# Thermal Management Systems for Civil Aircraft Engines: Review, Challenges and Exploring the Future
## 4 Detailed Design Phase on Thermal Management Systems
Another idea is to use a working fluid (e.g., water, Therminol, thermally neutral heat transfer fluid (TNHTF)) to absorb heat from engine fluids (oil and fuel). Two schematics of this idea are shown in Figure 7. A reversible heat pump is used in this embodiment to circulate a working fluid through heat exchangers. By employing this pump, the performance of the TMS system will improve operational flexibility that enables the heat transfer characteristics of the system to be more precisely controlled and more readily tailored to a particular engine across a greater range of operating conditions. The system will work in forward direction in normal operating conditions. As soon as one of engine fluids (oil or fuel or both) exceeds the temperature limit, the system will switch to reverse direction to protect the engine fluids from over-temperature and other physical limitations (e.g., cooking, lacquering, varnishing, etc.). However, the design and implementation of this system has its own complexity and considerations that should be taken into account. Figure 6: Thermal management system embodiments with separate line heat exchangers for geared systems. Figure 2:0 _Cont._  
In 2014, the von Karman institute for fluid dynamics in Belgium published an experimental study on thermal analysis of surface heat exchanger operating in transonic regime in which an integrated bypass-flow surface heat exchanger is modelled and tested to cope with the high cooling demands from innovative engine architectures. Their results showed that the investigated concept may provide up to 76% of the estimated lubrication cooling requirements during take-off of in a modern gas turbine power plant [51]. The idea of using thermally neutral heat transfer fluid (TNHTF) like Therminol and DOWTHERM for gas turbine engines TMS is becoming more widely focused in recent years. In 2016, United technologies Corporation presented an embodiment containing a fuel/TNHTF heat exchanger, an oil/TNHTF heat exchanger and an air/TNHTF heat exchanger to manage the excess thermal loads of GTEs [52]. The main advantage of this idea (shown in Figure 8) is the high potential of TNHTFs as heat sink in comparison with engine fluids (oil and fuel). However, the system should be designed precisely and the accurate flowrate control should be considered in order to satisfy the stability criteria of the system. In Figure 8, the proposed configuration of Fuel/TNHTF heat exchanger (1), oil/TNHTF heat exchanger (2) and air/TNHTF heat exchanger (3) are shown schematically. Another idea, is to cool the compressor bleed air using a TNHTF in order to decrease the required volume of airflow for cooling the high-pressure turbine and increase the engine efficiency [53]. Figure 2:1 Thermal management system embodiments with thermally neutral heat transfer fluid. Figure 2:2 Thermal management system embodiments with working fluid and reversible pump. FWD: Forward, Rev: Reverse. The schematic of this idea is shown in Figure 2:3 An accurate control system and a variable speed drive design should be considered in this idea. Moreover, the fuel lacquering and other deposit formation is an important concern in thermal management system design. To deal with this problem, the idea of controlling fuel temperature based on the aircraft altitude is presented in 2016 by [PERSON] and [PERSON] [PERSON] and [PERSON] (2016). The control strategy was proposed to manage fuel temperature by using the altitude sensor data. For instance, the controller may schedule a decreased temperature for the conditioned fuel at lower altitudes (respect to high fuel oxygen content at low altitudes) and an elevated temperature for conditioned fuel at higher altitudes (e.g., during cruise and high-altitude climb). In 2016, a comprehensive work on transient exergy analysis for thermal management of subsystem components was demonstrated and discussed [55].