Effect of Al2O3 Nanolubricant on R134a Pool Boiling Heat Transfer
Mark A. Kedzierski
This paper quantifies the influence of Al2O3 nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a roughened, horizontal, flat surface. Nanofluids are liquids that contain dispersed nano-size particles. A lubricant based nanofluid (nanolubricant) was made by suspending nominally, 10 nm diameter Al2O3 particles in a synthetic ester to roughly a 1.6 % volume fraction. The nanoparticles enhanced the boiling heat transfer relative to that for R134a/polyolester mixtures without nanoparticles for the three lubricant mass fractions that were tested. The enhancement occurred for the lowest heat fluxes, which gives the opportunity for designing chillers for lower approach temperatures. For the 0.5 % nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) as large as 400 % for the lowest tested heat flux. The average heat flux improvement for heat fluxes less than 40 kW/m2 was approximately 105 %, 49 %, and 155 % for the 0.5 %, the 1 %, and the 2 % mass fractions, respectively. The heat flux enhancement for all of the mixtures increased with respect to decreasing heat flux. A semi-empirical model was developed to predict the boiling enhancement as cause by the interaction of the nanoparticles with the bubbles. The model suggests that small particle size and large nanoparticle volume fraction improves boiling enhancement. Continued research with nanolubricants and refrigerants are required to further validate the model with measurements with other nanoparticle materials, sizes and concentrations.