# Exchange Rate Analysis for Ultra High Bypass Ratio Geared Turbofan Engines
### Uhbr Thermal Management System  
A typical engine thermal management system (TMS) comprises two oil circuits, namely one for the generator and one for other engine components, such as power gearbox, accessory gearbox, and bearings. The primary function of the oil is to lubricate the engine components, with a secondary function being cooling and transporting heat away from the heat source to available heat sinks. As a result of thermal cycling and high temperatures, the oil degrades because of oxidative and thermal breakdown. This manifests itself as increased viscosity and higher total acidity, which leads to lower oil performance. Unacceptably high oil temperature reduces oil viscosity and increases the oxidation rate or breakdown of the oil. It also affects component life because of the higher component temperatures coupled with reduced oil lubricating film thickness on contacting gear teeth and bearing surfaces, which increase wear. High oil temperature can also reduce the life of the shaft oil seals. Therefore, to achieve longer component design life, the operating oil temperature should be maintained within the working temperature range of the oil. Typically, oil temperature should be limited not to exceed 140 \({}^{\circ}\)C to 150 \({}^{\circ}\)C, depending on the engine rating.  
The heat generated from these oil system components is managed by the TMS (Figure 1).  
The primary function of the TMS is to cool the oil and prevent it from exceeding limiting temperatures and to enable engine components that generate excess heat to operate safely and reliably. The secondary function of the TMS is to manage excess heat in the most efficient manner, thus to minimize the impact of the TMS on fuel consumption.  
The TMS typically manages engine excess heat with fuel as the primary heat sink on the engine, and bypass-air as the secondary heat sink. It is preferable to transfer heat primarily into fuel prior to combustion as by doing so, the energy is maintained within the engine cycle, i.e., the fuel is pre-heated and less fuel is needed for the same combustor outlet gas temperature. This improves the thermal efficiency and the specific fuel consumption. However, there is a limit on the value of the fuel temperature before combustion due to fuel cracking effects. This limits the maximum amount of heat that the TMS can transfer into the fuel. For kerosene, this limit is about 135 \({}^{\circ}\)C. The excess heat is transferred to the atmosphere by a typical arrangement of an air-oil heat exchanger installed in the bypass-air duct (Figure 2).  
The bypass-air duct heat exchange system will impose air pressure drop and air temperature rise. While heat addition to bypass air has a positive effect on the bypass thrust, this benefit is negligible as compared to the large SFC penalty due to the pressure drop across inlet and outlet ducts and the heat exchanger. In the design process, many other alternative cooling architectures and installations can be considered, each having their own impact on engine performance. Thus, it is important to understand  
Figure 1: Typical thermal management system.  
Figure 2: Bypass heat exchange system [32].  
the trade-offs between various thermal management system design options and the engine specific fuel consumption. Exchange rates presented in this paper provide a first-hand preliminary assessment tool to enable quick investigation of these trade-offs.