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Developing a Benchmark for NaturalGas Combustion Models

RDTPA 96-31

K.C. Kaufman and W.A. Fiveland Peter M. WalshBabcock & Wilcox Energy and Environmental Research Corp.Alliance, Ohio, U.S.A. 44601 c/o Sandia National Laboratories

Livermore, California, U.S.A. 94551Neal Fornaciari and Philippe GoixCombustion Research FacilitySandia National LaboratoriesLivermore, California, U.S.A. 94551

Presented at the Fourth International Conference onTechnologies and Combustion for a Clean Environment

July 7-10, 1997Lisbon, Portugal

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AbstractThe natural gas industry is currently involved in the development of new burner tech-nology to produce efficient, very low-emission burners for industrial and utility applica-tions. Evaluation of new designs becomes increasingly cost intensive as new burnerdesigns extend outside existing operating envelopes. Numerical combustion model-ing is evolving as a standard tool to reduce design cycle time and focus test pro-grams. Models currently in use are based largely on the fundamental description ofthe complex interacting processes that occur during fossil fuel combustion: turbulentflow, gas phase chemical reaction and heat transfer. However, representation ofsome of these phenomena still requires the use of empirical models to close physi-cally-based equations or to permit predictions to be completed in practical timeframes. Because of these limitations, these models must be thoroughly validated andtheir range of application understood before they can be routinely applied.

The validation process requires access to high-quality experimental data in key areasof the flame, with complete information on geometry, operating conditions, and bound-ary conditions which play a strong role in modeling predictions. To construct a com-prehensive validation document for use by the natural gas industry, Gas ResearchInstitute has funded a joint effort between Babcock and Wilcox and the Burner Engi-neering Research Laboratory (BERL) at Sandia National Laboratories. Key projectgoals are the collection of a definitive set of measurements for an industrial naturalgas flame, and the documentation of the test cases for model validation. To ensurethe viability of this data for model validation, combustion models have been applied

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throughout the program. In particular, modeling was used to guide the test plan, focusmeasurement requirements, and evaluate the data for validation use. The tests andthe use of models to drive the test campaign are discussed. Predictions are com-pared with the collected data. Modeling strengths and remaining challenges are dis-cussed in terms of design application to industrial burners. The need for additionalvalidation efforts is addressed.

Key Words: burner, natural gas, combustion, numerical models,validation, flame

IntroductionThe use of natural gas continues to grow as a key fuel for utility, industrial, and resi-dential services. This increased use is expected to continue in the United Statesbecause of availability and price. In addition, market demands and both federal andlocal regulations are placing more stringent requirements on efficiency and emissions.These motivations are driving the natural gas industry to design new burners whichwill operate beyond of existing design envelopes.

Previous design strategies have relied heavily on empirical data, but this approachbecomes risky for advanced design concepts. Methods are needed to scale burnerdesigns and to effectively study a variety of parameters such as burner configuration,burner location and orientation, fuel composition and multiple fuel requirements, fueland air staging, operational upsets, and specific applications such as flame shaping.While pilot-scale tests will continue to be used to design and test burner concepts, the

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cost of these tests is significant. Additional tools are needed to optimize and focusthe test matrix, and to aid in scale-up of the pilot-scale designs, thus reducing devel-opmental costs and risk of nonperformance.

Numerical modeling is a relatively new engineering tool that is ideal for evaluatingcombustion systems. The evolution of these methods since the mid-1970s has pro-duced computational tools that are now applicable to the prediction of the complexphysical processes involved in fossil fuel combustion: turbulent fluid dynamics, gas-phase species transport, chemistry, and heat transfer (Fiveland, et al., 1984; Fivelandand Wessel, 1991; Fiveland and Jessee, 1994). These models are now being usedmore frequently in both the utility and industrial burner markets to provide additionalinsight and guidance to the design and application of efficient, low emission naturalgas burners (Peters and Weber, 1994).

The use of these models introduces several new issues, since they are neither fool-proof, nor simple to use. These issues include suitability of the modeled physics tothe design, validation of the software, correct use, and interpretation of the results.Most issues rely on an experienced engineer to determine the suitability of the model,and to correctly apply it and interpret the predictions. The coupling of the complex,non-linear mechanisms simulated by these models, however, requires that the modelsmust be thoroughly validated before they can be routinely applied to design problems.This validation process must also be repeated as the models are refined and ex-tended.

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Validation of the software, however, requires a set of comparison data for a represen-tative condition with characteristics similar to the case of interest. For satisfactoryvalidation of the numerical model, this data must meet several criteria. The data mustbe of good quality: repeatable, measured with appropriate instrumentation to sufficientresolution, and with error bounds that are understood. Just as important as the qualityof the measurements is the need to provide information on all aspects of the physicsconcerned, and the need to characterize critical spatial areas where the physical pro-cesses are active. For an industrial natural gas flame, measurements are needed forvelocities, temperature, and species both within the flame and external to the flame,as well as characterization of the heat transfer around the flame.

In addition to the measurement data themselves, sufficient information must also beprovided on the geometry, operating conditions, and surrounding environment tomake the validation meaningful. The application of reasonable boundary conditions isessential to the validation process, since unrealistic or uncharacteristic boundaryinformation can result in misleading predictions. For a natural gas burner, informationmust be obtained on the burner and furnace geometry, oxidant and natural gas flowrates, velocities and turbulence levels, and heat transfer and material information onthe boundary. For a validation benchmark that is to be used consistently at differenttimes and for different analysis software, this information must be documented clearlyso that compatible predictions can be obtained and compared.

