10.25394/PGS.11209403.v1
Aakrati Jain
Aakrati
Jain
Characterization of Flow Freezing in Small Channels for Ice Valve Applications
Purdue University Graduate School
2019
heat transfer processes
microfluidic chip
flow freezing
response time
closing time
cryogenic cooling
ice modes
ice valve
Mechanical Engineering
2019-12-03 13:53:05
Thesis
https://hammer.purdue.edu/articles/thesis/Characterization_of_Flow_Freezing_in_Small_Channels_for_Ice_Valve_Applications/11209403
<p>Freezing of water flowing through a small channel can be used
as an efficient and cost-effective flow control mechanism for microfluidic platforms. Ice valves provide a leak-proof, non-invasive and
high-pressure-tolerant method of flow control compared to their conventional
micromechanical counterparts. To develop,
design and implement ice valves an understanding of the processes and
parameters that govern ice formation in small channels is required. The aim of this dissertation is to understand
the freezing process of water flowing in small diameter channels and the
factors affecting the same, so as to develop a simple physical model that predicts
the ice growth process and channel closing time. Further, a stand-alone ice valve formation
device is developed that is suitable for implementation in high-pressure
microfluidic applications.</p>
<p> </p>
<p>While
ice valves have various advantages over conventional microvalves, their successful
implementation is in part hindered due to their long response time. An understanding of the factors that affect
the ice growth process during flow freezing in small diameter channels (commonly
encountered in microfluidic devices) would allow reduction and control of the
response time of ice valves. In
this dissertation, freezing in a pressure-driven water flow through a channel is
investigated using measurements of external channel wall temperature and flow
rate synchronized with high-speed visualization. A test setup is designed and demonstrated to
control the external cooling boundary conditions during visualization of the
ice formation modes in a small channel; the external wall thermocouple and the
water flow rate readings are synchronized with the high-speed images. Firstly, the effect of water flow rate on the
freezing process is investigated in a glass channel of 500
m inner diameter in terms of the external wall
temperature, the growth duration of different ice modes, and the channel
closing time. Freezing initiates as a
thin layer of ice dendrites that grows along the inner wall and partially
blocks the channel, followed by the formation and inward growth of a solid
annular ice layer that leads to complete flow blockage and ultimate channel
closure. A simplified analytical model
is developed to determine the factors that govern the annular ice growth, and
hence the channel closing time. The
model identifies the
water flow rate and the channel diameter as the two key parameters that govern
the channel closing time. For a
given channel, the model predicts that the annular ice growth is driven purely
by conduction due to the temperature difference between the outer channel wall
and the equilibrium ice-water interface.
The flow rate affects the initial temperature difference, and thereby
has an indirect effect on the annular ice growth. Higher flow rates require a lower wall temperature
to initiate ice nucleation and result in faster annular ice growth (and shorter
closing times) than at lower flow rates.
</p>
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