R134a/Al2O3 Nanolubricant Mixture Pool Boiling on a Rectangular Finned Surface

Abstract

This paper quantifies the influence of Al2O3 nanoparticles on the pool boiling performance of R134a/polyolester mixtures on a rectangular finned surface. Nanolubricants having 10 nm diameter Al2O3 nanoparticles of various volume fractions (1.0%, 2.3%, and 3.6%, a.k.a., 1AlO, 2AlO, and 3AlO) in the base polyolester lubricant were mixed with R134a at two different mass fractions (0.5% and 1%). The study showed that nanolubricants can significantly improve R134a/lubricant boiling on a rectangular finned surface. For example, the R134a/1AlO (99/1), R134a/3AlO (99/1), and the R134a/2AlO (99/1) mixtures exhibited average enhancement of approximately 18%, 102%, and 113%, respectively. The nanoparticles had practically no effect on the heat transfer relative to that for R134a/polyolester mixtures without nanoparticles for R134a boiling with the 1AlO nanolubricant at a 0.5% mass fraction with the refrigerant. This confirms, what was shown in a previous publication for a smooth surface, that for a particular system, a critical loading of nanoparticles must be exceeded before an enhancement can be achieved. The present study suggests that passively enhanced surfaces are likely to require more nanoparticle loading than a smooth surface to achieve similar heat transfer enhancement. This is based on the finding that the boiling heat transfer enhancement was shown to be a strong function of the absolute nanoparticle surface density that resides on the heat transfer surface and not the nanoparticle concentration in the nanolubricant as previously believed. The enhancement was shown to increase for three different boiling surfaces (from three different studies) as more nanoparticles accumulate on the boiling surface. Accordingly, a previously developed model for predicting refrigerant/nanolubricant boiling on a smooth surface was corrected so as to be dependent on the nanoparticle surface density rather that the nanoparticle concentration. In addition, the model was modified in order to predict the refrigerant/nanolubricant boiling on the rectangular finned surface. The model and the measurements agreed to within 10% for all of the data with heat fluxes less than 100 kW m−2.