. . ; .111E .AMERICAN soeinv , OF MECHANICAL ENGIMEERS, ' 346 E 4701 Si, New York, N.Y. 10017 ' • .. • - , ., , . , • 97,-61371,' • • , . , The Society Wail not be responsible for statements or opinions advanced in papers,or discussion at meetings of the Society, or ot i tis DI;IsiOne or ‘, SectionS, or, printed In its publications. Discussion IS printed only if the paper—Is published to photabbni:..1 • Authorization' • • . . in....•an . 'ASME. Journal: material: for internal or personal' use under circumstance not falling within the fair use provisions of the Copyright Act IS granted by :ASMS to; .' ...-:. Hbrades and Other users registered with the Copyright Clearance Centei(CCC)Trallsactiohal Rationing Service provided that the base fee of 60.30 ‘.-..' per Page is paid directly to the CCC 27 Congress Street, Salem' MA 01970 Requests special permission or bulk tepaidiiiilonehoUldbe ,addtessecl , . to the ASMETrichileal RiblisNng dipartmeht 1 . r . • 1 4 • Thight 01997.by ASME . Copy " . ... • , 1. 0 5,I' • All. Rights Resolved -'... . '• . , , . •• 1 • ,•Frintecint1J.S.A ' .,' I- • APPLICATION OF CFD TO DLN COMBUSTION C. Hornsby & E. R. Norster III 1111111011,11111 11 111111 European Gas Turbines Ltd Industrial Turbine Group P.O. Box 1 Thorngate House Lincoln, LN2 5DJ United Kingdom ABSTRACT NOMENCLATURE This paper describes the methodology and application of Computational Fluid Dynamics (CFD) to Dry Low NOx (DLN) combustion systems throughout the range of small industrial gas turbines produced at European Gas Turbines (EGT) Lincoln UK. The use of CFD in the development of such systems has been encouraged not only by the availability of a variety of general purpose CFD codes, but also by the inherent difficulties associated with direct measurement in such a harsh environment. Combusting flow analyses provide detailed predictions of local temperature and velocity fields together with exhaust emissions, enabling numerous conceptual studies to be undertaken without the usual associated mechanical difficulties. In particular, the work EGT has concentrated on concerns the prediction of fuel / air mixing quality upstream of the flame front, in order to assess the effect of fuel injector design variables on NOx production. This methodology has accelerated injector development resulting in less than 10 ppmV NOx combustors. Validation of the detailed features of the flow field is currently underway, though parametric comparisons have already proved consistently accurate in displaying the trends necessary for the development of an ultra low NOx combustion system. Correlations of rig emissions data with overall predictions have shown to be in good agreement. Cl E Irrl) (-1 ) M p S T [-1 , ( kgs- ] [Pa] (-1 (K] (1) f area of cell face i mass fraction of fuel through cell face i mass flow weighted mean general function mass flow of fluid through cell face i pressure standard deviation temperature combustor primary zone stoichiometry 1. Introduction Despite the complexities of the internal flow associated with the gas turbine engine the pressing need for aerothermodynamic improvements has made the aggressive use of CFD inevitable. The specific problems associated with combustor flows differ from their external aerodynamic counterparts in three principal ways: (1) they usually involve complicated geometries and boundary condition constraints, (2) there is close coupling of flow elements involving three-dimensional unsteady secondary flows and (3) there is significant energy exchange from multi-phase chemical reactions. While this seems formidable and could cause restraint one finds quite the opposite. Since the combustor environment is difficult to produce experimentally CFD is being employed as a technology to give insight and control, and developing new solution methodologies employing economic computational techniques. The ultimate use of CFD is as a design/analysis tool which can provide substantiel Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Orlando, Florida — June 2-5, 1997 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms reductions in time and cost in the combustor development cycle. This paper describes the use of CFD in advancing the development of an injection system to optimise the low NOx performance of a generic combustor design. The combustor and injection system are analysed for fuel/air mixing, operating temperature and emission species. The measured NOx emissions from rig and engine tests are correlated with the predictions of a NOx model and also the improved mixing performance of the injector as indicated through CFD analyses. through this method allowed optimisation of designs to proceed in minimal time. 3. Combustion System Description The particular DLN combustion system under consideration is a can-annular arrangement in common with the conventional combustion systems across the range of EGT small gas turbines comprising the Typhoon, Tornado and Tempest. Figure 1 illustrates the Typhoon installation of the G30 DLN combustors with details of the burner arrangement. 