NASA/CR--2002-212082

Simultaneous Laser Raman-Rayleigh-LIF

Measurements and Numerical Modeling

Results of a Lifted Turbulent H2/N 2 Jet

Flame in a Vitiated Coflow

R. Cabra, J.'l'. Chen, and R.W. Dibble

University of California at Berkeley, Berkeley, California

T. Myhrvold

Norwegian University of Science and Techn.olog D Trondh.eim., Norway

A.N. Karpetis and R.S. Barlow

Sandia National Laboratories, Livermore, California

December 2002
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NASA/CR--2002-212082

Simultaneous Laser Raman-Rayleigh-LIF

Measurements and Numerical Modeling

Results of a Lifted Turbulent H2/N 2 Jet

Flame in a Vitiated Coflow

R. Cabra, J.Y. Chen, and R.W. Dibble

University of California, Berkeley, Berkeley, California

T. Myhrvold

Norwegian University of Science and Technology, Trondheim, Norway

A.N. Karpetis and R.S. Barlow

Sandia National Laboratories, Livermore, California

Prepared for the

29th International Symposium on Combustion

sponsored by the Combustion Institute

Sapporo, Japan, July 21-26, 2002

Prepared under Grant NAG3-2103

National Aeronautics and

Space Administration

Glenn Research Center

December 2002
Acknowledgments

The research is part of a project supported by NASA Glenn Research Center, Grant NAG3---2103. Research

conducted at Sandia was supported by the United States Department of Energ?; Office of Basic Energy Sciences.

T. Myhrvold worked in close collaboration with I.S. Ertesv_g and I.R. Gran at the Norwegian University of Science

and Technology and was supported by the Research Council of:Norway.

Trade names or manufacturers' names are used in this report for

identificatio_ only. This usage does not constitute a_ official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

The Aerospace Propulsio_ and Power Program at

NASA Glenn Research Center sponsored this work.

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Available electronically at http://gltrs.grcnasa.gov
Simultaneous Laser Raman-Rayleigh-LIF Measurements and Numerical Modeling Results of a

Lifted Turbulent H2/N2 Jet Flame in a Vitiated Coflow

R. Cabra, J.Y. Chen, and R.W. Dibble

University of California, Berkeley

Berkeley, California

T. Myl_'vold

Norwegian University of Science and Technology

Trondheim, Norway

A.N. Karpetis mid R.S. Barlow

Sandia National Laboratories

Livermore, California

ABSTRACT

An experimental and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in a coflow

of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics

exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical

investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are

reported for a lifted turbulent jet flame of H2/N2 (Re=23,600, H/d=lO) in a coflow of hot combustion products from

a lean H2/Air flame (d?=0.25, T=l,045K). The combination of Rayleigh scattering, Raman scattering, and laserinduced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major

species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated

coflow throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function

and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off

height (HpDF/d=7, H_Dc/d=8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments,

yields a reasonably accurate prediction (HK/d=I 1.4) of the lift-off height for the present flame. The vitiated coflow

admits the possibility of auto-ignition of mixed fluid, and the success of the present parabolic implementation of the

PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened

turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility

of turbulent premixed flame propagation by small scale (on the order of the flame thickness) recirculation and

mixing of hot products into reactants and subsequent rapid ignition of the mixture.

NASA/CR--2002- 212082 1
INTRODUCTION

Clearunderstanding ofturbulent flamestabilizationinanenvironment ofhotcombustionproductswillaid

theadvancement of combustiontechnology.Practicalcombustor designsemploytherecirculationof hot

combustionproductsoachieveflamestabilizationintheirintenselyturbulent flows.Numerical investigations of

theseflowsaredifficulttoexecutebecauseofthedetailed, fullycoupled, turbulent fluidmechanicsandchemical

kinetics.A challengefor combustionresearchers is to designexperiments thataddressflamestabilizationin

combustionproducts, whiledecouplingthechemical kineticsfromthecomplexrecirculatingflow.

Thesimplifiedflowof thevitiatedcoflowburnerprovidesexperimental andnumerical accessto

fundamental combustionfeaturesofrecirculationburners.Thedesign(Fig.1)consistsofajetflameinacoaxial

flowofhotcombustionproductsfromaleanpremixedflame(vitiatedcoflow).Thereactingflowassociated with

thecentral jet exhibitsimilarchemical kinetics,heatransfer,andmolecular transport asrecirculationburners

withouttherecirculatingfluidmechanics. Thewell-defined, uniformboundaryconditionsandsimplifiedflow

associated withthecoaxial jetdesignareattractivefaturesfornumerical modeling.Additionally, theopendesign

facilitatestheuseofopticaldiagnostics.

