Subcooled flow boiling heat transfer and critical heat flux of different nanofluids in a vertical tube based on boiling surface evaluation

In this experimental study, the subcooled flow boiling of various nanofluids including Al2O3/H2O (0.005, 0.01, 0.02 and 0.03 vol%), Ag/H2O (0.005 vol%), SiO2/H2O (0.005 vol%) and CeO2 0.005 vol%) were investigated in a vertical macro tube under atmospheric pressure. The subcooled flow boiling curve of nanofluids until the occurrence of critical heat flux was presented and the different regions of the heat transfer on the boiling curve were explained. The effect of parameters such as concentration, nanofluid type, and mass flux within the raw tube on critical heat flux, heat transfer coefficient, and output equilibrium vapor quality of nanofluids were investigated and compared to the boiling of deionized water. The boiling surface characteristics, such as surface roughness, morphology observed through Scanning Electron Microscopy (SEM), and contact angle, were analyzed. The critical heat flux increased as the Al2O3/H2O nanofluid concentration increased, reaching a 23.4% increase compared to deionized water in the raw tube when the Al2O3/H2O nanofluid concentration was 0.03 vol %. Also, the results show that critical heat flux of nanofluid increased with mass flux compared to deionized water. Heat transfer coefficient for all nanofluids (0.005 vol%) increased compared to that of deionized water in the raw tube and CeO2/H2O showed the highest increment in heat transfer coefficient due to small particle size and the uniform pattern of nanoparticle accumulation on the surface and an increase the activation of nucleation sites. Output equilibrium vapor quality exhibited similar behavior for all nanofluids an deionized water, in the single-phase region and close to the occurrence of critical heat flux. However, it had a lower performance compared to deionized water in the nucleation region. The boiling parameters of deionized water on nanocoated surfaces with various nanoparticles enhanced compared to the boiling of deionized water in the raw tube surface and it is competitive with the boiling of the corresponding nanofluid in the raw tube.