Influence of channel diameter on subcooled flow boiling burnout at high heat fluxes

Among the many technical challenges that fusion technology has given rise to over the recent past, particular interest was reserved to the handling of the plasma and the heat from fusion reactions. In particular, some components of fusion reactors, such as divertors, plasma limiters, neutral beam calorimeter, ion dump and first-wall armor, are estimated to be subjected to very high heat loads. Heat fluxes to be removed range from 2 to 60 MW m-2, and forced convective subcooled boiling can accommodate these very high heat fluxes. However, successful use of subcooled flow boiling for high heat fluxes removal requires the critical heat flux (CHF), which is described as a sharp reduction in the energy transfer from a heated surface, not to be reached. The occurrence of CHF, for the case of heat flux controlled systems, results in a significant increase of the wall temperature, which is usually well above that at which serious damage or 'burnout' of the heating surface occurs. The number of data points regarding the effect of diameter on CHF in subcooled flow boiling is limited and even more reduced information is available in operating ranges typical of fusion reactor thermal hydraulics (high liquid velocity and subcooling, i.e. high heat fluxes). The aim of this technical note is to provide a critical status of the art of experiments carried out so far under the above-mentioned conditions with regard to the effect of diameter. (Author)