Comparative experimental and computational studies for annular condensing and boiling flows in millimeter scale horizontal ducts

This paper describes experimental approaches for ensuring high heat-flux annular flow boiling and flow condensation under conditions for which shear and pressure forces dominate buoyancy effects. The paper also describes fundamental predictive tools for such flows. For annular flows, the liquid phase flows in the form of a wavy film (often micro-meter scale thin) that continuously irrigates the heat-exchange surface inside millimeter scale ducts. Controlled attainments of these annular flow configurations (which experience only second order effects of surface-tension forces) are essential to integration of functional boilers and condensers for certain space-based as well as micro-scale thermal systems. The experiments deal with flow condensation of FC-72 in a 2 mm gap horizontal channel of 1 m length and flow boiling of FC-72 in a 1.6 mm gap horizontal channel of 0.74 m length. For both boiling and condensing flow experiments, annularity of the respective flows is ensured by choice of an appropriate rate of through flow of vapor that does not actively participate in phase-change and has a flow rate which lies within a well defined range. The through flow of vapor is shown to ensure stability, annularity (by effectively suppressing nucleation in the case of flow boiling), and predictability. This fact is demonstrated by relevant flow visualization videos whose schematic and still pictures are included here. Two sets of annular condensing flow simulation results (one based on a full computational fluid dynamics based steady/unsteady simulations and another based on a quasi 1-D steady simulations) are compared against experimental heat-flux measurements obtained for annular shear driven condensing flows of FC-72 vapor. For quasi-steady annular boiling, only the quasi 1-D steady simulations approach is used for comparisons with experimental heat load measurements. The reasonableness of the proposed 1-D predictive engineering tool, with proper understanding of its scope and limitations, enables one to generate useful results for more sophisticated simulations. Furthermore the tool readily yields results/estimates for other working fluids, channel dimensions, and flow conditions