Droplets boiling crisis of ethanol water solution on duralumin substrate with SiO2 nanoparticles coating

Boiling crisis and evaporation of droplets of ethanol water solution on a horizontal ceramic heating surface were studied experimentally. The experiment was performed on duralumin substrate with SiO2. The coating was obtained by nanoparticles plasma spraying. Measurements of the contact angle of the aqueous solution onto the ceramic surface were performed. Most evaporation relates to an increase in the area of the wetting droplet surface and only 10-20% of evaporation relates to the effect of diffusion and change in the thermal-physical coefficients. Maximal instability of the bubble microlayer for alcohol solution with C-0 = 20-40 mass% is observed. The critical heat flux behaves non-monotonously at a change in alcohol concentration in solution, and there are two extremes. The maximal value of sustainability coefficient at evaporation of droplets of ethanol solution corresponds to alcohol concentration of 20-30 mass%. Concentration gradient leads to instability of a triple contact line at the bubble bottom and a decrease in the dry spot area; this promotes higher critical heat flux levels (CHF). The behavior of a binary mixture depends not only on the Marangoni effect at the droplet edges, as thought previously, but also on the unsustainable behavior of the bubbles contact line. The heat transfer coefficient of ethanol solution in the spheroidal state decreases with a rise of wall overheating and spheroid diameter. Experimental dependence of the vapor layer height on wall overheating at boiling crisis was derived. The height of this layer (60-80 mu m) at Leidenfrost temperature manifold exceeds the surface microroughness value (1-1.5 mu m). (C) 2016 Elsevier Inc. All rights reserved.