Due to their scientific significance and practical applications, turbulent flows and flames have been under extensive and intensive research for a long time. Turbulent flows and flames of interests to practice inherently have three-dimensional (3D) spatial structures, and therefore diagnostic techniques that can instantaneously resolve their 3D spatial features have long been desired and probably are needed to ultimately answer some of the open research questions. The goal of this dissertation thus is to investigate such diagnostics and demonstrate their capability and limitations in a range of turbulent flows/flames. To accomplish this goal, this dissertation developed and evaluated the following three diagnostic methods: tomographic chemiluminescence (TC), volumetric laser induced fluorescence (VLIF), and super-resolution planar laser induced fluorescence (SR-PLIF).
First, 3D flame topography of well-controlled laboratory flames was measured with TC method and validated by a simultaneous 2D Mie scattering measurement. The results showed that the flame topography obtained from TC and the Mie scattering agreed qualitatively, but quantitative difference on the order of millimeter was observed between these two methods. Such difference was caused by the limitations of the TC method. The first limitation involves TC's reliance on chemiluminescence of nascent radicals (mainly CH*) in reacting flows, causing ambiguity in the definition of flame front and limiting its applications to certain types of reactive flow only. The second limitation involves TC's inability to study an isolated region of interest because the chemiluminescence is emitted everywhere in the flame.
Based on the above understanding of the TC technique, the second part of this dissertation studied a VLIF method to overcome the above limitations of the TC technique. Compared with the TC technique, the VLIF method can be used in either reacting or non-reacting flow and on any particular region of interest. In the VLIF technique, the fluorescence signal was generated by exciting a target species with a laser slab of certain thickness. The signal was recorded by cameras from different perspectives, and then a VLIF tomographic algorithm was applied to resolve the spatial distribution of the concentration of the target species. An innovative 3D VLIF algorithm was proposed and validated by well-designed experiment. This model enables analysis of VLIF performance in terms of signal level, size of the field of view in 3D, and accuracy. However, due to the limited number of views and the tomographic reconstruction itself, the spatial resolution of VLIF methods is limited.
Hence, the third part of this dissertation investigated a SR-PLIF method to provide a strategy to improve the spatial resolution in two spatial directions, and also to extend the measurement range of scanning 3D imaging strategies. The SR-PLIF method used planar images captured simultaneously from two (or more) orientations to reconstruct a final image with resolution enhanced or blurring removed. Both the development of SR algorithm, and the experimental demonstration of the SR-PLIF method were reported. / Ph. D. / Optical diagnostics have become indispensable tools for the study of the turbulent flows and flames. Due to the inherently 3D structure of turbulent flows and flames, diagnostic techniques which can provide 3D measurements have been long desired. Therefore, this dissertation reports the development of three optics diagnostic methods that can provide such measurement capability, with a detailed discussion of their capabilities and limitations. The methods studied are tomographic chemiluminescence (TC), volumetric laser-induced fluorescence (VLIF), and super-resolution planar laser induced fluorescence (SR-PLIF). For the TC technique, the emission of light from combustion radicals (CH* and OH*) was recorded by multiple cameras placed at different orientations. A numerical algorithm was then applied to reconstruct the 3D flame structure. For the VLIF technique, a laser slab was used to excite a specific chemical species in the flame, which were captured from different perspectives to reconstruct the flow or flame structure in 3D. For the SR-PLIF technique, a series of planar images were recorded from multiple orientations to reconstruct a target image with higher resolution or to extend the measurement volume of scanning 3D diagnostics.
It is expected that the results obtained in this dissertation lay the groundwork for further development and expanded application of 3D diagnostics for the study of turbulent flows and combustion processes.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/79912 |
| Date | 01 November 2017 |
| Creators | Xu, Wenjiang |
| Contributors | Aerospace and Ocean Engineering, Ma, Lin, Lowe, K. Todd, Philen, Michael K., Xiao, Heng |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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