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:
Experimental data from BASS and numerical simulation both showed that within the time period
between ignition until the flame tip reaches the shoulder of the sample, the flame length and time
have almost a linear relation.
Decreasing forced flow velocity increases the radiative loss ratio whereas decreasing oxygen mole
fraction decreases the radiative loss ratio. This finding must be considered in the effort to replicate
the behavior of flame spread over thick solid fuels in microgravity on earth.
Although the standoff distance will increase when the forced flow velocity is decreased as well as
when the oxygen mole fraction is decreased, the forced flow velocity has a much stronger effect on
the standoff distance than the oxygen mole fraction.
Unlike the previous two comparisons, the effect of forced flow velocity and oxygen mole fraction on
the maximum flame temperature was at similar level, reduction of either parameter would result in
lowering the maximum flame temperature.
The effect of preheating on the flame spread rate becomes stronger when either the oxygen flow
rate or forced flow velocity becomes larger. Depending on which element is more important, we can
distinguish 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.