# Influence of Oxygen Impurity on Nitrogen Atmospheric-Pressure Plasma Jet
## 3 Results and Discussion
### Temperature Profile
#### 3.1.2 Comparison with Experimental Data  
To validate the simulated results, an experiment was performed with a nitrogen DC pulse APPJ with a quartz tube design [6; 7] and the same dimensions and inlet gas setting (N\({}_{2}\) with 10\({}^{-3}\) O\({}_{2}\)) as those in the computing model, and the temperature was measured at the plasma equipment. In Figure 4, the inlet gas was supplied from the top of the nozzle exit and excited by electron and heat inside the nozzle. Then, the plasma flowed from the nozzle to the working plate inside the quartz tube, passed through the gap under the quartz tube, and finally mixed with outside air.  
The input voltage was 269 V, and the cycle time was 40 \(\upmu\)s with 7 \(\upmu\)s on. A K-type thermometer and NI9211 DAQ was used along with Labview data capture software for measuring the temperature. The typical and maximum errors for the K-type thermometer when used with the NI-9211, which account for gain errors, offset errors, differential and integral nonlinearity, quantization errors, noise errors, and isothermal errors, were in the range of 1-1.6 \({}^{\circ}\)C and 2-3 \({}^{\circ}\)C in the measured temperature range of 0-800 \({}^{\circ}\)C. The thermometers were placed on the surface at equal distances, as shown in Figure 4. One set of probes was directly exposed to the air to measure the plasma temperature, and silver glue was applied to another set of probes to obtain the treated surface temperature.  
Then, the data on the computing model were divided into two parts: the temperature at FT in Figure 1 on the solid surface and the temperature 0.025 mm above the surface, which mimics the plasma thermal characteristic in air.  
The simulated and experimental temperature measurement results were plotted using MATLAB R2022b, as shown in Figure 5. In the experiment, the temperature below the exit (r = 0 mm) was 643 \({}^{\circ}\)C in air and 500 \({}^{\circ}\)C on the surface, and these temperatures respectively decreased to 538 \({}^{\circ}\)C and 417 \({}^{\circ}\)C at a radius of 9 mm. However, the simulated temperature profiles were slightly lower than the experimental result below the exit and were slightly overestimated on the surface at a radius of 9 mm. This might be because of the error arising from model domain simplification. For instance, we ignored the heat capacities of some materials and assumed the downstream surface material (FGW in Figure 1) to be limited to an only 300 mm radius plate even though the radius is approximately 100 cm in reality.  
Figure 4: Experimental measurement setup for temperature measurement.  
Both the simulated and experimental temperature profiles agreed with a previous experimental study when different impurities were added to the inlet gas [21; 22]. The temperature of \(10^{-2}\) O\({}_{2}\) is slightly lower than that of \(10^{-4}\) O\({}_{2}\); this could be attributed to the endothermic reaction with abundant atomic O, O\({}_{2}\), and O\({}_{3}\) in the quartz tube. Thus, the temperature difference remained within 9 \({}^{\circ}\)C, and it could be neglected under the plasma working conditions.  
Furthermore, the temperature plot of the simulation data above the surface exhibited a jump at approximately 12.77 mm, where a vortex separation point is also seen in Figure 3. This point could be considered a demarcation between the plasma jet and the incorporation of air from the gap under the quartz tube.