Thevitiatedcoflowburnerenablesinvestigationofstabilizationmechanisms forliftedturbulent jetflames

inenvironments thatarerelevanttocombustionapplications buthavenotbeenstudiedsystematically. Typically,

liftedturbulent jetflameexperiments areconductedonfueljetsincoolair,wheretheairiseitherquiescent oralow

velocitycoflow.Therangeofflowsthatcanbestudiedislimitedbecausearelativelysmallincreaseinaircoflow

velocitycanresultin flameblow-off.In turbulent jet flameresearch, thislimitationof flamestabilityis often

circumvented withtheuseofpilots[1,2],whichintroducethecomplexityofathirdstream(jet,pilot,andcoflow)

andnon-uniformityof fluidentrainedbythejet. Thevitiatedcoflowisa largepilotthatprovidesauniform

environment fortheinvestigationofhighlyturbulent jetflameswithlowDamk{Shler numbers.Suchturbulent flow

characteristics areexemplaryofpractical combustors.

Liftedturbulent jetflameshavereceivedsignificant atentionithecombustionliterature, andcompeting

theoriesofflamestabilizationhavebeenproposed[e.g.,3-5].ArecentreviewinthetextbyPeters[6]presentsthe

viewthatstabilizationfaliftedflameinvolvesthepropagationof aturbulent partiallypremixedflameataspeed

thatbalancesthelocalstreamwiseconvectivevelocity.Thisdescriptionconsolidates ideasinvolvingturbulent

premixedflamepropagationandtripleflame(oredgeflame)propagation, andcomputations baseduponthisconcept

NASA/CR--2002- 212082 2
haveyieldedsuccessful predictionsofliftoffheight[7].Ithasalsobeenproposed[8,9]thathot,productcontaining

eddiesrotateupstream, entrainpreheated reactants, andfacilitatereaction.Recentstudiesusingscalarfieldimaging

[10]andPIV[11]showedevidenceoftheimportanceofsuchrecirculationatthestabilizationregion.

In thepresentstudy,multiscalar laserdiagnosticsandnumerical modelsareusedto investigatealifted

turbulent H2/N2jet flameinacoflowof leanH2/aircombustionproducts.Thisliftedflamehassomefeatures

consistent withthetheoryof stabilizationbypropagationf aturbulentpartiallypremixedflame.However, the

conditionsof thevitiatedcoflowadmitheinterestingpossibilitythatmixturescanautoigniteastheyconvect

downstream. In addition,therearefeaturesin thelasermeasurements thatsuggest thereactionzonein the

stabilizationregionisthickened(thickerthanthesmallest estimatedscalesofturbulence) ormayincludelocalized

ignitionevents.Experimental andcomputational resultsarediscussedinthecontext ofthesenovelconditions.

EXPERIMENTALMETHODS

Experimentswereconductedonaliftedturbulent H2/N2jet flamein avitiatedcoflow(Table1). The

combustor consistsofacentralH2/N2turbulent jetwithacoaxialfowofhotcombustionproductsfromalean

premixedH2/Airflame(Fig.1). ThedesignisanadaptationfthedesignbyChenetal.[2]. Thecentral jetexit

diameterisd 4.57 mm and the coflow flame is stabilized on a perforated disk with 87% blockage and an outer

diameter of 210 mm. The central jet extends 70 mm above the surface of the perforated disk. For the conditions

listed in Table 1, the observed lift-off height was H/d-lO, and the total flame length was HF/d=30.

Simultaneous, temporally and spatially resolved measurements of temperature and species mass fractions

were obtained using laser diagnostic systems of the Turbulent Diffusion Flame laboratory at the Combustion

Research Facility of Sandia National Laboratories. Details of the experimental setup and calibration techniques

have been previously presented [12-15]. Spontaneous Raman scattering and Rayleigh scattering, using two

frequency-doubled Nd:YAG lasers, were combined to measure temperature and the concentrations of N2, 02, H20,

and H2. Concentrations of OH and NO were measured using laser-induced fluorescence (LIF). The LIF systems

were operated in the linear fluorescence regime, and corrections were applied on a shot-to-shot basis to account for

local variation in Boltzmarm fraction and collisional quenching rates for OH and NO. These multiscalar

measurements were virtually simultaneous, with delays of about lOOns between the different laser pulses. The

spatial resolution of the system is 750gm. NO concentrations were consistently very low in this flame (YNo<3ppm),

and they will not be presented here.