Although a significant amount of experimental data is available for natural gas firedburners, the data is typically collected as part of specific burner test programs, anddoes not provide the detailed in-flame data and boundary condition information re-

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quired to verify combustion models. As a result, no comprehensive data sets areavailable to validate combustion models. To create a validation document for indus-trial natural gas burners, Gas Research Institute (GRI) has funded a joint effort be-tween Babcock and Wilcox and the Burner Engineering Research Laboratory (BERL)at Sandia National Laboratories in Livermore, California.

The goal of the program is to provide the measurements and documentation requiredto produce a complete validation document for use in the natural gas industry. Thedocument will be constructed to provide a consistent set of operational and boundarycondition information. This will allow predictions to be compared from various sourceswithout contradictions or confusion resulting from incompatible assumptions forboundary or initial conditions. This paper describes the background for the selectedtest case, the measurements collected at the BERL facility, and the use of numericalmodeling to guide and evaluate the validation data.

The BERL MeasurementsBackground and Test FacilityAt the start of the program, a review of measurements for industrial natural gas burnerflames was conducted to identify existing data that could be used. Little data wasfound in the available literature that included flow and combustion measurements,although the work completed under the GRI funded SCALING 400 program (Weber,et al, 1993) provided the best set of measurements for industrial scale gas burners

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under controlled conditions. In particular, the 300kW measurements completed in theBurner Engineering Research Laboratory (BERL) at Sandia National Laboratories(Sayre, et al, 1994), provided in-flame data in a well-instrumented experimental fur-nace. This set of measurements were selected as the baseline set.

The initial burner tests, completed by the IFRF/Sandia/EER team, collected in-flamedata for both a hot-wall and cold-wall furnace configuration. Measurements werecollected at six radial traverses - 27, 109, 177, 191, 343, and 432mm downstream ofthe burner quarl exit. Axial and tangential velocity components, gas temperature andmajor species were measured at five traverses each. The data collected by Sayre etal (1994) were of excellent quality and included very good radial resolution across theflame sheet at each traverse. However, since only five traverses were completed forall of the variables, additional measurements were planned to expand the data set,resolving the flame internal and external recirculation zones and the post-flame re-gion. Based on the existing data, the cold-wall furnace case was selected for furthermeasurements, since this configuration offered some advantages for flame visualiza-tion, and also provided a well-defined furnace wall condition. Data from both sets ofmeasurements will be included in the final validation document. As with the originalmeasurements, the follow-on measurements were collected in the BERL facility.

The Sandia/BERL furnace (Forniciari et al, 1994) and associated equipment weredesigned by the Energy and Environmental Research Corp. and the University ofCalifornia at Irvine Combustion Laboratory for the Gas Research Institute and the U.S.

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Department of Energy. The furnace is vertical, with an octagonal cross section, 1.08m across and 2.00 m tall and is assembled from five or six identical 333 mm highspool segments. A cross-sectional schematic of the furnace is shown in Figure 1.The burner is mounted in the floor and the area surrounding it is insulated with refrac-tory board. The furnace is topped by a 390 mm tall, refractory-lined 45o cone connect-ing the octagonal furnace to a 305 mm I.D. exhaust duct. The first 690 mm of theexhaust duct are refractory lined. A water-cooled sampling probe used to monitor thecomposition of furnace exit gas is located 340 mm from the exit of the cone. Thepressure in the furnace was adjusted to 12.5 Pa above ambient using a damper in theexhaust, upstream from the induced draft fan.

The 300 kW swirl-stabilized natural gas burner, shown schematically in Figure 2, wasbuilt by the International Flame Research Foundation (IFRF) for the Gas ResearchInstitute’s SCALING 400 research program. The burner is circumferentially symmet-ric with a bluff center body containing 24 radial natural gas injection holes. Combus-tion air is supplied by a blower and introduced through the annular air zone andswirled using IFRF swirl blocks. The burner has the capability for flue gas recirculation(FGR) and fuel staging, but was operated only in the single-stage mode without FGRwhile making the measurements presented here. The fuel and air flows were mea-sured using laminar flow elements.

Each of the eight sides of each furnace spool has an opening (406 mm wide x 267mm high) for a refractory-lined or bare stainless steel panel or a frame holding a fusedsilica window. For the cold-wall measurements, all of these openings contained

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water-cooled bare metal panels, as shown in Figure 1, except when optical accesswas required for laser Doppler velocimetry (LDV). For the original hot-wall measure-ments, the panels were refractory lined. Where probes are inserted, special panelsare installed that have 51 or 127 mm wide vertical slots fitted with covers having 32mm wide openings for the probes. These openings are filled with refractory blanketto minimize leakage of gas into or out from the furnace. The heat extracted throughthe furnace wall is determined from cooling water flowrates and temperaturechanges.

In-flame MeasurementsThe follow-on measurements duplicated the 15% excess air (3.0% exit O2, dry) cold-wall baseline flame as measured by Sayre et al (1994). A summary of typical operat-ing conditions is given in Table I. Table II contains a typical fuel analysis obtainedduring the tests. The data was collected using the six-spool BERL configuration in-stead of the five-spool configuration to limit facility modifications. The change inBERL configuration and the location of the original and follow-on measurementtraverses are shown in Figure 3. Combustion modeling cases completed prior to thestart of the tests indicated that this change had a negligible effect on the data.

