Corrosion-Fatigue Failure of Gas-Turbine Blades in an Oil and Gas Production Plant

materials

Article Corrosion-Fatigue Failure of Gas-Turbine Blades in an Oil and Gas Production Plant

Mojtaba Rajabinezhad 1, Abbas Bahrami 1, Mohammad Mousavinia 2, Seyed Jalil Seyedi 3 and Peyman Taheri 4,*

1 Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran; [email protected] (M.R.); [email protected] (A.B.) 2 Inspection and Corrosion Engineering Department, Pars Oil and Gas Company (POGC), Assalouyeh 1414713111, Iran; [email protected] 3 National Iranian South Oilﬁelds Company (NISOC), South Turbine Maintenance Centeral Shop, Omidieh 61735-1333, Iran; [email protected] 4 Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands * Correspondence: [email protected]; Tel.: +31-15-278-1683

Received: 10 January 2020; Accepted: 14 February 2020; Published: 18 February 2020

Abstract: This paper investigates the root cause of a failure in gas-turbine blades, made of Nimonic-105 nickel-based superalloy. The failure was reported in two blades in the second stage of a turbine-compressor of a gas turbine in the hot section. Two failed blades were broken from the root and from the airfoil. The failure took place after 20 k h of service exposure in the temperature range 700–850 ◦C, with the rotating speed being in the range 15,000–16,000 rpm. The microstructures of the failed blades were studied using optical/electron microscopes. Energy dispersive X-ray spectroscopy (EDS) was employed for phase identiﬁcation. Results showed that failure ﬁrst initiated from the root. The dominant failure mechanism in the root was concluded to be corrosion-fatigue. The failure scenario was suggested based on the results obtained.

Keywords: gas turbine; corrosion-fatigue; blades; failure

1. Introduction Gas-turbine blades are known to be extremely critical and important components in power plants. They are expected to operate in harsh working conditions, such as high temperature, high pressure and complex dynamic loading conditions. Degradation and failure of gas-turbine blades can obviously have severe negative implications for the integrity and functionality of gas turbines and hence the output of a power plant. Failures of gas-turbine blades can be due to creep damage, high-temperature corrosion, high-temperature oxidation, fatigue, erosion, and foreign object damage [1–15]. Obviously, in most cases the failure is due to the interrelation of more than one failure mechanism [2]. As a case in point, creep and fatigue can simultaneously result in failure in gas-turbine blades [2]. Increasing the operating temperature in gas turbines has always been a target, given that any increase in service temperature is associated with an increase in the efficiency and output of a turbine. Among different candidate materials for harsh high-temperature working conditions in turbines, nickel- and cobalt-based superalloys have long been known as the best options on the grounds that these alloys are capable of working at high temperature, while they keep their properties [16]. These alloys benefit from a unique and exceptional combination of high-temperature mechanical properties (i.e., superior creep resistance), excellent resistance to high-temperature degradation/oxidation, and superior microstructural stability during high-temperature service [17]. Nickel-based superalloys

Materials 2020, 13, 900; doi:10.3390/ma13040900 www.mdpi.com/journal/materials Materials 2020, 13, 900 2 of 8 are extensively used in critical components in aerospace and gas-turbine engines. In fact, about 50 wt.% of aero/gas turbine-engines are made of nickel-based superalloys [18]. Other applications of nickel-based superalloys include chemical/petrochemical plants, high-temperature reactors, and marine applications. Diﬀerent grades of nickel-based superalloys are now currently used in diﬀerent applications, amongst which Inconel 718 is one of the most widely used. Other widely used grades include FGH95, ME-16, RR1000, IN-100, Udimet 720LI, Nimonic 80A, Inconel 825, Nimonic C-263, Nimonic-75, and Nimonic-105 [19]. The latter alloy is extensively used in hot combustion chambers of gas turbines [20], with service temperatures being as high as 950 ◦C. Nimonic-105 is often cast in air. For more demanding and sensitive applications, vacuum melting is highly recommended. This research investigates a failure in gas-turbine blades in the second stage of a turbine-compressor of a gas turbine (i.e., the gas producer or hot section) in an oil and gas production plant. The failed blades were made of Nimonic-105 nickel-based superalloy. The main focus in this investigation is on the microstructural and metallurgical aspects of the failure. Even though the consequences of failures in the third row might not be as severe as those in the first and second rows, they need to be evaluated and analyzed carefully. Failures of blades in a gas turbine are often catastrophic in the sense that this causes severe damages in the turbine, associated with a significant economic loss. Therefore, it is necessary to conduct a failure investigation to analyze the root cause of failure and eventually to mitigate similar failures in any oil- and gas-production and power plants. This certainly has positive implications for the system reliability and service life of a gas turbine. Lessons learnt from a failure study will certainly be useful in planning failure mitigation strategies in similar plants/set-ups [21–30]. It will be shown that the failure, reported in this study, is a corrosion-fatigue failure. Given that not so many corrosion-fatigue failures have been reported in the literature, this manuscript will certainly be useful, especially when it comes to analyzing/controlling the same form of damage in similar gas turbines.