NASA/CR--2002- 212082 3
Theprecisionandaccuracyof theRaman-Rayleigh-LIF systemis determinedfromcalibrationflame

measurements [15].Theprecisionofsingle-shot measurements ofanH2flamewithnofluorescence interferences is

limitedbythephotoelectronshotnoise[16],andit is indicatedbythestandarddeviations(RMS)of theflat

(Hencken) flamemeasurements: temperature1.2%,N23.2%,H205.4%,OH12%andNO10%.Experimental

uncertaintiesnaveragedscalarvalueswereestimatedfromtherepeatability ofcalibrationresultsanduncertainties

inreferencequantities. Representative uncertainties forthepresent resultsare:temperature 3%,N23%,H204%,

andOH10%.Uncertaintyinthe02isbestrepresented asanabsoluteerrorinmassfractionof about+0.005,

regardless of the local value of Yo2.

NUMERICAL METHODS

Presented are two turbulent combustion models, the Eddy Dissipation Concept (EDC) and the Probability

Density Function (PDF). These models are coupled with either the standard k-c or Reynolds stress fluid dynamic

model for turbulent flow.

Eddy Dissipation Concept (EDC) Numerical Method

The general-purpose CFD code Spider [17] with EDC was developed at the Norwegian University of

Science and Technology division of Thermodynamics in Trondheim. The turbulent reacting flow is modeled by the

density-weighted Reynolds-averaged conservation equations for momentum components, energy, and mass fractions

of species. Turbulence was modeled either by the standard k-c model or by two versions of Reynolds-stress models

by Launder, Reese and Rodi (LRR) [18] or by Jones and Musonge (JM) [19,20]. In the present calculations Spider

employs a 2-D axisymmetric geometry.

The mean reaction rate of chemical species] is modeled by EDC [21,22] as

-- _*rh*

(wo_w;),

where 7* is the mass fraction of turbulence in fine structures and rh* is the reciprocal of the fine-structure

residence time (_* = 1/rh* ). These two quantities are expressed as functions of the turbulence energy and the

turbulence energy dissipation rate [21,23]. Specifically, the fine-structure residence time is assumed proportional to

the Kolmogorov time scale, yO and Yj are the mass fractions of species] in the surrounding fluid state and the

NASA/CR--2002- 212082 4
finestructurestate.Thefinestructureisregardedasaperfectlystirredreactorandthemassbalanceforspeciesj in

thereactorismodeledas

dYj*_Rj _4(yO_yj.)

dt p

Thesespeciesmassbalances, together withequationsforenergyandmomentum, areintegratedintimeuntilsteady

stateisreached.AdetailedH2mechanismtakenfromGRI-Mech2.11[24]isused(carbonspeciesandreactions

excluded).

Probability Density Function (PDF) Numerical Method

The model utilizes the joint scalar PDF for composition only and the k-c turbulence model for a parabolic

flow [25]. The turbulent flux and scalar dissipative terms appearing in the PDF transport equation are modeled by a

gradient diffusion model and the Curl mixing model [26], respectively. Monte Carlo simulation is used to compute

the transport equation for the PDF [27]. Four hundred stochastic particles per grid are used. A 7-step reduced

chemistry (6 steps for combustion and 1 for NO formation) is integrated directly in time for each particle. The

reduced chemistry model has been thoroughly tested and performs well in calculations of laminar opposed-flow

nonpremixed flames, laminar premixed flames, perfectly stirred reactors, and ignition. Therefore, we expect

satisfactory performance here.

RESULTS AND DISCUSSION

The structure of the lifted turbulent H2/N2 jet flame is investigated by examining the measured temperature

and species concentrations profiles. Centerline measurements were taken from z/d=l to 34 downstream of the

nozzle exit. Radial profiles were obtained at several axial locations (z/d=l, 8, 9, 10, 11, 14 and 26). The radial

domain covered by these profiles was -3mm to 50mm with spacing typically between 1 and 3mm. The single-shot

data was processed and the Favre averages and RMS fluctuations were generated. The following formulation,

modified for the current H2/N2 system, determines the mixture fraction [28].

f_ (1/2MH)(YH - YH,2)- (1/Mo)(Yo - Yo,2)

(1/2MH)(YH,1- YH,2)-(1/mo)(Vo,1- go,2)

The stoichiometric mixture fraction for the present fuel composition is f_=0.474.