The measurements reported here were made using a suction pyrometer with type Sthermocouple, an extractive gas sampling probe, and an ellipsoidal radiometer, allfrom the International Flame Research Foundation. Gas velocities, both mean andRMS fluctuating components, were measured by LDV using an argon ion laser andAerometrics Doppler Signal Analyzer, with the combustion air seeded using 0.05

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micrometer alumina particles. The probes and LDV system were mounted on amotor-driven optical table to traverse the flow. The entire furnace is moved up anddown in relation to the table in order to probe the flame gases at different heights.Gas composition was determined using nondispersive infrared (CO and CO2), flameionization (total hydrocarbons), paramagnetic (O2), and chemiluminescence (NO andNOX) detectors and a micro-gas chromatograph (H2, O2, CO, CO2, CH4, and N2).When making the temperature and gas composition measurements using the con-tinuous monitors, the output from each instrument was observed for approximatelyone minute and the highest, lowest, and typical readings were all recorded. Table IIIprovides a summary of all measured data for the three test conditions discussed be-low.

The tests began with a repeat of two of the original traverses at 27mm and 343mm toensure replication of the baseline flame characteristics. These two traverses dis-played excellent repeatability with the original data set for all measured quantities. Acomparison of several main variables from both the original and follow-on measure-ments is shown in Figure 4. This figure shows that only quantities such as carbonmonoxide and NOx, which are sensitive to small changes in fuel, show any significantvariations.

With successful repetition of the original baseline flame, data was then taken ateleven new traverse locations as shown in Figure 3: 0+, 16, 65, 131, 150, 208, 390,510, 695, 815, and 1156mm downstream of the quarl exit. Data collected at thesetraverses included axial and tangential velocity components, gas temperature, and

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major species as described above. The exceptions are the quarl exit traverse (x=0+),where axial, tangential, and radial velocity components were measured, the 16mmtraverse, where only gas temperature and species measurements were made, andthe 815mm and 1156mm traverse, where only velocity and gas temperature weremeasured. For a majority of the intrusive measurements, both minimum and maxi-mum values were recorded once the probe reached steady conditions. An example ofthis is shown in Figure 5 for gas temperature. This min-max range for key variablesprovides a valuable piece of information for model evaluation, since it provides anenvelope of possible values. As can be seen from Figure 5, this envelope can besignificant in some areas were turbulent fluctuations are large.

Limited data were also taken at seven traverses for a low excess air condition (2.5%excess air, 0.6% exit O2, dry). This case was selected to assess its usefulness toevaluate model sensitivity. The conditions reduce combustion air flow by ten percent,and result in a roughly 22% increase in exit NOx. However, although this conditiondoes show sensitivity in NOx levels, most other combustion quantities are largely unaf-fected, making the case useful primarily for evaluation of detailed nitrogen kineticsmechanisms. Traverses at this condition were made at the 0+, 27, 65, 131, 150, 343,and 510mm locations. Similar data as collected for the baseline case - velocity com-ponents, gas temperature, and major species - were collected for this condition.

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Cold Flow MeasurementsIn addition to the baseline flame measurements, data was also collected for non-reacting flow conditions. Seven traverses were completed for the non-reacting case(0+, 27, 65, 131, 208, 343, and 510mm) to provide turbulent flow validation data for arepresentative swirl number. This data was of considerable interest, since turbulenceplays a critical role in diffusion flame combustion, and these measurements provideddetailed flow validation data for the same burner configuration without the added diffi-culty of combustion chemistry and heat transfer. Burner air flows identical to thoseused for the baseline combustion case were used, here, with no change to the burnerswirl setting. For these conditions, the internal recirculation zone (IRZ) observed forthe baseline flame does not close, producing a large secondary recirculation zonedownstream along the burner centerline. Although these conditions produce a some-what different flow pattern than that observed with combustion, the result is a conser-vative test case for evaluation of turbulence models and numerically predicted swirlingflow.

Auxiliary MeasurementsIn addition to the described in-flame measurements for the three cases, a significantamount of auxiliary data was also collected to provide a thorough set of boundarycondition information and to provide qualitative visualization of the flame character.Burner operating conditions such as air and fuel flow and temperatures, as well asfuel composition, were monitored and documented throughout the campaign, asshown in Tables I and II. Furnace exit conditions were recorded and furnace

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massflow balance was monitored with pressure drop measured across an orificeinstalled in the exit duct. This orifice was duplicated in some modeling to successfullyconfirm minimal furnace leakage through pressure drop comparisons. A summary offurnace exit conditions over several test days is provided in Table IV. Temperaturesin the furnace refractory floor and on the outside of the furnace hood were recordedusing thermocouples. Radiative heat flux at the furnace wall was measured with anellipsoidal radiometer at 12 elevations.

