H 2 O – Ar plasma property functions for modeling of hybrid water – gas plasma torch Petr Krenek, Milan Hrabovsky Institute of Plasma Physics AS CR, Prague, Czech Republic Abstract: Equilibrium composition, thermodynamic and transport properties of H 2 O – Ar thermal plasma were calculated. Calculated material coefficients were used for evaluation of functions rep- resenting effect of plasma gas properties on characteristics of arc in hybrid plasma torch with com- bined stabilization by argon flow and water vortex. Keywords: Thermal plasmas, thermophysical properties, water-argon plasma. 1. Introduction Plasma torches with Gerdien arc (arc stabilized by water vortex) are used for plasma spraying and for waste and biomass gasification [1, 2]. The main characteristics of the torches are very high plasma enthalpy and temperature and low plasma mass flow rate. These properties result in spe- cial performance characteristics of the torches in plasma spraying, namely high spraying rates and possibility of spraying of materials with high melting point, and for the case of waste gasification - high efficiency and good con- trol of composition of gases. In hybrid gas/water stabilized torch [3] the principles of gas-stabilized and Gerdien arcs are combined and plasma composed of mixture of argon and steam is produced. The paper presents calculations of properties of plasma created by mixture of argon with steam. The effect of ratio plasma gas composition on basic arc characteristics is estimated on the basis of simple integral model of arc column. 2. Composition, thermodynamic and transport prop- erties of plasma The composition of the water – argon plasma has been defined as a stoichiometric combination of molar amounts (H 2 O) (1-q) Ar q . To describe the behavior of the whole sys- tem the values q were chosen from 0 to 1 step 0.1. The supposed decomposition products are e, H, O, Ar, O + , O 2+ , O 3+ , O 4+ , O 5+ , O 6+ , O - , O 2 , O 2 + , O 2 - , O 3 , H + , H - , H 2 , H 2 + , H 3 + , OH, OH + , OH - , HO 2 , HO 2 - , H 2 O, H 2 O + , H 3 O + , H 2 O 2 , Ar + , Ar 2+ , Ar 3+ , Ar 4+ , Ar 5+ , Ar 6+ . The temperature range has been chosen from 400 to 50000K, the pressure 0.1 MPa. The calculations were performed using the modified Newton method for the solution of nonlinear equations system which is composed of equations of Saha and mass action law type expressing individual complex compo- nents by the help of basic ones (e, H, O, Ar). ). The sys- tem is completed by the usual particle and charge balance assuming quasineutrality and equilibrium. The values of the equilibrium constants available were taken from com- monly accepted tables [4], [5]. For high temperatures and and especially for multiply charged ions it was necessary to perform calculations of internal partition functions. The energy levels were taken from NIST Atomic Spectra Da- tabase http://physics.nist.gov . After the calculations of the composition were completed, we estimated the lowering of the ionization potential (which differs slightly for different argon molar amounts q). Modified values of internal partition functions and new equilibrium constants were computed and the calculation of the composition was repeated. Fig.1a and Fig.1b show the molar fractions of components against temperature for the molar amount of Ar q=0.5 in both the dissociation and ionization regions. Fig. 1a Molar fractions of 50%Ar+50%H 2 O products of decomposition in the ionization region. The thermodynamic properties of individual components c p o (T), S o (T) and H o (T)-H o (T) available were taken from [4],[5]. For higher temperatures and for multiply charged ions they were calculated from the internal partition func- tions and their derivatives respecting the effect of the ionization potential lowering. Then, the enthalpy (Fig.2a), entropy, equilibrium heat capacity (Fig.2b), isobaric thermal expansion, isothermal compressibility and equi- librium sound velocity (Fig.2c) were calculated using numerical differentiation by temperature and pressure. In