Mechanisms for extreme heat transfer conditions in water-cooling of fusion reactor components

In existing fusion reactors conceptual designs, water-cooled impurity control components employ operating heat fluxes up to 15 MW m-2, coolant velocities above 5 m s-1, subcoolings more than 100 K, and pressures below 5 MPa. These conditions are quite different from those most used to obtain existing correlations for subcooled nucleate boiling and critical heat flux. In addition, some available data in the range of interest for fusion reactor components show that extrapolations of the correlations cannot be used. This paper suggests that two heat transfer mechanisms must be incorporated in developing correlations for fusion reactor high heat flux components.First, boiling can be suppressed, resulting in observed non-boiling wall temperatures far above those expected for nucleate boiling. Second, critical heat flux (CHF) can apparently occur with no prior boiling, when the wall temperature reaches the temperature of homogeneous nucleation (THN) while a single phase liquid is adjacent to the wall. A limit of this nature can apparently occur only under conditions of a very high mass flux and a very high bulk subcooling. These two mechanisms must be incorporated in fusion thermal-hydraulics analysis: suppression of nucleate boiling and critical heat flux caused by homogeneous nucleation. (orig.)