# Performance of a Supercritical CO2 Bottoming Cycle for Aero Applications
## 4 Discussion  
The geometric design and the performance of a recuperated bottoming cycle for a projected year 2050 geared turbofan engine was determined in an effort to conceptualise an aero engine capable of fulfilling the Flightpath 2050 goals. Previous studies have focused on ORCs [14] and open air cycles [10]. The current novel design features supercritical carbon dioxide as a working fluid, as did [PERSON] et al. [24]. The addition of a bottoming cycle, which recycles part of the core engine's exhaust heat and provides shaft power to the low pressure shaft of the main engine, makes the combined cycle engine approximately 1.9% more fuel burn effective than the reference TF2050. Although a non-negligible benefit, this will not be sufficient to reach the Flightpath 2050 goals. However, a considerable part of the RBC's potential for fuel burn reduction has not yet been capitalised on. The RBC will increase the specific power of the main engine's core and may facilitate lower specific thrust designs, with fewer penalties from extreme HPC exit conditions or large LPT expansion ratios. In fact, the optimum OPR of the main engine will likely be lower than that of the TF2050 due to the resulting favourable LPT exhaust gas conditions, alleviating some of the problem of excessive HPC or LPT designs. These effects will be investigated in combined cycle studies, with coupled TF2050 and bottoming cycle models. The creation of the integrated combined cycle engine model is the next step. It is very likely that further parametric studies and combined cycle optimisations will find more efficient designs. The current RBC design has been derived by exploring the designs of the three heat exchangers and the turbomachinery sequentially. Apart from the installed weight, the geometric constraints imposed by the main engine have only been included as absolute limits. Full parametric or optimisation studies that investigate the design parameters of all those components and the TF2050 together, will be able to more adequately capture the dependencies of each of the components' designs. The effect of designing the pre-cooler and MHX to feature four instead of two passes may also prove to increase the combined cycle efficiency of the engine, despite some increase in external pressure losses. Beside optimisations, there are two areas of this engine concept's further development that need to be explored for its more definite evaluation. The first is the extension of the current RBC model's capabilities. Four main features will be developed further:  
* thermal and hydraulic performance of finned tubes in the MHX and pre-cooler;
* simple mechanical integrity models for the heat exchangers, especially the tubes;
* mean line design and improved weight estimation of the turbomachinery;
* estimation of the nacelle diameter increases due to the heat exchanger geometries and the corresponding drag penalties. The issue of the tube thicknesses walls needs addressing and there are a number of options to do so. The elliptical tubes could be strengthened through an internal support structure or webs, similar to \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(T_{cmp,in}\) & 8 K & \(dP_{cold,S-CO2,Rec}\) & 0.2\% \\ \(P_{cmp,in}\) & 8.1 MPa & \(dT_{cold,S-CO2,Rec}\) & 7 K \\ \(W_{\text{cmp,in}}\) & 19 kg/s & \(\epsilon_{air,Pre}\) & 32.1\% \\ PR & 4 & \(dP_{air,Pre}\) & 4.7\% \\ \(\varepsilon_{MHX}\) & 60.5\% & \(dT_{air,Pre}\) & 37 K \\ \(dP_{air,MHX}\) & 4.1\% & \(W_{air,Pre}\) & 97 kg/s \\ \(dT_{air,MHX}\) & 135 K & \(dP_{S-CO2,Pre}\) & 2.7\% \\ \(dP_{S-CO2,MHX}\) & 0.5\% & \(dT_{S-CO2,Pre}\) & 78 K \\ \(dT_{S-CO2,MHX}\) & 142 K & RBC \(\eta_{th}\) & 18.8\% \\ \(\varepsilon_{Rec}\) & 50.1\% & Shaft power & 0.84 MW \\ \(dP_{hot,S-CO2,Rec}\) & 0.8\% & total weight & 604.5 kg \\ \(dT_{hot,S-CO2,Rec}\) & 11 K & \#fuel-burn & \(-\)1.90\% \\ \hline \end{tabular}
\end{table} 
Table 10: Final RBC specifications at cruise. the elliptical tubes of MTU's Lancette heat exchanger [39].