## 3 Results and Discussion
Figure 4 shows a comparison of the maximum and the average temperatures for a SRU thermal reactor with different zone 1 fuel mass fractions. The air is kept at the design condition. Eight zone 1 fuel mass fractions are used, including 0.0625, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75 and 0.875 (for an oxygen-normal case) or 0.895 (for an oxygen-rich case). The zone 1 fuel mass fractions ranging from 0.0625 to 0.895 are calculated to determine the stoichiometric fuel mass fraction which produces the most complete combustion and the highest temperature. Figure 4a-c show that the maximum temperature and the average temperature reach maximum values at a zone 1 fuel mass fraction of about 0.375, which implies that the stoichiometric fuel mass fraction is around 0.375. The temperature decreases when the zone 1 fuel mass fraction deviates from the stoichiometric fuel mass fraction. In Figure 4c, the average temperature of the thermal reactor represents the average temperature for both zone 1 and zone 2. In a practical SRU thermal reactor, high temperature can damage the refractory. For example, the zone 1 corner has a suddenly expanded geometry and a recirculation zone forms in this region. The temperature in this region can exceed the maximum service temperature for the refractory and cause collapse or deformation of the zone 1 corner. The suggested maximum service temperature for the refractory varies for different manufacturers. A typical value is 1700 \({}^{\circ}\)C (\(\approx\)1973 K). Figure 4a shows that the maximum temperatures for all oxygen-rich cases exceed the suggested maximum service temperature. However, for oxygen-normal cases, when the zone 1 fuel mass fraction is less than 0.3 or greater than 0.6, the maximum temperature is lower than the suggested maximum service temperature. The current design condition for an oxygen-normal operation (zone 1 fuel mass fraction of 0.875), listed in Table 1, allows an acceptable temperature field. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Case** & **Practical Data** & **Simulation Results** & **Deviations** \\ \hline Oxygen-normal & & & \\ operation & 0.078 & 0.079 & 1.3\% \\ Oxygen-rich operation & 0.084 & 0.091 & 8.3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of the measured and simulation results. Figure 4: \(Cont.\)  
Figure 5 shows a comparison of the cross-sectional average temperature for a SRU thermal reactor with different zone 1 fuel mass fractions. It is seen that the average temperature in zone 2 is higher than the average temperature in zone 1 for zone 1 fuel mass fractions of 0.0625 and 0.125. A zone 1 fuel mass fraction lower than the stoichiometric fuel mass fraction (e.g., 0.0625 or 0.125) produces a lower zone 1 temperature and this helps protect zone 1; however, the zone 2 temperature is increased because a lower zone 1 fuel mass fraction represents a lower zone 1 fuel supply and a higher zone 2 fuel supply. It is worth noting that a high zone 2 average temperature can damage the heat exchanger tubes. For larger zone 1 fuel mass fractions, the average temperature in zone 1 is higher. An oxygen-rich supply also enhances the chemical reaction and produces a higher temperature. The temperature decreases abruptly across the choke ring because thermal energy is converted into kinetic energy when the flow is accelerated. It is also seen that the temperature is more uniform for the design zone 1 fuel mass fractions (0.875 for the oxygen-normal case and 0.895 for the oxygen-rich case). This can also be observed from Figure 6 which shows the temperature profile on the symmetric plane for a SRU thermal reactor with different zone 1 fuel mass fractions. Figure 4: A comparison of the maximum and average temperatures for a SRU thermal reactor with different zone 1 fuel mass fractions: (**a**) Maximum temperature; (**b**) The average temperature in zone 1; (**c**) The average temperature in the thermal reactor.