气化炉内柴油火焰光谱辐射特性及CH~*二维辐射特性研究
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摘要
火焰的自发辐射光谱与火焰的结构、温度分布等燃烧特征参数密切相关。对激发态自由基辐射的辐射强度与二维分布进行研究,可清晰地反映火焰燃烧状态而不对火焰产生扰动。基于多喷嘴对置式气流床气化实验平台,利用光纤光谱仪和配置CCD相机的高温内窥镜,对柴油扩散火焰的辐射光谱及CH~*辐射二维分布特性进行研究。考察了当量比和撞击作用对火焰辐射光谱和CH~*辐射分布的影响。结果表明,柴油火焰在306.47及309.12 nm处存在OH~*辐射特征峰,在431.42 nm处存在CH~*辐射特征峰,且存在明显的碱金属原子Na~*(589.45 nm), K~*(766.91和770.06 nm)发射光谱。此外,由于柴油不完全燃烧生成大量碳黑,在辐射光谱的可见光波段产生了强烈的连续黑体辐射。火焰中的黑体辐射对CH~*辐射特征峰的检测存在干扰,且当量比越低时背景辐射越强,对自由基特征峰检测干扰越大。基于普朗克定律利用插值法可扣除430 nm附近波段背景辐射。柴油火焰中CH~*辐射峰值随当量比的增加单调减小, CH~*辐射等值线沿火焰发展方向依次出现三峰状、双峰状及单峰状,最终收缩为以反应核心区为中心的圆核。随着当量比的提高,出现各个形状的CH~*辐射强度阈值不断降低,火焰主反应区面积减小且向下游移动,当量比增加到1.0附近时,理论上柴油完全燃烧, CH~*辐射强度显著降低,贫燃火焰的CH~*辐射强度及分布区域几乎稳定不变。利用CH~*辐射强度值判定火焰举升长度,对于单喷嘴射流火焰,火焰举升长度随当量比的增加经历了显著增加后小幅下降的过程。相同当量比时两喷嘴撞击火焰CH~*辐射强度峰值始终高于单喷嘴射流火焰对应值;火焰举升长度随当量比的增加小幅增加。火焰撞击的约束作用使得火焰举升长度不易随着当量比变化发生较大波动,燃烧更加稳定。这为定量判断火焰燃烧状态提供了一种直观、有效的方法,同时为柴油燃烧的化学动力学研究提供了实验依据。
The spontaneous emission spectra of flame are closely related to flame characteristics such as flame structure and temperature distribution. And the combustion state of flame can be reflected clearly by radiation intensity and distributions of excited radicals without being destabilized. Based on a bench-scale opposed multi-burner(OMB) gasification platform, a fiber spectrometer and a high-temperature endoscope coupled with a CCD camera were applied to investigate the two-dimensional distributions of CH~* of diesel diffusion flames. The effects of equivalence ratio and impingement on emission spectraand CH~* distributionsof flame were further compared. The results show that there exists OH~*(306.47, 309.12 nm), CH~*(431.42 nm), Na~*(589.45 nm) as well as K~*(766.91, 770.06 nm) radicalsin diesel flames. In addition, due to incomplete combustionofdiesel fuel, a lot of black carbon is emitted, which leads to strong continuous black-body radiation in visible wavelengths. The black-body radiation interferes with the detection of the CH~* characteristic peak, and the lower the equivalence ratio, the stronger the background radiation, and the greater the interference to the detection. According to Planck's law and interpolation method, background radiation can be subtracted from total radiation in the band around 430 nm. The peak intensity of CH~* decreases monotonically with the increase of equivalence ratio. Meanwhile, the contours of CH~* radiation appear in the form of three-peak, double-peak, and single-peak along the direction of flame development, and eventually shrink into a circular nucleus centered on the reaction zone. As the equivalence ratio increases, the thresholds of each form continuously decrease, andthe reaction zone gradually shrinks and moves downstream. When the equivalence ratio increases to 1.0, the diesel fuel burns completely, CH~* radiation intensity decreases significantly, and the intensity and distribution of CH~* chemiluminescenceof fuel-lean flame remain stable. The flame lift-off length can be evaluated by CH~* radiation. For one-burnerjet flame, the flame lift-off length increases significantly and then decreases slightly with the increase of the equivalence ratio. The peak intensity of CH~* of impinging flame is always higher than that of jet flame. The lift-off length of impinging flame increases slightly with the increase of the equivalence ratio. More obviously, the confining effect of impingement makes the lift-off length of impinging flame not easy to fluctuatewith the change of the equivalence ratio, which enables the combustion process to be stabler. This provides an intuitive and effective method for quantitatively judging the flame combustion state, as well as an experimental basis for the study of the chemical kinetics of diesel combustion.
The spontaneous emission spectra of flame are closely related to flame characteristics such as flame structure and temperature distribution. And the combustion state of flame can be reflected clearly by radiation intensity and distributions of excited radicals without being destabilized. Based on a bench-scale opposed multi-burner(OMB) gasification platform, a fiber spectrometer and a high-temperature endoscope coupled with a CCD camera were applied to investigate the two-dimensional distributions of CH~* of diesel diffusion flames. The effects of equivalence ratio and impingement on emission spectraand CH~* distributionsof flame were further compared. The results show that there exists OH~*(306.47, 309.12 nm), CH~*(431.42 nm), Na~*(589.45 nm) as well as K~*(766.91, 770.06 nm) radicalsin diesel flames. In addition, due to incomplete combustionofdiesel fuel, a lot of black carbon is emitted, which leads to strong continuous black-body radiation in visible wavelengths. The black-body radiation interferes with the detection of the CH~* characteristic peak, and the lower the equivalence ratio, the stronger the background radiation, and the greater the interference to the detection. According to Planck's law and interpolation method, background radiation can be subtracted from total radiation in the band around 430 nm. The peak intensity of CH~* decreases monotonically with the increase of equivalence ratio. Meanwhile, the contours of CH~* radiation appear in the form of three-peak, double-peak, and single-peak along the direction of flame development, and eventually shrink into a circular nucleus centered on the reaction zone. As the equivalence ratio increases, the thresholds of each form continuously decrease, andthe reaction zone gradually shrinks and moves downstream. When the equivalence ratio increases to 1.0, the diesel fuel burns completely, CH~* radiation intensity decreases significantly, and the intensity and distribution of CH~* chemiluminescenceof fuel-lean flame remain stable. The flame lift-off length can be evaluated by CH~* radiation. For one-burnerjet flame, the flame lift-off length increases significantly and then decreases slightly with the increase of the equivalence ratio. The peak intensity of CH~* of impinging flame is always higher than that of jet flame. The lift-off length of impinging flame increases slightly with the increase of the equivalence ratio. More obviously, the confining effect of impingement makes the lift-off length of impinging flame not easy to fluctuatewith the change of the equivalence ratio, which enables the combustion process to be stabler. This provides an intuitive and effective method for quantitatively judging the flame combustion state, as well as an experimental basis for the study of the chemical kinetics of diesel combustion.
引文
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