Abstract
In this work, the effects of thermochemical non-uniformity on line-of-sight (LOS) laser absorption thermometry in counterflow diffusion flame (CDF) was systematically investigated. Differences between the LOS-determined temperature and the true core flame temperature was quantified considering the coupled effects of thermal and gas concentration non-uniformities along the LOS. Experimentally validated computational fluid dynamics simulation was used to establish the flame thermochemical structure with wide ranges of non-uniformities that were subsequently used as test cases. Six absorption line pairs of H2O near 2.5 μm were selected to ensure sufficiently strong absorption and wide temperature sensitivity. The level of non-uniformity (Γ, an index to reflect the percentage of the non-uniform thermochemical region), core flame temperature (Tcore) and core species concentration (Xcore) were parametrically examined under various non-uniformity conditions (Γ = 5–80%, Tcore = 1200–2600 K, Xcore = 2–20%). In the presence of thermochemical non-uniformity, the LOS-determined temperature was consistently lower than the core flame temperature under typical CDF conditions. The temperature deviations were found to increase with the increase of Tcore and Γ; however, they remained unchanged when Xcore varied dramatically from 2% to 20%. Additionally, the thermal non-uniformity was observed to have a stronger impact on temperature deviations than the gas concentration non-uniformity does. Further studies in two representative CDFs with significant thermochemical gradients showed that the LOS measurement could lead not only to deviations in the core flame temperature but also has the risk of providing inaccurate peak temperature position and the axial distribution profile. The most significant deviations occurred in the region near the fuel nozzle sections. Finally, novel strategies were proposed to mitigate the temperature deviations to improve the accuracy of LOS thermometry.
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More From: Journal of Quantitative Spectroscopy and Radiative Transfer
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