2. Background The combustor is of the lean-premixed variety with 55% of the total flow admitted through a radial swirler to control the reaction zone stoichiometry. Limitations to the length of the can means that a separate premixing zone in common with other systems of this type is not possible, instead a "fast mixing" fuel injection scheme is incorporated prior to burning in order to achieve low emissions capability. In addition to the fast mixer, an additional diffusion flame pilot system is utilised for flexible machine operation. In recent years most manufacturers of gas turbine engines have developed lean-burn combustors with the basic low NOx production benefits of low flame temperatures associated with this approach. The level of NOx in these systems is known to be sensitive to the degree of mixing of fuel and air. However until recently the degree of mixing achieved in a given injection system has been poorly defined. In European Gas Turbines combustor development programmes extensive use has been made of CFD to improve the prediction of fuel-air mixing in addition to the internal flow pattern of the combustor. The introduction of lean-burn technology on EGT small engines has not only been a requirement to meet legislative commitments but also to address competitive forces. The accelerated introduction of the lean-burn approach is being applied across a range of engines; Typhoon, Tornado & Tempest, and the time to optimise such systems is at a premium. 4. 4.1 Facilities Software Rather than develop a CFD code in house, as has previously been popular for many turbomachinery applications, it was felt that a general purpose commercial CFD code including combustion modelling capabilities would be appropriate, allowing EGT to work on the engineering without the software development lead time, particularly where fully elliptic flow is concerned. The combustion factors influencing NOx production are normally expressed through time, temperature & turbulence. Since for lean-burn flames the most significant production of NOx occurs at the flame front (2 to 4 milli seconds ) post flame changes have little influence. Bulk temperature, however, which is dependent on overall stoichiometry has a major influence. In addition, turbulence or effectively mixing of the fuel and air provides localised regions of burning which are above and below the overall mixture ratio. The NOx production rate increases exponentially with temperature and it is not surprising that under such conditions a higher NOx production results than for the case of fully pre-mixed conditions. The overall sensitivity of NOx production to the degree of mixing can be established through a statistical mixture distribution parameter such as the standard deviation Ref. 1-4. The influence of both mixing and stoichiometry on NOx production can be quantified as illustrated typically in Ref. 5. Correlations of the influence of various injector factors determined through CFD and compared The particular code chosen for this purpose was STAR CD. This code provides not only the necessary physical modelling, but also incorporates an unstructured meshing format, allowing models to be built without the constraints of a rectangular coordinate system. Furthermore, embedded meshing techniques allow refinement to be carried out locally in areas of particular interest (e.g. around the injection holes). The recent addition to these meshing tools of "arbitrary interfacing", whereby blocks of mesh can be connected however they are configured, has further enhanced the speed with which the models can be constructed and analysed. 4.2 Computational Hardware The software was run on workstations and cluster machines. A Silicon Graphics Indigo R4000 machine (64 Mbytes RAM, 1 Gbyte disk space) was used for mesh building and post processing. Hewlett Packard cluster machines were used for the numerical 2 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms the sensitivity of the flow to these conditions. A further problem is that of geometry, in particular how accurately the model represents the geometry and if the grid is sufficiently refined to capture the nature of the flow. Again sensitivity checks can be made to either highlight or circumvent such problems. The physical modelling in the CFD code itself can also cause problems. If the parameter in question depends strongly on a phenomenon which is not included in the mathematical models then the analysis could prove futile. analysis, a HP735 (144 Mbytes RAM, 1 Gbyte disk space) for small jobs and a HPK200 (1 Gbyte RAM, 7 Gbytes disk space) for larger analyses. 4.3 Test Facilities Testing was conducted on the High Pressure Air Facility (HPAF) in Lincoln. Air is delivered at full engine pressure (up to 19 bara and 5 kg/s flow) from a compressor driven by an EGT T65000 mechanical drive Gas Turbine. Indirect heating of the air (up to 550 °C) allows engine conditions to be simulated at each of the three high pressure test rigs installed in the facility. 