Synchronic measurements of temperatures and concentrations of OH, NH, and NO in flames based on broadband ultraviolet absorption spectroscopy

Abstract

Temperature is an important parameter influencing the combustion reaction path and rate and determining the combustion and energy exchange efficiency. The OH, NH, NO and other species are involved in the key elementary reactions of combustion and determine the generation of NO<i>_x</i> pollutants. Therefore, temperature and concentration measurements of OH, NH, and NO are of great significance for combustion diagnostics and research on reaction or emission mechanisms. In this work, a measurement system with high spatial resolution based on broadband ultraviolet absorption spectroscopy is established to realize simultaneous measurements of the temperature and concentrations of OH, NH, and NO in flames. Low detection limits of these three species are achieved by using the established measurement method. The 1<i>σ</i> detection limit of NH is 1.8 ppb·m (1560 K), which is realized for the first time in atmospheric-pressure flames using absorption spectroscopy. The 1<i>σ</i> detection limits of OH and NO are 60 ppb·m (1590 K) and 1 ppm·m (1380 K), respectively, which are obviously better than the existing results obtained by using infrared laser absorption spectroscopy. Then, the distributions of temperatures and concentrations of OH, NO and NH are acquired at various heights in an atmospheric-pressure NH₃/CH₄/air premixed flat flame with a high spatial resolution of nearly 0.1 mm. The broadband absorption spectra of OH and NH are acquired simultaneously inside the flame front, and the spectra of OH and NO are acquired simultaneously above the flame front. Inside or near the flame front, the temperatures deduced from the spectra of OH, NH, and NO are consistent, verifying the ability of these three species to be used to measure temperature. In addition, OH, NH, and NO are found to be suitable for different regions in combustion. The OH absorption is suitable for the post-combustion region with temperatures higher than 1000 K, the NH absorption can be used to acquire the temperature inside the flame front in complex combustion, and the NO absorption was able to provide the temperature in the region before or outside combustion at lower temperatures. Additionally, the experimental temperature and concentration profiles are in good agreement with the computational fluid dynamics predictions based on the mechanism, exhibiting the accuracy of the simultaneous temperature and concentration measurements by using broadband ultraviolet absorption spectra. Moreover, the differences in temperature and OH concentration between experiments and simulations indicate that the carbon sub-mechanism in the mechanism given by Okafor et al. [Okafor E C, Naito Y, Colson S, Ichikawa A, Kudo T, Hayakawa A, Kobayashi H <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1016/j.combustflame.2017.09.002">2018 <i>Combust. Flame</i> <b>187</b> 185</ext-link>] should be further improved for more accurate predictions of NH₃/CH₄ combustion.