# Thermal Management Systems for Civil Aircraft Engines: Review, Challenges and Exploring the Future
## 4 Detailed Design Phase on Thermal Management Systems
So, a thermal management system with an air-to-oil heat exchanger to provide the first conditioned fluid and a second (fuel-to-oil) heat exchanger to provide a second conditioned fluid and a third air-to-oil heat exchanger in parallel flow communication with the second heat exchanger to meet the fluid temperature limits was proposed as shown in Figure 3. In this figure, the first heat exchanger (Number 1) provides a first conditioned fluid (Number 2), a second heat exchanger (Number 3) exchanges heat with the first conditioned fluid (Number 2) to provide a second conditioned fluid (Number 4) and a third heat exchanger (Number 5) selectively exchanges heat with the first conditioned fluid (Number 2) to provide a third conditioned fluid (Number 6). Therefore, the TMS in this embodiment enables the designer to have two cooling loops with different temperatures for engine thermal loads management. A turbomachine structural assembly in which the nose cone provides an aperture that communicates air to an interior of the nose cone was proposed by [PERSON] et al. in 2013 [46]. This embodiment enhances the thermal management performance of the turbomachinery. However, more accessories like a motor and a clutch and a control mechanism is required for implementation of this idea. Some other embodiments are  
Figure 3: Thermal management system embodiments with different temperature loops proposed by United Technologies Corporation. also focused on this topic and its pros and cons from practical point of view [47; 48; 49]. These patents focused on cooling requirements of GTE as well as engine fluids temperature limitations and based on defined boundary conditions, different structures and architectures for GTEs thermal management system were proposed in detail. The four different proposed architectures could be categorized as follow [50; 51]:  
1. The first system includes an air/oil heat exchanger and an oil/fuel heat exchanger as shown in Figure 4. A small percentage of the fan bypass duct air flow passes through the Air-to-Oil heat exchanger (AOH). This embodiment not only adds weight to the aircraft but also creates a pressure loss in the fan bypass duct airflow (BPA), resulting in a reduction in propulsive thrust. 2. The second idea is shown in Figure 5 includes an oil/fuel heat exchanger (OFH) that transfers heat from the hot engine oil. However, the oil returns directly to the engine from the OFH in this embodiment. In addition, this system includes an air/ fuel heat exchanger (AFH), which transfers heat from the fuel to a fraction of the aircraft inlet or nacelle airflow (NCA). It maintains the fuel at a sufficiently low temperature to adequately cool the engine oil under different engine operating conditions. This can be achieved because the design of the system is not compromised by constrains imposed on the systems shown in Figure 4. However, the system may still not be capable of handling all engine and aircraft operating conditions without exceeding either the fuel or engine oil operating temperatures limits, resulting in operational limits on the aircraft. Figure 4: Thermal management system embodiments with two heat exchangers and using bypass air (EOL: Engine Oil Line, BPA: Bypass Air). Figure 5: Thermal management system embodiments with two heat exchangers and recirculation path. Figure 6 shows another TMS architecture which is particularly adapted for gas turbine engines with separate gearing structures (\"gearboxes\"). It includes an air/oil heat exchanger (AOH) in a gearbox lubricating oil line (GOL). In a fashion like that employed with the engine lubricating oil, the gearbox oil re-circulates from a sum, through the AOH and then back to the gear box. A separate heat transfer line (EOL) for the engine lubricating oil passes through an oil/fuel heat exchanger (OFH), so that heat from the engine lubricating oil is transferred to the fuel, in a fashion similar to that described in connection with the OFH in Figures 4 and 5.