Analytical modeling of condensation in microchannels with experimental validation
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The condenser is a critical component in many energy intensive systems, such as HVAC, power plants, automobiles, and gas liquefaction plants. Microchannel geometries offer the potential for more efficient and compact configurations for condensers. Condensation in small hydraulic diameter channels yields high heat transfer coefficients, combined with larger surface area-to-volume ratios, leading to increased system-level efficiency. Internal convective condensation in microchannels typically occurs in annular and intermittent flow regimes. This study develops mechanistic models for these two regimes, validated through relevant experiments. A first principles model for laminar annular flow condensation is developed. It addresses some of the limitations of models found in the literature, which are mostly shape-specific or have assumptions that are not valid over broad ranges of geometries. The present model is developed for an arbitrary channel geometry. For intermittent flow, most of the models in the literature address the hydrodynamics, or at best, heat transfer without phase change, while others are highly empirical. Therefore, a framework for a mechanistic model of condensation in intermittent flow in microchannels is developed here. A transient Lagrangian bubble-tracking scheme is used. Experimental data are collected using synthetic refrigerants as working fluids on a test facility capable of measuring heat transfer at low mass fluxes. Data are collected using two microchannel shapes, square and circle with hydraulic diameters of 0.98 mm and 1.55 mm, respectively. The tests are conducted at different saturation temperatures, saturation-to-wall temperature differences, and a range of low mass fluxes. These results show reasonable agreement with the predictions of mechanistic models for annular and intermittent flows. The effects of operating conditions and channel geometry on condensation are discussed and interpreted based on the underlying flow mechanisms.