Theoretical analysis of pool boiling characteristics of Al2O3 nanofluid according to volume concentration and nanoparticle size

To investigate the effect of volume concentration and nanoparticle size on nanofluid boiling, a simulation with various concentrations of nanofluids and nanoparticle sizes was carried out using the HFP (heat flux partitioning) model with variations in the contact angle. The CHF (critical heat flux) of the nanofluid rose more than the base fluid. When the volume concentration was 0.025 vol% and the size of the Al2O3-nanoparticles was 50 nm, the maximum critical heat flux was 1515 kW/m2 and gradually decreased with the concentration of the nanofluid. Since the Energy and K factor both improved as the Al2O3-nanofluid concentration increased, the frequency of bubble departure as well as the bubble departure diameter subsequently increased. However, the effect of nanoparticle size was not remarkable. The nucleate site density at the CHF increased from 32.97 to 30.53 site/cm2 with the increase of volume concentration at 50 nm-nanoparticle size but the increase was significantly lower than that of the base fluid, which was 65.90 site/cm2. The temperature converted the natural convection boiling to the nucleate boiling increased and the dryout factor decreased as the concentration of the nanofluid increased. In addition, these parameters increased as the nanoparticle size became similar to the value for the surface roughness.