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

这是一个基本上未开发的燃烧器的策略，使用高强度，高频率的强制燃料射流，以实现基本部分预混合的最初nonpremixed反应物的调查。 初步结果表明，它是可能产生的湍流非预混射流火焰的结构和亮度的显着变化与应用程序的强强制射流。 通过用高速阀以与燃料管的风琴管共振频率一致的频率调制燃料流来实现强烈的脉动。 快速燃料流率调制与燃料管的风琴管共振的组合导致燃料气体压力和速度的非常大的振幅、高频调制。 产生的流动表现出高度的部分预混合的燃料和空气，可能是由于摄入（吸入）的环境空气进入燃料管，涡环夹带，和湍流产生的不稳定性。 虽然已经证明了高振幅、高频率强迫的影响，但导致火焰结构发生重大变化的基本物理机制尚不清楚。 这项工作的目的是确定的范围内的条件下，这种现象是有效的，并提供一个明确的概念模型的过程中，通过仔细的实验。 要确定的范围内的操作参数，导致强的部分预混，一系列的燃料管直径，共振管长度（因此激发频率），燃料喷射雷诺数，和燃料类型进行了研究。 全局参数，如井喷限制和平均火焰长度的测量。 废气污染物，如氮氧化物，一氧化碳和碳氢化合物，测量。 一旦确定了最有趣的操作条件，研究的第二阶段将涉及进行仔细的流场测量，目的是了解潜在的物理机制。 在这一阶段，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被用来量化燃料管的速度波动，以测量射流夹带，并揭示在振荡周期的每个阶段的流场结构。 种子丙酮蒸气的PLIF用于进行定量混合测量，以揭示部分预混合的程度，OH和CH的PLIF用于研究强迫对反应区结构的影响，高速成像提供了火焰中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一种基于分析化学反应器的模型，该模型可用于预测这些类型的火焰中的火焰长度和污染物形成。也许有可能利用强的不稳定的燃料流的周期性强迫来设计燃烧器，通过部分预混合产生较少的煤烟或烟雾，并可能减少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驱动力。 这种类型的燃烧器可能对用于过程加热、烟囱火炬、甚至用于增强传统燃气涡轮机发电机的紧凑型燃烧器具有显著影响。 此外，不稳定操作可实现常规（旋流稳定）燃烧不可能实现的燃烧器控制策略。 本科生和研究生教授先进燃烧器开发和表征的技能，并将本研究的结果纳入燃烧理论和燃烧诊断课程。 研究活动包括来自代表性不足的少数群体的学生。

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.