Partial Premixing by High-Intensity, High-Frequency Forcing of Jet Flames

这是对一种基本上尚未开发的燃烧器策略的研究，该策略使用高强度、高频的燃料喷射力来实现初始未预混反应物的大量部分预混。初步结果表明，在强射流的作用下，湍流非预混射流火焰的结构和亮度有可能发生剧烈变化。通过高速阀调节燃油流量，使其频率与燃油管的管风琴共振频率一致，从而实现强烈的脉动。燃油流量的快速调制与燃油管的风琴管共振相结合，可实现对燃气压力和速度的大振幅、高频调制。由此产生的流动表现出高度的燃料和空气的部分预混，可能是由于吸入（吸入）周围空气进入燃料管，涡环夹带，以及由不稳定产生的湍流。虽然已经证明了高振幅、高频强迫的影响，但导致火焰结构显著变化的潜在物理机制尚不清楚。这项工作的目的是定义这种现象有效的条件范围，并通过仔细的实验提供一个清晰的概念模型。为了确定导致强部分预混的工作参数范围，研究了燃料管直径、共振管长度（以及激励频率）、燃料射流雷诺数和燃料类型的范围。测量了井喷极限和平均火焰长度等全局参数。测量废气污染物，如氮氧化物、一氧化碳和碳氢化合物。一旦确定了最有趣的操作条件，研究的第二阶段包括进行仔细的流场测量，以了解潜在的物理机制。在这个阶段，使用了许多成像诊断，包括粒子图像测速（PIV）、平面激光诱导荧光（PLIF）、高速米氏散射和高速纹影摄像。PIV用于量化燃料管速度波动，测量射流夹带，揭示振荡周期各阶段的流场结构。利用种子丙酮蒸汽的PLIF进行定量混合测量，揭示部分预混程度；利用OH和CH的PLIF研究作用力对反应区结构的影响；高速成像提供火焰特征频率的全局信息。从这些测量中获得的数据有助于开发基于分析化学反应器的模型，该模型可用于预测这些类型火焰中的火焰长度和污染物形成。更广泛的影响可能是利用燃料流的强非定常周期性强迫来设计燃烧器，通过部分预混，产生更少的烟尘或烟雾，并可能降低氮氧化物和碳氢化合物等污染物。减少环境污染物是目前工业燃烧器研究的主要推动力。这种类型的燃烧器可能会对用于过程加热的紧凑型燃烧器，堆栈耀斑，甚至增加传统的燃气轮机发电机产生重大影响。此外，非定常运行可以实现传统（旋流稳定）燃烧无法实现的燃烧器控制策略。向本科生和研究生教授先进燃烧器开发和表征的技能，并将研究结果纳入燃烧理论和燃烧诊断课程。来自代表性不足的少数群体的学生也包括在研究活动中。

This is an investigation of an essentially unexploited burner strategy that uses high-intensity, high-frequency forcing of a fuel jet to achieve substantial partial premixing of initially nonpremixed reactants. Preliminary results have shown that it is possible to produce dramatic changes in the structure and luminosity of a turbulent nonpremixed jet flame with the application of strong forcing of the jet flow. Intense pulsations are achieved by modulating the fuel flow with a high-speed valve at a frequency that coincides with the organ-pipe resonance frequency of the fuel tube. The combination of rapid fuel-flow rate modulation coupled with the organ-pipe resonance of the fuel tube results in very large amplitude, high-frequency modulation of the fuel gas pressure and velocity. The resulting flow exhibits a high degree of partial premixing of the fuel and air, possibly due to the ingestion (sucking) of ambient air into the fuel tube, vortex-ring entrainment, and turbulence generated by the unsteadiness. While the effect of high-amplitude, high-frequency forcing has been demonstrated, the underlying physical mechanisms that are responsible for the significant changes in the flame structure are not known. The objective of this work is to define the range of conditions where this phenomenon is effective and to provide a clear conceptual model of the process through careful experimentation. To determine the range of operating parameters that result in strong partial premixing, a range of fuel-tube diameters, resonance tube lengths (and hence excitation frequencies), fuel-jet Reynolds numbers, and fuel type are investigated. Global parameters such as the blowout limits and mean flame length are measured. Exhaust-gas pollutants such as nitrogen oxides, carbon monoxide, and hydrocarbons, are measured. Once the most interesting operating conditions have been identified, a second phase of the study involves making careful flow-field measurements with the aim of understanding the underlying physical mechanisms. In this phase, a number of imaging diagnostics, including particle image velocimetry (PIV), planar laser-induced fluorescence (PLIF), high-speed Mie scattering and high-speed schlieren videography are used. PIV is used to quantify the fuel-tube velocity fluctuations, to measure jet entrainment and to reveal the flow-field structure at each phase of the oscillation cycle. The PLIF of seeded acetone vapor is used to make quantitative mixing measurements to reveal the degree of partial premixing, PLIF of OH and CH are used to investigate the effect of forcing on the reaction zone structure, and the high-speed imaging provides global information on characteristic frequencies in the flames. The data obtained from these measurements aids in the development of an analytical chemical-reactor-based model that is useful for predicting flame lengths and pollutant formation in these types of flames. Broader impacts It may be possible to use strong unsteady periodic forcing of the fuel stream to design burners that produce less soot or smoke, and possibly lower pollutants such as nitrogen oxides and hydrocarbons, through partial premixing. The reduction of environmental pollutants is the primary driver of research on industrial burners at this time. This type of burner could have a significant impact on compact burners used for process heating, stack flares, and even to augment conventional gas turbine power generators. Furthermore, the unsteady operation may enable burner control strategies that are not possible with conventional (swirl-stabilized) combustion. Undergraduate and graduate students are taught skills in advanced combustion burner development and characterization, and results from this study are incorporated into courses on combustion theory and combustion diagnostics. Students from under-represented minority groups are included in the research activities.