Heat transfer to boiling binary liquid mixtures

Abstract

Heat transfer in boiling liquid mixtures of water and methylethylketone has been measured throughout the ranges of convection and nucleate boiling at atmospheric pressure [43]. With some mixtures a considerably higher maximum heat flux in nucleate boiling has been found than in the case of water. This higher heat flux is obtained at the same temperature of the heating surface as for water, or alternatively, the same heat flux is obtained at a lower surface temperature. For mixtures of water with acetone, methylethylketone, the lower aliphatic alcohols and ethyleneglycol respectively, the maximum heat flux in nucleate boiling has been studied as a function of concentration. In addition, mixtures consisting of two organic lqiuids, i.e. dioxane-methanol and 2-chloroethanol—di-iso-propylether, were investigated. In all mixtures a maximum value of the maximum heat flux for nucleate boiling occurs at a certain concentration. The higher the number of carbon atoms in the alcohol and ketone series, the more this maximum is shifted towards lower concentrations of the organic component. A maximum of 3 1 2 times the value for water has been observed in the system water—1-pentanol. Partial miscibility has no influence on the continuity of the maximum heat transfer curve. The occurrence of the maximum is qualitatively explained by the higher dew point of the vapour formed by flash vaporization, as compared with the boiling point of the original liqiud mixture. At a certain concentration, the dew point of the vapour bubbles may become equal to the temperature of the surrounding superheated liquid, and in that case the vapour bubbles grow only by diffusional mass transfer. The average size of the bubbles leaving the heating surface is then smaller, and the maximum heat transfer is higher. For mixtures of water and alcohols, it has been calculated that 1 mole % of the liquid is immediately vaporized at the surface. For water—acetone and water—methylethylketone, only 0·3 and 0·1 mole % direct vaporization respectively is found.