Nanosecond-gated laser-induced breakdown spectroscopy (n-LIBS) has been used to quantify fuel mole fraction (Χ_C2H4) in the cavity flameholder of a model high-speed combustor. The measurement locations selected, in the vicinity of the cavity shear layer, have low fuel concentrations. Previous n-LIBS measurements showed unexpectedly high values at these locations with expected low average fuel mole fraction (Χ_C2H4) and intermittent fuel presence; thus, an effort was undertaken to understand potential sources of error with n-LIBS at low Χ_C2H4. An improved direct spectrum matching (DSM) calibration matrix was thus constructed, focusing on low Χ_C2H4 for both reacting and non-reacting conditions. Comparisons were made between n-LIBS measurements using two different laser systems for plasma generation, and the insensitivity of the signal to the specific laser was demonstrated for the first time. Uncertainty of the n-LIBS measurement technique at two different gas densities was analyzed through the processing of spectra acquired at known values of X_C2H4. Measurements in the windtunnel, around the shear layer of the cavity flameholder, were conducted with a Mach-2 or Mach-3 freestream, and comparisons were made between n-LIBS results—using two DSM calibration matrices with different increments in X_C2H4—and numerical results employing dynamic hybrid Reynolds-averaged Navier-Stokes and large-eddy simulation (DHRL). The high-resolution DSM matrix produced slightly lower X_C2H4 than those from the coarse matrix, and comparisons with the simulation showed good agreement overall. For Mach-3 conditions, comparisons were made for both a non-reacting and a reacting cavity flameholder. Here, results showed a systematic difference between measured and simulated values of the mean X_C2H4 within the shear layer but good (even excellent) agreement within the upper portion of the recirculation zone. For reacting condition, four reduced chemical kinetic models were implemented with the DHRL technique and compared to n-LIBS results. Overall, the four kinetic models produced a substantial range of results (e.g., temperatures and reaction progress), showing the importance of a validated diagnostic technique like n-LIBS for model testing.

纳秒门控激光诱导击穿光谱（正LIBS）已被用于定量燃料的摩尔分数（Χ _C2H4在模型高速燃烧器的火焰稳定器腔）。在腔体剪切层附近选择的测量位置的燃料浓度较低。先前的n-LIBS测量显示出这些位置处的出乎意料的高值，具有预期的低平均燃料摩尔分数（X _C2H4）和间歇性燃料存在；因此，人们进行了努力，以了解低X _C2H4下n-LIBS的潜在误差来源。因此，构建了改进的直接光谱匹配（DSM）校准矩阵，重点放在低X _C2H4适用于反应条件和非反应条件。使用两种用于产生等离子体的不同激光系统在n-LIBS测量之间进行了比较，并且首次证明了信号对特定激光的不敏感性。通过处理在已知的X _C2H4值下获得的光谱，分析了两种不同气体密度下n-LIBS测量技术的不确定性。在风洞测量，围绕腔火焰稳定器的剪切层，用马赫- 2或马赫- 3自由流进行，并进行了比较正LIBS结果的使用具有在不同的增量2个DSM校准矩阵之间进行X _C2H4-以及采用动态混合雷诺平均Navier-Stokes和大涡模拟（DHRL）的数值结果。高分辨率DSM矩阵产生的X _C2H4略低于粗糙矩阵产生的X _C2H4，与模拟的比较表明总体上一致性良好。对于Mach-3条件，对非反应型和反应型火焰架进行了比较。在这里，结果表明平均值X _C2H4的测量值与模拟值之间存在系统差异在剪切层中的流动性良好，但在再循环区域的上部具有良好的（甚至极好的）一致性。对于反应条件，采用DHRL技术实施了四个简化的化学动力学模型，并与n-LIBS结果进行了比较。总体而言，这四个动力学模型产生了大量结果（例如温度和反应进程），显示出已验证的诊断技术（如n-LIBS）对于模型测试的重要性。