Influence of Swirl Clocking on the Performance of Turbine Stage With

energies

Article Inﬂuence of Swirl Clocking on the Performance of Turbine Stage with Three-Dimensional Nozzle Guide Vane

Shinyoung Jeon 1, Changmin Son 2,* and Jinuk Kim 3

1 School of Mechanical Engineering, Pusan National University, Pusan 46241, Korea; [email protected] 2 Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA 3 Gas Turbine Center, Doosan Heavy Industries & Construction, Changwon 51711, Korea; [email protected] * Correspondence: [email protected]; Tel.: +1-540-231-1924

Abstract: The effect of the swirl clocking on three-dimensional nozzle guide vane (NGV) is investi- gated using computational fluid dynamics. The research reports the loss characteristics of leaned and swept NGVs and the influence of swirl clocking. The three-dimensional NGVs are built by stacking the same 2D profile along different linear axes, characterized by different angles with respect to the normal or radial direction: ε = −12◦ ~ +12◦ for the leaned and γ = −5◦ ~ +10◦ for the swept airfoils. A total of 40 models are analyzed to study the effects of lean and sweep on aerodynamic performance. To investigate the influence of swirl clocking, the analysis cases include the center of the swirl that was positioned at the leading edge as well as the middle of the passage. The prediction results show that the relationship of the changes in mass flow rate and throat area are not monotonic. Further

observation confirms the redistribution of loading and flow angle under different lean and sweep angles; thus, three-dimensional design is a key influencing factor on aerodynamic performance. In

Citation: Jeon, S.; Son, C.; Kim, J. the presence of swirl clocking, NGV performance is changed significantly and the findings offer new Influence of Swirl Clocking on the insight and opportunities to improve three-dimensional NGV airfoil design. Performance of Turbine Stage with Three-Dimensional Nozzle Guide Keywords: total pressure loss; stage efficiency; lean angle; sweep angle Vane. Energies 2021, 14, 5503. https://doi.org/10.3390/en14175503

Academic Editor: Andrea De Pascale 1. Introduction The Nozzle Guide Vane (NGV) of a turbine experiences a highly turbulent combustion Received: 11 August 2021 flow with a complicated swirl structure [1,2]. The role of NGV is to turn the flow to the right Accepted: 31 August 2021 angle for downstream rotor and stages while controlling the flow rate through its throat Published: 3 September 2021 area. In addition, further cooling and structural requirements should be considered to withstand high pressure and temperature operating conditions. Therefore, this motivates Publisher’s Note: MDPI stays neutral researchers to develop robust approaches for improving overall efficiency by controlling with regard to jurisdictional claims in flow through NGV. One of which is to implement lean and sweep angles to the NGV. Lean published maps and institutional affil- is a stacking-line modification in which NGV sections are moved relative to each other in iations. the circumferential direction. For sweep, the modification applies to the axial direction. Therefore, the influence of lean, sweep and swirl on the three-dimensional aerodynamics of turbine passage is of great interest to researchers. The very early concept of radial airfoil stacking was introduced about sixty years ago. Straight lean and compound lean Copyright: © 2021 by the authors. have been recognized as an effective way to control the reaction, loading and secondary Licensee MDPI, Basel, Switzerland. flows [3–11]. It has been reported that the straight lean and compound lean can reorganize This article is an open access article the vortices and reduce the secondary flow loss. As a result, the secondary flow loss distributed under the terms and decreased remarkably by using appropriate leaned blades [12–16]. The sweep commonly conditions of the Creative Commons occurred because the meridional passage of the main annulus is not typically at a constant Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ radius, whereas the NGV is stacked on a near radial line in the meridional views. The 4.0/). secondary flow structure and the pressure distribution of the NGV passage are closely

Energies 2021, 14, 5503. https://doi.org/10.3390/en14175503 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 26

views. The secondary flow structure and the pressure distribution of the NGV passage are closely linked so that the sweep angle affects the secondary loss mechanism [17,18]. However, the sweep angle may simply shift the loss from the endwall region to mid-span [19,20]. The potential impact and limitation of the understanding were reviewed as the technology developed [21,22]. It has been almost twenty years since the advanced computational fluid dynamics (CFD) and experimental approach have been brought into turbomachinery research. Comprehensive results from experiments and CFD are reported to enhance the understanding of the root cause [3–7,9–15,17–29]. Furthermore, the design optimization approach is being applied to offer more complicated control of design pa- Energies 2021, 14, 5503 rameters [8,16,30]. 2 of 25 While the previous research on lean and sweep reported its influence on three-dimensional aerodynamics of airfoil passage, there is rarely any report in conjunction with linked so that the sweep angle affects the secondary loss mechanism [17,18]. However, swirlthe sweep clocking. angle may Similarly, simply shift the the research loss from theon endwall the swirl region effect to mid-span on NGV [19,20 aerodynamic]. perfor- manceThe potential [21,23–26] impact and did limitation not consider of the understanding the lean and were sweep. reviewed The as the present technology study aims to investi- gatedeveloped the influence [21,22]. It has of beenthe almostthree-dimensionali twenty years sincety theof advancedNGV on computational the turbine stage considering fluid dynamics (CFD) and experimental approach have been brought into turbomachinery moreresearch. realistic Comprehensive operation results conditions. from experiments Therefore, and CFD a are1.5 reported stage turbine to enhance with the three-dimensional NGVunderstanding airfoil is of modeled the root cause for [3 simulating–7,9–15,17–29]. the Furthermore, swirl clocking. the design A optimization series of CFD is conducted to (1)approach confirm is being the applied findings to offer from more early complicated research control and of design (2) reveal parameters the [ 8further,16,30]. understanding of While the previous research on lean and sweep reported its influence on three- thedimensional impact aerodynamicsof swirl clocking of airfoil on passage, the stage there pe is rarelyrformance. any report A parametric in conjunction geometry is created forwith the swirl present clocking. study. Similarly, The the parameters research on theto vary swirl effectthe NGV on NGV geometry aerodynamic are the angles of lean (+12performance ~ −12, [21Δ ,23= –6°),26] did compound not consider lean the lean (+12 and ~ sweep. −12, TheΔ = present 6°), and study sweep aims to in-(+10 ~ −5, Δ = 5°). In vestigate the influence of the three-dimensionality of NGV on the turbine stage considering addition,more realistic the operation study conditions.extended Therefore, to include a 1.5 the stage swir turbinel clocking with three-dimensional by positioning the swirl core at theNGV leading airfoil is edge modeled and for simulatingpassage center the swirl of clocking. NGV. A For series the of CFDanalysis, is conducted the variation to of the shape of(1) NGV confirm is the achieved findings from by earlyusing research in-house and (2) tools. reveal Then, the further the understanding produced airfoil of geometries are the impact of swirl clocking on the stage performance. A parametric geometry is created transferredfor the present to study. TurboGrid The parameters (Version to vary 19.2 the R2, NGV ANSYS geometry Inc, are Canonsburg, the angles of lean PA) to generate computational(+12 ~ −12, ∆ grids.= 6◦), compound In CFX (Version lean (+12 ~ 19.2−12, ∆R2,= 6 AN◦), andSYS sweep Inc, (+10 Canonsburg, ~ −5, ∆ = 5◦). InPA), the properties of combustionaddition, the study gas extendedand total to includeto static the swirlpressure clocking ratio by positioning are applied the swirl for corethe atcalculation. Uniform the leading edge and passage center of NGV. For the analysis, the variation of the shape inletof NGV total is achieved temperature by using and in-house adiabatic tools. Then,walls the ar producede assumed. airfoil To geometries model turbulence, are SST (Shear Stresstransferred Transport) to TurboGrid is (Versionselected 19.2 with R2, ANSYS an inlet Inc., freestream Canonsburg, PA,turbulence USA) to generate intensity of 5%. In total, 108computational cases are grids.analyzed In CFX for (Version the present 19.2 R2, ANSYSinvestigation. Inc., Canonsburg, PA, USA), the properties of combustion gas and total to static pressure ratio are applied for the calculation. Uniform inlet total temperature and adiabatic walls are assumed. To model turbulence, 2.SST Computational (Shear Stress Transport) Model is selected and Approach with an inlet freestream turbulence intensity of 5%. In total,A nozzle 108 cases guide are analyzed vane for of the the present first investigation.stage turbine of an industrial gas turbine is used as a baseline2. Computational configuration Model and for Approach CFD validation. The airfoil at 50% span height is shown in Fig- ure 1.A The nozzle NGV guide has vane an of axial the first chord stage of turbine 123.6 of mm an industrial at 50% span gas turbine height is usedand a total of 48 airfoils aroundas a baseline the configurationannulus. The for CFDheight validation. of the The airfoil airfoil is at 157.20 50% span mm height at isthe shown leading edge (LE) and in Figure1. The NGV has an axial chord of 123.6 mm at 50% span height and a total of the48airfoils NGVaround exit angle the annulus. is 71.2° The at height 50% ofspan the airfoil height. is 157.20 The mm total at theto leadingtotal pressure edge ratio is 1.86 and the(LE) stage and the efficiency NGV exit angle is 91.8%. is 71.2◦ at The 50% inlet span height. Reynolds The total number to total pressure is 1.32 ratio × 10 is6 and the exit Mach × 6 number1.86 and the at stagedesign efficiency condition is 91.8%. is The 0.77 inlet at Reynolds mid-span number height. is 1.32 10 and the exit Mach number at design condition is 0.77 at mid-span height.

