Numerical Investigation on the Thermal Behaviour of a Lox/LCH4 Demonstrator Cooling System

aerospace

Article Numerical Investigation on the Thermal Behaviour of a LOx/LCH4 Demonstrator Cooling System

Daniele Ricci * , Francesco Battista and Manrico Fragiacomo

CIRA, Centro Italiano Ricerche Aerospaziali, Via Maiorise, 81043 Capua, Italy; [email protected] (F.B.); [email protected] (M.F.) * Correspondence: [email protected]; Tel.: +39-0823-623096; Fax: +39-0823-623100

Abstract: Reliability of liquid rocket engines is strictly connected with the successful operation of cooling jackets, able to sustain the impressive operative conditions in terms of huge thermal and mechanical loads, generated in thrust chambers. Cryogenic fuels, like methane or hydrogen, are often used as coolants and they may behave as transcritical fluids flowing in the jackets: after injection in a liquid state, a phase pseudo-change occurs along the chamber because of the heat released by combustion gases and coolants exiting as a vapour. Thus, in the development of such subsystems, important issues are focused on numerical methodologies adopted to simulate the fluid thermal behaviour inside the jackets, design procedures as well as manufacturing and technological process topics. The present paper includes the numerical thermal analyses regarding the cooling jacket belonging to the liquid oxygen/liquid methane demonstrator, realized in the framework of the HYPROB (HYdrocarbon PROpulsion test Bench) program. Numerical results considering the nominal operating conditions of cooling jackets in the methane-fuelled mode and the water-fed Citation: Ricci, D.; Battista, F.; one are included in the case of the application of electrodeposition process for manufacturing. A Fragiacomo, M. Numerical comparison with a similar cooling jacket, realized through the conventional brazing process, is Investigation on the Thermal addressed to underline the benefits of the application of electrodeposition technology. Behaviour of a LOx/LCH4 Demonstrator Cooling System. Keywords: liquid rocket engine; numerical analyses; thermal control; cooling jacket design; regener- Aerospace 2021, 8, 151. ative cooling; methane transcritical behaviour; electrodeposition technology; brazing process https://doi.org/10.3390/ aerospace8060151

Academic Editor: Konstantinos Kontis 1. Introduction In the last few years, an increasing interest has arisen in the utilization of the LOX/CH4 Received: 14 April 2021 propellant combination for space propulsion applications as testified by the efforts spent by Accepted: 24 May 2021 several academic and research institutions, international agencies and private companies. Published: 27 May 2021 The utilization of the LOX/CH4 combination for space propulsion applications provides many advantages, as indicated by several authors [1–3]: Publisher’s Note: MDPI stays neutral • high specific impulse; with regard to jurisdictional claims in • thrust-to-weight ratio performances; published maps and institutional affil- • good cooling capability; iations. • engine reusability and throttlability; • fewer storage, handling and insulation concerns; • reduced pollution impact on ground, atmosphere and space; • compatibility with ISRU (in situ resource utilization) purposes for lunar/Martian missions. Copyright: © 2021 by the authors. These capabilities result in a large number of applications and missions enabled Licensee MDPI, Basel, Switzerland. This article is an open access article by methane-based propulsion systems, from in-space systems (landing or descent ve- distributed under the terms and hicles, service modules, etc.) to space launchers (main stages or upper stages). In fact, conditions of the Creative Commons oxygen/methane couple represents a potential candidate to substitute hypergolic and Attribution (CC BY) license (https:// solid propellants in the future. Thus, its versatility makes methane a good candidate creativecommons.org/licenses/by/ for several applications, from in-space propulsion systems (service modules, landing or 4.0/). descent vehicles, and ascent stages) to accessing to space (first stages of launchers or upper

Aerospace 2021, 8, 151. https://doi.org/10.3390/aerospace8060151 https://www.mdpi.com/journal/aerospace Aerospace 2021, 8, x 2 of 21

Aerospace 2021, 8, 151 2 of 20 applications, from in-space propulsion systems (service modules, landing or descent vehicles, and ascent stages) to accessing to space (first stages of launchers or upper stages) [3]). Somestages) important [3]. Some programs important have programs recently been have launched recently in been Europe launched like the in LYRA Europe pro- like the ject (ASI/ItalianLYRA project Space (ASI/Italian Agency-Avio) Space [4], Agency-Avio) which provided [4], the which first providedimpulse to the the first develop- impulse to ment theof future development generation of futureof VEGA generation upper stage of VEGA engines, upper currently stage engines,on-going currently due to AVIO on-going effortsdue [5]; to Prometheus AVIO efforts engine [5]; Prometheus (100-t-thrus enginet) project, (100-t-thrust) led by CNES project, (Centre led by CNESNational (Centre d'EtudesNational Spatiales)/Airbus-Safran d’Etudes Spatiales)/Airbus-Safran Launcher (France), Launcher is inserted (France), in ESA is inserted FLPP Neo in ESAPro- FLPP gram Neo(European Program Space (European Agency SpaceFuture Agency Launchers Future Preparatory Launchers Programme) Preparatory and Programme) is devoted and is to developingdevoted tothe developing future launcher the future family launcher after familyAriane after 6 [6]. Ariane In the 6 [Russian6]. In the Federation, Russian Federa- severaltion, projects, several like projects, Energomash, like Energomash, KBKhM (KB KBKhM KhimMash/Volga (KB KhimMash/Volga staged combustion staged combustion de- rived derivedengine with engine up withto 10 upt of to thrust), 10 t of KBKhA thrust), (KB KBKhA KhimAvtomatika) (KB KhimAvtomatika) and Starsem and (So- Starsem yuz), (Soyuz),are historically are historically active with active this with kind this of kindissue of and issue involved and involved in several in several studies studies [7]. [7]. JapanJapan has reached has reached high levels high levels of readiness of readiness [8] through [8] through the consolidated the consolidated cooperation cooperation be- be- tweentween JAXA JAXA(Japan (Japan Aerospace Aerospace Exploration Exploration Agency) Agency) and IHI. and In IHI.the United In the UnitedStates, a States, lot of a lot playersof are players involved are involved in several in projects, several projects,oriented orientedto different to different applications. applications. It is worth It isre- worth memberingremembering the SpaceX the SpaceXRaptor Project Raptor [9], Project Blue-Origin [9], Blue-Origin (240-t-thrust (240-t-thrust BE-4 engine, BE-4 based engine, on based stagedon combustion staged combustion cycle) [10] cycle) and [NASA,10] and developing NASA, developing a 20-kN pressure-fed a 20-kN pressure-fed engine, in- engine, tendedintended for the Morpheus for the Morpheus lunar lander lunar (Armadil lander (Armadillolo Aerospace, Aerospace, Mesquite, Mesquite, Texas, USA) TX, USA) [11]. [11]. BesidesBesides the aforementioned the aforementioned initiatives, initiatives, in Italy in there Italy thereis a strong is a strong interest interest in chemical in chemical space spacepropulsion propulsion issues issues and in and the in liquid the liquid oxygen/liquid oxygen/liquid methane methane combination. combination. In fact, In the fact, the ItalianItalian Ministry Ministry of University of University and Research and Research is funding is funding a specific a specific program program devoted devoted to the to the consolidationconsolidation of current of current technologies, technologies, methodologies methodologies and manufacturing and manufacturing capabilities capabilities as as well aswell the as development the development of future of future propulsion propulsion systems. systems. The program The program is named is named HYPROB HYPROB (HYdrocarbon(HYdrocarbon PROpulsion PROpulsion test Bench) test Bench) and ha ands been has beenassigned assigned to the to Italian the Italian Aerospace Aerospace ResearchResearch Centre Centre (CIRA) (CIRA) [12]. Among [12]. Among the differ theent different goals, the goals, most the significant most significant objective objective is the design,is the realization design, realization and testing and of a testing LOX/LCH of a4 demonstrator LOX/LCH4 demonstrator (DEMO), capable (DEMO), of 30 kN capable of thrust.of 30 An kN incremental of thrust. Anapproach incremental strategy approach has been strategy adopted has to enlarge been adopted the comprehen- to enlarge the sion ofcomprehension critical physical of criticalaspects physical through aspects the design, through manufacturing the design, manufacturing and testing of andspecific testing of test articles.specific Basic test articles. activities Basic like activities the design like and the experiments design and experiments on injectors onor injectorsmaterial orchar- material acterizationscharacterizations have been have accomplished. been accomplished. Howeve However,r, central centralissues were issues represented were represented by the by the experimentalexperimental campaigns campaigns on igniters on igniters and andsubscale subscale mono-injector mono-injector engines engines (to (to investigate investigate the the comprehensioncomprehension of of supercritical supercritical combustion combustion,, heatheat release,release,injection injection and and mixing mixing phenomena, phenomena,etc.) etc.) as well as well the the experimental experimental studies studie ons methaneon methane transcritical transcritical behaviour behaviour as depictedas de- by pictedFigure by Figure1[13]. 1 [13].

Figure 1. Logical steps of LOX/CH4 demonstrator (DEMO) development. Figure 1. Logical steps of LOX/CH4 demonstrator (DEMO) development.

