The mechanisms responsible for ice propagation on surfaces undergoing dropwise
condensation have been determined and characterized. Based on experimental data
acquired non-invasively with high speed quantitative microscopy, the freezing process
was determined to occur by two distinct mechanisms: inter-droplet and intradroplet
ice crystal growth. The inter-droplet crystal growth mechanism was responsible
for the propagation of the ice phase between droplets while the intra-droplet
crystal growth mechanism was responsible for the propagation of ice within individual
droplets. The larger scale manifestation of these two mechanisms cooperating in
tandem was designated as the aggregate freezing process.
The dynamics of the aggregate freezing process were characterized in terms of
the substrate thermal di usivity, the substrate temperature, the free stream air humidity
ratio, and the interfacial substrate properties of roughness and contact angle,
which were combined into a single surface energy parameter. Results showed that for
a given thermal di usivity, the aggregate freezing velocity increased asymptotically
towards a constant value with decreasing surface temperature, increasing humidity,
and decreasing surface energy. The inter-droplet freezing velocity was found to be
independent of substrate temperature and only slightly dependent on humidity and
surface energy. The intra-droplet freezing velocity was determined to be a strong function of substrate temperature, a weaker function of surface energy, and independent
of humidity. From the data, a set of correlational models were developed
to predict the three freezing velocities in terms of the independent variables. These
models predicted the majority of the measured aggregate, inter- and intra-droplet
freezing velocities to within 15%, 10%, and 35%, respectively.
Basic thermodynamic analyses of the inter- and intra-droplet freezing mechanisms
showed that the dynamics of these processes were consistent with the kinetics
of crystal growth from the vapor and supercooled liquid phases, respectively. The
aggregate freezing process was also analyzed in terms of its constituent mechanisms;
those results suggested that the distribution of liquid condensate on the surface has
the largest impact on the aggregate freezing dynamics.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2010-05-8038 |
| Date | 2010 May 1900 |
| Creators | Dooley, Jeffrey B. |
| Contributors | O'Neal, Dennis L. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | thesis, text |
| Format | application/pdf |
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