# Thermal Management Systems for Civil Aircraft Engines: Review, Challenges and Exploring the Future
## 1 Introduction  
New designs of gas turbine civil aero-engines are increasingly complex. Nowadays, we are facing with More Electric Aircraft (MEA) with higher demands on engines for thrust and power generation resulting in hotter fluids, higher components temperature and higher heat generation, which means critical thermal management issues. So, it is time to think differently about how we can manage the thermal loads in modern gas turbine engines. In other words, future aircraft propulsion systems should be able to meet ambitious targets and severe limitations set by governments and organizations (e.g., the Advisory Council for Aviation Research and Innovation in Europe has a target of a 75% reduction in CO\({}_{2}\) emissions and a 90% reduction of NOx emissions by 2050) [1]. These targets cannot be achieved just through marginal improvements in turbine technology or aircraft design. High power and high heat flux cooling requirements of new aero-engine designs result in generation of more excess heat by Gas Turbine Engines (GTEs). The Thermal Management System (TMS) plays a key role in dealing with this heat to limit payload size and to enhance the engineperformance. The TMS utilizes engine fluids to transfer excess heat from the engine heat sinks like bearings, accessory gearbox, pumps, generators, constant speed drive and power gearbox in new geared turbofans. It is worthwhile to mention that the TMS structure is not usually discussing the turbine blade cooling and the cooling mechanisms used for this purpose as it has a separate circuit fed by the compressor bleed air. A comprehensive review about the cooling mechanisms for high level heat flux components [2; 3; 4], physic-based models and cooling mechanisms (convection, film, transpiration cooling, cooling effusion, pin fin cooling etc.) [5; 6; 7; 8] and control systems for the associated cooling mechanisms could be found widely in the literature [9; 10; 11]. Of course, there are limitations and constraints in TMS design and implementation. For instance, the total volume and weight of the TMS will be limited by engine design and geometry. Moreover, the operable temperature for fuel, oil and other fluids communicated throughout the engine should be maintained. Engine oil may undergo coking and fuel may undergo lacquering, gumming, or varnishing above certain temperature limits [12]. These constraints limit the heat sink capacity of the TMS and highlight the requirement of a smart optimized TMS design for modern GTEs. In addition, an optimized TMS should be able to increase engine performance by using the engine excess heat (e.g., heat may be transferred into the engine fuel in order to increase the fuel efficiency). In the last 60 years, many ideas, concepts, embodiments and research studies have been done on thermal management systems for gas turbine aero-engines to meet all thermal load management requirements and to achieve optimized performance for the TMS and the engine. The historical progress in the development of TMS for aerospace applications can for convenience be divided into three phases:  
* **Phase 1:** Pioneering work: The first two decades (between 1958 and 1978). The first theoretical studies and embodiments' presentation were undertaken, resulting in generation of the main ideas for the aerospace TMS: using two heat exchangers for the engine thermal management, using catalyst to increase the fuel heat capacity and oil flowrate tuning through the heat exchangers to control the engine fuel temperature. These ideas helped researchers to build a very strong integration phase in the field. * **Phase 2:** Integration: The second two decades (between 1978 and 1998). The idea of having integrated thermal management system for engine and airframe is presented and discussed in several embodiments in this phase. Moreover, the most practical idea for the TMS structure, having two separate cooling loops for the management of engine heat loads, is also proposed and developed. * **Phase 3:** Detailed Design: The last two decades (between 1998 and 2018).