The finalThe object ﬁnal object is represented is represented by a by30-k aN-class-thrust 30-kN-class-thrust demonstrator. demonstrator. The baseline The baseline thrustthrust chamber chamber assembly assembly concept concept is depicted is depicted in Figure in Figure2: it includes2: it includes an igniter, an igniter, an injector an injector head with 18 injectors and a regenerative cooling jacket with 96 axial channels and inlet/outlet manifolds. In Table1 the main parameters are detailed.

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Aerospace 2021, 8, 151 3 of 20 head with 18 injectors and a regenerative cooling jacket with 96 axial channels and inlet/outlet manifolds. In Table 1 the main parameters are detailed.

1—igniter; 2—injector head; 3—outlet manifold with collector; 4—thrust chamber including the cooling jacket; 5—inlet fuel collector and distributor.

FigureFigure 2. 2.DEMO DEMO thrust thrust chamber chamber assembly assembly view view with with the the main main components. components.

TableTable 1. 1.HYPROB HYPROB (HYdrocarbon (HYdrocarbon PROpulsion PROpulsion test test Bench) Bench) DEMO DEMO main main performance performance parameters. parameters.

O/FO/F 3.43.4 PPCCCC 5.5 MPaMPa

ReactionReaction efﬁciency efficiency 0.98 0.98 I Ispsp 286 ss ThrustThrust 30 kN30 kN AAcccc/A/At 4.04.0

BasedBased onon aa counter-flowcounter-flow architecture, methan methanee enters enters the the channels channels in in the the nozzle nozzle re- regiongion in in liquid liquid phase, phase, is isheated heated by by the the comb combustionustion gases gases along along the the chamber chamber and, and, then, then, in- injectedjected into into the the combustion combustion chamber chamber as asa supe a supercriticalrcritical gas gas through through the theinjection injection head. head. The Thecooling cooling jacket jacket represents represents the themost most critical critical component: component: it is itcomposed is composed of an of inner an inner liner, liner,made made of high of thermal high thermal conducti conductiveve materials materials (generally (generally copper copperalloys), alloys),and by a and close-out by a close-outstructure, structure, made up madeof robust up of alloys, robust like alloys, Inconel like or Inconel nickel, or and nickel, consists and 96 consists axial channels, 96 axial channels,surrounding surrounding the combustion the combustion chamber. chamber. The des Theign design activities activities have havebeen beeninternally internally con- conductedducted by by means means of ofin-house in-house codes, codes, as as repo reportedrted by by [14], [14], and supported byby CFDCFD analyses analyses (to(to characterize characterize the the thermal thermal and and fluid-dynamic fluid-dyna behaviourmic behaviour of the coolingof the cooling jacket) and jacket) thermo- and structuralthermo-structural verifications. verifications. In the process In the of process developing of developing the final DEMO the final version, DEMO a firingversion, test a campaignfiring test is campaign needed in is water-cooled needed in water-cooled mode to characterize mode to the characterize cooling jacket the behaviour cooling jacket and tobehaviour accomplish and qualification to accomplish of thequalification innovative of manufacturing the innovative methodmanufacturing (i.e., electroplating) method (i.e., selectedelectroplating) to join the selected copper to liner join with the thecopper nickel liner close-out. with the In fact,nickel CIRA, close-out. after accomplishing In fact, CIRA, aafter specific accomplishing technological a specific activity, technological decided to substituteactivity, decided the planned to substitute use of athe conventional planned use process,of a conventional based on brazing process, of based non-homogeneous on brazing of components,non-homogeneous with galvaniccomponents, deposition with gal- of coppervanic deposition and nickel of layers copper for theand realization nickel layers of thefor coolingthe realization jacket. Inof fact, the largecooling difficulties jacket. In werefact, encounteredlarge difficulties in the were repeatability encountered of thein the brazing repeatability process forof chambersthe brazing with process tens offor coolingchambers channels with tens working of cooling in tough channels conditions: working they in have tough to withstandconditions: pressure they have values to with- up tostand 16.0 pressure MPa, are values required up to 16.0 keep MPa, methane are required liquid as to long keep as methane possible liquid and hugeas long thermal as pos- 2 loadssible (fluxesand huge up tothermal tens of MW/mloads (fluxes); walls up should to tens be of as MW/m thin as2); possible walls should for efficient be as thermal thin as exchangepossible for and efficient weight thermal reduction exchange purposes, and as weight depicted reduction in Figure purposes,3a. According as depicted to the in new Fig- process,ure 3a. According grooves in theto the liner new part process, are milled grooves and thenin the overlaid liner part with are copper milled and and nickel then over- [15]: inlaid this with way, copper channels and are nickel generated [15]: in by this a combination way, channels of two are specialgenerated galvanic by a combination depositions of of puretwo copperspecial andgalvanic nickel, depositions as depicted of bypure Figure copper3b. and nickel, as depicted by Figure 3b.

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(a) (a) (b) (b)

FigureFigureFigure 3. Example 3. ExampleExample of a of oftypical a atypical typical liquid liquid liquid rocket rocket rocket engine engine engine (LRE) (LRE) (LRE)cooling cooling cooling jacket: jacket: jacket:(a) milled(a) milled (a) channels milled channels channels brazed brazed onto brazed onto the the ontoexternal external the close- external close- out; (b) schematics of the electroplating process applied in LRE cooling jacket manufacturing. out;close-out; (b) schematics (b) schematics of the electroplating of the electroplating process applied process in applied LRE cooling in LRE jacket cooling manufacturing. jacket manufacturing.

TheThe advantages advantagesadvantages are areare summarized summarizedsummarized in: in: in:(a) (a) (a)br brazingazing brazing and and and welding welding welding free free free process process process since since since cop- copper cop- perandper and and alloys alloys alloys could could could be be applied beapplied applied without without without thermal therma thermal stresses stressesl stresses and and and deterioration deterioration deterioration of of base baseof base materials mate- mate- rialsandrials and (b)and (b) high (b)high high reliability reliability reliability becausebecause because the the proc process processess repeatability repeatability repeatability can can can be be controlled,be controlled, controlled, improving improvingimproving mechanicalmechanical resistance, resistance, thermal thermal and and electrical electrical conductivity conductivity of the of the deposited deposited copper. copper. Thus, Thus, thethe use use of ofthisof thisthis advanced advancedadvanced process processprocess has hashas been beenbeen motivated motivatedmotivated by bybythe thethe possibility possibilitypossibility of ofavoidingof avoidingavoiding any anyany deteriorationdeterioration of baseof base materials, materials, and and by by a high a high level level of repeatability of repeatability and and reliability reliability [16]. [16]. [16 ]. However,However, this thisthis choice choicechoice led ledled to a toto plan a plan to optimi to optimizeoptimize activityze activity on onthe the cooling cooling system system arrangement arrangement to adaptto adaptadapt the thethe brazed brazedbrazed configuration configurationconfiguration to totheto thethe electroplated electroplatedelectroplated one. one.one. At Atthis thisthis point, point,point, the thethe DEMO-0A DEMO-0ADEMO-0A manufacturingmanufacturing phase phase has has been been completed completed and and the the product product has hashas been beenbeen accepted acceptedaccepted by bybymeans meansmeans of ofof leakleak and and proof proof test test (Figure (Figure 4).4 4).The). TheThe next nextnext step stepstep will willwill be bebethe thethe integration integrationintegration with withwith the thethe injector injectorinjector head, head,head, alreadyalready validated validated in a in previous a previous firing firingfiring campaign campaign,campaign, and, and the the igniter igniter in order in order to start to startstart the thethe final finalfinal testtest activity. activity.

FigureFigure 4. Pictures 4. Pictures of DEMO-0A of DEMO-0A in the in themanufacturing manufacturing phase phasephase and andan afterd afterafter the the thecompletion completioncompletion of realizationofof realizationrealization phase phasephase [17]. [[17].17 ].

