(230a) Spontaneously Oscillating Menisci: Maximizing Evaporative Heat Transfer By Inducing Condensation | AIChE

(230a) Spontaneously Oscillating Menisci: Maximizing Evaporative Heat Transfer By Inducing Condensation


Nguyen, T. - Presenter, Rensselaer Polytechnic Institute
Plawsky, J., Rensselaer Polytechnic Institute
Wayner, P. C. Jr., Rensselaer Polytechnic Institute
oscillating menisci: Maximizing evaporative heat transfer by inducing

Thao T.T. Nguyen, Joel L.
Plawsky, and Peter C. Wayner, Jr.

Department of Chemical
and Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA.


Understanding the fluid dynamics and
the phase change heat transfer process within a thin liquid film is important
to improve the performance of many industrial processes like coating or
distillation. Studies by our group and other research teams showed that thin
liquid films begin to oscillate spontaneously as the heat flux increases. We
also found that the oscillation amplitude and frequency increase with
increasing heat input. This implies that there is a heat transfer advantage to
an oscillating thin film. We developed a numerical model to try and understand
if there is an advantage to oscillation and under what conditions that
advantage occurs. We found that oscillation can enhance net evaporative heat
transfer but only if a short period of condensation exists within each
oscillation cycle. Such condensation can be driven by intermolecular forces,
capillary forces, Marangoni forces, or combinations of all three as we
concluded from recent heat pipe experiments.  Condensation increases the liquid
film thickness at the contact line, and therefore decreases the disjoining
pressure impediment to evaporation. These short condensation periods followed
by fast evaporation appear as “spikes” in the liquid film thickness over time.
These “spikes” were observed experimentally and mimicked by the simulations
(Fig. 1). Our calculations also show that the heat transfer efficiency
increases with increasing oscillation frequency and amplitude in qualitative
agreement with experiment.


The finding that condensation helps
with overall evaporation heat transfer process may explain why we observed
thick, oscillating, condensed liquid films on the wall surfaces at the heated end
of the Constrained Vapor Bubble (CVB) wickless heat pipe in micro gravity previously. The results from our simulation are very encouraging. An
experimental system should be developed so that both the liquid film thickness
and heat flux profiles can be acquired simultaneously to study the oscillation
phenomenon in a liquid meniscus.


(a)                                                                     (b)

Fig. 1. Oscillation in a liquid meniscus
over time. (a) The interferometry fringe pattern of a liquid meniscus at a
fixed location over time was extracted and stacked together to show the
instability in the liquid film with time. The spikes were observed. The data
were obtained from our experiment. (b) Simulate liquid film thickness ahead of
the contact line region and the total heat released over time with the “spikes”
in the thickness profile.


This material is based on the work
supported by the National Aeronautics and Space Administration (NASA) under Grant
No. NNX13AQ78G and the National Science Foundation under Grant No.