Film boiling: modelling, experiments and prediction of critical heat flux
Heat transfer by boiling is one of the most efficient heat transfer mechanism. Amongst others it occurs inside the reactor core of a nuclear power plant. Prevention of critical heat flux (CHF) is of special importance here.
The boiling process consists of different regimes (see figure below). With increasing heat flux the heat transfer mechanism changes from natural convection to partial nucleate boiling and fully developed nucleate boiling. Exceeding a critical value of the heat flux (CHF), which is often referred to as boiling crisis or departure from nucleate boiling, leads to film boiling can occur. As the thermal resistance of the gas film is high, the fuel rods do heat up and the cladding may lose integrity. This has to be avoided under all circumstances. Much effort has been spent in research so far for a proper modelling and prediction. However, there is yet no general model to securely predict critical heat flux under different boundary conditions.
In order to simulate the heat transfer in boiling processes with CFD methods the complex hydrodynamics of the fluid in the bundles of fuel rods with spacer must be considered and modelled. In the near future, stationary models remain preferred for this large-scale system due to the high computational resource requirement of the explicit transient calculations. However, the boiling process is not exactly macroscopically stationary when it reaches critical heat flux. Therefore, experiments are conducted to investigate the complex interplay between heat transfer and flow structure. The experimental facility utilizes x-ray imaging techniques developed at the Institute of Fluid Dynamics, especially the ultrafast x-ray tomography system ROFEX. Together with the theoretical part, this project contributes toward the development of calculation methods of boiling crisis in complex geometries.
Together with the other project partners the transient process from nucleate boiling to film boiling is studied both by means of experimental and numerical tools. Numerical method development includes the modelling of the development of gas fraction, bubble size and distribution and heat transfer in the Euler-Euler two-phase framework near or at CHF. Basing on the RPI heat flux partitioning model for subcooled boiling the approach is extended by further model parts describing the boiling sublayer. In the bulk the GENTOP-concept developed at HZDR will be applied with the inhomogeneous MUSIG model in ANSYS CFX, to describe development of larger gaseous structures in the slim subchannels. In order to validate the model and better understand the physical mechanisms of two-phase flow at CHF, we perform a series of experiments with a refrigerant-operated bundle facility and a steam/water facility.
- Technische Universität Dresden, Germany
- Helmholtz Zentrum Dresden Rossendorf, Germany
- Technische Universität München, Germany
- Karlsruher Institut für Technologie KIT, Germany
- Eidgenössische Technische Hochschule Zürich (ETH), Switzerland
- Gesellschaft für Reaktorsicherheit GRS, Germany
- AREVA NP, Germany
- Ansys GmbH, Germany
This work is part of the research project „NUBEKS“ and is funded by the German Federal Ministry of Economic Affairs and Energy (BMWi) with the grant number 1501473С on the basis of a decision by the German Bundestag. Responsibility for the content of this report lies with the authors."
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