ConcerningConcerning the thethe description descriptiondescription of of theof the the cooling cooling cooling jacket jacket thermal thermal behaviour,behaviour, behaviour, in in a regenerativelyain regenera- a regenera- tivelycooledtively cooled cooled LRE, LRE, the LRE, coolantthe the coolant coolant is represented is representedis represented by the by propellant. bythe the propellant. propellant. Moreover, Moreover, Moreover, in the casein thein of the cryogeniccase case of of cryogenicfluidscryogenic like fluids methanefluids like like methane or methane hydrogen, or orhydrogen, thehydrogen, propellant the the propellant behaviour propellant behaviour in behaviour terms of in phase termsin terms and of workingphaseof phase andconditionsand working working conditions evolves conditions rapidly evolves evolves in rapidly the rapidly cooling in thein jacket. the cooling cooling Thus, jacket. jacket. fluid Thus, isThus, injected fluid fluid is into injectedis injected the cooling into into system in liquid state (characterized by values of pressure and temperature higher and thethe cooling cooling system system in liquidin liquid state state (character (characterizedized by byvalues values of pressureof pressure and and temperature temperature lower than critical values, respectively) and generally undergoes a “pseudo-phase change” higherhigher and and lower lower than than critical critical values, values, resp respectively)ectively) and and generally generally undergoes undergoes a “pseudo- a “pseudo- from liquid-like state to a vapour-like one, as carried out by [18,19]. Thermo-physical phasephase change” change” from from liquid-like liquid-like state state to toa vapour-likea vapour-like one, one, as ascarried carried out out by by[18,19]. [18,19]. properties change quickly around the near critical zone (Tcr = 190.56 K and Pcr = 4.59 MPa Thermo-physicalThermo-physical properties properties change change quickly quickly around around the the near near critical critical zone zone (Tcr (T =cr 190.56 = 190.56 K K for methane), as reported in Figure5a, but gradually if subcritical phase change phenomena andand Pcr P= cr4.59 = 4.59 MPa MPa for for methane), methane), as reportedas reported in Figurein Figure 5a, 5a, but but gradually gradually if subcritical if subcritical phase phase are compared [20–22]. As aforementioned, methane thermo-physical properties inside changechange phenomena phenomena are are compared compared [20–22]. [20–22]. As Asaforementioned, aforementioned, methane methane thermo-physical thermo-physical the LRE channels may change rapidly and a heat transfer deterioration phenomenon propertiesproperties inside inside the the LRE LRE channels channels may may change change rapidly rapidly and and a heat a heat transfer transfer deterioration deterioration may be observed from to the axial coordinate where the specific heat at constant pressure phenomenonphenomenon may may be beobserved observed from from to theto the axial axial coordinate coordinate where where the the specific specific heat heat at con-at con- exhibits the peak (pseudo-critical conditions, Tpc > Tcr)[23–25]. Pseudo-critical temperature stantstant pressure pressure exhibits exhibits the the peak peak (pseudo-critical (pseudo-critical conditions, conditions, Tpc T>pc T >cr )T [23–25].cr) [23–25]. Pseudo-crit- Pseudo-crit- increases as pressure increases while the specific heat peak value reduces, up to vanishing icalical temperature temperature increases increases as aspressure pressure increa increasesses while while the the specific specific heat heat peak peak value value re- re- at very high values of pressure, as depicted by Figure5b. Designers generally pay a lot of duces, up to vanishing at very high values of pressure, as depicted by Figure 5b. Designers duces,attention up to tovanishing properly at conceiving very high values the cooling of pressure, passages as todepicted avoid the by aforementionedFigure 5b. Designers critical generally pay a lot of attention to properly conceiving the cooling passages to avoid the generallyphenomenon. pay a lot In of fact, attention due toto theproperly thermal conceiving stratification the cooling and disadvantageous passages to avoid physical the aforementioned critical phenomenon. In fact, due to the thermal stratification and disad- aforementionedconditions, low critical convective phenomenon. heat transfer In fact, coefficient due to the values thermal can stratification be detected because and disad- a thin vantageous physical conditions, low convective heat transfer coefficient values can be de- vantageouslayer (behaving physical as conditions, a vapour and low having convective low thermal heat transfer conductivity) coefficient may values divide can the be heated de- tectedtected because because a thin a thin layer layer (behaving (behaving as aas vapour a vapour and and having having low low thermal thermal conductivity) conductivity)

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wall from the core,may composed divide the of heated liquid-like wall from layers. the Incore, the comp viewosed of developing of liquid-like robust layers. In the view of design solutionsdeveloping for cooling robust jackets, design risks, solutions linked to for the cooling heat transfer jackets, deterioration risks, linked on-to the heat transfer set, can be reduceddeterioration either by increasing on-set, can the be coolant reduced pressure either orby byincreasing increasing the the coolant surface pressure or by in- roughness [24]. creasing the surface roughness [24].

3 ρ (density) [kg/m ] ρ [kg/m3] 18.0

16.0 400 Supercritical vapour 14.0 350

12.0 300

10.0 250

P [MPa] 8.0 200

6.0 150

4.0 Critical Point 100

2.0 50

1100 150 200 250 300 350 400 450 T[K] (a)

(b)

Figure 5.Figure Methane 5. Methane thermo-physical thermo-physical properties, properties, as a function as a function of temperature of temperature and pressure: and pressure: (a) density (a) density (kg/m3); (b) specific heat (J/kg(kg/m K). 3); (b) speciﬁc heat (J/kg K).

Given this background,Given thethis coolant background, flow analysis the coolant and the flow deep analysis comprehension and the ofdeep fluid comprehension of transcritical behaviour represent central points in the design activity of LO /LCH rocket fluid transcritical behaviour represent central points in theX design4 activity of LOX/LCH4 engines: the predictionrocketof engines: surface the temperature prediction and of surface heat flux te frommperature the combustion and heat flux gases from to the combustion the engine wall isgases directly to dependentthe engine onwall heat is directly transfer dependent capabilities on of theheat coolant transfer [25 capabilities]. More- of the coolant over, classical semi-empirical[25]. Moreover, correlations classical forsemi-empirical the evaluation correlati of heatons transfer for the coefficients evaluation do of heat transfer co- not work in the deterioratedefficients do modenot work and in relatively the deteriorated high values mode of and roughness, relatively which high may values of roughness,

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whichoccur sincemay occur the typical since the dimension typical dimension of channels of (in channels the order (in ofthe some order mm) of some also mm) represent also representchallenging challenging conditions conditions from a numerical from a numerical point of point view. of Thus, view. an Thus, accurate an accurate investigation inves- tigationabout configurations about configurations of rocket-engine-like of rocket-engine-like cooling channels cooling waschannels needed was before needed designing before designingthe DEMO the jacket. DEMO For thisjacket. reason, For this a specific reason, test a article,specific named test article, MTP-BB named (Methane MTP-BB Thermal (Me- thaneProperties Thermal Breadboard) Properties and Breadboard) shown by Figure and 6shown, has been by Figure designed 6, has by CIRA been anddesigned tested by at CIRAdifferent and conditions tested at different [26]. The conditions breadboard, [26]. madeThe breadboard, of a copper made alloy, of was a copper provided alloy, with was a providedsingle rectangular with a single channel rectangular on the top, channel connected on the to the top, facility connected supplies to the through facility mechanical supplies throughflanges, wheremechanical temperature flanges,and where pressure temperature sensors and were pressure accommodated. sensors were Liquid accommo- methane dated.was injected Liquid inmethane the rectangular was injected passage in the (mass rectangular flow rate passage ranging (mass from flow 0.01 torate 0.06 ranging kg/s, fromTin ranging 0.01 to 0.06 from kg/s, 120 T toin 140ranging K and from Pin 120in theto 140 range K and 6.0–16.0 Pin in the MPa) range and 6.0–16.0 gradually MPa) heated and graduallyby means heated of the by electrical means of cartridges, the electrical placed cartridges, on the placed bottom on part, the tobottom reach part, transcritical to reach transcriticalconditions beforeconditions exiting. before Several exiting. internal Several thermocouples internal thermocouples were placed were inside placed the inside body theto collect body to temperature collect temperature data at different data at different axial positions axial positions and at a distanceand at a distance from the from channel the channelbottom upbottom to 4 mm.up to 4 mm.

FigureFigure 6. 6. MTPMTP breadboard: breadboard: sketch sketch of of the the test test article. article.

TheThe collected collected experimental experimental data data were were useful useful to to conduct conduct a a rebuilding rebuilding activity activity in in order order toto set set the the thermal thermal numerical numerical models models and and support support the the design design of of the the DEMO DEMO cooling cooling jacket. jacket. InIn fact, fact, the the channel channel located located on on the the top top of th ofe theMTP MTP breadboard breadboard has si hasmilar similar dimensions dimensions with respectwith respect to theto DEMO the DEMO cooling cooling jacket; jacket; as well as as well, input as, inputheat flux heat (imposed flux (imposed from the from base- the ment),basement), inlet inletconditions conditions and andmass mass flow flow rates rates are are in inthe the typical typical range range of of DEMO DEMO operating operating conditions.conditions. Details Details on on the the experimental experimental rebuilding rebuilding and and validation validation activity activity are are reported reported in [26].in [26 ]. InIn this this paper, paper, both methane-cooled and and water-cooled modes of the DEMO cooling systemsystem have have been been analysed analysed through through a 3-D CFD model regarding a singlesingle channel.channel. The validationvalidation of the numericalnumerical procedureprocedure has has been been accomplished accomplished indirectly indirectly through through the the exper- ex- perimentalimental results results obtained obtained in thein the MTP MTP breadboard breadboard campaign campaign for for methane methanewhile while waiting waiting for forfuture future firing firing tests. tests. However, However, the presentthe present investigation investigation allows allows describing describing of the transcriticalof the tran- scriticalbehaviour behaviour in an LRE in typical an LRE cooling typical jacket, cooling on onejacket, hand, on and one to hand, compare andthe to compare response ofthe such re- sponsesystems, of manufactured such systems, by manufactured means of different by means technological of different process, technological on the other process, one. on In fact,the othercomparisons one. In fact, between comparisons the brazed between configuration the brazed of theconfiguration demonstrator of the and demonstrator the electroplated and theone, electroplated named DEMO-0A, one, named are carried DEMO-0A, out to are underline carried theout achievedto underline improvements the achieved under im- the thermal point of view. CFD results were adopted as input for the thermo-structural provements under the thermal point of view. CFD results were adopted as input for the simulations, needed to evaluate the lifecycle of the thrust chamber assembly. thermo-structural simulations, needed to evaluate the lifecycle of the thrust chamber assembly.2. Materials and Methods The final demonstrator architecture provides a regeneratively cooled ground engine, 2. Materials and Methods which implements a typical counter-flow architecture. The cooling jacket is characterized by severalThe final narrow demonstrator axial channels, architecture defined provides in the bottom a regeneratively part by a copper cooled alloy ground liner, coveredengine, whichwith electrodeposited implements a typical layers counter-flow of pure copper architecture. and pure The nickel, cooling or by jacket a brazed is characterized nickel part, bywhich several represent narrow the axial close-out channels, part. defined in the bottom part by a copper alloy liner, cov- ered Anwith in-house electrodeposited design tool, layers based of onpure typical copper correlations, and pure nickel, was developed or by a brazed to design nickel the part,cooling which jacket represent in terms the of close-out channel part. dimension, liner and rib thickness [14]. The adopted strategyAn in-house considers design a constant tool, based number on of typical channels correlations, and a constant was developed value for theto design rib width the cooling(w) while jacket a variable in terms value of ofchannel the rib dimensio height (h)n, has liner been and considered, rib thickness according [14]. The to Figureadopted7a. strategyIn this way, considers an optimization a constant of number the cooling of channels performances and a wasconstant achieved, value taking for the into rib account width

