Dynamic surface effect on the boiling of mixtures

It is possible that the critical boiling heat flux of mixtures exceeds that of the components considerably. This effect has been attributed to the slowing down in bubble growth rate caused by the exhaustion of the volatile component near the vapor-liquid interface (the F effect) and the retardation of bubble coalescence due to surface tension gradient caused by the evaporation of component of lower surface tension (the M effect). The growth and coalescence of vapor bubbles are dynamic processes. The surface tension of a fast stretching surface of a binary mixture is higher than the static value because the component of lower surface tension cannot diffuse to the adsorbed layer promptly. This dynamic effect is represented quantitatively by a Y function. The Y values for various binary systems are plotted versus concentration along with the F and M values in comparison with the critical heat flux values. The resemblance between these curves shows that F, Mand Y effects are all partial reasons for the increasing of the critical heat flux. They are interrelated and further studies are desirable. The agreement between variation of the tray efficiencies of a fractionation distillation column and that of the Y values shows another example of that the dynamic surface effect is an important factor in many heat and mass transfer operations.