Modelling of solute concentration into crud deposits under subcooled boiling conditions

Blondel, P., Girardin, G.
AREVA, Centre Technique Framatome ANP, Dpt Corrosion-Chimie Porte Magenta, Le Creusot France

The modelling of the incorporation of non-volatile solutes from liquid coolant into porous crud deposited on a heating surface under subcooled boiling conditions has been investigated.

A suitable model for the concentration of solutes in porous deposits with steam chimneys, proposed by Pan et al., has been studied and implemented with the FEMLAB® software. With this model, the porous structure of the deposit provides the capillary force that draws the liquid through the interparticulate spaces toward the evaporation heating surface. The resulting vapour escapes from the porous layer by vapour channels (or 'chimneys') of larger cross-sections than the liquid feed capillaries. In this way, 'wick boiling' constitutes a very efficient heat transfer mechanism resulting in relatively small temperature differences between the heating surface and the bulk coolant.

However, the non-volatile solutes carried into the porous deposits by the liquid flow may in fact be concentrated near the chimney wall, especially in the vicinity of the heating surface. This establishes a concentration gradient within the crud and induces possible back-diffusion of the solute across the porous deposits away from the chimney wall and from the heating surface The 2D model is based on the coupling of a heat transfer model and a momentum transfer model in a porous medium (Darcy's law) with a convection-diffusion equation of transport of solutes. Implementation and numerical solving with FEMLAB® reproduce rather successfully the original results presented by Pan et al.: liquid flux velocity field, temperature, pressure and concentration factor (CF) profiles across the crud layer, thus allowing to conduct similar parametric studies.