Interferometric investigation of nanofluid natural convection heat transfer: Addressing existing inconsistencies

Despite extensive research on nanofluids, their widespread adoption remains limited due to conflicting findings, with optical studies reporting up to 75 % enhancement and heat balance methods showing less enhancement or even some reduction. This study employs Mach-Zehnder Interferometry (MZI) to investigate the heat transfer characteristics of Al₂O₃–water nanofluids in an inclined rectangular cavity heated from below, with an aspect ratio of 1.5 and an inclination angle of 9.3°. The experiments were conducted under natural convection at Rayleigh numbers of 9.7 $\times$ 10⁵ and 1.8 $\times$ 10⁶, within the steady laminar flow regime. Leveraging the ability of MZI to simultaneously visualize and quantify temperature and concentration fields, the study aims to address possible reasons behind the inconsistencies reported in the literature. Three concentrations of Al₂O₃–water nanofluids provided by different preparation methods are examined: 0.05, 0.16, and 0.23 wt%. Their stability and thermal conductivity, both essential for accurate heat transfer measurements using MZI, are evaluated prior to the natural convection experiments. The optical path length of the experimental model is chosen to be short enough to ensure distinguishable fringes near the target surface, minimize the effects of surface refraction, reduce temperature measurement errors, and mitigate the impact of nanofluid instability. With the improvements made in this study, the results show that within the measurement uncertainty, the local Nusselt number distributions and average heat transfer rates for dilute Al₂O₃–water nanofluids with concentrations below 0.23 wt% (0.06 vol %) are the same as those of deionized water, regardless of the preparation method.