**2. Mechanism of surface heating due to DBD plasma actuation**

As that shown schematically in **Figure 1**, when a high voltage (i.e., either in alternating current (AC) or nanosecond pulses) is applied to the electrodes of an DBD plasma actuator, the air over the encapsulated electrode would be ionized to generate a streak of plasma charges. It has been reported that DBD plasma actuation would have significant thermal effects [24, 30–32, 40]. While substantial thermal energy is generated along with the formation of ionic airflow for AC-DBD plasma actuation [40], ns-DBD plasma discharge was found to induce an ultra-fast gas heating (FGH), which can dramatically affect the kinetics of chemical reactions, leading to the development of shockwaves in the near-surface gas layer [41–43].

It is well known that, when the high-voltages are applied to the electrodes, a high-intensity electric field would be generated between the exposed electrode and the grounded electrode separated by the dielectric layer. Driven by the electric field, the free electrons and ions in the air are responsible for energy transmission from the external power source to gas heating [32, 42]. As suggested by Popov [44] and Aleksandrov et al. [20], the dynamic gas heating during the plasma discharge is mainly caused by the complex collisions, reactions, and interactions between electrons, ions, and the excited molecules in the electrical field, as summarized in **Figure 2**.

The free electrons get energy from the electric field through acceleration, and then collide with neutrals and ions in the air. If an elastic collision occurs, there is an immediate, but only a rather small portion of total energy release, while in inelastic collisions, ionized particles and excited molecules can be produced, which are the main sources of energy heating the gas. Collision between ions and neutrals and

**Figure 1.** *Schematic of a typical DBD plasma actuator.*

*An Experimental Investigation on the Thermodynamic Characteristics of DBD Plasma… DOI: http://dx.doi.org/10.5772/intechopen.100100*

### **Figure 2.**

*Heating mechanisms of DBD plasma actuation.*

electrons is another source that should be taken into consideration [45]. **Figure 3** summarizes the primary reactions contributing to the gas heating in DBD plasma actuation. When the free electrons impact the molecules in the air (e.g., *N*2 and O2), these molecules would be excited from the ground states to the electronic states. Then, dissociation of the excited molecules would occur, which can generate a significant amount of thermal energy [44]. When electrons impact with the molecular ions in the electrical field, recombination would also occur [20], in which process, the energy would be released between the electronic and translational degrees of freedom of the produced atoms [44]. During the dissociation processes of the electronically excited molecules, while the energy released in the collisions is expended on the rotational excitation of molecules and gas heating, the rotational energy is relaxed into the translational degrees of freedom during the multiple collisions,


### **Figure 3.**

*Primary reactions for gas heating in DBD plasma actuation.*

### *Plasma Science and Technology*

which is termed as quenching of the excited molecules. The kinetic energy produced in the quenching processes is rapidly converted into gas heating [44]. It should be noted that, a large number of excited oxygen atoms are produced in the dissociation-recombination-quenching reactions. These excited atoms can also be quenched by the molecules in the air, i.e., N2 and O2. It was suggested about 70% of the excitation energy of the excited atoms O(1 *D*) is expended on gas heating [44]. Along with the above dissociation and quenching reactions, the excited oxygen atoms O(3 *P*) would also lead to the reaction of VT relaxation, which is considered to be a significant reaction contributing to the gas heating [44].
