Plasma kinetics in a nanosecond pulsed filamentary discharge sustained in Ar–H₂O and H₂O

Abstract

The plasma kinetics of Ar-H 2 O and H 2 O at atmospheric pressure are of interest for applications in biotechnology where rare-gas plasma jets treat liquid surfaces and in water treatment where discharges are generated in bubbles or directly in liquid water. Due to evaporation resulting from heat transfer to the liquid, for many conditions the mole fraction of water in the plasma can be large-approaching nearly pure water. In this paper, results are discussed from a combined experimental and computational investigation of the chemical kinetics in a high electron density plasma filament sustained in Ar-H 2 O at atmospheric pressure. The chemical kinetics were simulated using a 0D global model, validated by measurements of the absolute OH and H densities by laser induced fluorescence (LIF) and two-photon absorption LIF. The primary sources of H and OH during the discharge pulse are dissociative excitation transfer from metastable Ar atoms and Ar dimer excimers at low water concentration and electron impact dissociation of H 2 O at high water concentration. In spite of their similar sources, the density of OH was measured to be two orders of magnitude smaller than that of H at power densities on the order of 10 5 Jm −3 . This disparity is due to electron impact dissociation of OH during the discharge pulse and rapid reactions of OH in the presence of high H and O densities in the afterglow. It is often assumed that OH is the dominant non-selective reactive species in water-containing plasmas. These results reinforce the importance of atomic species such as H and O in water containing high energy density plasmas. A numerical parametric study revealed that the lowest energy cost for H 2 O 2 production is achieved at low energy densities in pure water. The high concentration of atomic radicals, which rapidly recombine, results in an overall lower energy efficiency of reactive species production. In particular, the selectivity of H 2 O 2 production decreases with increasing power density which instead favors H 2 and O 2 production.

Two-temperature balance equations implementation, numerical validation and application to H2O–He microwave induced plasmas

“…where U n+1 m is an updated heavy particles energy in each iteration of the global model, U T m t (T g ) is the total translation energy-first term on the rhs of ( 14)-and U int m t (T g ) is the total internal energy-contribution of electronic and ro-vibrational, second term on the rhs of (14). A temperature range is defined for the Brent algorithm in such a way that the above equation gives the opposite sign for the lowest temperature and highest temperature in the defined range.…”

“…Therefore, the heavy particles energy balance is solved self-consistently along with the electron energy balance and particle balances for each involved species in the model. Equation (12) in accompanied by equation (14) for estimation of the heavy particles energy is added to the ODE system of this global model. As is explained in sections 5.4 and 5.5, the first and fourth terms on the rhs of equation ( 12) are assumed to be transport related frequencies for the heavy particles energy balance in this model.…”

“…A partition sum calculation for each species is done in order to have a proper heavy particles energy representation, see equation (14). For atomic species, only translational energy is considered.…”

“…Two examples for this condition are H 2 (b 3 Σ + u ) and HeH + . Note that for all explained cases above the translational energy are taken into account besides internal energy (14).…”

“…The main chemical pathways were recognized for some important species like O, OH, OH(A), and H 2 O 2 . Recently, Luo et al [14] investigated the plasma kinetics in a nanosecond pulsed filamentary discharge in atmospheric Ar-H 2 O and pure H 2 O plasmas. They studied the main mechanisms in the production of H 2 O 2 , which is one of the most important species in biomedical applications of the plasmas.…”