Visualization of dynamic boiling processes using high-speed optical coherence tomography

Abstract

Investigating microscale nucleate boiling processes with high heat flux requires experimental visualization and quantification with high spatial resolution in the micrometer range as well as a sufficient temporal resolution. Numerous measurement techniques are employed for providing comprehensive experimental data on microscale boiling processes and other multiphase flows. In this context, optical coherence tomography (OCT) has been introduced recently for the visualization of quasistatic growing vapor bubbles in turbid fluids with a high spatial resolution. Since OCT detects backscattered light, only one optical access is necessary and OCT is feasible for measurements in turbid media, where other imaging techniques fail. Within this study, a high-speed OCT system is utilized for visualizing dynamic nucleate boiling processes at a heated surface with a frame rate of about 234 Hz. The bubble contour is extracted out of the OCT images using segmentation and tracking algorithm, which provide bubble contours and the course of the bubble area for individual vapor bubbles over time. Additionally, high-speed Doppler OCT imaging is presented revealing the velocity component of the fluid in beam direction up to 30 mm/s unambiguously. The present proof of principle study suggests high-speed OCT imaging as a promising and alternative technique for the simultaneous measurement of bubble geometries and fluid velocities in dynamic processes with a high spatial resolution of 16 µm. Due to the ongoing development and availability of ultra high-speed OCT systems, the perspective temporal resolution will be comparable to the frame rates provided by presently established techniques, such as particle image velocimetry or high-speed camera imaging.