Images of burner flow were obtained using Mie scattering from fine particles of tita-nium dioxide formed by reaction of water vapor with titanium tetrachloride vaporadded to the combustion air. The 532 nm beam from a frequency-doubled Nd:YAGlaser was formed into a sheet approximately 1mm thick using cylindrical lenses toexpand the beam in one dimension. Introduction of titanium tetrachloride was startedimpulsively to seed the combustion air. An image was then obtained of the seededflow entering and mixing with the unseeded furnace flue gas by proper timing of thecamera shutter with respect to the impulse. A pulse of 6ns duration from the lasereffectively freezes the flow. A narrow bandpass filter centered at 532nm was used tominimize flame and background radiation. The illuminated sheet of particles wasfocused using a 105mm, f/4.5 UV lens. An electronic shutter with a 1 microsecondgate time was used with a high resolution Photometrics 14-bit AT200, 1024x1024pixel CCD detector coupled with an electronically gated image intensifier. The digi-tized signal was process using PMIS software for Microsoft Windows.

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Mie scattering images were collected for the non-reacting measurements as well asboth the baseline and low excess air flames. The relative time lapse between intro-duction of the titanium tetrachloride and the camera shutter allows the mixing to bevisualized at several stages. Although these images represent nearly instantaneoussnapshots of the flame structure, they are very useful in defining overall flame charac-ter and shape; for example the closed internal recirculation zone of the baselineflame is readily visible in Figure 6, compared with the open central recirculation zonesformed by the non-reacting flow. The images are also useful in defining key mixinglayers and characterizing larger-scale turbulent structures.

The blue chemiluminescence from excited CH radicals in the flame zone provides amethod for obtaining images of flames in furnaces where background radiation ren-ders the flame practically invisible to the naked eye. In this technique, the intensifiedCCD camera was used with a filter centered at 431nm and having a bandwidth of10nm (full width at half maximum). The resulting image provided a time-averagedflame envelope that may be used to qualitatively define flame length and regions ofhigh reaction rate, e.g. the flame front.

Design of the Validation TestsPrior to proceeding with the follow-on measurements, a number academic and com-mercial modeling groups were surveyed to provide feedback on the kind of measure-ments and information needed to aid in validation of multi-dimensional combustionmodels. Information obtained in this survey was then incorporated into the construc-tion of the second set of measurements and the planned validation document. In

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addition, feedback was solicited from the same groups on an initial release of thevalidation document based on the original measurements. This feedback resulted inadditional improvements in the overall document and test plan.

Predictions were completed with Babcock & Wilcox’s combustion model, COMO(Fiveland and Jessee, 1994, and Jessee and Fiveland, 1995a) prior to testing andduring the follow-on measurements to provide input to the test plan (Kaufman andFiveland, 1995). Two-dimensional (2-D) axisymmetric and three-dimensional (3-D)predictions were compared with the original data of Sayre et al (1994) to assess theuse of the hot- and cold-wall cases for validation before beginning the additional mea-surements. A burner and furnace geometry that could be used to evaluate bothaxisymmetric and full 3-D predictions was desired, and the initial modeling indicatedthat the IFRF burner and BERL geometry worked well for both types of applications.

Discrepancies between the 2-D predictions, 3-D predictions, and the data were thenused to determine where additional measurements were required. These assess-ments lead to the additional traverses near the quarl and through the internal recircu-lation zone. Based on observed differences in predicted downstream mixing betweenthe 2-D and 3-D models, as shown by the predicted oxygen contours in Figure 7,several profiles were also added just past the main combustion zone, with additionalprofiles in the post-flame region to evaluate downstream mixing and further establishthe external recirculation zones. The LDV traverses at the quarl exit were added toprovide additional definition of the velocity field as it left the burner. The modeling

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results, together with the existing data, were used to define the desired level of mea-surement resolution across the flame at each traverse. A preliminary assessment ofthe low excess air condition was also completed using axisymmetric models prior totesting.

The models were also used to evaluate the change from a 5-spool BERL configura-tion to a 6-spool configuration. The 5-spool furnace configuration had been used forthe original measurements. However, the use of the existing 6-spool BERL configura-tion at the time of the additional measurements would save considerable effort andexpense, while providing more time for collecting data. Comparisons between 5-spool and 6-spool cold-wall predictions showed only minor changes in the down-stream flow patterns, with no significant effects on the near-flame region. This evalu-ation was borne out by the subsequent measurements.

Application to Model ValidationModel DescriptionThe COMO model was used throughout this effort to predict the natural gas flames.COMO is a modular, fundamentally based multi-dimensional flow and combustionmodel. Both structured, orthogonal and unstructured body-fitted versions are currentlyavailable. Gas-phase equations are solved using a collocated, finite volume formula-tion for the Eulerian equations. The flow algorithm is built around a cell-vertex finitevolume formulation of the steady, incompressible Navier-Stokes equations (Jesseeand Fiveland, 1996). Turbulent flow is predicted using a standard two-equation k-ε

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turbulence model (Launder and Spalding, 1974). A solution the radiation heat transferin the system is found using the discrete ordinates method of Fiveland and Jessee,1995, while the combustion model is similar to that described in Fiveland and Jessee(1994). Solutions are obtained using algebraic multigrid techniques to enhance con-vergence. The model has been validated in a step-wise manner as described inFiveland, et al, 1996.

For the results reported here, flow and combustion models were completed using both2-D axisymmetric and 3-D cylindrical structured orthogonal models and 2-Daxisymmetric and 3-D unstructured models. Grid sensitivity studies were completedover a range of grid sizes from 7130 nodes to 47,600 nodes for 2-D models and65,500 nodes to 153,500 nodes for 3-D models. Further discussion of grid sensitivityis reported in Kaufman and Fiveland (1995).

