EVAPORATIVE HEAT TRANSFER IN THE CONTACT LINE REGION OF A MIXTURE

The evaporative heat transfer processes in the contact line region of a stationary evaporating thin film in the form of an extended meniscus were studied experimentally and modeled. The effects of small changes in the composition of the fluid on the heat transfer characteristics were evaluated. Results obtained for the evaporation of pure decane (98% min) on a silicon substrate were taken as a standard for comparison. Experimental results were also obtained for the following liquid systems on silicon: pure decane (98% min) plus 2% tetradecane by weight; pure decane plus small percentages of practical grade nonanoic acid; research grade (99.95% min) hexane; and technical grade hexane (96% min).;The results demonstrate that, as a result of distillation, the concentration in the immediate vicinity of the contact line is significantly different from the bulk concentration. The resulting concentration and temperature gradients give interfacial shear stresses and recirculation flow patterns that enhance contact line stability. In general it is proposed that the proper use of a "second component" in the evaporating liquid should lead to enhanced control and improved heat transfer characteristics in the contact line region of an evaporating thin film.;The transport processes were found to be a function of the gradients in the film thickness, the composition and the temperature which could be varied by changing the bulk composition of the liquid and the integral evaporation rate. The liquid film thickness profile was determined using a scanning microphotometer to measure the relative location of approximately fifty interference fringes. Thermocouples were used to obtain the integral value of the evaporation rate. A distillation model was used to determine local values of the composition and temperature. Using the conservation of mass and momentum principles, a heat balance was obtained with the experimental data and a constant heat flux model.