# Thermal Management Systems for Civil Aircraft Engines: Review, Challenges and Exploring the Future
## 4 Detailed Design Phase on Thermal Management Systems  
After integration phase, the research studies and proposed ideas have been focused on the detailed design and novel ideas during the last two decades. Some of the most important achievements of this phase are covered in this section:  
An interesting idea is to use the Cooled Cooling Air (CCA) system to improve the gas turbine engine efficiency. In 1999, air force research laboratory suggested that when utilizing the available heat sinks in a gas turbine engine, the overall engine performance could be improved [35]. The main novelty was to use a heat exchanger to cool the compressor bleed air to decrease the required air flow rate for the turbine blade cooling. Four different embodiments were investigated in detail and the results of the study showed that the CCA system with a fuel-to-air heat exchanger offers the greatest potential for improved engine performance while rescuing some of the dependence on advanced materials. The schematic of this idea is shown in Figure 2. However, the main drawback of this idea is its complexity and weight as well as safety and reliability considerations. In 2002, [PERSON] et al. focused on the fuel-cooled thermal management for advanced aero engine by utilizing endothermic cracking and reforming of hydrocarbon fuels [36; 37]. By developing a bench-scale test rig, they presented a conceptual design to demonstrate the technologies necessary for utilizing conventional multi-component hydro-carbon fuels for fuel-cooled thermal management system including the development of the endothermic potential of JP-7 and JP-8+100, a demonstration of the combustion of supercritical/endothermic fuel mixture and conceptual design of a fuel-air heat exchanger [38; 39]. However, the requirement of high fuel temperature (700 to 1200 \({}^{\circ}\)F) is still the limit of this concept [40; 41]. Between 2004 and 2007, two comprehensive research studies were published on the remaining challenges for future aircraft power systems thermal management [42; 43]. The main conclusion of these studies could be summarized as follow:  
1. The first step in designing an optimized TMS for an aircraft engine is to precisely define the boundary conditions and to recognize the requirements and constraints of the specific system. Figure 2: CCA concept using a Fuel-to-Air Heat Exchanger Concept. 2. Different approaches should be taken for different applications and aircraft platform in order to optimize the power and TMS. 3. Different levels of heat flux should be discussed in detail and suitable approaches for each level should be proposed. 4. Since all power system components such as batteries, capacitors, power semiconductors, generators, pulsed power sources and beam conditioners have thermal design issues, partial solutions have been sought by way of increased heat transfer through the use of spray cooling, micro channels and subcooled boiling, loop heat pipes, capillary pumped loops, energy storage and spray cooling arrays. In 2009, [PERSON] et al. presented a thermal management modelling approach for integrated power system. Based on the control-volume method they developed a dynamic model for the TMS and the propulsion system in Numerical Propulsion System Simulation (NPSS) software and linked it to an electrical model created in MATLAB/Simulink [44]. However, in their model they assumed that the engine oil cooling would be dealt with using a separate cooling system. Moreover, the effect of the air bleed cooling system was not considered in this study. In 2012, United Technologies Corporation presented another schematic of a two integrated loops TMS for gas turbine engines [45]. The main idea of this patent is to have two cooling loops with different temperature to deal with different levels of heat flux in flight mission.