2. Experimental The microstructural analyses in this case were performed using scanning electron microscopy (SEM, Philips, Eindhoven, The Netherlands) and optical microscopy (OM, Nikon, Japan). Energy-dispersive X-ray spectroscopy (Philips, The Netherlands) was employed to study chemical compositions of different phases in the microstructure. Failed blades were first visually evaluated. Macro images were then taken by Stereo Microscope Nikon SMZ 800 (Nikon, Tokyo, Japan). For optical microscopy, samples were first ground with grinding papers. Samples were then polished with diamond paste (3 and 1 µm). Fractography was also performed on failed specimens to determine dominant failure mechanisms.

3. Case Background This failure is related to a gas turbine where its input air volume can be as high as 5220 m3/h. Approximately 40% of this sucked-in air is used for combustion. The sucked-in air pressure is close to 100 psi. The combustion air to fuel ratio is 15. The fuel is a methane/ethane gas mixture which has been dried before entering into the gas turbine. The failure in this case was in blades in the second stage in which 69 blades are assembled on a disk. Based on the available speciﬁcation, blades are made of Nimonic-105. Nickel-based rotating blades are expected to operate under extreme working conditions, where erosion and corrosion phenomena come into play. Besides, blades in nickel-based superalloys in aggressive working conditions are expected to show great resistance against cavitational erosion. Blades in the second stage are expected to have service life over 100 k h up to 120 k h. In this case, blades failed after approximately 20 k h service, which makes this failure a typical premature incident. A major drawback of having a blade fail in a whole assembly is that all the blades have to be replaced with brand new blades or blades with similar service lifetime. This has to do with the fact that it is a routine practice to make sure all blades are in similar conditions in terms of service life. This makes the interpretation of non-destructive testing (NDT) results much easier. The working temperature in the hot section of the turbine is 700–850 ◦C. The rotating speed of the blades is in Materials 2020, 13, 900 3 of 8

the rangeMaterials 15,000–16,000 2020, 9, x FOR PEER rpm. REVIEW The blades are made of Nimonic-105 nickel-based superalloy3 withof 8 the following chemical composition (Table1). easier. The working temperature in the hot section of the turbine is 700–850 °C. The rotating speed of the blades is Tablein the 1.rangeChemical 15,000–16,000 composition rpm. of Nimonic-105The blades are nickel-based made of Nimonic-105 superalloy. nickel-based superalloy with the following chemical composition (Table 1). Element C Si Cu Fe >Mn Cr Ti Al Co Mo Zr Ni Table 1. Chemical composition of Nimonic-105 nickel-based superalloy. wt. % 0.1 0.5 0.1 0.5 0.6 15.0 1.0 4.5 19.0 5.0 0.1 Bal. Element C Si Cu Fe Mn Cr Ti Al Co Mo Zr Ni 4. Results and Discussionwt. % 0.1 0.5 0.1 0.5 0.6 15.0 1.0 4.5 19.0 5.0 0.1 Bal.

