Transient heating of a bio-oil droplet within the pre-explosion stage

The study is concerned with a suitable heating model for the prediction of the pre-explosion time of a single bio-oil droplet. Diffusion and distillation limit models of the internal droplet liquid transport are considered. The diffusion limit model only allows diffusion, while the distillation limit model implies the existence of an infinitely fast transport rate, therefore it represents the fastest possible transport limit. It is most likely that after the droplet ignition, the water and the lighter fuel fractions evaporate and burns firstly at almost constant droplet temperature in accordance with the distillation limit model. The increase of the liquid viscosity leads to domination of the diffusion limit model. According to the diffusion limit model, shortly after initiation of gasification the droplet surface becomes more concentrated of high-boiling point components, so the droplet surface reaches very high temperature, while the droplet core has a higher concentration of low-boiling components, accumulating a substantial amount of heat at temperatures near the superheat limit. Low thermal diffusivity values of bio-oil allow simplification of the solution of the equations that govern the diffusion limit model. This, in turns, allows estimating of the effective value of the bio-oils superheat limit and prediction of the droplets pre-explosion lifetime.