NASA/CR--2002- 212082 5
Inlet and Far-Field Boundary Conditions

Radial profiles of Favre averaged temperature at z/d=l, 14 and 26 are plotted in Fig 2. The measured mean

temperature in the coflow at z/d=l is uniform (2% RMS), indicating a well-mixed mixture. Also, the far-field

(coflow) measurements of temperature do not change with axial distance. Thus, the integrity of the coflow is

maintained in the entire test region. The same well-defined boundary conditions are observed for the species

measurements. These results demonstrate that the flame can be modeled as a jet flame issuing into an infinite hot

coflow, and they attest to the success of the experimental design.

Flame Structure and Lift-Off'Height

The OH mass fraction, YOH, is used as a marker of the average flame lift-off height in both measured and

modeled results. Fig. 3a shows a contour map generated using an aggregate of the point measurements (white dots).

Several points were taken in the flame stabilization region to provide adequate resolution for determination of the

lift-off height, H/d-i O, which was taken to be the location where the Favre average YOHreaches 600 ppm.

The numerical models each predict a lifted flame structure, which is a significant result in itself, regardless

of the accuracy of the predicted lift-off height. It is was not obvious, a priori, that the PDF model would predict a

lifted flame because the present calculation proceeds in a downstream marching solution and includes no mechanism

for propagation of a turbulent premixed or partially premixed flame into the convecting flow. However, it is

apparent from Fig. 3b that there is reaction progress for some fraction of the PDF particles well upstream of the

flame stabilization location at the YOH=600 ppm contour. This result is associated with auto-ignition of mixed fluid,

a process that would not occur with a cold air coflow. There were no visually obvious auto-ignition events well

below the lift-off height; perhaps they would have been revealed by radial profiles taken at intermediate locations

(l<z/d<8). However, the flame does spontaneously ignite in the laboratory, starting at a far downstream location,

when the coflow is operating and jet flow is turned on. This possibility for auto-ignition leading to flame

stabilization in the vitiated burner and the PDF calculation are worthy of further exploration.

Results from three numerical simulations are shown. Fig. 3b shows the OH contours from the PDF

combustion model and the standard k-c model, which yields a lift-off height of HpDF/d=7. Using the same k-c

model, the EDC model predicts H_Dc/d=8.5, as shown in Fig. 3c. The standard k-cmodel is known to over-estimate

the turbulent diffusivity and, consequently, over predict the spreading rate of round jets. This may account for the

wide flame predictions in these two calculations (Figs. 3b and 3c) relative to the experimental results (Fig. 3a). The

NASA/CR--2002- 212082 6
third simulation, using the EDC model and the LRR Reynolds stress model, predicted a shorter lift-off height of

Hr,d=5 (Fig. 3d). The overall flame shape is narrower than that predicted by the standard k-cmodel and in better

agreement with the measured flame width, even though the lift-off height is under predicted.

Using the present EDC model, a parametric study was conducted to explore the sensitivity of the predicted

lift-off height to boundary conditions, turbulence models, and grid resolution. The base case employed the standard

k-c model, and the boundary conditions consisted of a fully developed turbulent pipe flow (fij=107m/s) for the jet

and a uniform flow field (fic=3.5m/s) for the coflow. The inlet coflow turbulence variables k and cwere determined

using rough estimates of the integral length scale and turbulence intensity 0vo=lmm and u'/Uc=5%). Most cases

were calculated with a mesh of 60x55 (axial x radial) with grids clustered near the jet exit. The predicted lift-off

height was found to be insensitive to changes in the coflow turbulence parameters (increased to )Vo=3mm and

u'/Uc=10%) and unaffected by the use of detailed modeling of the nozzle wall turbulence. Increases in the jet

velocity to fij=120m/s and the coflow velocity to ac=10m/s caused the predicted lift-off height to increase by roughly

25% and 50%, respectively. Refinement of the grid revealed a sensitivity of the calculated lift-off height to grid

resolution, confirming that fine grids in the near field region are needed for adequate accuracy. The mesh was

refined to 150x55, while keeping the computational domain for all EDC simulations fixed (z/d=40 x r/d=12).

However, further increases in grid resolution (beyond 150x55) produce insignificant changes in model results. The

fine grid resolution was also used for the computational domain of Reynolds stress models. The results in Figs. 3

and 4 were calculated using the fine grid. Predictions of lift-off height by the Reynolds stress turbulence models

were consistently lower than those by the k-cmodel.

