Optimization of Heat Exchangers for Intercooled Recuperated Aero Engines

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1 aerospace Article Optimization Heat Exchangers Intercooled Recuperated Aero Enges Dimitrios Misirlis 1, *, Zon Vlahostergios 2, Michael Flouros 3, Christa Salpgidou 2, Stefan Donnerhack 3, Apostolos Goulas 2 Kyros Yakthos 2 1 Department Mechanical Engeerg, Technological Educational Institute (TEI) Central Macedonia, Serres 62124, Greece 2 Laboratory Fluid Mechanics & Turbomachery, Department Mechanical Engeerg, Aristotle University ssaloniki, ssaloniki 54124, Greece; zonv@eng.auth.gr (Z.V.); csalpgidou@eng.auth.gr (C.S.); goulas@auth.gr (A.G.); kyak@auth.gr (K.Y.) 3 MTU Aero Enges AG, Munich 80995, Germany; michael.flouros@mtu.de (M.F.); stefan.donnerhack@mtu.de (S.D.) * Correspondence: misirlis@eng.auth.gr; Tel.: Academic Editors: Simon Blakey, Sigrun Mats, Paul Brok, Volker Grewe Simon Christie Received: 29 December 2016; Accepted: 26 February 2017; Published: 13 March 2017 Abstract: In framework European research project LEMCOTEC, a section was devoted to furr optimization recuperation system Intercooled Recuperated Aero enge ( enge) concept, MTU Aero Enges AG. This concept is based on an advanced rmodynamic combg both tercoolg recuperation. present work is focused only on recuperation process. This is carried out through a system exchangers mounted side hot-gas exhaust nozzle, providg fuel economy reduced pollutant emissions. optimization recuperation system was permed usg computational fluid dynamics (CFD) computations, experimental measurements rmodynamic analysis a wide range enge operatg. A customized numerical tool was developed based on an advanced porosity model approach. exchangers were modeled as porous media predefed transfer loss behaviour could also corporate major critical exchanger design decisions CFD computations. optimization resulted two completely new novative exchanger concepts, named as CORN (COnical Recuperative Nozzle) STARTREC (STraight AnnulaR rmal RECuperator), provided significant benefits terms fuel consumption, pollutants emission weight reduction compared to more conventional exchanger designs, thus provg that furr optimization potential this technology exists. Keywords: exchangers; porosity model; recuperation; aero enge optimization 1. Introduction enhancement aero enge permance reduction fuel consumption pollutant emissions have always been focal pot tense engeerg optimization efts both environmental economic reasons. Currently, as air traffic is growg at an annual rate 5% global awareness regardg environmental issues arises, necessity to improve efficiency aero enges becomes constantly more tense evident. For se reasons, a large number research projects have been itiated funded by European Union collaboration between ma European aero enge manufacturers, Universities Research Institutes. se projects have been maly aimg at design novative aero enge concepts achievg improved permance, reduced fuel consumption reduced pollutant emissions, thus promotg fulfillment year 2020 ; doi: /aerospace

