Post-CHF Swirl Flow Heat Transfer with Low Wall Superheat

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The objectives of this investigation are to: 1) measure and predict heat transfer to post-critical heat flux (post-CHF) two-phase flow under swirling flow conditions at low wall superheat over a broad parameter range, 2) verify the mass flux effect recently identified, and 3) identify physical conditions in the flow field. The benefit of flow swirl to the post-CHF regime is substantial, and recent data obtained at UIC, for application to heat exchangers with swirl flow induced by inserts inside tubes, suggest that direct drop-wall heat transfer is an important contributor when the wall superheat is low and the mass flux is high. Minimal post-CHF data exist in the literature at low wall superheat axial or swirl flow - and most data at high wall superheat and axial flow do not exhibit this effect. The relatively unique liquid-heated flow boiling experimental facility at UIC, which allows steady-state testing in the post-CHF region, will be used to obtain data over an extended range of mass flux to provide overlap with limited existing data necessary to verify the mass flux effect on this post-CHF heat transfer and to support modeling verification of this two-phase flow. Flash X-ray, laser Doppler velocimetry and photographic instrumentation available at UIC will be used to measure drop size distributions in the flow for use with an expanded modeling effort. A second experimental facility will measure swirl flow drop size in air/water dispersed flows. Experiments to verify the mass flux effect will also broaden the data base by including an environmentally acceptable refrigerant as the boiling fluid. Test section orientation and size will also be varied to produce predictions based on a wider range of parameters. Experimental results will be used in the development of both semi-empirical phenomenological models and basic analysis. Both approaches have been formulated and successfully verified over a limited data range, and significant advancement in both generality and modeling approach is expec ted from this combined experimental/analytical effort.