# Thermal Management Systems for Civil Aircraft Engines: Review, Challenges and Exploring the Future
## 3 Integration Phase on Thermal Management Systems Design  
From 1978, the integration phase in TMS design and development started. The idea of having integrated TMS for the engine and airframe was proposed for the first time by [PERSON], et al. in 1981 [23]. The main idea was to refrigerate fuel to deal with all cooling requirements that resulted in necessitating additional power for the refrigeration system. However, as the cooling demand of the airframe and engine increase, the required quantity of heat absorbing fuel is increased as well. So, the cooling capability will be reduced significantly near the end of mission because of the minimal quantity of the fuel. Also, the other prior art based on fuel refrigeration was proposed in 1985 that had the same disadvantage as discussed above [24; 25]. As a milestone, United Technologies Corporation registered a patent in 1987 on fuel and oil heat management system for a gas turbine engine [26]. This patent presented two different embodiments both contained two oil cooling loops, two fuel-oil heat exchangers and two air-oil heat exchangers to deal with cooling requirements of gas turbine engines. The heat exchangers characteristics and design is used from previous embodiments presented in References [27; 28; 29]. The embodiments are designed to manage the excess heat from different sources efficiently. The schematic of the architecture is shown in Figure 1. This structure has been used widely in real-world applications for thermal management of gas turbine aero-engines. The main working strategy of this embodiment could be described as follow:  
1. The main heat sources of the engine like bearings and accessory gearbox will be cooled (and also lubricated) by main oil loop in which the oil will be distributed and collected throughout the main engine structure and then it will be returned to a collection point after absorbing excess heat generated by the engine components. 2. Another oil loop is designed to lubricate and to dissipate extra heat from accessory drive like constant speed drive for the aircraft service electrical generator. This method is simple, straightforward and easy to design, implement and control. However, since the maximum temperature of the fuel is limited, an extra air flow from bleed valve is required to cool the oil loop in some situations and it results in increasing the overall engine power demand for a given level of useful thrust. This power penalty increases the engine thrust specific fuel consumption. In 1988, [PERSON] and [PERSON] proposed an integrated aircraft fuel thermal management system to optimize the TMS by integrating airframe and engine fuel heat sink systems into a composite heat sink and a secondary power source [30]. They used several accessories like boost pumps, bypass relief valves, temperature control valves and filters to control the fuel temperature and to reduce the dependency on supplemental ram air for the fuel cooling heat exchanger. Moreover, the engine control unit (ECU) is designed to be cooled by the airframe environmental control system (ECS) to increase its reliability. However, in this embodiment, the integrated drive generator (IDG), accessory drive gearbox (ADG) and airframe hydraulic system (Hyd) have not been considered by thermal management system with this assumption that they do not have critical temperature requirements. Figure 1: Thermal management system embodiments proposed by United Technologies Corporation. The rest of the studies in the integration phase were focused on the above milestones and tried to sort out the disadvantages of the proposed ideas [31; 32; 33; 34]. These efforts resulted in emerging another phase in TMS development for gas turbine aero-engines: the detailed design phase. The main presented ideas of the integration phase could be summarized as follow:  
1. Integrated cooling system for the engine and airframe
2. Designing two separate cooling paths for the engine thermal management
3.