2 Aerospace 2017, 4, 4, promotg fulfillment year 2020 ACARE (Advisory Council Aviation Research novation ACARE (Advisory Europe) Council targets Aviation 20% reduction Research novation Europe) targets 20% reduction CO2 80% NOx emissions, compared to year CO levels, 80% as mentioned NO x emissions, also compared [1,2]. to year 2000 levels, as mentioned also [1,2]. In In this direction, direction, significant significant eft eft has been has focused been focused on development on development alternative technologies alternative technologies ir subsequent ir subsequent corporation corporation novative novative aero enge aero concepts enge concepts high-bypass high-bypass turban turban enges. enges. Such a technology Such a technology concept concept is implemented is implemented Intercooled Intercooled Recuperative Recuperative Aero enge Aero enge ( enge) ( configuration, enge) configuration, is presented is presented 1a, combes 1a, both tercoolg combes both tercoolg recuperation recuperation is one key-objectives is one EU key-objectives funded LEMCOTEC EU Collaborative funded LEMCOTEC Project [3], Collaborative targetg reduction Project [3], air-traffic targetg emissions reduction by creasg air-traffic emissions rmalby efficiency creasg aero rmal enges. efficiency present aero workenges. refers to activities present carried work refers out with to activities this project, carried part out with this has project, been presented part also has [4]. been presented also [4]. ` Intercooled Intercooled Recuperative Recuperative Aero Aero enge enge [5]; [5]; A matrix matrix MTU- MTU- exchanger exchanger 4/3/4 4/3/4 elliptical elliptical tubes tubes staggered staggered arrangement arrangement [6]. [6]. enge concept has been also vestigated with framework previously enge concept has been also vestigated with framework previously successfully completed major European research projects such as Component Validator successfully completed major European research projects such as Component Validator Environmentally-friendly Aero-Enge CLEAN/FP5, NEW Aero enge Core concepts NEWAC/FP6 Environmentally-friendly Aero-Enge CLEAN/FP5, NEW Aero enge Core concepts NEWAC/FP6 Low Emissions Core-Enge Technologies LEMCOTEC/FP7. Additional mation regardg Low Emissions Core-Enge Technologies LEMCOTEC/FP7. Additional mation regardg this concept can be found works [7 9]. this concept can be found works [7 9]. More specifically, this concept is based on a system exchangers mounted side More specifically, this concept is based on a system exchangers mounted side hot-gas exhaust nozzle an tercooler placed between compressor stages. hot-gas exhaust nozzle an tercooler placed between compressor stages. exhaust gases is recovered downstream exit low- turbe pres air exhaust gases is recovered downstream exit low- turbe pres air enters combustion chamber with creased enthalpy, providg fuel economy enters combustion chamber with creased enthalpy, providg fuel economy reduced reduced pollutant emissions. pollutant emissions. Regardg recuperation system, exchangers operate under harsh Regardg recuperation system, exchangers operate under harsh combg both high temperatures. As a result, ir rmo-mechanical permance combg both high temperatures. As a result, ir rmo-mechanical permance reliability is primary importance, as presented [10]. Furrmore, imposed reliability affect is potential primary benefit importance, as aero presented enge [10]. rmodynamics Furrmore, imposed while exchanger s affect own potential weight reduces benefit a part aero enge rmodynamic rmodynamics efficiency while improvement. exchanger s For se own reasons, weight reduces optimization a part rmodynamic recuperation system efficiency is critical improvement. importance For se beneficial reasons, operation optimization enge recuperation concept, system as poted is out critical [11 13]. importance beneficial operation In enge addition, concept, use as poted tercoolg, out [11 13]. a technology perms at its best rmodynamic In addition, use facilitate tercoolg, recuperation, a technology contributes perms to a reduction at its best rmodynamic high compressor work leadg facilitate to a furr recuperation, improvement contributes to aero a reduction enge efficiency. high Intercoolg compressor also makes work leadg recuperation to a furr more improvement efficient as it creases aero enge temperature efficiency. Intercoolg difference between also makes recuperation hot exhaust gas more efficient high as it creases compressor temperature (HPC) difference exit air. between Both tercoolg hot exhaust recuperation gas high are utilisg specially designed tegrated exchangers.

3 3 16 compressor (HPC) exit air. Both tercoolg recuperation are utilisg specially designed tegrated exchangers. 2. Optimization Heat Exchanger Concepts present work is focused only on recuperation process more specifically on optimization both geometry exchangers ir stallation with exhaust nozzle, order to maximize recuperation benefits, specifically targeted an enge. More details about enge concept can also be found [2,9,14,15]. implementation recuperation an enge is permed through mountg a number exchangers side hot-gas exhaust nozzle, downstream low- turbe. basic exchanger (HEX) enge, was vented developed by MTU Aero Enges AG was used itial HEX permance studies, is presented 1b. It consists elliptically priled tubes placed a 4/3/4 staggered arrangement targetg high transfer rates reduced. Additional mation HEX operation can be found [10]. HEX tubes geometry arrangement can significantly affect turbe expansion thus degrade produced turbe work due to imposed. overall exchanger design plays a critical role sce above mentioned are lked directly to available exchange surface its geometry, turn strongly affects HEX effectiveness exhaust gas waste exploitation. As a result, to achieve maximum recuperation benefits, a compromise between HEX design parameters is required. Towards this direction, development accurate validated numerical tools is particular importance sce y can provide time- cost-efficient design solutions can lead to a priori estimation HEX major operational characteristics (i.e., effectiveness). se operational characteristics can n be tegrated a rmodynamic analysis aero enge order to assess recuperation effects on aero enge efficiency fuel consumption. Thus, se tools can significantly contribute to development, assessment optimization various novative exchanger concepts orwise could not be afdable laboratory (due to time cost limitations) Heat Exchanger Recuperator Porosity Model Approach development optimization novative exchanger concepts, focused on Intercooled Recuperated Aero Enge, are an evolution origal MTU HEX design, are presented here. vestigation optimization reported this work were permed with use 2D 3D CFD computations, experimental measurements rmodynamic analysis, a wide range enge operatg. optimization activities were maly based on use an novative customizable 3D numerical tool could efficiently model transfer loss permance exchangers enge stallation. numerical tool was based on an advanced porosity model approach exchangers were modeled as porous media predefed transfer loss behavior, was determed by correlations specifically developed recuperator HEX. use a porous media methodology modelg exchangers provides significant advantages sce it can facilitate corporation exchangers macroscopic transfer loss behavior 3D CFD models overall aero enge stallation (cludg hot-gas exhaust nozzle precise mountg recuperator system side aero enge). In addition, use numerical tool can be corporated CFD models, by beg tegrated to fluid momentum energy transport equations 3D CFD computations. This tool is able to provide consistent numerical solutions complicated side recuperator nozzle stallation. Without use presented numerical tool, 3D CFD computations precise recuperator detailed geometry could not be achieved due to extremely high CPU memory requirements sce more than one

