Thermal Management Systems for Civil Aircraft Engines: Review, Challenges and Exploring the Future

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Review Thermal Management Systems for Civil Aircraft Engines: Review, Challenges and Exploring the Future

Soheil Jafari * and Theoklis Nikolaidis

Centre for Propulsion Engineering, School of Aerospace Transport and Manufacturing (SATM), Cranfield University, Cranfield MK43 0AL, UK; t.nikolaidis@cranfield.ac.uk * Correspondence: s.jafari@cranfield.ac.uk

Received: 26 September 2018; Accepted: 22 October 2018; Published: 24 October 2018

Abstract: This paper examines and analytically reviews the thermal management systems proposed over the past six decades for gas turbine civil aero engines. The objective is to establish the evident system shortcomings and to identify the remaining research questions that need to be addressed to enable this important technology to be adopted by next generation of aero engines with complicated designs. Future gas turbine aero engines will be more efficient, compact and will have more electric parts. As a result, more heat will be generated by the different electrical components and avionics. Consequently, alternative methods should be used to dissipate this extra heat as the current thermal management systems are already working on their limits. For this purpose, different structures and ideas in this field are stated in terms of considering engines architecture, the improved engine efficiency, the reduced emission level and the improved fuel economy. This is followed by a historical coverage of the proposed concepts dating back to 1958. Possible thermal management systems development concepts are then classified into four distinct classes: classic, centralized, revolutionary and cost-effective; and critically reviewed from challenges and implementation considerations points of view. Based on this analysis, the potential solutions for dealing with future challenges are proposed including combination of centralized and revolutionary developments and combination of classic and cost-effective developments. The effectiveness of the proposed solutions is also discussed with a complexity-impact correlation analysis.

Keywords: thermal management systems; next generation of gas turbine engines; aero engines; engine efﬁciency; performance enhancement

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 CO2 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 engine

Appl. Sci. 2018, 8, 2044; doi:10.3390/app8112044 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2044 2 of 16 performance. 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 [2–4], physic-based models and cooling mechanisms (convection, film, transpiration cooling, cooling effusion, pin fin cooling etc.) [5–8] and control systems for the associated cooling mechanisms could be found widely in the literature [9–11]. 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). More comprehensive studies based on detailed testing and real applications were published in this phase. The ideas of new advanced structures for the TMS, fuel temperature control in TMS and using different cooling fluids like water, Therminol, which is a synthetic heat transfer fluid and thermally neutral heat transfer fluid (TNHTF) are presented and discussed as well.

Obviously, the above-mentioned phases are concentrated on different ranges of power and technology development that was the state of the art in the date of research. Table1 shows the quantitative analysis of different phases that would be discussed in next chapters.

Table 1. Quantitative analysis of different phases [12–17].

Turbine Entry Turbine Blade Thrust (lbf) Bypass Ratio Temperature (◦C) Cooling Efﬁciency Phase I ≤40,000 ≤1200 ≤5 ≤0.55 Phase II 40,000–55,000 1200–1400 5–7.6 0.55–0.65 Phase III ≥55,000 ≥1400 ≥8 ≥0.65 Appl. Sci. 2018, 8, 2044 3 of 16

Moreover, taking the new advances in material sciences into account, it could be seen that the turbine entry temperature has had an average increase of about 8 ◦C rise per year over the last 20 years [17]. It is due to the new advanced materials used for the turbine manufacturing in new aircraft engines (e.g., SC cast alloys and ceramics). So, more increase in thermal loads is expected for the future engines. Consequently, designing an optimal thermal management system architecture is a key factor for the heat management in future civil engines. The most important publications and milestones during phases 1 to 3 are discussed in detail in next sections. Section2, focuses on pioneering work and concepts dating back to 1958. Section3 describes the integration phase in thermal management system architecture design that occurred between 1978 and 1998. Section4 concentrates on detailed design phase in the last two decades addressing the state of the art technologies for thermal load managements in civil aero engines. Based on the results of these sections, different classes of development for the future thermal management systems design are classiﬁed and described in detail and the pros and cons of each class is discussed in Section5. Based on this discussion, two potential solutions for dealing with future challenges of thermal management systems are proposed and illustrated. The conclusion remarks are also presented in Section6.

2. Pioneering Work on Thermal Management Systems Design First, in 1958 an invention presented on oil cooling and drag reducing system for high speed vehicles such as aircraft, sleds, or boats. It is the ﬁrst publicly available publication that presents thermal management system architecture for aircraft engine systematically [18]. Moreover, another invention related to lubricating oil cooling systems for turbojet engines presented in 1963 [19]. The main idea of these two patents were to utilize two heat exchangers, Air/Oil Heat Exchanger (AOHE) and Fuel/Oil Heat Exchanger (FOHE), to dissipate extra heat generated by engine components. These two heat exchangers could be mounted in the cooling loop in series or parallel architecture. In 1969, a patent was registered by Nodding et al. in which a cooling system for engine and airframe is proposed where fuel is used as the heat sink to absorb frictional heat produced by airstream passing over wing and fuselage surface [20]. They showed that by using a catalyst an endothermic chemical reaction would be promoted, allowing fuel to absorb more heat. The main idea is to break down the fuel into smaller molecules and to alter the fuel’s characteristics as a propellant. However, this embodiment requires temperature of 700 to 1000 ◦F for operation and is consequently limited in high supersonic or hypersonic applications. Finally, fuel delivery control system and its effects in oil cooling system of gas turbine aero engines was discussed at last years of the pioneering work phase [21,22]. The main idea of these embodiments was to use a position control valve to tune the oil ﬂowrate in FOHE in order to protect the engine from fuel over temperature and malfunctions. Consequently, the main presented ideas of the pioneering work phase could be summarized as follow:

(1) Using two heat exchangers to manage the excess heat of the engine: FOHE and AOHE (2) Using a catalyst to increase the fuel heat absorption capacity (3) Tuning the oil ﬂow rate through the FOHE to control the engine fuel temperature

These interesting ideas were the main motivations for the next phase.

