Nucleate pool boiling of R134a on cold sprayed Cu-CNT-SiC and Cu-CNT-AlN composite coatings

Ternary copper-carbon nanotube-silicon carbide (Cu-CNT-SiC) and copper-carbon nanotube-aluminum nitride (Cu-CNT-AlN) composite coatings, which were fabricated by mechanical alloying and cold gas dynamic spraying, were used as heating surfaces for boiling heat transfer enhancement. The surfaces of these composite coatings had randomly distributed micro- and nano-sized cavities, which served as active sites for bubble nucleation. Nucleate boiling heat transfer tests on the composite coatings were investigated in a pool of saturated R134a refrigerant at 4.8 ± 0.04 °C. Boiling inception on the Cu-CNT-SiC and Cu-CNT-AlN coatings started at heat flux values of about 10-20 kW/m2 and at wall superheats that were about 4 °C lower than that of the plain Cu plate. The Cu-CNT coating had maximum boiling heat transfer enhancement ratio Eh,max of 1.48 as compared to the plain Cu plate. With the addition of SiC and AlN particles into the composition of the Cu-5CNT composite, the enhancement was augmented further. The coating with 20 vol.% AlN exhibited the highest enhancement with an Eh,max of 2.83. This was followed by the coating with 10 vol.% SiC with an Eh,max equal to 2.52. Although the Cu-CNT-ceramic coatings had high Eh,max, the Eh decreased at high heat fluxes. The maximum enhancement by achieved by each of the composite coatings was observed at heat fluxes within 100-200 kW/m2.