SUMMARY

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 the 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. Initial results from these two models have shown good agreement with the literature.

Data will be collected for model comparison using a synthetic refrigerant as working fluid. For the data to be consistent with the assumptions in the models, a low mass flux and heat duty condensation facility has been constructed. The data will be collected using two microchannel shapes, square and circle (hydraulic diameters of order 1 mm). Different manufacturing techniques are used to fabricate a single square microchannel. The tests will be conducted at saturation temperatures above 40 C and low mass fluxes. These results will be used to validate and refine the mechanistic models of annular and intermittent flows. This study will provide a comprehensive model to further the understanding of condensation in microchannels and elucidate the effect of channel geometry, flow regimes, heat transfer and pressure drop.