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(w) while a variable value of the rib height (h) has been considered, according to Figure 7a. In this way, an optimization of the cooling performances was achieved, taking into someaccount significant some significant sections, suchsections, as the such nozzle as the (NZ), nozzle throat (NZ), (CT) throat and (CT) cylindrical and cylindrical part (CP), part highlighted(CP), highlighted in Figure in 8Figure. Furthermore, 8. Furthermore, Figure 8Figure depicts 8 depicts the dimensionless the dimensionless profile profile of the of thrustthe thrust chamber, chamber, the variations the variations of cooling of cooling channel channel hydraulic hydraulic diameter diameter (dh) along (dh) thealong axial the axisaxial and axis the and applied the applied heat flux heat profile flux (defined profile (defined as “nominal” as “nominal” in the “Results in the “Results and Discussion” and Dis- section),cussion” representing section), representing the design profile. the design It has prof beenile. calculated It has been through calculated reactive through simulations reactive insidesimulations the combustion inside the chamber combustion [15]. chamber [15]. ConsideringConsidering L, L, the the overall overall thrust thrust chamber chamber length, length, as the as reference the reference length, length, the geometric the geo- parametersmetric parameters are equal are to: equal to: • • channelchannel height height (h/L), (h/L), ranging ranging from from 0.0018 0.0018 to to 0.0061; 0.0061; • • channelchannel width width (b/L), (b/L), ranging ranging from from 0.0019 0.0019 to to 0.0107; 0.0107; • • ribrib width width (w/L) (w/L) = = 0.0032; 0.0032; • • linerliner thickness thickness (h (h1/L)1/L) = = 0.0020;0.0020; • • coppercopper layer layer height height (h (hcucu/L)/L)= = 0.0023;0.0023; • • nickelnickel layer layer height height(h (nih/L)ni/L)= = 0.0034.0.0034. InIn the the case case of of the the DEMO DEMO brazed brazed version, version, the the copper copper layer layer and and nickel nickel close-out close-out are are substitutedsubstituted by by a a unique unique Inconel Inconel part, part, joined joined with with the the liner liner rib rib through through the the brazing brazing process. process.

Close-out Close-out

Copper Layer Symmetry Symmetry Fluid Fluid

Liner Liner

Thermal input Thermal input

(a) (b) Figure 7. Sketch of the model concerning: (a) geometry description of the DEMO-0A cooling jacket—cross-section; (b) AerospaceFigure 2021 7. Sketch, 8, x of the model concerning: (a) geometry description of the DEMO-0A cooling jacket—cross-section;8 of 21 (b) materialsmaterials and and boundary boundary conditions conditions for for electroplated electroplated (left) (left) and and brazed brazed (right) (right) DEMO DEMO versions. versions.

0.12 thrust chamber profile Inlet 50 dh q (reactive CFD results) hg,input 45 0.1 40 Section NZ

0.08 35 ] 2

[m] Section CP h

0.06 25 [MW/m Outlet y/L, d 20 hg, input

0.04 q 15

0.02 throat

Section CT 5

0 0 0.2 0.4 0.6 0.8 1 x/L FigureFigure 8. DEMODEMO assembly assembly details details on chamber on chamber profile, profile, hydraulic hydraulic diameter diameter of cooling of channels cooling channelsand and input heat flux profile. input heat flux profile. The numerical investigations on a single DEMO cooling channel, extracted from the

complete model, were performed by means of ANSYS Fluent v17 (Canonsburg, Pennsyl- vania, USA) [27]. The solution of the governing equations, such as continuity, momentum and energy in three-dimensional form were accomplished in a steady state regime and considering an NIST (National Institute of Standard and Technology) real gas model and turbulent flow with thermo-physical properties, calculated through REFPROP v7.0 database [28]. Conduction effects have been contemplated. Rough channel walls have been taken into account while k-ω sst turbulence model was assumed [27,29]. A pressure-based method was selected to solve the energy and momentum equations and a second-order upwind scheme was chosen together with PISO (Pressure-Implicit with Splitting of Oper- ators) coupling to couple pressure and velocity, respectively. Concerning the convergence criteria, residuals of velocity components and energy values were considered equal to 10−6 and 10−9, respectively. Initialization was performed at inlet section conditions, Pin = 16.0 MPa and Tin = 110 K for methane-cooled mode (overall mass flow rate is equal to 1.92 kg/s). For the water-cooled mode (overall mass flow rate is equal to 4.5 kg/s and 5.0 kg/s) Pin = 16.0 and 12.0 MPa (to take into account different test conditions offered by the test facility) and Tin = 293 K, respectively, were adopted. A NIST real gas model was applied and the single-species flow form was selected to eventually handle both liquid and vapour phases in supercritical pressure conditions [27,28], in the case of methane. In fact, if methane is considered, it is fundamental to the transcritical operating conditions of the working fluid since both the simulation initialization and the convergence history can be influenced. According to Figure 7a,b, the considered solid materials were: (a) a copper alloy (Cu- CrZr) for the liner, chosen because of its high thermal conductivity and good mechanical properties, for both modes; and (b) Inconel or pure nickel, due their mechanical performances, for the close-out part in the case of brazed and electroplated versions, respectively. For DEMO-0A, a copper layer was deposited between the liner and the external close-out. The thermo-physical properties of all the materials were assumed to be dependent on temperature and they were calculated through a characterization campaign [14].

Aerospace 2021, 8, 151 8 of 20

The numerical investigations on a single DEMO cooling channel, extracted from the complete model, were performed by means of ANSYS Fluent v17 (Canonsburg, Pennsylva- nia, USA) [27]. The solution of the governing equations, such as continuity, momentum and energy in three-dimensional form were accomplished in a steady state regime and considering an NIST (National Institute of Standard and Technology) real gas model and turbulent flow with thermo-physical properties, calculated through REFPROP v7.0 database [28]. Conduction effects have been contemplated. Rough channel walls have been taken into account while k-ω sst turbulence model was assumed [27,29]. A pressure-based method was selected to solve the energy and momentum equations and a second-order upwind scheme was chosen together with PISO (Pressure-Implicit with Splitting of Operators) coupling to couple pressure and velocity, respectively. Concerning the convergence criteria, residuals of velocity components and energy values were considered equal to 10−6 and −9 10 , respectively. Initialization was performed at inlet section conditions, Pin = 16.0 MPa and Tin = 110 K for methane-cooled mode (overall mass flow rate is equal to 1.92 kg/s). For the water-cooled mode (overall mass flow rate is equal to 4.5 kg/s and 5.0 kg/s) Pin = 16.0 and 12.0 MPa (to take into account different test conditions offered by the test facility) and Tin = 293 K, respectively, were adopted. A NIST real gas model was applied and the single-species flow form was selected to eventually handle both liquid and vapour phases in supercritical pressure conditions [27,28], in the case of methane. In fact, if methane is considered, it is fundamental to the transcritical operating conditions of the working fluid since both the simulation initialization and the convergence history can be influenced. According to Figure7a,b, the considered solid materials were: (a) a copper alloy (Cu- CrZr) for the liner, chosen because of its high thermal conductivity and good mechanical properties, for both modes; and (b) Inconel or pure nickel, due their mechanical performances, for the close-out part in the case of brazed and electroplated versions, respectively. For DEMO-0A, a copper layer was deposited between the liner and the external close-out. The thermo-physical properties of all the materials were assumed to be dependent on temperature and they were calculated through a characterization campaign [14]. Simulations contemplate a half channel to limit the computational efforts; in fact, a symmetry condition is adopted (due to the geometrical, thermal and fluid-dynamic symmetry); top wall was considered adiabatic (for a conservative approach) as well as the left one to take into account half rib. The thermal load was applied on the liner bottom surface by means of specific user-defined functions (udf ). Moreover, the following boundary conditions have been assigned: • inlet section: uniform velocity (at a fixed mass flow) and uniform temperature profile; • outlet section: pressure outlet in order to define the static pressure at flow outlets; • channel walls: velocity components equal to zero. The nominal heat flux profile, adopted for the design, is depicted in Figure8 and was obtained through reactive CFD simulations of the combustion chamber side, considering the thrust walls at a temperature of 300 K [15]. However, this approach is conservative but useful to adopt suitable margins in the cooling jacket design; then, a weak thermal coupling was taken into account for further analyses, as suggested by [18]. In this case, the thermal load was applied by means of a hot gas side convective heat transfer coefficient, calculated by scaling the “design” heat flux (obtained at a constant value of Twg equal to 300 K) and considering the average hot gas temperature inside the chamber (Taw), according to Equation (1). Taw values, given in Figure9, are evaluated through RPA code [ 30] for four significant sections, corresponding to inlet (1), throat (2), convergent/cylindrical part interface (3) and outlet (4). Then, at a fixed position, Taw is calculated by means of a linear interpolation between the values of adjacent sections.

