Heat Transfer in Post Dryout and Its Augmentation

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1. Boiling heat transfer coefficient for flow boiling in a uniformly heated vertical tube was measured for freon 11, freon 113 and their mixture in the various range of pressure, mass velocity, inlet subcooling, and heat flux.2. Heat transfer coefficient in the regions of nucleate boiling and two-phase forced convection was not predicted well by the available correlations proposed so far. In the two-phase forced convection region in particular, there was not observed an appreciable increase of heat transfer coefficient with increasing quality, as in the case of water.3. A semi-empirical correlation was proposed for heat transfer coefficient in the regions of nucleate boiling and two-phase forced convection.4. A transition from the two-phase forced convection to the post dry out region resulted in an abrupt increase of tube wall temperature. The heat flux and quality for this transition were well predicted from the Katto correlation for the critical heat flux.5. Wall temperature in the post dry out region increased abruptly right after the dry out and thereafter asymptotically approached to the wall temperature predicted for pure vapor, while the pattern of wall temperature in this process depended upon mass velocity, heat flux, and a distance from the dry point.6. Heat transfer coefficient was greatly degradated in the post dry out region. Thus an augmentation of heat transfer coefficient in this region is very important from the practical point of view.