Dr. Dirk Lucas

Head Computational Fluid Dynamics
Phone: +49 351 260 2047

Flashing flow

Flash boiling is a kind of phase change from liquid to vapour due to depressurization, which has a fundamental and decisive presence in many industrial and technical applications. For example, it can be encountered in flow through divergent nozzles, blow-off valves or cracks at pressurized liquid containers or in natural circulation cooling systems.

Since the middle of last century there have arisen many theoretical and experimental investigations on flash boiling flow owing to great concern in nuclear safety. So far one-dimensional approaches or system codes are routinely applied to deal with such kind of flow configurations. However, the flashing two-phase mixture has a strongly three-dimensional nature, which is accompanied with large heterogeneous gaseous structures and high gas volume fractions. All of these features along with the micro-scale bubble dynamic processes require a more sophisticated prediction tool with high time and space resolution such as the CFD technology. Encouraging CFD researches on flashing flows have been published recently. They are all based on the framework of a simplified two-fluid-model for bubbly flow. Here, one major challenge is to reproduce bubble dynamics such as nucleation, growth, coalescence and breakup via reliable closure models.

Although wall nucleation is dominant, bulk nucleation is shown to be non-negligible in many practical systems due to the presence of impurities or dissolved gases.

Flashing evaporation of water inside a large vertical pipe under pressure release transients is simulated using the iMUSIG-approach. The temporal evolution of bubble size distribution is successfully reproduced, which provides a reliable Sauter mean bubble diameter for the computation of interfacial exchange rates.

The models were applied to the investigation of boiling flows in large volumes of nuclear reactors.

Selected publications

  • Liao, Y.; Lucas, D.
    Possibilities and limitations of CFD simulation for flashing flow scenarios in nuclear application.
    Energies 2017, 10, 139; doi:10.3390/en10010139
  • Liao, Y.; Lucas, D.
    Computational modelling of flash boiling flows: A literature survey.
    International Journal of Heat and Mass Transfer, 111, 246-265
  • Janet, J.; Liao, Y.; Lucas, D.
    Heterogeneous nucleation in CFD simulation of flashing flows in converging-diverging nozzles.
    International Journal of Multiphase Flow, 74, 106–117
  • Liao, Y.; Lucas, D.
    3D CFD simulation of flashing flows in a converging-diverging nozzle.
    Nucl. Eng. Des., 292, 149-163
  • Liao, Y. et al
    Interfacial heat transfer models for flashing flows: a review.
    13th international conference Multiphase Flow in Industrial Plant, Sept.17-19, Genova, Italy
  • Liao, Y.; et al.
    Assessment of CFD predictive capacity for flash boiling.
    CFD4NRS-5, Zurich, Switzerland, Sept. 09 -11, 2014
  • Liao, Y., et al.
    Flashing evaporation under different pressure levels.
    Nucl. Eng. Des. 265, 801-813
  • Liao, Y.; et al.
    CFD simulation of flashing boiling flow in the containment cooling condenser system of KERENA reactor.
    21st International Conference on Nuclear Engineering, ICONE21, Chengdu, China, July 29 - Aug. 02, 2013
  • Liao, Y.; et al.
    CFD simulation of water evaporation under pressure release transients.
    15th International Topical on Nuclear Reactor Thermalhydraulics, NURETH15, Pisa, Italy, May 12 -17, 2013