An experimental and numerical investigation of post-CHF heat transfer for one-sided heat load with highly sub-cooled flow boiling

One-sided high heat fluxes are very commonly found in fusion reactor plasma facing components for proposed long-pulse devices, such as ITER. Heat fluxes between 5 and 30 MW m−2 are removed by a highly sub-cooled flow of boiling water. For one-sided heat loads, all boiling regimes (single-phase, nucleate, fully developed sub-cooled, transition and film boiling) can be present under a stable condition which, in certain cases, can exceed the local critical heat flux (CHF) value. A thorough numerical and experimental analysis of the post-CHF phenomenon under a one-sided heat load has been carried out. The entire boiling curve was described with available correlations for all the regimes, except for the transition regime, where an assumed path was used. Numerical results for a circular channel under a non-uniform, one-sided heat load showed stable post-CHF temperature distributions. The sensitivity of the numerical analysis to the assumed transition boiling region was studied. During electron beam heating experiments, temperature distributions in a circular channel and a diverter plate (DP) were measured, and the ultimate load (“burnout”) was observed in circular channels with and without twisted tapes. The enhancement of the ultimate heat flux over the local CHF value was found to be 2 for a DP and 1.5 for a round channel. It was shown that the enhancement factor η is highly dependent on the wall thickness and flow parameters. Good agreement was found when the numerical results for the temperature distributions were compared with the experimental data. An increase of about 100% in the burnout heat flux was observed in circular channels with a twisted tape. The overall results show that safety margins in actively cooled plasma facing components can be increased when conditions for stable post-CHF exist.