Heat transfer enhancement observed in nucleate boiling of alcohol aqueous solutions at very low concentration

Experiments on saturated pool boiling were conducted by using alcohol aqueous solutions of 1-Propanol/Water, 2-Propanol/Water and Water/Ethylene glycol mixtures at atmospheric pressure. The heating surface was a horizontal upward-facing circular flat plate of 40mm in diameter. Despite that only the deterioration of heat transfer coefficient due to the existence of diffusion resistance was reported so far in nucleate boiling of mixtures, enhancement of heat transfer coefficients was observed for 1-Propanol/Water and 2-Propaonl/Water mixtures at extremely low alcohol concentrations. In these concentration ranges, Marangoni force acting on the vapor-liquid interface towards the three-phase interlines was expected to enhance the heat transfer, and this heat transfer enhancement can overcome the heat transfer deterioration due to the mass transfer resistance inherent in mixture boiling. In the intermediate concentration range, however, no heat transfer enhancement but only deterioration was observed, where Marangoni effect had no important role. On the other hand, the measured CHF values were gradually decreased with the increase of alcohol concentration in the moderate concentration range. In the low alcohol concentration range, however, CHF values were clearly decreased for 1-Propanol/Water and 2-Propanol/Water. To confirm the observed heat transfer trends of three different alcohol aqueous solutions, Marangoni effect on the nucleate boiling heat transfer mechanisms was discussed.