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 Frosch, 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 Appl. Sci. 2018, 8, 2044 4 of 16 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]. Appl. AsSci. a2018 milestone,, 8, x FOR PEER United REVIEW Technologies Corporation registered a patent in 1987 on fuel and oil4 heat of 16 management system for a gas turbine engine [26]. This patent presented two different embodiments deal with cooling requirements of gas turbine engines. The heat exchangers characteristics and design both contained two oil cooling loops, two fuel-oil heat exchangers and two air-oil heat exchangers to is used from previous embodiments presented in References [27–29]. The embodiments are designed deal with cooling requirements of gas turbine engines. The heat exchangers characteristics and design to manage the excess heat from different sources efficiently. The schematic of the architecture is is used from previous embodiments presented in References [27–29]. The embodiments are designed shown in Figure 1. This structure has been used widely in real-world applications for thermal to manage the excess heat from different sources efficiently. The schematic of the architecture is shown management of gas turbine aero-engines. The main working strategy of this embodiment could be in Figure1. This structure has been used widely in real-world applications for thermal management of described as follow: 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 (1) Thelubricated) main heat by sourcesmain oil of loop theengine in which like the bearings oil will and beaccessory distributed gearbox and collected will be cooled throughout (and also the lubricated)main engine by structure main oil and loop then in which it will the be oilreturn willed be to distributed a collection and point collected after absorbing throughout excess the mainheat generated engine structure by the engine and then components. it will be returned to a collection point after absorbing excess (2) heatAnother generated oil loop by is the designed engine components.to lubricate and to dissipate extra heat from accessory drive like (2) Anotherconstant oilspeed loop drive is designed for the aircraft to lubricate service and electrical to dissipate generator. 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, sinceThis the methodmaximum is simple,temperature straightforward of the fuel is and limited, easy toan design,extra air implement flow from andbleed control. valve is However, required sinceto cool the the maximum oil loop temperaturein some situations of the fueland isit limited,results in an increasing extra air flow the fromoverall bleed engine valve power is required demand to coolfor a the given oil loop level in someof useful situations thrust. and This it resultspowerin penalty increasing increases the overall the engine engine powerthrust demandspecific fuel for aconsumption. given level of useful thrust. This power penalty increases the engine thrust specific fuel consumption.

FigureFigure 1.1.Thermal Thermal managementmanagement systemsystem embodimentsembodiments proposedproposed byby UnitedUnited TechnologiesTechnologies Corporation.Corporation.

InIn 1988,1988, HudsonHudson andand LevinLevin proposedproposed anan integratedintegrated aircraftaircraft fuelfuel thermalthermal managementmanagement systemsystem toto optimizeoptimize thethe TMSTMS byby integratingintegrating airframeairframe andand engineengine fuelfuel heatheat sinksink systemssystems intointo aa compositecomposite heatheat sinksink andand a secondary secondary power power source source [30]. [30 ].They They used used several several accessories accessories like likeboost boost pumps, pumps, bypass bypass relief reliefvalves, valves, temperature temperature control control valves valves and and filters ﬁlters to tocontrol control the the fuel fuel temperature temperature and and to reducereduce thethe dependencydependency on on supplemental supplemental ram ram air air for thefor fuelthe coolingfuel cooling heat exchanger.heat exchanger. Moreover, Moreover, the engine the controlengine unitcontrol (ECU) unit is (ECU) designed is designed to be cooled to be by cooled the airframe by the environmental airframe environmental control system control (ECS) system to increase (ECS) its to reliability.increase its However, reliability. in thisHowever, embodiment, in this embodi the integratedment, the drive integrated generator drive (IDG), generator accessory (IDG), drive accessory gearbox (ADG)drive gearbox and airframe (ADG) hydraulic and airframe system hydraulic (Hyd) have system not been (Hyd) considered have not by thermalbeen considered management by thermal system withmanagement this assumption system thatwith they this doassumption not have criticalthat they temperature do not have requirements. critical temperature requirements. 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 [31–34]. These efforts resulted in emerging another phase in TMS development for gas turbine aero-engines: the detailed design phase.

Appl. Sci. 2018, 8, 2044 5 of 16

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 [31–34]. These efforts resulted in emerging another Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 16 phase in TMS development for gas turbine aero-engines: the detailed design phase. TheThe main main presented presented ideas ideas of of the the integrat integrationion phase phase could could be be summarized summarized as as follow: follow: (1)(1) IntegratedIntegrated cooling cooling system system for for the the engine engine and and airframe airframe (2)(2) DesigningDesigning two two separate separate cooling cooling paths paths for for the the engine engine thermal thermal management management (3)(3) IntegratingIntegrating airframe airframe and and engineengine fuel fuel heat heat sink sink systems systems into ainto composite a composite heat sink heat and sink a secondary and a secondarypower source power source