4.1. Visual4. Results Observations and Discussion The4.1. Visual reported Observations failure in this case was related to two turbine blades as shown in Figure1, with one failing fromThe the reported root and failure the in other this fromcase was the related airfoil. to These two turbine two blades blades were as shown located in Figure in the 1, vicinity with of each other.one failing There from are the a fewroot observationsand the other from worth the mentioning.airfoil. These two First blades of all, were there located was noin the severe vicinity surface corrosionof each on other. either There side ofare the a few blades, observations except for worth a brownish mentioning. spot First shown of all, in Figurethere was1. Thisno severe does not appearsurface to be corrosion a severely on corrodedeither side spot.of the Also,blades, there except was for noa brownish excessive spot deformation shown in Figure (i.e., 1. noticeable This thinningdoes or not buckling) appear to in be the a profileseverely ofcorroded the failed spot. airfoil. Also, Onethere canwas also no excessive see that theredeformation was no (i.e., surface irregularitynoticeable observed thinning in or the buckling) failed blades. in the profile This includesof the failed surface airfoil. cracking One can or also pitting. see that In there one was edge no of the failedsurface airfoil, irregularity one can see observed a dent, whichin the failed seems blades. to be dueThis toincludes the collision surface ofcracking a high-energy or pitting. flying In one object. edge of the failed airfoil, one can see a dent, which seems to be due to the collision of a high-energy The last important visual observation to mention is that the fracture in the airfoil has two areas: the flat flying object. The last important visual observation to mention is that the fracture in the airfoil has surfacetwo and areas: the the part flat with surface 45-degree and the inclination.part with 45-degree This is inclination. clearly shown This is in clearly Figure shown1c. in Figure 1c.

FigureFigure 1. Visual 1. Visual observation observation of of broken broken blade, blade, ( a(a)) andand overview of of blades blades in inthe the assembled assembled condition, condition, (b) failed(b) failed blade blade from from the the root, root, and and (c, d(c),d broken) broken blade blade fromfrom the airfoil. airfoil.

4.2. Microstructure4.2. Microstructure of Failed of Failed Blades Blades FigureFigure2 depicts 2 depicts the typicalthe typical microstructure microstructure in in the the failedfailed blades. The The microstructure microstructure is a istypical a typical dendriticdendritic cast structurecast structure where where dendrites dendrites have have grown grown from from two two sides side towardss towards the the centerline centerline of of the the blade, blade, as expected. Fine white particles in-between dendrites are carbide particles. as expected. Fine white particles in-between dendrites are carbide particles.

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FigureFigure 2. (2.a) ( Macroa) Macro and and ( b(b)) optical optical microscope images images of offailed failed blades. blades.

4.3. Fractography4.3. Fractography of Failed of FigureFailed Blades Blades2. (a) Macro and (b) optical microscope images of failed blades.

Figure4.3. FractographyFigure3 shows 3 shows stereomicroscope of Failed stereomicroscope Blades and and SEM SEM micrographs micrographs of of the the fracture fracture surface surface from from thethe bladeblade that failedthat from failed the from root. the As root. shown As inshow Figuren in3 Figurea, the fracture3a, the fracture surface surface has two has areas: two areas: the dominant the dominant grayish Figure 3 shows stereomicroscope and SEM micrographs of the fracture surface from the blade area andgrayish the restarea ofand the the surface. rest of This the issurface. a typical This fracture is a typical surface fracture when fatiguesurface iswhen the governingfatigue is damagethe governingthat failed damagefrom the mechanism. root. As show Figuren in Figure3b,c show 3a, theSEM fracture micrographs surface ofhas two two areas, areas: highlighted the dominant in mechanism. Figure3b,c show SEM micrographs of two areas, highlighted in Figure3a. The former Figuregrayish 3a. area The and former the imagerest of (Figurethe surface. 3b) shows This typiis a caltypical beach fracture marks, surfaceindicating when that fatiguefatigue isis thean imageactivegoverning (Figure mechanism.3 b)damage shows Further mechanism. typical down beach Figureto marks,the edge,3b,c indicating show there SEMis a thatclear micrographs fatiguechange isin of anthe two active fracture areas, mechanism. surface highlighted (Figure Further in down3c).Figure to theThe 3a. edge,surface The there formerat the is edge aimage clear of (Figurethe change failed 3b) in blade theshows fractureis coveredtypical surface beachwith some (Figuremarks, deposits, indicating3c). The which surface that were fatigue at analyzed the is edge an of the failedbyactive EDS. blade mechanism. Results is covered of FurtherEDS with analysis down some of to deposits, thethe microstructureedge, whichthere is were a at clear the analyzed changeedge (presented in by the EDS. fracture in Results Figure surface of 4) EDS showed (Figure analysis of thethat3c). microstructure Thesurface surface deposits at at the the are edge edge rich of in (presentedthe Cl, failed K, Na, blade in and Figure is Ca. covered 4This,) showed togetherwith some that with surfacedeposits, the fact deposits which that therewere are isanalyzed rich a high in Cl, K, Na, andconcentrationby EDS. Ca. This,Results of together oxygenof EDS in withanalysis the theanalysis, of fact the thatindicates microstructure there that is athese high at thedeposits concentration edge are(presented in fact of corrosion in oxygen Figure products.in 4) theshowed analysis, It that surface deposits are rich in Cl, K, Na, and Ca. This, together with the fact that there is a high indicatesis highly that likely these that deposits the aforemention are in facted corrosion elements products. come from It the is highlyfuel/gas. likely that the aforementioned concentration of oxygen in the analysis, indicates that these deposits are in fact corrosion products. It elements come from the fuel/gas. is highly likely that the aforementioned elements come from the fuel/gas.

