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Applied Thermal Engineering 66 (2014) 318e327

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Numerical investigation on flow structures of a laboratory-scaletrapped vortex combustor

Yi Jin a,b,*, Xiaomin He a,b, Jingyu Zhang a, Bo Jiang a,c, Zejun Wu a

aCollege of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu 210016, PR ChinabCo-Innovation Center for Advanced Aero-Engine, Beijing 100191, PR ChinacDepartment of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA

h i g h l i g h t s

� The standard keε turbulence model is proved to be able to provide a satisfactory prediction of flow structures of trapped vortex combustors.� Both the single-vortex and the dual-vortex flow pattern are observed to be exist in cavities.� The difference in flow patterns of different planes is thought to be helpful to the mixing in the spanwise direction.� The flow structure obtained by the present work could help widening understandings of trapped vortex combustor.

a r t i c l e i n f o

Article history:Received 28 August 2013Accepted 14 February 2014Available online 23 February 2014

Keywords:Trapped vortex combustorFlow structureNumerical investigationTurbulence model

* Corresponding author. Co-Innovation Center for A100191, PR China.

E-mail addresses: pde_jy@nuaa.edu.cn, jinyi1984@

http://dx.doi.org/10.1016/j.applthermaleng.2014.02.031359-4311/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Flow fields of combustors have been commonly used to help understand combustion characteristics. Inthis paper, numerical simulations with validated methodology are employed to provide insight of theflow structures of a laboratory-scale trapped vortex combustor (TVC). Turbulence model determinationand numerical method validation are accomplished with the help of experimental data from particleimage velocimetry (PIV) measurements. A comparison of numerical and experimental results suggeststhat the standard keε turbulence model is able to provide a satisfactory prediction of the flow structures.Both of the two typical cavity flow patterns mentioned are observed: in a plane between two radialstruts, the cavity flow features the dual-vortex pattern, however, in a plane along a radial strut, the cavityflow is dominated by the single-vortex pattern. This difference in flow patterns of different planes in-dicates the difference in cavity streamemainstreammixing mechanism, which further, is believed to leadto enhanced mixing in spanwise direction.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The combustor is one of the key components in a gas turbineengine and the stable operation of the combustor is essential forgood performance of engines. Combustion stability is often ach-ieved through the use of recirculation zones to provide continuoussources of ignition by mixing hot products with the fresh air andfuel. Conventionally, swirlers are commonly used in primary zonesto establish recirculation zones for flame stabilization, at higherinlet velocities, however, these zones become less stable, leading toinferior combustor performance in terms of flame stability, com-bustion efficiency and emissions. Recently, Hsu et al. [1,2] proposed

dvanced Aero-Engine, Beijing

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a new flame stabilization concept known as trapped vortexcombustor (TVC), in which cavities are employed to stabilize theflame instead of swirlers. The TVC has become one of the mostpromising combustors for future aero engines for its potential toovercome the difficulty in flame stability associated with conven-tional combustors.

The TVCs can be operated either in rich burn-quick quench-lean burn (RQL) mode or in staged mode. One of the key issues inTVC design is the good vortex structure in cavity zone which actsas the continuous ignition source both for the fresh air and fuel incavities and in the mainstream flow. If the cavity zone is designedproperly, the vortex established in cavity is well shielded fromthe mainstream flow and as a result, the flame will be leastsensitive to the mainstream flow. Another key issue involvestransporting and mixing the heat and burnt gases from cavityzone into the mainstream flow, which helps in effective ignition

Nomenclature

TVC trapped vortex combustorPIV particle image velocimetryLES large eddy simulationRANS Reynolds-averaged NaviereStokesCFD Computational fluid dynamicsL axial length of cavity, mmMa inlet Mach numberT3 inlet temperature, KP3 inlet pressure, atmx axial coordinatey radial coordinatez spanwise coordinate

Y. Jin et al. / Applied Thermal Engineering 66 (2014) 318e327 319

of the mainstream premixed flow. Therefore, a detailed study andthorough knowledge of the flow structures both in cavity zoneand in mainstream are of crucial importance for the ultimatesuccess of a TVC design.

