To understand boiling heat transfer characteristics of nanofluid, the experiment and theoretical study about effect on nanofluid concentration, nanoparticle size and surface roughness were carried out by using Al2O3 nanofluid. Both the smooth surface(Ra=177.48 nm) and rough surface(Ra=292.79 nm), the critical heat flux(CHF) and superheat to reach the CHF of Al2O3 nanofluid increased according to the increase of nanofluid concentration. Maximum CHF on the smooth surface and rough surface was 722.52 kW/m2 and 834.9 kW/m2, respectively, at 0.05vol%-concentration and those were improved one more than 22 kW/m2 and 350 kW/m2 compared to that of water as base fluid.
However, the variation trend of boiling heat transfer coefficient was different according to surface roughness. In case of smooth surface, the boiling heat transfer coefficient at the CHF was 0.9, 17.4, 16.9, 19.2, 17.4 kW/m2?K at the nanofluid concentrations of 0, 10-3, 10-2, 5×10-2, 10-1vol%, respectively. The boiling heat transfer coefficient of nanofluid was improved more than water and the improvement of boiling heat transfer coefficient decreased over the nanofluid concentration of 0.05vol%. Besides, for the rough surface, the boiling heat transfer coefficient at the CHF was 15.5, 14.5, 18.1, 14.2, 15.2 kW/m2?K, respectively. at the nanofluid concentrations of 0, 10-3, 10-2, 5×10-2, 10-1vol%, which were similar to water or a little decreased one.
In all experiments, the nanoparticle deposition on the heated surface was observed but the influence of nanoparticle deposition to heat transfer performance was different. Both the smooth and rough surface, the improvement of wettability was observed compared to water during boiling. However, the nanoparticle deposition was uniformly made on the smooth surface but was irregularly made on the rough surface for the investigation of heated surface geometry change by nanoparticle deposition after boiling.
To investigate the influences on nanofluid boiling according to volume concentration and size of nanoparticle, the theoretical study was carried out when the volume concentration was varied from 0vol% to 0.1vol% and nano particle sizes were 25, 50, 75 nm. The critical heat flux of nanofluid improved more than base fluid. When the volume concentration of 0.025vol%?Al2O3 nanoparticle size of 50 nm, the maximum critical heat flux was 1,515 kW/m2 and then it gradually decreased. However, in case that volume concentration was 0.025vol%, the critical heat fluxes were 1,499, 1,515, and 1,509 kW/m2 for the nanoparticle sizes of 25, 50, and 75 nm, respectively. It was small different. For Al2O3 nanoparticle of 50 nm, as the volume concentration increased from 0.025vol% to 0.1%, the bubble departure diameter increased from 1.235 to 1.805 mm but the bubble departure diameter did not show a significant change according to nanoparticle size. It is because the energy factor and K improved according to the increase of volume concentration of Al2O3 nanoparticle but it did not change a lot with nanoparticle size. The nucleate site density at the critical heat flux increased from 32.97 to 30.53 site/cm2 with the increase of volume concentration for 50 nm-Al2O3 nanoparticle size, but it was seriously lower than that base fluid with the nucleate site density of 65.90 site/cm2. As nanoparticle size increased from 25 to 75 nm at the volume concentration of 0.025vol%, the nucleate site density at the critical heat flux decreased from 35.59 to 31.3 site/m2. The volume concentration and nanoparticle size affects to the natural convection boiling. At the volume concentration of 0.025vol%, the portion of natural convection boiling was extended because the increase of dryout fraction, which was caused by the improvement of wettability before and after boiling. However, as nanoparticle size was similar to surface roughness, the critical heat flux was increased because the nucleate site density increased while the bubble departure diameter was slightly increased.