A numerical model is demonstrated for the pyrolysis of a spherical thermoplastic sample in microgravity including effects of bubbles. The model combines nucleation, growth, and migration of individual bubbles in three-dimensional space with finite element model that solves the one-dimensional radial equation for the temperature field. Energy calculations include surface losses due to radiation and convection, conductive heat transfer through the mixture of gaseous and condensed phase material, and the chemistry of gasification. Gases released by bursting bubbles determine the mass loss rate from the sample. Results demonstrate the thermally insulating properties of bubbles as they transport gases to the surface of heated polymeric sample. The mass loss rate is particularly sensitive to the bursting process, since slow drainage from the thin film defining the bubble at the surface maintains gases of low thermal conductivity as thermal barrier to slow the transport of heat. The behaviors of pyrolyzing PMMA and PP spheres are investigated.
Helpful conversations with Dr. Sandra Olson of NASA, Dr. Indrek Wichman of Michigan State University, and Drs. Jiann Yang, Anthony Hamins, Takashi Kashiwagi, and Nicos Martys of NIST are gratefully acknowledged. This research was funded by the NASA Glenn Research Center under NASA Interagency Agreement C32033E.