A classical TVC geometry which consists of two concentrictubes, a forebody and an afterbody was proposed by Hsu et al. [1,2].The cavity length, which is defined as the axial distance betweenthe forebody and afterbody, is adjustable. Fuel and primary air aredelivered through concentric tubes into separate chambers in theafterbody. This TVC geometry has been extensively studied. Hsuet al. [1,2] used particle image velocimetry tomeasure the flow fieldand found that when the length of the cavity is 0.59 of the forebodydiameter, a vortex is trapped in the cavity, furthermore, this lengthalso gives the minimum pressure drop. For realistic applications, asufficient amount of fuel and air must be injected directly into thecavity, by employing numerical simulations, Katta and Roquemore[3,4] studied the influence of this mass injection on the dynamiccharacteristics of the flow inside and around the cavity, in addition[5], they also studied the entrainment and residence-time charac-teristics of cavity flows for different cavity and spindle sizes. Stur-gess and Hsu [6] used experimental-data based indirect methods to

Fig. 1. 2-D schematic of the combustor, computational domain

assess mainstream mass entrainment into the cavity, the resultsrevealed that the entrainment is controlled by the ratio of cavitymomentum to that of themainstream. Large-Eddy Simulation (LES)was carried out by Stone and Menon [7] with particular focus onthe effects of Reynolds number on fuel/air mixing and combustionproperties, enhanced mixing rates are obtained with higherannular air velocity, and consequently, the reactions are found to beentirely contained inside the cavity. Mancilla et al. [8] analyzed theperformance of a TVC both experimentally and computationally,the geometry of their combustor is similar to that of Hsu et al.,however, liquid fuel was used instead of gas fuel. More recently,numerical study conducted by Kumar and Mishra [9,10] took intoaccount the momentum flux ratiodthe ratio of cavity fuel and airjet momentum to the mainstream momentum, and concluded thatthe momentum ratio plays an important role in altering the flowand flame structures within the cavity, thus affecting the perfor-mance of the combustor.

Attempts to develop an aeronautical gas turbine engine TVChave been conducted by US Air Force Research Laboratory(AFRL) and General Electric Aircraft Engines (GEAE). The TVCprogram in AFRL started in the early 1990s and three genera-tions of TVC have been developed. Since 1996, GEAE and theAFRL have been jointly developing a novel TVC concept foraircraft engines, their efforts led to the fabrication of a 12-inch-wide rectangular sector test rig which was also known as highpressure TVC sector test rig [11e13]. This test rig demonstratedthat the ignition, blow out, and altitude relight were up to 50%improved over swirl stabilized combustors. The NOx emissionswere in the range from 40% to 60% of the International CivilAviation Organization (ICAO) standard. The combustion effi-ciency was maintained at or above 99% over a 40% wider oper-ating range than a conventional aircraft gas turbine enginecombustor. Unfortunately, detailed information of the flow fieldof this test rig was not available.

It can be seen that up to now, work of trapped vortex combustorflow field mainly focuses on Hsu’s combustor model or modelswhich are very similar to Hsu’s model. As for workable TVC modelsfor aeronautical gas turbine engines, attentions are paid mostly to

and photograph of the PIV test rig. All dimensions in mm.

Fig. 2. Schematics of the mainstream bluff body and radial struts. All dimensions in mm.

Y. Jin et al. / Applied Thermal Engineering 66 (2014) 318e327320

emissions and combustion characteristics, few detailed flow fieldanalysis are available [11e18]. In particular, Hendricks et al. [14,15]investigated the high pressure TVC sector numerically, but theconfiguration was modeled without casing and diffuser, air fed intodifferent injection locations were separately controlled, which in-troduces differences compared to realistic combustors. This paperproposes a new workable TVC geometry for aeronautical gas tur-bine engines and investigates the detailed flow structure. The pri-mary focus of the present study was put on the prediction ofdetailed flow structures, especially flow structures in cavities andthe interplay between the cavity stream and the mainstream. Somefundamental flow physics were then analyzed to improve the un-derstanding of the flow dynamics in TVC. Secondly, the Reynolds-averaged NaviereStokes (RANS) is still the most computationallyaffordable approach for engineering design now, however, differentturbulence models have been employed by RANS approach forcomputation of TVC flow field. A comparison between results offour turbulence models and PIV measurements was conducted,thus the abilities of the four turbulence models to capture the flowstructures of the TVC could be effectively evaluated.

Fig. 3. Geometry of the combustor liner down

2. Computational domain and numerical method

2.1. Computational domain

Fig. 1 shows a 2-D schematic of the combustor, a photograph ofthe PIV test rig and the computational domain.

