posted on 2019-02-13, 00:00authored byShakerur Ridwan, Matthew McCarthy
High porosity nanostructured
coatings have been extensively studied
for their use in enhancing liquid-to-vapor phase change due to their
ability to wick liquids laterally across surfaces during boiling.
Although the effect of these coatings on the maximum heat transfer
rate achievable (the critical heat flux) is now well understood, the
impact on boiling efficiency (the heat transfer coefficient) is less
clear. In this work, a novel experimental apparatus is used to take
heat transfer measurements beneath growing and departing bubbles on
nanostructured surfaces. By independently tuning the surface heat
flux and bubble departure time, IR thermography is used to directly
visualize and characterize surface superheat, heat flux, and heat
transfer coefficient during the highly transient bubble ebullition
cycle. It is shown that although flat surfaces exhibit large spatial
and temporal variations in surface temperature and heat flux, the
nanostructured coatings produce a uniform temperature profile with
enhanced heat transfer due to evaporation from the nanostructure-supported
liquid films beneath the bubble. This work demonstrates the relative
importance of advancing and receding contact lines, as well as the
quenching process, on the overall thermal performance of structured
and nonstructured surfaces. It is seen that the combined effects produce
a net increase in heat transfer coefficient of over 30%, averaged
over the entire ebullition cycle and throughout the entire area of
influence. Additionally, the impact of viscous resistance and the
importance of the nanostructure dry-out has been studied by tuning
the ebullition cycle time to create dry spots. This work shows for
the first time the role of nanostructured coatings and thin-film evaporation
during nucleate boiling, and it provides a framework to understand
the complicated nature of nanostructured boiling across all portions
of the boiling curve from nucleation to critical heat flux.