Richard J. Gessman

EXPERIMENTAL STUDY OF KINETIC MECHANISMS OF RECOMBINING ATMOSPHERIC PRESSURE AIR PLASMAS

Results from experimental and numerical investigations of the mechanism of ionizational nonequilibrium in recombining plasmas of air, nitrogen/argon, and air/argon are presented. Measurements of electron and excited state concentrations in plasmas produced with a 50 kW RF torch operating at atmospheric pressure are compared with numerical simulations performed with three reaction mechanisms widely used for air plasma kinetics. It is shown that, as electron recombination in molecular plasmas occurs primarily through inherently fast two-body dissociative recombination, ionizational nonequilibrium is ultimately caused by slow three body neutral recombination. The rates of the controlling three-body neutral recombination reactions are assessed through a comparison between the predictions of the kinetic models and the measured nonequilibrium concentrations of electrons, of the A, C, and D states of NO, of the B and C state ofN2, and of the A state of OH.

Collisional-radiative modeling of nonequilibrium effects in nitrogen plasmas

A new kinetic model and improvements to the vibrationally specific collisional-radiative (CR) model of Ref 1 are presented for nonequilibrium nitrogen plasmas. The rate coeficients of both models are calculated with the Weighted Rate Coeficient method based on the same elementary cross-section data and depend explicitly on the different kinetic (T,, T,) and population (Teb TV, Tr) temperatures of nonequilibrium plasmas. The CR model is first exercised to study plasmas in chemical nonequilibrium and thermal equilibrium. Good agreement with measurements of vibrational level populations in the recombining Nz /Ar plasma experiment of Gessman et al2 is obtained by adjusting two controlling rates. The CR and the kinetic models are then exercised to calculate steady-state species concentrations under conditions of thermal nonequilibrium. It is shown that a transition takes place between a regime where vibrational levels are equilibrated at Ts and a regime where they are equilibrated at TV Therefore, kinetic models that only consider reactions between chemical species can be used to accurately predict steady-state concentrations in nonthermal plasmas, provided that the vibrational regime is known a priori.

Measurement and Modeling of OH, NO, and CO Infrared Radiation at 3400 K

The infrared emission spectrum of an air plasma containing small quantities of CO 2 and H 2 O was measured and modeled in absolute intensity in the spectral range 2.4-5.6 μm. A 50-kW radio-frequency inductively coupled plasma torch was used to produce the air plasma in local thermodynamic equilibrium. The temperature profile measured by emission spectroscopy peaks at 3400 K. The absolute intensity emission spectrum was measured and compared with numerical simulations obtained with the line-by-line radiation code SPECAIR. Spectroscopic models incorporated into the SPECAIR code include the infrared rovibrational bands of OH, NO, and CO. Absorption of the plasma emission by room-air CO 2 and H 2 O in the optical path between the plasma and the detector is taken into account. Plasma emission from CO 2 (v 1 + v 3 ) and (v 3 ) bands is also modeled, using a correlated-k model

Ionization nonequilibrium induced by neutral chemistry in air plasmas

The present numerical study of air plasma kinetics yields the unexpected result that,as electron recombination occurs primarily via fast two-body dissociative recombination,ionizational nonequilibrium is caused under certain circumstances by slow neutral recombination.

Mechanisms of ionizational nonequilibrium in air and nitrogen plasmas

Experimental and numerical investigations are presented of the mechanism and degree of ionizational nonequilibrium in recombining plasmas of air and nitrogen produced by a 50 kW RF plasma torch operating at atmospheric pressure. It is shown that, as electron recombination in molecular plasmas occurs primarily via inherently fast two-body dissociative recombination, ionizational nonequilibrium is ultimately limited by slow neutral recombination. This is unlike the case for atomic plasmas, wherein ionizational nonequilibrium is caused by finite three-body electron recombination rates. Electron and heavy particle concentrations were measured and compared with the results of numerical simulations performed with three reaction mechanisms widely used for air and nitrogen plasma kinetics. The results support the proposed recombination model and provide a preliminary assessment of the kinetic rates of importance in recombining air and nitrogen plasmas

Numerical Simulation of Nonequilibrium Nitrogen and Air Plasma Experiments

Computational fluid dynamics is used to simulate recent nonequilibrium plasma experiments performed at Stanford University. In these experiments, hightemperature nitrogen and air plasmas are generated and then forced to cool under controlled conditions in a test-section. Measurements of temperature and electron concentration are made. The computational model includes a 10-species, 14-reaction finite-rate chemical kinetics model for the nitrogen plasma and an 11-species, 19-reaction finite-rate model for the air plasma. It uses a finite rate vibration-electronic energy relaxation model. The influence of the plasma temperature profile and swirl is studied. The simulations do not agree with the experiments; the centerline plasma cooling rate is under-predicted, resulting in the electron concentration remaining too large. Possible reasons for this difference are discussed.

Simulation of nonequilibrium plasma experiments

Summary form only given. We present numerical simulations of a series of experiments at Stanford University. These experiments made detailed measurements of the temperature and the nonequilibrium recombination of high-temperature nitrogen and air plasmas. The experiments, in conjunction with simple one-dimensional simulations, showed that the recombination of the plasma is limited by the speed of the three-body recombination of N/sub 2/ and NO in the nitrogen and air plasmas, respectively. The simulations allow us to determine these rates quantitatively. Also the effect of swirl on the radial distribution of energy and chemical composition is assessed.

Experimental investigation of ionizational nonequilibrium in atmospheric pressure air plasmas

Summary form only given. In air plasmas, the presence of molecular ions, electronegative species, dissociative recombination, charge exchange, and finite-rate heavy-particle chemistry produces a complex situation in which the balance between these various effects is not currently well understood, mainly because of a lack of understanding of the dominant mechanisms and the large differences between the reaction rates proposed in the literature. To understand the ionization/recombination mechanisms and to assess the rates of the controlling reactions, experiments have been conducted in our laboratory with atmospheric pressure plasmas of either pure air or 10% air in argon. In these experiments, electron recombination was measured as a function of residence time as the plasmas flowed through water-cooled test-sections mounted on the exit nozzle of a 50 kW RF plasma torch. In the case of the recombining pure air plasma, electron number densities were found to remain close to equilibrium. In contrast, large electron over populations appeared in the air/argon plasma as it cooled from 7900 K to 2500 K within approximately 1.3 ms: at 2500 K, the electron density was measured to be -2/spl times/10/sup -3/, which is /spl sim/500 times larger than the equilibrium density. Results from these experiments and from kinetic analyses indicate that electrons recombine primarily via the fast two-body dissociative recombination reaction NO/sup +/+e/spl hArr/N+0, and that the slow subsequent three-body recombination reaction N+O+M/spl hArr/NO+M ultimately controls the degree of ionizational nonequilibrium. Thus, ionizational nonequilibrium results from and is controlled by the recombination of neutrals. Inhibiting the three-body recombination of NO may then be one key to sustaining elevated electron number densities in low temperature air plasmas.