Sprayable Thin and Robust Carbon Nanofiber Composite Coating for Extreme Jumping Dropwise Condensation Performance

TL;DR

This paper introduces a sprayable, durable, and thermally conductive nanocomposite coating that enables sustained jumpwise condensation on metallic surfaces, significantly enhancing heat transfer efficiency in energy systems.

Contribution

A novel thin polytetrafluoroethylene-carbon nanofiber coating with improved durability and scalability that maintains jumpwise condensation under demanding conditions.

Findings

Achieves up to 900% increase in heat transfer coefficient.

Sustains jumpwise condensation for over 60 hours under high shear steam.

Can be applied via spray for versatile and economical fabrication.

Abstract

Condensation of water on metallic surfaces is critical for multiple energy conversion processes. Enhancement in condensation heat transfer efficiency often requires surface texturing and hydrophobicity, usually achieved through coatings, to maintain dropwise condensation. However, such surface treatments face conflicting challenges of minimal coating thermal resistance, enhanced coating durability and scalable fabrication. Here we present a thin (~ 2 {\mu}m) polytetrafluoroethylene - carbon nanofiber nanocomposite coating which meets these challenges and sustains coalescence-induced jumping droplet condensation for extended periods under highly demanding condensation conditions. Coating durability is achieved through improved substrate adhesion by depositing a sub-micron thick aluminum primer layer. Carbon nanofibers in a polytetrafluoroethylene matrix increase coating thermal conductivity and promote spontaneous surface nano-texturing to achieve superhydrophobicity for condensate microdroplets. The coating material can be deposited through direct spraying, ensuring economical scalability and versatility for a wide range of substrates. We know of no other coating for metallic surfaces that is able to sustain jumping dropwise condensation under shear of steam at 111 degC flowing at ~ 3 m s-1 over the surface for 10 hours and dropwise condensation for an additional 50 hours. Up to ~ 900% improvement in condensation heat transfer coefficient is achieved compared to conventional filmwise condensation.