Technical
All images by the University of Pretoria
Silica nanoparticle structures .
Two types of nanostructures were investigated . The first consisted of applying silica nanoparticles to the outside of a copper tube . These silica nanoparticles are made up of the same material as beach sand but are only 20 nm wide ( so 0.00002 mm ). By applying these small balls to the copper surface , intricate cavities and voids can be created on the surface , which
traditionally has been found to improve heat transfer under certain conditions .
The second nanostructure type was applied using a special chemical formulation , which after encountering the copper surface , causes a chemical reaction that forms a forest of tiny nanospikes . These nanospikes change the surface properties of the copper , giving the surface the ability to wick the refrigerant , in effect absorbing refrigerant in the same way paper towels can absorb water when cleaning up a spill . The research 7 , 8 so far has led to several innovative findings . The nanospikes increased boiling heat transfer by up to three times that of a plain copper tube . But the silica nanoparticles decreased heat transfer . This showed that the simple fact that the nanostructures create voids and cavities is not enough to guarantee improved heat transfer , and careful selection and understanding of the nanostructures and their geometries is needed .
The nanostructures also changed the way the heat transfer from the copper tubes to the refrigerant occurred , with new heat transfer mechanisms noted that had not been seen before . “ Using high-speed camera footage , we were able to see that the refrigerant liquid was wicked into the surface , and it appeared to allow refrigerant to actually creep under the bubbles created during boiling , allowing the bubbles to grow larger and quicker ,” suggests Dr . Bock . “ But more work is needed to really figure out more precisely what is going on .”
Despite these promising initial findings , more work will need to be done if nanostructures are to compete with existing
7 https :// doi . org / 10.1016 / j . ijheatmasstransfer . 2020.120452 8 https :// doi . org / 10.1016 / j . ijheatmasstransfer . 2020.120387
Forest of tiny nanospikes .
technologies . “ These nanostructures don ’ t currently compete with commercially enhanced tubes ,” Dr Bock points out , “ But with further research we can optimise these nanostructures and hopefully unlock their full potential .”
“ 3D printed heat exchangers are a particularly promising application of these nanostructures ,” Dr Bock also noted . Nanostructures can be applied to the complex geometries that can be produced with 3D printing by simply pumping the chemicals through the heat exchangers , increasing the capacity of these already enhanced heat transfer devices .
The team at the University of Pretoria are continuing their research on this topic , not only in the hope of making the next big breakthrough in the HVAC & R industry , but to ensure Africa is at the forefront of the new technologies that will be built over the next decades on its shores . RACA
Nanostructured coated copper tube boiling refrigerant R-134a .
www . refrigerationandaircon . co . za RACA Journal I March 2023 49