Structured-illumination visualization and fluorescence thermometry of flow boiling in dielectric fluids

Flow boiling in high aspect ratio minichannels, or microgaps, with one-sided heating models transport in ultra-compact heat exchangers. Visualizing such two-phase flows is challenging because of the refractive-index difference between the liquid and vapor phases. Although spatially modulated light sheets (e.g., SLIPI) have been used to mitigate higher-order scattering in sprays, here structured illumination (SI) across a plane normal to the optical axis is used instead to visualize streamwise slices with a thickness of O(10 mm) of flow boiling in 1 mm deep microgaps with no optical access through the side walls. The working fluid is a fluorescent HFE 7200 solution. This, and other, hydrofluoroethers are used for cooling silicon-based electronics because they are dielectric fluids with relatively low boiling points. The results demonstrate that SI visualizations of flow boiling have significantly higher contrast and image a thinner slice than standard visualizations obtained with “unstructured” uniform illumination. The temperature-sensitive emissions from the fluorescent HFE 7200 solution are used to estimate liquid-phase temperatures. Structured illumination appears to improve the accuracy of these fluorescence thermometry results compared with uniform illumination.