Effect of non-uniform nanofluid concentration on interferometric heat transfer measurements

The effect of non-uniform nanofluid concentration on the accuracy of interferometric heat transfer measurements has been investigated using a Mach-Zehnder interferometer. Because the refractive index is a function of concentration as well as temperature, concentration variations within the nanofluid can produce unwanted interference fringes, leading to temperature measurement errors. Measurement errors in the temperature gradient are demonstrated for conduction within a cavity heated from top to bottom, filled with an Al₂O₃-water nanofluid (0.16 wt%) produced using a standard two-step method. The results of the current measurements show that the temperature gradient can be overestimated by up to 100 % due to near-wall concentration gradients in an unstable nanofluid. The measurement problem is delineated, and several approaches to mitigate this source of measurement error are outlined. The technical trade-offs associated with designing interferometric heat transfer experiments to reduce the sensitivity to concentration differences are discussed. These trade-offs include selecting the nanofluid type and concentration, temperature differences, and the optical path length of the experimental model. It is shown that many interferometric studies in the literature were much more sensitive to concentration-induced errors than the current experiment. An isothermal stability test is recommended to detect nanofluid concentration gradients prior to temperature-based interferometry experiments.