Figure 4 compares centerline profiles of mixture fraction and oxygen mass fraction and shows the

sensitivity of predicted mixing rates to the turbulence model. The peak in oxygen seen near z/d_14 (Fig 4b)

illustrates the upstream penetration (increased with lift-off height) and subsequent consumption of oxygen by the

flame. By presenting an entrainment rate profile similar to the Yo: centerline profile, Han and Mungal [29]

observed a similar correlation between mixing and lift-off height. Since the standard k-c model over-predicts

turbulent diffusion for round jets, the predicted oxygen penetration is higher than the data, as evident by the early

(PDF) or high (EDC) centerline Yo: peaks and the rapid decay in mixture fraction. While the lift-off height

predicted by the EDC with the Reynolds stress model is low, both predicted centerline profiles agree well with the

data.

NASA/CR--2002- 212082 7
Combustion Statistics at Flame Stabilization

Scatter data of temperature vs. mixture fraction are shown in Fig. 5. Approximately 4,000 point

measurements from different radial positions were grouped together to form a probability density map for three axial

positions (z/d=8,11,14). There is a clear progression from a predominantly mixing condition (z/d=8) to vigorous

flame burning (z/d=14) that corresponds to the transition from mixing only to mixing combined with ignition and

flame stabilization. Since the flame is not attached to the nozzle, the central fuel jet entrains hot oxidizer from the

coflow, evolving into a partially premixed flow with fluid temperatures corresponding to the mixing line between

the jet and coflow boundary conditions in Fig. 5c. Beyond the potential core of the jet there is progressive dilution

of the richest samples, such that the fuel-rich boundary condition for combustion at z/d=14 has decreased from f=l.0

to values between f_3.9 and f_3.6. Also plotted in Fig. 5a are the results from a series of laminar opposed flow

flame calculations with equal molecular and thermal diffusivities. Corresponding calculations with full transport

(i.e. with differential diffusion included) poorly matched the data, suggesting that turbulent stirring is more

important than differential molecular diffusion in determining the relative mass fractions of major species in the

measured flame. The fuel side boundary condition for the laminar flame calculations was set at f=0.8, to represent

this measured departure from the pure jet composition.

The range of scalar dissipation rates in the turbulent flame above the stabilization region can be estimated

by determining the strain rates whose corresponding opposed flow laminar flame solutions match the upper and

lower bounds of the laser shot measurements [30], particularly on the fuel-lean side. In Fig. 5a, the laser shot data is

approximately bounded by solutions with strain rates of 100s -a and 5,000s -1. The computed strain rate prior to

laminar flame extinction was 13,000s -a, and a number of data points are below the 5,000s -1 solution. It should be

noted that the low strain rate calculation adequately describes the lean side results as expected, since these results

correspond to the hot co-flow where low strain should prevail and where viscosity is still relatively high. The richside experimental results cannot be adequately represented by this limited set of laminar flame calculations because

of the broad range of fuel-side boundary conditions produced by mixing upstream of the reaction zone.

Perhaps the most interesting condition can be seen in Fig. 5b, where the data is scattered throughout the

zone between the mixing (lower) and fast chemistry (upper) limits on temperature. This axial position (z/d=l 1) is

one diameter above the observed lift-off height. The behavior of the scatter data in Fig. 5b is qualitatively different

from that reported for lifted H2 jet flames in air [31,32], where there is clear bimodality between unreacted and

NASA/CR--2002- 212082 8
reacted samples in the region of flame stabilization. Results for the present burner suggest the existence of a

thickened turbulent flame in the stabilization region. A laminar premixed flame simulation predicts an unstrained

flame thickness of 8L=2.5mm, while the turbulence models suggest a Kolmogorov scale of roughly 50-500 microns

in the stabilization region. Therefore, it may be argued that there are turbulent eddies of order lmm within a

thickened reaction zone. The observed scatter could also result from a process of small-scale mixing and ignition at

the flame base, as outlined below.

