Effect of particles on nonequilibrium relaxation phenomena in high temperature gas-nozzle flows

Abstract

This thesis presents an exhaustive study of the effects of solid particles on the relaxation phenomena of various nonequilibrium processes, such as vibrational, dissociation and ionization, of the gas phase at high temperature expanding through a nozzle. Starting from the basic one-dimensional unsteady governing equations for the flow, the steady-state results are computed using time-marching numerical technique. A general computer code is developed based on MacCormack's predictor-corrector finite difference numerical scheme. This code, after proper validation, is used to investigate all the problems reported in this thesis. The thesis consists of 8 Chapters. A general review of the gas-particle flow studies with particular emphasis on the nozzle flows is given in Chapter-1. Previous work in this field is critically reviewed and major conclusions are highlighted. The set of basic equations governing the gas-particle nozzle flow in the unsteady form along with the review of existing methods of solving these equations is presented in Chapter-2. Merits and demerits of different solution methods are reviewed and the advantage of using the time-marching technique which has been used in the present study is highlighted. The problem of gas-particle nozzle flows including vibrational nonequilibrium effects is studied in Chapter-3. The details of computer code are also given in this chapter. Numerical computations are carried out for various particle sizes and particle mass concentrations in hyperbolic and contour nozzles. It is observed that vibrational energy relaxation phenomena along the nozzle length is unaffected by the different particle sizes at lower particle concentrations (Particle loading ratio n < 0.2). As the particle concentration is increased, the vibrational energy distribution in the nozzle very rapidly tends towards the frozen limit. Thus higher particle concentration adversely affects the thermal nonequilibrium processes of high-temperature gases. In general, similar observation is obtained for the gas-particle nozzle flows with dissociation nonequilibrium which is discussed in Chapter-4. Dissociation recombination process in the nozzle is unaffected by the particle sizes at lower particle concentrations (n < 0.2). However, the atom mass fraction variation in the nozzle tends rapidly towards the frozen limit as the particle concentration is increased. In Chapter-5, the effect of particles on argon plasma expanding through a hyperbolic nozzle is studied and it is found that the ionization fraction is similarly affected by the particle sizes and concentrations as observed in Chapter-4 for the case of dissociation nonequilibrium. Furthermore, as the particle concentration is increased, the electron temperature tends towards the translational temperature. Effect of solid particles on the small signal optical gain of a CO₂-N₂ gasdynamic laser system is evaluated in Chapter-6 and it is found that the small signal gain decreases with the increase of particle concentrations and even becomes negative at higher concentrations. The effect of particles on gain is significant at lower particle sizes and higher concentrations, whereas it is insignificant at lower concentrations (n < 0.2) and higher particle diameters (> 2 μm). The effects of finite volume fraction and virtual mass of the particles are investigated in Chapter-7 for the case of vibrational nonequilibrium. It is found that for all practical purposes, their effects are found to be negligible; and hence the assumptions made in earlier Chapters are justified. The general conclusions of the work presented in this thesis are given in Chapter-8.