Figure 1. Nozzle guide vane proﬁle.

Figure 1. Nozzle guide vane profile.

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TheThe computationalcomputational domaindomain isis shownshown in in Figure Figure2 ,2, which which starts starts from from 0.6 0.6 C Caxaxupstream upstream ofof thethe leadingleading edgeedge (LE) (LE) and and ends ends at at 0.1 0.1 C Caxax downstreamdownstream ofof thethe trailingtrailingedge. edge. TheThe principalprincipal geometricgeometric parametersparameters ofof thethe NGVNGV andand boundaryboundary conditionsconditions utilizedutilized inin thisthis researchresearch areare shownshown inin TableTable1 .1.

FigureFigure 2.2.Computational Computational domaindomain andand surfacesurface meshmesh distribution.distribution.

TableTable 1. 1.NGV NGV geometric geometric parameters parameters and and boundary boundary conditions conditions at at 50% 50% span span height. height.

ParametersParameters UnitUnit NGVNGV BladeBlade AirfoilAirfoil count count - - 36 36 71 71 AxialAxial chord, chord, Cax Cax mmmm 123.6123.6 113.2113.2 ExitExit metal metal angle, angle,β β ◦ ° 71.271.2 56.556.5 TipTip clearance clearance mm mm - - 2.2 2.2 Inlet total pressure Bar 22 - InletInlet total total temperature pressure K Bar 1873 22 - - StageInlet pressure total ratio, temperature p01/p02 - K 1.86 1873 - - StageExit Mach pressure number, ratio, M2 p01/p02 -- 0.771.86 - - × 6 InletExit Reynolds Mach number, number, Re CaxM2 - - 1.32 0.7710 - - Span height to axial chord, Hspan/Cax - 1.43 2.69 Inlet Reynolds number, ReCax - 1.32 106 - Span height to axial chord, Hspan/Cax - 1.43 2.69 The definition of total to total isentropic efficiency in this paper is The definition of total to total isentropic efficiency in this paper is h02 − h01 ηt = (1) h02ℎ0−ℎh01 η (1) ℎ ℎ where ηt is total to total isentropic efficiency, h is static enthalpy, respectively. where η is total to total isentropic efficiency, h is static enthalpy, respectively. (p01 − p02) C = 𝑝 𝑝 (2) p 𝐶 2 0.5ρ2u2 (2) 0.5𝜌𝑢

wherewhereC Cpp isis totaltotal pressurepressure lossloss coefﬁcient,coefficient,p p00 isis totaltotal pressure, ρ isis densitydensity andand u isis velocity. 𝑄𝐶𝛺2 𝑆2 (3) Q = CQ Ω − S (3) where CQ = 0.25, S is the absolute value of the strain rate (s−1) and Ω is the absolute value −1 whereof vorticityCQ = 0.25,(s−1). S is the absolute value of the strain rate (s ) and Ω is the absolute value of vorticity (s−1).

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For this systematic parametric study, a total of 40 NGV configurations applying straightFor lean, this compound systematic lean parametric and sweep study, angl aes total are ofconstructed. 40 NGV configurationsThe baseline model applying has straightstraight lean lean, = compound0°, and sweep lean = and0°. The sweep parametric angles aregeometry constructed. is based The on baseline the baseline model NGV has ◦ ◦ configuration.straight lean = The 0 , andgeometrical sweep = 0representation. The parametric of NGV geometry consists is based of a ontotal the of baseline 11 sections NGV includingconfiguration. hub, mean The geometrical and tip. Each representation cross section of the NGV airfoil consists represented of a total by of the 11 pressure sections sideincluding (PS) and hub, suction mean side and (SS) tip. Eachis designed cross section using the of the Pritchard airfoil represented 11-parameter by method the pressure [31]. Theside baseline (PS) and NGV suction geometry side (SS) uses is a designed simple radi usingal stack the Pritchard around the 11-parameter NGV trailing method edge with [31]. noThe lean baseline and sweep. NGV geometryBy changing uses the a simpleshape of radial stacking stack line, around a new the NGV NGV model trailing is edgeobtained. with no leanThe and other sweep. 39 NGVs By changing are constructed the shape by of appl stackingying line,different a new straight NGV model lean, compound is obtained. lean andThe sweep. other 39 In NGVs the case are of constructed straight lean by models, applying the different stacking straight lines are lean, leaned compound in the lean and sweep. In the case of straight lean models, the stacking lines are leaned in the circumferential direction (Figure 3a). The compound lean has an elliptical tangential circumferential direction (Figure3a). The compound lean has an elliptical tangential stacking profile within the angle inclination at both end-walls (Figure 3a). Positive and stacking profile within the angle inclination at both end-walls (Figure3a). Positive and negative straight lean angles are applied to study its influence and the same approach is negative straight lean angles are applied to study its influence and the same approach is applied to the airfoils with compound lean angles. The sweep is defined as the deviation applied to the airfoils with compound lean angles. The sweep is defined as the deviation of the stacking axis of the airfoil from a line perpendicular to the axisymmetric stream of the stacking axis of the airfoil from a line perpendicular to the axisymmetric stream surface in the meridional view (Figure 3b). As summarized in Table 2, the sweep angle surface in the meridional view (Figure3b). As summarized in Table2, the sweep angle chosen for the present investigation is in the range of −5° ~ +10°. For lean angle, a slightly chosen for the present investigation is in the range of −5◦ ~ +10◦. For lean angle, a slightly wider range angle of −12° ~ +12° is selected. These lean angles provide sufficient geometry wider range angle of −12◦ ~ +12◦ is selected. These lean angles provide sufficient geometry variation to drives the changes in aerodynamic characteristics including loading, second- variation to drives the changes in aerodynamic characteristics including loading, secondary ary flow and mass-flow distribution. Figure 4 shows the overlapped cross-sectional ge- flow and mass-flow distribution. Figure4 shows the overlapped cross-sectional geometries ometriesof four selected of four three-dimensionalselected three-dimensional NGV airfoils. NGV airfoils.

(a) (b)

FigureFigure 3. 3. DefinitionsDeﬁnitions of of lean lean and and sweep sweep angle: angle: ( (aa)) Circumferential Circumferential view—Straight view—Straight lean lean angle angle and and compound compound lean lean angle; angle; ((bb)) Meridional Meridional view—Sweep view—Sweep angle. angle.

Table 2. Lean and sweep angle range. Table 2. Lean and sweep angle range. Parameter Abbreviation Angle range Parameter Abbreviation Angle Range Straight lean angle (εs) SL −12°, −◦6°, 0°,◦ +6°,◦ +12°◦ ◦ Straight lean angle (εs) SL −12 , −6 , 0 , +6 , +12 Compound lean angle (εc) CL −12°, −6°, 0°, +6°, +12° Compound lean angle (ε ) CL −12◦, −6◦, 0◦, +6◦, +12◦ Sweep angle (γ) c SP −5°, 0°, +5°, +10° Sweep angle (γ) SP −5◦, 0◦, +5◦, +10◦

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(a) (b)

FigureFigure 4. 4.Airfoil Airfoil proﬁles profiles of of spanwise spanwise direction: direction: ( a(a)) SL0 SL0 SP0; SP0; ( b(b)) SL0 SL0 SP10; SP10; ( c()c) SL12 SL12 SP0; SP0; ( d(d)) CL12 CL12 SP0. SP0.

TheThe computational computational domain domain and and mesh mesh (5 (5× ×10 1066 nodes)nodes) andand thethestation stationnumber number of of the the turbineturbine stagestage areare shownshown inin FiguresFigure 2 and 5Figure, respectively. 5, respectively. The steady-state The steady-state calculations calcula- weretions conductedwere conducted using using the commercial the commercial CFD package,CFD package, CFX, CFX, which which solves solves the Reynolds- the Reyn- averagedolds-averaged Navier–Stokes Navier–Stokes equations equations using using a finite-volume, a finite-volume, node-centered node-centered approach approach with high-resolutionwith high-resolution schemes schemes for convective for convective fluxes fluxes and implicit and implicit time integration. time integration. The k- Theω SST k-ω turbulentSST turbulent is used is used and underand under the no-slip the no-slip wall wall conditions. conditions. A periodic A periodic boundary boundary condition condi- istion applied is applied in the in circumferential the circumferential interface. interfac Thee. mixingThe mixing plane plane approach approach is adopted is adopted to the to interfacesthe interfaces between between stator stator and rotorand rotor as shown as sh inown Figure in Figure5. The boundary5. The boundary layer is layer resolved is re- + bysolved achieving by achieving the y of the approximately y+ of approximately 1 throughout 1 throughout the domain the asdomain required as required to resolve to the re- near-wallsolve the regionnear-wall and region to avoid and the to useavoid of wallthe use functions. of wall Thefunctions. difference The indifference NGV efficiency in NGV −4 predictionsefficiency predictions due to mesh due density to mesh is density in the order is in ofthe 10 order. Figureof 10−46. Figurecompares 6 compares numerical nu- andmerical experimental and experimental results in results terms ofin theterms surface of the pressure surface distributionpressure distribution and total pressureand total loss at vane exit (z/C = 1.1). Figure6 shows good agreement between predicted and pressure loss at vane axexit (z/Cax = 1.1). Figure 6 shows good agreement between predicted measuredand measured data. data. To study swirl clocking, the swirl intensity which is characterized by swirl number, SN (the ratio of the axial momentum flux and a characteristic radius [32]) is implemented for the present study. The applied inlet swirl profile and clocking positions (Figure7) are representing a swirl number of approximately 0.5. The overall analysis matrix is shown in Table3.

