Modelling the dynamic behaviour of gas bubbles in thermoplastic pyrolysis

This study aims to reveal the detailed bubble development and bursting processes driven by the pyrolysis of plastic fuels. The evolution of a single bubble within the thermoplastic pool consists of two distinct stages: (I) the seed stage, characterised by a tiny constant-radius bubble, and (II) the subsequent growth stage, with a rapid increase in bubble volume. In the seed stage, a bubble core emerges with a fixed initial diameter due to the influence of the strong viscosity of the plastic melts. Simultaneously, the continuous release of gaseous volatiles through the pyrolysis process from the condensed phase results in a substantial pressure increase inside the bubble. After a critical development time, the volume of the bubble grows rapidly, whereas the pressure decreases to slow down growth. The numerical solution of bubble size agrees well with experimental measurements of an externally heated PMMA plate. With bubbles of 0.14–0.18 mm radius seen in a burning PMMA, the bursting jet velocity, estimated by Bernoulli's equation, is around 100–125 m/s. Such velocities could potentially cause the ejection of condensed fuel particles. This study offers a fundamental understanding of polymer bubbling and bursting dynamics and helps quantify the burning rate enhanced by fuel ejection during thermoplastic combustion.