By Josh Perry, Editor
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Researchers at the University of Hawaii at Manoa have created a novel concept to overcome the critical heat flux (CHF) of boiling by 10 percent to utilize the latent heat of converting water to vapor to cool high-heat surfaces, such as those in nuclear power plants.

Bending of the nano-bimorph under temperature change. (University of Hawaii at Manoa)

According to a report from the university, previous efforts at increasing the CHF have focused on using nanowires to roughen the surface. Roughening the surface increases the number of sites where bubbling occurs.

This new study involves coating the hot surface with nanoscale bimorphs, a piece of metal that bends when exposed to heat. Thermal expansion causes the bimorphs to deform spontaneously and makes the surface more conducive to boiling.

Researchers will work on new geometries for the bimorphs to see what works best in different applications.

The research was recently published in Nano Letters. The abstract read:

“We present a new concept of a structured surface for enhanced boiling heat transfer that is capable of self-adapting to the local thermal conditions. An array of freestanding nanoscale bimorphs, a structure that consists of two adjoining materials with a large thermal expansion mismatch, is able to deform under local temperature change.

“Such a surface gradually deforms as the nucleate boiling progresses due to the increase in the wall superheat. The deformation caused by the heated surface is shown to be favorable for boiling heat transfer, leading to about 10% of increase in the critical heat flux compared to a regular nanowire surface.

“A recently developed theoretical model that accounts for the critical instability wavelength of the vapor film and the capillary wicking force successfully describes the critical heat flux enhancement for the nanobimorph surface with a good quantitative agreement.”