Dryout stability and inception at low flow rates

Abstract

A theoretical model for the heat flux required to maintain a stable hot patch on a heated surface cooled by a falling liquid film is developed. The theoretical model is based on an existing analytical solution of the two-dimensional heat equation with boundary conditions supplied by heat transfer coefficient correlations appropriate to the fluid conditions at the hot dry patch and at the wet region from the patch. Using physically reasonable correlation forms for the dry patch and wet region heat transfer coefficients, the heat flux is expressed as a function of the liquid film Reynolds number. Existing experimental data were compiled, collected, and used to obtain the correlating lines. The data cover the one-dimensional (low Biot number) region, and indicate that laminar and turbulent liquid film conditions occur. Corresponding correlating equations are obtained from the analytical results. For laminar flow, the dryout heat flux is q d = 0.27 Re 0.5, and for turbulent flow q d = 0.017 Re 0.9, with an uncertainty of the order of ± 50%.