Abstract
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.
Highlights
After each experiment, the chamber was flushed with zero air and emptied to a pressure of
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Summary
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. They suggest that at short time scales (prior to plasma recombination) vorticity is generated through a baroclinic torque induced in the flow by misaligned pressure and density gradients, while at longer time scales additional vorticity is created by roll-up of the plasma core (similar to the model of Bradley et al.) Similar mechanisms to those discussed here (in the context of laser ignition) are responsible for kernel dynamics induced by conventional spark plugs and in nanosecond discharges between electrode pairs. Endo et al have reported a comparative study between laser breakdown ignition and discharge spark plugs indicating that the plasma-driven fluid dynamics play an important role in flame kernel augmentation[24] They suggest that flame vorticity entrains the surrounding combustible mixture which leads to an increase of the effective kernel energy in the early stages of flame development. The CL images indicate that suppression of the third lobe leads to more robust flame kernel growth, and it is for these conditions that the dual-pulse method shows extension of the lean limit
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