Effects of two-phase flow conditions on flow boiling CHF enhancement of magnetite-water nanofluids

Experimental studies were conducted to understand the flow boiling CHF (critical heat flux) characteristics of a magnetite-water nanofluid for a wide range of exit qualities, especially for intermediate and relatively high exit qualities. In contrast to previous studies, a wide range of exit qualities was considered for various heating lengths, mass fluxes, and inlet sub-cooling. CHF enhancement was observed when the exit quality was low and a DNB-like thermal crisis occurred: these results were consistent with the previous studies reporting a delay in DNB when using nanofluids. Meanwhile, this CHF enhancement did not occur for high exit qualities. In annular flow and high exit qualities, an LFD-type thermal crisis occurred. This CHF enhancement is not expected using a nanofluid under the LFD condition because LFD phenomena are nearly unaffected by the surface conditions. As the exit quality increased from 0.07 to 0.74, the amount of CHF enhancement gradually decreased and approached zero. In conclusion, this CHF enhancement using a nanofluid is not expected for annular flow with a relatively high exit quality.