Bubble dynamics of water-ethanol mixture during subcooled flow boiling in a conventional channel

•Bubble dynamic analysis is carried for water ethanol mixture in the subcooled boiling region.•The bubble departure diameter is measured and its relation with heat transfer coefficient is discussed.•A new empirical correlation is developed to predict the heat transfer coefficient for water.•The bubble lift-off and bubble sliding are determined.•Force balance is carried out to determine the possible mechanisms governing the bubble dynamics. In this paper, bubble dynamics in subcooled flow boiling of water-ethanol mixture in horizontal rectangular channels is investigated through visualization. The subcooled flow boiling heat transfer coefficient of water ethanol mixtures are determined for various heat flux, mass flux and ethanol volume fraction. A new empirical correlation is proposed to predict the heat transfer coefficient of pure water based on the parameters like heat flux, bubble departure diameter, waiting period and the growth period. Two types of bubble behaviours are observed after nucleation: (i) Sliding for a distance along the bottom wall of the channel surface before lift-off and (ii) Lift-off from the bottom wall of the channel surface without sliding. Force balance analysis is carried out to determine the reason for bubble lift-off and bubble sliding. The bubble lift-off without sliding is observed at higher ethanol volume fraction, lower heat flux and higher channel inlet temperature. The bubble sliding and lift-off are observed at higher heat flux and lower channel inlet temperature for water and water-ethanol mixture of 25% ethanol volume fraction. However, the effect of mass flux on the bubbles sliding or bubble lift-off is not significant.