Effect of Oxygen Injection into Argon Induction Plasmas on Chemically Non-Equilibrium Conditions

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1 Proceedings of 17th Internationa Symposium on Pasma Chemistry, Toronto, Canada, August 7-12, 25 Effect of Oxygen Injection into Argon Induction Pasmas on Chemicay Non-Equiibrium Conditions Nobuhiko Atsuchi 1, Masaya Shigeta 2, and Takayuki Watanabe 2 1 Department of Nucear Engineering, Tokyo Institute of Technoogy, Tokyo, Japan 2 Department of Environmenta Chemistry and Engineering, Tokyo Institute of Technoogy, Yokohama, Japan Abstract Effective generation of chemica reactive species in therma pasmas has been required in the fied of materia processing and waste treatment. The effect of oxygen injection into argon induction pasmas was investigated by numerica anaysis without chemica equiibrium assumptions for the dissociation and ionization. Oxygen dissociation and heat fux to torch wa can be controed by oxygen injection ocation. Therefore, suitabe oxygen injection needs to be chosen according to the appication requirement. 1. Introduction Therma pasmas have been used for a number of appications of materia processing and waste treatment. In these appications, effective generation of chemica reactive species is required. The modeing of induction therma pasmas with severa injection methods of nitrogen or hydrogen was performed previousy [1]. The purpose of this work is to investigate the effect of oxygen injection into argon induction pasmas on chemicay non-equiibrium (CNE) conditions by numerica anaysis. Reaction kinetics rates of the dissociation and recombination of oxygen as we as the ionization were taken into account in this modeing. The transport properties were estimated using higher-order approximation of Chapman-Enskog method [2]. 2. Numerica formuation 2-1 Thermodynamic and transport properties Higher-order approximation of the Chapman-Enskog method is used according to the required accuracy for transport properties. Modeing of induction therma pasmas has been performed with the first-order approximation because higher-order of Sonine poynomia expansion requires many kinds of coision integras resuting in the compex formua. The first-order approximation may cause errors especiay for eectrica conductivity and therma conductivity of eectron transationa contribution over 1 K [3]. The coision integras were taken from Ref. [4-11] which provide intermoecuar potentia and fitting data. Detaied descriptions of the cacuation method of viscosity, therma conductivity and eectrica conductivity are given by Watanabe and Sugimoto [3]. In CNE mode, the transport and thermodynamic properties are reated to the temperature and the composition of pasmas. Therefore, the transport and thermodynamic properties shoud be estimated considering the diffusion of species in pasmas at each cacuation step unti the convergence. The thermodynamic properties, enthapy and specific heat at constant pressure, were obtained from the equiibrium properties. This woud be oversimpification, however, the exact estimation of the non-equiibrium thermodynamics properties is very compex to satisfy the sef-consistent conditions. The radiative intensity of argon and oxygen was taken from Ref. [12-13]. 2-2 Kinetic rate constants The equiibrium composition was estimated by FACT (Center for Research in Computationa Thermochemistry), considering six species of Ar, Ar +, O 2, O, O +, and e -. Tabe 1 summarizes the reactions of Ar-O 2 mixture considered in this study. Tweve reactions comprising 6 forward reactions in Tabe 1 and their backward reactions were taken into account. The reaction rates for forward direction are given by Arrhenius equation. f b k a T Tabe 1 Chemica reactions in argon-oxygen mixtures. Reaction O 2 + O 2 O + O + O O 2 + O O + O + O O 2 + Ar O + O + Ar O 2 + e - O + O + e O + e - O + + e - + e Ar + e - Ar + + e - + e - f k was cacuated by Eq. (2) = exp( c / T ) (1) a b c

