Quantitative Investigation on the Transient Evaporation of Fuel Droplet Using High Speed Imaging and Digital Image Processing

Evaporation, flame formation and liquid-phase dynamics of isolated fuel droplet shares similar characteristics with the droplets within fuel spray. Hence, isolated droplet studies which involves simple mass and energy transport are relevance to the complex spray environment in combustion. Motivations in the present work evolves around the evaluation of disruptive, transient liquid and gas phase of evaporating liquid fuel between analytical and experimental method. These disruptive and transient effects were analysed by several droplet conditions and arrangements including the experimentation on neat fuel with large difference in volatility and sooting propensities, stable and unstable emulsion droplets, soot contaminated diesel and multi-droplet combustion. Fuel droplets experimented in present work were suspended on 100 μm silicon carbide fibre and ignited in normal gravity, atmospheric pressure and ambient air. Backlighting was utilised to image the droplet and the flame was imaged directly by high speed cameras. The discrepancies between experimental and analytical works were divided into two conditions. Prolonged droplet heating affected the prediction accuracy of burning rate whilst prolonged fuel vapour accumulation affected the prediction accuracy of flame stand-off ratio. Higher amount liquid mass was ejected from water emulsion compared to ethanol emulsion droplet resulting 100% higher burning rate in each additive loading. It was found that microexplosion only occurs once all three conditions are present; the droplet temperature reaches the superheat limit of its lower boiling point component, a complete phase separation of emulsion components and the location of dispersed phase near the centre of the droplet. Compared to the unchanged reduction in burning rate of surface-contaminated droplet, the burning rate of volume-contaminated droplet was further reduced when the particle loading was higher. The difference was determined to be factored by the agglomeration rate of particles. Surface-contaminated droplet had a complete particle agglomeration upon ignition whilst volume-contaminated droplet had gradual agglomeration of particles, with faster particle agglomeration in higher loadings. In both contamination conditions, the droplet heating effect was longer due to high particle absorbance and the accumulated fuel vapour is found to be reduced due to the supressed evaporation rate. During the combustion of closely packed fuel droplets, the critical distance is found to be longer for lower volatility fuel factored by its tendency to have fuel vapour accumulation effect which enlarges the flame size when there is a starvation of oxygen. Transient droplet heating during multi-droplet combustion was found to be affected by the availability of oxidiser and heat transfer from the flame of nearby droplets. On the other hand, the combustion stability depended on the inter-droplet distance between neighbouring droplet. Overall, it is found that during transient droplet heating, longer duration taken for a fuel droplet to reach its boiling point would prolong its effect whilst the effect of fuel vapour accumulation is highly depended on the evaporation rate and volatility of fuel.