Puffing/micro-explosion in rapeseed oil/water droplets: The effects of coal micro-particles in water

The paper is focused on detailed experimental investigation of puffing and micro-explosions in composite water/rapeseed oil droplets in the presence of lignite and bituminous coal micro-particles in water. Gas temperature was measured using a high speed National Instruments 9219 for data collection and an S-type thermocouple. Video recording of droplet micro-explosions was performed using a Phantom Miro M310 high-speed camera. Droplets with radii in the range 1–2 mm were placed in a hot chamber with air velocities 3–7 m/s and temperatures up to 600 °C. The time to puffing/micro-explosion and average radii of child droplets generated during puffing and micro-explosions are shown to decrease with increasing gas temperature. The presence of bituminous coal led to a visible decrease in these radii. The observed times to puffing/micro-explosion were interpreted in terms of the recently developed model of the phenomenon based on the assumption that a single spherical water sub-droplet is located in the centre of a spherical fuel droplet. The time to puffing/micro-explosion in this model is associated with the time instant when the temperature at the water/fuel interface reaches the water nucleation temperature. The model predicts a decrease in time to puffing/micro-explosion in agreement with experimental observations. The effect of coal particles on this time is shown to be weak in agreement with observations at gas temperatures above 300 °C. It is shown that the times to puffing predicted by the model are close to those predicted by a simpler model in which these times are identified with the time instants when the temperature at the water/fuel interface reaches the boiling temperature of water.