## 3 Calculation Results and Discussion
### Hybrid System Off-Design Performance Evaluation under Variable Operation Parameters
#### 3.3.1 Effect of O/C Ratio Variation on the Hybrid System Performance
This limitation necessitates a balance to prevent graphite fouling while maximizing syngas production. Consequently, the shaded region denoted as \"S\" represents the preferred range for CPOx reactions, striking a balance between syngas production, carbon formation, and reactor temperature considerations for optimal system performance. The region \"S\" correlates to a bound acceptable for optimal CPOx performance ranging between 0.85 and 0.95. A crucial aspect to consider for this analysis is the effect this outlet reformate fuel flow has on the fuel utilization (FU) of the SOFC. Only H\({}_{2}\) is considered for the assessment of the SOFC FU. Figure 6 shows the FU increase with O/C ratio, showing that O/C ratio affects the SOFC FU. Beyond O/C = 1.1, the FU is 100%. This implies complete fuel consumption in the stack at O/C ratio > 1.1, necessitating an adjustment of the O/C ratio setting to maintain the fuel utilization within the ideal 75% range. A safe range of the SOFC utilization is \(\pm\)10% of the ideal fuel utilization, as shown in shaded region \"P\". Anything higher or lower than this safety margin will yield performance issues in the SOFC, either excessive fuel or fuel deprivation, which affects the longevity of the stack through overheating, inefficient electrochemical reactions and carbon deposition, ultimately making the fuel cell performance inefficient. The impact of varying this parameter on key performance parameters of the hybrid system is evaluated. Given the established influence of the O/C ratio on SOFC FU, we extend our investigation to observe its effects on other hybrid system performance parameters. Certain SOFC performance metrics are directly linked to SOFC FU, and any variation in the latter will consequently impact these metrics. These metrics encompass SOFC cell voltage (representing SOFC power), and electric efficiency. The SOFC working temperature is also assessed, as it is an important parameter that influences the electric power generation in the SOFC. Figure 7 illustrates that an increase in the O/C ratio results in higher SOFC voltage and electric efficiency for the stack. The working temperature exhibits a modest variation within the range of 1070-1073 K. This is attributed to the optimal selection of the HPC outlet air split fraction to the cathode (AS-cathode), maintaining the stack temperature within this range to achieve a stack output power ranging between 97 and 108 kW. The AS-cathode air fraction serves dual purposes in the stack, facilitating electrochemical reactions and providing cooling. Most of this air fraction is utilized for stack cooling, leading to minimal variation in the stack temperature. However, if the air fraction is suboptimal,  
Figure 6: O/C variation with CPOx inlet fuel flow rate and SOFC fuel utilization. the temperature variation in the SOFC would be more significant. In this assessment, the cathode inlet temperature and AS-cathode are held constant, resulting in a low variation in stack temperature. From Figure 7, it is evident that with higher O/C ratios, the SOFC output power increases, electric efficiency also rises. However, in this operational region (O/C > 1.1), the high SOFC FU will cause performance issues. Therefore, it does not represent the optimal operational region for achieving optimal hybrid system performance. The selected O/C ratio at the design point is 0.85, resulting in the SOFC performance detailed in Table 2. For the chosen optimal region, the design point cell voltage required to meet the power demand is 0.7594 V, yielding a stack output of 102 kW and an electric efficiency of 45.89%. This leads to the conclusion that a suboptimal O/C ratio influences the electric power generation of the hybrid system. Considering the hybrid system's intended use for aircraft propulsion, it is essential to analyze the impact of a variable O/C ratio on thrust generation capabilities.