6.2 Time Constraints A single combustor Typhoon test rig comprising an uncooled combustor casing with the combustor exhausting into a water cooled drum and exhaust section was used for testing. Provision was made for instrumentation to measure the relevant combustor parameters. The purpose of the CFD analysis is to provide a faster route for development. Hence if the analysis required is so complex that the CPU time it takes is greater than the time to manufacture and test the same thing then it becomes redundant. Sufficient computing power must be available to keep the CPU time low enough, or a simpler analysis must be considered. 5. 6.3 Experimental Data Quality Iteration Loop To calibrate the analyses effectively the test data must be consistent and reliable. Data quality problems also carry the more global issue of how to assess if the product is improved, and indeed what the original performance was. Figure 2 illustrates the procedure used for CFD parametric studies. The initial step is to perform a baseline study. This consists of analysing a current configuration and comparing the results with available test data. Commonly the test data and analysis will not agree, this can be for a variety of reasons ranging from the coarseness of the computational grid to the applicability .of the physical models used. In such circumstances it is necessary to perform a further analysis of a different configuration also with corresponding test data, and for which the parameter in question is measurably different from the original case. Comparison of the variation in the parameter allows a calibration of the modelling to be made. Further modified models are subsequently analysed and changes in the parameter noted. Where a notable improvement in the parameter occurs the modified geometry is tested, and the data compared to the analysis. If this still proves consistent the product is improved and taken as a new baseline for further enhancements. 6. Potential Problems 7. Modelling 7.1 Approach CFD modelling was used as a tool to aid engineering development, rather than a detailed analysis to provide quantitative performance data. The approach taken was to use a number of relatively simple models to provide parametric comparisons with a base model. This approach is by far the most cost effective and widespread use of CFD, it enables a number of comparative analyses to be performed in a relatively short space of time, providing the engineer with a fast "what if" tool, The altemative approach of detailed quantitative analysis not only carries with it a large time penalty, but also a requirement for correspondingly detailed data measurement both upstream of the combustor for boundary condition definition, and inside the combustor for validation purposes. A number of problems can occur in performing such a procedure, which can be sectioned into three distinct areas:- With this aim the following physical models, together with their associated asSumptions, were applied:- 6.1 CFD Models The first, and possibly most common, problem with CFD is the availability of adequate boundary conditions. This does not necessarily mean that all the details of the flow entering the calculation domain must be accurately known, but where they are not checks should be performed as to • • • • Analysis is steady state k-s turbulence model Density, p calculated as an ideal gas f(T,p) Wall functions 3 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms • • • Properties at model inlets assumed to be flat based upon bulk flow properties from test data. Combined Eddy Break-Up / chemical kinetics reaction rate model. Fuel is CH 4 using the 2-step reaction:- These models were run non-combusting under full engine inlet temperature and pressure conditions in order to assess the fuel/air mixing characteristics upstream of the burning zone (test results and fluid velocity calculations both suggesting that no burning could take place in the swirler slots). CH4 + 3/2 02 -> CO + 2H 20 CO + 1/202 -> CO2 The analyses were parameterised using the mass flow weighted standard deviation of fuel distribution and fraction of fuel coverage at the exit of the slot defined by :- using the reaction rate data of Dupont et al (Ref. 6):- std deviation, S = E(C 7) - E7(C) ) where 4IQmci441 = dt A1 (CH41° 7 102] 8 eXP('E81 A2 [CO]' 5 Pe R-r) E(c) = mean = ECM/ M EMi & E(C7) = EC2M; / EMi eXP(-Ea2 / RD dt fractional coverage = S f(C i , A ) / SA; where where = 5.0114 x 10" Eal A2 • = 202.32x 106 = 1.2604 x 10' 7 = 179.75 x 106 f(0,A1 )=A; =0 m' 5 kmora5 s-1 J kmo1-1 m725 kmo1475 J kmoll The boundary conditions for this model were applied as flat profiles derived from bulk flow test data as detailed measurements of these conditions were not available. Sensitivity checks of turbulent intensities applied at model inlet revealed the model to be very insensitive to such variations as the turbulence generated downstream of the model inlet swamped the convected quantities. Combustion is adiabatic and radiation is not significant. 7.2 Single Swirler Slot Fuel/Air Mixing Models A particularly fast and useful parametric method was achieved by modelling only part of the combustor. In this case only a single slot of the radial swirler was modelled thereby imposing the assumptions that :• the flow of air and fuel into each swirler slot is identical, and • the downstream interactions of the swirler slots are of little consequence to the mixing at the injection point dose to the swirler entry. 