Flame Stabilization

The experimental and numerical results present an opportunity to discuss possible flame stabilization

mechanisms for the vitiated coflow burner. The presented EDC and k-c model results (Fig. 3c) indicate an average

flow velocity of 10m/s at the flame base, while the maximum laminar premixed flame speed was calculated to be

Sr=3m/s for reactants at f=0.4 along the mixing line. A factor of 3 increase in velocity for a propagating turbulent

premixed or partially premixed flame is plausible [6,33,34]. Interestingly, the correlation by Kalghatgi [4], which is

based upon scaling arguments:

HK= 50/v jet82 jet U "//Tc°fl°wMjet '/1.5 ,

L,max )\ TjetMcoflow

yields a relatively accurately prediction of our measured lift-off height. For the present flame the jet mixture

viscosity is Vjet=2X 10 -5 m:/s, and molar mass of the jet and coflow mixtures are Mjet=21.5 and M_onow=27.6. For the

given conditions, this engineering correlation predicts a lift-off height of Hx/d=l 1.4, indicating the robustness of the

correlation and suggesting that stabilization of the present flame may be controlled by the same mechanisms that

control lifted flames in cold air. However, the potential for auto-ignition of mixed fluid and the behavior of the

temperature scatter data noted above both allow for speculation on variations or additional mechanisms that may

contribute to stabilization of the present flame.

It is possible that auto-ignition of mixed fluid is only important for the transient startup of the laboratory

flame and that, once ignited, a propagating partially premixed flame advances upstream to a lift-off height for which

convective times from the nozzle are shorter than ignition delay times for any fluid sample along the mixing line. It

is also possible that auto-ignition in the stabilization region serves to augment or anchor the propagating flame. A

specific mode of ignition-enhanced turbulent flame propagation is illustrated in Figs. 5d,e and is based on the

observation that the ignition delay for mixtures of products and reactants can be short compared to the timescale of

NASA/CR--2002- 212082 9
smalleddiesinthestabilizationregion.Theillustrationshowsmixingofproductandreactant samples(Fig.5d),

eachatf=0.4.Suchmixingcouldoccuraroundanauto-ignitionkernelorbyeddyturnoverinthethickenedpartially

premixedflame.Afterrapidignitionofthemixedfluid,areactionfrontpropagateshroughneighboringmixtures

(Fig.5e).Thissortofmixing/ignition/propagation mechanismwouldbeconsistent withthemeasureddistribution

oftemperature scatterdatainFig.5b.However, multiscalar imagingmeasurements wouldbeneededtodetectsuch

events.

CONCLUSIONS

1. Thevitiatedcoflowburnerprovidesasimplifiedflowwithwell-definedboundaryconditions. Measurements

confirmthatthecoflowpropertiesareuniformthroughout thetestregion.Theconfigurationenablesthe

examinationf turbulentmixingandchemical kineticsrelevantto themodelingof advancedcombustors

withoutthecomplexrecirculatingfluidmechanics.

2. Simultaneous multiscalar measurements ofaliftedH2/N2jet(H/d_-lO) werepresentedandcomparedtoaseries

ofnumerical simulationswithvariouscombustionandturbulencemodels.Numerical resultsfromaPDFand

anEDCcombustionmodelreasonably predictthelift-offheight(HpDF/d=7, H_Dc/d=8.5).

3. Features of the instantaneous scalar measurements in the stabilization region, together with information on

laminar flame thickness and turbulence quantities from calculations, suggest that a thickened turbulent partially

premixed flame exists at the flame stabilization location.

4. The vitiated coflow admits the possibility of auto-ignition of mixed samples. Such ignition is the only possible

stabilization mechanism in the PDF calculation, which proceeds by a parabolic marching solution. While there

is no clear experimental evidence of auto-ignition events below the lift-off height, it is plausible that autoignition or turbulent mixing of products and reactants in the stabilization, followed by rapid ignition, may

augment the stability of the present flame.

NASA/CR--2002- 212082 10
REFERENCES

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2. Chen, Y.G., Peters, N., Schneemarm, G.A., Wruck, N., Renz, U., & Mansour, M.S., Combust. Flame 107:223

(1996).