4.4. Detailed Detailed Design Design Phase Phase on on Thermal Thermal Management Management Systems Systems AfterAfter integration integration phase, thethe researchresearch studies studies and and proposed proposed ideas ideas have have been been focused focused on the on detailed the detaileddesign anddesign novel and ideas novel during ideas the lastduring two decades.the last Sometwo decades. of the most Some important of the achievements most important of this achievementsphase are covered of this in phase this section:are covered in this section: AnAn interesting interesting idea idea is isto to use use the the Cooled Cooled Cooling Cooling Air Air (CCA) (CCA) system system to to improve improve the the gas gas turbine turbine engineengine efficiency. efﬁciency. In In 1999, 1999, air air force force research research laboratory laboratory suggested suggested that that when when utilizing utilizing the the available available heatheat sinks sinks in in a agas gas turbine turbine engine, engine, the the overall overall en enginegine performance performance could could be be improved improved [35]. [35 ].The The main main noveltynovelty was was to to use use a heat a heat exchanger exchanger to to cool cool the the comp compressorressor bleed bleed air air toto decrease decrease the the required required air air flow ﬂow raterate for for the the turbine turbine blade blade cooling. cooling. Four Four different different embodiments embodiments were were investigated investigated in in detail detail and and the the resultsresults of of the the study study showed showed that that the the CCA CCA system system with with a afuel-to-air fuel-to-air heat heat exchanger exchanger offers offers the the greatest greatest potentialpotential for for improved improved engine engine performance performance while while rescuing rescuing some some of of the the dependence dependence on on advanced advanced materials.materials. The The schematic schematic of of this this idea idea is is shown shown in in Figure Figure 22.. However, thethe mainmain drawbackdrawback of of this this idea idea is isits its complexitycomplexity and and weight weight as as well well as as safety safety and and reliability reliability considerations. considerations.

FigureFigure 2. 2.CCACCA concept concept using using a Fuel-to- a Fuel-to-AirAir Heat Heat Exchanger Exchanger Concept. Concept.

InIn 2002, 2002, Haung Haung et et al. al. focused focused on on the the fuel-cooled fuel-cooled thermal thermal management management for for advanced advanced aero aero engine engine byby utilizing utilizing endothermic endothermic cracking cracking and and reforming reforming of ofhydrocarbon hydrocarbon fuels fuels [36,37]. [36,37 By]. Bydeveloping developing a bench-scalea bench-scale test test rig, rig, they they presented presented a conceptual a conceptual design design to todemonstrate demonstrate th thee technologies technologies necessary necessary forfor utilizing utilizing conventional conventional multi-component multi-component hydro-carbon hydro-carbon fuels fuels for for fuel-cooled fuel-cooled thermal thermal management management systemsystem including including the the development development of of the the endothermi endothermicc potential potential of of JP-7 JP-7 and and JP-8+100, JP-8+100, a demonstration a demonstration of the combustion of supercritical/endothermic fuel mixture and conceptual design of a fuel-air heat of the combustion of supercritical/endothermic fuel mixture and conceptual design ◦of a fuel-air heat exchangerexchanger [38,39]. [38,39 ].However, However, the the requirement requirement of of high high fuel fuel temperature temperature (700 (700 to to 1200 1200 °F)F) is isstill still the the limit limit ofof this this concept concept [40,41]. [40,41 ]. BetweenBetween 2004 2004 and and 2007, 2007, two two comprehensive comprehensive research research studies studies were were published published on on the the remaining remaining challengeschallenges for for future future aircraft aircraft power power systems systems ther thermalmal management management [42,43]. [42,43 ].The The main main conclusion conclusion of of thesethese studies studies could could be be summarized summarized as as follow: follow: (1)(1) TheThe first first step step in indesigning designing an an optimized optimized TMS TMS for for an an aircraft aircraft en enginegine is isto to precisely precisely define define the the boundaryboundary conditions conditions and and to torecognize recognize the the requir requirementsements and and constraints constraints of of the the specific specific system. system. (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. Appl. Sci. 2018, 8, 2044 6 of 16

(2) Different approaches should be taken for different applications and aircraft platform in order to optimize the power and TMS. (3)Appl. Sci.Different 2018, 8, x levels FOR PEER of heat REVIEW ﬂux should be discussed in detail and suitable approaches for each level6 of 16 should be proposed.

(4)(4) Since all powerpower systemsystem componentscomponents suchsuch asas batteries,batteries, capacitors,capacitors, powerpower semiconductors,semiconductors, generators, pulsed power sources andand beambeam conditionersconditioners havehave thermalthermal designdesign issues,issues, partialpartial solutions have been sought by way of increased heat transfertransfer through thethe useuse ofof sprayspray cooling,cooling, micro channels and subcooled boiling,boiling, looploop heatheat pipes,pipes, capillarycapillary pumpedpumped loops,loops, energyenergy storagestorage and spray cooling arrays. In 2009, Maser et al. presented a thermal management modelling approach for integrated power In 2009, Maser 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 system. Based on the control-volume method they developed a dynamic model for the TMS and propulsion system in Numerical Propulsion System Simulation (NPSS) software and linked it to an the propulsion system in Numerical Propulsion System Simulation (NPSS) software and linked it to electrical model created in MATLAB/Simulink [44]. However, in their model they assumed that the an electrical model created in MATLAB/Simulink [44]. However, in their model they assumed that engine oil cooling would be dealt with using a separate cooling system. Moreover, the effect of the 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. air bleed cooling system was not considered in this study. In 2012, United Technologies Corporation presented another schematic of a two integrated loops 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 TMS for gas turbine engines [45]. The main idea of this patent is to have two cooling loops with different different temperature to deal with different levels of heat flux in flight mission. So, a thermal temperature to deal with different levels of heat flux in flight mission. So, a thermal management management system with an air-to-oil heat exchanger to provide the first conditioned fluid and a system with an air-to-oil heat exchanger to provide the first conditioned fluid and a second (fuel-to-oil) second (fuel-to-oil) heat exchanger to provide a second conditioned fluid and a third air-to-oil heat heat exchanger to provide a second conditioned fluid and a third air-to-oil heat exchanger in parallel exchanger in parallel flow communication with the second heat exchanger to meet the fluid flow communication with the second heat exchanger to meet the fluid temperature limits was proposed temperature limits was proposed as shown in Figure 3. In this figure, the first heat exchanger as shown in Figure3. In this figure, the first heat exchanger (Number 1) provides a first conditioned (Number 1) provides a first conditioned fluid (Number 2), a second heat exchanger (Number 3) fluid (Number 2), a second heat exchanger (Number 3) exchanges heat with the first conditioned fluid exchanges heat with the first conditioned fluid (Number 2) to provide a second conditioned fluid (Number 2) to provide a second conditioned fluid (Number 4) and a third heat exchanger (Number 5) (Number 4) and a third heat exchanger (Number 5) selectively exchanges heat with the first selectively exchanges heat with the first conditioned fluid (Number 2) to provide a third conditioned conditioned fluid (Number 2) to provide a third conditioned fluid (Number 6). fluid (Number 6).

