity on earth.[3]Although the standoff distance will increase when the forced flow velocity is decreased as well aswhen the oxygen mole fraction is decreased, the forced flow velocity has a much stronger effect onthe standoff distance than the oxygen mole fraction.[4]Unlike the previous two comparisons, the effect of forced flow velocity and oxygen mole fraction onthe maximum flame temperature was at similar level, reduction of either parameter would result inlowering the maximum flame temperature.[5]The effect of preheating on the flame spread rate becomes stronger when either the oxygen flowrate or forced flow velocity becomes larger. Depending on which element is more important, we candistinguish oxygen flow rate driven flame spread from preheating driven flame spread. Findings of this research are being utilized in the design of the upcoming space experiment, Growth and Extinction Limits of solid fuel (GEL) project. This research is supported by the National Aeronautics and Space Administration (NASA). This work made use of the High Performance Computing Resource in the Core Facility for Advanced Research Computing at Case Western Reserve University and the Ohio Supercomputer Center.

This dissertation is an investigation into the effects of natural convection on the combustion process of a spreading flame in a gravitational environment. The flame is spreading into an opposing flow of oxidizer over a solid fuel. This is approached as a steady state problem with coordinates fixed at the tip of the flame. This investigation incorporates the use of experimental data, numerical simulations and a simplified approach to develop a better understanding of combustion. The focus of the material presented can be separated in two components: First, a well validated forced flow numerical model is used to evaluate flame structure for the natural convection configuration. A simplified approach is developed and compared to the numerical model for flame structure and flame spread rates in chapters 2 and 3. Critical parameters controlling flame spread such as pressure, fuel thickness, oxygen concentration, and strength of gravitational field are widely varied. In the thermal regime, where this simplified approach applies, comparisons between experimental data, numerical solutions and simplified approach predictions are excellent. The numerical model is also compared to experimental data outside the thermal regime including a prediction of the regression rate of the solid fuel and gas phase characteristics. Second, a hybrid two-color pyrometry technique is developed and used to analyze flame structure for experiments in a microgravity environment. Images of flame intensity are calibrated and converted into temperature profiles for various opposed flow velocities and oxygen concentration. Numerical simulations are used to demonstrate various approximate techniques and their accuracies. The experimental images are used in conjunction with the numerical simulation to determine the temperature profiles and the partial pressure of carbon dioxide. Techniques are discussed on how to improve the results for future experiments by modifying the filter bandwidth selections. Through a greater understanding of the physics and controlling mechanisms for flame spread, the ability to control fire and the establishment of comprehensive guidelines for fire safety will be realized. This dissertation is another step toward that goal.