The TVC consists of a diffuser, cowls, a central bluff body,radial struts, cavities, liners and casings. The diffuser comprises ofa straight walled pre-diffuser, of area ratio 1.472, which projectsinto a step region, where the flow is divided into three streams.The cavity has a length of 50 mm and a depth of 43 mm. Cavityair is introduced through four 4 mm-width slots which arelocated in the cavity fore walls and the cavity after walls,respectively. The 180-mm wide (z direction) PIV test rig is madeentirely of plexiglass, thus allowing high quality optical mea-surements. The computational domain corresponds to one thirdsof the PIV test rig with translational periodicity on the two lateralsides. The dome of the mainstream, as shown in Fig. 2, iscomprised of a bluff body and two radial struts. The mainstreamdome is designed to be flush with the cavity fore walls. The

stream of cavities. All dimensions in mm.

Table 1Details of the four grids used for grid independency study.

G1 G2 G3 G4

Number of cells 1,700,000 2,165,566 2,572,194 3,077,402Average

yþ of most near-wall cells63 55 52 51

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mainstream is introduced through the passages formed by thebluff body and radial struts. Fig. 3 illustrates the geometry of theliner downstream of cavities, the cooling slots are all 1 mm wide,and the inclination angle to the liner surface is 45�, the dilutionholes are perpendicular to the liner surface.

Fig. 6. Photograph of the PIV test system.

2.2. Numerical method

In the present work, the numerical simulations were finished byusing commercial CFD software FLUENT 6.3.26. To simulateisothermal non-reacting flow field of the combustor, the steady-state continuity and momentum equations are discretized overthe computational domain using a finite volume method. Convec-tion and diffusion terms are discretized by the second-order

Fig. 4. Radial profiles of mean axial velo

Fig. 5. 3-D view of the grid determin

upwind scheme. The well-known SIMPLE algorithm is applied forpressureevelocity coupling. Translational periodical boundary

city at x/L ¼ 0.2, 0.4, and 0.6 in PM.

ed by grid independency study.

Fig. 7. PIV measured and computational streamlines in PM (Ma ¼ 0.25).

Fig. 8. PIV measured and computational streamlines in PS (Ma ¼ 0.25).

Y. Jin et al. / Applied Thermal Engineering 66 (2014) 318e327322

conditions are applied to the two lateral sides of the domain.Massflow and pressure boundary conditions are employed at theinlet and the outlet respectively. The standard wall functions areutilized to take care of the near-wall region.

The solution was ensured to be independent from the grid byperforming a grid independency study on four different grids. Thedetails of the four grids, which are all of hexahedral cells, are givenin Table 1.

Fig. 9. Comparison of the experimentally measured and stand

It should be noted that, PM (xey plane) and PS (xey plane) referto the plane between two radial struts and the plane along radialstruts, respectively. Fig. 4 provides the radial profiles of the meanaxial velocity at x/L ¼ 0.2, 0.4, and 0.6 in PM (where x is the axialdistance and L ¼ 50 mm is the axial length of cavity). It is observedthat there is approximately little difference between the profilespredicted by G3 and G4, therefore, the grid G3 was adopted for thepresent work. The uncertainty due to discretization in the

ard keε predicted vortex structures in cavity (Ma ¼ 0.25).

Fig. 10. Radial profiles of axial velocity at x/L ¼ 0.2, 0.4, 0.6 and 0.8 in PM: d standard keε predicted; B experiments.

Y. Jin et al. / Applied Thermal Engineering 66 (2014) 318e327 323

simulation is estimated to be 1.6% [19]. The G3 grid is shown inFig. 5. The convergence criteria for the computation in the presentwork is 10�6.

3. Results and discussions

3.1. Turbulence model determination and numerical methodvalidation

The turbulence model determination and numerical methodvalidation are accomplished with help of our PIV measurements.PIV is a whole-field measurement technique, providing quantita-tive flow visualization of a two-dimensional velocity field. Fig. 6shows a photograph of the PIV experimental system. The PIV sys-tem utilized for the present experiments consists of a 200-mJdouble pulsed Nd:YAG laser and a CCD camera. The repetitionrate of the laser is 15 Hz. The typical time separation between thetwo laser pulses is set to be about 20 ms for the flow conditionsstudied. The CCD camera has a 2048 � 2048 array. Solid tracerparticles of nominal diameter of 5 mmwere introduced into the coldair flow 1000 mm upstream of the combustor inlet using a simplefluidized bed particle dispenser. Image mapping, calibration, andparticle cross-correlations were completed using the commercialsoftware La Vision DaVis 7.2. The estimated uncertainty is about10% in the wake region of the center bluff body and 2% in otherregions [20].