7.3 Full Combustor Fuel/Air Mixing Models Following models, similar the single slot non-combusting models of a full combustor and transition duct were developed. These models remove the assumptions of symmetry imposed by the single slot models, but retain the simplistic approach to the physics of the problem thereby keeping time penalties for the extended model to a minimum. The base model is a three dimensional geometry of a complete Typhoon DLN combustor and transition duct. It includes an entry plenum upstream of the swirler in common with the single slot models, and comprises approximately 200,000 fluid cells (fig 4). The assumption is made that the pressure drop is identical over each fuel injection orifice consistent with the single slot models, this ignores any distribution bias in the fuel feed manifold. Although this model is constructed using almost double the number of cells as the single slot models the definition in the swirler slot is reduced, it is however still possible to make qualitative comparisons between the two models. The base model is a three dimensional geometry of a 30 0 sector of the G30 12 slot swirler with entry plenum. It is determined to have cyclic symmetry along an axial plane either side of the slot, and comprises approximately 118,000 fluid cells (fig 3). Typically modifications to the base model involved changes to:• • • C1 >0 C1 =0 fuel injection location and orifice size fuel injection pressure swirler inlet geometry 4 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms fuel coverage by using three injection points in place of two. These models were run at similar conditions to the single slot models. The fuel/air mixing characteristics upstream of the buming zone were assessed in two ways:• The full combustor fueVair mixing models were similarly successful in the prediction of fuel distribution, the patterns at slot exit being consistent at slot exit if not quantitatively similar. The NOx predictions at prechamber exit, using a correlation via the standard deviation of fuel distribution, proved reliable up to a point but as the mixing improved and the fuel pattern altered the predictions became inconsistent due to the recirculatory nature of the flow. Thus the more precise method of NOx prediction using the NOx correlation at zero standard deviation became necessary. This proved successful and carried with it the added bonus of predicting a burning zone via the subset of the model created as shown in figure 7. Mass flow weighted standard deviations were taken at the exit of the prechamber and NOx values derived using the overall stoichiometry and a NOx / Mixedness correlation (illustrated later.) • A subset of the domain was taken under viable conditions for combustion (as regards flame speed and stoichiometry). A NOx value per cell was then extracted using the NOx correlation, assuming the fuel in each computational cell to be perfectly premixed. A volume integration was then carried out to obtain a bulk NOx value. This NOx value was then used with the stoichiometry and correlation to obtain a standard deviation. The final more rigorous method of full combusting flow analysis was less accurate in predicting NOx due to underprediction of the temperature field, though predictions were more consistent but at a significant time penalty. This method did however prove useful in providing detailed predictions of the CO and temperature patterns (figs 8 and 9). 7.4 Full Combusting Flow Models A more rigorous approach in order to predict temperatures and CO production was also used. This was done using full combusting flow models as a further extension to the scalar transport models above. NOx predictions based on the CFD models above are illustrated in figure 10 using the NOx correlation chart (Ref. 5) and compared to those obtained directly from test. The test results use the design point stoichiometry, (1), the measured NOx value and the correlation to predict a standard deviation, S. Where the CFD predictions use the design point stoichiometry, and either : • the standard deviation, S. and the correlation to predict a NOx value, or • the correlation at zero standard deviation and volume integrate over the field to predict values for both NOx and standard deviation. Data from both GE and Siemens is also included in figure 10 as comparison. Details of temperature, CH4 and CO fields were obtained directly from these models. Values for NOx and standard deviation were extracted in a similar manner to those above by:• Taking a subset of the domain based on the temperature field (e.g. T> 1800K) • Obtaining a NOx value per cell via the correlation assuming the fuel to be perfectly premixed in each computational cell. • Performing a volume integral over the subset to obtain a bulk NOx value. • Using the calculated bulk NOx value, overall stoichiometry and the NOx correlation to obtain a standard deviation of fuel distribution. 