To look at the effects of including bubbles in the pyrolysis calculations for polypropylene, the same two cases are considered as for PMMA in sections 6.1 and 6.2. For the base case, all gases generated during a timestep immediately escape the sample. For the simplest case including bubbles, the problem is initialized with one nucleation site per element and new bubbles are added only to empty elements. The bubbles burst as soon as they contact the surface of the spherical sample. The number of elements in the sphere is 41 for these runs, and the timestep is 0.25 ms.
Figure 32: Sample radius vs. time for a spherical PP sample exposed to an external heat flux of 60 W/cm2 with bubbles (orange) and without bubbles (red).
Figure 33: Mass loss rate vs. time corresponding to Figure 32.
The effects for bubbles in PP are considerably larger than for PMMA. Figures 32 and 33 show that the presence of bubbles causes mass loss rate to drop considerably after 10 s and doubles the pyrolysis time. Compare this to the roughly 10 % increase in pyrolysis time for bubbling PMMA under identical assumptions, as shown in Figure 13. The thermally insulating effect of the bubbles related to this drop in mass loss rate is shown in Figure 34. Although the outer surface of the sample heats to the same temperature as the case without bubbles, the center of the sample remains cooler after the initial pre-heating. An extended quasi-steady period of slow interior heating follows.
Figure 34: Temperature vs. time at the sample center (lower plots) and outer surface (upper plots) corresponding to Figure 32.