Numerical simulation of radiation losses in a decaying laser sparks using LBL method. (MS)

Abstract

An investigation has been done to numerically simulate the radiation losses which occur due to laser energy deposition. Although the fluid dynamic effects due to laser energy deposition has been studied by a number of researchers independently but the effect of radiation has either been neglected or has not been highlighted effectively. When a laser beam is focused on a small focal volume of gas, the energy is absorbed by the gas causing breakdown of gas resulting in collision of energetic electrons which results in very high pressure and temperature gradients at the end of plasma formation. These gradients lead to fluid dynamic phenomenon such as formation of detonation, and the propagation of the pressure wave into the surrounding gas. Chemical changes also occur which result in dissociation, ionization and recombination of different species as N2, O2, NO, N, O, etc. At such high temperature, the diatomic air species may become highly dissociated and emission from the resulting two mono atomic species N and O cannot be neglected and is the major source of radiation loss. An Open Source CFD software Open Foam has been used to study the above effects. To define the chemistry model five species namely O2, O, N2, N, NO and eleven elementary reaction steps are considered for dissociation and recombination of air. For radiation modeling, detailed line by line spectral model has been developed. Accurate simulation of radiation heat transfer is carried out by line-by-line radiation modelling which requires very accurate absorption coefficient data at hundreds of thousands of wavelengths. The RTE must be solved at each wavelength, and the total intensity is calculated by applying a suitable integration scheme in wavelength space. Spectroscopic data required for the calculation of emission and absorption coefficients was taken from NIST database. This model includes features as thermodynamic data from Gordan and McBride, Boltzmann distribution, line broadening effects, absorption and emission coefficients calculations. It has been integrated with Open FOAM to solve RTE using P1 solver. Radiation losses obtained are almost 500 times higher as compared to the past reported data but still these losses are very less as compared to the deposited energy. Since the most accurate method LBL also showed that radiation losses are negligible so our study validates the assumption of neglecting the radiation losses which was assumed in all previous studies without any proper validation.