3. Vanquickenborne, L. & Van Tiggelen, A., Combust. Flame 10:59 (1966).

4. Kalghatgi, G.T., Comb. Sci. Tech. 41:17 (1984).

5. Pitts, W.M., Proc. Comb. Inst. 22:809 (1988).

6. Peters, N., Turbulent Combustion, Cambridge University Press, 2000 p.237-261.

7. Chen, M., Herixnann, M., and Peters, N., Proc. Comb. Inst. 28:167 (2000).

8. Broadwell, J.E., Dahm W.J.A., & Mungal, M.G., Proc. Comb. Inst. 20:303 (1984).

9. Dahm, W.J.A., & Dibble, R.W., Proc. Comb. Inst. 22:801 (1988).

10. Tacke, M.M., Geyer, D., Hassel, E.P., & Janicka J., Proc. Comb. Inst. 27:1157 (1998).

11. Upatnieks, A. Driscoll, J. F., Ceccio, S. L., Proc. Comb. Inst. 29:?? (2002).

12. Nooren, P. A., Versluis, M., van der Meer, T. H., Barlow, R. S., and Frank, J. H., Appl. Phys. B 71:95 (2000).

13. Barlow, R.S., Karpetis, A.N., Frank, J.H. & Chen, J.Y., Combust. Flame 127:2102 (2001).

14. Nguyen, Q.V., Dibble, R.W., Carter, C.D., Fiechtner, G.J., & Barlow, R.S., Combust. Flame 105:499 (1996).

15. Barlow, R.S., Fiechtner, G.J., Carter, C.D., & Chen, J.-Y., Combust. Flame 120:549 (2000).

16. Dibble, R.W., Masri, A.R., & Bilger, R.W., Combust. Flame 67:167 (1987).

17. Melaaen, M.C., Ph.D. Dissertation, University in Trondheim, 1990.

18. Launder, B. E., Reece, G. J., & Rodi, W.,3. Fluid. Mech. 68(3):537 (1975).

19. Jones, W. P., Turbulent Reacting Flows, Academic Press Ltd., 1994 p. 171-189.

20. Jones, W. P., Musonge, P., Phys. Fluids 31(12):3589 (1988).

21. Ertesvfig, I. S., Turbulent Flow & Combustion (in Norwegian), Tapir Academic Publisher, 2000 p. 309-372.

22. Gran, I. R., Magnussen B. F., Comb. Sci. Tech. 119:191 (1996).

23. Ertesvfig, I. S., Magnussen, B. F., Comb. Sci. Tech. 159:213 (2000).

24. Bowman, C.T., Hanson R. K., Davidson, D. F., Gardiner Jr., W. C., Lissianski V., Smith, G. P., Golden, D. M.,

Goldenberg, M., Frenklach, M., Gri-Mech 2.11, http://www/me.berkeley.edu/gri_mech/(1999).

25. Smith, N.S.A., Bilger, R.W., Carter, C.D., Barlow R.S., & Chen, J.Y., Comb. Sci. Tech. 105:357 (1995).

NASA/CR--2002- 212082 11
26. Pope, S.B., Proc. Comb. Inst. 23:591 (1990).

27. Chen, J.-Y. & Kollmarm W., Proc. Comb. Inst. 22:645 (1988).

28. Bilger, R.W., Stfimer, S.H., & Kee, R.J., Combust. Flame 80:135 (1990).

29. Han, D.H. & Mungal, M.G., Combust. Flame 124:370 (2001).

30. Wamatz, J., Maas, U., & Dibble, R.W., Combustion, Physical & Chemical Fundamentals, Modeling &

Simulation, Experiments, Pollutant Formation, 3rd Edition, Springer-Verlag, 2000 p. 195.

31. Cheng, T. S., Wehrmeyer, J. A., and Pitz, R. W., Combust. Flame 91:323 (1992).

32. Brockhinke, A., Andresen, P., Kohse-HOinghaus, K., Appl. Phys. B 61:533 (1995).

33. Lififin, A., & Williams, F.A., Fundamental Aspects of Combustion, Oxford University Press, 1993 p. 120.

34. Ruetsch, G.R., Vervisch, L., & Lififin, A., Phys. Fluids 7:1447 (1995).

Table 1 Flame and flow conditions.

Central Jet Coflow

QH2 (slm) 25 QH2 (slm) 225

QN2 (slm) 75 QAIR (slm) 2,100

TjE T (K) 305 TcovLow (K) 1,045

VJE T (m/s) 107 VcovLow (m/s) 3.5

Re JET 23,600 RecovLow 18,600

djg T (mm) 4.57 DcovLow (mm) 210

+ 0.25

XH2 0.2537 Xo2 0.1474

XN2 0.7427 XH2 O 0.0989

XN2 0.7534

Q: volumetric flow rate; x: mole fraction; Re: Reynolds number; d and D: diameter.

d?:equivalence ratio.

NASA/CR--2002- 212082 12
N2/air H2/air

coflow H2/N2 coflow

jet

Figure 1.--Vitiated cofiow burner: a lifted H2/N 2 jet flame in a coflow

of hot combustion products from a lean premixed H2/air flame.

