This model verifies that the presence of bubbles has a considerable impact on heat and mass transfer. The results illustrate areas where further research is most needed to provide an accurate depiction of the effects of bubbling within burning thermoplastic materials.
Pyrolysis is found to be particularly sensitive to the bursting process. Because of the insulating effects of bubbles that retain a significant amount of gas within the sample, increasing the delay time before bursting by tens of milliseconds causes a significant decrease in mass loss rate. To accurately predict these effects, a much better physical model of the bursting process is needed. The improved model should include both degradation and drainage of the polymeric thin film in the heated environment. In addition, the bursting process results in a fine droplet spray as the thin film ruptures, with the potential to expel a center droplet as the liquid surface rebounds. In microgravity, these events will carry polymeric melt away from the sample, thus increasing the effective mass loss rate. Experiments and computational models that improve our understanding of these phenomena are needed to determine their effects on microgravity combustion of thermoplastic materials.
An improved model of bursting behavior will naturally provide a better physical description of bubble merging, and vice versa. Although it was not used in this model, a spring constant to maintain distance between bubbles has been applied successfully to studies of foam flow [55] and could be used in similar models to keep bubbles separate during the thin-film drainage process.
As polymers burn, gaseous degradation products are generated in-depth. Some gaseous molecules are created at sufficiently close proximity to form bubble nucleation sites, some are distributed among existing bubbles, and others remain in solution in the melt. Gases may not be generated uniformly within the heated material, as demonstrated by recent research on secondary nucleation in which rapid gasification was discovered in regions of high elastic strain surrounding growing bubbles. More work to understand the dynamic behavior of gases at the molecular level is necessary to be able to predict bubble behavior with a greater degree of confidence.
Nucleation, bubble merging, and gas partitioning among bubbles did not appear to have strong effects on the pyrolysis process in the results from this model. However, these phenomena are critical in determining the number and size distribution of bubbles. Their influence may be much more apparent if it is determined, for example, that time to bursting is strongly dependent on radius.