Annular/Stratified Internal Condensing Flows in Millimeter to Micrometer Scale Ducts

This paper presents computational simulations for internal condensing flows over a range of tube/channel geometries — ranging from one micro-meter to several millimeters in hydraulic diameters. Over the mm-scale, three sets of condensing flow results are presented that are obtained from: (i) full computational fluid dynamics (CFD) based steady simulations, (ii) quasi-1D steady simulations that employ solutions of singular non-linear ordinary differential equations, and (iii) experiments involving partially and fully condensing gravity driven flows of FC-72 vapor. These results are shown to be self-consistent and in agreement with one another. The paper demonstrates the existence of a unique solution for the strictly steady equations for gravity and shear driven flows. This paper also develops useful correlations for shear driven and gravity driven annular stratified internal condensing flows (covering some refrigerants and common operating conditions of interest). A useful map that marks various transitions between gravity and shear dominated annular stratified flows is also presented. For the micro-meter scale condensers, computations indentify a critical diameter condition (in non-dimensional terms), below which the flows are insensitive to the orientation of the gravity vector as the condensate is always shear driven. Large pressure drop, importance of surface tension, and vapor compressibility for μm-scale flows are also discussed. With the help of comparisons with 0g flows, the paper also discusses effects of transverse gravity on the solutions for horizontal channel flows.