R uwr2dr SN = (4) R Ru2rdr

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FigureFigureFigure 5. Swirl 5. SwirlSwirl center center center location location location and and measurement measurement measurement plane. plane.plane.

Figure 6. Comparison of surface pressure distribution of NGV. Figure 6. Comparison of surface pressure distribution of NGV.

To study swirl clocking, the swirl intensity which is characterized by swirl number, Figure 6. Comparison of surface pressure distribution of NGV. SN (the ratio of the axial momentum flux and a characteristic radius [32]) is implemented for the present study. The applied inlet swirl profile and clocking positions (Figure 7) are To study swirl clocking, the swirl intensity which is characterized by swirl number, representing a swirl number of approximately 0.5. The overall analysis matrix is shown SN (thein Table ratio 3. of the axial momentum flux and a characteristic radius [32]) is implemented for the present study. The applied inlet swirl profile and clocking positions (Figure 7) are representing a swirl number of approximately 0.5. 𝑢𝑤𝑟 The𝑑𝑟 overall analysis matrix is shown 𝑆𝑁 (4) in Table 3. 𝑅𝑢 𝑟𝑑𝑟 𝑢𝑤𝑟𝑑𝑟 𝑆𝑁 (4) 𝑅𝑢𝑟𝑑𝑟

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(a) (b)

FigureFigure 7. Swirl 7. Swirl vector vector at inletat inlet plane: plane: (a) ( Swirla) Swirl at LE;at LE; (b) ( Swirlb) Swirl at passageat passage center. center.

TableTable 3. 3D 3. 3D NGV NGV geometry geometry combination combination matrix. matrix.

StraightStraight Lean Lean (◦) (°) Sweep Sweep (°) (◦) Swirl Swirl −12, −6, 0, 6, 12 −5,0,5,10 No swirl, swirl at LE, swirl at passage center −12, −6, 0, 6, 12 −5, 0, 5, 10 No swirl, swirl at LE, swirl at passage center Compound Lean (°) Sweep (°) Swirl Compound Lean (◦) Sweep (◦) Swirl −12, −6, 0, 6, 12 −5,0,5,10 No swirl, swirl at LE, swirl at passage center −12, −6, 0, 6, 12 −5, 0, 5, 10 No swirl, swirl at LE, swirl at passage center 3. Results 3. Results 3.1. Stage Performance with and without Swirl Clocking 3.1. Stage Performance with and without Swirl Clocking Figure 8 compares the stage efficiencies of a total of 108 cases analyzed for the present Figure8 compares the stage efficiencies of a total of 108 cases analyzed for the present study. For comparison, the results of straight lean with sweep are collected in Figure 8a, study. For comparison, the results of straight lean with sweep are collected in Figure8a, and compound lean with sweep are shown in Figure 8b, respectively. The baseline airfoil and compound lean with sweep are shown in Figure8b, respectively. The baseline airfoil geometry (lean angle = sweep angle = 0°) of NGV is highlighted using a solid green-col- geometry (lean angle = sweep angle = 0◦) of NGV is highlighted using a solid green-colored ored triangle. The variation in lean angle (±12°) is noted in the figure. The changes in triangle. The variation in lean angle (±12◦) is noted in the figure. The changes in sweep sweep angle (−5° ~ +10°) is distinguished by different colors. Finally, the case of swirl cen- angle (−5◦ ~ +10◦) is distinguished by different colors. Finally, the case of swirl center ter aligned at the leading edge of NGV is expressed using a dotted line. The dash-dotted aligned at the leading edge of NGV is expressed using a dotted line. The dash-dotted line line is used to represent the alignment of the swirl center in the middle of passage (be- is used to represent the alignment of the swirl center in the middle of passage (between leadingtween edges). leading In edges). Figure In8a, Figure the group 8a, the of group the triangle of the triangle symbols symbols in solid in line solid shows line theshows highestthe highest efficiency. efficiency. These areThese the are cases the without cases wi swirlthout (No swirl swirl) (No butswirl) showing but showing the influence the influ-

of leanence andof lean sweep and anglessweep toangles stage efficiency. to stage efficiency. Both positive Both lean positive and sweep lean anglesand sweep improve angles stageimprove efficiency stage but efficiency negative but angles negative reduce angles the reduce efficiency. the efficiency. The improvement The improvement due to the due positiveto the sweeppositive angle sweep (+10 angle◦) is the(+10°) highest is the (0.127%p) highest (0.127%p) when there when is no there lean angleis no appliedlean angle toapplied the airfoil. to the A similarairfoil. A observation similar observation can be made can be for made the influencefor the influence of lean of angle lean asangle the as improvementthe improvement due to due positive to positive lean anglelean angle (+12 ◦(+12°)) is the is highestthe highest (0.174%p) (0.174%p) without without sweep sweep angle.angle. However, However, the the influence influence of leanof lean angle angle is lessis less sensitive sensitive to compoundto compound lean lean cases cases shown shown in Figurein Figure8b. This8b. characteristicThis characteristic makes makes compound compound lean more lean robust more design robust choice design as known.choice as Anknown. increase An in increase sweep angle in sweep still improves angle still the improves stage efficiency. the stage efficiency. By introducing the swirl clocking, the trend of stage efficiency is changed dramatically (Figure8). The two swirl clocking positions representing the core of the combustor exit swirl are aligned at (1) the leading edge of NGV and (2) the center of the passage. First of all, the stage efficiency is reduced significantly in general. The maximum reduction is about 1.362%p for the model of SL12 SP10 compared to the no-swirl case. For straight lean cases, in Figure8a, the stage efficiency curves with a swirl at passage center are simply shifted down. However, the trend with a swirl at leading edge is reversed. The detailed three-dimensional flow field will be followed to explain the difference in loss mechanism. The models with compound lean angles shown in Figure8b are less sensitive to the lean angle in the presence of swirl clocking compared to straight lean. However, the influence of the lean angle is recognizable when the swirl center is positioned at the leading edge. The variation in sweep angle influences the stage efficiency in a similar way to the straight lean.

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(a) (b)

Figure 8. Comparison of stage efﬁciencies:efficiencies: (a) Straight lean; ((b)) CompoundCompound lean.lean.

ByIt will introducing also be important the swirl clocking, to monitor the the tren changed of stage in the efficiency throat area is changed due to leandramati- and callysweep, (Figure as it will8). The influence two swirl mass clocking flow rate. positi Figureons representing9 shows the comparisonthe core of the between combustor the exitthroat swirl area are and aligned mass at flow (1) the rate leading of all cases.edge of It NGV is interesting and (2) the to center observe of the that passage. the increase First ofin all, throat the areastage is efficiency not always is reduced the cause signifi of thecantly increase in general. in mass The flow maximum rate. The reduction slopes ofis aboutnormalized 1.362%p mass for flow the model rate and of throatSL12 SP10 area compared are different to the in many no-swirl cases, case. except For straight Figure9 dlean of cases,the straight in Figure lean 8a, with the sweep stage angle. efficiency Nevertheless, curves with the a mass swirl flow at passage rate increases center asare lean simply and shiftedsweep anglesdown. increase.However, Therefore, the trend the with increase a swirl in at stage leading efficiency edge is of reversed. straight lean The airfoils detailed is three-dimensionalinfluenced by lean flow and sweepfield will angles. be followed With compound to explain lean the (Figuredifference9c,d), in theloss variations mechanism. in Energies 2021, 14, x FOR PEER REVIEW 9 of 26 throatThe area models and mass with flow compound rate are lean much angles smaller show thann in that Figure of the 8b straight are less lean. sensitive This will to the be leanone ofangle the reasonsin the presence for less variationof swirl clocking in the stage compared efficiency to duestraight to the lean. lean However, and sweep. the influence of the lean angle is recognizable when the swirl center is positioned at the leading edge. The variation in sweep angle influences the stage efficiency in a similar way to the straight lean. It will also be important to monitor the change in the throat area due to lean and sweep, as it will influence mass flow rate. Figure 9 shows the comparison between the throat area and mass flow rate of all cases. It is interesting to observe that the increase in throat area is not always the cause of the increase in mass flow rate. The slopes of normal- ized mass flow rate and throat area are different in many cases, except Figure 9d of the straight lean with sweep angle. Nevertheless, the mass flow rate increases as lean and sweep angles increase. Therefore, the increase in stage efficiency of straight lean airfoils is influenced by lean and sweep angles. With compound lean (Figure 9c,d), the variations in throat area and mass flow rate are much smaller than that of the straight lean. This will be one of the reasons for less variation in the stage efficiency due to the lean and sweep.

(a) (b)

Figure 9. Cont.