Figure 3. Thermal management system embodiments with different temperature loops proposed by Figure 3. Thermal management system embodiments with different temperature loops proposed by United Technologies Corporation. United Technologies Corporation. Therefore, the TMS in this embodiment enables the designer to have two cooling loops with differentTherefore, temperatures the TMS for in engine this embodiment thermal loads enables management. the designer to have two cooling loops with differentA turbomachine temperatures structural for engine assembly thermal in whichloads themanagement. nose cone provides an aperture that communicates air toA an interiorturbomachine of the nose structural cone was assembly proposed byin Suciuwhich et al.the in nose 2013 [46cone]. This provides embodiment an aperture enhances that the thermalcommunicates management air to performancean interior of of the turbomachinery.nose cone was proposed However, by more Suciu accessories et al. in like 2013 a motor [46]. This and aembodiment clutch and a controlenhances mechanism the thermal is required management for implementation performance of of this the idea.turbomachinery. Some other embodiments However, more are accessories like a motor and a clutch and a control mechanism is required for implementation of this idea. Some other embodiments are also focused on this topic and its pros and cons from practical point of view [47–49]. 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]: Appl. Sci. 2018, 8, 2044 7 of 16 also focused on this topic and its pros and cons from practical point of view [47–49]. 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]: Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 16 (1) TheAppl. Sci. first 2018 system, 8, x FORincludes PEER REVIEW an air/oil heat exchanger and an oil/fuel heat exchanger as7 of shown16 (1) The first system includes an air/oil heat exchanger and an oil/fuel heat exchanger as shown in in(1) FigureFigureThe first4 4.. AAsystem small includespercentage percentage an ofair/ oftheoil the fanheat fanbypass exchanger bypass duct ductandair flow an air oil/fu passes flowel passesheatthrough exchanger through the Air-to-Oil as the shown Air-to-Oil heat in heatexchangerFigure exchanger 4. A (AOH). small (AOH). percentage This This embodiment embodiment of the fan not bypass only not onlyductadds air addsweight flow weight passesto the tothroughaircraft the aircraft butthe Air-to-Oilalso but creates also heat createsa a pressurepressureexchanger loss loss (AOH). in in the the fanfanThis bypass embodiment duct duct airflow airflow not only(BPA), (BPA), adds resulting resulting weight in toa in reductionthe a reduction aircraft in propulsivebut in propulsivealso creates thrust. thrust.a pressure loss in the fan bypass duct airflow (BPA), resulting in a reduction in propulsive thrust.

FigureFigure 4. Thermal 4. Thermal management management system systemembodiments embodiments with with two two heat heat exchangers exchangers andand using using bypass bypass air air (EOL:(EOL:Figure Engine Engine 4. Thermal Oil Oil Line, Line, management BPA: BPA: Bypass Bypass system Air). Air). embodiments with two heat exchangers and using bypass air (EOL: Engine Oil Line, BPA: Bypass Air). (2) (2)The The second second idea idea is shownis shown in in Figure Figure5 5includes includes anan oil/fuel heat heat exchanger exchanger (OFH) (OFH) that thattransfers transfers heat(2) heatThe from fromsecond the the hot idea hot engine engineis shown oil. oil. However,in However, Figure 5 theincludes oil oil retu returns anrns oil/fuel directly directly heat to to theexchanger the engine engine from(OFH) from the that OFH the transfers OFH in this in this embodiment.embodiment.heat from the In addition,Inhot addition, engine this oil.this systemHowever, system includesincludes the oil retuan an air/ air/rns fuel directly fuel heat heat exchangerto the exchanger engine (AFH), from (AFH), which the OFH which transfers in transfersthis heatheatembodiment. from from the the fuel fuel In addition, to to aa fractionfraction this systemof of the the aircraft includes aircraft inlet an inlet orair/ nacelle fuel or nacelle heat airflow exchanger airﬂow (NCA). (AFH), (NCA). It maintains which It maintains transfers the fuel the atheat a sufficientlyfrom the fuel low to atemperature fraction of the to aircraft adequately inlet orcool nacelle the airflowengine (NCA).oil under It maintainsdifferent theengine fuel fuel at a sufﬁciently low temperature to adequately cool the engine oil under different engine operatingat a sufficiently conditions. low This temperature can be achieved to adequately because thecool design the engine of the systemoil under is not different compromised engine operatingbyoperating constrains conditions. conditions. imposed This This on can the can besystems be achieved achieved shown becausebeca inuse Figure the the design4. design However, of ofthe the thesystem system system is not ismay notcompromised still compromised not be by constrainscapableby constrains of imposedhandling imposed all on onengine the the systems systems and aircraft shownshown oper in atingFi Figuregure conditions 4.4 .However, However, without the the system exceeding system may may stilleither stillnot the be not be capablefuelcapable or of engine handling of handling oil operating all engineall engine temperatures and and aircraft aircraft limits, operating oper resultingating conditions conditions in operational without without limits exceedingexceeding on the aircraft. either the fuel or enginefuel or oil engine operating oil operating temperatures temperatures limits, limits, resulting resulting in operational in operational limits limits on on the the aircraft.aircraft.

