Internal convective jumping-droplet condensation in tubes

Abstract

Water vapor condensation governs the efficiency of a number of industrial processes. Jumping-droplet condensation of water has recently been shown to have a 10× heat transfer enhancement compared to filmwise condensation due to the removal of condensate at much smaller length scales (∼1µm diameter). However, the removal efficiency of jumping droplets can be limited by return to the surface due to gravity, entrainment in bulk convective vapor flow, and entrainment in local condensing vapor flow. If used appropriately, convective condensation has the potential to entrain droplets and impede their return to the surface. In this work, a comprehensive model of internal convective jumping-droplet condensation in a superhydrophobic tube has been developed for constant heat flux boundary conditions. Laminar boundary layer theory was used to model the vapor flow inside the tube with condensation modeled as vapor suction. We analyzed the effects of jumping droplet size (1<Rd<100µm), condensation heat flux (0<q<10W/cm2), initial jumping location axially along the tube (0<x<5m) and radial position (0<theta<2π), entering vapor mass flux (0.05<G<1.5kg/m2s), and pipe radius (1mm<a<30cm), on droplet trajectory, overall heat transfer performance, and pressure drop. By linking droplet return with droplet jumping (multi-hop), we develop a framework to predict macroscopic droplet motion along the tube, and offer guidelines for the minimization of drag force and maximization of overall condensation heat transfer. The reduced theoretical pressure drop and enhanced heat transfer performance of convective internal jumping-droplet condensation shows potential of applicability in air-cooled condensers, and presents a framework for novel flow humidification and dehumidification technologies with appropriate design and flow separation.