Multi-scale modeling of laser induced breakdown in non-equilibrium plasmas

Laser;Plasma;Non-Equilibrium Flows;Plasma-Coupled Combustion;Supersonic Drag Reduction

Alberti, Andrea

关键词: Laser;    Plasma;    Non-Equilibrium Flows;    Plasma-Coupled Combustion;    Supersonic Drag Reduction;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/107940/ALBERTI-DISSERTATION-2020.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This dissertation discusses the modeling of breakdown of gases under nano-second laser discharges, the plasma kernel dynamics, the plasma core decay, and the subsequent post-dischargee hydrodynamics, together with some of its engineering applications (e.g., flow control and ignition of combustible mixtures). A non-equilibrium model for laser-plasma interaction has been developed and validated for the purpose of this study. The computational framework has been, then, used to investigate the plasma kernel dynamics under different ambient conditions (e.g., pressure), laser parameters (e.g., power density and beam wavelength), mode-beating temporal pulse histories (e.g., single-mode and multi-mode nano-second discharges), and pulse con figurations (e.g., single-pulse and dual-pulse nano-second discharges). The hydrodynamics is described based on the chemically reactive Navier-Stokes equations, and non-equilibrium effects are accounted for using multi-temperature models. Laser-plasma interactions in the nano-second time-scale are modeled using a kinetic approach for the photons of the laser beam via the Radiative Transfer Equation (RTE), where multiphoton ionization (MPI) and inverse Bremsstrahlung (IB) have been included self-consistently. Validation was performed with an extensive comparison of quantities of interest (e.g., absorbed energy and plasma emission) predicted by the Laser-Induced Breakdown (LIB) model with the corresponding experimental measurements in a wide range of operating conditions. For single-pulse and single-mode discharges, the laser generated plasma exhibits a two-lobe structure (as also observed from the experiments) with the plasma waves developing both in the backward (i.e., toward the laser source) and forward (i.e., away from the laser source) directions; the analysis of the dynamics suggests that the kernel evolution results from a radiation-driven wave that is: (i) triggered by MPI, (ii) sustained by energy deposition in IB interactions, which compensates for the energy lost by free-electrons in ionizing collisions, and (iii) guided by both MPI and ionization by electron impact (IE). When the single-pulse operates in a multi-mode configuration, the LIB model predicts the formation of periodic plasma kernel structures (similar to experimental observations), with the periodic scattered spots being functions of the modulating frequency, and their phenomenology following the same mechanism (guided by both MPI and IE). Results also show that the nano-second discharge is followed by the formation of a strong ellipsoidal shock wave that propagates outwardly (away from the focal point) with a shock strength that decreases with time, until degeneration into a spherical acoustic wave (within a few microseconds). The plasma core decay is driven by a strong vorticityeld that is generated in the ionized gas as a result of a baroclinic torque associated with the misalignment between the radial gradient of density (due to the sudden gas expansion) and the strong gradient of pressure (initiated by the radiative energy transfer). A dual-pulse con figuration (with the energy deposition separated in afirst ultraviolet laser pulse for pre-ionization purposes, and a second near-infrared pulse for the main energy deposition shifted in both time and space) can be used to control the dimension, shape, and maximum temperature of the plasma kernel, as well as the vorticity generation process and plasma core decay. Finally, this work also investigated some engineering applications of LIB, such asflow control. The simulation results showed that the laser-generated thermal spot can be used effectively for controlling the aerodynamic forces and the shock-wave/boundary-layer interaction in supersonic/hypersonic flows over complex geometries. Future directions are also highlighted for the investigation of the effectiveness of laser energy deposition on the enhancement of the reaction kinetics in combustible mixtures, and on the use of lasers to control the Deflagration-to-Detonation-Transition (DDT) for applications such as Pulse Detonation Combustor (PDC).