4 4 16 billion computational pots are estimated be necessary accurate representation overall nozzle recuperator geometry. This approach has been successfully applied past as presented [14,15] it was shown that use porous media methods combation with CFD computations can lead to accurate computationally afdable CFD models can provide basis optimization overall geometrical configuration. In present approach, most important part is accurate corporation, through appropriate correlations, overall transfer macroscopic permance exchangers. Here, se correlations have been derived through detailed CFD computations use experimental measurements, as presented detail [16,17], were numerically corporated CFD models overall aero enge stallations by addg appropriate source terms momentum energy transport equations. se correlations cluded effect both currents, i.e., ner-cold air outer-hot-gas s, on, toger with achieved transfer between m Hot Side Pressure Loss Model hot-gas, per unit length exchanger, were modeled usg followg equation ( an isotropic mulation where same coefficients are used all three directions space): P/L = [(a 0 + a 1 ν)µu + (b 0 + b 1 ν + b 2 ν 2 )ρu 2 ]/L (1) Equation (1) is a modified mulation Darcy Forchheimer drop law with drop coefficients beg functions kematic viscosity thus, functions both temperature. values coefficients a 0, a 1 b 0, b 1, b 2 correspond to viscous ertial loss coefficients were derived through a trend le curve fittg process. coefficients were derived through detailed 2D CFD computations on exchanger core outer ( hot gas ), were validated agast both experimental isormal non-isormal measurements. se coefficients were calibrated outer coverg whole extent operational range exchanger (i.e., Take-Off, Average Cruise Max Climb ) Cold Side Pressure Loss Model Regardg ner, cold-air, exchanger, y were modeled by usg a CFD-derived loss per tube length coefficient, f, given by followg equation: f = C 1 (ln Re) C 2 (2) This was calibrated relation to ner mean Reynolds number based on detailed 3D CFD computations, were carried out takg to account precise ner geometry tubes Heat Transfer Model clusion transfer modelg exchanger required calculation ner outer transfer coefficients. An approach based on a Nusselt Prtl Reynolds number correlation was used, given by Equation (3): Nu = CPr m Re n (3) where coefficients C, m n were calibrated through detailed CFD computations experiments separately each ner outer streams, while all properties were calculated at mean temperatures. It must be mentioned that similar analysis was permed various tube core geometries, correspondg to different number tubes staggered arrangement (e.g., exchanger cores 3/2/3, 4/3/4, 5/4/5, 6/5/6 tubes staggered arrangement different tubes spacg have been tested, tube spacg has been almost doubled relation to itial

5 5 16 MTU design) various loss transfer correlations were derived each tube s core configuration, Aerospace 2017, 4, 14 were exploited optimization recuperator geometrical characteristics se analyses also provided data most efficient selection exchangers core both CORN STARTREC (COnical (STraight Recuperative AnnulaR Nozzle) rmal RECuperator) STARTREC (STraight concepts, AnnulaR rmal different RECuperator) numbers concepts, tubes arrangement different were numbers used tubes recuperators arrangement cores. were used recuperators cores. As next step analysis, transfer correlations were cluded momentum energy Reynolds Averaged Navier Stokes equations Fluent CFD stware [18] as as additional source terms, as as presented 2, through 2, through specially specially programmed User Defed User Defed Functions Functions (UDF). (UDF). 2. Implementation loss transfer source terms momentum 2. Implementation loss transfer source terms momentum energy energy transport equations. transport equations. set equations presented 2 is solved outer. rmal energy exchange set equations two exchanger presented streams 2 is is solved achieved through outer. energy source rmal term energy given exchange by Equation (4), two exchanger streams is achieved through energy source term given by Equation (4), U overall Uoverall (T( TT ner T )S ) S exchange (4) (4) where T ner T to refers to temperature ner, S exchange S is exchange surface per unit volume exchanger (m (m 2 2 /m 3 3 ), ), calculatedas as ratio ratio total totalouter outersurface surface exchanger tubes tubes to to occupied occupied volume volume exchanger, exchanger, while while U overall U corresponds overall corresponds to to overall transfer coefficient is calculated by Equation (5), overall transfer coefficient is calculated by Equation (5), 1 1 = 1 = (5) U overall h (5) U outer h h ner overall where h where ner h h outer are ner outer transfer coefficients calculated separately each ner h outer are ner outer transfer coefficients calculated separately current usg Equation (3). each current usg Equation (3). For appropriate modelg ner (cold air ), two additional 1D transport For appropriate modelg ner (cold air ), two additional 1D transport equations, Equations (6) (7), were coupled with equations presented 2. se equations, Equations (6) (7), were coupled with equations presented 2. se transport equations model transport total specific enthalpy total transport equations model transport total specific enthalpy total ner were also implemented CFD computations through use UDF Fluent ner were also implemented CFD computations through use UDF CFD stware. Fluent CFD stware. For ner, calculation Re Pr numbers at every computational cell For ner, calculation numbers at every computational cell temperature T temperature ner must be known. ner temperature, T T must be known. ner temperature, ner is provided by 1D transport ner T ner is provided by 1D Equation (6), via calculation total specific enthalpy, given Equation (8). transport Equation (6), via calculation total specific enthalpy, given Equation (8). l [ρu nerh [ ρ u total ] = U nerhtotal ] = (T Uoverall ( T)S Tner T ) S exchange (6) exchange (6) l 2 l P tot_ner = f ρu2 ner (7) f 2D ρu ner Ptot _ ner = (7) l 2D outer ner