It should be noted that both the numerical simulations andexperiments were conducted under non-reacting conditions.Although the flow field under non-reacting conditions will bedifferent from that of reacting conditions, investigation in non-reacting conditions provide insight to better understand the flowphysics in the combustor. PIV measurements were carried out bothin PM and in PS for inlet Ma ¼ 0.25, temperature T3 ¼ 300 K,pressure P3 ¼ 1 atm. The Mach number 0.25 is selected for thisanalysis because it is a typical value for modern aero-engines. Athigher pressure and temperature conditions, flow fields similar tothe present results can be expected. The experimental andcomputational results were compared in terms of global flowstructures and detailed velocity distributions. Working conditionsof the numerical simulations are identical to that of the PIV ex-periments. Fig. 7 exhibits the PIV measured and the computedcavity streamlines in PM. It can be seen from the PIV results that thedual-vortex flow pattern was established in the cavity, as shown inFig. 7(a). The standard keε model provides a good solution of theflow field, as shown in Fig. 7(b), the dual-vortex cavity flow patternwas successfully predicted. Fig. 7(c), (d) and (e) depicts the resultsof the SST keu model, the RNG keε model and the realizable keεmodel respectively, these three results are similar in the presenceof a large scale vortex near the downstream bottom corner of thecavity. This corner vortex was formed mainly because the mo-mentum of the cavity fore wall air jet was underestimated. How-ever, it is noteworthy that this vortex was not found in PIV results.

Fig. 11. Radial profiles of axial velocity at x/L ¼ 0.2, 0.4, 0.6 and 0.8 in PS: d standard keε predicted; B experiments.

Y. Jin et al. / Applied Thermal Engineering 66 (2014) 318e327324

In PS, as shown in Fig. 8, the standard keε model predicted thesingle vortex flow pattern which was determined by PIV mea-surement, the other three models also failed.

A more detailed comparison of vortex structure between PIVresults and standard keε results was displayed in Fig. 9. It is foundthat in PM, the predicted vortex core locations in the radial direc-tion are in very good agreement with that of PIV results. In the axialdirection, however, difference was observed: the predicted core ofthe large vortex was located about 3 mm downstream of that in thePIV results, and the displacement of the small vortex compared tothe PIV data was about 10 mm. The large vortex is the dominantflow feature in cavities, therefore, a slight deviation in the predic-tion of the large vortex could result in a remarkable deviation in theprediction of the small vortex. In PS, the predicted location of thevortex agrees well with the PIV results.

Figs. 10 and 11 show the radial profiles of axial velocity atvarious axial locations, in PM and PS respectively. It is evident thatthe computational results in PM reproduce the flow feature, qual-itatively and quantitatively with only a modest overshoot in anarrow area near y ¼ 0. A better agreement is attained between thecomputed results and the PIV results in PS.

In conclusion, the great differences in flow structures betweenthe SST keu model, the RNG keε model, the realizable keε modeland the experimental results suggest that these three models failedto capture the primary flow features of the present combustor. Onthe contrary, the standard keε model predicted the overall flow

structure reasonably well although it did not predict the very exactlocation of vortex cores. Therefore, one can conclude that the nu-merical method associated with standard keεmodel in the presentwork is relatively valid.

3.2. Detailed flow structure and mixing analysis

Fig. 12 shows the streamlines overlaid on the counters of ve-locity magnitude both in PM and in PS forMa ¼ 0.25. It is clear fromFig. 12(a) that, in PM, the dual-vortex flow pattern is established incavities: a main vortex is located deeply in the cavity, a secondsmall vortex is located between the main vortex andmainstream. Itis believed that the small vortex serves as a barrier to protect themain vortex from the mainstream, as well as a mixing stage of themain vortex and the mainstream. Also clear from Fig. 12(a) is thatthe cavity and the low-velocity wake region of the bluff body aredisconnected by the high-velocity mainstream. In contrast to thedual-vortex pattern in PM, the single-vortex pattern is formed inthe cavities in PS, as illustrated in Fig. 12(b). The cavity streamtravels along radial struts and eventually, reach the wake region ofthe bluff body. This interlocked nature of the different flow struc-tures in the spanwise direction indicates the three dimensionalnature of the flow in the combustor.

The calculated streamlines in cross sectional planes (yez plane)at various axial locations are presented in Fig. 13, from which theevolution of streamwise vortices along the axial direction can be

Fig. 12. Streamlines overlaid on the contours of velocity magnitude for Ma ¼ 0.25 in: (a) PM; (b) PS.

