# Performance of a Supercritical CO2 Bottoming Cycle for Aero Applications
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Case 1** & **Case 2** & **Case 3** \\ \hline EGT (K) & 615.34 & 615.34 & 615.34 \\ W\({}_{\text{S-CO2}}\) (kg/s) & 19 & 19 & 19 \\ Shaft power (MW) & 0.84 & 1.22 & 1.22 \\ MHX hot-side area increase (\%) & 0 & 200 & 200 \\ Pre-cooler air-side area increase (\%) & 0 & 100 & 100 \\ Additional dP\({}_{\text{air,HMX}}\) (\%) & 0 & 0 & 2 \\ Additional dP\({}_{\text{air,Pre}}\) (\%) & 0 & 0 & 2 \\ Weight increase of MHX and & 0 & 0 & 100 \\ Pre-cooler (\%) & 1.9 & 5.1 & 2.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Approximate influence of improved design boundaries at cruise. The S-CO\({}_{2}\) turbomachinery component efficiencies assumed herein are ambitious, but should not be untenable considering the 35-year prelude to the estimated entry into service of the engines. Studies into S-CO\({}_{2}\) cycles for the close- to mid-term future [19; 41] assume isentropic efficiencies of 85% and 90% for the compressor and turbine respectively, though for larger mass flows. Figure 2b in Section 2.1 illustrated the sensitivity of the cycle to the turbomachinery efficiencies. The fuel burn benefit of the bottoming cycle augmented engine would reduce from 1.9% to 1.62%, if the compressor efficiency were 85% rather than 89%, if all other input parameters were kept constant. Besides dedicated models for finned tubes and four-pass configurations for the MHX and the pre-cooler, mechanical integrity models are under development to more realistically estimate wall thicknesses of elliptical heat exchanger tubes. The potential effects of over-optimistic component weights due to underestimated tube wall thicknesses can be inferred from the trends in Table 11.  
The current investigation illustrates the effects of the most important design parameters of the RBC components. There are numerous additional parameters such as surface finish in the heat exchangers, the inlet and outlet duct shapes and the positioning of all the components. These parameters will be accounted for in future sensitivity studies. Possibly the most influential of these parameters are the material choices. The heat exchangers are all assumed to be made of the light, but relatively expensive, Ti-6 Al-4V [36]. The thermal boundary conditions of the components would allow for stainless steels to be used, but the bottoming cycle weight would increase by up to 70%, possibly making the designs untenable. Changing the material of a single component would alter the percentage of the overall RBC weight that that component accounts for. This may then necessitate the redesign of other components for optimum performance. The goals of Flightpath 2050 specify emissions as well as fuel burn reduction targets. Bottoming cycles do not alter the combustion processes or the combustor's design, but they reduce the amount of fuel burnt. Reductions in CO\({}_{2}\) emissions depend on the consumption of Jet-A1 and therefore correlate to any fuel burn improvements made, but NO\({}_{x}\) may be reduced further, particularly if ITR is employed and the main combustor flame temperature is reduced. No CFD studies or rig tests have been conducted yet on the effect of the heat exchangers on engine noise, but the lower core EGT combined with a muffling effect of the heat exchanger matrix behind the LPT, could reduce noise from the core of the engine. Overall, it can be stated that the addition of an S-CO\({}_{2}\) bottoming cycle to a geared turbofan engine promises considerable performance benefits that may be further increased through synergies with other novel core concepts.