Pool boiling on nano-finned surfaces
Abstract
The effect of nano-structured surfaces on pool boiling heat transfer is explored in this
study. Experiments are conducted in a cubical test chamber containing fluoroinert
coolant (PF5060, Manufacturer: 3M Co.) as the working fluid. Pool boiling experiments
are conducted for saturation and subcooled conditions. Three different types of ordered
nano-structured surfaces are fabricated using Step and flash imprint lithography on
silicon substrates followed by Reactive Ion Etching (RIE) or Deep Reactive Ion Etching
(DRIE). These nano-structures consist of a square array of cylindrical nanofins with a
longitudinal pitch of 1 mm, transverse pitch of 0.9 mm and fixed (uniform) heights
ranging from 15 nm – 650 nm for each substrate. The contact angle of de-ionized water
on the substrates is measured before and after the boiling experiments. The contact-angle
is observed to increase with the height of the nano-fins. Contact angle variation is also
observed before and after the pool boiling experiments.
The pool boiling curves for the nano-structured silicon surfaces are compared with that
of atomically smooth single-crystal silicon (bare) surfaces. Data processing is performed
to estimate the heat flux through the projected area (plan area) for the nano-patterned
zone as well as the heat flux through the total nano-patterned area, which includes the surface area of the fins. Maximum heat flux (MHF) is enhanced by ~120 % for the nanofin
surfaces compared to bare (smooth) surfaces, under saturation condition. The pool
boiling heat flux data for the three nano-structured surfaces progressively overlap with
each other in the vicinity of the MHF condition. Based on the experimental data several
micro/nano-scale transport mechanisms responsible for heat flux enhancements are
identified, which include: “microlayer” disruption or enhancement, enhancement of
active nucleation site density, enlargement of cold spots and enhancement of contact
angle which affects the vapor bubble departure frequency.
Citation
Sriraman, Sharan Ram (2007). Pool boiling on nano-finned surfaces. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /ETD -TAMU -2091.