Effect of CuO nanolubricant on R134a pool boiling heat transfer

Abstract

This paper quantifies the influence of CuO nanoparticles on the boiling performance of R134a/polyolester mixtures on a roughened, horizontal, flat surface. A lubricant based nanofluid (nanolubricant) was made with a synthetic ester and CuO particles. 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) of between 50% and 275%. A smaller enhancement was observed for the R134a/nanolubricant (99/1) mixture, which had a heat flux that was on average 19% larger than that of the R134a/polyolester (99/1) mixture. Further increase in the nanolubricant mass fraction to 2% resulted in a still smaller boiling heat transfer improvement of approximately 12% on average. Thermal conductivity measurements and a refrigerant/lubricant mixture pool-boiling model were used to suggest that increased thermal conductivity is responsible for only a small portion of the heat transfer enhancement due to nanoparticles.

Introduction

Government research initiatives have supported an explosion of research in recent years including the investigation of the heat transfer properties of liquids with dispersed nano-size particles called nanofluids. Prior to these initiatives, nanofluids research was mainly confined to thermal conductivity investigations. Eastman et al. (2001) found that the thermal conductivity of some nanofluids, with nanoparticles at a volume fraction of less than 0.4%, was more than 40% greater than that of the pure base fluid. Herein lies what is believed to be a great potential for the enhancement of liquid heat transfer by the addition of nanoparticles to the base fluid.

Water, ethylene glycol, and lubricants have been successfully used as base fluids in making stable nanofluids where the particles remain suspended in the liquid. Although water based nanofluids are the least stable of the three liquids because of the relatively low viscosity of water, most of the boiling heat transfer studies have been conducted with water based nanofluids (Bang and Chang, 2004, Wen and Ding, 2005, You et al., 2003). Of these previous studies, You et al. (2003) and Bang and Chang (2004) did not observe a pool-boiling enhancement with nanofluids; however, Wen and Ding (2005) did. Consequently, boiling heat transfer improvements can be obtained with nanofluids even though the mechanisms that govern the improvement are not fully understood.

Currently, there are no published measurements to determine if nanoparticles can be used to improve refrigerant/lubricant boiling heat transfer. Bi et al. (2007) examine the effect of nanoparticles on the performance of a refrigerator, thus, giving an indirect account of the effect of nanoparticles on refrigerant/lubricant flow boiling. One reason for the lack of pool-boiling heat transfer investigations might be the expectation that once the nanolubricant is mixed with the refrigerant, the nanoparticles will become unstable with respect to the refrigerant/lubricant mixture because the relatively low viscosity of the mixture discourages Brownian motion. This potential outcome, however, may not prohibit the application of nanoparticles to air-conditioning equipment because the mechanism of the boiling heat transfer of refrigerant/lubricant mixtures is strongly governed by the lubricant excess layer that resides at the boiling surface (Kedzierski, 2003a). Similar to the way a lubricant excess layer is established, the boiling will drive the nanoparticles to the heat transfer surface where they will become stable and remain within the viscous lubricant excess layer. Some of the particles will also be entrained in the vigorous boiling of the fluid. If the nanoparticles significantly change the thermal conductivity of the lubricant excess layer, that may cause an enhancement or a degradation in heat transfer depending on whether the increased conduction causes a reduced available superheat or whether it increases the thermal boundary layer thickness. The potential for a boiling heat transfer enhancement is likely to depend on the material of the particles, their shape, size, distribution, and concentration.

In order to investigate the influence of nanoparticles on refrigerant/lubricant pool boiling, the boiling heat transfer of three R134a/nanolubricant mixtures on a roughened, horizontal, flat (plain), copper surface was measured. A commercial polyolester lubricant (RL68H¹), commonly used with R134a chillers, with a nominal kinematic viscosity of 72.3 μm²/s at 313.15 K was the base lubricant that was mixed with nominally 30 nm diameter copper (II) oxide (CuO) nanoparticles. Copper (II) oxide (79.55 g/mol) has many commercial applications including use as an optical glass-polishing agent. A manufacturer used a proprietary surfactant at a mass between 5% and 15% of the mass of the CuO as a dispersant for the RL68H/CuO mixture (nanolubricant). The manufacturer made the mixture such that 9% of the volume was CuO particles. The mixture was diluted in-house to a 1% volume fraction of CuO by adding neat RL68H (Kedzierski and Gong, 2007) and ultrasonically mixing the solution for approximately 24 h. The particle size and dispersion were verified by a light scattering technique several weeks after mixing. The particles were approximately 35 nm and well dispersed with little particle agglomeration (Sung, 2006). The RL68H/CuO (99/1)² volume fraction mixture, a.k.a. RL68H1Cu, was mixed with pure R134a to obtain three R134a/RL68H1Cu mixtures at nominally 0.5%, 1%, and 2% nanolubricant mass. In addition, the boiling heat transfer of three R134a/RL68H mixtures (0.5%, 1%, and 2% mass fractions), without nanoparticles, was measured to serve as a baseline for comparison to the RL68H1Cu mixtures.