6 6 16 h total = h static u2 ner (8) In Equation (6), right h side is calculated from computations outer. This is a parameter reflectg geometrical structure exchanger core is directly lked to selection tubes staggered arrangement number. ner cold air velocity, u ner, is calculated from prescribed ner mass side tubes, takg to consideration density variations due to temperature along tubes. ner total along tube were obtaed by numerical tegration Equation (7). ma advantage presented porosity model relation to previous porosity models used similar setups, as presented [19,20], is that both currents are cluded model equations thus, derived model can simultaneously provide both ner outer. In addition, specific model can calculate 3D distribution achieved exchange between outer hot-gas ner-cold air, somethg cannot be computed through a literature-based effectiveness-ntu method. Furrmore, direct effect exchanger core geometry, such as shape size tubes, on achieved transfer can be straightwardly computed; actual exchanger effectiveness can be calculated n be taken to consideration aero enge rmodynamic analysis. Some additional details about customizable numerical tool can be found [21]. Additionally, this novative customizable 3D numerical tool can also corporate major critical exchanger design parameters CFD computations, by supportg numerical tegration exchanger geometrical characteristics (e.g., tubes collector numbers, streams splittg mixg, tubes core arrangement). It should be mentioned aga here that use currently presented, porosity-model-based approach exchanger geometry, is almost obligatory sce clusion precise exchanger geometry a CFD model would result an extremely high number computational pots could not be computationally afdable terms CPU memory (RAM) resources (more than 1 billion computational pots would be required accurate modelg overall exhaust nozzle stallation with exchangers recuperators stallation). On or h, use a porosity model approach where recuperator exchangers are modeled as regions predefed loss transfer characteristics, allows use an afdable computational grid to obta an accurate modelg overall exhaust nozzle configuration can be used furr engeerg analysis. Moreover, if someone attempted to model a small part exchanger core through detailed CFD modelg both ner outer s, toger with modelg tubes walls, attempted to model same part exchanger core with use currently presented approach, required time convergence detailed CFD model would be larger by a factor 80 while both models would provide results similar accuracy, as presented detail [21]. 3. Results Recuperation Concepts As a first stage present vestigation, aementioned numerical model was applied to NEWAC nozzle configuration, shown 3a, related to NEW Aero enge Core concepts/newac research project.