. q CFD,Twg = 300K hc,hg = (1) Taw − Twg Aerospace 2021, 8, x 9 of 21

Simulations contemplate a half channel to limit the computational efforts; in fact, a symmetry condition is adopted (due to the geometrical, thermal and fluid-dynamic symmetry); top wall was considered adiabatic (for a conservative approach) as well as the left one to take into account half rib. The thermal load was applied on the liner bottom surface by means of specific user-defined functions (udf). Moreover, the following boundary conditions have been assigned: • inlet section: uniform velocity (at a fixed mass flow) and uniform temperature profile; • outlet section: pressure outlet in order to define the static pressure at flow outlets; • channel walls: velocity components equal to zero. The nominal heat flux profile, adopted for the design, is depicted in Figure 8 and was obtained through reactive CFD simulations of the combustion chamber side, considering the thrust walls at a temperature of 300 K [15]. However, this approach is conservative but useful to adopt suitable margins in the cooling jacket design; then, a weak thermal coupling was taken into account for further analyses, as suggested by [18]. In this case, the thermal load was applied by means of a hot gas side convective heat transfer coefficient, calculated by scaling the “design” heat flux (obtained at a constant value of Twg equal to 300 K) and considering the average hot gas temperature inside the chamber (Taw), according to Equation (1). Taw values, given in Figure 9, are evaluated through RPA code [30] for four significant sections, corresponding to inlet (1), throat (2), convergent/cylindrical part interface (3) and outlet (4). Then, at a fixed position, Taw is calculated by means of a linear interpolation between the values of adjacent sections.

. Aerospace 2021, 8, 151 q 9 of 20 h = CFD ,Twg= 300K (1) c,hg − ()TTaw wg

T = 2456 K Taw= 3543 K Taw = 3538 K Taw = 3370 K aw 4 3 2 1

Section CP Section CT Section NZ

Outlet Inlet throat

FigureFigure 9.9. Sketch reporting reporting the the hot hot gas gas average average temper temperatureature (K) (K) values values along along the the engine engine for forthe the cylindricalcylindrical part, part, convergent, convergent, throat throat and and nozzle nozzle sections, sections, evaluated evaluated through through RPA RPA code. code.

WithWith regards regards to to the the adopted adopted mesh, mesh, a a structured structured grid, grid, composed composed of of about about 2.7 2.7 million million nodes,nodes, waswas chosenchosen and a sketch sketch is is reported reported in in Figure Figure 10. 10 A. Amesh mesh sensitivity sensitivity analysis, analysis, de- describedscribed by by Table Table 2, 2was, was conducted conducted on onthe themodel model considering considering the nominal the nominal conditions conditions (nom- (nominalinal input input heat heatflux, flux,Tin = 110 Tin =K, 110 Pin K,= 16.0 Pin MPa= 16.0 and MPa m = and 0.01m kg/s= 0.01 for kg/ssingle for half single channel). half channel).Three structured Three structured mesh distributions mesh distributions (“coarse”, (“coarse”, “fine” and “fine” “finest”), and generated “finest”), generated by follow- bying following the mesh thesuggestions mesh suggestions given by Ansys given Fluent by Ansys user’s Fluent guideuser’s in the case guide of inrough the casechannel of roughwalls channel[27], were walls considered: [27], were they considered: have about they 1.3, have 2.7 aboutand 5.6 1.3, million 2.7 and nodes, 5.6 million respectively. nodes, respectively. Finally, the second grid case was chosen to perform the investigations, pre- Aerospace 2021, 8, x Finally, the second grid case was chosen to perform the investigations, presented10 ofin 21 the sentedpresent in paper, the present because paper, it ensured because a it good ensured compromise a good compromise between the between machine the computa- machine computationaltional time and time the andaccuracy the accuracy requirements. requirements.

(a) (b)

FigureFigure 10. 10. NumericalNumerical modelling: modelling: (a ()a global) global view view of of cooling cooling channel channel model model ex extractedtracted from from cooling cooling system system geometry; geometry; (b ()b ) meshmesh distribution distribution with with a adeta detailil on on inlet inlet zone zone (axial (axial direction) direction) and and on on aa transversal transversal section section (divergent (divergent zone). zone).

Table 2. Grid-independence test results. Table 2. Grid-independence test results. Outlet Fluid Bulk Liner Maximum Channel Bottom Wall ΔP Outlet Fluid Bulk Liner Maximum Channel Bottom Wall Type of Mesh ∆P Temperature Temperature Maximum Temperature Type of Mesh (MPa) Temperature Temperature Maximum Temperature (MPa) Tb,f out (K) Tw, hg max Tw, ch max T (K) T T 1—coarse 5.051 420.1b,f out 600.4w, hg max 555.4w, ch max 2—fine1—coarse 5.094 5.051 420.4 420.1 610.8 600.4 562.7 555.4 3—finest2—ﬁne 5.096 5.094 420.5 420.4 611.5 610.8 563.4 562.7 3—ﬁnest 5.096 420.5 611.5 563.4 The numerical approach and settings were validated through results obtained in the rebuildingThe numericalactivity of approach the test campaign and settings conducted were validated on the throughaforementioned results obtained MTP bread- in the board,rebuilding described activity in Figure of the test 6, with campaign regard conducted to the transcritical on the aforementioned behaviour of methane MTP breadboard, [15,26]. Indescribed fact, the test in Figurearticle6 was, with provided regard towith the a transcriticalchannel, having behaviour dimensio ofns methane representative [ 15,26]. of In onesfact, adopted the test in article the DEMO was provided and this with is also a channel, true for havingthe operating dimensions conditions. representative Figure 11 of reportsones adopted the comparisons in the DEMO between and the this experimental is also true fordata the (when operating stationary conditions. thermal Figure regime 11 wasreports reached) the comparisons and the numerical between results the experimental for two relevant data (whentest cases: stationary test #24 thermal and test regime #26, characterizedwas reached) by and mass the flow numerical rates equal results to for20.87 two g/s relevant and 20.57 test g/s, cases: Tin test= 137.1 #24 K and and test 140.8 #26, K, and Pin = 11.21 MPa and 12.89 MPa, respectively. Numerical results fitted the experimental data very well, as depicted by Figure 11a and Figure 11b, with regards to pressure and fluid bulk temperature axial profiles, which are compared with test data. Moreover, very low discrepancies are observed in terms of solid temperature results if the numerical temperature profile at a distance of 4 mm from the channel base is compared with the related experimental data for test #24, as reported in Figure 11b. Further details on CFD simulations, modelling, solving strategies and both numerical and experimental results are given in [26].

Aerospace 2021, 8, 151 10 of 20

characterized by mass flow rates equal to 20.87 g/s and 20.57 g/s, Tin = 137.1 K and 140.8 K, and Pin = 11.21 MPa and 12.89 MPa, respectively. Numerical results fitted the experimental data very well, as depicted by Figure 11a,b with regards to pressure and fluid bulk temperature axial profiles, which are compared with test data. Moreover, very low discrepancies are observed in terms of solid temperature results if the numerical temperature profile at a distance of 4 mm from the channel base is compared with the related experimental data for test #24, as reported in Figure 11b. Further details on CFD Aerospace 2021, 8, x simulations, modelling, solving strategies and both numerical and experimental results are11 of 21 given in [26].

inlet flange sensor

12 outlet flange sensor

inlet flange sensor

rsue[MPa] Pressure 11

Test n.24 - Numerical Rebuilding Test n.26 - Numerical Rebuilding Test n.24 - experimental data Test n.26 - experimental data outlet flange sensor 10 0 0.2 0.4 0.6 0.8 1 x/L (a)

center line of channel upper wall center line of channel bottom wall 500 solid T - line at 4 mm depth CFD results at sensors locations T experimental data - 4 mm depth 450 experimental data - Tf

Tb,f 400 e[K] r 350 atu r 300

Tempe 250

200 T cr,f Test n.24 150 steady state regime

0 0.2 0.4 0.6 0.8 1 x/L (b)

Figure 11. MTP breadboardFigure 11. MTP rebuilding breadboard of hot rebuildingtests: (a) Pressure of hot tests: drops (a (test) Pressure #24 and drops #26); (test and #24 (b) andFluid #26); temperature and (b) Fluid and bottom wall temperature profiles (test 24). temperature and bottom wall temperature proﬁles (test 24). 3. Results and Discussion

In the present paper, results for numerical simulations ran at nominal conditions (Tin, f = 110 K, Pin = 16.0 MPa and overall m = 1.92 kg/s) for methane mode for both the DEMO configurations, characterized by the electrodeposition and brazing process, are presented. Moreover, since the validation of the electrodeposition process will be accomplished by means of a specific test campaign considering water as refrigerant evolving in the demonstrator cooling system, the following initial conditions are also taken into account: Tin, f = 293 K, Pin = 16.0 and 12.0 MPa and overall m = 5.0 and 4.5 kg/s. These conditions were set according to the capabilities expressed by the test bench.