1400,,.°°%

4OO

0 2 4 6 8 10

r/d

Figure 2.--RadiN profile of Favre averages for temperature at

axial locations z/d = 1, 14, 26. Well-defined boundary conditions

are shown by the z/d = 1 profile and the matching far-field

measurements.

NASA/CR--2002-212082 13
20

15

"%3

1,4

lO

o 1 2 3 1 2 3 1 2 3 1 2 3

r/d

Figure 3.--The flame structure is represented by the averaged

OH mass fraction fields. Presented are experimental results

(a), PDF combustion with standard k-s turbulence model

(b), EDC combustion with k-c turbulence model (c) and

EDC combustion with LRR Reynolds stress model (d). The

white dots in plot (a) denote the locations of the laser

based multiscalar measurements.

NASA/CR--2002-212082 14
1.0

0.8

0.6

0.4

0.2

(b)

O

0 10 20 30

z/d

Figure 4.--Axial profiles of the mixture fraction (a) and oxygen

mass fraction (b). Presented are experimental results (solid

40

circles), PDF combustion with k-_ turbulence model (dotted line),

EDC combustion with k-s turbulence model (solid line), and EDC

combustion with LRR Reynolds stress model (dashed line).

NASA/CR--2002-212082 15
15O0

1200

900

6OO

3OO

1500

1200

9oo

600

3OO

1500

1200

v_ 900

F60O

z/d = 11

(b)

z/d = 8

(c)

3OO

1500

1200

900

6OO

3OO

1500

\

I

(d)

12OO

v_ 900

6OO

300

0 0,2 0.4 0.6 0,8 1,0

f

Figure &--Scatter plots of temperature vs. mixture fraction from three

axial positions (z/d = 14, 11,8) showing the progress from mixing

(c) to fully burning (a}, Data from complete radial profiles were

grouped together to form each plot, Lines in (a) show the results of

steady strained opposed-flow nonpremixed laminar flame

calculations with equal species and thermal diffusivities, Schematic

of the fastest possible auto-ignRion (f = 0.4) and flame-holding (d).

Schematic of the classic ignition scenario [6, p. 219], starting at

f = 0.4 and propagating in time and space (e),

NASA/CR--2002-212082 16
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December 2002 Final Contractor Report

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Simultaneous Laser Raman-Rayleigh-LIF Measurements and Numerical

Modeling Results of a Lifted Turbulent H2/N 2 Jet Flame in a Vitiated Coflow

6. AUTHOR(S)

R. Cabra, J.Y. Chen, R.W. Dibble, T. Myhrvold, A.N. Karpetis, and R.S. Barlow

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

University of California, Berkeley

Berkeley, California 94720

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WB S-22-708-90-01

NAG3-2103

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-13735

10. SPONSORIN G/MONITORING

AGENCY REPORT NUMBER

NASA CR--2002-212082

11. SUPPLEMENTARY NOTES

Prepared for the 29th International Symposium on Combustion sponsored by the Combustion Institute, Sapporo, Japan,

July 21-26, 2002. R. Cabra, J.Y. Chen, and R.W. Dibble, University of California Berkeley, Berkeley, California 94720;

T. Myhrvold, Norwegian University of Science and Technology, Trondheim, Norway; and A.N. Karpetis and R.S. Barlow,

Sandia National Laboratories, Livermore, California 94550. Project Manager, James D. Holdeman, Turbomachinery and

Propulsion Systems Division, NASA Glenn Research Center, organization code 5830, 216-433-5846.

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Available electronically at http://gltrs.grc.nasa.gov

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

An experiment and numerical investigation is presented of a lifted turbulent H2/N2jet flame in a coflow of hot, vitiated gases. The

vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the

recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the

complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N 2 (Re = 23,600, Hid = 10)

in a coflow of hot combustion products from a lean H2/Air flame (_ = 0.25, T = 1,045 K). The combination of Rayleigh scattering,

Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of

the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow

throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off height (HpDFld = 7, HEDcId = 8.5).

Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction

(HK/d = 11.4) of the lift-off height for the present flame. The vitiated coflow admits the possibility of auto-ignition of mixed find,

and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such

ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical

investigations support the plausibility of turbulent premixed flame propagation by small scale (on the order of the flame thickness)

recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.

14. SUBJECT TERMS

Combustion chamber; Gas turbines; Flames; Burners; Raman spectroscopy; Laser induced

fluorescence; Numerical analysis; Hydrogen; Nitrogen; Turbulent; Non-premixed;

Partially premixed

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