Recent achievements in the thermal hydraulics of high heat flux components in fusion reactors

Some components of fusion thermonuclear reactors, such as divertors, plasma limiters, or first-wall armor, are believed to be subjected to operating conditions characterized by extremely high thermal loads. It is therefore necessary to remove from the surface of these components very high heat fluxes, ranging from 2 to 60 MW/m². Water subcooled flow boiling, under conditions of high mass flux, high liquid subcooling, and small to intermediate channel diameter, can accomodate these very high heat fluxes. Further enhancement of the upper limit of cooling, the critical heat flux (CHF), can be obtained by making use of turbulence promoters such as twisted tapes and coiled wires even if coupled with a relevant increase in pressure drop. An overview is presented of recent achievements obtained in water subcooled flow boiling CHF under operating conditions of interest to the thermal hydraulic design of fusion reactors. Observed basic parametric trends—CHF as a function of mass flux, pressure, subcooling, and channel geometry—are outlined, together with findings on the use of CHF enhancement techniques. From experiments it was seen that water subcooled flow boiling allows CHF conditions as high as 228 MW/m² to be achieved under extreme geometric and thermal hydraulic conditions. On the other hand, design and engineering boundary conditions limit variation in these conditions, and a suitable compromise has not yet been reached. Predictive tools are presented for the evaluation of subcooled flow boiling CHF both in straight tubes and with twisted tapes, and are assessed with reference to recent available experimental data.

Although several indications for practical applications can be found in recent achievements, a full understanding of the basic mechanisms of heat transfer and CHF in subcooled flow boiling has not yet been achieved. Future research to overcome the present lack of knowledge in this field is suggested.