This paper addresses how NH and NO absorption along the laser axis disrupts NO-LIF measurements made around 225 nm in NH/O/N premixed flames confined at low pressure (10 kPa). Measurements were performed in 3 flames (equivalence ratio = 0.87, 1.1 and 1.3) with 24 % N dilution. Together with the LIF measurements, single-pass absorption measurements were obtained in all flames as a function of the height above the burner (HAB) and of the wavelength. A significant NH absorption of the laser beam is found in the pre-heat zone and flame front of each flame, as well as in the burned gas in the rich flame. In the lean flame, whatever the HAB, an additional significant absorption is due to the NO present in the gas surrounding the flame. Whereas NH absorbance only occurs inside the flame under all conditions, NO absorbance is mainly due to the high concentration of NO produced in the lean flame and spread in the entire confined vessel around the flame. From this analysis, relative NO rotational population profiles were recorded by measuring the temporal peak of the LIF signals (prompt-LIF) which were corrected for absorption to account for the available energy at burner's center. NO-LIF thermometry was conducted to determine the temperature in the flames through collecting excitation LIF spectra and performing spectral fitting using a dedicated software (thermo NO-LIF, ). The procedure developed to collect the NO profiles could not be transposed to determine the temperature in the lean flame. Indeed, the attenuation of the laser beam depends on whether the laser is tuned on a “cold” transition (strongly impacted by absorption outside the flame) or on a “hot” transition of NO, little affected. Thus, two tubes (with N flushing) were installed inside the vessel along the laser axis and surrounding the flame to suppress the NO absorbance.

中文翻译：

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关于解释受限氨火焰中 226 nm 附近 NO-LIF 测量值的困难

本文讨论了 NH 和 NO 沿激光轴的吸收如何干扰在低压 (10 kPa) 限制的 NH/O/N 预混合火焰中在 225 nm 附近进行的 NO-LIF 测量。在 24% N 稀释的 3 个火焰（当量比 = 0.87、1.1 和 1.3）中进行测量。与 LIF 测量一起，在所有火焰中获得单程吸收测量，作为燃烧器上方高度 (HAB) 和波长的函数。在每个火焰的预热区和火焰前锋以及富火焰中的燃烧气体中发现了激光束的显着 NH 吸收。在稀薄火焰中，无论 HAB 是什么，额外的显着吸收是由于火焰周围气体中存在的 NO 造成的。 NH 吸收在所有条件下仅发生在火焰内部，而 NO 吸收主要是由于稀薄火焰中产生高浓度的 NO 并扩散到火焰周围的整个密闭容器中。根据该分析，通过测量 LIF 信号 (prompt-LIF) 的时间峰值来记录相对 NO 旋转种群分布，并针对吸收进行校正，以考虑燃烧器中心的可用能量。 NO-LIF 测温是通过收集激发 LIF 光谱并使用专用软件（thermo NO-LIF，）进行光谱拟合来确定火焰中的温度。为收集 NO 分布而开发的程序无法转换来确定稀薄火焰中的温度。事实上，激光束的衰减取决于激光是在“冷”跃迁（受到火焰外部吸收的强烈影响）还是在 NO 的“热”跃迁（几乎不受影响）上调谐。因此，沿激光轴在容器内安装两根管（用 N 冲洗）并围绕火焰以抑制 NO 吸收。