Critical heat flux enhancement in pool boiling through increased rewetting on nanopillar array surfaces
Boiling is a central phenomenon in technological and industrial applications as diverse as thermal management in electronics, power generation and chemical processing. In such applications, it is of immense importance to enhance the energy efficiency by increasing the critical heat flux (CHF), the highest heat flux a boiling substrate can achieve, as well as reducing operational risks caused by the notorious “boiling crisis”, a catastrophic failure in boilers or heat exchanging devices. The boiling crisis occurs on a boiling substrate when excessive vaporization of liquid forms a vapour layer that severely impedes heat transfer through the substrate. This leads to abrupt jump in surface temperature and subsequently irreversible damages to the substrate of the boiling equipment. The temperature T c at which CHF occurs therefore is directly connected to the boiling crisis; a boiling system operating at temperature higher than T c inevitably drifts towards the boiling crisis. As a result, an enhancement in boiling performance, without triggering the boiling crisis, requires significant increases in both the critical heat flux and critical temperature T c.
In this paper, Paul and colleagues have shown that nanopillars fabricated on boiling substrates induce a substantial effect on the boiling behaviour in the nucleate boiling regime. In particular, increasing the height of nanopillars effectively leads to considerable enhancement in both heat flux and surface temperature at CHF. Such enhancement to the nanopillar-induced increase in rewetting velocity, which can be measured in a separate wetting experiment. Based on the observations that the rewetting velocity increases for substrates with higher nanopillars, as well as the assumption that the enhancement in heat flux dominantly takes place at the three-phase contact line, they develop a mechanistic model to predict both the heat flux and the temperature at CHF of nanopillar substrates. This model takes into account the nanoscales of the pillars, and thus, excludes the wicking motion, or imbibition of fluid, as plausible mechanisms for heat transfer enhancement. As a result, the enhanced capillary force due to the presence of nanopillars is the major cause of the intensified rewetting process and subsequent increases in heat flux and temperature at CHF.