Water beats air in Hong Kong-led scientific heat transfer breakthrough

·3-min read

A Hong Kong-led research team has cracked a centuries-old problem that has made it impossible to use water for cooling extremely hot surfaces, from rocket engines to frying pans.

Lead researcher Wang Zuankai, a chair professor of mechanical engineering at City University of Hong Kong, first came across the phenomenon as a 10-year-old in his hometown in China’s eastern province of Shandong.

“A magician came to our village and performed walking barefoot on red-hot iron pillars. I didn’t dare to watch,” Wang said.

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“Now I know it’s the Leidenfrost effect – he wasn’t hurt because he dipped his feet in water before dashing across the pillars, creating a layer of vapour which served as protection.”

Named after the German doctor Johann Gottlob Leidenfrost, who first described it in the 18th century, the effect is often observed in kitchens – when water droplets bounce like beads on hitting a hot pan.

A vapour barrier is formed between the liquid and surface which prevents the water from boiling and evaporating, reducing heat transfer at high temperatures.

While the effect is a boon to firewalkers, it has been a costly problem in steelmaking, aerospace industries and nuclear plants, where rapid cooling from extremely high temperatures is a common need, usually solved by using air.

Now Wang and his team of scientists in Hong Kong, mainland China and France say they have solved the 265-year-old thermal cooling challenge, in a paper published on Thursday in the peer-reviewed journal Nature.

The research team has developed a low-cost, thin and flexible film – described by Wang as a “structured thermal armour” – that can withstand temperatures up to 1,150 degrees Celsius (2,102 Fahrenheit).

The film can be welded to flat or curved surfaces and makes it possible for the first time to overcome the Leidenfrost effect and efficiently cool them at ultra-high temperatures using nothing more than water.

The scientists said their new material can potentially improve safety in aerospace engines, defence weapons and next-generation nuclear plants.

The film is formed by tiny conductive steel pillar arrays holding an insulating fibrous membrane that absorbs liquid, which then evaporates. The pillars serve as heat transfer bridges and between them are U-shaped grooves operating as underground channels for the removal of the vapour.

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“The structure is like a sophisticated public transport system that has metro – the vapour – running underground and buses – the liquid – on highways to ease traffic and divert it from crowded roads,” Wang said.

“In our case, vapour and liquid are decoupled – the vapour escapes vertically and liquid goes sideways, so that cooling can happen continuously.”

According to Wang, the new material could be applied to rocket surfaces, as a better alternative to their current thermal insulation layer which is technically challenging to produce, and can easily fall off after launch.

With the film applied, water sprayers could be used to cool a rocket’s surfaces during launch, when they are exposed to extreme temperatures of more than 1,000 degrees Celsius (1,832 Fahrenheit).

Wang said another possible application is engine turbines, which currently use air cooling. The technology could also be applied in closed channels and at very small scale – right down to the creation of tiny water pipes inside quantum computer chips, he suggested.

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