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Temperature stratification and mixing: Transient two-phase boiling in a side wall heated tank

Temperature stratification and mixing: Transient two-phase boiling in a side wall heated tank. E. Krepper 4 th NC IAEA Research Coordination Meeting Vienna, Sept. 10 th -13 th 2007. 1. The problem. Safety related problem.

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Temperature stratification and mixing: Transient two-phase boiling in a side wall heated tank

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  1. Temperature stratification and mixing: Transient two-phase boiling in a side wall heated tank E. Krepper 4th NC IAEA Research Coordination Meeting Vienna, Sept. 10th-13th 2007

  2. 1. The problem Safety related problem • Safety assessment of barrels with dangerous fluids under the influence of an external fire • Evaporation of the fluid and release of toxic gases into the environment

  3. 1. The problem Mode of heating • Heating from the bottom: • fluid is well mixed • Heating from the side walls: • strong temperature stratification • Boiling at the upper surface occurs much earlier, than with regular mixing • Realistic for barrels standing on the bottom, which are exposed to an external fire

  4. 2. Experiments Experiments • Cylindrical water tank • D = 0.25 m, • H = 0.25 m • M = 10 kg • Side walls electrical heated: P = 4 kW

  5. 2. Experiments Measurements At different height levels: • Temperatures • Local Void fractions

  6. 2. Experiments Measured Temperatures

  7. 3. CFD-Simulations ANSYS/CFX concept modelling subcooled boiling at a heated wall • Constant pressure  given Tsat • Overall heat flux Qw given • Heat flux partition: Qw = Qf + Qe + Qq Qf - single phase convection Qe - evaporation Qq - quenching: (departure of a bubble from the heated surface  cooling of the surface by fresh water)

  8. 3. CFD-Simulations Measured and calculated temperatures in the centre (CFX-11 calculation)

  9. 3. CFD-Simulations Interpretation of the observed phenomena

  10. 3. CFD-Simulations Interpretation of the observed phenomena • An upper region with well mixed fluid and a lower region with remaining temperature stratification is established • The boundary between the two regions gradually moves downward • When the boundary passes a thermocouple • temperature jump

  11. 3. CFD-Simulations Influence of the near wall mesh resolution on the calculated temperature stratification 0.5 mm; y+=5 2.5 mm; y+=12

  12. 3. CFD-Simulations Doubled tank diameter

  13. 3. CFD-Simulations Doubled diameter

  14. References • A. Aszodi, E. Krepper, H.-M. Prasser: Experimental and numerical investigations of one and two phase natural convection in storage tanks; Heat and Mass Transfer 36 (2000) 497-504 • E. Krepper, A. Aszodi. H.-M. Prasser: Experimental and numerical investigations of one and two phase natural convection in storage tanks; 4th International Conference on Multiphase Flow, New Orleans, Mai 2001, paper 515

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