Falling-film evaporation over horizontal rectangular tubes
Bustamante, John Gabriel
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The present study is the first investigation of falling-film evaporation over horizontal rectangular tubes. This geometry is representative of the external profile of microchannel tubes. Incorporating these designs into shell-and-tube heat exchangers has the potential to provide compact, high-performance components for a wide range of applications. This fluid flow was investigated experimentally, targeting three areas: measurements of heat transfer coefficients, quantification of flow characteristics, and the performance of flow distributors. Falling-film evaporation experiments were conducted using water on a rectangular test section with dimensions of 203 × 1.42 × 27.4 mm (length × width × height), measuring heat transfer coefficients over a range of saturation temperatures, test section spacings, heat fluxes, and film Reynolds numbers. This was supported by a flow visualization study that quantified droplet and wave parameters using image analysis of high speed videos. Finally, the performance of eight liquid distributors, which are used to establish falling-film flows, was quantified and the size of the generated droplets and jets was measured. Three models were developed to predict the flow regime, wetted tube area, and heat transfer coefficient. The flow regime model is based on a thermodynamic analysis, while the wetted tube area is found with a hydrodynamic model based on idealized flow assumptions. Finally, the heat transfer model relies on a relationship with the classic Nusselt (1916) film theory. Each of these models demonstrated good agreement with the experimental data, as well as trends in the literature. The increased understanding of falling-film evaporation gained in this study will enable the accurate design of shell-and-tube heat exchangers with microchannel tubes.