Figure 5. Thermal management system embodiments with two heat exchangers and recirculation FigurepathFigure 5.. Thermal 5. Thermal management management system system embodiments embodiments with with two two heat heat exchangers exchangers and and recirculation recirculation path. path. Figure 6 shows another TMS architecture which is particularly adapted for gas turbine engines with separateFigure 6 gearingshows another structures TMS (“gearboxes”). architecture whichIt incl udesis particularly an air/oil adaptedheat exchanger for gas (AOH)turbine inengines a gearbox with lubricatingseparate gearing oil line structures (GOL). In (“gearboxes”).a fashion like that It incl emudesployed an with air/oil the heat engine exchanger lubricating (AOH) oil, inthe a gearboxgearbox lubricating oil line (GOL). In a fashion like that employed with the engine lubricating oil, the gearbox Appl. Sci. 2018, 8, 2044 8 of 16

Figure6 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 Appl.a gearbox Sci. 2018, lubricating8, x FOR PEER oilREVIEW line (GOL). In a fashion like that employed with the engine lubricating8 of 16 oil, the gearbox oil re-circulates from a sump, through the AOH and then back to the gear box. oil re-circulates from a sump, through the AOH and then back to the gear box. A separate heat transfer Appl.A separate Sci.line 2018 (EOL), 8 heat, x for FOR the transfer PEER engine REVIEW linelubricating (EOL) oil for passes the throug engineh an lubricating oil/fuel heat oilexchanger passes (OFH), through so that an heat oil/fuel8 of 16 heat exchangerfrom the (OFH), engine solubricating that heat oil from is transferred the engine to th lubricatinge fuel, in a fashion oil is transferred similar to that to described the fuel, in a fashion oil re-circulates from a sump, through the AOH and then back to the gear box. A separate heat transfer connection with the OFH in Figures 4 and 5. similarline (EOL) to that for described the engine lubricating in connection oil passes with throug theh OFH an oil/fuel in Figures heat exchanger4 and5 .(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.

FigureFigure 6. Thermal 6. Thermal management management system system embodiments embodiments with with separate separate line line heat heat exchangers exchangers for gearedfor geared systems. systems. AnotherFigure idea 6. Thermal is to usemanagement a working system fluid embodiments (e.g., water, with Therminol,separate line heat thermally exchangers neutral for geared heat transfer Another idea is to use a working fluid (e.g., water, Therminol, thermally neutral heat transfer fluid fluidsystems. (TNHTF)) to absorb heat from engine fluids (oil and fuel). Two schematics of this idea are (TNHTF)) to absorb heat from engine fluids (oil and fuel). Two schematics of this idea are shown in shownFigure in 7. Figure A reversible7. A reversibleheat pump is heatused in pump this embodiment is used in to this circulate embodiment a working fluid to circulatethrough heat a working Another idea is to use a working fluid (e.g., water, Therminol, thermally neutral heat transfer fluid fluidexchangers. through By heat employing exchangers. this pump, By employingthe performance this of pump,the TMS thesystem performance will improve ofoperational the TMS system (TNHTF)) to absorb heat from engine fluids (oil and fuel). Two schematics of this idea are shown in willflexibility improve that operational enables the heat flexibility transfer thatcharacteristics enables of the the heat system transfer to be more characteristics precisely controlled of the system Figure 7. A reversible heat pump is used in this embodiment to circulate a working fluid through heat and more readily tailored to a particular engine across a greater range of operating conditions. The to beexchangers. more precisely By employing controlled this pu andmp, the more performance readily tailoredof the TMS to sy astem particular will improve engine operational across a greater system will work in forward direction in normal operating conditions. As soon as one of engine fluids rangeflexibility of operating that enables conditions. the heat transfer The system characteristics will workof the insystem forward to be more direction precisely in controlled normal operating (oil or fuel or both) exceeds the temperature limit, the system will switch to reverse direction to protect conditions.and more Asreadily soon tailored as one to a of particular engine engine fluids ac (oilross or a greater fuel or range both) of operating exceeds conditions. the temperature The limit, the engine fluids from over-temperature and other physical limitations (e.g., cocking, lacquering, the system will will work switch in forward to reverse direction direction in normal operating to protect conditions. the engine As soon fluids as one from of engine over-temperature fluids varnishing, etc.). However, the design and implementation of this system has its own complexity and (oil or fuel or both) exceeds the temperature limit, the system will switch to reverse direction to protect andconsiderations other physical that limitations should be taken (e.g., into cocking, account. lacquering, varnishing, etc.). However, the design the engine fluids from over-temperature and other physical limitations (e.g., cocking, lacquering, and implementation of this system has its own complexity and considerations that should be varnishing, etc.). However, the design and implementation of this system has its own complexity and takenconsiderations into account. that should be taken into account.

Figure 7. Cont. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 16

Appl. Sci. 2018, 8, 2044 9 of 16 Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 16

Figure 7. Thermal management system embodiments with working fluid and reversible pump. FWD: FigureFigure 7. 7.Thermal Thermal management system system embodiments embodiments with working with working fluid and ﬂuid reversible and pump. reversible FWD: pump. Forward, Rev: Reverse. FWD:Forward, Forward, Rev: Rev: Reverse. Reverse.

