# Performance of a Supercritical CO2 Bottoming Cycle for Aero Applications
## 3 Results
### Standalone Bottoming Cycle  
Following the geometric definition of the three heat exchangers in the recuperated bottoming cycle, four more cycle parameters are investigated to maximise the fuel burn benefit. Those are the compressor inlet temperature and pressure, the S-CO\({}_{2}\) mass flow and the compressor pressure ratio. The effects of the compressor inlet pressure and temperature on the cycle performance are strongly correlated. Figure 9a,b illustrates that relationship. The shaft power of the RBC, shown in Figure 9a, strongly depends on the compressor inlet conditions, as they determine the proximity of the compressor's operating point to the critical point of S-CO\({}_{2}\). The plot shows that slightly higher inlet conditions (approximately 8 MPa and 308 K) may be more favourable than those previously presumed ideal (7.5 MPa and 305 K). The compressor pressure ratio was kept constant during this study, which may be problematic from the mechanical integrity point of view of the MHX. Attempting to maintain compressor inlet conditions very close to the critical point even during off-design operation would probably require very sophisticated and expensive control systems. Higher inlet conditions not only improve the shaft power output of the bottoming cycle in this particular case, but they also simplify its control systems. However, the inlet temperature should not be increased too much, as Figure 9a shows that changes as small as 10 K can mean a loss in shaft power of 25% to 30%. The inlet pressure has a less severe, though still noticeable impact. Figure 9b illustrates how the inlet conditions affect the overall cycle weight. Although non-negligible, the changes in weight, relative to a cycle designed for inlet conditions of 305.15 K and 7.5 MPa, are not as significant as the changes in shaft power. The final cycle is therefore likely to feature slightly hotter and denser S-CO\({}_{2}\) than previously assumed. Figure 9: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(MN_{fr,c}\) & 0.12 & RBC shaft power & 0.82 MW \\ \(N_{columns}\) & 2 & Pre-cooler weight & 101.7 kg \\ \(b_{tube}\) & 1.2 mm & \(\Delta\)fuel burn & \(-\)1.73\% \\ \(d_{ifr,c}\) & 1.02 m & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Parameters of the final pre-cooler design at cruise. The S-CO\({}_{2}\) mass flow of the final cycle is likely to be between 18 and 23 kg/s for the current component designs, as can be derived from Figure 9c. Lower mass flows trade considerable losses in shaft power against modest improvements in weight and bypass flow disturbance. Higher mass flows on the other hand, lead to minor improvements in shaft power while resulting in larger and heavier cycles. Either way, the power-to-weight ratio of the bottoming cycle decreases. The required air flow in the pre-cooler increases noticeably with increasing S-CO\({}_{2}\) mass flow and the amount of heat that needs to be rejected. The geometric implications of having a larger air mass flow in the pre-cooler will impose another constraint on the feasible S-CO\({}_{2}\) mass flow. Furthermore, the RBC's thermal efficiency is lower in the higher mass flow designs, because the peak temperature in the cycle drops. When trying to maximise shaft power, the range of compressor pressure ratios depicted in Figure 9d may be too narrow. However, a pressure ratio of four is currently assumed to be the maximum possible with the projected technology levels, as the components and their casings may otherwise get too heavy. Thermal and mechanical loads may already be too high with the current designs, which is why more detailed mechanical design tools are under development. The thermal efficiency of the RBC also increases with increasing pressure ratio, which is why the required pre-cooler air mass flow reduces slightly.