Convective heat transfer in droplets of fuel microemulsions during conductive heating

The experimental findings of the convective heat transfer in drops and films of fuel microemulsions heated on hot surfaces are presented. The study examines the Rayleigh-Benard-Marangoni convection in the films of fuel microemulsions heated from below under the conditions close to saturation and the forced convection in the drops of the same liquids in the Leidenfrost state. Laser-induced phosphorescence method is used to record the temperature of drops. Features and characteristics of vaporization and coalescence of the dispersed phase of microemulsions are analyzed by optical microscopy. The experiments are performed with the heated substrates made of sapphire and stainless steel. The fuel microemulsions are manufactured from diesel fuel, fatty acids methyl esters of rapeseed and sunflower oils, distilled water, surfactant – polyethylene glycol ether of isononylphenol, and co-surfactant – 2-ethylhexanol. The results show that the evaporation rate of the dispersed phase of microemulsions largely depends on the total concentration of emulsifier and distilled water. At surface temperatures below 80 °C, the mean diameter of droplets of the dispersed phase decreases linearly with increasing temperature; when the temperature is above 80 °C, the droplets are enlarged about two times, initiating the bubble boiling. The number of the large coalesced droplets of the dispersed phase decreases linearly with time; the process occurs more intensively at surface temperatures above 80 °C. At a local liquid temperature of about 120–150 °C, an intensely heated microemulsion drop partially disintegrates (puffing). With an increase in the surface temperature of the substrate from 360 °C to 425 °C, the evaporation rate of microemulsion drops practically does not change, i.e., it reaches certain asymptotic values. The significant heterogeneity of temperature fields of microemulsion drops in the Leidenfrost state is shown.