Effects of nanofluids and nanocoatings on the thermal performance of an evaporator with rectangular microchannels

In this study, the effect of Al2O3–R141b nanofluids as the working fluid on the thermal performance of micro-channel surfaces at different pressures and nanofluid particle concentrations were experimentally investigated. The operational pressures and volume concentration of nanoparticles were varied among 0.86 × 105, 0.91 × 105, 0.96 × 105, 1.0 × 105, and 2.0 × 105 Pa, and 0.001, 0.01, and 0.1 vol%, respectively. The experimental results showed that the operational pressure and volume concentration of nanoparticles have significant effect on the heat transfer characteristics for nanofluids of Al2O3/R141b. For operating pressure in the range 0.86–0.96 × 105 Pa and nanofluid concentrations of 0.001 and 0.01 vol%, the corresponding heat transfer coefficients were larger than those of the base fluid. For the concentration of 0.1 vol%, when the superheat degree was higher than that for the pure fluid, the heat transfer coefficient was lower. At high pressures of 1.0 and 2.0 × 105 Pa, for the concentration of 0.1 vol%, heat transfer deteriorated for the entire range of measured data. Under the same operating pressures, for the concentrations of 0.001 and 0.01 vol%, the heat transfer enhancement was not apparent. Further, the heat transfer characteristics of the nanofluids for Al2O3-nanoparticle-covered surfaces with rectangular microchannels, which were produced by the evaporation of nanofluids with various concentrations ranging from 0.001 to 0.1 vol%, were experimentally studied. The heat transfer curves for the nanoparticle-covered surfaces were compared with those for nanofluids on bare surfaces, and the results demonstrated that the nanoparticle covering has a strong influence on the heat transfer behavior.