Picosecond Imaging Of Bubbly Flows

Abstract

Despite their ubiquity, bubbly flows are notoriously challenging to instrument. In high-void fractions, most optical measurement techniques fail due to intense scattering and occlusion. Ultrafast imaging with picosecond exposure times offers an opportunity, allowing measurements before scattering obscures the field of view. Furthermore, detecting the time of arrival of photons allows measurement of bubble sizes, even for those partially occluded. Through implementing the Optical Kerr Effect (OKE), using a 70 fs 800 nm pulsed laser and a Carbon Disulfide cell, images are acquired with an exposure of 2.9 ps. Gating these images with an optical delay stage further allows the decomposition of scenes with 33 fs time increments. Proof of concept is demonstrated by applying this approach to static scenes of submerged bubbles at low and high void fractions. The experimental temporal decomposition shows the arrival of photons that travel through different media (air, water, and glass) and those scattered within the turbid media. A new visualization is presented to display this breakdown concisely, the Temporally Tagged Shadowgraph (TTS), presenting the time of arrival with the context of position and surroundings. Bubble sizes are measured using this temporal information and through edge detection, with good agreement for single unoccluded bubbles. With polydisperse bubble plumes, time delay-based measurements tend to overestimate bubble diameter; however, this could also be due to greater uncertainty in edge detection with occlusion. This demonstrates the potential of Ultrafast Imaging when it comes to probing high-void fraction flows. By further shortening exposure times and improving the robustness of processing algorithms, the scope of applicability of this technique can be widened.