(c) (d) Figure 9. Variation of throat area and mass flow rate of the 3D NGVs: (a) Sweep with straight lean angle; (b) Straight lean with sweep angle; (c) Sweep with compound angle; (d) Compound lean with sweep angle.

3.2. Effect of Straight Lean, Compound Lean and Sweep There are early studies to investigate the influence of lean and sweep angles on turbine aerodynamics [21,22]. Prior to discuss about its combined effect and also swirl clocking, it will be worthwhile to observe the influence of straight lean, compound lean and sweep on the flow field of turbine passage. Figure 10 shows the distribution of total pressure loss coefficient varying those angles. The straight lean of +12° model shows reduction in loss at lower endwall region while the loss is increase near the upper endwall compares to the baseline model. The compound lean model with the same +12° minimizes the loss at the upper endwall region. By applying the sweep angle (10°), the level of loss near the upper endwall region is reduced but the wake is widely spread than the baseline model. The same trend is also observed for the swept airfoils with straight and compound lean angles.

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(a) (b)

FigureFigure 9. 9. VariationVariation of of throat throat area area and and mass mass flow ﬂow rate rate of of the the 3D 3D NGVs: NGVs: ( (aa)) Sweep Sweep with with straight straight lean lean angle; angle; ( (bb)) Straight Straight lean lean withwith sweep sweep angle; angle; ( (cc)) Sweep Sweep with with compound compound angle; angle; ( (d)) Compound Compound lean lean with sweep angle.

3.2.3.2. Effect Effect of of Straight Straight Le Lean,an, Compound Compound Lean Lean and and Sweep Sweep ThereThere are are early studies toto investigateinvestigate the the influence influence of of lean lean and and sweep sweep angles angles on on turbine tur- bineaerodynamics aerodynamics [21,22 [21,22].]. Prior Prior to discuss to discuss about about its combined its combined effect effect and alsoand also swirl swirl clocking, clock- it ing,will it be will worthwhile be worthwhile to observe to observe the influence the influence of straight of lean,straight compound lean, compound lean and sweeplean and on sweepthe flow on fieldthe flow of turbine field of passage. turbine passage. Figure 10 Figu showsre 10 the shows distribution the distri ofbution total pressureof total pres- loss surecoefficient loss coefficient varying thosevarying angles. those The angles. straight The leanstraight of +12 lean◦ model of +12° shows model reduction shows reduction in loss at inlower loss at endwall lower endwall region while region the while loss the is increase loss is increase near the near upper the upper endwall endwall compares compares to the tobaseline the baseline model. model. The compound The compound lean model lean mode withl thewith same the +12same◦ minimizes +12° minimizes the loss the at loss the atupper the upper endwall endwall region. region. By applying By applying the sweep the sw angleeep angle (10◦), the(10°), level the oflevel loss of near loss the near upper the upperendwall endwall region region is reduced is reduced but the but wake the wake is widely is widely spread spread than than the baseline the baseline model. model. The Thesame same trend trend is also is also observed observed for the for swept the swept airfoils airfoils with straightwith straight and compound and compound lean angles. lean angles.In Figure 11, the circumferentially averaged parameters of flow rate, exit flow angle and total pressure loss coefficient are compared at the exit plane of NGV. The comparison is intended to confirm the findings from the previous researches [5,19,20,23]. Straight lean angle is varied from +12◦ to −12◦ with the interval of 6◦. Figure 11a–c shows the influence of straight lean angle of +12◦, 0◦ (baseline model) and −12◦. The positive lean of 12◦ (red-colored line) makes more mass flow rate passes through the lower half of the span height so that it leads to under turning of the exit flow. Considering the exit metal angle near the trailing edge in the region of span height of 15% is 70◦, it concludes less total pressure loss as shown in Figure 11c. However, the trend is reversed in the upper half of the span height. The observation is changed when the negative lean of 12◦ (blue-colored line) is applied. For the compound lean, Figure 11d–f, the benefit of the lean angle applies to the lower and upper regions of the span height and keep the flow field of the mid-span region similar to the baseline airfoil. In comparison of positive and negative compound lean models, the positive 12◦ leaned model (red-colored line) showed reduced loss at the lower and upper regions of the span height. As the results show, the compound leaned airfoil is expected to be more robust design to de-sensitize the overall loss mechanism in turbine passage. For the cases only with sweep angle, Figure 11g–i show the characteristics of the NGV passage flow. The positive sweep angle of 10◦ increases mass flow rate through the lower 30% of span height and it produces a similar effect as it was observed from the positive straight lean angle of 12◦. However, there are two main differences as (1) the location of the highest total pressure loss coefficient is moved upward slightly from lower endwall region and (2) the loss is not high as it was for the positive straight lean angle of 12◦ in the upper endwall region. Energies 2021, 14, 5503 10 of 25

These changes in the flow field are also associated with the changes in the airfoil loading. Figure 12 shows the radial variations of the static pressure distributions of the baseline NGV and blade at 10%, 50% and 90% span heights. It will be regarded as a reference in comparison with other models. The loading distributions of negative and positive 12◦ of straight lean models are compared in Figure 13. The negative lean angle contributes to make bigger radial changes in the loading distribution while the positive lean angle moves the loading toward the front of the axial chord of the airfoil. The compound leaned NGV shows different characteristics in loading distribution as shown in Figure 14. The positive compound leaned model put more loading in the mid-span region but the negative angle allocated a higher loading towards the upper and lower endwalls. Interestingly, the positive sweep redistributes the loading more toward the front of the airfoil while the negative sweep keeps the loading similar to the baseline but shows slightly higher gradient along the radial direction as compared in Figure 15. Finally, the loading Energies 2021, 14, x FOR PEER REVIEW 10 of 26 distributions of the two models with maximum and minimum efficiencies are compared in Figure 16.

Sweep 0° Sweep 10°

Baseline

(a) (b)

Straight lean 12°

Compound lean 12 °

(e) (f)

FigureFigure 10. 10.Distributions Distributions of of total total pressure pressure loss loss coefﬁcient coefficient at at NGV NGV exit, exit, z/C z/Caxax = 1.06 (rear view): ( (aa)) SL0 SL0 SP0; SP0; ( (bb)) SL0 SL0 SP10; SP10; (c) (c)Scheme SL12 SP0; 12. ( dSP0;) SL12 (d) SP10;SL12 SP10; (e) CL12 (e) SP0;CL12 ( fSP0;) CL12 (f) SP10.CL12 SP10.

In Figure 11, the circumferentially averaged parameters of flow rate, exit flow angle and total pressure loss coefficient are compared at the exit plane of NGV. The comparison is intended to confirm the findings from the previous researches [5,19,20,23]. Straight lean angle is varied from +12° to −12° with the interval of 6°. Figure 11a–c shows the influence of straight lean angle of +12°, 0° (baseline model) and −12°. The positive lean of 12° (red- colored line) makes more mass flow rate passes through the lower half of the span height so that it leads to under turning of the exit flow. Considering the exit metal angle near the trailing edge in the region of span height of 15% is 70°, it concludes less total pressure loss as shown in Figure 11c. However, the trend is reversed in the upper half of the span height. The observation is changed when the negative lean of 12° (blue-colored line) is applied. For the compound lean, Figure 11d–f, the benefit of the lean angle applies to the lower and upper regions of the span height and keep the flow field of the mid-span region similar to the baseline airfoil. In comparison of positive and negative compound lean models, the positive 12° leaned model (red-colored line) showed reduced loss at the lower and upper regions of the span height. As the results show, the compound leaned airfoil is expected to be more robust design to de-sensitize the overall loss mechanism in turbine passage. For the cases only with sweep angle, Figure 11g–i show the characteristics of the NGV passage flow. The positive sweep angle of 10° increases mass flow rate through the lower 30% of span height and it produces a similar effect as it was observed from the positive straight lean angle of 12°. However, there are two main differences as (1) the location of the highest total pressure loss coefficient is moved upward slightly from lower endwall region and (2) the loss is not high as it was for the positive straight lean angle of 12° in the upper endwall region.

Energies 2021, 14, 5503 11 of 25 Energies 2021, 14, x FOR PEER REVIEW 11 of 26

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

FigureFigure 11. Comparison11. Comparison of circumferentiallyof circumferentially averaged averaged staticstatic pressure pressure,, exit exit flow ﬂow angle, angle, and and total total pressure pressure loss losscoefficient coefﬁcient due todue the to straightthe straight lean, lean, compound compound lean lean and and sweep: sweep: ((aa)) StaticStatic pressure pressure with with straight straight lean; lean; (b) ( bFlow) Flow angle angle with with straight straight lean; (c) Total pressure loss with straight lean; (d) Static pressure with compound lean; (e) Flow angle with compound lean; (c) Total pressure loss with straight lean; (d) Static pressure with compound lean; (e) Flow angle with compound lean; Energies 2021lean;, 14 , (xf FOR) Total PEER pressure REVIEW loss with compound lean; (g) Static pressure with sweep; (h) Flow angle with sweep;12 of 26(i ) Total (f) Totalpressure pressure loss losswith withsweep. compound lean; (g) Static pressure with sweep; (h) Flow angle with sweep; (i) Total pressure loss with sweep. These changes in the flow field are also associated with the changes in the airfoil loading. Figure 12 shows the radial variations of the static pressure distributions of the baseline NGV and blade at 10%, 50% and 90% span heights. It will be regarded as a reference in comparison with other models. The loading distributions of negative and positive 12° of straight lean models are compared in Figure 13. The negative lean angle contributes to make bigger radial changes in the loading distribution while the positive lean angle moves the loading toward the front of the axial chord of the airfoil. The compound leaned NGV shows different characteristics in loading distribution as shown in Figure 14. The positive compound leaned model put more loading in the mid-span region but the negative angle allocated a higher loading towards the upper and lower endwalls. Interestingly, the positive sweep redistributes the loading more toward the front of the airfoil while the negative sweep keeps the loading similar to the baseline but shows slightly higher gradient along the radial direction as compared in Figure 15. Finally, the loading distributions of the two models with maximum and minimum efficiencies are compared in Figure 16.

