Advances in Engineering Advances in Engineering features breaking research judged by Advances in Engineering advisory team to be of key importance in the Engineering field. Papers are selected from over 10,000 published each week from most peer reviewed journals.
Significance
Non-intrusive optical diagnostic techniques are indispensable tools in studying complex and hostile environments associated with propulsion and combustion processes. To increase combustion efficiency and reduce associated environmental impacts, in-depth knowledge of the processes governing turbulent flames is vital. Although established techniques like spontaneous Raman scattering provide great insights into combustion, turbulence and associated interactions, most of these techniques are limited to single-point or planar measurements. Since turbulent flames are inherently three-dimensional (3D), understanding flame characteristics require time-resolved 3D measurement techniques.
With the advances in high-speed imaging and reconstruction algorithms, the dynamic behaviors of turbulent flames have been achieved with remarkable spatiotemporal resolutions. Numerous approaches have been explored, and they fall under two categories: sweeping planar technique and computer tomography. While sweeping planar can achieve comprehensive measurements, it involves high costs and the complexity of multiple lasers. Likewise, computer tomography requires multiple high-speed cameras to achieve comprehensive measurement despite having a simplified setup.
Recently, 3D Multiplexed Structured Image Capture (3D-MUSIC) has drawn considerable attention as a promising technique for characterizing the behaviors of turbulent flames. Before imaging, 3D-MUSIC utilizes spatial light modulation to encode the target into spatial frequency shifts corresponding to unique temporal images. It allows the individual tagging of targeted elements as sub-images. This technique has proved effective for multi-view and multi-spectral 3D imaging applications. In most cases, Fourier transform of the images is used to resolve the spatial frequency shift problem associated with the overlapping of tagged sub-images in a single-shot multiplexed exposure.
Herein, PhD candidate Walker McCord, Postdoctoral fellow Dr. Cary Smith and led by Professor Zhili Zhang from the University of Tennessee in Knoxville a new passive 3D imaging technique based on a single-camera 3D-MUSIC stereoscopic imaging system. This system was used to obtain a 3D fuel-to-air ratio (FAR) mapping of methane-air Bunsen flame at standard atmospheric conditions. Specifically, the 3D-MUSIC was adopted to reconstruct a target object to facilitate 3D FAR mapping. Their research work is currently published in the journal, Optics and Lasers in Engineering.
The research team showed the spatial modulation and capturing of the images of CH* (430nm) and C2*(500nm) into a single image on the camera without special illumination. Low-pass filtering in the spatial frequency domain was used to recover the C2*/CH* ratio and it exhibited a linear relationship with the FAR map of the flame. The unique FAR map of each channel was combined to generate a line-of-sight 3D FAR map by triangulating the points from the individual channels.
The system optimization was achieved by examining the different modulation gratings, focal lengths as well as object depth that yielded dense 3D reconstructions. A 3D reconstruction algorithm was established to process the recovered images. The algorithm matched the features and values required for visualizing a 3D FAR map of methane/air flames at different equivalence ratios. In the end, the 3D FAR map with ±0.05 mm measurable placement errors were successfully reconstructed.
In summary, University of Tennessee scientists developed a new stereoscopic imaging ability of the MUSIC technique using a single camera. Unlike most existing techniques, 3D-MUSIC technique could perform 3D imaging by simply increasing the view per camera while maintaining full size of the sensor per view. This overcomes the limitations associated with sensor splitting. Besides its ability to obtain multi-spectral multi-view 3D images, 3D-MUSIC could be expanded beyond two channels. In a statement to Advances in Engineering, Professor Zhili Zhang stated that 3D-MUSIC is a promising technique for improved and low-cost stereo imaging applications.
Reference
McCord, W., Smith, C., & Zhang, Z. (2022). Single-camera stereoscopic 3D multiplexed structured image capture for quantitative fuel-to-air ratio mapping. Optics and Lasers in Engineering, 152, 106945.
