## 3 Results and Discussions
However, the further increase in the frequency up to 270 kHz led to a slight reduction in the \(T_{vib}\) down to 6310 K. The increase in the direct air flow also affects the vibrational temperatures of N\({}_{2}\)* species. The temperature was -5140 K when the air flow rate was 6.7 L/min. The increase in the air flow rate from 8.9 L/min to 11.2 L/min (at 130 V) resulted in a slight increase in the vibrational temperatures from 5220 K to 5430 K, respectively. The enhancement of the air flow rate up to 13.4 L/min resulted in a drastic increase in the N\({}_{2}\)* vibrational temperature up to 6310 K. However, the temperature only slightly increased up to 6330 K when the air flow rate of 15.6 L/min was  
Figure 3.1 Variation in intensities of the N\({}_{2}\)* (**a**) and NO\({}_{\gamma}\), O, and N\({}_{2}\)+ (**b**) species on the voltage, when the frequency was 270 kHz and the direct air flow rates were 11.2 L/min and 13.4 L/min. used. It should be noted that the vibrational temperature of the excited N\({}_{2}\)* particles does not demonstrate the true plasma temperature [30]. The variation in the vibrational temperatures of N\({}_{2}\)* with the voltage at air flow rates of 11.2 L/min and 13.4 L/min is presented in Figure 3.2 The calculation of the vibrational temperature demonstrated that the increase in the air flow rate from 11.2 L/min to 13.4 L/min enhanced the temperatures of the excited N\({}_{2}\)* particles. The temperature rose from 5350 K to 7030 K at an air flow rate of 13.4 L/min as the output voltage increased from 90 V to 250 V. Similarly, when the air flow was reduced to 11.2 L/min, the temperature exhibited a linear increase from 5190 K to 5850 K with the rise in the voltage. The increase in the intensity of the emission lines with the increase in the air flow rate is related to the propagation of the arc length to higher distances. As the optical emission spectra of air plasma were measured at a constant distance (at 35 mm), with the increase in the air flow rate, the discharge arc is passing through an optical fiber. The excitation and ionization processes are dominant in the arc; meanwhile, outside the arc region, the probability of electron collision with the oxygen or nitrogen molecules is reduced. It was demonstrated that the intensity of nitrogen emission lines strongly depends on the spatial locations of the optical fiber when the gliding arc discharge is used [32]. Also, it will result in a huge difference in the rotational and vibrational temperatures of the nitrogen plasma in the axial direction. Depending on the axial distance, the rotational temperature varied from 1250 K up to 1900 K. The vibrational temperature of N\({}_{2}\)* was highest when the arc length reached 70-80% of its propagation distance [32]. [PERSON] et al. [30] observed that the intensities, as well as the vibrational temperatures of the N\({}_{2}\)* species, strongly depend on the measurement location in the air plasma. The vibrational temperature of N\({}_{2}\)* particles ranged from 2400 K to 7000 K, depending on the measured x and y directions. [PERSON] et al. [33] observed that the increase in the O\({}_{2}\) gas concentration in the nitrogen-oxygen plasma increased the intensities of the atomic oxygen and NO species in the plasma. Studies by these authors [30;32] showed that the temperatures of various species and intensities of emission lines depend quite strongly on the location of the measurements in plasma. It is well known that an increase in the power (supply voltage) and reduction in the gas flow rates should increase the intensity of the emission lines of dominant species in air or air-nitrogen plasmas [23;24]. The increase in the air flow rate shortens the arc lifetime and also the propagation length [34;35]. At higher air flows, the arc is slightly shortened, and it  
Figure 3.3 The dependence of temperature on the voltage, when the direct air flow rates were 11.2 L/min and 13.4 L/min. is likely that the maximum temperatures are reached near the measurement location.