Ignition and combustion of pulverized solid fuel is investigated in a laminar burner. The two-dimensional OH radical field is measured in the experiments, providing information on the first onset of ignition and a detailed characterization of the flame structure for the single particle. In addition, particle velocity and diameter are tracked in time in the experiments. Simulations are carried out with a Lagrangian point-particle approach fully coupled with an Eulerian solver for the gas-phase, which includes detailed chemistry and transport. The numerical simulation results are compared with the experimental measurements in order to investigate the ignition characteristics. The effect of the slip velocity, i.e. the initial velocity difference between the gas-phase and the particle, is investigated numerically. For increasing slip velocity, the ignition delay time decreases. For large slip velocities, the decrease in ignition delay time is found to saturate to a value which is about 40% smaller than the ignition delay time at zero slip velocity. Performing a simulation neglecting the dependency of the Nusselt number on the slip velocity, it is found that this dependency does not play a role. On the contrary, it is found that the decrease of ignition delay time induced by the slip velocity is due to modifications of the temperature field around the particle. In particular, the low-temperature fluid related to the energy sink due to particle heating is transported away from the particle position when the slip velocity is non-zero; therefore, the particle is exposed to larger temperatures. Finally, the effect of particle swell is investigated using a model for the particle swelling based on the CPD framework. With this model, we observed negligible differences in ignition delay time compared to the case in which swelling is not included. This is related to the negligible swelling predicted by this model before ignition. However, this is inconsistent with the experimental measurements of particle diameter, showing a significant increase of diameter even before ignition. In further simulations, the measured swelling was directly prescribed, using an analytical fit at the given conditions. With this approach, it is found that the inclusion of swelling reduces the ignition delay time by about 20% for small particles while it is negligible for large particles.

中文翻译：

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层流燃烧器中固体颗粒点火的数值模拟和实验：滑移速度和颗粒膨胀的影响

在层流燃烧器中研究粉状固体燃料的点火和燃烧。在实验中测量二维 OH 自由基场，提供有关首次点火的信息和单个粒子的火焰结构的详细表征。此外，在实验中及时跟踪粒子速度和直径。模拟是使用拉格朗日点粒子方法与气相欧拉求解器完全结合进行的，其中包括详细的化学和传输。将数值模拟结果与实验测量结果进行比较，以研究点火特性。滑移速度的影响，即气相和颗粒之间的初始速度差，进行了数值研究。为了增加滑移速度，点火延迟时间减少。对于大的滑移速度，发现点火延迟时间的减少饱和到一个值，该值比零滑移速度下的点火延迟时间小约 40%。执行忽略努塞尔数对滑移速度的依赖性的模拟，发现这种依赖性不起作用。相反，发现由滑移速度引起的点火延迟时间的减少是由于粒子周围温度场的改变。特别是，当滑移速度非零时，与粒子加热引起的能量汇相关的低温流体被运离粒子位置；因此，颗粒暴露在更高的温度下。最后，使用基于 CPD 框架的颗粒膨胀模型研究了颗粒膨胀的影响。使用这个模型，我们观察到与不包括膨胀的情况相比，点火延迟时间的差异可以忽略不计。这与该模型在点火前预测的可忽略不计的膨胀有关。然而，这与粒子直径的实验测量不一致，甚至在点火之前显示出直径的显着增加。在进一步的模拟中，在给定条件下使用分析拟合直接规定测量的膨胀。通过这种方法，发现包含溶胀使小颗粒的点火延迟时间减少了约 20%，而对于大颗粒则可以忽略不计。与不包括膨胀的情况相比，我们观察到点火延迟时间的差异可以忽略不计。这与该模型在点火前预测的可忽略不计的膨胀有关。然而，这与粒子直径的实验测量不一致，甚至在点火之前显示出直径的显着增加。在进一步的模拟中，在给定条件下使用分析拟合直接规定测量的膨胀。通过这种方法，发现包含溶胀使小颗粒的点火延迟时间减少了约 20%，而对于大颗粒则可以忽略不计。与不包括膨胀的情况相比，我们观察到点火延迟时间的差异可以忽略不计。这与该模型在点火前预测的可忽略不计的膨胀有关。然而，这与粒子直径的实验测量不一致，甚至在点火之前显示出直径的显着增加。在进一步的模拟中，在给定条件下使用分析拟合直接规定测量的膨胀。通过这种方法，发现包含溶胀使小颗粒的点火延迟时间减少了约 20%，而对于大颗粒则可以忽略不计。这与颗粒直径的实验测量不一致，表明即使在点火之前直径也显着增加。在进一步的模拟中，在给定条件下使用分析拟合直接规定测量的膨胀。通过这种方法，发现包含溶胀使小颗粒的点火延迟时间减少了约 20%，而对于大颗粒则可以忽略不计。这与颗粒直径的实验测量不一致，表明即使在点火之前直径也显着增加。在进一步的模拟中，在给定条件下使用分析拟合直接规定测量的膨胀。通过这种方法，发现包含溶胀使小颗粒的点火延迟时间减少了约 20%，而对于大颗粒则可以忽略不计。