Interfacial shear stress, heat transfer and bubble appearance in falling film evaporation

In the falling film evaporation of dairy products, the phase change process occurs via two major phenomena: surface convective evaporation and boiling evaporation. Previous studies have shown that under certain conditions, the heat transfer mechanism can be greatly improved when the evaporation is dominated by the presence of bubbles or foam clusters. In the present work, the influence of the surface bubbling phenomenon on the heat transfer coefficient has been studied. The effect of the co-flowing vapor rate inside the evaporative tube has been experimentally related to the presence of the surface bubbles for dairy products characterized by different dry solid contents (DC = 0%, 13%, 30%, 40%, 51%). The results show that at low dry solid contents (DC <= 30%), the co-flowing vapor negatively affects the heat transfer when the structure and the dimension of the bubbles are modified or inhibited by the action of the co-flow. At high dry solid contents (DC = 40%, 51%), the sweeping effect of the co-flow plays a positive role on the heat transfer coefficient by promoting a more even circumferential distribution of the falling film and by increasing its velocity