Film boiling
Film boiling is a form of evaporation of liquids with very high heat flux densities . A continuous vapor film forms on the heating surface, which, in contrast to the well-mixed liquid, has a high heat-insulating effect during nucleate boiling . The heat transfer coefficient is significantly lower with film boiling than with nucleate boiling, since with film boiling the heat transport is dominated by the thermal radiation. As a result, the wall temperature also rises significantly if the heat flow density remains the same and, if the heat flow density is impressed on the system, this can lead to the destruction of the heating surfaces. If the heating surface is only partially covered by a vapor film, it is referred to as partial film boiling, if the heating surface is completely wetted with a vapor film, it is referred to as complete film boiling.
Film boiling in the stable or even unstable range is permissible if the temperature of the heat source is below the maximum permissible operating temperature or if the heat transfer coefficient in film boiling is the controlling resistance for the heat flow.
In technical applications, care must therefore be taken to ensure that the heat flow densities are well below the transition point from bubble boiling to film boiling. The critical heat flow density depends on the pressure, the saturation pressure of the steam and the surface structure of the heating surface. At atmospheric pressure it is > 1000 kW / m² for the media pairing water vapor / steel . Steam boilers are therefore designed in such a way that a heat flow density of 300 kW / m² is not exceeded, in order to avoid the onset of film boiling, especially in the case of a compact boiler design with a tight boiler tube arrangement. The transition to film boiling is reduced by oils or a high salt content in the boiler water.
The film boiling can be clearly understood through the behavior of a drop of water on a hot stove (see Leidenfrost effect ). A vapor film forms under the drop, on which the drop moves across the plate. The water droplet evaporates very slowly due to the poor heat transfer. The value of the heat flux density assumes a local minimum at the so-called Leidenfrost point.
In the case of water-cooled nuclear reactors , it is a critical phenomenon that is usually countered with high pressure. Exceeds the heat flux density at the fuel rod surface , e.g. B. by an accident-related increase in reactor power, a certain value, the so-called critical heat flux density or critical heating surface load, the nucleate boiling changes almost suddenly into stable film boiling. A coherent vapor film forms on the rod surface, which drastically impairs the heat transfer. The surface temperature rises by leaps and bounds and reaches values that can be above the melting point of the cladding tube material. For this, the term burn out has become naturalized from English , as the fuel shell burns through or melts at the affected area.
Individual evidence
- ↑ a b Ruben Steinhoff: Condensation and evaporation on structured tubes. Construction of a test stand to investigate heat transfer coefficients . Springer-Verlag, 2015, ISBN 978-3-658-09530-7 , pp. 14 ( limited preview in Google Book search).
- ↑ a b Helmut Schaefer: VDI-Lexikon Energietechnik . Springer-Verlag, 2013, ISBN 978-3-642-95748-2 , pp. 603, 1316 ( limited preview in Google Book search).
- ↑ Karl Stephan: Heat transfer when condensing and boiling . Springer-Verlag, 2013, ISBN 978-3-642-83159-1 , p. 211 ( limited preview in Google Book search).
- ^ Association of German Engineers, VDI Society Author: VDI-Wärmeatlas . Springer-Verlag, 2013, ISBN 978-3-662-10743-0 , pp. 233-IA192 ( limited preview in Google Book search).
- ↑ Ralf Goedecke: Fluid process engineering basics, methodology, technology, practice . John Wiley & Sons, 2006, ISBN 3-527-31198-X , pp. 281 ( limited preview in Google Book search).
- ↑ TU Dresden Incineration, heat and mass transfer: incineration and steam generation (distance learning)