Figure 12. Loading distribution of NGV loading at 10, 50 and 90% span heights. Figure 12. Loading distribution of NGV loading at 10, 50 and 90% span heights.

(a) (b) Figure 13. Comparison of NGV loading due to the straight lean angle: (a) Span height 10%; (b) Span height 90%.

Energies 2021, 14, x FOR PEER REVIEW 12 of 26

Energies 2021, 14, 5503 12 of 25 Figure 12. Loading distribution of NGV loading at 10, 50 and 90% span heights.

Energies 20212021,, 1414,, xx FORFOR PEERPEER REVIEWREVIEW 1313 ofof 2626

(a) (b)

Figure 13. Comparison of NGV loading due to the straight lean angle: ( a) Span height 10%; ( b) Span height 90%.

((a)) ((b))

Figure 14.14. Comparison of NGV loading duedue toto thethe compoundcompound leanlean angle:angle: ((aa)) SpanSpan heightheight 10%;10%; ((bb)) SpanSpan heightheight 90%.90%.90%.

((a)) ((b))

Figure 15.15. Comparison of NGV loadingloading duedue toto sweepsweep angle:angle: (a)) SpanSpan heightheight 10%;10%; ((bb)) SpanSpan heightheight 90%.90%.

((a)) ((b)) Figure 16. Comparison of NGV loading of the models with maximum efficiency (SL0 SP0 No swirl) and minimum effi- ciencyciency (SL-12(SL-12 SP-5SP-5 NoNo swirl)swirl) model:model: ((a)) SpanSpan heightheight 10%;10%; ((b)) SpanSpan heightheight 90%.90%.

Energies 2021, 14, x FOR PEER REVIEW 13 of 26

(a) (b) Figure 14. Comparison of NGV loading due to the compound lean angle: (a) Span height 10%; (b) Span height 90%.

Energies 2021, 14, 5503 (a) (b) 13 of 25 Figure 15. Comparison of NGV loading due to sweep angle: (a) Span height 10%; (b) Span height 90%.

(a) (b)

Figure 16. Comparison ofof NGVNGV loadingloading of of the the models models with with maximum maximu efﬁciencym efficiency (SL0 (SL0 SP0 SP0 No swirl)No swirl) and minimumand minimum efﬁciency effi- Energies 2021, 14, x FOR PEER REVIEW 14 of 26 ciency(SL-12 (SL-12 SP-5 No SP-5 swirl) No model:swirl) model: (a) Span (a) height Span height 10%; (b 10%;) Span (b) height Span height 90%. 90%.

3.3. Combined Effect of Lean with Sweep 3.3. Combined Effect of Lean with Sweep The influence of the combined angles of lean and sweep on aerodynamic characteristics is comparedThe influence in this of section. the combined The comparison angles of lean is made and forsweep the on combination aerodynamic of maximumcharacter- positiveistics is compared (+12◦) and in negative this section. (−12◦ )The lean comparison angles with sweepis made angles for the of 0combination◦ and +10◦. Figure of maxi- 17 mumshows positive the comparison (+12°) and of circumferentiallynegative (−12°) lean averaged angles masswith flowsweep rate, angles exit flowof 0° angle, and +10°. and Figuretotal pressure 17 shows loss the coefficient comparison due toof thecircumferentially combined effect averaged of the straight mass flow lean withrate, sweep.exit flow It angle,is interesting and total to observepressure that loss the coefficient radial variations due to the of thecombined presented effect parameters of the straight are similar lean withto the sweep. trend ofIt theis interesting individual to influence observe ofthat lean the and radial sweep variations angles. Theof the superposition presented param- of the eterstwo effects are similar is what to can the be trend seen of in the comparison.individual influence This trend of andlean comparison and sweep are angles. seen alsoThe superpositionin Figure 18 where of the the two combined effects is effects what ofcan compound be seen in lean the andcomparison. sweep are This compared trend and for comparisonthe selected angles.are seen Figure also in 19 Figure shows 18 the wher statice the pressure combined distributions effects of atcompound 10%, 50% lean and 90%and sweepspan heights. are compared Comparing for the Figures selected 13 bangles. and 14 Fibgure shows 19 theshows loading the static distribution pressure by distribu- adding tionsthe sweep at 10%, angle 50% ofand +10 90%◦. As span observed heights. already, Comparing the positive Figures 13b sweep and tends 14b shows to increase the load- the ingloading distribution at the lower by adding endwall. the This sweep influence angle is of added +10°. to As the observed combined already, models the as expected. positive Thesweep loading tends isto slightly increase increased the loading at 10% at the span lower height endwall. by adding This the influence sweep angleis added of +10 to ◦the. combinedThe present models data as expected. observation The provides loading useful is slightly insights increased on how at each 10% of span the combinedheight by addingangles of the straight sweep and angle compound of +10°. lean and sweep influence the aerodynamics characteristics.

(a) (b) (c)

Figure 17.17. Comparison ofof circumferentiallycircumferentially averaged averaged mass mass flow flow rate rate (a ),(a exit), exit flow flow angle angle (b), (b and), and total total pressure pressure loss loss coefficient coeffi- (cientc) due (c) to due the to straight the straight lean with lean sweep.with sweep.

The present data observation provides useful insights on how each of the combined angles of straight and compound lean and sweep influence the aerodynamics characteristics.

(a) (b) (c) Figure 18. Comparison of circumferentially averaged mass flow rate (a), exit flow angle (b), and total pressure loss coefficient (c) due to the compound lean with sweep.

Energies 2021, 14, x FOR PEER REVIEW 14 of 26

3.3. Combined Effect of Lean with Sweep The influence of the combined angles of lean and sweep on aerodynamic characteristics is compared in this section. The comparison is made for the combination of maximum positive (+12°) and negative (−12°) lean angles with sweep angles of 0° and +10°. Figure 17 shows the comparison of circumferentially averaged mass flow rate, exit flow angle, and total pressure loss coefficient due to the combined effect of the straight lean with sweep. It is interesting to observe that the radial variations of the presented parameters are similar to the trend of the individual influence of lean and sweep angles. The superposition of the two effects is what can be seen in the comparison. This trend and comparison are seen also in Figure 18 where the combined effects of compound lean and sweep are compared for the selected angles. Figure 19 shows the static pressure distributions at 10%, 50% and 90% span heights. Comparing Figures 13b and 14b shows the loading distribution by adding the sweep angle of +10°. As observed already, the positive sweep tends to increase the loading at the lower endwall. This influence is added to the combined models as expected. The loading is slightly increased at 10% span height by adding the sweep angle of +10°.

(a) (b) (c) Figure 17. Comparison of circumferentially averaged mass flow rate (a), exit flow angle (b), and total pressure loss coefficient (c) due to the straight lean with sweep.

The present data observation provides useful insights on how each of the combined Energies 2021, 14, 5503 14 of 25 angles of straight and compound lean and sweep influence the aerodynamics characteristics.

(a) (b) (c) Energies 2021, 14, x FOR PEER REVIEW 15 of 26 FigureFigure 18. 18. ComparisonComparison of circumferentially averagedaveraged mass mass flow flow rate rate (a (),a), exit exit flow flow angle angle (b ),(b and), and total total pressure pressure loss loss coefficient coefficient(c) due (c) todue the to compound the compound lean withlean with sweep. sweep.

(a) (b)

FigureFigure 19. 19. ComparisonComparison of of NGV NGV and and Blade Blade load loadinging distributions distributions of of SL12 SL12 SP10 SP10 ( aa)) and and CL12 CL12 SP10 SP10 ( (b)) models. models.

