# Performance of a Supercritical CO2 Bottoming Cycle for Aero Applications
## 4 Discussion
The need for larger external heat transfer surface areas makes a case for finned elliptical tubes [40], which could have the added benefit of the fins stiffening the tubes themselves, reducing the necessary wall thickness. As a third option, the tube geometry could be altered towards lower aspect ratio elliptical tubes or even circular tubes, either with or without fins. These tube shapes will enable thinner walls, though that mechanical benefit will be traded against the likely higher drag and larger internal hydraulic diameters, resulting in lower heat transfer. However, if the tube arrangement were to be changed, together with the tube shape, and if the fins were larger, then finned circular tubes in a non-staggered arrangement may prove to be a more attractive option. An exploration of a number of these options is currently underway and will not only help to down-select a preferred design, but will also generate an analytical model to appropriately size the tube walls for that design. The second area of future development is the investigation of the synergies bottoming cycles may have with other novel core technologies for aero engines. The bottoming cycle's shaft power output is largely constrained by the relatively low exhaust gas temperature of the TF2050 at cruise. In an effort to increase the bottoming cycle's efficiency and at the same time reduce the main engine's core size, the addition of inter-turbine reheat (ITR) in the main engine will be investigated. Inter-turbine combustion has been shown to improve the efficiency of stationary gas turbines in Alstom's GT24/26, especially at part-load and in combination with a bottoming cycle that utilises the increased exhaust gas temperatures. If ITR were employed in the TF2050, the core mass flow would be reduced and hence the HP component efficiencies would be penalised. The bypass ratio would also increase for a given specific thrust, reducing the transfer efficiency of the engine. The fuel-burn benefit of a combined cycle TF2050 with ITR will likely exceed 1.9%, as the increased efficiency of the bottoming cycle and weight reductions in the core engine and the bottoming cycle are likely to outweigh these penalties. ITR should furthermore help reducing NO\({}_{x}\), as the peak temperature in the main combustor could be reduced. Inter-turbine combustion can also remedy the EGT-lowering effect of increased OPR in an intercooled main engine. It should therefore be worth investigating a combined cycle engine featuring ITR, intercooling and an S-CO\({}_{2}\) bottoming cycle. Table 11 highlights the approximated effects of greater external heat transfer surface areas for the MHX and the pre-cooler in two case studies (Cases 2 and 3). All parameters not mentioned in Table 11 were kept constant. Case 1 represents the RBC configuration presented in this study. Case 2 illustrates the maximum achievable benefit the RBC could provide, if the heat transfer surface areas of the MHX and pre-cooler could be increased by 200% and 100%, respectively, and if there would be no weight or pressure loss penalty. Case 2 is an unrealistic yet illustrative scenario. However, if the additional heat transfer surfaces doubled the weight increase of the MHX and pre-cooler, and if the respective external pressure losses were increased by two percentage points, then the fuel burn benefit of the larger heat transfer surface configuration over Case 1 would drop from 3.2% to 0.5%. Case 3 also indicates the weight and pressure loss penalties that should not be exceeded, for \"enhanced\" external transfer surface designs to still be profitable.