falling-film heat exchanger, flow pattern
Horizontal-tube, falling-film heat exchangers are used in many air-conditioning and refrigeration systems. Depending on the geometry, flow rate and fluid properties, when a liquid film falls over a series of horizontal tubes several distinct flow patterns can be manifested: the droplet mode, the jet mode, and the sheet mode. A thermodynamic analysis is undertaken to predict the transitions between these modes. By seeking thermodynamic equilibrium between two neighboring modes, a scaling relation is developed for the transitional Reynolds number. This theoretical framework is successful in explaining the well-established empirical correlation of transitional Reynolds number with modified Galileo number. The scaling relation also suggests a tube-spacing effect on the mode transitions. Using limited data and prior results from the literature it is found that this effect is likely to exist. The implications and limitations of this thermodynamic approach to predicting two-phase flow patterns are discussed, as is its incompleteness.