The intertube falling-film modes: Transition, hysteresis, and effect on heat transfer

Abstract

Horizontal-tube falling-film heat exchangers enjoy wide application because they offer superior thermal performance and operate with less fluid charge than conventional shell-and-tube or flooded heat exchangers. Recent developments vitiate the heuristic design approach of the past, and a clearer grasp of the relevant flow and heat transfer interactions is needed. This research is directed at developing a deeper understanding of the falling-film modes, their transitions, and their effect on heat transfer.

Five different working fluids were used in an experimental apparatus that allowed detailed observations of the falling-film flow. Three distinct falling-film patterns were observed--the droplet, jet, and sheet modes--along with two transitional patterns--the jet-droplet and jet-sheet modes. Jet departure sites were found to exhibit either in-line or staggered arrangements, and the free surface shapes of the jets were predicted using a simplified model of the flow. The effects of fluid properties, flow rate, tube diameter and spacing, and vapor shear on the falling-film modes were quantified. The mode transitions depended on the Reynolds number based on film thickness and a modified Galileo number based on the capillary constant. This behavior reflected the importance of inertial, viscous, surface tension and gravitational effects. Using the experimental results and a dimensional analysis, a map for predicting the falling-film flow pattern was developed. These unique observations, the new flow classifications, and the novel flow regime maps provide the clearest and most complete description of the intertube falling-film mode to date.

Heat transfer experiments were conducted with three working fluids for all flow patterns. Detailed measurements of the local heat transfer revealed the importance of the impingement, development, and jet-interaction regions. Far all three modes, the local Nusselt number was highest in the flow impingement region. For the sheet mode, small axial and large circumferential variations due to flow development were measured. For the jet and droplet modes, large axial and circumferential variations in the Nusselt number were measured and related to the flow characteristics. New heat transfer correlations were developed far each of the flow modes.