Numerical Study of the Effects of Confinement on Large-Scale Fires in Microgravity

Wohan Cui, Case Western Reserve University
Ya-Ting Tseng Liao, Case Western Reserve University

Abstract

Confinement has been shown to play an important role on burning behavior of solid materials in microgravity. Previous studies using small flow ducts (duct height < 7.6 cm) concluded that the flame spread rate is linear to the inverse of the duct height. The underlying physics for this correlation is the combustion thermal expansion that leads to different flow acceleration in different flow duct cross-section areas. In recent NASA microgravity fire experiments, Saffire, wide solid fuel samples were burned in two large flow ducts. In these experiments, the duct heights (30 cm and 50 cm) were significantly larger than the flame standoff distance (∼1 cm) and the effect of thermal expansion was expected to be minimal. However, the experiment results showed that both flame spread rate and pyrolysis length increased more than 95 % when the duct height decreased by 40 %. To understand the underlying physics of the experimental results, two-dimensional transient numerical simulations are performed. The model configuration is based on the Saffire experiments. The model successfully predicts the transient flame development processes and the flame spread rates observed at both duct heights. In addition, a methodology is developed to deduce net heat flux distribution on the sample surface using thermocouple data obtained in the experiments. The deduced heat flux profiles in the experiments and in the numerical results agree qualitatively and quantitively. After the model is validated against the experimental data, a parametric study on the duct height is performed. When duct height decreases, flame spread rate increases. It is found that, at large duct heights (> 20 cm), the inverse of the flame spread rate has a linear dependency on the inverse of the duct height. Analytical analysis of cold flow (without combustion) demonstrates that this relationship is due to the different flow profiles on duct cross-section plane when duct height varies (i.e., a hydrodynamic effect). Effects of duct ceiling radiation properties are also considered. Radiation reflection from the duct ceiling increases the heat input on the sample surface, resulting in an increased flame spread rate. This effect is stronger at a smaller duct height.