Where m is the mass, cP is the specific heat capacity of water, A is the cross-sectional diameter of the drop, Δt is the time between deposition and nucleation, ΔTi is the initial temperature difference between the drop and the plate, and ΔTf is the final temperature difference between the nucleation temperature and the plate. The nucleation temperature was separately measured to be –2.2 ºC ± 0.5 ºC. From the data we gathered from experiments, the freezing HTC was measured to be: There is relatively high uncertainty in this measurement because it is based on the nucleation time. By nature, nucleation, whether homogeneous or heterogeneous, is a stochastic phenomenon so there is not a time at which nucleation will always occur. A constant HTC throughout the freezing process is a critical part of improving ice thermal energy storage. Whereas a traditional ice-oncoil HEX has a transient HTC, in which the HTC value decreases over time due to increasing thermal resistance, our new HEX operates under steady state conditions. Over the course of the entire freezing cycle, this results in up to more ice made for the same amount of area.
Conclusions Observations of a new icephobic phenomenon have been reported. Water drops frozen in a silicone oil bath on a PTFE coated cold plate have demonstrated very low adhesion with the substrate. The maximum ice adhesion stress for a small drop was calculated to be 5.21 Pa. Frozen water drops that passively liftoff from the heat exchanger surface allow for elimination of the thermal resistance of ice growing on the heat exchanger surface. The freezing heat transfer coefficient of drops sliding down the heat exchanger surface was measured and recorded. The average freezing HTC for a 50 µL
References
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drop using the HEX was measured to be: Because this HTC is a constant value over the duration of the freezing process, it drastically reduces the required surface area to make ice, and thus reduces overall costs.
Acknowledgements
We would like to thank the Innovation Crossroads program run by Oak Ridge National Laboratory and the NEXUS-NY Clean Energy Seed Accelerator for their support.
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CTI Journal, Vol. 40, No. 2