In part I, a three-zone flow boiling model is formulated to describe evaporation of elongated bubbles in micro-channels. The heat transfer model describes the transient variation in local heat transfer coefficient during the sequential and cyclic passage of (ⅰ) a liquid slug, (ⅱ) an evaporating elongated bubble and (ⅲ) a vapor slug. In this part, the time-averaged local heat transfer coefficient is compared to experimental data taken from seven independent studies covering the following seven fluids: R-11, R-12, R-113, R-123, R-134a, R-141b and CO_2. The 1591 test data cover tube diameters from 0.77 to 3.1 mm, mass velocities from 50 to 564 kg/m~2s, pressures from 124 to 5766 kPa, heat fluxes from 5 to 178 kW/m~2, and vapor qualities from 0.01 to 0.99. The new model predicts 67% of the database to within +-30%. The new model illustrates the importance of the strong cyclic variation in the heat transfer coefficient and the strong dependency of heat transfer on the bubble frequency, the minimum liquid film thickness at dryout and the liquid film formation thickness.
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Laboratory of Heat and Mass Transfer (LTCM), Faculty of Engineering Science, Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland;