2 Proceedings of 17th Internationa Symposium on Pasma Chemistry, Toronto, Canada, August 7-12, 25 O 2 Quartz tube Cooing water O 2 O 2 O 2 O 2 O 2 O 2 Type (A) Type (B) Type (C) Fig. 1 Schematic diagram of oxygen injection ocation types. Type (D) For Ar ionization reaction, the reaction rate can be cacuated by Eq. (2) [14] = T (1353 / T + 2) exp( 1353 / T ) k f (2) 2-4 Cacuation mode Four injection ocations of oxygen were considered in the modeing as iustrated in Fig. 1; (A) center injection; (B) radia injection at the center of the coi; (C) inner injection from the top; (D) sheath gas injection. The geometry of cacuation domain of induction pasma torch is shown in Fig. 2 and the operating conditions are summarized in Tabe 2. Gas fow rate of each oxygen injection type was shown in Tabe 3. The pasma torch consists of a water-cooed quartz tube, and is surrounded by a water-cooed induction coi. The coi consists of three turns and appies the induction frequency at 4 MHz to the pasma. The actua power eve was assumed to be 5 kw. The cacuation are based on the foowing assumptions to derive the governing equations: steady-state aminar fow; (b) axia symmetry; (c) opticay thin; (d) negigibe viscous dissipation in energy equation; (e) negigibe dispacement current in comparison with the conductive current; (f) negigibe fow-induced eectric fied; (g) same temperature of heavy partices and eectrons. Q 1 Q 2 Q 3 +Q 4 T ype(a) (C) (D) Q 1 Q 2 Q 3 +Q 4 Type(B) Tabe 2 Torch characteristic dimensions and operationa condition. r 3 R t r 1 r 2 r 4 Fig. 2 L 1 L 2 L 3 L 4 r 3 R t r 1 r 2 r 4 R c R c L 5 Q 5 d L 1 L 2 L 3 L 4 Geometry of cacuation domain of induction pasma torch. Torch Power 5 kw Work Frequency 4 MHz Reactor Pressure 11.3 kpa Coi Radius 32 mm Coi turn number 3 Wa thickness 1.5 mm L 1 19 mm L 2 45 mm L 3 65 mm L 4 19 mm L 5 4 mm r 1 6.5mm r 2 21 mm r 3 1 mm r mm R t 4.5 mm d.1 mm

3 Proceedings of 17th Internationa Symposium on Pasma Chemistry, Toronto, Canada, August 7-12, 25 The effect of turbuence has been reported by Chen and Bouos [15]. They reported that most of the fow fied in induction pasmas was aminar at the Reynods number 625 at the gas inet, whie the turbuence effect is arge at the Reynods number The fow fied of the present induction pasmas can be considered as aminar, because the inet Reynods number is 35 in this study. Tabe 3 Fow rate of each injection type. (L/min) Type (A) Type (B) Type (C) Type (D) Q1 O 2 :.1 Q (O 2 :.1) 3 Q3 + Q (O 2 : 1) Q5 - O 2 : Tota Governing equations and boundary conditions The fied of fow, temperature and concentration in induction therma pasmas were cacuated by soving the two-dimensiona continuity, momentum, energy, and species conservation equations couped with the Maxwe s equations. Reactions of the dissociation and recombination as we as the ionization were taken into account in this modeing. Continuity: ( ρu)= (3) Momentum: ρu u = p+ τ + J B (4) Energy: λ ρu h = h C + J E q (5) r p Species: Eectromagnetic: ρu Y i = ( ρd Y i )+ R ri (7) 2 E ξσ E t = (8) The boundary conditions aong the centerine were set to insure axia symmetry. At the wa of the pasma torch, no sip conditions are maintained for the veocity, and the concentrations have zero gradient. The temperature at the inside wa of the pasma torch was cacuated assuming that the outside wa was maintained at 3 K by water cooing. The injection tube was assumed to be at 5 K. The outfow boundary conditions at the torch were assumed that the gradient of the variabes are zero. The sheath gas has swir veocity component. Each gas stream has constant axia veocity with zero radia veocity having temperature at 3 K. 2-5 Cacuation procedure The governing conservation equations were soved using SIMPLER (Semi-Impicit Method for Pressure Linked Equation Revised) agorithm [16]. The governing equations and the eectric fied intensity equation with the associated boundary conditions were discretized into finite difference from using contro-voume technique. In oxygen injection types (A), (C), and (D), non-uniform grid points 3 by 3 were used for radia and axia directions, respectivey. In oxygen injection type (B), non-uniform grid points 3 by 54 were used for radia and axia directions, respectivey. Grids are made finer cose to the center and the coi region. Thermodynamic and transport properties were cacuated from the temperature and compositions at each position in the cacuation domain at each iteration step. 3. Resut and discussion The effect of oxygen injection of type (A: center injection) on pasma characteristics is presented in Fig. 3. At the oxygen injection region, the temperature decreases due to oxygen dissociation as shown in Fig. 3. Higher heat capacity incuding oxygen dissociation approximatey 4 K eads to a decrease in the pasma temperature. The fow fied in Fig. 3 (b) exhibits the characteristic recircuation caused by the radia Lorentz force above the coi region. The corresponding concentration profies of oxygen atoms and moecues are iustrated in Fig. 3 (c) and (d), respectivey. Injected oxygen moecues are dissociated quicky because oxygen is injected directy into the high-temperature region. Therefore, dissociated oxygen

4 Proceedings of 17th Internationa Symposium on Pasma Chemistry, Toronto, Canada, August 7-12, 25 Radia Position [ mm ] Radia Position [ mm ] Radia Position [ mm ] Radia Position [ mm ] Axia Position [ mm ] Axia Position [ mm ] (b) (c) (d) 18 (b) 18 (c) (d) 3 1 K (b) (c)..13 (d) K (b) (c)..15 (d). 1. Fig. 3 Isotherms, (b) streamines, (c) O atom distributions in an Ar-O 2 pasma; Type (A): center injection at.1 L/min. Fig. 4 Isotherms, (b) streamines, (c) O atom distributions in an Ar-O 2 pasma; Type (B): radia injection at 3 L/min. atoms can be found near the center of the torch. Numerica resuts of oxygen injection type (B: radia injection) is shown in Fig. 4. The cacuated temperature fied in Fig. 4 shows the severe decrease in temperature at the oxygen injection region aong the torch wa due to oxygen dissociation. The streamines shown in Fig. 4 (b) demonstrate the modified fow fied from other injection types in the coi region because of the radia injection toward the high-temperature region. The concentration profies of oxygen atoms and moecues are presented in Fig. 4 (c) and (d), respectivey. Oxygen moecues and dissociated oxygen atoms exist ocay near the torch wa owing to the negigibe diffusion of dissociated oxygen atoms toward the torch center. The temperature fied in Fig. 5 and the fow fied in Fig. 5 (b) in the case of oxygen injection type (C: inner injection) are comparabe to types (A) and (D). Thus, the effect of inner injection from the top is weak on the pasma characteristics. The corresponding concentration profies of oxygen atoms in Fig. 5 (c) and moecues in Fig. 5 (d) indicate that injected oxygen moecues are dissociated quicky above the coi region. Dissociated oxygen atoms exist broady except the ow-temperature region near the torch wa. The widespread mixing of oxygen is due to the recircuation caused by the Lorentz force in the coi region. The effect of oxygen injection type (D: sheath gas injection) is presented in Fig. 6. High-temperature region above 8 K is amost the same with an argon pasma, whie the temperature decreases sighty near the torch wa region as shown in Fig. 6. The concentration profies of oxygen atoms and moecues are shown in Fig. 6 (c) and (d), respectivey. Injected oxygen moecues exist near the torch wa through the downstream region. The dissociated oxygen atoms exist broady to the torch center owing to the recircuation. Oxygen injection from the top of the torch resuts in the broad distribution of dissociated oxygen atoms.

