| NSF Org: |
CBET Div Of Chem, Bioeng, Env, & Transp Sys |
| Recipient: |
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| Initial Amendment Date: | July 31, 2012 |
| Latest Amendment Date: | July 31, 2012 |
| Award Number: | 1235881 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Jose Lage
CBET Div Of Chem, Bioeng, Env, & Transp Sys ENG Directorate For Engineering |
| Start Date: | September 1, 2012 |
| End Date: | June 30, 2017 (Estimated) |
| Total Intended Award Amount: | $324,823.00 |
| Total Awarded Amount to Date: | $324,823.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
A-153 ASB PROVO UT US 84602-1128 (801)422-3360 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
UT US 84602-1231 |
| Primary Place of Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | TTP-Thermal Transport Process |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
CBET-1235881
Maynes
Superhydrophobic surfaces are highly water repellent surfaces and are generated by combining microscale structuring and a hydrophobic coating such that liquids will only be in contact with a fraction of the solid surface. Their use in engineering devices occurs anywhere non-wetting surfaces are desired, although future uses are likely to be more expansive. The decrease in surface contact area between the liquid and solid phases, and thus increase in contact with a trapped gas, leads to distinct alterations of the thermal boundary conditions at the plane of the surface. Consequently, the fundamental convection physics for liquid flow over these surfaces differs drastically from classical behavior. A comprehensive understanding of the local thermal transport physics is necessary to utilize such surfaces in emerging technologies and is the primary goal of this research project. In general, it is expected that convective heat transfer coefficients will be reduced, droplets will change phase slower, and the boiling curve will be significantly altered for liquid in contact with such surfaces. The objectives of this research project are to characterize the departure from classical transport behavior due to the superhydrophobic nature of a surface. Specifically convective heat transfer is explored for: 1) Heat transfer to static liquid droplets resting on superhydrophobic surfaces, 2) Mini- and microchannel, developing and developed transport in channels with SH walls, 3) Influence on free convection dynamics for vertical SH surfaces adjacent to a liquid layer, and 4) Alteration of the classical boiling curve for liquids on SH surfaces. The research is conducted using complementary laboratory based experimental and analytical approaches to explore these topics for a range of typical superhydrophobic surface topologies. The results will be a knowledge base allowing prediction of convective heat transfer coefficients for the scenarios explored and as a function of the superhydrophobic surface topologies.
This project will explore the influence of superhydrophobicity on convective heat transfer at liquid-solid surface interfaces. Uses of superhydrophobic surfaces include self-cleaning surfaces, drag reducing surfaces, non-wetting surfaces in condensers, microfluidic manipulators in lab-on-a-chip concepts, microscale heat exchangers, and many more. For many of these, and a wide range of other potential applications, the issue of thermal transport at superhydrophobic surfaces is of fundamental importance for their implementation into high performance thermal systems. This project will advance basic knowledge related to heat transfer at superhydrophobic surfaces and provide increased understanding for implementation of such surfaces into optimized engineering devices.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This work has explored thermal transport at superhydrophobic surface for channel flows, pool boiling and heating, and jet/droplet impingement. For all scenarios considered superhydrophobic surfaces yield a reduction in the heat transfer that occurs. In essence a temperature jump at the wall occurs. This is caused by the air-filled cavity regions that lie in between microscale surface structuring, which are required to render a surface superhydrophobic. The temperature jump is a strong function of the surface feature size and spacing on the surface. Below are specific insights gained by the work.
Heat Transfer in Microchannels
Analytical and computation modeling suggest large reductions in heat transfer may occur in microscale flows, that coincide with reductions in the flow friction that exists. These reductions are strong functions of the channel size and the size of the superhydrophobic features. Modeling of several common superhydrophobic surface types were considered and have provided a comprehensive understanding of heat transfer in superhydrophobic microchannels in the absence of mass transfer. However, experiments reveal that mass transfer is a dominant mechanism and heating of channel walls results in large nucleation of air on the surfaces. These air bubbles significantly distort the flow field and cause very large pressure fluctuations and a dramatic increase in flow resistance. Further, the heat transfer is reduced beyond what would be expected for a superhydrophobic surface without nucleation. This result is the first experimental evidence of this phenomena. The surface feature orientation and geometry play a pivotal role in how the air bubbles grow and then are flushed downstream during periodic events.
Heat Transfer in a Pool at Temperatures Below the Boiling Point
Experiments for this scenario also reveal large influence of nucleation and mass transport that makes separating the thermal transport and mass transport process impossible for consideration of real systems. The nucleation is more manifest at larger surface temperatures and greater superhydrophobicity.
Pool Boiling
Experiments considering classical pool boiling reveal a marked deviation in the boiling dynamics compared to typical surfaces. For nearly all superhydrophobic surfaces considered, transition from nucleate to film boiling occurs at a small temperature difference between the surface and the pool. The overall heat transfer is reduced by 80-90% in the typical nucleate boiling regime and Leidenfrost behavior occurs at surface temperatures only 5-8 degrees higher than the boiling point. This is in contrast to traditional surfaces where the Leidenfrost point occurs at greater than 100 degrees above the boiling point.
Droplet Impingement
Droplets that impact surfaces at temperatures much higher than the boiling temperature undergo rapid atomization. The amount of atomization is significantly suppressed for superhydrophobic surfaces, concomitant with decreased heat transfer during the impingement process. Mechanisms for why this happens were deduced and maps showing under what conditions atomization would occur were developed.
Jet Impingement
Analytical modeling of the heat transfer for an impinging jet was performed. The results show heat transfer behavior that is much less in magnitude than for a jet impinging on a smooth surface. The overall heat transfer coefficient decreases with increasing cavity fraction of the superhydrophobic surface for all cases considered.
Last Modified: 10/13/2017
Modified by: R Daniel Maynes
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