## 4 Experimental Results and Analysis  
### Experimental Results and Analysis of Optical Cavity Boundary Layer Control  
#### 4.1.1 Solid Plate Experiment Results  
Before performing the pressure recovery analysis with supersonic diffuser experiment on active gas pressurization, an optical cavity boundary layer control experiment of the SCRP was performed to find a more efficient boundary layer control method for the optical cavity. Before the experimental study of each group of optical cavity experiments, the SCRP was run under the condition that the optical cavity wall board was the original solid plate, and pressure changes in the optical cavity were observed. Figure 5 shows the change curve of the optical cavity pressure with time during the SCRP operation with a solid plate. The pressure in the optical cavity gradually increased with time during the energy discharge. The P\({}_{\mathrm{D}}\) in the figure represents the design pressure of the optical cavity, and the initial back pressure was approximately 1.25 P\({}_{\mathrm{D}}\). When the SCRP was in the early stage of operation, the pressure of the optical cavity was sharply dropped to approximately 0.6 P\({}_{\mathrm{D}}\), and a supersonic flow field in line with expectations was established. As the experiment progressed, the back pressure increased gradually, pushing the shock wave structure in the SCRP airflow channel upstream, while the heat released by the chemical reaction accumulated in the channel, increasing the temperature of the air stream. The reasons for this were that the air velocity in the optical cavity decreased, the pressure increased, the supersonic flow field in the optical cavity was destroyed, and the operating state of the SCRP was affected. If the pressure in the optical cavity is excessively high, it can cause the chemical reaction platform to malfunction. Therefore, during the operation of the chemical reaction platform, a continuous increase in pressure in the optical cavity was not conducive to the long-term operation of the platform. There were two reasons for the continuous increase in the pressure in the optical cavity. First, the volume of the vacuum tank is limited. As the experiment progressed, the pressure in the vacuum tank continued to increase, which increased the inverse pressure gradient in the airflow channel, thickened the boundary layer, and forced the flow field structure to move upstream, increasing the pressure in the optical cavity. Second, part of the energy generated by the non-residual chemical reaction was converted into the internal energy of the air stream, which increased the pressure in the optical cavity. Figure 6 shows the power curve of the extracted energy when the chemical reaction platform operates independently, where N\({}_{\mathrm{D}}\) is the designed reaction energy power of the platform. The figure shows that the energy power increases continuously within 0-1 s and reaches its maximum value. Within 1-3 s, the energy power decreased, and there was a small fluctuation; within 3-5 s, the energy power decline is more apparent, and the amplitude of the fluctuation becomes larger. The state of the airflow in the optical cavity was an important factor affecting the chemical reaction. During the energy extraction process, the increasing pressure in the optical cavity was the main reason for the decrease in energy power. In addition, owing to the thickening of the boundary layer in the optical cavity, the disturbance of the airflow in the optical cavity also increases, increasing the fluctuation of the power curve. The results when the SCRP works independently demonstrated that the ability of the SCRP to resist back pressure and airflow disturbances was weak when it was directly connected to the vacuum tank. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Value** & **Unit** \\ \hline Ma & 4.5 & \\ T\({}_{0}\) & 300 & K \\ \(\gamma\) & 1.4 & \\ R & 287 & J/(kg\(\cdot\)K) \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of the active gas flow.