# Exchange Rate Analysis for Ultra High Bypass Ratio Geared Turbofan Engines
## 5 SFC Exchange Rate Analysis
### Synthesis Exchange Rates  
In this case, the DP was at T/O (DP parameters, as in Table 2), and the exchange rates were calculated at cruise, which is an OD operating point (cruise parameters in Table 4). Therefore, the engine geometry was fixed and the cycle changed. The engine was sized at T/O conditions, and the geometry was fixed. Then, the simulations for the exchange rates were performed at cruise conditions as OD steps. For the above operating condition, different bypass duct P\({}_{\text{loss}}\) values and mass flow extraction points were simulated for constant power settings (i.e., COT), and the impact on performance was calculated. The results showed similar trends as the fixed cycle cases (Figures 13 and 14). There was a considerable impact when P\({}_{\text{loss}}\) increased in the bypass duct. SFC deteriorated linearly when P\({}_{\text{loss}}\) increased (from 1% to 4%, SFC deteriorated 7%; Figure 15). Regarding the air mass bleed, the following cases were investigated (1% to 4% variation, stations refer to the engine model in Figure A1):  
* LPC bleed to the bypass flow (station 6)
* HPC bleed to the bypass flow (station 8)
* Bypass bleed overboard (station 24)
* LPC bleed overboard (station 7)
* HPC bleed overboard (station 9)  
It was found that any air mass flow extraction either from the bypass duct or the core flow had a considerable deterioration in SFC. This was justified by the fact that air mass flow had a direct effect on the bypass thrust. This was well evident when mass flow bleed from the bypass duct, causing a 3.3% SFC reduction for 1% bleed, which corresponded to 1.9 kg/s. However, the magnitude of the impact on engine performance was different and depended strongly on the bleed port location for the same bleed mass flow percentage. It was observed that apart from extracting mass flow from the bypass duct, having an HPC overboard bleed was also detrimental for engine performance. For a 4% HPC bleed, SFC deteriorated by 4.7% and 6.6% when air was driven to bypass and overboard, respectively  
Figure 14: Added heat effect on SFC (DP study). (Figure 16). Extracting high pressure air from the core had a detrimental effect, as the turbine work (both HPT and LPT) were reduced. When the air was driven to the bypass duct, there was a small compensation due to bypass mass flow and pressure increase. For constant power settings (i.e., COT), the FN was reduced for the different bleed port locations and extracted mass flow values. HPC and bypass overboard had a dominant effect on FN reduction, being 11.73% and 11.8%, respectively, for 4% mass flow bleed (Figure 17). Figure 16: SFC exchange rates vs. bleed mass. Figure 15: Effect of bypass duct pressure loss. With regard to the added heat due to the utilization of AOHE and FOHE, it was shown that preheating the fuel offered a greater reduction to the SFC compared to the case in which the heat was directed to the bypass duct. The synthesis exchange rates showed a reduction of 1.32% for 150 kW added to the fuel before combustion (Figure 18). For the same amount of heat, the reduction was 0.38% in the case of AOHE. Therefore, for the thermal management point of view, it is better to cool the oil by using FOHE. However, there is a limitation on the maximum temperature the fuel can have before combustion due to cracking problems. Consequently, the use of AOHE is necessary in combination with FOHE. Figure 17: FN change vs. bleed mass. Figure 18: Added heat effect on SFC (synthesis exchange rates). The effect of HPT power offfake on engine performance at cruise conditions was also investigated. At DP, HPT auxiliary work for this engine model is 37.5 kW. A number of cases were investigated for constant power setting at cruise conditions, with a range of HPT power offfake up to 133% to reflect the additional auxiliary power requirements from the aircraft and engine systems. The increase of power offfake caused an increase in SFC, as the HPT expands to lower P in order to provide the extra power.