A heat transfer model for evaporation of coalescing bubbles in micro-channel flow

The current study presents a one-dimensional model of confined coalescing bubble flow for the prediction of micro-channel convective boiling heat transfer. Coalescing bubble flow has recently been identified as one of the characteristic flow patterns to be found in micro-scale systems, occurring at intermediate vapor qualities between the isolated bubble and the fully annular regimes. As two or more bubbles bond under the action of inertia and surface tension, the passage frequency of the bubble–liquid slug pair declines, with a redistribution of liquid among the remaining flow structures. Assuming heat transfer to occur only by conduction through the thin evaporating liquid film trapped between the bubbles and the channel wall, the present model includes a simplified description of the dynamics of the formation and flow of the liquid film and the thin film evaporation process, taking into account the added mass transfer by breakup of the bridging liquid slugs. The new model has been confronted against experimental data taken within the coalescing bubble flow mode that have been identified by a diabatic micro-scale flow pattern map. The comparisons for three different fluids (R-134a, R-236fa and R-245fa) gave encouraging results with 83% of the database predicted within a ±30% error band. Furthermore, the new model is able to predict a “nucleate boiling curve” with an exponent of 0.74 typical of numerous micro-channel flow boiling studies, thus suggesting film evaporation as the controlling heat transfer mechanism rather than nucleate boiling.