Ignition and hydrodynamic ejections by laser-induced breakdowns
A focused laser can cause optical breakdown of gas, forming a high-temperature plasma that rapidly absorbs laser energy. As a source of heat and radical species, laser-based energy deposition can be an effective ignition source, and its relative versatility and precision make it an attractive alternative to conventional ignition methods. Understanding of the breakdown-induced flow, its dependence on details of the plasma generation, and how it can alter and tailor ignition and subsequent flame growth will facilitate incorporation into engineering applications such as internal combustion engines and supersonic propulsion systems. Direct numerical simulation with five gas models, in conjunction with an energy-deposition model that reproduces key experimental observations, is used to analyze the post-breakdown flow and ignition dynamics by direct numerical simulation. A perfect-gas model shows that the breakdown-induced flow occurs primarily by hydrodynamic processes that explain a curious experimental observation: hot gas ejects from the breakdown region along the laser axis, traveling up to distances several times the plasma kernel size, and can reverse direction for small changes in breakdown conditions. Vorticity-generating mechanisms are quantified and shown to be sensitive to asymmetries in the kernel's hydrodynamic expansion, and changes to the kernel geometry can lead to the observed ejection or its reversal. Even subtle alterations—such as a 20% increase in aspect ratio—can lead to qualitative differences in the vorticity dynamics and ultimate flow pattern. Rich flow phenomenology is caused by dual-pulse configurations, and mechanisms by which the ejection can be disrupted or enhanced by the second deposition are analyzed. For simultaneous depositions, vorticity generation can be suppressed by even a small secondary kernel, preventing the ejection from forming, whereas for time-delayed depositions, asymmetry in the kernel expansion can amplify ejection or precipitate its reversal. The time delay and spatial offset between two depositions are controlled to enhance the dispersal of hot gas, which is shown to increase the burning rate of a nascent hydrogen flame. This sensitivity of the breakdown-induced flow to energy deposition warrants an assessment of nonequilibrium-plasma effects on the hydrodynamics, which are analyzed using a two-temperature argon-plasma model. We show that electron recombination can occur on the time scale of the plasma's hydrodynamic expansion and provide an avenue for the formation energy of ions to be converted to mechanical work on the gas, thereby enhancing the expansion and altering the vorticity distribution. Finally, we analyze how these flow mechanisms can couple with the ignition of a fuel–oxidizer interface. We first consider the detailed hydrogen-combustion model, and results are generalized to heavy fuels with a reduced model. Heat release by radical recombination is a primary mechanism by which the ejected gas maintains the temperature ignition threshold as it is transported towards fuel. For depositions close to a fuel lighter than oxygen, the expanding kernel interacts with the mixture density mismatch to produce a flow that repels hot gas away, in some cases leading to ignition failure. This flow pattern is absent for heavy fuels, which more readily ignite in this non-premixed configuration due to suppression of the adverse flow response.
Laser-induced breakdown, ignition, combustion, laser-energy deposition, nonequilibrium plasma, compressible flow, fluid dynamics
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign