Experimental investigation of pool boiling characteristics in Al2O3 nanofluid according to surface roughness and concentration

To investigate the influence of the surface roughness on the boiling characteristics of nanofluids, a pool boiling experiment was carried out for different concentrations of Al2O3 nanofluid using the surfaces with Ra = 177.5 nm and Ra = 292.8 nm. As the concentration of nanofluid increased from 0 vol% to 0.05 vol%, it was found that the CHF (critical heat flux) at the surfaces with Ra = 177.5 nm and Ra = 292.8 nm improved by 224.8% and 138.5%, respectively. However, when the concentration was over 0.05 vol%, the improvement of the CHF decreased the surfaces with Ra = 177.5 nm and Ra = 292.8 nm. In the case of the same heat flux at the heated surface, the BHTC (boiling heat transfer coefficient) of Al2O3 nanofluid for the surface with Ra = 177.5 nm was higher than that for the surface with Ra = 292.8 nm, although it was lower than that of the base fluid, water. The BHTC at CHF of Al2O3 nanofluid was higher than that of water. After boiling, as the concentration increased from 0 vol% to 0.05 vol%, the contact angle for the surfaces with Ra = 177.5 nm and Ra = 292.8 nm decreased from 50.9° to 38.7° and 49.6°-40.4°, respectively. The contact angles increased above a concentration of 0.05 vol%. For a concentration of 0.1 vol%, the roughness for the surface with Ra = 177.5 nm was changed to 530 nm and that with Ra = 292.8 nm was changed to 920 nm after boiling. For the surface with Ra = 292.8 nm, the change in surface roughness was relatively large compared to that of for the surface with Ra = 177.5 nm. Overall, the performance of boiling heat transfer for Al2O3 nanofluid is closely related to the surface roughness and concentration of nanofluid.