9. Discussion 8. The methodology used to apply CFD to DLN combustion systems has proved fast and effective in optimising the emissions characteristics of the combustor. The flexibility and cost effectiveness of this approach has more than justified its use, and provided a good deal of insight into the operation of the system. There are drawbacks to such analysis work, notably that CFD remains relatively specialised and the inability to provide very fine flow details, or quantitative performance data, without a good deal of time and effort still leaves many unconvinced of its usefulness despite the parametric success. Results The single slot models proved a good guide as to the distribution of fuel in the swirler slot, in particular the penetration of fuel jets into the high velocity air stream could easily be assessed. Figure 5 depicts an isosurface of gas fuel mass fraction as it is injected into the slot providing a clear picture of the fuel penetration. The trends of the parameters corresponded to the trends of the NOx produced though it was not possible to extract values for NOx directly from these models. Figure 6 shows the mass fraction of fuel for sections taken at the slot exit and highlights the improvement in 5 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms However, there are a variety of ways of increasing the'accuracy of the predictions including:- Acknowledgement The authors would like to acknowledge the support of their colleagues of the DLN team, Lincoln. • 'Further grid refinement • Enhanced turbulence models i.e. Chen k s, second moment closure. • Enhanced combustion models i.e. 3-step reaction schemes, probability density functions. • Transient analyses. - References 1 Mikus T, Heywood J B & Hicks R E Nitric Oxide Formation in Gas Turbine Engines: A Theoretical and Experimental Study; NASA C R 2977 April 1978 Though these, and many other, additional models would undoubtedly improve the accuracy of the results, almost all would be at a considerable time penalty, which must be weighed-up against available computing power. Clearly as the trend towards faster computers continues, more complex and accurate models will become feasible. 2 Lyon V J Fuel-Air Nonuniformity Effect on Nitric Oxide Emissions. AIAA Journal Vol. 20 pp 660-665 1982; NASA Technical Paper 1978, 1981. 3 Fric T F Effect of Fuel-Air Unmixedness on NOx Emissions, AIM 92-3345 July 1992. 10. Further Plans And Developments Current developments of the reported CFD modelling has resulted in predictions of fuel/air mixedness of N ° 2 distillate both in liquid and vapour forms via Lagrangian 2-phase evaporative models. Hopefully in the near future this will lead to the G30 DLN combustor operating on liquid fuels without the need for diluent injection for NOx suppression. 4 Kesseli J, Norster E R & Landau M Low NOx Combustor Design and Test with a Recuperated Gas Turbine Engine; ASME Cogen-Turbo Vol. 7 Book No. 100333, 1992. 5 Norster E R and De Pietro S M Dry Low Emissions Combustion System for EGT Small Gas Turbines, Institute of Diesel and Gas Turbine Engineers Publication No 495, March 1996. 6 Dupont, V. Pourkashanian, M. Williams, A. Modelling of Process Heaters Fired by Natural Gas: J. Inst. Energy, pp 20 - 28 March 1993. 6 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms Figures Figure 1 - Installation of G30 DLN combustor Figure 4 - Full combustor CFD mesh PROITOM 11.10/4.1/ AV, 4.4 °Z.iirrs own. Figure 2 - Flowchart detailing CFD procedure BASELINE ANALYSIS AND COMPARISON WITH TEST MCA er MODIFIED MODEL ANALYSIS AND COMPARISON WITH TEST Figure 5 - CFD predicted penetration of fuel jet C ;CAMAY C (WRY I TEST THE BEST I Figure 3 - Single slot CFD mesh FRO370117a Figure 6 - CFD pred'cted fuel distribution at swirler slot exit v 1•444-0 YEW IMO 5, - AMIE -MI 0 T3 mama 1110,3 warn ZOE .4.3 10 man not OOLOONIU WNTISATIOPI OS GEDIETRY )-- C ICIMNSIV Ma OLE C0411119TOR - MOLE Pileilfffi SIDT Pill WM FM1%ClION DOTREP171170 SIOT OUT 2 Injection points 3 Injection points 7 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms Figure 9 - CFD predicted temperature distribution in combustor Figure 7 - NOx calculation subset j. 1131103a1CU MAYO= 100111111001 SET CMOS/ V5 LI" Figure 8 - CFD predicted CO distribution in combustor Figure 10 - NOx / Mixedness con-elation 0.35 100 mom. MOW% 11.11111011 1111/111till =1•11riallIMIa ,. r ernammo. IIIMIralM,Mill 111111/1111 I IA IVO .—vA gi Ma N.7* 1,12511111 LB 11-3101.13 SC t-00 wanes nrof-in LOCOLDII. APO ream OLE COMUSION MOS ROCIE01-100D0 OF 0111043011 CMOS/ L. . 30 05 .00 - "Pr 04 ■■ TR 1 t it 00 1100 1000 100 A - EGT, 2 njection points, high pressure rig data B - EGT, 2 njection points, prechamber exit mbcing CEO C - EGT, 2 injection points, zonal mixing CFD D - EGT, 2 injection points, zonal combusting CEO E - EGT, 3 injection points, high pressure rig data F - GE high pressure rig data, ASME 92-GT-121 G - Siemens theory (S=0.15), IMechE Sept 95 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 12/29/2014 Terms of Use: http://asme.org/terms

© Copyright 2018