Aerospace 2021, 8, 151 11 of 20

3. Results and Discussion In the present paper, results for numerical simulations ran at nominal conditions (Tin, f = 110 K,Pin = 16.0 MPa and overall m = 1.92 kg/s) for methane mode for both the DEMO conﬁgurations, characterized by the electrodeposition and brazing process, are presented. Moreover, since the validation of the electrodeposition process will be accomplished by means of a speciﬁc test campaign considering water as refrigerant evolving in the demonstrator cooling system, the following initial conditions are also taken into account: Tin, f = 293 K, Pin = 16.0 and 12.0 MPa and overall m = 5.0 and 4.5 kg/s. These conditions were set according to the capabilities expressed by the test bench. The test matrix is reported by Table3.

Table 3. Test matrix of present numerical campaign.

T P m Imposed Heat Flux Manufacturing Run in,f in Fluid (K) (MPa) (kg/s) (MW/m2) Process 1 110 16.0 1.92 Methane Nominal Electrodeposition 2 110 16.0 1.92 Methane Weak Coupling Electrodeposition 3 110 16.0 1.92 Methane Nominal Brazing 4 293 16.0 5.0 Water Nominal Electrodeposition 5 293 16.0 5.0 Water Nominal Brazing 6 293 16.0 4.5 Water Nominal Electrodeposition 7 293 12.0 5.0 Water Nominal Electrodeposition 8 293 12.0 4.5 Water Nominal Electrodeposition

Results are presented in terms of axial profiles for the liner and channel wall temperature, fluid bulk temperature, convective heat transfer coefficient and pressure drops as well as fluid significant properties. Moreover, some temperature and fluid properties’ field plots, including some slices of significant interest, are presented. Figures 12 and 13 depict the axial profiles of hot gas walls, channel bottom walls and fluid bulk temperature for water-cooled and methane-cooled modes, respectively. Both figures point out that DEMO-0A, the optimized electroplated configuration of DEMO, presents a general reduction in terms of thermal stresses with respect to the brazed version. This is mainly due to the insertion of the copper layer to overlay the liner part and the possibility of changing the close-out material from Inconel to pure nickel. For the water- cooled mode, temperature peaks in the throat region and in the cylindrical section of the chamber resulted in reductions of about 30–40 K for a fixed mass flow rate (i.e., 5.0 kg/s), as depicted by Figure 12. A little decrease in terms of mass flow rate (4.5 kg/s, i.e., −10%) causes DEMO-0A profiles to overlap with ones obtained for the brazed version at mass flow rate equal to 5.0 kg/s. Moreover, fluid bulk temperature increases from about 400 to 420 K as mass flow rate decreases; water constantly remains in liquid phase, because of high pressure conditions as confirmed by the fluid bulk profiles and maximum values of channel walls. If methane is considered as refrigerant, the thermal behaviour of the DEMO-0A and the “brazed” versions of the DEMO detach significantly from each other, as depicted by Figure 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux is imposed, reduces from about 640 to 540 K while the absolute maximum, observed at about x/L = 0.13 (in correspondence with the re-attachment zone of hot gases on the combustion chamber wall) decreases by about 90 K in DEMO-0A. Moreover, it is worthy to emphasise that in that zone, methane flows as a supercritical gas. A third relative maximum, observed in the brazed configuration and located in the divergent zone, tends to disappear in DEMO- 0A, due to a reduced local thermal stratification of the fluid. In fact, all the channel walls are made of copper alloys or pure copper, including the upper wall, and this determines a more homogeneous heat distribution inside the material and, consequently, in the fluid. Moreover, from that region fluid begins to locally change its conditions from a “liquid-like” to a “vapour-like” one, as depicted by the fluid bulk temperature profile: the working Aerospace 2021, 8, 151 12 of 20

fluid, after entering in liquid conditions, reaches critical temperature (about 190 K) near the throat region. From this section, methane tends to behave like a highly compressible fluid near the hot walls of the channel, and like a liquid near the cold ones, especially in the upper part of the channel. However, moving towards the outlet section, a larger fluid fraction behaves like a vapour. In the cylindrical part, methane is completely composed of a supercritical gas since the temperature is very high and the pressure is much higher than the critical value (4.6 MPa). If the “scaled” thermal load (considering a weak coupling approach) is applied, the liner results in being less thermally stressed as expected and, Aerospace 2021, 8, x FOR PEER REVIEWin fact, a reduction of about 40 K is evaluated in terms of temperature peaks,12 of 20 located at Aerospace 2021, 8, x 13 of 21 x/L = 0.61 and x/L = 0.13.

600 If methane is consideredDEMO "brazed" as refrigerant, version - mthe = 5.0thermal kg/s behaviour of the DEMO-0A and the “brazed” versionsDEMO-0A of the DEMO - m = 4.5 detach kg/s significantly from each other, as depicted by DEMO-0A - m = 5.0 kg/s Figure 13. Temperature peak in the throat section (x/L = 0.61), where the highest heat flux Tw,hg is imposed, reduces Tfromw,ch about 640 to 540 K while the absolute maximum, observed at about x/L = 0.13 (in correspondenceTb,f with the re-attachment zone of hot gases on the com- bustion500 chamber wall) decreases by about 90 K in DEMO-0A. Moreover, it is worthy to emphasise that in that zone, methane flows as a supercritical gas. A third relative maximum, observed in the brazed configuration and located in the divergent zone, tends to

disappear in DEMO-0A, due to a reduced local thermalTw,hg stratification of the fluid. In fact,

allT[K] the channel walls are made of copper alloys or pure copper, including the upper wall, and this determines a more homogeneous heat distribution inside the material and, con- 400 sequently, in the fluid. Moreover, from that region fluid begins to locally change its con- T ditions from a “liquid-like” to a “vapour-like”w,ch one, as depicted by the fluid bulk temperature profile: the working fluid, after entering in liquid conditions, reaches critical tem- Tb,f perature (about 190 K) near the throat region. Fromthroat this section, methane tends to behave like a highly compressible fluid near the hot walls of the channel, and like a liquid near water cooled mode the cold300 ones, especially in the upper part of the channel. However, moving towards the outlet section, a larger fluid fraction behaves like a vapour. In the cylindrical part, methane is completely0 0.1composed0.2 of0.3 a supercritical0.4 0.5 ga0.6s since0.7 the temperature0.8 0.9 is1 very high and the pressure is much higher than the criticalx/L value (4.6 MPa). If the “scaled” thermal load Figure(considering 12. 12. Water-cooledWater-cooled a weak coupling mode: mode: axial approach) profiles axial is proﬁles of applied, hot gas of walls,the hot liner gaschannel results walls, bottom in channel being walls less and bottom thermally fluid wallsbulk and ﬂuid temperature.stressed as expected and, in fact, a reduction of about 40 K is evaluated in terms of tem- bulk temperature. perature peaks, located at x/L = 0.61 and x/L = 0.13. If methane is considered as refrigerant, the thermal behaviour of DEMO-0A and “brazed” methane-cooledversion of DEMO modedetaches significantly from each other, as depicted by Figure 13. Temperature700 peak in the throat section (x/L = 0.61), where the highest heat flux is imposed, reduces from about 640 K to 540 K while the absolute maximum, observed at about x/L = 0.13 (in correspondence of the re-attachment zone of hot gases on the combustion 600 T chamber wall) decreases of about 90 K in DEMO-0A.w,hg Moreover, it is worthy to underline that in that zone, methane flows as a supercritical gas. A third relative maximum, ob- served500 in the brazed configuration and located in the divergent zone, tends to disappear in DEMO-0A, due to a reduced local thermal stratification of the fluid. In fact, all the channel walls are made of copperT alloys or pure copper, including the upper wall, and this 400 w,ch

determinesT[K] a more homogeneous heat distribution inside the material and, consequently, in the fluid. Moreover, from that region fluid begins to locally change its conditions from 300 a “liquid-like” to a “vapour-like”T one, as depicted by the fluid bulk temperature profile: the working fluid, afterb,f entering in liquid conditions, reaches the critical temperature (about 190 K) nearDEMO the throat "brazed" region. version From this section, methane tends to behave like a 200 highly compressibleDEMO-0A fluid near the hot walls of the channel, and like a liquid near the cold DEMO-0A scaled ones, especially in the upper part of the channel. However, moving towards the outlet Tw,hg

section,100 a larger fluidTw,ch fraction behaves like a vapour. In the cylindrical part, methane is T throat completely composedb,f by a supercritical gas since temperature is very high and pressure is much 0higher0.1 than0.2 the critical0.3 0.4value0.5 (4.6 MPa).0.6 If0.7 the “scaled”0.8 0.9 thermal1 load (considering x/L a weak coupling approach) is applied, the liner results to be less thermally stressed as Figureexpected 13. 13. and,Methane-cooledMethane-cooled in fact, a reduction mode: mode: axial of profiles about axial proﬁles40 of Khot is gas evaluated of walls, hot gaschannel in walls,terms bottom of channel temperature walls bottomand fluid peaks, walls and ﬂuid bulk temperature. bulklocated temperature. at x/L = 0.61 and x/L = 0.13. Figure 14 depicts the axial profiles for average convective heat transfer both for DEMO-0A and DEMO in the brazed configuration. It is shown that values are generally higher in the case of DEMO-0A if methane is considered as refrigerant and it is particularly evident in the divergent part and in the cylindrical part of the cooling system. Alt- hough, in the divergent zone, the fluid is in large part in liquid form, especially near the inlet section where the heat transfer coefficient values are lower than the ones obtained in the cylindrical part, where methane behaves like a supercritical vapour. This is mainly