7 Aerospace 2017, 2017, 4, 14 4, 14 Aerospace (c) (d) 3. NEWAC nozzle configuration; computational grid nozzle; (c,d) swirl effects 3. NEWAC nozzle configuration; computational grid nozzle; (c,d) swirl effects NEWAC nozzle, different colors dicate velocity magnitude value configuration (red: high, blue: (c) (d)configuration (red: high, low). NEWAC nozzle, different colors dicate velocity magnitude value blue: low). 3. NEWAC nozzle configuration; computational grid nozzle; (c,d) swirl effects This nozzle configuration tovelocity a quarter overall nozzle stallation takg NEWAC nozzle, differentcorresponds colors dicate magnitude value configuration (red: high, blue: to account symmetric arrangement exchangers. NEWAC nozzle configuration low). This nozzle configuration corresponds to a quarter overall nozzle stallation takg to consists four exchangers (HEXs) per exchangers. quarter exhaust nozzle. configuration HEXs are account symmetric arrangement NEWAC nozzle consists This nozzle configuration corresponds a quarter overall nozzle stallation takgwere to placed at angles relation toto axial direction. CFD computations four exchangers (HEXs) per quarter exhaust nozzle. HEXs are placed at account symmetric turbulence exchangers. NEWAC configuration permed usg SST arrangement (Shear Stress Transport) model Menter, nozzle details can be found -17 anglesconsists relation to (HEXs) axial direction. CFDnozzle. computations were permed four model exchangers per quarter exhaust HEXs are [22], a CFD overall nozzle stallation, is presented 3b, a wide usgrange SST (Shear Stress Transport) turbulence model Menter, details can be found [22], a placedathex angles relation axial direction. CFD computations as presented to [10] exchangers permance were was CFD cluded model through overall nozzle stallation, turbulence is presented 3b, a wide HEX permed usg (Shear Stress Transport) model Menter, details can beenergy found SST implementation additional source terms momentum range as presented [10] exchangers permance was cluded through [22], a CFD model overall nozzle stallation, is presented 3b, a wide equations. Additional details can be found [23 25]. As it can be seen 3c,d, strong swirl is range HEX as presented [10]momentum exchangers was implementation additional terms energy Additional details med side NEWACsource nozzle, creases equations. causes apermance deterioration cluded through implementation additional source terms momentum energy found HEXs permance be recuperation benefits aero swirl engeis. In s can be [23 25]. As it can seen 3c,d, strong med side 3c,d NEWAC equations. Additional details can be found [23 25]. As it can after be seen 3c,d,turbe. strong swirl also s 4 7, white arrow dicates hot-gas low Someis nozzle, creases causes a deterioration HEXs permance med side NEWAC nozzle, creases side aexhaust deterioration additional mation velocity temperature s nozzle, recuperation benefits regardg aero enge. In 3c,d alsocauses 4 7, white HEXs permance recuperation enge non-dimensionalized with let, benefits is presented aero 4a,b.. In s 3c,d arrow dicates hot-gas after low turbe. Some additional mation regardg also s 4 7, white arrow dicates hot-gas after low turbe. Some velocity temperature side exhaust nozzle, non-dimensionalized with let, additional mation regardg velocity temperature side exhaust nozzle, is presented 4a,b. non-dimensionalized with let, is presented 4a,b. 4. Cont.

8 NEWAC nozzle configuration static statictemperature temperature non-dimensional 4. NEWAC configuration velocity; velocity; non-dimensional 4. NEWAC nozzle nozzle configuration velocity; static temperature non-dimensional distributions. distributions. distributions. waswas followed by second stepaimg aimg to optimize HEXs stallation side above followed byaaasecond secondstep step aimg to HEXs stallation side above above was followed by tooptimize optimize HEXs stallation side hot-gas exhaust nozzle, through additional similar CFD computations use hot-gas exhaust nozzle,through through additional additional similar similar CFD use use hot-gas exhaust nozzle, CFD computations computations customizable numerical tool. optimization optimization efts efts resulted completely newnew novative customizable numerical tool. resulted two completely novative customizable numerical tool. optimization efts resulted two two completely new novative HEX concepts, named as CORN (COnical Recuperative Nozzle) STARTREC (STraight AnnulaR HEX concepts, named as CORN (COnical Recuperative Nozzle) STARTREC (STraight HEX concepts, named as CORN (COnical Recuperative Nozzle) STARTREC (STraight AnnulaR AnnulaR rmal RECuperator), presented s 5 66 respectively. respectively. rmal rmal RECuperator), RECuperator), presented presented s s55 6 respectively. Both concepts were based on MTU MTU HEX origal origal tube geometry. tubes staggered Both Both concepts concepts were were based based on on MTU HEX HEX origal tube tube geometry. geometry. tubes tubes staggered staggered arrangement number rows were optimized through use presented numerical arrangement number rows were optimized through use presented numerical arrangement number rows were optimized through use presented numerical tool. In addition, number tube collectors was also optimized order to achieve optimum tool. number was also order to tool. In In addition, addition, number tube tube collectors collectors was also optimized optimized order to achieve achieve optimum optimum combation ner regardg transfer. This resulted combation ner regardg transfer. This resulted combation nertube regardg transfer.(this Thisconcept resulted use eight collectors CORN four tube collectors STARTREC use eight tube collectors CORN four tube collectors STARTREC (this concept consists consists two banks different core staggered density). As a result, use eight tube collectors tube CORN four arrangement tube collectors STARTREC (this concept two banks different core arrangement density). As adensity). result, As acfd models, CFDmodels, a 45 tube sector wasstaggered used core CORN a 90 arrangement sector STARTREC, as shown s consists two banks different tube staggered result, sector 8a, by applyg appropriate periodicity computations. acfd 45 7a sector was used CORN a 90 STARTREC, as shown s 7a 8a, models, a 45 sector was used CORN a 90 sector STARTREC, as shown s by periodicity periodicity computations. 7a applyg 8a, byappropriate applyg appropriate computations. 5. CORN (COnical Recuperative Nozzle) geometry currents per 45 sector CORN CORN (COnical (COnical Recuperative Recuperative Nozzle) Nozzle) geometry geometry currents currents per per sector. sector.