InIn 2014,2014,In 2014, the the vonthe von von Karman Karman Karman institute instituteinstitute for forfor ﬂuid fluidfluid dynamics dynamics dynamics in in Belgium inBelgium Belgium published published published an an experimental experimental an experimental study onstudy thermalstudy on onthermal analysis thermal analysis ofanalysis surface of of heat surfacesurface exchanger heat exchangerexchanger operating operating operating in transonic in transonicin regimetransonic regime in whichregime in which an inintegrated which an an bypass-ﬂowintegratedintegrated bypass-flow surface bypass-flow heat surface surface exchanger heat heat exchanger isexchanger modelled is is modelled andmodelled tested and and to tested cope tested to with cope to thecope with high with the coolinghigh the coolinghigh demands cooling demands from innovative engine architectures. Their results showed that the investigated concept fromdemands innovative from innovative engine architectures. engine architectures. Their results Th showedeir results that showed the investigated that the conceptinvestigated may provideconcept may provide up to 76% of the estimated lubrication cooling requirements during take-off of in a upmay to provide 76% of the up estimated to 76% of lubrication the estimated cooling lubrication requirements cooling during requirements take-off of induring a modern take-off gas turbineof in a modernmodern gas gas turbine turbine power power plant plant [51]. [51]. power plantThe [idea51]. of using thermally neutral heat transfer fluid (TNHTF) like Therminol and The idea of using thermally neutral heat transfer fluid (TNHTF) like Therminol and DOWTHERMThe idea of using for gas thermally turbine engines neutral TMS heat transferis becoming fluid more (TNHTF) widely like focused Therminol in recent and years. DOWTHERM In 2016, for DOWTHERM for gas turbine engines TMS is becoming more widely focused in recent years. In 2016, gas turbineUnited enginestechnologies TMS isCorporation becoming morepresented widely an focused embodiment in recent containing years. In 2016,a fuel/TNHTF United technologies heat CorporationUnitedexchanger, technologies presented an oil/TNHTF anCorporation embodiment heat exchanger presented containing and an an air/TNHTF a fuel/TNHTFembodiment heat heatexchanger containing exchanger, to manage a anfuel/TNHTF oil/TNHTF the excess heatheat exchangerexchanger,thermal and loadsan anoil/TNHTF of air/TNHTF GTEs [52]. heat The heat exchanger main exchanger advantage and to manage anof thisair/TNHTF idea the excess(shown heat thermal in exchangerFigure loads 8) is oftheto GTEs managehigh [potential52]. the The excess main advantagethermalof TNHTFs loads of this ofas ideaGTEsheat (shownsink [52]. in Thecomparison in Figure main8 advantage) with is the engine high of potentialfluids this idea(oil of and (shown TNHTFs fuel). inHowever, as Figure heat sink the8) is system in the comparison high should potential with engineof TNHTFsbe designed fluids as (oil heat precisely and sink fuel). andin However,comparison the accurate the with flowrate system engine shouldcontrol fluids beshould designed(oil beand considered fuel). precisely However, in and order the theto accurate satisfy system the flowrate should stability criteria of the system. In Figure 8, the proposed configuration of Fuel/TNHTF heat exchanger controlbe designed should precisely be considered and the in order accurate to satisfy flowrate the stability control criteriashould of be the considered system. In in Figure order8, theto satisfy proposed the (1), oil/TNHTF heat exchanger (2) and air/TNHTF heat exchanger (3) are shown schematically. configurationstability criteria of Fuel/TNHTFof the system. heatIn Figure exchanger 8, the (1),proposed oil/TNHTF configuration heat exchanger of Fuel/TNHTF (2) and air/TNHTF heat exchanger heat exchanger(1), oil/TNHTF (3) are heat shown exchanger schematically. (2) and air/TNHTF heat exchanger (3) are shown schematically.

Figure 8. Thermal management system embodiments with thermally neutral heat transfer fluid.

Figure 8. Thermal management system embodiments with thermally neutral heat transfer fluid. Figure 8. Thermal management system embodiments with thermally neutral heat transfer fluid. 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]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 16

Appl. Sci.Another2018, 8, 2044idea, is to cool the compressor bleed air using a TNHTF in order to decrease the required10 of 16 volume of airflow for cooling the high-pressure turbine and increase the engine efficiency [53]. The Theschematic schematic of this of this idea idea is shown is shown in inFigure Figure 9.9 .An An accurate accurate control control system andand aa variablevariable speed speed drive drive designdesign should should be be considered considered in in this this idea. idea.