Non-reacting FlowPredictions for the non-reacting flow case were completed using 27,000 nodes onboth structured and unstructured 2-D axisymmetric meshes. As noted before, thisnon-reacting flow produces the primary central recirculation zone immediately behindthe burner center body, but also creates a large secondary recirculation zone alongthe burner centerline. Figure 8 shows the comparison between measurements andpredictions of axial and tangential velocity at five traverse locations. Both predictionsshow very good agreement with the data with several exceptions. At the 27mm loca-tion, all features of the flow are captured by the model except for the magnitude of the

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recirculation zone, which is under predicted, and the pronounced double peak on thetangential velocity component. These are both known artifacts of the k-ε turbulencemodel, which under predicts the strength of the recirculation zone.

In Figure 9, predicted axial, radial, and tangential velocity components are plotted atthe quarl exit and compared with measured LDV profiles for the mean velocity compo-nents. In these figures, the profiles are bounded by the envelope of measured RMSfluctuations. The predictions show good agreement with the measurements, and fallwell within the RMS envelope, again with the exception of the IRZ strength.

Simple Chemistry ModelsFor the bulk of industrial and utility combustion applications, gas phase chemistry ismodeled in two steps: (1) oxidation of fuel producing a pool of carbon monoxide andother products; and (2) oxidation of carbon monoxide to carbon dioxide. In COMO,this step is accomplished using a rate limiting step between the turbulent mixing(Magnussen and Hjertager, 1976) and the kinetics. This eddy dissipation model(EDM) provides reasonable results for many large applications and is time efficientwhen large models are required to resolve the geometry and flow, temperature, andspecies gradients.

Results from combustion predictions of the cold-wall case with the 2-step mechanismare shown in Figures 10-12. Figures 10 and 11 show 2-D predictions of axial andtangential velocity, temperature, O2 and CO at the 27mm and 208mm traverses com-

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pared with the measurements. These profiles are 0.3 and 2.4 burner diameters (Do)downstream of the quarl exit. These figures indicate that the 2-D model does well incapturing the trends near the burner. Downstream, however, the 2-D model does notcompare as well, indicating a longer flame, with slower mixing near the burnercenterline. This is the result of the axisymmetric assumption requiring the use anannular natural gas inlet stream, as opposed to discrete gas jets that can be correctlymodeled in 3-D. This idealization for 2-D models has a significant impact on mixingpatterns near the burner, which in turn affect downstream predictions. Understandingthese impacts is important, since simplified 2-D models are often used since theyprovide faster turn-around time for parametric studies. Even with the noted limitation,however, the 2-D model still provides a reasonable representation of the overall flamefeatures and trends.

In Figure 12, a Mie scattering image obtained for the baseline flame is compared withthe velocity field, gas temperature, and local stoichiometric ratio contours from anaxisymmetric prediction. Although the Mie scattering image portrays an instanta-neous image of the flame, while the predictions are steady, time-averaged representa-tions of the flame, obvious similarities in shape and structure are apparent. This com-parison, though not quantitative, does provide additional verification of the model’sability to correctly simulate the characteristics of the flame.

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Advanced ModelsAlthough two-step chemistry mechanisms have been a standard approach for manycombustion modeling applications, this approach ignores the minor and radical spe-cies which are strongly linked to the generation of pollutants such as NOx. NOx pre-dictions for these types of solutions require “global” pollutant models that rely on em-pirical rates and constants that must be tuned from case to case. These global mod-els have been used successfully for predicting overall furnace emissions (Fivelandand Wessel, 1991), but are much less reliable for burner or local variations.

To correctly predict in-flame NOx, a kinetics mechanism with sufficient detail to capturethe key NOx reactions must be used. This approach, which replaces the two-stepchemistry with an increased number of detailed, elementary kinetics mechanisms, canprovide information on minor and radical species included in the mechanism. How-ever, to model hydrocarbon chemistry and nitrogen chemistry to correctly predict nitro-gen oxides, a large number of reactions (over 200) must be included. The addition ofthis large number of stiff equations to the solution significantly increases the computa-tional time required for even a modest two-dimensional prediction. To use these mod-els effectively, a reduced mechanism set must be sought that provides sufficient detailfor combustion and pollutant formation, while minimizing computer time. This is bestaccomplished by comparison of reduced mechanisms with complete mechanismssuch as GRI-Mech (Frenklach, et al, 1995). Practical mechanisms may be derivedthat will consist of between twenty and seventy reactions.

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As part of an on-going internal program at Babcock & Wilcox, several reduced kinet-ics mechanisms are being evaluated against combustion solutions completed with thefull 279 reactions of GRI-Mech 2.11. Predictions with these mechanisms have beencompared with the 2-step EDM predictions and the BERL data. An example of such acomparison is shown in Figure 13, which shows the standard 2-step mechanism(EDM), as well as a revised 2 reaction mechanism (EDC 2-Rx) and a 20 reactioncombustion mechanism (EDC 20-Rx) compared with the full 279 reaction mechanismof GRI-Mech (EDC 279-Rx) and the measured data at two traverse locations. Thiscomparison demonstrates significant improvement in the combustion prediction usingthe 20 reaction mechanism; however, this mechanism is not sufficiently detailed topermit prediction of in-flame NOx as shown for the full GRI-Mech prediction. The useof the BERL data set in this way provides a strong argument for the need for suchdata when evaluating advanced combustion chemistry models.