Aerospace 2021, 8, 151 13 of 20

Figure 14 depicts the axial profiles for average convective heat transfer both for DEMO- 0A and DEMO in the brazed configuration. It is shown that values are generally higher in the case of DEMO-0A if methane is considered as refrigerant and it is particularly evident in the divergent part and in the cylindrical part of the cooling system. Although, Aerospace 2021, 8, x 14 of 21 in the divergent zone, the fluid is in large part in liquid form, especially near the inlet section where the heat transfer coefficient values are lower than the ones obtained in the cylindrical part, where methane behaves like a supercritical vapour. This is mainly due todue the to high the high velocity velocity attained attained by the by fluidthe fluid in the in the last last sections sections of the of the cooling cooling jacket, jacket, while while in mostin most of theof the divergent divergent part part velocity velocity remains remains low becauselow because of fluid of fluid state andstate the and channel the channel cross sectioncross section dimension. dimension. Maximum Maximum values arevalues attained are attained in the throat in the region, throat where region, the where minimum the dimensionsminimum dimensions of passages of arepassages designed, are designed and there, and are verythere little are very differences little differences between thebe- versions.tween the Moreover, versions. Moreover, a profile for a theprofile water-cooled for the water-cooled DEMO-0A modeDEMO-0A is plotted mode in is order plotted to comparein order to data, compare underlining data, underlining that the convective that the heat convective transfer heat values transfer are larger values in the are case larger of waterin the becausecase of water of its basicbecause thermo-physical of its basic thermo-physical properties. properties.

800000 DEMO-0A - methane-cooled DEMO (brazed conf.) - methane-cooled 700000 DEMO-0A - water-cooled (m = 5 kg/s)

600000

500000 K] 2

400000 [W/m av h

300000 throat

200000

100000

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L Figure 14.14. Convective heat transfer axialaxial proﬁles.profiles.

Regarding staticstatic pressurepressure profiles, profiles, plotted plotted for for the the DEMO-0A DEMO-0A version version in in Figure Figure 15 ,15, it isit clearis clear that that pressure pressure drops drops in the in nozzlethe nozzle are very are lowvery because low because methane methane behaves behaves like aliquid. like a Theliquid. highest The highest increase increase is localized is localized in the convergent in the convergent part and part the cylindricaland the cylindrical one, as pointed one, as outpointed in Figure out in 15 Figure, because 15, densitybecause is density very low is very and velocitylow and tendsvelocity to increasetends to towardsincrease theto- outletwards section.the outlet As section. a result, As largea result, pressure large pressure drops are drops evaluated are evaluated although although methane methane mass flowmass rateflow is rate about is about 2.5 times 2.5 ti lowermes lower than waterthan water in the in water-cooled the water-cooled mode. mode. In fact, In fact, in the in water-cooledthe water-cooled mode, mode, the refrigerantthe refrigerant remains remain in liquids in liquid form form throughout throughout the system.the system. Figure 16 depicts the temperature field for DEMO-0A, including channel walls and some slices, pointing out that maximum temperature values are attained in correspondence with throat region and re-attachment point while Figure 17 gives a comparison with DEMO brazed configuration considering some significant cross sections. It is evident that the liner part is hotter if the brazed version of DEMO is compared with DEMO-0A in all of the considered cross sections. Furthermore, for both versions, it is shown that the thermal stratification of fluid is significant: near the bottom of the channel and near the rib walls the refrigerant is very hot while moving up towards the upper part of the channel, the temperature tends to decrease as it is possible to observe the sections until the throat region). Moreover, after entering the jacket as a compressed liquid, the fluid is heated

Aerospace 2021, 8, 151 14 of 20

by the hot gases and from the throat region exhibits temperature values higher than the critical one on average. In this region and in the ﬁrst part of convergence, it is possible to Aerospace 2021, 8, x observe a gas-like ﬂuid moving near the bottom walls of the channels while a liquid-like15 of 21

one is present in the upper parts.

Water - m = 5.0 kg/s +07 1.6x10 Water - m = 4.5 kg/s Methane - m = 1.92 kg/s DEMO-0A

Pin = 16.0 and 12.0 MPa 1.4x10+07

1.2x10+07 e[Pa] r essu r +07

P 1.0x10

8.0x10+06 throat 6.0x10+06 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L

Aerospace 2021, 8, x FigureFigure 15. 15.DEMO-0A: DEMO-0A: axial axial proﬁles profiles of of static static pressure pressure for for both both methane-cooled methane-cooled (P 16(P inof= = 21 16.0 16.0 MPa) MPa) and and in water-cooled versions (Pin = 16.0 MPa and 12.0 MPa). water-cooled versions (Pin = 16.0 MPa and 12.0 MPa).

Figure 16 depicts the temperature field for DEMO-0A, including channel walls and some slices, pointing out that maximum temperature values are attained in correspondence with throat region and re-attachment point while Figure 17 gives a comparison with DEMO brazed configuration considering some significant cross sections. It is evident that the liner part is hotter if the brazed version of DEMO is compared with DEMO-0A in all of the considered cross sections. Furthermore, for both versions, it is shown that the thermal stratification of fluid is significant: near the bottom of the channel and near the rib walls the refrigerant is very hot while moving up towards the upper part of the channel, the temperature tends to decrease as it is possible to observe the sections until the throat region). Moreover, after entering the jacket as a compressed liquid, the fluid is heated by the hot gases and from the throat region exhibits temperature values higher than the critical one on average. In this region and in the first part of convergence, it is possible to observe a gas-like fluid moving near the bottom walls of the channels while a liquid-like one is present in the upper parts.

FigureFigure 16. DEMO-0A: 16. DEMO-0A: Temperature Temperature ﬁeld of field channel of channel walls, including walls, including some slices—methane-cooled some slices—methane-cooled mode. mode.

After the convergence zone, the fluid appears to be in supercritical gas state. This is also confirmed by the density axial profiles, strictly linked to temperature ones, and by the specific heat distribution, given in Figure 18a: from the throat region, the fluid core is characterized by low values of density and high values of specific heat. In particular, this is evident in the convergence section where a large part of the fluid exhibits the highest values of specific heat while relatively low values of thermal conductivity can be detected. The pseudo-critical condition is observed at about x/L = 0.50 and in that region low thermal conductivity values are observed as depicted by Figure 18b. However, a deterioration mode, characterized by a sort of a thermal barrier between the fluid near to the hot wall and the cold upper part of the channel [22], is not significant for both the demonstrator versions. In fact, aforementioned profiles of the liner temperature do not exhibit this phenomenon and this is due to the channel dimensions (optimized by means of the design codes), relatively high value of the wall roughness and fluid pressure, sufficiently far from the critical value [23].

Aerospace 2021, 8, 151 15 of 20 Aerospace 2021, 8, x 17 of 21

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Figure 17. Methane-cooledFigure 17. Methane-cooled mode—temperature mode—temperature ﬁelds fields plotted plotted for for some some signiﬁcant significant slices. slices.

After the convergenceDEMO-0A zone, the fluid appears to be in supercritical gas state. This is DEMO "brazed" version also confirmed by the densityDensity axial profiles, strictly linked to temperature5000 ones, and by 400 the specific heat distribution,Specific Heat given in Figure 18a: from the throat region, the fluid core is characterized bymethane-cooled low values of mode density and high values of specific heat. In particular, this is ρ evident in the convergence section where a large part of the fluid exhibits4500 the highest values ] of specific3 300 heat while relatively low values of thermal conductivity can be detected. The pseudo-critical condition is observed at about x/L = 0.50 and in that region low thermal conductivity values are observed as depicted by Figure 18b. However, a deterioration cp 4000 mode, characterized by a sort of a thermal barrier between the fluid near to the hot wall 200

and the cold [kg/m Density upper part of the channel [22], is not signiﬁcant for both the demonstrator

versions. In fact, aforementioned profiles of the liner temperature3500 doSpecific Heat [Jkg/K] not exhibit this phenomenon and this is due to the channel dimensions (optimized by means of the design codes), relatively100 high value of the wall roughnessthroat and fluid pressure, sufficiently far from the criticalFigure 17. value Methane-cooled [23]. mode—temperature fields plotted for some3000 significant slices.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 DEMO-0A x/L DEMO "brazed" version (a) Density 5000 400 Specific Heat methane-cooled mode ρ 4500 ] 3 300

cp 4000 200 est [kg/m Density

3500 Specific Heat [Jkg/K]

100 throat

3000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L (a)

Figure 18. Cont.

