Visualization of dynamic boiling processes using high-speed optical coherence tomography
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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.
KeywordsOptical Coherence Tomography Particle Image Velocimetry Vapor Bubble Optical Coherence Tomography Image Particle Tracking Velocimetry
This research was funded by the German Federal Ministry of Education and Research (BMBF) within the joint project “fundamental research Energy 2020+”, project number 02NUK010C and 02NUK010l. The corresponding author was jointly supported by the European Social Fund and Micro-Epsilon Optronic GmbH, Germany.
Conflict of interest
The authors declare that they have no conflict of interest.
Online Resource 1 (ESM_1.mpg) The sequence shows 384 pairs of OCT cross sections (right) and camera images (left) of a 1.64 s long image sequence. The frame rate is 234 Hz corresponding to an acquisition time of 4.3 ms per OCT cross section. The scale bar in each image represents 1 mm in every direction for camera and OCT images (refractive index of 1 assumed). The lateral position of the OCT cross section is in the middle of the corresponding camera image, which is indicated by the red vertical line in each camera image. (MPG 19014 kb)
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