ConclusionsA set of in-flame and supporting data for an industrial natural gas flame has beensuccessfully collected at the Sandia BERL test facility. The test was designed to pro-vide a complete, well documented data set for validation of numerical combustionmodels of natural gas burners. The validation document builds on measurementsfrom the GRI sponsored SCALING 400 program, and successfully used combustionmodeling as an integral part of the program to guide and focus experimental testing.The collected data provides a baseline for evaluation of differences between idealized

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2-D axisymmetric models, typically used for parametric studies, and more detailed 3-Dmodels. The data also provides a vehicle for evaluation of advanced methods suchas the implementation of detailed kinetics mechanisms. However, to provide a com-prehensive validation data set, additional testing is still required. These measure-ments must provide quantitative information and visualization of in-flame radicals andfurther insight into importance of turbulence and improved turbulence models for com-bustion. In addition, a strong sensitivity case is required to quantify the model’s abilityto predict changes in performance with changes in operating conditions.

Comparisons to date between predictions and data indicate that existing combustionmodels are capable of replicating many of the experimental trends and predictingmain features of the natural gas flames, but that additional work must still be done toimprove model performance. Physical models of chemistry and turbulence must beimproved. This improvement remains a significant challenge. The implementation ofdetailed kinetics mechanisms has already shown great promise using the describeddata set; however, significant work remains to reduce the increased computationaltimes required by these models. This effort must combine identification of reducedreaction chemistry that will provide pollutant predictions, and implementation of thealgorithms on parallel or distributed computing systems.

AcknowledgmentsThe authors would like to thank Gas Research Institute and Dr. R.V. Serauskas forsupport of this work through GRI contract 5093-260-2729. The experiments wereperformed at the Burner Engineering Research Laboratory (BERL) located in the

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Combustion Research Facility at Sandia National Laboratories in Livermore, CA. TheBERL is funded by the Gas Research Institute (GRI) and the U.S. Department ofEnergy, Office of Industrial Technologies, and the Office of Basic Energy Sciences,Division of Chemical Sciences. The BERL is operated by Sandia, The Energy andEnvironmental Research Corporation, and the University of California, Irvine, Com-bustion Laboratory under contract to GRI. The authors would also like to thank Dr.Roman Weber and his colleagues at the IFRF for their work on the SCALING 400program which led to the original data set, and Mr. Robert Gansman, Babcock andWilcox ARC for his work on the detailed kinetics predictions for the BERL flames.

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Magnussen, B.F., and Hjertager, B.H., 1976, “On Mathematical Modeling of Turbu-lent Combustion with Emphasis on Soot Formation and Combustion,” 16th Sympo-sium (International) on Combustion, The Combustion Institute, 719-729.

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Peters, A.A.F., and Weber, R., 1994, “Mathematical Modeling and Scaling of FluidDynamics and NOx Characteristics of Natural Gas Burners,” Proceedings of theASME/EPRI Joint Power Generation Conference, Phoenix, Arizona.

Sayre, A., Lallemant, N., Dugue, J., and Weber, R., 1994, “Scaling Characteristicsof Aerodynamics and Low-NOx Properties of Industrial Natural Gas Burners: TheSCALING 400 Study. Part IV: The 300 kW BERL Test Results,” The Gas ResearchInstitute, GRI Contract No. 5090-298-1977.

Weber, R., Driscoll, J.F., Dahm, W.J.A., and Waibel, R.T., “Scaling Characteristicsof Aerodynamics and Low-NOx Properties of Industrial Gas Burners. The SCALING400 Study. Part I: Test Plan,” IFRF Doc No. F 40/y/, September, 1993.

u

5 33.4 mm

45o

16 51.0 mm

Wat er- cooled Pa nel

2 19.0 mm

45o

Wate r Colu mns

Wind ow Pa nel

Burne r

Spool Cross-section A-A

Side View

Furna ce Exit Pr obe Loca tion

A A

30 0.0 mm ID

3 81.0 mmCo nical

Fur nace Ho od

Spo ol Segme nt1 o f 5

Octago nal

Exhau st Duct

Figure Not To Scale7 62.0 mm

Figure 1 Schematic of BERL 5-spool configuration

u

2.00 Ro

1.21 Ro

1.15 Ro

6 .0 mm X

R

24 Gas Injection HolesEach @ 1.8 mm Di a.

Burner Quarl

Duct Insert

1.33 Ro

1.66 R

Axial Locat ion ofSwirl block Calibrat ion Port

195 mmBurner Axis

R

M easurement Traverse Stat ions

Located f rom the Quarl E xit

NaturalGas

Flue Gas

CombustionAir

Swirling

StagingGas

Burner Radius = 43.5 mmRo

20o

20o

o

o

0.66 Ro

Figure 2 IFRF 300 kw swirl-stabilized natural gas burner

uFigure 3 BERL traverse locations for the 300 kW flame measurements

432

IFRF Burner

FurnaceSpool

NewTraverseLocations

(mm)

OriginalTraverseLocations

(mm)

