HEAT-TRANSFER ANALYSIS OF PREHEATED TURBULENT FALLING FILMS IN VERTICAL TUBE EVAPORATORS

A theoretical investigation is conducted to study the hydrodynamics an d thermal aspect of preheated turbulent free-falling liquid films in l ong vertical tube evaporators. A formal analytical solution of the gov erning energy problem is developed based on a splitting-up procedure. This leads to homogeneous and non-homogeneous simple problems that can be solved to yield a more convenient solution far computational purpo ses. The eigenvalues associated with the homogeneous problem are effic iently computed by the sign-count algorithm. A new correlation for the local heat transfer coefficient is developed over wide ranges of Reyn olds and Prandtl numbers. Also, a correlation for the average heat tra nsfer coefficient is established to include the effect of interfacial shear forces exerted by gas Bow at the film surface. This influence is more pronounced in long tube applications such as desalination indust ries. Furthermore, a correlation for the fully developed heat transfer coefficient is developed as well. The theoretical predictions obtaine d in the present study compare well with existing experimental results reported in the literature with similar conditions. A sensitivity stu dy is also performed in which the influence of various participating f low parameters are explored for their general effect on velocity and h eat transfer of sensibly heated turbulent falling films.