Subcooled flow boiling heat transfer of γ-Al₂O₃/water nanofluids in horizontal microtubes and the effect of surface characteristics and nanoparticle deposition

Highlights: • Subcooled boiling heat transfer characteristics of nanofluids were investigated in micro scale. • Dilute alumina/water nanofluids had no considerable enhancement effect at low mass flux. • With the increase in mass flux, enhancements in heat transfer were observed for dilute nanofluids. • At high mass fractions, deterioration in heat transfer and change on the surface were present. • The deterioration beyond a specific mass fraction was linked to the change in surface characteristics. - Abstract: In this study, subcooled flow boiling heat transfer characteristics of nanofluids were investigated at micro scale. For this purpose, the effect of γ-Al₂O₃ (gamma-alumina) nanoparticles with an average solid diameter of 20 nm was considered. In the experiments, various mass fractions were considered in horizontal smooth stainless steel microtubes with inner and outer diameters of ∼502 µm and ∼717 µm, respectively, at mass fluxes of 1200 and 3400 kg m⁻² s⁻¹. Nanoparticles were added to distilled water (base fluid) at five mass fractions (low mass fractions 0.05 wt% and 0.2 wt%; high mass fractions 0.5 wt%, 1 wt% and 1.5 wt%). According to our results, subcooled flow boiling heat transfer coefficients for nanofluids with low mass fractions were nearly the same as those of the pure water. However, heat transfer deteriorated for nanofluids with high mass fractions. Observations of dynamic light scattering measurements for low and high mass fractions before and after the experiments revealed that agglomeration of nanoparticles is an important parameter in deterioration of heat transfer at higher concentrations. Besides, Scanning Electron Microscopy images of microtube inner surfaces showed that deposition of nanoparticles and agglomerated nanoparticles on the inner surface of the microtubes also contributed to the heat transfer deterioration at high mass fractions. Generally, the deterioration in heat transfer beyond a specific mass fraction value was linked to the disturbance in the stability of suspended nanoparticles and deposition of nanoparticles upon boiling.