9 STARTREC (STraight AnnulaR rmal RECuperator) geometry; currents per 6. STARTREC (STraight AnnulaR rmalrecuperator) geometry; currents per 90 sector. 90 sector. 6. STARTREC (STraight AnnulaR rmal RECuperator) geometry; currents per 90 sector. se concepts were based on use novative exchanger designs, followg an annular tubes arrangement exhaust nozzle, leadg to reduction (followg STARTREC se concepts were basedside on use novative exchanger designs, an annular se concepts were based on use novative exchanger designs, followg an concept) or complete elimation ( CORN concept) swirl effects achievg a tubes arrangement side exhaust nozzle, leadg to reduction ( STARTRECmore concept) annular tubes arrangement side exhaust nozzle, leadg to reduction ( STARTREC homogeneous distribution with reduced secondary relation to NEWAC or complete elimation ( CORN concept) swirl effects achievg a more homogeneous concept) or completeaselimation ( CORN concept) swirl effects achievg a more nozzle configuration, presented s 7 8. distribution secondary relation NEWAC homogeneouswith reduced distribution with reduced secondary to relationnozzle to configuration, NEWAC as presented s 7 as 8. nozzle configuration, presented s Cont.

10 (c) (c) 7. CORN computational grid grid (45 (45 sector); non-dimensional sector); 7. CORN computational static statictemperature temperature non-dimensional 7. CORN computational distribution; (c) velocity streamles. grid (45 sector); static temperature non-dimensional distribution; (c) velocity streamles. distribution; (c) velocity streamles. 8. Cont.

11 11 16 Aerospace 2017, 4, 14 Aerospace 2017, 4, (c) (c) sector); 8. STARTREC computational grid (90 sector); static temperature non-dimensional 8. STARTREC computational grid (90 static temperature non-dimensional 8. STARTREC computational grid (90 sector); static temperature non-dimensional distribution; (c) velocity streamles. distribution; velocity streamles. distribution; (c) (c) velocity streamles. fal stage study, basic characteristics three exchanger concepts (i.e., AtAt fal stage study, basic characteristics three exchanger concepts (i.e., At fal stage study, basic characteristics three exchanger concepts (i.e., NEWAC, CORN STARTREC) were corporated to GasTurb 11 stware [26]. More NEWAC, STARTREC)were were corporated GasTurb 11 stware [26]. More NEWAC,CORN CORN STARTREC) corporated toto GasTurb 11 stware [26]. More specifically, specifically, CFD-computed resultswere were corporated calculation calculation specifically, CFD-computed results corporated CFD-computed results were corporated calculation rmodynamic rmodynamic is presented 9, through coefficients ner rmodynamic is presented 9, through coefficients (cold-air), ner is presented 9, through appropriate coefficients appropriate appropriate ner (cold-air), outer (hot-gas) exchanger (cold-air), outer effectiveness. (hot-gas) exchanger outer (hot-gas) exchanger corporation se effectiveness. corporation se coefficients rmodynamic analysis effectiveness. corporation se coefficients rmodynamic analysis coefficients rmodynamic analysis quantifies recuperator effect on waste quantifies recuperator effect waste exploitation from hot-gas nozzle preg quantifies recuperator effect onon waste exploitation from hot-gas nozzle preg exploitation from hot-gas nozzle preg compressor discharge air bee combustion, compressor discharge air bee combustion, leadg to reduced fuel mass requirement, leadg compressor discharge air bee combustion, leadg to reduced fuel mass imposed requirement, to reduced fuel mass requirement, impact recuperator impact recuperator imposed both (ner-cold impact recuperator imposed As both currents (ner-cold air,air, outer-hot both currents (ner-cold air, outer-hot gas). a result, currents rmodynamic isouter-hot properly gas). As a result, rmodynamic is properly adapted, from necessary data gas). As a result, rmodynamic is properly adapted, fuel necessary data adapted, from necessary data calculation from specific consumption can be. calculation specific fuel consumption can be extracted from Equation (9) where is extracted calculation Equation specific consumption can be extracted from Equation (9) wheredetails is can m f is mass from (9) fuel where consumed fuel. Additional be mass consumed fuel. Additional details can be found [20]. mass [20]. consumed fuel. Additional details can be found [20]. found. mf = = =SFC Thrust ℎ ℎ (9) (9)(9) Recuperation Recuperation 9. Cont.