3.4.3.4. Effect Effect of of Swirl Swirl Clocking Clocking AsAs observed observed in in Figure Figure 88,, the application ofof the swirl clocking toto 3D NGV changed thethe stagestage performance performance characteristics characteristics quite quite dramat dramatically.ically. The The two two swirl swirl clocking clocking positions positions are are introduced,introduced, one atat thethe centercenter of of the the leading leading edge edge and and another another shifted shifted half half of theof the pitch pitch in thein theclockwise clockwise direction direction (front (front view); view); hence, hence, at the at the center center of passage of passage 1. Figure 1. Figure 20 visualizes 20 visualizes the thedevelopment development of vorticity of vorticity and and interaction interaction with wi passageth passage flow dueflow to due the to swirl the clocking.swirl clocking. When Whenthe swirl the coreswirl is core aligned is aligned to the leadingto the leadin edge ofg edge NGV, of Figure NGV, 20 Figureb,e, the 20b,e, vortex the (red-coloured) vortex (red- coloured)stays on the stays suction on the side suction of NGV side and of thenNGV it an isd pushed then it towards is pushed the towards upper endwall the upper (this end- can wallbe observed (this can from be observed the rear view)from the as itrear flows view) downstream. as it flows Thedownstream. development The ofdevelopment the counter ofvortex the counter (blue-colored) vortex (blue-colored) is also observed is nearalso theobserved upper near endwall. the upper From theendwall. surface From oil flow the surfacetrajectory, oil theflow stagnation trajectory, lines the atstagnation the leading lines edge at the of both leading NGVs edge are of strongly both NGVs deflected are stronglytoward thedeflected pressure toward side. the Figure pressure 20c,f side. show Fi thegure vortex 20c,f structureshow the whenvortex thestructure swirl corewhen is thealigned swirl atcore the is center aligned of at passage the center 1. The of pass combustorage 1. The exit combustor swirl remains exit swirl strong remains through strong the throughcenter of the the center passage. of the The passage. counter The vortex counter is vo alsortex stronger is also stronger than the than swirl the center swirl center at the atleading the leading edge. edge. The stagnation The stagnation lines of lines the leadingof the leading edge of edge both of NGVs both areNGVs deflected are deflected toward towardthe pressure the pressure side. Stronger side. Stronger downwash downwash flow is observed flow is onobserved the pressure on the surface pressure from surface the oil fromflow the trajectory. oil flow As trajectory. shown in As the shown results, in athe stronger results, vortex a stronger on the vortex suction onsurface the suction of NGV1 sur- faceis also of NGV1 observed is also in Figure observed 20f. in These Figure will 20f. contribute These will to contribute increasing to secondary increasing flow secondary loss of flowthe cases loss of with the the cases swirl with clocking the swirl at theclocking passage at the center. passage center. Another interesting observation from Figure 8 is that the characteristics of the stage performance are reversed when the swirl clocking is positioned at the center of the leading edge. Particularly for the straight lean of −12°, it shows higher stage performance with the swirl cockling at the leading edge than at the passage center. To understand the mechanism, Figure 21 is presented by comparing the influence of the positive and negative lean angles to the swirl clocking. It is clearly seen that the straight lean of -12° splits the combustor swirl from the leading edge so that weaker vortices are passing through the pressure and suction side of the NGV1. With maximum lean and sweep angles, the SL12 SP10 model shows minimum sensitivity to the swirl clocking (see Figure 8). Figure 22 shows the development of the vorticity depends on the swirl clocking positions. It only provides qualitative comparison but shows the strength of vortices of the two swirl clocking cases. These findings contribute to improving the flow field characterization in the presence of a combustor swirl. Therefore, the traditional understanding and 3D design approach should be reconsidered if swirl clocking is introduced as it dominates the loss mechanism in the turbine passage.

EnergiesEnergies 20212021,, 1414,, x 5503 FOR PEER REVIEW 1615 of of 26 25

(a) (b) (c)

(d) (e) (f)

Figure 20. Comparison of vortex structure due to the swirl clocking−SL0 SP0 model (Baseline): (a) No swirl (front view); Figure 20. Comparison of vortex structure due to the swirl clocking−SL0 SP0 model (Baseline): (a) No swirl (front view); (b) SW@LE (front view); (c) SW@PC (front view); (d) No swirl (rear view); (e) SW@LE (rear view); (f) SW@PC (rear view). (b) SW@LE (front view); (c) SW@PC (front view); (d) No swirl (rear view); (e) SW@LE (rear view); (f) SW@PC (rear view). Another interesting observation from Figure8 is that the characteristics of the stage performance are reversed when the swirl clocking is positioned at the center of the leading edge. Particularly for the straight lean of −12◦, it shows higher stage performance with the swirl cockling at the leading edge than at the passage center. To understand the mechanism, Figure 21 is presented by comparing the inﬂuence of the positive and negative lean angles to the swirl clocking. It is clearly seen that the straight lean of −12◦ splits the combustor swirl from the leading edge so that weaker vortices are passing through the pressure and suction side of the NGV1. With maximum lean and sweep angles, the SL12 SP10

model shows minimum sensitivity to the swirl clocking (see Figure8). Figure 22 shows (a) the development of the vorticity(b) depends on the swirl clocking positions.(c) It only provides qualitative comparison but shows the strength of vortices of the two swirl clocking cases. These findings contribute to improving the flow field characterization in the presence of a combustor swirl. Therefore, the traditional understanding and 3D design approach should be reconsidered if swirl clocking is introduced as it dominates the loss mechanism in the turbine passage.

3.5. Downstream of NGV Passages and Mixing Plane

It should be noted that the flows through passage 1 and 2 experience different inlet (d) swirl due to the clocking positions(e) as shown in Figure 23. This shows(f) the combined effect of 3D NGV and swirl clocking on the total pressure loss coefficient at the exit plane of NGV. Comparison with Figure 10 provides clear observation on the influence of the

swirl clocking. Although it looks complicated, the previous understanding from Figure 20 Figure 21. Swirl clocking interactionsupports with the passage analysis flow of− the influence swirl interaction of straight with lean angle: 3D NGV (a) SL12 models. SP0 No A similarswirl; (b) interaction SL12 SP0 SW@LE; (c) SL12 SP0 SW@PC;mechanism (d) SL− drives12 SP0 theNo ﬂowswirl;through (e) SL−12 the SP0 passage SW@LE; so (f) that SL−12 the SP0 loss SW@PC. is more concentrated at the upper endwall and at the center of the passage depends on swirl clocking position.

Energies 2021, 14, x FOR PEER REVIEW 16 of 26

(a) (b) (c)

(d) (e) (f)

Energies 2021, 14, 5503 16 of 25 Figure 20. Comparison of vortex structure due to the swirl clocking−SL0 SP0 model (Baseline): (a) No swirl (front view); (b) SW@LE (front view); (c) SW@PC (front view); (d) No swirl (rear view); (e) SW@LE (rear view); (f) SW@PC (rear view).

(a) (b) (c)

(d) (e) (f)

EnergiesFigure 2021, 14 21. , x FORSwirlSwirl PEER clocking clocking REVIEW interaction with passage ﬂowflow−−thethe influence inﬂuence of of straight straight lean angle: ( (a)) SL12 SL12 SP0 SP0 No No swirl; swirl; ( b) SL12 SL1217 of 26

SP0 SW@LE; ( c) SL12 SP0 SW@PC; ( (d)) SL SL−1212 SP0 SP0 No No swirl; swirl; (e (e) )SL SL−−1212 SP0 SP0 SW@LE; SW@LE; (f) ( fSL) SL−12− 12SP0 SP0 SW@PC. SW@PC.

(a) (b) (c)

FigureFigure 22. 22. ComparisonComparison of of vortex vortex structure structure due due to to the the swirl swirl clocking clocking−SL12−SL12 SP10 SP10 model: model: (a () aSL12) SL12 SP10 SP10 No No swirl; swirl; (b ()b SL12) SL12 SP10SP10 SW@LE; SW@LE; (c ()c SL12) SL12 SP10 SP10 SW@PC. SW@PC.

3.5. DownstreamFigure 24 ofshows NGV thePassages comparison and Mixing of circumferentiallyPlane averaged total pressure loss coefﬁcientsIt should for be passagenoted that 1 (blue-coloredthe flows through line) andpassage 2 (red-colored 1 and 2 experience line). The different results inlet show swirldifferent due to loss the distributions clocking positions downstream as shown of the in twoFigure passages 23. This as shows expected the fromcombined the previous effect ofobservation. 3D NGV and For swirl reference clocking purposes, on the thetotal loss pressure distribution loss coefficient averaged over at the the exit two plane passages of NGV.is also Comparison presented with in a dottedFigure black10 provides line. It clear proves observation that such on an the average influence approach of the swirl is not clocking.appropriate Although for distinguishing it looks complicated, the inﬂuence the previous of the inlet understanding swirl on each from passage. Figure The20 sup- solid portsblack the line analysis represents of the the swirl loss distribution interaction ofwith the 3D baseline NGV modelmodels. without A similar inlet interaction swirl. mechanism drives the flow through the passage so that the loss is more concentrated at the upper endwall and at the center of the passage depends on swirl clocking position. Figure 24 shows the comparison of circumferentially averaged total pressure loss coefficients for passage 1 (blue-colored line) and 2 (red-colored line). The results show different loss distributions downstream of the two passages as expected from the previous observation. For reference purposes, the loss distribution averaged over the two passages is also presented in a dotted black line. It proves that such an average approach is not appropriate for distinguishing the influence of the inlet swirl on each passage. The solid black line represents the loss distribution of the baseline model without inlet swirl. The present study also aims to understand the influence of 3D NGV and swirl clocking on stage performance. Hence, the computational domain includes 1.5 stages of the turbine. Ideally, unsteady CFD applying a sliding mesh interface will be more physically representative. However, evaluating the average performance is still meaningful to support the present research interest and saves computational time to complete a large number of calculations. For these reasons, steady CFD with mixing plane interface is applied for the series of calculations. Figure 25 shows the distributions of total pressure loss coefficient at the mixing plane depends on swirl clocking positions. It still captures the key flow characteristics of NGV downstream and passes the effect of wake flow to the computational domain of the blade. As seen in the results, the loss distributions at the exit of the blade are influenced by the swirl clocking. This observation supports the value of the present study applying steady CFD with a mixing plane interface.

