Study of Gas Heating by a Microwave Plasma Torch

A complementary simulation and experimental study of an atmospheric pressure microwave torch operating in pure argon or argon/hydrogen mixtures is presented. The modelling part describes a numerical model coupling the gas dynamics and mixing to the electromagnetic field simulations. Since the numerical model is not fully self-consistent and requires the electron density as an input, quite extensive spatially resolved Stark broadening measurements were performed for various gas compositions and input powers. In addition, the experimental part includes Rayleigh scattering measurements, which are used for the validation of the model. The paper comments on the changes in the gas temperature and hydrogen dissociation with the gas composition and input power, showing in particular that the dependence on the gas composition is relatively strong and non-monotonic. In addition, the work provides interesting insight into the plasma sustainment mechanism by showing that the power absorption profile in the plasma has two distinct maxima: one at the nozzle tip and one further upstream.

Section: On the Interplay Of Gas Dynamics And The Electromagnetic Fie...mentioning

confidence: 99%

“…Few authors have complemented electromagnetic field simulations with a gas dynamics model for a single gas [12,13]. As far as self-consistent modelling of comparable discharges is concerned, there have been very advanced works focusing on Plasma Sources Science and Technology…”

Section: Introductionmentioning

confidence: 99%

On the interplay of gas dynamics and the electromagnetic field in an atmospheric Ar/H₂microwave plasma torch

Synek¹,

Obrusník²,

Hübner³

et al. 2015

“…Gas temperature calculations in low-temperature plasmas have always been a subject of intense research [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. These works present theoretical studies [1][2][3][4][5], experimental measurements [9][10][11][12][14][15][16] or both [2, 6-8, 13, 17-19], involving different plasma compositions, such as helium [7], argon [4,8,15], hydrogen [14], nitrogen [1,12,18], N 2 -O 2 [1,19], air [1-3, 6, 9, 11, 13, 16, 17], Ar-N 2 -NO [10], Ar-CF 4 [5] and air-fuel mixtures [17], as well as different plasma sources: nanosecond [11,13,17], microsec...…”

Section: Introductionmentioning

confidence: 99%

Study of gas heating mechanisms in millisecond pulsed discharges and afterglows in air at low pressures

Pintassilgo

Guerra

Guaitella

et al. 2014

116

A self-consistent model is developed to study the temporal variation of the gas temperature in millisecond single dc pulsed discharges and their afterglows in air-like mixtures (N 2 -20%O 2 ) at low pressures. The model is based on the solutions to the time-dependent gas thermal balance equation, under the assumption of a parabolic gas temperature profile across the discharge tube, coupled to the electron, vibrational and chemical kinetics. Modelling results provide a satisfactory explanation for recently published time-resolved experimental data for the gas temperature in a 5 ms pulsed air plasma with a current of 150 mA and the corresponding afterglow at a pressure of 133 Pa (1 Torr). It is shown that the main heating mechanisms during the first millisecond of the pulse come predominantly from O 2 dissociation by electron impact through the pre-dissociative excited state O 2 (B 3 − u ) and the quenching of nitrogen electronically excited states N 2 (A 3 + u , B 3 g , a 1 − u , a 1 g , w 1 u ) by O 2 , agreeing with other studies on fast gas heating in air plasmas. As the pulse duration increases, other gas heating sources become important, namely V-T N 2 -O energy exchanges, recombination of oxygen atoms at the wall, N 2 (A) quenching by O( 3 P) and reaction N( 4 S) + NO(X) → N 2 (X, v ∼3) + O, contributing altogether to an additional smooth increase in the gas temperature until the end of the pulse. In the first instants of the early afterglow, the gas temperature decreases very rapidly as a consequence of the minor role played by electronic collisions and due to a fast decay of N 2 electronic states. For afterglow times up to 10 ms, the gas temperature continues to decrease, following the time-dependent kinetics of [N 2 (X,v)], [N( 4 S)], [O( 3 P)] and [NO(X)]. Sensitivity of the model to different input parameters such as thermal accommodation coefficient and probabilities for atomic recombination at the wall are reported.

“…Accordingly, the study of gas temperature in different plasma discharges has been carried out in numerous experimental works [3][4][5][6][7][8][9][10][11][12], modelling studies [13][14][15][16][17][18][19][20][21][22][23][24] or both [25][26][27][28][29][30][31], as well as in the study of nozzle, shock wave and boundary layers of a body hit by a hypersonic flow [32][33][34] that behave similarly to discharges. Owing to the large number of works in this field, the list of references should be regarded as indicative.…”

Section: Introductionmentioning

confidence: 99%

On the different regimes of gas heating in air plasmas

Pintassilgo

Guerra

2015

Simulations of the gas temperature in air (N 2 -20%O 2 ) plasma discharges are presented for different values of the reduced electric field, E/N g , electron density n e , pressure and tube radius. This study is based on the solutions to the time-dependent gas thermal balance in a cylindrical geometry coupled to the electron, vibrational and chemical kinetics, for = E N / 50 g and 100 Td (1 Td = 10 −17 V cm 2 ), 10 9 ⩽ n e ⩽ 10 11 cm −3 , pressure in the range 1-20 Torr, and also considering different tube radius, 0.5, 1 and 1.5 cm. The competing role of different gas heating mechanisms is discussed in detail within the time range 0.01-100 ms. For times below 1 ms, gas heating occurs from O 2 dissociation by electron impact through pre-dissociative excited states, e + O 2 → e + * O 2 → e + 2O( 3 P) and … → e + O( 3 P) + O( 1 D), as well as through the quenching of N 2 electronically excited states by O 2 . For longer times, simulation results show that gas heating comes from processes N( 4 S) + NO( X) → N 2 ( X, v ~ 3) + O, N 2 (A) + O → NO( X ) + N( 2 D), V-T N 2 -O collisions and the recombination of oxygen atoms at the wall. Depending on the given E/N g and n e values, each one of these processes can be an important gas-heating channel. The contribution of V-T N 2 -O exchanges to gas heating is important in the analysis of the gas temperature for different pressures and values of the tube radius. A global picture of these effects is given by the study of the fraction of the discharge power spent on gas heating, which is always ~15%. The values for the fractional power transferred to gas heating from vibrational and electronic excitation are also presented and discussed.