Fluid Dynamic Characterization and Control of Turbulent Plasma Jets

本研究的目的是推导出湍流等离子体射流中关键参数的详细物理描述，并设计出控制湍流的方法。现代测量技术的发展相对良性的燃烧环境，揭示了等离子体射流的复杂流场。该策略是使用激光散射技术对射流中的三维速度分布进行高时间分辨率的详细测量，模拟等离子体射流的情况，考虑射流流体与环境之间的密度比以及电弧附着动力学引起的流动强迫。利用氦作为核心射流气体，氩气作为边界层气体的装置，获得了合适的模拟射流密度场。这种变密度的喷流排放到六氟化硫的环境中。通过迫使上游压力和速度波动产生从混沌到周期性的条件来模拟等离子体的不稳定性。这些研究是通过对等离子体喷枪的测量来补充的，首先得到适当的强迫函数，然后通过在温度较低的等离子体射流条纹中进行类似的测量来验证在冷流条件下测量的速度分布。射流特性使我们更好地理解了射流不稳定的原因，这反过来又允许设计流体动力学手段来控制等离子体射流的大规模湍流，例如周围微射流阵列、被动喷嘴罩或使用共流的剪切层控制。更广泛的影响大气压等离子射流中的大规模湍流仍然是许多等离子过程中不可复制行为的原因，例如等离子喷涂和切割，粉末合成（包括纳米级结构的等离子合成）和冶金加工。可以定性地理解，这些湍流波动是由(a)热等离子体和周围气体之间的极端密度差异引起的，以及(b)由于电弧阳极附着物的变化而引起的上游流动波动。然而，由于难以获得等离子体条件下具有高时间分辨率和足够信噪比的局部速度等数据，迄今尚未实现详细的表征。燃烧流体动力学和等离子体科学技术的专业知识的协同结合将提高对大气压等离子体湍流的理解。预计这些结果不仅可以通过改进等离子体火炬设计影响许多热等离子体过程，而且还可以应用于具有类似密度梯度的其他喷气机，例如航空航天推进系统。此外，预期改进的湍流控制可以显著降低等离子体处理设备和推进系统的噪声水平。

The objective of this research is to derive a detailed physical description of the crucial parameters in turbulent plasma jets and to devise means for controlling the turbulence. Modern measurement techniques developed for the relatively benign environment of combustion are exploited to unravel the complicated flowfield of the plasma jet. The strategy is to use laser scattering techniques to perform detailed measurements of three-dimensional velocity distributions with high time resolution in jets that simulate the plasma jet situation with respect to density ratios between jet fluid and environment and with respect to flow forcing due to arc attachment dynamics. The appropriate density field in the simulated jet is achieved using a novel facility that supplies helium as the core jet gas and argon as a boundary layer gas. This variable-density jet exhausts into an ambient environment of sulfur hexafluoride. The plasma instabilities are simulated by forcing upstream pressure and velocity fluctuations to produce conditions ranging from chaotic to periodic. These studies are supplemented by measurements on plasma spray torches, first to derive appropriate forcing functions, then to verify the velocity distributions measured under cold-flow conditions by performing similar measurements in the plasma jet fringes where the temperatures are lower. The jet characterization leads to an improved understanding of the causes of the jet instabilities, which in turn allows the design of fluid-dynamic means to control the large-scale turbulence of the plasma jet, such as arrays of surrounding microjets, passive nozzle shrouds, or shear-layer control using coflow.Broader impactLarge-scale turbulence in atmospheric pressure plasma jets remains a cause of irreproducible behavior in a number of plasma processes, such as plasma spraying and cutting, powder synthesis including plasma synthesis of nanoscale structures, and metallurgical processing. It is qualitatively understood that these turbulent fluctuations are caused by (a) the extreme density differences between the hot plasma and the surrounding gas, and (b) the upstream fluctuations of the flow due to varying arc-anode attachments. However, a detailed characterization has not been achieved so far because of the difficulty of obtaining data such as local velocities with high time resolution and sufficient signal-to-noise ratio under plasma conditions. Improved understanding of turbulence in atmospheric-pressure plasmas will result from the synergistic combination of expertise in combustion fluid dynamics and plasma science and technology. It is expected that the results impact not only a number of thermal-plasma processes through improved plasma-torch designs, but also that the knowledge is applicable to other jets with similar density gradients such as in aerospace propulsion systems. Furthermore, the expected improved turbulence control can lead to significant reductions in noise levels both for plasma-processing equipment as well as for propulsion systems.