Early Evaporation of Microlayer for Boiling Heat Transfer Enhancement

Abstract

For over five decades, enhancement in pool boiling heat transfer has been achieved by altering the surface wetting, wickability, roughness, nucleation site density and providing separate liquid/vapor pathways. In this work, a new enhancement mechanism based on the early-evaporation of the microlayer is discovered and validated. The microlayer is a thin liquid film present at the base of a vapor bubble. Presence of micro-ridges on the silicon-dioxide surface partitions the microlayer and disconnects it from bulk liquid causing it to evaporate sooner, thus leading to increase in bubble growth rate, heat transfer, departure frequency and critical heat flux (CHF). Compared to a plain surface, ~120% enhancement in CHF is obtained with only ~18% increase in surface area. A CHF enhancement map is developed based on ridge height and spacing, resulting in three regions of full, partial and no enhancement. The new mechanism is validated by comparing the growth rate of a laser created vapor bubble on ridge-structured surface and plain surface, and the corresponding prediction of CHF enhancement is found to be in good agreement with experimental boiling data. This discovery opens up a new field of CHF enhancement and can be coupled with existing techniques to further push the limits of boiling heat transfer.

Transition between thin film boiling and evaporation on nanoporous membranes near the kinetic limit

“…The critical heat flux (CHF) in pool boiling represents the maximum heat flux that can be dissipated under safe operation conditions, which is usually on the order of 100 W cm -2 for a flat surface for water at the atmospheric pressure. Extensive efforts have been made to enhance the CHF and decrease the wall superheat using various strategies, including managing the bubble nucleation sites [5][6][7], increasing the surface wettability/capillarity using micro/nanostructures (which usually also increases the nucleation site density and provides a fin effect with an enhanced heat transfer area) [8,9], increasing the contact line length [10,11], providing separate liquid-vapor pathways for enhanced macroconvection [12,13], preventing bubble coalescence by pinning the contact line [14], and combination of multiple mechanisms stated above. Significant CHF enhancement has been demonstrated up to ~400 W cm -2 [15], but the CHF is still relatively low compared with flow boiling and other new configurations using water [16,17].…”

Simulations of microlayer formation in nucleate boiling

“…Predicting boiling heat transfer has therefore garnered significant attention, but remains complicated by the need to consider phenomena occurring over multiple scalessee Figure 1, from the adsorbed liquid layer at the wall at the nanometer scale -see Churaev (1975) and Chung et al (2011), up to the bubble diameter at the millimeter scale, see Dhir (1998), Stephan & Kern (2004), Kim (2009), Kunkelmann & Stephan (2010), Guion et al (2013), Jiang et al (2013), Jung & Kim (2014), Sato & Niceno (2015), Giustini et al (2016), Zou et al (2016a), and Zou et al (2016b).…”

Microscale Morphology Effects of Copper–Graphene Oxide Coatings on Pool Boiling Characteristics

“…2. Increasing evaporation rates from the microlayer under a bubble has been found to increase the boiling performance as demonstrated by Kandlikar [12], Maroo [28], and Zou et al [29]. This can be achieved by increasing the microlayer volume under a growing bubble [8,11].…”

“…The nucleation activity is further promoted by the enhanced evaporation through microlayer partitioning mechanisms. Maroo's group introduced this mechanism by fabricating silicon ridges in the nano/microscale [11,29]. The microlayer is a thin liquid film that forms underneath a growing bubble.…”

“…The microlayer is a thin liquid film that forms underneath a growing bubble. By employing ridges, Zou and Maroo [11,29] demonstrated that the microlayer gets partitioned which results in additional liquid volume available for evaporation. Through analytical models, they demonstrated that this behavior results in increased bubble volume and frequencies.…”