EnergiesEnergies2021 2021, 14, 14, 5503, x FOR PEER REVIEW 1718 ofof 25 26

Energies 2021, 14, x FOR PEER REVIEW 18 of 26

Swirl @LE Swirl @Passage Center Swirl @LE Swirl @Passage Center

Baseline Baseline

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

Straight lean 12° Straight lean 12°

Compound lean 12° Compound lean 12°

(e) (f) (e) (f)

Sweep 10° Sweep 10°

(g) (h) (g) (h)

FigureFigure 23. 23.Total Total pressure pressure loss loss coefﬁcient coefficient at at vane vane exit, exit, z/C z/Caxax=1.06= 1.06 (rear (rear view): view): (a ()a SL0) SL0 SP0 SP0 SW@LE; SW@LE; (b ()b SL0) SL0 SP0 SP0 SW@PC; SW@PC; (c) Figure(cSL12) SL12 23.SP0 SP0 Total SW@LE; SW@LE; pressure (d ()d SL12) loss SL12 SP0coefficient SP0 SW@PC; SW@PC; at (vanee) (CL12e) exit, CL12 SP0 z/C SP0 SW@LE;ax=1.06 SW@LE; (rear (f) (CL12 fview):) CL12 SP0 (a SP0 )SW@PC; SL0 SW@PC; SP0 (SW@LE;g) SL0 (g) SL0 SP10 (b SP10) SL0SW@LE; SW@LE;SP0 SW@PC; (h) SL0 (h) SL0 SP10(c) SL12 SP0 SW@LE; (d) SL12 SP0 SW@PC; (e) CL12 SP0 SW@LE; (f) CL12 SP0 SW@PC; (g) SL0 SP10 SW@LE; (h) SL0 SP10 SP10SW@PC. SW@PC. SW@PC.

Figure 24. Comparison of circumferentially averaged total pressure loss coefficients for passage 1 FigureFigureand 2. 24. 24. ComparisonComparison of of circumferentially circumferentially averaged averaged total total pressure pressure loss loss coefficients coefﬁcients for for passage passage 1 1 andand 2. 2.

Energies 2021, 14, 5503 18 of 25

The present study also aims to understand the influence of 3D NGV and swirl clocking on stage performance. Hence, the computational domain includes 1.5 stages of the turbine. Ideally, unsteady CFD applying a sliding mesh interface will be more physically representative. However, evaluating the average performance is still meaningful to support the present research interest and saves computational time to complete a large number of calculations. For these reasons, steady CFD with mixing plane interface is applied for the series of calculations. Figure 25 shows the distributions of total pressure loss coefficient at the mixing plane depends on swirl clocking positions. It still captures the key flow characteristics of NGV downstream and passes the effect of wake flow to the computational domain of the blade. As seen in the results, the loss distributions at the exit of the blade are Energies 2021, 14, x FOR PEER REVIEW 19 of 26 influenced by the swirl clocking. This observation supports the value of the present study applying steady CFD with a mixing plane interface.

No swirl Swirl @LE Swirl @Passage Center

Blade Inlet (Mixing Plane)

(a) (b) (c)

Blade Exit

(d) (e) (f)

FigureFigure 25. 25. TotalTotal pressure pressure loss loss coefficient coefficient at at the the inlet inlet an andd exit exit of of blade blade (rear (rear view), view), Baseline Baseline NGV: NGV: (a (a) )Blade Blade inlet, inlet, No No swirl; swirl;(b) Blade (b) Blade inlet, SW@LE;inlet, SW@LE; (c) Blade (c) Blade inlet, inlet, SW@PC; SW@PC; (d) Blade (d) Blade exit, No exit, swirl; No swirl; (e) Blade (e) Blade exit, SW@LE; exit, SW@LE; (f) Blade (f) Blade exit, SW@PC. exit, SW@PC 3.6. Stage Performance with Swirl Clocking 3.6. Stage Performance with Swirl Clocking From Figure8, it is interesting to observe that the stage performance is strongly From Figure 8, it is interesting to observe that the stage performance is strongly in- influenced by the proposed swirl clocking. Detailed analysis is made for the models fluenced by the proposed swirl clocking. Detailed analysis is made for the models show- showing the highest stage performance at the proposed swirl clocking positions as shown ing the highest stage performance at the proposed swirl clocking positions as shown in in Table4. The models performing better than others are with the largest lean (+12 ◦) and Table 4. The models performing better than others are with the largest lean (+12°) and sweep (+10◦) angles, and also the one with a negative lean angle of −12◦. The SP-12 SP10 sweep (+10°) angles, and also the one with a negative lean angle of −12°. The SP-12 SP10 model shows the highest stage performance when the swirl core is aligned at the leading modeledge of shows NGV. the In Figurehighest 21 stage, the swirlperformance interaction when with the 3D swirl NGV core was is shown aligned to explainat the leading the loss edgemechanism. of NGV. In In this Figure section, 21, the the swirl detailed intera characteristicsction with 3D of theNGV key was parameters shown to are explain presented the lossto confirm mechanism. the findings In this section, from the the previous detailed observation. characteristics of the key parameters are presented to confirm the findings from the previous observation.

Table 4. Highest stage performance models at proposed swirl clocking.

Swirl Clocking Type of Model No swirl Swirl @LE Swirl @Passage Center Straight Lean with SL12 SP10 SP-12 SP10 SL12SP10 Sweep Compound Lean with CL12 SP10 CL12 SP10 CL12 SP10 Sweep

Figure 26 shows the influence of swirl clocking on the distribution of the total pressure loss coefficient at the exits of NGV. Compared to the distributions shown in Figure 23, the loss is more widely spread as the models have a combined effect of lean and sweep. As observed in Figures 17 and 18, the combined effects can be seen as the superposition of the effect of individual angles. The distorted NGV exit flow continues to influence the blade aerodynamics as shown in Figures 27 and 28.

Energies 2021, 14, 5503 19 of 25

Table 4. Highest stage performance models at proposed swirl clocking.

Swirl Clocking Type of Model No Swirl Swirl @LE Swirl @Passage Center Straight Lean with Sweep SL12 SP10 SP-12 SP10 SL12SP10 Compound Lean with Sweep CL12 SP10 CL12 SP10 CL12 SP10

Figure 26 shows the influence of swirl clocking on the distribution of the total pressure loss coefficient at the exits of NGV. Compared to the distributions shown in Figure 23, the loss is more widely spread as the models have a combined effect of lean and sweep. As Energies 2021, 14, x FOR PEER REVIEWobserved in Figures 17 and 18, the combined effects can be seen as the superposition of20 the of 26 effect of individual angles. The distorted NGV exit flow continues to influence the blade aerodynamics as shown in Figures 27 and 28.

Swirl @LE Swirl @Passage Center

Straight lean 12°/ Sweep10 °

(a) (b)

Straight lean −12°/ Sweep 10°

Compound lean 12°/ Sweep 10°

(e) (f)

FigureFigure 26. 26.Total Total pressure pressure loss loss coefﬁcient coefficient at vane at vane exit, exit, z/C z/Cax =ax 1.06=1.06 (rear (rear view): view): (a ()a SL12) SL12 SP10 SP10 SW@LE; SW@LE; (b )(b SL12) SL12 SP10 SP10 SW@PC; SW@PC; (c)( SLc) −SL12−12 SP10 SP10 SW@LE; SW@LE; (d ()d SL) SL−12−12 SP10 SP10 SW@PC; SW@PC; ( e()e) CL12 CL12 SP10 SP10 SW@LE; SW@LE; ( f()f) CL12 CL12 SP10 SP10 SW@PC. SW@PC.

EnergiesEnergies2021 2021, 14, 14 5503, x FOR PEER REVIEW 2021 of of 25 26

No swirl Swirl @LE Swirl @Passage Center

Straight lean 12°/ Sweep10

(a) (b) (c)

Straight lean -12°/ Sweep10°

(d) (e) (f)

Compound lean 12°/ Sweep 10°

(g) (h) (i)

FigureFigure 27. 27.Total Total pressure pressure loss loss coefﬁcient coefficient at at blade blade exitexit at z/C axax=1.2= 1.2(rear (rear view): view): (a) SL12 (a) SL12 SP10 SP10 No swirl; No swirl; (b) SL12 (b) SP10 SL12 SW@LE; SP10 SW@LE;(c) SL12 (c )SP10 SL12 SW@PC; SP10 SW@PC; (d) SL (−d12) SLSP10−12 No SP10 swirl; No ( swirl;e) SL− (12e) SLSP10−12 SW@LE; SP10 SW@LE; (f) SL−12 (f) SP10 SL−12 SW@PC; SP10 SW@PC; (g) CL12 (g SP10) CL12 No SP10 swirl; No(h swirl;) CL12 (h )SP10 CL12 SW@LE; SP10 SW@LE; (i) CL12 (i )SP10 CL12 SW@PC. SP10 SW@PC.