Figure 3. (a) Stereomicroscope and (b,c) scanning electron microscope (SEM) micrographs of the

fracture surface of the blade, failed from the root. FigureFigure 3. (a )3. Stereomicroscope(a) Stereomicroscope and and (b (,bc,)c) scanning scanning electronelectron microscope microscope (SEM) (SEM) micrographs micrographs of the of the fracturefracture surface surface of the of blade,the blade, failed failed from from the the root. root.

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FigureFigure 4. Energy 4. Energy dispersive dispersive X-ray X-ray spectroscopy spectroscopy (EDS)(EDS) analysisanalysis of of corrosion corrosion prod productsucts at the at theedge edge of of broken blade. brokenFigure blade. 4. Energy dispersive X-ray spectroscopy (EDS) analysis of corrosion products at the edge of broken blade. Results of EDS analysis of the microstructure further away from the edge is presented in Figure Results of EDS analysis of the microstructure further away from the edge is presented in Figure5. 5. There is hardly any oxygen, Na, Ca, K, and Cl in the analysis, inferring that the aforementioned There is hardlyResults anyof EDS oxygen, analysis Na, of the Ca, microstructure K, and Cl in furt theher analysis, away from inferring the edge thatis presented the aforementioned in Figure 5.corroded There is area hardly is limited any oxygen, to few Na,hundred Ca, K, micrometers and Cl in the from analysis, the edge. inferring that the aforementioned corroded area is limited to few hundred micrometers from the edge. corroded area is limited to few hundred micrometers from the edge.

