Session *BB5.1

3:15 PM *BB5.1
PHYSICAL PRINCIPLES AND PROPERTIES OF WATER-STABILIZED PLASMA GENERATORS. Milan Hrabovsky, Inst. of Plasma Physics, Prague, CZECH REPUBLIC.

Plasma torches with liquid stabilized arcs provide the alternative to commonly used sources of thermal plasmas based on gas stabilized arcs or RF discharges. In the torches with liquid stabilization the arc column is confined inside the vortex which is created in an arc chamber with tangential liquid injection. Evaporation from the inner wall of the vortex surrounding the arc column and heating and ionization of the vapor are basic mechanisms that produce arc plasma. The relations between the power absorbed in the vapor sheath surrounding the arc column, the power absorbed in the boiling layer on the inner surface of the vortex and the power transferred into the liquid are decisive for all arc and plasma parameters. Arc voltage and thus arc power are high for water stabilized torch. Typically the arc voltage is 300 V at arc current 300-600 A. The mean mass enthalpy of generated plasma is 160-270 MJ/kg. Centerline plasma temperature measured 2 mm downstream of the nozzle exit was 28 000 K for arc current 600A, plasma flow velocity at the same point was 7 km/s. Mean bulk plasma temperature was 16 200 K and mean bulk plasma velocity was more than 4 km/s. High flow velocity and extremely low ratio of the jet density to ambient density lead to enhanced mixing between the jet fluid and ambient gas and to high level of turbulence. Consequently, the spread angle of mixing zone is large, and fully turbulent flow was identified at short distances from the torch exit. Experimentally determined characteristic frequency of production of vortex structures in the shear layer on the jet boundary was 60-100 kHz. High plasma temperature and velocity, high level of turbulence, very low characteristic time constants of mixing processes and large volume of mixing zone are principal causes of special performance characteristics in plasma spraying, especially extremely high deposition rates.