Y. Jin et al. / Applied Thermal Engineering 66 (2014) 318e327 325

clearly identified. The formation of streamwise vortices is observedat x/L ¼ 0.2, which is located 10 mm downstream of the cavity forewall. The streamwise vortices grow as the flow marches down-stream, to x/L ¼ 0.6. However, further downstream at x/L ¼ 0.8, thestreamwise vortices get small. The inducement of the streamwisevortices is mainly ascribed to the set of radial struts. The set ofradial struts leads to partial blockage in the flow paths, the low-pressure wake regions corresponding to radial struts favors inestablishment of spanwise pressure gradient. However, It should benoted that, as the limited axial length of the wake regions of radialstruts, the pressure field is relatively uniform in the downstreamarea far away from the struts, thus the smaller streamwise vorticesin x/L¼ 0.8 could reasonably be explained. Also shown in Fig. 13 arethe rotating directions of streamwise vortices. These vorticesentrain fluids from mainstream and eject them towards the radialstruts, which is thought to be lead to mixing enhancement in thespanwise direction. Distribution of mean streamwise vorticity as afunction of the axial location is shown in Fig. 14. The meanstreamwise vorticity varies non-monotonically and peaks at theregion near x/L ¼ 0.5.

The schematic in Fig. 15 is an attempt to summarize the mainflow features identified by the numerical simulations in the presentstudy. In a plane along the radial struts, the single-vortex flowpattern is observed in cavities, the mixing of the cavity stream andmainstream is accomplished with the help of the wake regions ofradial struts. With enough momentum, the cavity stream couldtravel farther along the struts to the wake region of the bluff body,then laterally spread across the bluff body. The two mixing stages,which occur in the wake region of radial strut and bluff body

respectively, are schematically marked in Fig. 15. In a plane be-tween two radial struts, the dual-vortex flow pattern is establishedin cavities, the mixing of the cavity stream and mainstream ismainly accomplished by the small vortex, however, the mixing islimited as the small vortex is relativelyweak. It can be inferred fromFig. 13 that the mixing strength of these two planes are remarkablydifferent, which further, is thought to lead to enhanced mixing inthe spanwise direction.

An obvious feature of the flow is the formation of the regions ofalternate cavity stream and mainstream along the spanwise di-rection, which suggests that, under reacting conditions, there areregions of alternate hot combustion gases originating from cavitiesand cold air/fuel mixtures from mainstream. This provides a largeinterface area for heat and mass exchange between the twostreams. We believe that such conditions are very conducive for thecold air/fuel mixtures from mainstream to be ignited by the cavityhot stream.

Following the previous analysis, an issue focuses on the width ofan individual radial strut is then put forward for discussion. Themain benefit of a wider strut is that it helps to establish a widerwake region, and consequently achieve higher momentum for in-dividual cavity stream. This momentum favors the travel of thecavity flow to the wake region of the bluff body, where the secondmixing stage occurs. However, for a constant blockage area, theincrease of individual strut width accompanies with decrease innumber of struts, which, consequently would reduce the interfacearea between the cavity stream and the mainstream. Therefore, theoverall effect of varying the width of radial struts is hard to beanticipated and further investigations are needed.

Fig. 13. Streamlines in cross sectional planes at various axial locations, the rectangular areas refer to radial struts.

Fig. 14. Mean streamwise vorticity at different axial locations.

Y. Jin et al. / Applied Thermal Engineering 66 (2014) 318e327326

4. Conclusions

In this paper, a newworkable TVC geometry for aeronautical gasturbine engine was described and numerical simulations withvalidated methodology were carried out with the objective toprovide insight of detailed flow structures in this combustor. Thecapabilities of four turbulence models in modeling the TVC flowfield were evaluated by comparison with PIV experimental results,and the superiority of the standard keε model was effectivelyrevealed. Flow field analysis demonstrates that the two typicalcavity vortex flow patterns mentioned in literature, the dual-vortexpattern and the single-vortex pattern, both exist in the combustoralthough within different planes. Ascribe to the formation of wakeregions and the inducement of streamwise vortices, the radialstruts are credited to play a key role in mixing process in a TVC. Theflow structure obtained by the present work could help wideningour understandings of TVC and therefore, is valuable for TVC designand optimization.

Prospective research efforts shall focus on flow field measure-ment under reacting conditions and combustion characteristicsexploration.

Fig. 15. Schematic of the flow structures in the TVC.

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