Role of nanoscale roughness in the heat transfer characteristics of thin film evaporation

Abstract

Thin film evaporation yields high local heat fluxes that contributes significantly to the total heat transfer rate during various two-phase transport processes including pool boiling, flow boiling, and droplet evaporation, among others. Recent studies have shown a strong correlation between the roughness of a surface and its two-phase heat transfer characteristics, but the underlying role of nanoscale surface roughness in thin film evaporation is not fully understood. In the present work, a thin film evaporation model is developed that accounts for the role of the roughness-affected disjoining pressure and flow permeability in determining the film thickness profile and heat transfer rate. Nanoscale surface roughness leads to a flatter evaporating meniscus profile when the effect of disjoining pressure is more pronounced of the two and promotes evaporation, consistent with previous experimental observations. However, our results reveal that surface roughness may also inhibit evaporation and lead to a steeper evaporating meniscus profile when flow permeability has the more pronounced influence on thin film evaporation. It is important to identify the specific surface roughness characteristics that determine whether disjoining pressure or flow permeaiblity has the stronger influence. To this end, a parametric study is performed that analyzes thin film evaporation on V-grooved surfaces of different depths and pitches. While the heat transfer rate increases monotonically with groove depth, there exists an optimal groove pitch that leads to a maximized evaporation rate. Also, when the groove pitch is smaller than a critical value, surface roughness inhibits thin film evaporation.

A Review of Nano and Microscale Heat Transfer: An Experimental and Molecular Dynamics Perspective

“…This surface roughnessinduced evaporation results in a flatter evaporating meniscus profile. Hu et al [245], in one of their studies, revealed that flow permeability has the most significant impact on thin-film evaporation. Their findings indicate that higher average surface roughness may also restrict evaporation and result in a steeper evaporating meniscus profile.…”

Heat Transfer Enhancement-a Brief Review of Literature in 2020 and Prospects

“…They also reported an optimum pit-to-pit spacing for maximum boiling enhancement, which was identical to capillary length. Hu et al (2020a) reported that roughness enhances thin-film evaporation when the effect of disjoining pressure is more pronounced and it inhibits thick-film evaporation when the effect of flow permeability is more pronounced.…”

Laser‐Tuned Surface Wettability Modification and Incorporation of Aluminum Nitride (AlN) Ceramics in Thermal Management Devices

“…[24,25] At the same time, wettability engineering (bydesign modulation of the physicochemical surface propertiesboth surface chemistry and physical microstructure) has been known to enhance phase-change heat transfer both in condensation (often by orders of magnitude in the absence of noncondensable gases) and evaporation configurations. [26][27][28][29][30] In view of the above, wettability engineering of AlN surfaces becomes an obvious path for exploiting the favorable properties of these ceramics in developing next-generation thermal management devices that have high CTE compatibility with Silicon. Such properties are apposite for use in hermetically sealed thermal management devices, such as vapor chambers (VCs) and heat pipes, which rely on the phase change of a working fluid.…”