# Exchange Rate Analysis for Ultra High Bypass Ratio Geared Turbofan Engines
## 5 SFC Exchange Rate Analysis
### Design Point Exchange Rates  
To conduct the simulation in this scenario, the engine was sized at cruise conditions as the engine operates at this point for the majority of the flight mission duration, and the cycle was fixed (i.e., all the simulations were performed as DP). The operating parameters are shown in Table 4.  
In these cases, the components design parameters (pressure ratios (PRs), efficiencies (ETAs), COT, etc.) were fixed, and only the bypass duct pressure loss (P\({}_{\text{loss}}\)) and extraction air mass flow from the core (W\({}_{\text{air}}\)) varied. The engine geometry changed to meet the design point requirements. The value of SFC was calculated and compared with the initial DP (P\({}_{\text{loss}}\) 1% and no W\({}_{\text{air}}\) extraction). It was found that in all cases, there was a detrimental effect on SFC when P\({}_{\text{loss}}\) and bleed W\({}_{\text{air}}\) increased.  
In terms of the bypass duct P\({}_{\text{loss}}\) (station 25-26, Figure A1), there was a linear relation due to the effect of pressure force in the thrust calculation, as the nozzle operated at choked conditions (Figure 12). SFC values were normalized to the initial DP (i.e., P\({}_{\text{loss}}\) 1%), which was shown by when the pressure loss in the bypass duct increased from 1% to 4%, there was an SFC deterioration by 6.7%. This additional P\({}_{\text{loss}}\) caused a reduction of PR across the bypass nozzle and consequently a reduction of thrust contribution from the bypass stream.  
Similar effects were found when airflow was extracted from LPC and HPC, and it was driven to the bypass duct (stations 6 and 8, respectively, in Figure A1). One more case was also examined: the extraction of airflow from the bypass air duct to the aircraft cabin (station 24 in Figure A1).  
Air was extracted from the LPC and HPC and driven to the bypass duct. The air, which was extracted from the bypass duct, was driven to the aircraft systems, i.e., it escaped from the engine air path. It is worth noting that the cases were considered independently. It was shown that the bypass mass flow extraction had a great influence on engine exchange rates (3% bleed causes an 11% SFC  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value—Units** \\ \hline Altitude & 10,000 m \\ Mach number & 0.78 \\ Mass flow & 200 kg/s \\ Combustion Outlet Temperature (COT) & 1624 K \\ PR & 54 \\ BPR & 15.6 \\ Bypass P\({}_{\text{loss}}\) & 1\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Design engine parameters at cruise conditions.  
deterioration, as shown in Figure 13). This was explained by the first order effect the bypass mass flow had on the thrust calculation, and the high value of bypass air mass flow compared with the core flow.  
The utilization of AOHE using air from the bypass duct and FOHE using the fuel to cool the oil is common in an engine's thermal management architecture. The added heat of 50 kW to 150 kW either to the bypass duct (reflecting AOHE installation) or to the fuel (reflecting the FOHE utilization to pre heat the fuel) was investigated. Taking into account the fuel's low heating value (LHV) and based on enthalpy calculations, the added heat to the fuel caused a reduction in fuel flow for the same value of combustor temperature rise. The cases were examined independently (not in any combination of them) for constant FN. Additionally, the Ploss for the AOHE was assumed constant at 1%.  
The results in the case of the design point exchange rates showed that the use of FOHE had a greater beneficial effect than the use of AOHE on the SFC exchange rates. Having preheated the fuel to an equivalent of 150 kW, the SFC was reduced by 1.33%, while this reduction was 0.45% when the  
Figure 12: Bypass duct P loss effect on SFC (DP at cruise).  
Figure 13: Bypass duct mass flow effect on SFC (DP study).  
bypass air was heated by the same amount (Figure 14). This was justified by the fact that adding heat to the bypass duct had a minor effect on the bypass air flow T due to the high value of mass flow.