Figure 9. Thermal management system embodiments with thermally neutral heat transfer fluid for coolingFigure the9. Thermal compressor management bleed air. system embodiments with thermally neutral heat transfer fluid for cooling the compressor bleed air. Moreover, the fuel lacquering and other deposit formation is an important concern in thermal managementMoreover, system the fuel design. lacquering To deal withand other this problem, deposit formation the idea of is controlling an important fuel temperatureconcern in thermal based onmanagement the aircraft system altitude design. is presented To deal inwith 2016 this by problem, Teicholz the and idea Stearns of controlling [54]. The fuel control temperature strategy based was proposedon the aircraft to manage altitude fuel is temperature presented in by 2016 using by theTeicholz altitude and sensor Stearns data. [54]. For The instance, control the strategy controller was mayproposed schedule to manage a decreased fuel temperaturetemperature forby theusing conditioned the altitude fuel sensor at lower data. altitudes For instance, (respect the to controller high fuel oxygenmay schedule content a at decreased low altitudes) temperature and an elevated for the temperatureconditioned forfuel conditioned at lower altitudes fuel at higher(respect altitudes to high (e.g.,fuel duringoxygen cruisecontent and at high-altitude low altitudes) climb). and an elevated temperature for conditioned fuel at higher altitudesIn 2016, (e.g., a comprehensive during cruise and work high-altitude on transient exergyclimb). analysis for thermal management of subsystem componentsIn 2016, was a demonstratedcomprehensive and work discussed on transient [55]. Using exergy experimental analysis results,for thermal this study management formulated of asubsystem methodology components for system was level demonstrated optimization and to discuss developed a [55]. dynamic Using air experimental cycle machine results, (ACM) this model study atformulated the Air Force a methodology Research Laboratory’s for system level Modelling, optimization Simulation, to develop Analysis a dynamic and Testing air cycle (MSAT) machine lab. The(ACM) main model outcome at the of thisAir studyForce Research was a “total Laboratory’s efficiency parameter”Modelling, definedSimulation, by combining Analysis and the exergyTesting analysis.(MSAT) lab. This The parameter main outcome would helpof this future study simulation was a “total studies efficiency to be parameter” implemented defined on various by combining system architecturesthe exergy analysis. to generate This accurate parameter models would and help predictive future analysis. simulation studies to be implemented on variousIn 2017, system Rolls-Royce architectures proposed to generate different accurate embodiments models and for predictive high bypass analysis. geared turbofan engines TMS.In This 2017, patent, Rolls-Royce proposed proposed a heat exchange different system embodime for a powernts for gearboxhigh bypass in an geared aircraft turbofan engine in engines which atTMS. least This one heatpatent, transfer proposed device a isheat enclosed, exchange embedded system and/orfor a power attached gearbox with thein an casing aircraft of the engine power in gearbox,which at wherein least one at heat least transfer one airflow device is directed is enclosed to the, embedded one heat transfer and/or device attached for with thermally the casing controlling of the thepower power gearbox, gearbox. wherein Different at least heat one transfer airflow devices is directed can also to be the used one in heat this transfer idea for device cooling/heating for thermally the powercontrolling gearbox the in power different gearbox. operating Different conditions heat [56transfer,57]. devices can also be used in this idea for cooling/heatingFinally, in 2018, the power Rolls Royce gearbox has in presented different operating a novel idea conditions on a closed [56,57]. dynamic cooling circuit for engineFinally, thermal in management2018, Rolls Royce system. has In presented this embodiment, a novel idea a Thermal on a closed Energy dynamic Storage cooling (TES) systemcircuit for is designedengine thermal with two management cooling circuit system. to manage In this theembodi dynamicment, thermal a Thermal load Energy of the engine.Storage So,(TES) each system circuit is hasdesigned its own with compressor two cooling and circuit heat exchanger to manage to the be dyna controlledmic thermal to fully load absorb of the thermal engine. energy So, each received circuit byhas the its TESown [ 58compressor]. The idea and is veryheat promisingexchanger to as be both controlled steady state to fully and absorb transient thermal situations energy are received being considered.by the TES However,[58]. The idea the implementationis very promising considerations as both steady and state stability and transient issue should situations be considered are being with more details before the embodiment could go for the practical application. Appl. Sci. 2018, 8, 2044 11 of 16

The main ideas and milestones presented in the detailed design phase could be summarized as follow:

(1) CCA thermal management system to increase the performance of the gas turbine engine by cooling the bypass air (2) Fuel cooled system with two different thermal management loops with different temperature levels to deal with all flight points efficiently (3) Using different coolants (e.g., water, Therminol and thermally neutral heat transfer fluid (TNHTF)) to manage the thermal loads in different points of the gas turbine engine.

5. TMS Design Challenges and Potential Solutions for the Next Generation of Aircraft Engines From the historical review presented plus some general consideration of efﬁciency, energy management and coolant properties, it is possible to identify four different systematic concepts for the future of the TMS design and development as follows:

• Class I. Classic Development. This class of development would concentrate on the improvement of conventional TMS structure (Figure1) for GTEs. This improvement could be applied on minor system structure changes in order to get better efficiency, replacement of the components with more advanced and smart ones (e.g., new, effective heat exchangers [59,60]) and changing the oil and fuel path to get better thermal management efficiency. The advantages of this approach are that the theoretical fundamentals and the implementation infrastructures are well-developed and these modifications would not dictate huge cost to manufacturers. However, the main disadvantage of this approach is that probably it would just bring marginal improvement for the TMS as this area is already well investigated. • Class II. Centralized Development. This class would investigate the integrated TMS for the engine and airframe. It results in a centralized system that manages the thermal loads of the airframe and the engine simultaneously. So, the control algorithm and strategy would have more degree of freedom to deal with thermal loads. However, the main disadvantage is that it would be a high dimension multidisciplinary design problem with many parameters to select and tune simultaneously. Especially, in new advanced aircraft and engines it could be hardly affordable. • Class III. Revolutionary Development. This class of development would cover new ideas with major changes in the TMS. Using new coolants for the engine ([50] and Figure7) and ACC system ([35] and Figure2) are examples of this class. The positive points about this class of development is that it may be possible to get a noticeable jump in TMS effectiveness. In other words, a revolutionary approach may result in a revolutionary achievement. But, the problem with this class of development is that implementation of these approaches requires a significant change in the system manufacturing and infrastructures and it may not be interesting for the airliners and manufacturers. • Class IV. Cost-effective Development. This type of approaches will focus on the cost-effective solutions to enhance the TMS behaviour in the GTEs. The driving idea is to keep the hardware structure of the TMS unchanged (if possible) and to work on the solutions that could be implemented easily and/or with minimum changes in the current working systems. Enhancement of the control algorithms [61], splitters, valves and mixers [62] are some examples of cost-effective solutions. The relative merits of this class to the other classes is clear as the cost consideration is one of the most important issues in real-world applications. However, the correlation between the complexity and impact is the matter that should be discussed in these methods.

With respect to the above systematic approaches, the main drawbacks of the TMS development classes for the future aircraft engines are summarized in Table2. Appl. Sci. 2018, 8, 2044 12 of 16

Table 2. Drawbacks of the classes of TMS development for the future aero-engines.

Class of Main Drawbacks Development

• The new aircraft engines designs are being more complicated with much more extra heat to be dissipated [59,62]. • Geared Turbofans have power gearbox which makes the extra heat I Classic of the engine at least twice [59,62]. • Current TMS for aircraft engines are already working on their limits and any modiﬁcation would probably lead to just marginal improvements.

• New architectures for airframe and engines are more compact and have less space to accommodate conventional TMS components. • A multidisciplinary design optimization problem with many II Centralized parameters should be solved in this approach. • A centralized complicated control algorithm is required to satisfy both engine and airframe TMS requirements simultaneously.

