Power Transfer to Gas Heating in Pure N₂ and in N₂–O₂ Plasmas

Abstract

A time-dependent kinetic model is developed to study the energy transfer to gas heating in N 2 and N 2 −O 2 plasmas. The model is based on the coupled solutions to the gas thermal balance equation and a system of rate balance equations for the most important neutral (including vibrational kinetics) and ionic heavy species produced in these plasmas. A complete set of gas heating mechanisms is taken into account, together with gas cooling by heat conduction to the wall. Calculations are performed for a cylindrical geometry and fractional concentrations of oxygen molecules [O 2 ]/N g (N g being the gas number density) up to 80%. Modeling simulations are carried out for fixed values of the reduced electric field E/N g and electron density n e with the purpose of focusing on the role of adding molecular oxygen O 2 into the N 2 −O 2 mixture. It is shown that the fractional discharge power and energy transferred to the translational mode (gas heating) increase with [O 2 ]/N g . The present work also shows that when the variation of E/N g and n e with the mixture composition is considered, the gas temperature has a maximum for [O 2 ]/N g ∼ 20%, which is qualitatively in line with experimental observations. Gas heating mechanisms are identified and discussed in detail.

Relaxation of heavy species and gas temperature in the afterglow of a N2 microwave discharge

“…This work consolidates recent modelling calculations we have undertaken in the study of gas heating mechanisms in pure nitrogen plasmas [1,2] by considering now a direct comparison with experimental results obtained in the afterglow of a flowing N 2 microwave discharge [3]. As a matter of fact, our previous simulations in pure N 2 plasmas described the temporal evolution of the gas temperature and corresponding heating mechanisms by solving a system of rate-balance equations for the heavy species coupled to the gas thermal balance equation for a relatively wide range of values for the reduced electric field and electron density observed on a typical glow discharge [2], but considering only a qualitative comparison with experiment.…”

“…As in our previous modelling works on gas heating in pure N 2 plasmas [1,2] we have carried out sensitivity tests with the purpose to analyse the effect of considering different percentages of these energy values channelled to gas heating on the value of T g . Note that this kind of analysis has been also undertaken recently [16,17] in the study of nanosecond pulsed discharges in nitrogen and air in order to explain the magnitude of experimental values for the gas temperature.…”

“…This equation determines the radially averaged gas temperature T g assuming a parabolic radial profile for the gas temperature and considering further that heat conduction is the main cooling mechanism. Furthermore, this equation must account for all available energy to gas heating (translational mode) from volume and wall processes, described with detail in [1,2,[9][10][11][12].…”

“…In what concerns the afterglow region, once the stationary concentrations of the heavy species produced in the discharge and the gas temperature are calculated, we consider these values as initial conditions in a set of coupled time-dependent kinetic rate-balance equations, including the vibrational distribution function (VDF) of N 2 (X, v) molecules and all the other heavy-particles, solved simultaneously with the time-dependent gas thermal balance equation [1,2].…”

“…To shorten this paper to an appropriate length, the reader should refer to references [4][5][6][7]9,11] for more details on collisional data, modelling aspects concerning the electron Boltzmann equation, microwave discharge and post-discharge conditions, to references [1,2,10,11] for additional information on gas heating mechanisms, gas thermal balance equation and energy data, as well as to [1,[13][14][15] for the values of the molar heat capacity and thermal conductivity under non-equilibrium conditions.…”