Detailed visualization of spray dynamics using Light Sheet Fluorescence Microscopic imaging

Abstract:
We demonstrate the use of Light Sheet Fluorescence Microscopic (LSFM) imaging for viewing the dynamic of atomizing sprays with high contrast and resolution. The technique presents several advantages: first liquid fluorescence gives a more faithful representation of the structure of liquid bodies, droplets and ligaments than Mie scattering does. The reason for this is that the signal is emitted by the fluorescing dye molecules inside the liquid itself and not generated at the air-liquid interfaces. Second, despite the short depth of field (~0.2 mm) obtained when using the long range microscope, the contribution of out-of-focus light is much smaller on a light sheet than on a line-of-sight configuration providing more clearly sectioned images. Finally by positioning the light sheet on the spray periphery, toward the camera objective, the effects due to multiple light scattering phenomena can be reduced to some extent. All those features provide, for many spray situations, images with high fidelity of the liquid fluid, allowing the extraction of the velocity vectors at the liquid boundaries. Here, double frame images were recorded with a sCMOS camera with a time delay of 5 μs between exposures. A typical pressure-swirl atomizer is used here producing a water hollow-cone spray which was imaged between 20 bars and 100 bars in liquid injection pressure. Such data are important for the validation of CFD models simulating liquid breakups in the near-field spray region.