Effects of two-phase inlet quality, mass velocity, flow orientation, and heating perimeter on flow boiling in a rectangular channel: Part 2 - CHF experimental results and model.

This study is the second part of a two-part study exploring flow boiling of FC-72 along a rectangular channel with either one wall or two opposite walls heated for saturated inlet conditions. While the first part examined flow boiling interfacial behavior, boiling curves, local and average heat transfer coefficients, and pressure drops, this part is focused entirely on CHF measurement, flow visualization and modeling. Both single-sided and double-sided heating configurations are tested in horizontal flow, vertical upflow, and vertical downflow. For low mass velocities, high speed video analysis shows gravity has a dominant influence on interfacial behavior, with single-sided top-wall heating yielding the lowest CHF values, and bottom-wall heating the highest. For both single-sided heating and double-sided heating, increasing mass velocity decreases the influence of orientation on CHF, with identical CHF values achieved at high mass velocities irrespective of orientation, and increasing inlet quality serves to decrease the mass velocity value required for inertia to completely overcome gravity effects. A separated flow model for two-phase inlet conditions is proposed to predict key flow variables necessary for CHF modeling. With a MAE ≤ 14%, this study proves that the combination of separated flow model and Interfacial Lift-off Model is highly effective at predicting CHF for saturated inlet conditions as it did in prior studies for sub-cooled inlet conditions.