Phase change

The phase transition from one physical state to another is associated with the absorption or release of a substance- and state-dependent amount of energy, the entropy of the substance changes accordingly. For pure substances the phase transition takes place at a certain temperature, for alloys or mixtures of substances the transformation often takes place within a temperature range. The amount of energy required to exceed this temperature limit is significantly greater than that required to heat the same substance by the same temperature difference without the simultaneous phase change.

If this energy necessary for changing the state of aggregation is not otherwise introduced into the material, it must be withdrawn or supplied from the adjacent components or the adjacent fluid.

Computation time / model size

For the simulation of temperature fields this means a rapid change of the system equations. The thermal conductivity at the phase transition can well increase by two to three powers of ten in the corresponding temperature and time range. This strong non-linearity leads to a significantly increased numerical effort and may require consideration in the model size.

Relevance

In many technical applications, phase transition is deliberately used to increase heat flux densities, e.g. in the cooling of nuclear power plants and internal combustion engines or during hardening and quenching in an oil or water bath. If the effect is present, it must always be taken into account because of its significance.