Colorado State University, Department of Mechanical Engineering, Fort Collins, 80523, USA.
Colorado State University, Department of Chemistry, Fort Collins, 80523, USA.
Sci Rep. 2017 Aug 31;7(1):10239. doi: 10.1038/s41598-017-10457-0.
The present contribution examines the impact of plasma dynamics and plasma-driven fluid dynamics on the flame growth of laser ignited mixtures and shows that a new dual-pulse scheme can be used to control the kernel formation process in ways that extend the lean ignition limit. We perform a comparative study between (conventional) single-pulse laser ignition (λ = 1064 nm) and a novel dual-pulse method based on combining an ultraviolet (UV) pre-ionization pulse (λ = 266 nm) with an overlapped near-infrared (NIR) energy addition pulse (λ = 1064 nm). We employ OH* chemiluminescence to visualize the evolution of the early flame kernel. For single-pulse laser ignition at lean conditions, the flame kernel separates through third lobe detachment, corresponding to high strain rates that extinguish the flame. In this work, we investigate the capabilities of the dual-pulse to control the plasma-driven fluid dynamics by adjusting the axial offset of the two focal points. In particular, we find there exists a beam waist offset whereby the resulting vorticity suppresses formation of the third lobe, consequently reducing flame stretch. With this approach, we demonstrate that the dual-pulse method enables reduced flame speeds (at early times), an extended lean limit, increased combustion efficiency, and decreased laser energy requirements.
本研究考察了等离子体动力学和等离子体驱动的流体动力学对激光点火混合物火焰增长的影响，并表明可以采用新的双脉冲方案来控制核心形成过程，从而扩展贫燃点火极限。我们对（传统）单脉冲激光点火（λ=1064nm）和基于结合紫外（UV）预电离脉冲（λ=266nm）与重叠近红外（NIR）能量添加脉冲（λ=1064nm）的新型双脉冲方法进行了比较研究。我们采用 OH*化学发光来可视化早期火焰核心的演化。对于贫燃条件下的单脉冲激光点火，火焰核心通过第三叶瓣分离，对应于熄灭火焰的高应变率。在这项工作中，我们通过调整两个焦点的轴向偏移来研究双脉冲控制等离子体驱动的流体动力学的能力。特别地，我们发现存在一个束腰偏移，由此产生的涡度抑制了第三叶瓣的形成，从而降低了火焰拉伸。通过这种方法，我们证明了双脉冲方法能够降低火焰速度（在早期）、扩展贫燃极限、提高燃烧效率和降低激光能量需求。