• The chance of successfulness of a new idea is not too much in comparison with the time that should be spent on its development and investigation. III Revolutionary • The industry is reluctant to change the infrastructure as it would be expensive and time consuming. • A new system requires its own certiﬁcates and reliability tests before it can go into the production.

• Keeping the main structure and architecture and achieving noticeable improvement in the efﬁciency is not a simple straightforward task. IV Cost-effective • Detailed and probably complicated control algorithms should be developed, simulated and tested in different scenarios to conﬁrm the effectiveness of these methods.

Considering that the above-listed drawbacks in some classes could be sorted out with using ideas of other classes, the potential solutions for the improvement of the aircraft engine TMS for the future could be proposed as follow:

(1) The first effective solution could be utilizing both class I and IV simultaneously. It means that concentration of new studies should be on some minor modification on the hardware as well as working on algorithms and strategies. The superposition of these classes of development would enable the TMS designers to deal with several challenges in parallel. In one hand, the main architecture of the TMS would be kept approximately unchanged as the manufacturers and airliners prefer. On the other hand, minor changes in the components and cooling paths will give a huge degree of freedom to the control structure designers to play with more parameters in order to enhance the performance indices of the TMS and the engine noticeably. As an illustrative example, change the circulation path of the oil in parallel with defining a smart control algorithm for the variable valve positions may result in a noticeable enhancement in the TMS performance. (2) Another solution could be defining the classes II and III as a unique approach in an engineering optimization problem format. The main idea would be to define the structure of the integrated TMS, the order of components, the oil and fuel paths, the air flowrate and path and so forth, as the indices of a comprehensive objective function; then to use a powerful optimizer to deal with the Appl. Sci. 2018, 8, 2044 13 of 16

defined objective function. By tuning the mutation operator probability, the potential evolutionary developments would be investigated as well. However, definition of very smart constraints, penalty functions, penalty factors and objective function indices coefficients is a very vital step in successfulness of this approach. So, the main step in this solution is to define the problem statement very clear and with high level of details and accuracy; and then to select a suitable optimizer which can deal with such a big optimization problem with affordable run time.

Appl. Sci.Associated 2018, 8, x FOR with PEER this REVIEW classiﬁcation, it is helpful to consider some form of evaluation and ranking13 of 16 of proposedAssociated solutions. with For this this classification, purpose, it isit worthwhileis helpful to to consider analyse some the complexity-impact form of evaluation (CI) and correlation ranking ofwhich proposed is a practical solutions. index For in this selecting purpose, the feasibleit is worthwhile approach forto analyse dealing withthe complexity-impact challenges [63–65]. (CI) It is correlationan assessment which to valueis a practical the effort index that in should selecting bespend the feasible on a problem approach with for respectdealing towith impacts challenges of the [63–65].results. BasedIt is an on assessment whole review, to value analyses the effort and discussion that should presented be spend so on far, a Figure problem 10 summarizeswith respect theto impactsdevelopment of the classes results. and Based proposed on whole solutions review, with anal theiryses CI and index. discussion presented so far, Figure 10 summarizes the development classes and proposed solutions with their CI index. • Class I. Classic development is a straightforward approach with low complexity. However, it is • Classnot expected I. Classic that development it could have is a straightforward great impact on TMSapproach efﬁciency with low for the complexity. future engines. However, it is • notClasses expected II and that III. it couldCentralized have a andgreat revolutionary impact on TMS developments efficiency for are the complicated future engines. approaches • Classesin formulation II and III. and Centralized implementation. and revolutionary If they are developments successful, are they complicated would impact approaches the TMS in formulationsector noticeably. and implementation. If they are successful, they would impact the TMS sector • noticeably.Class IV. Cost-effective development is also directed towards design and implement and if the • Classbasic ideaIV. Cost-effective has great potential, development the impact is also would directed be noticeable towards asdesign well. and implement and if the basic idea has great potential, the impact would be noticeable as well. Potential Solutions: Potential Solutions: (1)(1) SolutionSolution I Iwill will promote promote the the effectiveness effectiveness of of class class I I by by combining combining it it with with class class II II of of development development (2)(2) SolutionSolution II II will will promote promote classes classes II II and and III III to to ac achievehieve more more impact impact to to the the TMS TMS development in in aerospaceaerospace sector. sector.

BothBoth solutions solutions are are lying lying on on the the diagonal diagonal direction direction of of the the CI CI matrix matrix which which makes makes sense sense from from computationalcomputational effort effort and and cost cost compensation compensation points points of of view view and and introduce introduce them them as as practical practical solutions solutions toto deal deal with with the the TMS TMS challenges challenges of of the future aircraft engines.

FigureFigure 10. 10. CICI analysis analysis of of development development classes classes and and proposed proposed solutions. solutions.

6. Conclusions A comprehensive historical review on design and development of thermal management systems for aircraft engines has been covered to identify the future challenges, opportunities and potential solutions for dealing with extra heats caused by advanced next generation of aircraft engines. The new ideas and approaches to enhance the TMS performance have been categorized in four different classes: classic, centralized, revolutionary and cost-effective developments. Each of these classes has its own pros and cons and will be faced with practical challenges in the development phase. Future engines involving more electric components will need the development of more sophisticated methods to deal with the extra heat. Consequently, two potential solutions have been proposed to deal with these challenges in the future: Appl. Sci. 2018, 8, 2044 14 of 16

(1) Combination of classes I (classic) and IV (cost-effective) to cope with much more extra heat generated by new advanced and geared turbofan engines with power gearbox, (2) Combination of classes II (centralized) and III (revolutionary) to deal with needs for energy management in the system and focusing on new smart components for more advanced smart thermal management systems.

Author Contributions: Conceptualization, Methodology, Investigation, S.J. and T.N.; Writing-Original Draft Preparation, S.J.; Writing-Review & Editing, T.N. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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