Shear-driven annular flow-boiling in millimeter-scale channels: Direct numerical simulations for convective component of the overall heat transfer coefficient

Many contemporary high heat-flux cooling applications are facilitated by controlled operation of millimeter-scale flow-boilers that operate in the steady annular or steady-in-the-mean (with superposed large amplitude standing waves in the liquid film) annular regimes – with micrometer-scale liquid film thicknesses. That is, a thin evaporating liquid film flow covers the heated boiling-surface – with or without superposed micron/sub-micron-scale nucleate boiling regime. Therefore, to begin with, to characterize convective boiling component of experimentally measured values of heat transfer coefficient (HTC), it becomes important to fully characterize the underlying steady annular flows under the assumption of suppressed nucleation. For such steady cases, and liquid thickness values in the range of tens to hundreds of micrometers that are much smaller than the mm-scale hydraulic diameter of the ducts, this paper presents a direct numerical simulations (DNS) approach for laminar liquid and laminar vapor flows. Representative detailed steady solutions for annular flow-boiling of FC-72 in a horizontal channel (heated from below) are presented, the flow-physics is studied, and HTC values are correlated. Furthermore, a one-dimensional correlations-based design tool is developed and discussed, along with its future extensions for covering laminar liquid and turbulent vapor are annular flow realizations that may also occur in the aforementioned operations of flow-boiling