1

2

3

4

5

6

65131

150

208

16

390

0+

1156

510

695

815

27

109177

191

343

SpoolSegments

1993 BERL

Configuration

u

0

10

20

30

-10

-5

0

5

10

15

-0.1 0.0 0.1 0.2 0.3 0.4 0.50

20

40

60

80

2

4

6

8

10

12

2.00E4

4.00E4

6.00E4

8.00E4

500

1000

1500

2000

-0.1 0.0 0.1 0.2 0.3 0.4 0.50

5

10

15

20

1.00E4

2.00E4

3.00E4

4.00E4

Original

New

New - MinimumNew - Maximum New - Maximum

OriginalOriginal

Axial (m/s)

Tangential (m/s)

T (K)

O2 (%, dry)

CO2 (%, dry)

NOx (ppm, dry)

Radial Distance (m) Radial Distance (m)

New - Minimum

UHC (ppm, wet)

CO (ppm, dry)

Figure 4 Comparison of original and follow-on measurementprofiles at the 27mm traverse

u

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5500

1000

1500

2000

Radius (m)

Traverse Location

208mm (2.4Do)

510mm (5.9Do)

Gas

Tem

pera

ture

(K

)

T-min

T-max

Tavg

T-min

T-max

Tavg

Figure 5 Temperature variations at two traverses from the baseline flame

u

Figure 6 Mie scattering images for non-reacting and baseline reacting conditions

Non-reacting flow Baseline flame

u

12

34

5

6

5

43

2

1

3-D Cylindrical Model

2-D Axisymmetric ModelO2 Contours (%vol)

Figure 7 Comparison of 2-D and 3-D predictions for oxygen concentration

u

x=27mm (0.3Do)

x=131mm (1.5Do)

x=208mm (2.4Do)

x=343mm (3.9Do)

x=510mm (5.9Do)

MeasuredStructuredUnstructured

Axi

al V

eloc

ity (

m/s

)

radius (m)

-10

0

10

20

-10

0

10

20

-10

0

10

20

-10

0

10

20

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5

-10

0

10

20

-10

-5

0

5

10

-10

-5

0

5

10

-10

-5

0

5

10

-10

-5

0

5

10

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5

-10

-5

0

5

10x=27mm (0.3Do)

x=131mm (1.5Do)

x=208mm (2.4Do)

x=343mm (3.9Do)

x=510mm (5.9Do)

MeasuredStructuredUnstructured

Tan

gent

ial V

eloc

ity (

m/s

)

radius (m)

Figure 8 Predicted and measured velocity profiles for non-reacting flow

u

-10

0

10

20

30

-10

0

10

-0.05 0.00 0.05

-20

-10

0

10

20

RMS EnvelopePrediction

Measured

Axi

al (

m/s

)R

adia

l (m

/s)

Tan

gen

tial (

m/s

)

radius (m)

Figure 9 Predicted and measured profiles at the quarl exit

u

Radius (m)

Axial (m/s)

Tangential (m/s)

T (K)

O2 (%dry)

Predicted

Max Measured

Min-Measured

CO (ppm dry)

-10

0

10

20

30

40

-10

0

10

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.50.00E0

2.00E4

4.00E4

6.00E4

0

5

10

15

500

1000

1500

2000

Figure 10 Measured and predicted baseline profiles at the 27mm traverse

u

Radius (m)

Axial (m/s)

Tangential (m/s)

T (K)

O2 (%dry)

Predicted

Max Measured

Min-Measured

CO (ppm dry)

500

1000

1500

2000

0

5

10

15

-10

0

10

20

30

40

-20

-10

0

10

20

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.50.00E0

2.00E4

4.00E4

Figure 11 Measured and predicted baseline profiles at the 208mm traverse

u

Instantaneous Mie Scattering Image

Predicted Time-averaged Stoichiometric RatioPredicted Time-averaged Gas Temperatures

Predicted Time-averaged Velocity Field

Figure 12 Predicted baseline velocity, temperature, and stoichiometriccontours compared with mie scattering visualization

u

0.00 0.10 0.20 0.30 0.40 0.500

25

50

75

100

125

Data

EDC 279-Rx

0

25

50

75

100

1250

25

50

75

100

1250

25

50

75

100

1250

25

50

75

100

125

Radial Position (m)

300 kW Flame

NO

X M

ole

Fra

ctio

n (%

dry

)

x = 432 mm (5.0Do)

x = 343 mm (3.9Do)

x = 191 mm (2.2Do)

x = 109 mm (1.3Do)

x = 27 mm (0.3Do)

Radial Position (m)

O2

Mol

e F

ract

ion

(% d

ry)

300 kW Flame

x = 432 mm (5.0Do)

x = 343 mm (3.9Do)

x = 191 mm (2.2Do)

x = 109 mm (1.3Do)

x = 27 mm (0.3Do)0

5

10

15

200

5

10

15

200

5

10

15

200

5

10

15

20

0.00 0.10 0.20 0.30 0.40 0.500

5

10

15

20Data

EDM

EDC 2-Rx

EDC 20-Rx

EDC 279-Rx

0.00 0.10 0.20 0.30 0.40 0.50

500

1000

1500

2000

Data

EDM

EDC 2-Rx

EDC 20-Rx

EDC 279-Rx

500

1000

1500

2000

500

1000

1500

2000

500

1000

1500

2000

500

1000

1500

2000

Tem

pera

ture

(K

)

Radial Position (m)

