Temperature measurements near the heating surface at high heat fluxes in pool boiling of 2-propanol/water mixtures

To examine the mechanism of CHF enhancement with binary mixtures, this paper measured local temperatures close to a heating surface using a micro-thermocouple at high heat fluxes for pool boiling of a 3 mol% of 2-propanol/water mixture on an upward-facing 12 mm diameter circular surface. The variations in temperature traces measured at various locations above the heating surface indicate that the distributions of 2-propanol concentration have strong non-uniformities in the vertical and radial directions: 2-propanol concentration is higher in the vicinity of the heating surface at the central part of the heating surface and lower in the periphery of the heating surface. The local concentration of 2-propanol was determined based on the assumption that low peak values of temperature fluctuations correspond to the local saturation temperatures of the mixture. This assumption enabled a quantitative representation of the concentration distributions in the vertical and radial directions. The variations in the surface tension of the mixture were evaluated based on the deduced local values of the saturation temperature and the composition of the mixture. It was found that near the heating surface the surface tension increases steeply in the direction of the heating surface, suggesting that intensive Marangoni convection is induced at the liquid/vapor interface near the heating surface.