Measurement of inter-scale molecular fluxes in supercritical reacting turbulence

了解湍流中特定长度尺度上的波动如何影响其他长度尺度上的现象，对于通过数值模拟准确地预测流动特性是至关重要的。湍流燃烧是将化学能转化为热能的主要方式，在运输和发电应用中，与非反应湍流相比，出现了额外的挑战，因为波动的化学反应速率发生在非常小的长度尺度上，通常无法通过模拟来解决。在越来越高的压力下运行燃烧系统的愿望加剧了这个问题，这种压力通常会延伸到超临界压力条件。虽然增加压力对效率和功率密度有重要的好处，但它也将波动集中在越来越小的尺度上，并在临界点附近引入热力性质的强烈变化。为了更好地了解这一现象，该项目将使用激光诊断和数据分析技术的新组合来测量在初始超临界条件下燃烧反应物的预混火焰中，分子氮密度的波动如何在长度尺度之间移动。空间滤波后的氮数密度和速度场的时间分辨测量将在交叉平面内使用猝发模式的自发拉曼散射进行。观测过程的滤波效果将通过图像反褶积来确定和调整。因此，所得到的数据将允许基于滤波场的动态通过滤波尺度明确地确定子滤波波动的通量。这些信息将确定传统的建模范例是否适用于超临界反应湍流，并在必要时允许开发更精确的物理模型。它还将提供一个独特的数据集，作为超临界湍流燃烧模拟的基准。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Understanding how fluctuations at a specific length scale in a turbulent flow affect the phenomena at other length scales is critical to accurately predict flow properties by numerical simulation. In turbulent combustion, which is the predominant mode of converting chemical energy to thermal energy for transportation and power generation applications, additional challenges arise in comparison to non-reacting turbulent flows because the fluctuating chemical reaction rates occur at very small length scales that typically are not resolved by simulations. This problem is exacerbated by the desire to operate combustion systems at increasingly high pressures, often spanning in to the supercritical pressure conditions. While increasing the pressure has important benefits for efficiency and power density, it also concentrates fluctuations at increasingly small scales and introduces strong variations in thermodynamic properties around the critical point. To better understand this phenomenon, this project will use a novel combination of laser diagnostic and data analysis techniques to measure how fluctuations in molecular nitrogen density move between length scales in premixed flames burning reactants at initially supercritical conditions. Time-resolved measurements of the spatially filtered nitrogen number density and velocity fields will be made in crossed planes using burst-mode spontaneous Raman scattering. The filtering effect of the observation process will be determined and tuned through image deconvolution. Hence, the resultant data will allow explicit determination of the flux of sub-filter fluctuations through the filter scale based on the dynamics of the filtered fields. This information will identify whether traditional modeling paradigms hold for supercritical reacting turbulence and, where necessary, enable the development of more physically accurate models. It also will provide a unique data set against which to benchmark simulations of supercritical turbulent combustion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.