Figure 5. EDS analysis of the area with beach marks in the failed blade, broken from the root. FigureFigure 5. EDS 5. EDS analysis analysis of of the the area area with with beach beach marks in in the the failed failed blade, blade, broken broken from from the root. the root. From the results obtained, it can be concluded that the failure in the blade, broken from the root, was controlled by the simultaneous activation of corrosion and fatigue. Corrosion-fatigue is a FromFrom the the results results obtained, obtained, it it cancan be be concluded concluded that that the thefailure failure in thein blade, the broken blade, from broken the root, from the common degradation mechanism in moving components exposed to high-temperature harsh root,was controlled by by the the simultaneous simultaneous activation activation of ofcorrosion corrosion and and fatigue. fatigue. Corrosion-fatigue Corrosion-fatigue is a is a commonworking conditionsdegradation [21]. mechanism Corrosion fatiguein moving essentiall compy onentstakes place exposed under to the high-temperature synergistic activation harsh of common degradation mechanism in moving components exposed to high-temperature harsh working workingcorrosion conditions and dynamic [21]. loading. Corrosion This fatigue degradation essentiall mechanismy takes place is one under of the the most synergistic common activation reasons forof conditionscorrosionthe failure [21 ].and of Corrosion turbinedynamic blades loading. fatigue [22]. essentiallyThis Corrosion degradation takesfatigu mechanism placee is undoubtedly under is one the of synergisticone the of most the mostcommon activation complex reasons of failure corrosion for and dynamicthemechanisms. failure loading. of turbineBoth Thismechanical blades degradation [22]. and Corrosion chemical mechanism fatiguaspectse is isof one undoubtedlyfailure of thein this most one case commonof are the equally most reasons compleximportant. for failure the Any failure of turbinemechanisms.mitigation blades measure [Both22]. mechanical Corrosion to avoid this and fatigue failure chemical is necessitate undoubtedly aspectss ofa multi-disciplinaryfailure one ofin thethis mostcase approach.are complex equally Corrosion-fatigue failure important. mechanisms. Any Bothmitigation mechanicalfailure of themeasure andblade chemical toroot avoid in this this aspects case failure has of necessitate failurepossibly in originateds thisa multi-disciplinary case from are equallycorrosion approach. important. pits/products Corrosion-fatigue Any deposited mitigation measurefailureat the to root/airfoilof avoid the blade this interface. failureroot in this necessitatesThe case results has showedpossibl a multi-disciplinaryy th originatedat corrosion from products/deposits approach. corrosion pits/products Corrosion-fatigue in this case deposited mostly failure of theatcontained blade the root/airfoil root CaO, in Na this interface.2O, case KCl, has Theand possibly resultsNaCl. The showed originated sensitivity that corrosion of from turbine corrosion products/deposits blades to pits corrosion-fatigue/products in this deposited case initiation mostly at the contained CaO, Na2O, KCl, and NaCl. The sensitivity of turbine blades to corrosion-fatigue initiation root/airfoil interface. The results showed that corrosion products/deposits in this case mostly contained CaO, Na2O, KCl, and NaCl. The sensitivity of turbine blades to corrosion-fatigue initiation largely depends on the integrity of the passive ﬁlm on the surface [22]. The breakdown of this passive ﬁlm Materials 2020, 13, 900 6 of 8

Materials 2020,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 6 of 8 together with the deposition of oxide/corrosion deposits create an ideal condition for corrosion-fatigue largelylargely dependsdepends onon thethe integrityintegrity ofof thethe passivepassive ffilmilm onon thethe surfacesurface [22].[22]. TheThe breakdownbreakdown ofof thisthis crack initiation. The film breakdown and deposition of corrosion products causes anodic/cathodic sites, passive film together with the deposition of oxide/corrosion deposits create an ideal condition for leading to an accelerated localized anodic dissolution reaction and, hence, localized corrosion [23]. corrosion-fatigue crack initiation. The film breakdown and deposition of corrosion products causes The formationanodic/cathodic of corrosion-induced sites, leading to surfacean accelerated irregularities localized creates anodic stress dissolution riser centers reaction on and, the surfacehence, [24]. In caselocalizedlocalized these surfacecorrosioncorrosion irregularities [23].[23]. TheThe formationformation exceed ofofa corrosion-icorrosion-i critical size,nduced a fatigue surface crack irregulariti is likelyes creates to initiate. stress Theriser crack thencenters propagates on the under surface dynamic [24]. In case loading these condition, surface irre finallygularities leading exceed the a critical failure size, of thea fatigue turbine crack blade. is Figurelikelylikely toto6 showsinitiate.initiate. stereomicroscope TheThe crackcrack thenthen propagatespropagates/SEM micrographs underunder dynamicdynamic ofthe loadingloading fracture condition,condition, surface finallyfinally of the leadingleading sample, thethe failed fromfailure the airfoil of the (see turbine Figure blade.1c,d). SEM micrographs, shown from di fferent areas, indicate that there is Figure 6 shows stereomicroscope/SEM micrographs of the fracture surface of the sample, failed hardly anyFigure difference 6 shows between stereomicroscope/SEM fracture modes micrographs in different of areas. the fracture More importantly,surface of the theresample, is failed absolutely from the airfoil (see Figure 1c,d). SEM micrographs, shown from different areas, indicate that there is no indicationfrom the airfoil that fatigue (see Figure has been1c,d). anSEM active micrographs, mechanism shown in this from blade, different i.e., areas, there indicate is no beach that markthere is on the hardly any difference between fracture modes in differentdifferent areas.areas. MoreMore importantly,importantly, therethere isis fracture surface. Figure7 shows SEM micrographs of the fracture surface with higher magnification, absolutely no indication that fatigue has been an active mechanism in this blade, i.e., there is no again confirming that there is no fatigue involved in this case. Fracture in this sample has elements beach mark on the fracture surface. Figure 7 shows SEM micrographs of the fracture surface with of ductilehigher and magnification, brittle fractures. again confirming There are that some there areas is no with fatigue colonies involved of dimplesin this case. and Fracture some in areas this with cleavagesample fracture. has elements The fact of thatductile there and is brittle no fatigue fracture in thiss. There blade are is some indicative areas with that thiscolonies blade of hasdimples fractured due toand a suddensome areas overloading with cleavage and fracture./or collision. The fact The that logical there is failure no fatigue scenario in this is blade that is the indicative blade that that failed fromthis the blade root has has first fractured undergone due to aa corrosion-fatiguesudden overloading failure. and/or The collision. broken The piece logical has failure then hitscenario the adjacent is blade,that causing the blade the that fracture failedof from the the blade root from has firs thet airfoil.undergone The a dentcorrosion-fatigue observed at failure. the edge The of broken the airfoil piece has then hit the adjacent blade, causing the fracture of the blade from the airfoil. The dent (highlightedpiece has in then Figure hit the1c,d) adjacent has possibly blade, causing formed the as afracture result of the a collision blade from between the airfoil. the flyingThe dent broken observed at the edge of the airfoil (highlighted in Figure 1c,d) has possibly formed as a result of a pieceobserved and the airfoil.at the edge of the airfoil (highlighted in Figure 1c,d) has possibly formed as a result of a collision between the flying broken piece and the airfoil.