300 kW Flame

x = 432 mm (5.0Do)

x = 343 mm (3.9Do)

x = 191 mm (2.2Do)

x = 109 mm (1.3Do)

x = 27 mm (0.3Do)

0.00 0.10 0.20 0.30 0.40 0.50-20

0

20

40Data

EDM

EDC 2-Rx

EDC 20-Rx

EDC 279-Rx

-20

0

20

40

-20

0

20

40

-20

0

20

40

-20

0

20

40

Radial Position (m)

300 kW Flame

Axi

al V

eloc

ity (

m/s

)

x = 432 mm (5.0Do)

x = 343 mm (3.9Do)

x = 177 mm (2.1Do)

x = 109 mm (1.3Do)

x = 27 mm (0.3Do)

Figure 13 Predictions for hot-wall flame with several chemistry mechanisms

u

Table I Summary of Typical Burner OperatingConditions for 300 kW Flame

Condition Baseline FlameLow Excess Air

Flame

FuelLHV 48.20 MJ/kg 46.97 MJ/kg

Stoichiometric Air/FuelRatio

16.75 16.27

Fuel Temperature 312.0 K 310.8 K

Gauge Pressure 11.85 kPA 11.70 kPa

Fuel Flow 0.006229 kg/s 0.006420 kg/s

Combustion AirAmbient Pressure 99.01 kPa 99.63 kPa

Mole Fraction H2O 0.0166 0.0125

Gauge Pressure 2.60 kPa 2.20 kPa

Air Temperature 311.4 K 306.4 K

Air Flow 0.1200 kg/s 0.1075 kg/s

Swirl Number 0.56 0.56

Stoichiometric Ratio 1.15 1.029

Condition Baseline FlameLow Excess Air

Flame

FuelLHV 48.20 MJ/kg 46.97 MJ/kg

Stoichiometric Air/FuelRatio

16.75 16.27

Fuel Temperature 312.0 K 310.8 K

Gauge Pressure 11.85 kPA 11.70 kPa

Fuel Flow 0.006229 kg/s 0.006420 kg/s

Combustion AirAmbient Pressure 99.01 kPa 99.63 kPa

Mole Fraction H2O 0.0166 0.0125

Gauge Pressure 2.60 kPa 2.20 kPa

Air Temperature 311.4 K 306.4 K

Air Flow 0.1200 kg/s 0.1075 kg/s

Swirl Number 0.56 0.56

Stoichiometric Ratio 1.15 1.029

u

Table II Typical Natural Gas Normalized Fuel Analysis

Species Mole Fr (%)

Methane 93.448

Ethane 4.603

Propane 0.263

i-Butane 0.038

n-Butane 0.042

i-Pentane 0.013

n-Pentane 0.008

>C6 0.026

Nitrogen 0.859

CarbonDioxide

0.700

Species Mole Fr (%)

Methane 93.448

Ethane 4.603

Propane 0.263

i-Butane 0.038

n-Butane 0.042

i-Pentane 0.013

n-Pentane 0.008

>C6 0.026

Nitrogen 0.859

CarbonDioxide

0.700

y

u

Table III Summary of Measured Data for BERL Test Conditions

TraverseStations

TraverseLocation

s(mm)

OriginalSCALING

400Measurement

s

BaselineMeasurement

s

LowExcess Air

Measurements

Non-reactingMeasurement

s

1 0 - V V V

2 16 - T,SP - -

3 27 V,T,SP V,T,SP V,T,SP V

4 65 - V,T,SP V,T V

5 109 V,T,SP - - -

6 131 - V,T,SP V,T,SP V

7 150 - V,T,SP V,T -

8 177 V - - -

9 191 T,SP - - -

10 208 - V,T,SP - V

11 343 V,T,SP V,T,SP V,SP V

12 390 - V,T,SP - -

13 432 V,T,SP - - -

14 510 - V,T,SP V V

15 695 - V,T,SP - -

16 815 - V,T - -

17 1156 - V,T - -

V=LDV velocities, T=gas temperature, SP=species

TraverseStations

TraverseLocation

s(mm)

OriginalSCALING

400Measurement

s

BaselineMeasurement

s

LowExcess Air

Measurements

Non-reactingMeasurement

s

1 0 - V V V

2 16 - T,SP - -

3 27 V,T,SP V,T,SP V,T,SP V

4 65 - V,T,SP V,T V

5 109 V,T,SP - - -

6 131 - V,T,SP V,T,SP V

7 150 - V,T,SP V,T -

8 177 V - - -

9 191 T,SP - - -

10 208 - V,T,SP - V

11 343 V,T,SP V,T,SP V,SP V

12 390 - V,T,SP - -

13 432 V,T,SP - - -

14 510 - V,T,SP V V

15 695 - V,T,SP - -

16 815 - V,T - -

17 1156 - V,T - -

V=LDV velocities, T=gas temperature, SP=species

u

Table IV Summary of Baseline Furnace Exit Conditions

Date 5/3 5/23 6/6

O2 (%, dry) 3.00 3.00 3.00

CO2 (%, dry) 10.20 10.10 10.10

CO (ppm, dry) 10.4 10.5 10.6

NO (ppm, wet) 21.50 21.00 22.50

NOx (ppm, wet) 22.00 21.50 23.50

Exit Tem perature(K)

1036 1026 1022

Orifice dP (Pa) 113.3 110.8 -