Parametric Trends for Post-CHF Heat Transfer in Rod Bundles

A 3 × 3 rod bundle with a heated shroud was developed to study post-critical-heat-flux (post-CHF) dispersed-flow boiling. The hot-patch technique was applied to a rod bundle, which successfully arrested the quench front at the test section inlet. Measurements included mass flux, wall heat flux, inlet equilibrium quality, wall temperatures along the bundle axis, and actual vapor temperatures upstream and downstream of a spacer grid. The vapor superheat (up to 600°C) increased with increasing wall heat flux and decreasing mass flux and vapor quality. The heat partition ratio (fraction of total heat input that goes toward evaporation) was found to increase with increasing mass flux and decreasing inlet quality but remained essentially independent of heat flux. The results for the rod bundle were found to be in good agreement with trends previously reported for post-CHF heat transfer in single tubes.