Structured illumination studies of flow boiling in dielectric fluids*

Flow boiling in a 2D microgap is the fundamental flow for heat transfer in ultracompact heat exchangers.  The hydrofluoroether HFE7200, which has a boiling point compatible with silicon-based microelectronics, is visualized in a glass minichannel (of cross-section H = 1 mm x 25 mm) with an indium tin oxide (ITO) thin-film heater.  Flow boiling in the slug and annular flow regimes is studied at Reynolds numbers Re = 100-500 (mass fluxes G < 500 kg/(m²-s) and heat fluxes q² ≤ 500 W/m²) using structured illumination (SI) imaging, which can reduce refraction and reflection at liquid-vapor interfaces, to better resolve this highly nonisothermal two-phase flow. Thin slices of flow and pool boiling of HFE7200 dyed with fluorous rhodamine (FRh) are obtained with two-pulse structured laser illumination planar imaging (2p-SLIPI) [DOI 10.1364/OL.41.005422], which uses a light sheet with a sinusoidally modulated intensity, at frame rates as great as 9.5 kHz. The images reconstructed from two successive frames show previously inaccessible details, such as liquid-vapor interfaces and the interactions between bubbles—flow features that are difficult to resolve using high-speed imaging with uniform illumination. Structured-illumination visualizations therefore provide more accurate identification of phase boundaries, and hence more accurate estimates of vapor quality. We also exploit the temperature sensitivity of the FRh to estimate time-averaged liquid-phase temperature fields using fluorescence thermometry (FT) in single-phase flows of HFE 7200.