Optical Kerr Effect Gated Imaging of High Void Fraction Bubbly Flows*

High void-fraction bubbly flows are highly probable in natural and man-made environments; hence, they are of critical importance in mass and energy transfer between phases. Despite this importance, physical and experimental data with high void fractions is limited, with significant scatter in data leading to a dependence on empirical models. This lack of data results from increased optical thickness and scattering, limiting the ability to measure the bubble size distribution spectra, the most critical parameter with bubbly flows. With significant scattering and occlusion, spatial and geometric information is lost, producing a white appearance readily visible in white caps. Due to the difference in refractive index, however, a measure of the speed of light, there is a time delay between the arrival of unscattered light and scattered photons. Capturing only the initial photons will recover information lost through scattering.

Moreover, by quantifying the differences in the time of arrival of photons, bubble size may also be interpreted. Through implementing the Optical Kerr Effect, with ultrashort femtosecond pulses, high void fraction bubble plumes have been imaged with exposure times less than three picoseconds, allowing the temporal decomposition shadowgraph images and removing of high order scattering modes. Observed scattering behavior shows good agreement with computationally derived scattering profiles. Equally, time-of-arrival analysis of bubble sizes agrees with traditional visual inspection methods. With further progression, this ultrafast method may prove key for probing these vital flow conditions.