Computation of and a bit of experimentation
on interfacial flows with evaporation
Steffen Hardt
Institut für Nano- und Mikroprozesstechnik
Leibniz Universität Hannover
Abstract:
An evaporation model compatible with interface-capturing schemes for vapor-liquid flow is presented. The model formulation is largely independent of the specific realization of interface capturing and relies on a continuum-field representation of the source terms implementable in a broad class of CFD models. In contrast to most other numerical methods for evaporating interfacial flows, the model incorporates an evaporation source term derived from a physical relationship for the evaporation mass flux. It is shown that especially for microscale evaporation phenomena this implies significant deviations of the interface temperature from the saturation temperature. The mass source-term distribution is derived from the solution of an inhomogeneous Helmholtz equation that contains a free parameter allowing to tune the spatial localization of the source. The evaporation model is implemented into the volume-of-fluid scheme with piecewise linear interface construction. Results are obtained for various analytically or semi-analytically solvable model problems, showing a good overall agreement between the numerical and the analytically computed data. As more realistic validation cases, film and nucleate boiling problems are considered. In that context, very thin thermal boundary layers and parasitic currents are identified as the most challenging issues. A major advantage of the developed evaporation model is that it does not refer to intrinsic details of the interface capturing scheme, but relies on continuum field quantities that can be computed by virtually any CFD approach.
On the experimental side, results on flow boiling in a micro glass tube with circular cross section are presented. Video sequences of the boiling process were recorded with a high speed camera. Even in such a comparatively simple channel geometry amazingly complex flow phenomena are found, including bubble nucleation, film boiling, film instabilities, and dryout. The future goal is to gain a better understanding of these phenomena by complementing the measurements by numerical computations.