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John O | October 2017

Researchers develop nanoporous membrane evaporator with high heat fluxes


Researchers from the University of Tokyo (Japan) and the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. have published research about the development of an ultrathin, nanoporous membrane evaporator that has a membrane thickness of around 200 nanometers and a pore radius of 65 nanometers, while allowing high interfacial heat fluxes with pure evaporation.

 


Researchers developed an ultrathin membrane evaporator. (University of Tokyo)

 

According to a report from Nanowerk, the researchers were able to create this evaporator by reducing the thermal and fluidic resistance in the liquid phase and the clogging risk of non-evaporative contaminants.

 

“With an evaporation into air experiment, the researchers experimentally demonstrate the validity of the Maxwell-Stefan equation when the interfacial heat flux is high,” the article explained. “They note that the high flux evaporative transport was assisted by the small boundary layer thickness δ. If scaling up the system, this δ will become larger, which can increase the vapor diffusion resistance.”

 

The evaporator works by flowing liquid across the membrane into the nanopores where it is heated by a gold layer and evaporates into the air.

 

The article continued, “The team set the input heating power and waited for the system to equilibrate at a certain temperature. The system inherently contained a feedback loop. As the heating power was set to a higher value, the membrane temperature also increased, which gave rise to more intense evaporation and a higher cooling rate.”

 

When the cooling rate and heating power were equal, then the system achieved a steady state. The system took once second to respond and was held at a steady state for five minutes to record the data.

 

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

 

“Evaporation is a ubiquitous phenomenon found in nature and widely used in industry. Yet a fundamental understanding of interfacial transport during evaporation remains limited to date owing to the difficulty of characterizing the heat and mass transfer at the interface, especially at high heat fluxes (>100 W/cm2).

 

“In this work, we elucidated evaporation into an air ambient with an ultrathin (≈200 nm thick) nanoporous (≈130 nm pore diameter) membrane. With our evaporator design, we accurately monitored the temperature of the liquid–vapor interface, reduced the thermal–fluidic transport resistance, and mitigated the clogging risk associated with contamination.

 

“At a steady state, we demonstrated heat fluxes of ≈500 W/cm2 across the interface over a total evaporation area of 0.20 mm2. In the high flux regime, we showed the importance of convective transport caused by evaporation itself and that Fick’s first law of diffusion no longer applies.

 

“This work improves our fundamental understanding of evaporation and paves the way for high flux phase-change devices.”

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