12 rmodynamic rmodynamic Gas Gas Turb Turb Enge Enge Average Average Cruise Cruise schematic schematic illustration illustration enge enge basic basic components components.. stations stations numberg numberg is is followg followg stard stard GasTurb GasTurb numeration. numeration. optimization studies aimed to design optimized exchanger concepts stallation optimization studies aimed to design optimized exchanger concepts stallation enge. For comparison results, ma data enge were kept enge. For comparison results, ma data enge were kept same while recuperator characteristics were updated each time to correspond to new same while recuperator characteristics were updated each time to correspond to new proposed proposed exchanger characteristics. exchanger characteristics. 4. Discussion 4. Discussion All CFD results analysis presented current paper aim at accurate calculation All CFD results analysis presented current paper aim at accurate calculation different recuperator efficiencies accurate quantification ner outer different recuperator efficiencies accurate quantification ner outer, order to assess ir combed impact on overall rmodynamic an, order to assess ir combed impact on overall rmodynamic an aero enge, will utilize recuperation technology, such as enge. As a result, aero enge, will utilize recuperation technology, such as enge. As a result, permance three recuperator concepts was quantified compared relation to a permance three recuperator concepts was quantified compared relation to a conventional non-tercooled non-recuperated aero enge similar technology level with conventional non-tercooled non-recuperated aero enge similar technology level with. outcome this assessment is summarized Table 1.. outcome this assessment is summarized Table 1. Table Comparative results recuperation concepts. SFC Reduction ( Relation to a Heat Exchangers Weight Reduction Case SFCConventional Reduction ( Aero Relation Enge) to a Heat Exchangers ( Relation Weight to NEWAC) Reduction Case Conventional Aero Enge) ( Relation to NEWAC) NEWAC nozzle 12.3% 0 NEWAC CORN nozzle 12.3% 13.1% ~5% 0 STARTREC CORN 13.1% 9.1% ~5% ~50% STARTREC 9.1% ~50% From Table 1, it can be seen that two new recuperator concepts provide significant benefits, relation From Table to 1, reference it can be seen NEWAC that recuperator, two new recuperator regardg concepts specific provide fuel consumption significant benefits, (SFC) relation weight two to reference very important NEWAC recuperator, parameters regardg an environmentally specific fuel friendly consumption aero enge (SFC) sce weight two SFC is directly very related important to parameters aero enge fuel an environmentally burn thus, to friendly pollutant aero enge emissions sce while SFC is directly recuperator relatedweight to aero is a enge parameter fuel burn strongly thus, affectg to pollutant implementation emissions while potential recuperator this weight technology. is a parameter strongly affectg implementation potential this technology. More specifically, NEWAC concept resulted a 12.3% SFC reduction relation to a conventional non-tercooled aero enge. furr improvement NEWAC recuperator hence permance improvement, was materialized by two new troduced concepts, provided some some terestg fdgs. fdgs. First First all, all, CORN CORN concept concept showed showed best behavior best regardg behavior regardg SFC reduction SFC by reduction reachgby 13.1% reachg ( relation 13.1% to ( arelation non-recuperated to a non-recuperated aero enge) aero aenge) small weight a reduction small weight ~5% reduction relation ~5% to NEWAC. relation On to NEWAC. or h, On STARTREC or h, concept, STARTREC although itconcept, presented although an SFCit reduction presented an only SFC 9.1%, reduction provided aonly significant 9.1%, provided recuperator a significant weight reduction recuperator that can weight reachreduction 50% relation that can toreach NEWAC 50% relation recuperator concept. NEWAC recuperator concept.