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1.6E-04 0.25 DEMO-0A DEMO "brazed" version 1.4E-04 Thermal Conductivity Viscosity methane-cooled mode 0.2 1.2E-04

1.0E-04 0.15

8.0E-05

0.1 6.0E-05 Viscosity [Pa s]

4.0E-05 λ

0.05 Thermal Conductivity [W/mK] 2.0E-05 μ throat

0.0E+00 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x/L (b)

Figure 18. FigureMethane-cooled 18. Methane-cooled mode, axial mode, profiles: axial (a proﬁles:) average ( adensity) average and density specific and heat; speciﬁc (b) thermal heat; (b conductivity) thermal and viscosity. conductivity and viscosity.

4. Conclusions4. Conclusions In this paper, theIn this thermal paper, and the fluid-dynamicthermal and fluid-dyna analyses,mic supporting analyses, thesupporting design ofthe design of the the cooling system of the 30-kN thrust class HYPROB LOX/LCH4 demonstrator, were cooling system of the 30-kN thrust class HYPROB LOX/LCH4 demonstrator, were dis- discussed. In particular, the final demonstrator cooling jacket configuration, based on cussed. In particular, the final demonstrator cooling jacket configuration, based on elec- electroplating manufacturing process was analysed by means of 3-D CFD simulations troplating manufacturing process was analysed by means of 3-D CFD simulations and and compared with the behaviour of the original brazed configuration. An NIST real compared with the behaviour of the original brazed configuration. An NIST real gas gas model was adopted to describe the behaviour of evolving fluids and, in particular, of model was adopted to describe the behaviour of evolving fluids and, in particular, of the the transcritical conditions experimented by methane flowing in the cooling system. The transcritical conditions experimented by methane flowing in the cooling system. The se- selected arrangement was discussed by means of results, given in terms of temperature, lected arrangement was discussed by means of results, given in terms of temperature, fluid bulk temperature and pressure profiles. Simulations supported the optimization fluid bulk temperature and pressure profiles. Simulations supported the optimization of of the design of the cooling jacket, realized through electrodeposition: in fact, the most the design of the cooling jacket, realized through electrodeposition: in fact, the most ther- thermally stressed zones, such as the throat region and the re-attachment point zone, mally stressed zones, such as the throat region and the re-attachment point zone, were were identified. The adoptionidentified. of the electrodeposition process, instead of the brazing one, led to significant thermalThe benefits adoption all over of the the electrodeposition cooling jacket. Inproce fact,ss, in instead the case of the of nominal brazing one, led to sig- operating conditionsnificant and thermal if methane benefits cooled all over versions the cooling are considered, jacket. In afact, decrease in the ofcase about of nominal operat- 100 K is observeding bothconditions in the throatand if regionmethane (x/L cooled = 0.61) versions and at are the considered, end of the cylindricala decrease of about 100 K part of the chamberis observed (x/L = both 0.13), in as the shown throat by region Figure (x/L 13. In= 0.61) fact, and convective at the end heat of transfer the cylindrical part of values are generallythe chamber higher in(x/L the = case0.13), of as DEMO-0A shown by and Figure the differences13. In fact, withconvective the brazed heat transfer values configuration areare significantgenerally higher in the in divergent the case partof DEMO-0 (t timesA higherand the at differences maximum) with and the in brazed config- the cylindricaluration part of theare significant cooling system in the (1.2 divergent times on part average), (t times ashigher can beat maximum) observed in and in the cylin- Figure 14. Moreover,drical part a different of the cooling behaviour system is observed (1.2 times in on the average), divergent as zone,can be where observed a in Figure 14. thermal relaxationMoreover, of the liner a different is reported, behaviour since at is x/Lobserved = 0.70, in a the difference divergent of about zone, 180where K is a thermal relax- evaluated comparingation of DEMO-0A the liner resultsis reported, with thesince brazed at x/L configuration = 0.70, a difference ones. This of about is due 180 to K is evaluated the reduced thermalcomparing stratification DEMO-0A of fluid results in the with case the of brazed the electroplated configuration demonstrator, ones. This as is due to the re- also evident fromduced the temperature thermal stratification field given of for fluid the cross-sectionin the case of at the x/L electroplated = 0.70 in Figure demonstrator, 17. as also In addition, theevident water-cooled from the mode temperature was considered field given since for the the first cross-section firing campaign at x/L will = 0.70 be in Figure 17. In conducted consideringaddition, water the aswater-cooled refrigerant tomode accomplish was considered the acceptance since teststhe first of DEMO-0A firing campaign will be and to validateconducted the adopted considering technological water process. as refrigerant In the water-cooled to accomplish mode, the temperatureacceptance tests of DEMO- peaks in the throat0A and region to validate and in the the cylindrical adopted technological section of the process. chamber In decreases the water-cooled by about mode, temperature peaks in the throat region and in the cylindrical section of the chamber decreases by

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30–40 K for a fixed mass flow rate in the electroplated version of demonstrator, as reported in Figure 12. Results about the most significant thermo-physical properties have been presented to describe the transcritical behaviour of methane inside the cooling jacket. Furthermore, temperature fields in Figure 17 have been discussed to underline the thermal stratification occurring inside the channel and the thermal response of the liner. Methane, injected as a compressed liquid, tends to be in those conditions until half divergent, then the fluid layers near the bottom part of the channel tends to behave like a vapour. In the throat region the fluid bulk temperature reaches critical values (about 190 K on average) and all the thermo-physical properties dramatically change in the near-critical conditions as depicted by Figure 18 while the pseudo-critical conditions are attained in the convergent part (at x/L = 0.50). No peaks or temperature irregularities are observed in that zone; thus, no evident thermal deterioration phenomena are significant since pressure is two times the critical values. In the cylindrical part of the chamber, the fluid is fully composed of a supercritical vapour. The present activity was propaedeutic to perform thermo-structural simulations, nec- essary to estimate the lifecycle of the thrust chamber assembly, whose manufacturing has been recently completed and is ready to withstand the firing test campaign. The adoption of the electroplating process, instead of the brazing one, to join the liner and the close-out has provided evident benefits in terms of the thermal response of the demonstrator (besides the other advantages linked to the process repeatability and the absence of heat treatments). It is expected that the objective of 5 firing tests with a duration of 30 s (considering a safety margin of 4) will be fulfilled. After completing the test activity, experimental results will be extracted to run further simulations and accomplish a wide numerical rebuilding activity.

Author Contributions: For the present research paper, authors declare as following: Conceptualiza- tion, D.R., F.B. and M.F.; methodology, D.R., F.B. and M.F.; software, D.R.; validation, D.R. and F.B.; formal analysis, D.R., F.B. and M.F.; investigation, D.R., F.B. and M.F.; resources, D.R. and F.B.; data curation, D.R. and F.B.; writing—original draft preparation, D.R., F.B. and M.F.; visualization, D.R.; supervision, F.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was performed in the framework of the HYPROB program, funded by the Italian Ministry of University and Research. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data presented in this study are available on request from the corresponding author. Data are not publicly available due to CIRA policy on intellectual copyright. Acknowledgments: This work was performed in the framework of the HYPROB Program, financed by the Italian Ministry of University and Research. The authors would like to thank our colleagues, Daniele Cardillo, Pasquale Natale and Michele Ferraiuolo, whose efforts were thoroughly appreciated, as well as Andrea Ceracchi, Luca Manni and CECOM personnel for their professionalism and competences, proved throughout the cooperation activities. Conflicts of Interest: The authors declare no conflict of interest. Aerospace 2021, 8, 151 18 of 20

Nomenclature

b Cooling channel width: m BB Breadboard CFD Computational Fluid-Dynamic –1 –1 cp Specific heat, J kg K CP Cylindrical part of the combustion chamber CT Convergent section d Diameter, m DEMO Demonstrator FSBB Full Scale BreadBoard h Channel height of DEMO, m –2 –1 hc Convective heat transfer coefficient, W m K HS Heat Sink HYPROB Hydrocarbon PROpulsion test Bench I Impulse, s k Turbulence kinetic energy, m2 s–2 L Length of test articles, m LCH4 Liquid methane LOX Liquid oxygen LRE Liquid rocket engine m Mass flow rate, kg s–1 MTP Methane thermal properties NIST National Institute of Standard and Technology NZ Nozzle section O/F Mixture ratio (oxidizer mass/fuel mass), - P Pressure, Pa q Input heat flux, W m–2 SSBB SubScale BreadBoard sst shear stress transport T Temperature, K w Rib width, m x, y, z Spatial coordinates, m Greek symbols λ Thermal conductivity, W m–1 K–1 µ Viscosity, Pa s ρ Density, kg m3 ω Specific dissipation rate of turbulence kinetic energy, s–1 Subscripts av average aw adiabatic wall b bulk cc combustion chamber ch channel cr critical cu copper f fluid h hydraulic hg hot gas side in inlet l liner ni nickel out outlet pc pseudo-critical s static sp specific t throat w wall Aerospace 2021, 8, 151 19 of 20

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