Quantitative measurement of chemiluminescence is a challenging work that limits the development of combustion diagnostics based on chemiluminescence. Here, we present a feasible method to obtain effective quantitative chemiluminescence data with an integrating sphere uniform light source. Spatial distribution images of OH* and CH* radiation from methane laminar diffusion flames were acquired using intensified charge-coupled device (CCD) cameras coupled with multiple lenses and narrow-band-pass filters. After the process of eliminating background emissions by three filters and the Abel inverse transformation, the chemiluminescence intensity was converted to a radiating rate based on the uniform light source. The simulated distributions of OH* and CH* agree well with the experimental results. It has also been found that the distribution of OH* is more extensive and closer to the flame front than that of CH*, demonstrating that OH* is more representative of the flame structure. Based on the change in the reaction rate of different formation reactions, OH* distributions can be divided into three regions: intense section near the nozzle, transition section in the middle of the flame, and secondary section downstream the flame, whereas CH* only exists in the first two regions. In addition, as the velocity ratio of methane and co-flowing air increases, the main reactions become more intense, while the secondary reaction of OH* becomes weaker.
Chemiluminescence information is of great significance for characterization of flame structure and combustion characteristics. An atmospheric low swirl burner was developed to investigate the chemiluminescence characteristics of OH* and CH* in low swirl flames, with the equivalence ratio varying from 0.8 to 1.2 and the swirl number from 0.2 to 0.6. The chemiluminescence images were captured via ICCD cameras coupled with narrow-bandpass filters, and an Abel inversion method was introduced to transform the line-of-sight-integrated image into two-dimensional radial distributions. The results show that the distribution of CH* is smaller than that of OH* and concentrated more upstream of the flame near the burner. The equivalence ratio has a relatively more direct influence on chemical reactions, while the swirl number has a more evident effect on the flame structure. As the equivalence ratio increases, the peak value of OH* and CH* increases and the peak position moves downstream of the flame, suggesting that the chemical reactions become more intense. In contrast, the height and width of chemiluminescence distribution increase linearly with increasing swirl number. Moreover, it is found that the equivalence ratio and swirl number can be feasibly estimated based on chemiluminescence measurement results, using the correlation between them derived from this study.
Direct numerical simulation of a spatially developing supersonic mixing layer with a convective Mach number of 1.0 is conducted. The present work focuses on the structural evolution and the turbulent statistics, and both instantaneous and time-averaged data are utilized to obtain further insight into the dynamical behaviors of the flow. The full development process of instability, including the shear action at the flow early stage, the generation of kinds of typical vortex structures in the flow transition region, and the establishment of self-similar turbulence, is clearly presented. The formation and evolution mechanisms of multiple ring-like vortices are reported and analyzed using the Helmholtz first law in compressible mixing layers, and the role they play in the mixing process in the flow transition stage is researched. The mean velocity distribution and the turbulent intensities are found to have close relations with the evolution of the multiple ring-like vortices. The presence of multiple ring-like vortices leads to local strong ejection and sweep regions that create pockets of partially mixed fluid near the tips of the vortices, which contributes much to the huge energy and momentum transfer of the upper and lower streams. Some anisotropy coefficients and turbulent structure parameters are described and analyzed to better reveal the effects of multiple ring-like vortices on flow behaviors. Our results indicate that with the increase in compressibility, though in a fully turbulent region, mixing layer growth and turbulent intensities are both suppressed, the appearance of multiple ring-like vortices and their evolutions can significantly promote mixing in the transition stage, which is usually ignored by previous researchers. Therefore, employing flow control methods to extend the flow transition stage and help sustain multiple ring-like vortices over a longer distance is a possible technique to enhance mixing.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.