In the search for ever improved thermal efficiencies of refrigeration equipment, researchers at the University of Pretoria have coated nanoscale structures onto a variety of different finned copper tubes and found that this combination works only some of the time unfortunately, depending on the structure of the finned surfaces.

The experimental investigation, conducted by Dian Dickson (UP) as part of his Master’s thesis under the supervision of Dr Bradley D. Bock (UP) and Prof. John R. Thome (EPFL, Switzerland), was spurred by previously encouraging results where nanocoatings on plain unfinned tubes improved the heat transfer of refrigerant boiling through mechanisms not previously possible.

The nanocoating, produced by dipping copper surfaces in a special chemical bath, produced a series of nanoscale rods, sheets and flowers made of copper oxide on the copper surface of the tubes that drastically altered the surface chemistry. The structures in particular are able to wick liquid, much in the same way paper towels do, and the addition of these structures adds a new dimension to the boiling physics that usually take place on the copper tubes. The coating was subsequently applied to tubes with a variety of fins of differing shapes and sizes.

The nanocoating used was found to produce a forest of copper oxide ( CuO ) structures of varying types, with rods, sheets and flowers all present.

Two sets of tests were conducted, with coated and uncoated tubes either submerged in a pool of refrigerant and boiled, or with a thin liquid film drizzled over the tubes and then boiled. This “falling film boiling” mode has the advantage of needing far less refrigerant in a system, which is particularly advantageous when the refrigerant is expensive, or dangerous.

A plain copper tube (left) and a copper oxide nanocoated tube (right) under pool boiling conditions.

A plain copper tube (left) and a copper oxide nanocoated tube (right) under falling film boiling conditions.

The study found that the finned tubes with simple shapes benefitted the most from the combination of nanocoating and fins, while finned tubes with more intricate shapes actually had worse heat transfer, with the nanocoatings interfering with the heat transfer. It’s though that the nanocoatings impeded some of the useful flow on the intricately finned tubes.

So, while combining these difference scale enhancements can be beneficial, careful design and experimentation needs to first take place, as the process is complex and not without pitfalls.

For more information, the journals papers documenting this work are

• Heat transfer of uncoated and nanostructure coated commercially micro-enhanced refrigeration tubes under pool boiling conditions
• Multiscale enhancement of refrigerant falling film boiling by combining commercially enhanced tubes with nanostructures