**3. Chemical and hydrodynamics activation of gases using corona microdischarge**

### **3.1 Introduction**

During the past two decades several studies have shown that non-thermal plasmas reactor working in ambient air are very efficient sources of active species like charged particles, radicals and excited species. In fact, and as already explained in the previous sections, in the non-thermal plasma reactor, the majority of the injected electrical energy goes into the generation of energetic electrons, rather than into gas heating. The energy in the microplasma is thus directed preferentially to electron-impact dissociation, excitation and ionization of the background gas to generate active species that, in turn, induce the chemical activation of the medium. As a consequence, the non-thermal plasma reactors at atmospheric pressure are used in many applications such as flue gas pollution control (Fridman et al., 2005, Urashima et Chang, 2010), ozone production (Ono & Oda b, 2004), surface decontamination (Clement et al., 2001, Foest et al., 2005) and biomedical field (Laroussi, 2002, Villeger et al., 2008, Sarrette et al., 2010). For many applications, particularly in the removal of air pollutants, decontamination or medicine field, the non-thermal plasma approach would be most appropriate because of its energy selectivity and its capability for simultaneous treatment of pollutants, bacteria or cells for example.

In micro-discharges the active species are created by energetic electrons during the primary and the secondary streamer propagation that last some hundred of nanoseconds. Despite these very fast phenomena, the energy transferred to the gas can initiate shock waves starting from the stressed high voltage electrodes. Furthermore, a part of the electronic energy is stored in the vibrational energy that relaxes in thermal form after some tens of microseconds. Anyway, it is worth to notice, that all the initial energy (chemical, thermal, among others) is transferred inside a very thin discharge filament i.e. in a very small volume compared with the volume of the plasma reactor. Therefore, the efficiency of the processes is correlated to the radical production efficiency during the discharge phase, the number of micro-discharges that cross the inter-electrode gap, the repetition frequency of the discharge and how the radicals are diffused and transported from the micro-discharge towards the whole reactor volume.

In the following sections, the discharge and the post-discharge phase are simulated using the hydrodynamics models presented in section 2.5 in the case of a DC positive pin-to-plan corona reactor in dry air at atmospheric pressure.
