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Boiling on Nano-Structured Surfaces
Keywords: nanotechnology, boiling, critical heat flux
The impact of nano-structured surfaces on pool boiling heat transfer rates is experimentally investigated in this study. Several experimentss are performed using bare silicon wafers and wafers coated with Multi-Walled Carbon Nanotubes (MWCNT) of 9-m (Type-A) and 25-m (Type-B) in height and 8-16 nm diameters. The experiments are performed under saturation and subcooling conditions of 5C, 10C and 20C. Experiments are also performed using silicon wafers with etched nano-structures which are 100-200 nm diameter, 100-650 nm height and 800 nm 1 micron pitch. The wafers are patterned using Step and Flash Imprint Lithography (SFIL) process with Reactive Ion Etching (RIE). The experiments on the imprinted wafers are performed under conditions of saturation and subcooling of 10C and 20C. The experimental setup is of constant heat flux type and the test fluid in all cases is PF-5060, a perfluorocarbon refrigerant. Enhancements in heat transfer rates range from 37.8-62.6% for Type-A CNT and 27.1-32.8% for Type-B CNT at a wall superheat of 10 C for varying levels of liquid undersaturation in the nucleate boiling regime. At CHF these enhancements range from 36.2-52.4% for Type-A CNT and from 20.7-57.9% for Type-B CNT. Type-A CNT shows little to no enhancement in heat transfer rates in the film boiling regime. Type-B CNT however, shows heat transfer rate enhancements ranging from 62.24-147.92% in the film boiling regime. Contact angle measurements performed on the CNT wafers indicate that the presence of CNT changes the surface from hydrophilic to hydrophobic. Contact angle measurements on the silicon nanostructures show a similar yet anomalous trend in changing the surface behavior from hydrophilic to hydrophobic.
Sharan R. Shriraman, Graduate Assistant Researcher
Texas A&M University
College Station, TX

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