Modification and enhancement of cryogenic quenching heat transfer by a nanoporous surface

摘要

Since the average chilldown efficiency for cryogenic systems is only about 8%, significant improvements to heat transfer are needed for many applications. An experiment was performed to evaluate the modification and enhancement on the quenching heat transfer by a nanoporous heat transfer surface in this study. For comparison purposes, two sample surfaces were used. One is the mechanically polished conventional normal aluminum surface serving as the base case and the other is an aluminum surface with the anodized aluminum oxide (MO) nanoporous finish. In this work, the effect of the nanoporous surface on the heat transfer during chilldown in a liquid nitrogen pool is investigated. The results indicated that the nanoporous surface completely modified and enhanced the phase-change heat transfer in all three quenching regimes. Comparing to the conventional surface case, the Leidenfrost temperature was increased by 32 K and the critical heat flux (CHF) was raised by 160% due to the nanoporous surface. However, the most significant modification on the boiling mechanisms by the nanoporous surface was found in the transition regime that is composed of transitional film and transitional nucleate sub-regimes with quite different quenching curve slopes. For cryogenic quenching applications, it is estimated that the nanoporous surface could save 20% in the amount of cryogen consumption by shortening the chilldown time. The modification and enhancement are mainly attributed to the superhydrophilic property and nanoscale nucleation sites offered by the nanoporous surface.