5 Proceedings of 17th Internationa Symposium on Pasma Chemistry, Toronto, Canada, August 7-12, 25 Radia Position [ mm ] Radia Position [ mm ] Radia Position [ mm ] Radia Position [ mm ] Axia Position [ mm ] Axia Position [ mm ] (b) 18 (c) (d) 18 (b) 18 (c) (d) 3 1 K (b) (c). 1. (d) K (b) (c)..3 (d)..3 Fig. 5 Isotherms, (b) streamines, (c) O atom distributions in an Ar-O 2 pasma; Type (C): inner injection at.1 L/min. Fig. 6 Isotherms, (b) streamines, (c) O atom distributions in an Ar-O 2 pasma; Type (D): sheath gas injection at 1 L/min. Degree of CNE, δ, was introduced to evauate chemica non-equiibrium of pasmas. d Degree of CNE for dissociation: δ O2 = [O]2 [O 2 ]K d (9) i Degree of CNE for ionization: δ O = [O + ][e ] (1) [O]K i In these definitions, vaue of δ = 1 represents compete equiibrium. In non-equiibrium, δ > 1 represents overpopuation of products and underpopuation of reactants for δ < 1. Fig. 7 presents the radia profies of the degree of CNE due to dissociation of injected oxygen with type (B) at the center of the coi. The degree of CNE deviates strongy from the equiibrium vaue at the oxygen injection region. This indicates that the chemica non-equiibrium occurs at the injection region due to the effect of diffusion and finite reaction rate. In the downstream region, the chemica non-equiibrium occurs near the torch wa. In a injection types, the strong chemica non-equiibrium was found at the oxygen injection region and near the torch wa where the concentration gradient is high. Fig. 8 shows the effect of oxygen injection on axia profies of the degree of dissociation. High degree of dissociation is obtained by oxygen injection types (A) and (C). The high degree of dissociation in types (A) and (C) is attributed to the direct oxygen injection into the high-temperature region of pasmas. Whie, the degree of dissociation is ow in the case of types (B) and (D), because oxygen moecues exist ocay near the torch wa. Oxygen injection type (B) which is direct radia injection from the torch wa provides the owest degree of dissociation. The owest degree of dissociation resuts from the insufficient penetration of

6 Proceedings of 17th Internationa Symposium on Pasma Chemistry, Toronto, Canada, August 7-12, 25 injected oxygen into the high-temperature region of pasmas, therefore higher injection veocity is required to obtain higher degree of dissociation of oxygen. Fig. 9 presents the effect of oxygen injection on axia profies of heat fux to the torch wa. The ow heat fux in the coi region in type (B) and (D) is due to the arge amount oxygen fow aong the torch wa, resuting in the severe decrease in the temperature. In contrast, the heat fux to the torch wa in type (A) and (C) is high. The contribution of the center or inner oxygen injection is sma for owering the heat fux to the torch wa by the oxygen injection. Non-equiibrium modeing with finite reaction rates for various oxygen injection ocations demonstrates the foowing resuts; chemica non-equiibrium exists at the region of high-concentration gradient such as oxygen injection region and near the torch wa. The degree of dissociation of oxygen and the heat fux to torch wa can be controed by oxygen injection ocation. Therefore, suitabe oxygen injection ocation needs to be chosen according to the appication requirements. The present modeing woud give the guidance for the rationa design of new materia processing with effective reactive gas injection into pasmas. 4. References [1] T. Watanabe, T. Honda, and A. Kanzawa Proc. ISPC-1, , Bochum (1991) [2] J. O. Hirschfeder, C. F. Curtiss, and R. B. Bird, Moecuar Theory of Gases and Liquids, John Wiey, New York, (1964) [3] T. Watanabe and N. Sugimoto, Thin Soid Fims, 457, 21 (24) Degree of CNE [ - ] Degree of Dissociation [ - ] Fig. 7 Radia profies of degree of CNE due to dissociation; (B) radia injection Center of the coi Rear end of the coi 1 mm from the torch inet Radia Position [mm].2 A: Center injection B: Radia injection C: Inner injection D: Sheath gas injection Axia Position [mm] [4] E. Mason, J. Chem. Phys., 22,169 (1954) Fig. 8 Effect of oxygen injection on axia [5] L. Monchick, Phys. Fuids, 2, 695 (1959) profies of degree of dissociation. [6] T. Kihara, et a., Phys. Fuids, 3, 715 (196) [7] F. Smith, et a., J. Chem. Phys., 41, 356 (1964) [8] R. Devoto, Phys. Fuids, 1, 354 (1967) [9] H. Mioy, Aust. J. Phys., 3, 61 (1977) [1] M. Capitei, et a., J. Thermophys., 14, 259 (2) [11] J. Aubreton, C. Bonnefoi, and J. M. Mexmain, Rev. Phys App., 21, 365 (1986) [12] R. Krey and J. Morris, Phys. Fuids, 13, 1483 (197) [13] R. Mier and R. Ayen, J. App. Phys., 4, 526 (1969) [14] M. I. Hoffert and H. Lien, Phys. Fuids, 1, 1769 (1967) [15] K. Chen and M. I. Bouos, J. Phys. D: App. Phys. 27, 946 (1994) [16] S. V. Patanker, Numerica Heat and Fuid Fow, McGraw-Hi, New York, (198) Heat Fux [ W / m 2 ] A: Center injection B: Radia injection C: inner injection D: Sheath gas injection Axia position [ mm ] Fig. 9 Effect of oxygen injection on axia profies of heat fux to torch wa.