3.7. Straight Lean (SL+12 SP0, SL-12 SP0 and SL-12 SP10) with Swirl Clocking To analyze the influence of swirl clocking to 3D NGV, the aerodynamic characteristics of the three models with positive (+12◦) and negative (−12◦) straight lean angles with and without sweep (+10◦) are compared in detail. These are the models with green and black-colored symbols as shown in Figure8. The stage performances of these three models are changed by the swirl clocking, especially for the model with a negative (−12◦) straight lean angle. As shown in Figure 29, the circumferentially averaged total pressure loss coefficients are compared at the exits of NGV and blade. The comparison confirms the influence of negative lean angle (−12◦) explored in Figure 21.

Energies 20212021,, 1414,, 5503x FOR PEER REVIEW 2221 of of 2526

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 28. Comparison of vortex structure due to the swirl clocking—Blade passage: (a) SL12 SP10 No swirl; (b) SL12 SP10 Figure 28. Comparison of vortex structure due to the swirl clocking—Blade passage: (a) SL12 SP10 No swirl; (b) SL12 SP10 SW@LE; (c) SL12 SP10 SW@PC; (d) SL−12 SP10 No swirl; (e) SL−12 SP10 SW@LE; (f) SL−12 SP10 SW@PC; (g) CL12 SP10 SW@LE; (c) SL12 SP10 SW@PC; (d) SL−12 SP10 No swirl; (e) SL−12 SP10 SW@LE; (f) SL−12 SP10 SW@PC; (g) CL12 SP10 No swirl; (h) CL12 SP10 SW@LE; (i) CL12 SP10 SW@PC. No swirl; (h) CL12 SP10 SW@LE; (i) CL12 SP10 SW@PC.

Energies 2021, 14, x FOR PEER REVIEW 23 of 26

3.7. Straight Lean (SL+12 SP0, SL-12 SP0 and SL-12 SP10) with Swirl Clocking To analyze the influence of swirl clocking to 3D NGV, the aerodynamic characteristics of the three models with positive (+12°) and negative (−12°) straight lean angles with and without sweep (+10°) are compared in detail. These are the models with green and black-colored symbols as shown in Figure 8. The stage performances of these three models are changed by the swirl clocking, especially for the model with a negative (−12°) straight lean angle. As shown in Figure 29, the circumferentially averaged total pressure loss co- Energies 2021, 14, 5503 22 of 25 efficients are compared at the exits of NGV and blade. The comparison confirms the influence of negative lean angle (−12°) explored in Figure 21.

SL12 SP0 SL-12 SP0 SL-12 SP10

NGV

(a) (b) (c)

Blade

(d) (e) (f)

FigureFigure 29. 29. ComparisonComparison of of circumferentially circumferentially averaged averaged mass mass flow flow rate rate,, exit exit flow flow angle, angle, and and total total pressure pressure loss loss coefficient coefficient duedue to to swirl swirl clocking: clocking: ( (aa)) SL12 SL12 SP0—NGV; ((b)) SLSL−−1212 SP0—NGV; SP0—NGV; ( c) SL−1212 SP10—NGV; SP10—NGV; ( (dd)) SL12 SL12 SP0—Blade; (e) SL−1212 SP0—Blade;SP0—Blade; ( (ff)) SL SL−−1212 SP10—Blade. SP10—Blade.

3.8. Compound Lean with Swirl Clocking 3.8. Compound Lean with Swirl Clocking The SL12 SP10 model showed the highest stage efficiency with no swirl and its per- The SL12 SP10 model showed the highest stage efficiency with no swirl and its perfor- formance is less sensitive to the swirl clocking (see Figure 8). Therefore, it is an interesting mance is less sensitive to the swirl clocking (see Figure8). Therefore, it is an interesting model to investigate further. For this purpose, the comparison of circumferentially aver- model to investigate further. For this purpose, the comparison of circumferentially averaged mass flow rate, exit flow angle, and total pressure loss coefficient with swirl clocking aged mass flow rate, exit flow angle, and total pressure loss coefficient with swirl clocking isis shown shown in in Figure Figure 30. 30 It. reflects It reflects the vortex the vortex formation formation through through the passage the passage shown shownin Figure in 22.Figure With 22 swirl. With clocking swirl clocking at the leading at the leadingedge (red-colored edge (red-colored lines), spanwi lines), spanwisese variations variations of the parametersof the parameters are more are uniform more uniform than the than results the resultswith swirl with at swirl passage at passage center centerand also and with- also outwithout swirl. swirl.However, However, the averaged the averaged performance performance is very ismuch very the much same the as same shown as in shown Figure in 8.Figure 8. ByBy extending extending the the observation observation to to the the compound compound lean lean models, models, it it provides provides better better insight insight intointo the the differences differences in in stage stage performance. performance. The The CL12 CL12 SP10 SP10 model model shows shows the the highest highest stage stage efficiencyefficiency with with the the proposed proposed swirl swirl clocking clocking (see (see Figure Figure 88).). There is no dramatic change in thethe stage stage efficiency efficiency as as is observed withwith straightstraight leanlean models.models. Furthermore, Furthermore, the the lean lean angle angle is isnot not sensitive sensitive to to the the stage stage efficiency efficiency exceptexcept inin thethe casescases with swirl clocking at at the the leading leading edge.edge. However, However, the the sweep sweep angle angle is is sensitive sensitive to to the the stage stage efficiency. efficiency. The The mass mass flow flow rate rate andand exit exit flow flow angle angle are are more more uniform uniform and and consistent consistent with with the the two two swirl swirl clocking clocking positions positions asas shown shown in in Figure Figure 31. 31 .The The trends trends can can also also be be compared compared with with the the SL12 SL12 SP10 SP10 model model (see (see FigureFigure 30). 30). Overall, Overall, it it can can be be stated stated that that the the compound compound lean lean is is more more robust robust than than the the other other models,models, meaning meaning it it is is less less sensitive sensitive to to the the operating operating conditions. conditions.

Energies 2021, 14, x FOR PEER REVIEW 24 of 26 Energies 2021, 14, 5503x FOR PEER REVIEW 2423 of of 26 25

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

(d) (e) (f) (d) (e) (f) Figure 30. Comparison of circumferentially averaged mass flow rate (a,d), exit flow angle (b,e), and total pressure loss Figure 30. ComparisonComparison of of circumferentially circumferentially averaged averaged mass mass flow flow rate rate (a (,da,),d ),exit exit flow flow angle angle (b, (eb),,e and), and total total pressure pressure loss loss coefficient (c,f) of the SL12 SP10 model due to swirl clocking coefficientcoefficient (c,f) of the SL12 SP10 modelmodel duedue toto swirlswirl clocking.clocking

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

(d) (e) (f) (d) (e) (f) Figure 31. Comparison of circumferentially averaged mass flow rate (a,d), exit flow angle (b,e), and total pressure loss Figure 31. Comparison of circumferentially averaged mass flow rate (a,d), exit flow angle (b,e), and total pressure loss Figurecoefficient 31. (Comparisonc,f) of the CL12 of circumferentiallySP10 and CL−12 SP10 averaged model mass due flowto swirl rate clocking. (a,d), exit flow angle (b,e), and total pressure loss coefficientcoefficient ( c,f) of the CL12 SP10 and CL−1212 SP10 SP10 model model due due to to swirl swirl clocking. clocking.

4. Conclusions The present study aims to investigate the inﬂuence of swirl clocking on turbine stage performance. A total of 108 CFD cases are analyzed to investigate the independent and combined effects of lean, and sweep angles on turbine aerodynamics. Furthermore,

Energies 2021, 14, 5503 24 of 25

the study is extended to consider the influence of swirl clocking. The key findings are summarized as follows. • The influence of combined angles of lean and sweep inherits their individual effects. • Positive lean and sweep (SL12 SP10 model) shows improved stage performance of +0.208%p with no swirl and +0.101%p with swirl clocking positioned at passage center against the baseline configurations. • Negative lean with sweep (SL-12 SP10 model) shows the enhanced stage efficiency of +0.277%p with swirl clocking positioned at the leading edge compared to the baseline model. • Compound lean is less sensitive to the swirl clocking while straight lean is strongly influenced. The present study reveals that there is a strong influence of swirl clocking on the turbine stage performance with 3D NGV. Therefore, designers are asked to consider realistic operating conditions and reflect the findings from the present study to the future design and performance improvement. In terms of robust design, compound lean can be the favored choice for turbine designers even for the presence of swirl and swirl clocking. For future research, its impact on heat transfer applying inlet temperature profile, cooling flow and unsteadiness will be necessary to provide more comprehensive aerothermal design guidelines.

Author Contributions: S.J. conducted analysis and drafted the paper. C.S. supervised the project and revised the paper. J.K. provided concept of the project and reviewed the paper. All authors have read and agreed to the published version of the manuscript. Funding: The authors would like to thank Doosan Heavy Industries & Construction for the sponsor- ship on this project. Furthermore, the authors would like to recognize the financial support received from Virginia Tech in conducting the present research. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: This work was supported by the Korea Institute of Energy Technology Evalua- tion and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20181110100400). Conflicts of Interest: The authors declare no conflict of interest.

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