Thermal and nonthermal regimes of gliding arc discharge in air flow

Gliding discharges comprising both equilibrium and nonequilibrium plasma conditions offer high energy efficiency and selectivity for chemical processes. Prevailing parameters satisfying nonequilibrium plasma conditions at relatively high power levels should be well understood and characterized. In the present work, gliding discharges formed between diverging electrodes in air flow were studied experimentally over a wide range of gas velocities and power levels. Depending on the system parameters the following discharge regimes were observed: low power nonequilibrium discharge; thermal quasiequilibrium discharge; and gliding discharge with equilibrium to nonequilibrium transition. The effect of system parameters on discharge characteristics is analyzed. The equilibrium to nonequilibrium transition was experimentally observed as a change of voltage increase rate with discharge length growth. The local electric field, defined as dV/dl, increased up to three times, indicating the change of plasma conditions. However, previously reported phenomenon of length explosion was not supported by our experimental data. The co-existence of equilibrium and nonequilibrium phases is also discussed in the frame of phenomenological theory, assuming formation of a growing nonequilibrium fragment inside the gliding discharge channel. It was found that high flow velocities provide intensive cooling, an increase of electric field, and a decrease of gas temperature, promoting equilibrium to nonequilibrium transition at high power levels.