13 13 16 basic advantage se two new recuperator designs, comparison to NEWAC concept, is that due to ir optimized geometrical shape, is based on an annular axisymmetric recuperator idea, y are more suitable tegration exhaust nozzle an aero enge are thus able to completely elimate secondary swirl effects, are a major contributor to external hot-gas. Additionally, y both reta itial geometrical elliptical tubes shape NEWAC recuperator, is an advantage regardg mimization external sce external (hot-gas) separation around recuperator tubes core recuperator is mimized transfer between two recuperator streams is much more effective. Apart from external, annular design positiong ternal collectors also favor decrease ternal also affect SFC reduction. Regardg weight reduction, a compromise must be made between recuperator effectiveness, weight. Sce recuperator effectiveness is strongly lked with recuperator exchange surface, a lighter recuperator would have worse effectiveness as a result would exploit less hot-gas waste energy, but on or h would provide a better behavior regardg external ternal. As a result, a general conclusion cannot be drawn regardg recuperator design (CORN or STARTREC) is always optimal sce this depends on specific mission is designed. Fally, it must be mentioned that studies concepts presented current work constitute fundamental basis new novative recuperator designs that are currently beg furr developed assessed (toger with some or new recuperator concepts designs) with framework Ultra Low emission Technology Innovations Mid-century Aircraft Turbe Enges (ULTIMATE) Horizon2020 EU project [27], somethg that also proves strong optimization potential recuperation technology. se studies have, so far, provided new novative aero enge architectures that will employ currently evolvg recuperator technology will set technology level year 2050 aero enges. se new aero enge architectures are designed order to be le with ACARE goals dem greener environmentally friendly aero enges. 5. Conclusions In present paper, ma activities regardg corporation recuperation aero enges, permed EU funded LEMCOTEC research project are presented. se activities were focused on furr optimization exchangers recuperation system Intercooled Recuperated Aero enge ( enge) concept, was developed by MTU Aero Enges AG. vestigation optimization efts present work were permed with use 2D/3D CFD computations, experimental measurements rmodynamic analysis a wide range enge operatg. ma activities that can be highlighted current work are followg: optimization procedure reference MTU exhcanger was based on development a customizable numerical tool can efficiently model exchanger permance regardg transfer enhancement mimization, specially designed aero enge applications. described numerical tool is based on an advanced porosity model approach where exchanger core is modeled as a porous media predifed transfer behavior. Additionally, derived porosity model is able to provide accurate results to a wide range (from laboratory to flight ) to corporate major critical recuperator design decisions CFD computations, through couplg momentum energy transport equations. optimization efts resulted two completely new novative recuperator design concepts, named as CORN (COnical Recuperative Nozzle) STARTREC (STraight AnnulaR rmal RECuperator). se concepts were followg an annular tubes recuperator axisymetric core design side hot-gas exhaust nozzle.

14 14 16 proposed recuperators were assesed a rmodynamic analysis providg improved SFC significant benefits terms pollutants emission weight reduction comparison to origal NEWAC recuperator stallation enge, thus promotg fullfillment ACARE goals towards greener environmentally friendly aero enges. Acknowledgments: This work is fancially supported by EU under LEMCOTEC Low Emissions Core-Enge Technologies, a Collaborative Project co-funded by European Commission with Seventh Framework Programme ( ) under Grant Agreement No Christa Salpgidou would like to thank Alexer S. Onassis Public Benefit Foundation scholarship. Author Contributions: Dimitrios Misirlis, Zon Vlahostergios, Christa Salpgidou, Kyros Yakthos Apostolos Goulas contributed to development numerical tool, CFD computations, rmodynamic analysis, data post processg analysis paper writg. Michael Flouros Stefan Donnerhack provided dustrial feedback advice, technical specifications regardg enge. Conflicts Interest: authors declare no conflict terest. Nomenclature a 0, a 1 Viscous loss coefficients b 0, b 1, b 2 Inertial loss coefficients C, m, n Nusselt number coefficients C 1, C 2 Calibration constants ner f Pressure loss per tube length coefficient h outer, h ner Outer ner transfer coefficients, W m 2 K 1 h total, h static Total static specific enthalpy, J kg 1 L Length exchanger, m. m f Mass consumed fuel Nu Nusselt number P tot_ner Total ner, Pa Pr Prtl number Re Reynolds number S exchange Heat exchange surface per unit volume exchanger, m2 m 3 T Temperature, K T ner Temperature ner, K v Kematic viscosity, m2 s 1 U i Cartesian velocity vector, m s 1 U overall Overall transfer coefficient, W m 2 K 1 u i u j Reynolds stresses, m2 s 2 u ner Inner cold air velocity, m s 1 x i Cartesian coordates P Hot-gas, Pa µ Molecular viscosity, kg m 1 s 1 ρ Fluid density, kg m 3 CORN COnical Recuperative Nozzle HEX Heat EXchanger Intercooled Recuperative Aero-enge LEMCOTEC Low Emissions Core-Enge Technologies NEWAC NEW Aero enge Core concepts SFC Specific Fuel Consumption STARTREC STraight AnnulaR rmal RECuperator ULTIMATE Ultra Low emission Technology Innovations Mid-century Aircraft Turbe Enges References 1. Bock, S.; Horn, W.; Wilfert, G.; Sieber, J. Active Core Technology with NEWAC Research Program Cleaner More Efficient Aero Enges. In Proceedgs European Air Space Conference, Berl, Germany, September 2007; CEAS

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