08_Regime_Specific_Phenomena_web

08_Regime_Specific_Phenomena_web - ENU 4134 Regime Specific...

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Unformatted text preview: ENU 4134 Regime Specific Phenomena D. Schubring Fall 2011 Learning Objectives I 1-f-i Characterize the various regimes of two-phase flow I 1-f-iii Identify the importance of coherent structures (waves, droplets, bubbles, etc.) in two-phase flow and in the differentiation among regimes I 1-f-iv Explain the qualitative argument behind the interfacial area transport equation (IATE), frequently used to model bubbly flow I 1-f-v Use and critically evaluate the traditional modeling paradigm for annular flow (triangular relationship, etc.) I 1-f-vi Develop a basic understanding of other flow regimes in horizontal and vertical co-current flow I 1-g-i Explain the nature and origin of counter-current flow and identify the nuclear-relevance of this condition I 1-g-ii Use correlations to models counter-current flow reversal and flooding Bubbly Flow Topics I Applicability of previously discussed models. I Other phenomena. I Interfacial area concentration and transport. This discussion will be qualitative. (The sort of thing that might show up in a short essay question.) Applicability of Analytical Models Many of the models discussed in the first two-phase flow block are especially useful for the bubbly flow regime (and into slug flow). The concepts of two-phase averaged parameters, such as { } and S , are most useful in these regimes, as is 2 lo . { } is most useful here because its value is far away from 1 and, often, far away from 0 less fractional uncertainty on data. At high mass flux and pressure, bubbly flow is well-modeled with the HEM. At lower mass fluxes and BWR pressures, the drift flux model is recommended in the bubbly regime. Other Bubbly Flow Phenomena Well cover some of these in more detail on the heat transfer section: I Over-pressure required for bubble formation (surface tension). I Contact angles (surface). I Bubbly break-up and coalescence. Recall Momentum Transport Equation d dt [ h k ~ v k i k V h k i ]- X j { k ~ v k ~ v k } kj ~ A j { k } + { k ~ v k ( ~ v k- ~ v s ) } ks ~ A ks = h k i k ~ g h k i V + X j ~ F jk- X j { p k } kj ~ A k { k } + ~ F sk (1) Observe: some terms are dependent on volume , others on interfacial area . In T&K, a 1-D equation is developed by taking V as A z z and dividing through by z . An alternate treatment divides through by V . Interfacial Area Concentration When this version is considered, area-based terms include an interfacial area concentration term, with dimensions of inverse length. Physically, the interfacial area concentration is inversely related to a characteristic distance between gas-liquid interfaces. Since mass, momentum, and energy transfer between phases occurs at these interfaces, this term is important for understanding the relative importance of the interphase area terms. Example: Big vs. Small Bubbles Consider a section of a tube volume with length of 0.1 m and diameter of 0.1 m....
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This note was uploaded on 10/22/2011 for the course ENU 4134 taught by Professor Schubring during the Fall '11 term at University of Florida.

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08_Regime_Specific_Phenomena_web - ENU 4134 Regime Specific...

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