FigureFigure 6. Images6. ImagesImages of ofof fracture fracturefracture surface surfacesurface ofof thethe the blade, blade, broken broken from from the the airfoil. airfoil.

FigureFigure 7. ( a7.,b ((a),, SEMb)) SEMSEM micrographs micrographsmicrographs of ofof fracture fracturefracture surfacesurface ofof of thethe the blade,blade, blade, brokenbroken broken fromfrom from thethe airfoil. theairfoil. airfoil.

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5. Concluding Remarks This paper investigates a failure in gas-turbine blades made of Nimonic-105 nickel-based superalloy. Two broken blades were investigated, with one having failed from the root and the other from the airfoil. Blades are from the second row of the gas producer/hot section of a gas turbine with working temperature being in the range 700–850 ◦C and rotating speed in the range 15,000–16,000 rpm. The failure was reported after 20 k hours of service, which is much shorter than the anticipated service life. The following conclusions can be drawn, based on the data obtained:

Nimonic-105 superalloy in this case had a typical dendritic cast structure, with dendrites growing − from the edges towards the center. There were no surface irregularities, excessive deformation, buckling, or surface cracking observed − on the surface of the blades, except for a dent on the edge of one of the broken blades. Results showed that corrosion fatigue is the governing failure mechanism in the blade that failed − from the root. In this case, there are corrosion deposits at the edge of the blade, rich in Cl, Ca, O, Na, and K. Fatigue cracks have initiated from corroded spots at the edge of the blade. Fractography of the blade that failed from the airfoil showed that, contrary to the former case, − there is hardly any fatigue involved in the failure of this blade. It was concluded that the blade that failed from the root first underwent a corrosion-fatigue − failure, finally ending up in the breakage of the airfoil from the root. The broken piece then hit the adjacent blade in its trajectory, causing the fracture of the adjacent blade from the airfoil. The dent observed at the edge of the airfoil possibly formed as a result of the collision between the flying broken piece and the airfoil. Given that both corrosion and fatigue mechanisms were involved in this failure, it is important to − minimize mechanical vibrations and to reduce contaminants in the system. The latter necessitates further investigation on the origins of the Cl, Na, and Ca-containing residues and how they can be controlled/eliminated from the system.

Author Contributions: Conceptualization, M.R. and M.M.; methodology, M.R., S.J.S. and A.B.; formal analysis, A.B., S.J.S. and M.M.; investigation, M.R. and M.M.; writing—original draft preparation, A.B.; writing—review and editing, A.B. and P.T.; visualization, A.B.; supervision, A.B., M.M. and P.T.; project administration, P.T. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: Authors would like to thank Delft University of Technology for support in publishing this manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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