High-speed direct observations of the multiphase process in a micro particle of polyethylene terephthalate resin (abbreviated to PET in the following) under abrupt heating are made along with stereoscopic imaging of solid residuals. High-speed reflection images and stereoscopic images give conclusive evidence of occurrence of multiple micro-bubbling within the PET droplet, which is triggered by thermal decomposition of PET resin and flash vaporization of the decomposed products. Multiple and irregular explosions of micro bubbles follow just after the internal bubbling and cause irregular and random micro jets, resulting in randomly fluctuating micro diffusion flames downstream of the micro jets. It is concluded that the key factor to such a high expansion rate of the particle diameter as exceeds that due to thermal expansion can be attributed to the multiphase process due to the multiple and internal flash vaporization of thermally decomposed products, being particular to plastic resin combustion.
