Advanced Turbulence Models for Emission Modeling in Gas Combustion

Transcription

1 1 Advanced Turbulence Models for Emission Modeling in Gas Combustion Ville Tossavainen, Satu Palonen & Antti Oksanen Tampere University of Technology Funding: Tekes, Metso Power Oy, Andritz Oy, Vattenfall R&D AB (Sweden) Network: ÅA University (Combustion and Materials Chemistry), HUT (Laboratory of Applied Thermodynamics), Stanford University (Computational Energy Sciences)

2 2 Outline of Presentation Motivation Goals and contribution Measurements Computations Conclutions Plans for future research

3 3 Motivation for Research Computational Fluid Dynamics (CFD) already powerful design tool in many engineering fields Provides compete information on flow conditions Modifications quite easily performed Accuracy limited by models and computer capacity Computational power increases exponentially according to Moore s law and single CPU load can be reduced using computer clusters Comprehensive models can be applied to even more complex and larger applications

4 4 Motivation for Research ( continues) Direct Numerical Simulation (DNS) remains in academic realm for a long time Intermediate approach so-called Large-Eddy Simulation (LES) technique gotten more attention LES promising tool for combustion studies and academic research work is very active Industry has great interest to lower emissions in combustion processes (burners, internal combustion engines, turbines, furnaces )

5 Why a Crossflow Reactor? What is 5 SNCR process? Jet-in-crossflow coon phenomenon in combustion applications Staged combustion and good mixing lower emissions Selective non-catalytic reduction (SNCR) process a secondary method to reduce NO x emissions Nitric oxide + Aonia Nitrogen + Water SNCR process sensitive to temperature: Too high temperature more NO formed Too low temperature NH 3 passes unreacted

6 6 Goals and Contribution of Research Predict emission formation (NO x ) and unburned fuel using LES technique with appropriate turbulence-chemistry interaction closure Contribution to study SNCR process with LES technique Detailed measurements with crossflow pilot reactor: 1) Cold flow + secondary air, 2) propane flame + secondary air, 3) Case 2 + aonia injection Different crossflow-to-jet velocity ratios studied

7 7 Measurements

8 8 Crossflow Pilot reactor OSBOURNE built in 3-4 Burner: Oilon GP-6.2 H Power: 6-16 kw Liquefied Petroleum Gas, LPG (98% propane, 2% butane) Dimensions: Inside width: 4 Outside width: 76 Total height: 283 OSBOURNE pilot reactor

9 9 Crossflow Pilot Reactor (continues...)

10 1 Crossflow Pilot Reactor (...continues) Air splitting Primary air : 7% Secondary air: 3% Secondary air system 8 nozzles on both sides 3 nozzle diameters: 8, 1, 12 Modified Oilon LPG burner Secondary air pipe lines

11 11 Feedings Seeding particles for LDA measurements: Ceriumoxide (CeO 2 ), d p = 1 µm Secondary air: one inlet on both sides Compounds in measurement Case 3: NO in primary air NH 3 in secondary air Particle flow to the reactor Pressurized air

12 12 Measuring parameters Velocities with Laser-Doppler Anemometer (LDA) Two velocity components Concentrations with emission analyzers CO, CO 2, O 2, NO, NO 2, HC Temperature measurements K-type thermocouples Flow rates Primary and secondary air Fuel Optical measurement inlets

13 13 Combustion case - LDA measurements MEASUREMENT POINTS: Secondary air: Width of the reactor: 4 Nozzle diameter: 8 x Temperature: 15 C X= Mean velocity: 3 m/s 26 Primary air: Mean velocity: ~1 m/s 13 LDA: Secondary air level 11 measurement points Sample limit: 5, measurement time s in each point Measurements started when temperature reaches 1 C in the reaction zone Temperature of the reaction zone increases to the 1 C during the measurements LINE 3 LINE 2 LINE 1

14 14 Velocity Vector Fields First LDA measurements were done at the centerline of two opposing jets (1.6 and 2.6 at the picture) Jet 1.1 Jet 2.1 Jet 1.2 Jet 2.2 Jet 1.3 Jet 2.3 Jet 1.4 Jet 2.4 LDA measurement line 2 Jet 1.5 (Between) Jet 2.5 Centerline Jet 1.6 Jet 2.6 LDA measurement line 1 Jet 1.7 (Centerline) Jet 2.7 Jet 1.8 Jet 2.8 Measurement window Measurements between two opposing jet pairs were done to find out the behavior of jets better The jets seems to be interlaced! Between

15 15 Velocity Histograms on Jet Centerline Horizontal velocity histograms on line 1 Frequency Histogram x = Bin Frequency Cumulative % 12, %, % 8, % 6, % 4, % 2, %, % Velocity [m/s] Mean horizontal velocity Distance [] Frequency ,5-4 Histogram x = 35-37, , , , ,5-15 Bin Frequency Cumulative % -12,5-1 -7,5-5 -2,5 2,5 5 7,51 12, %, % 8, % 6, % 4, % 2, %, % Histogram x = 15 Histogram x = 25 Frequency Bin Frequency Cumulative % , %, % 8, % 6, % 4, % 2, %, % Frequency Bin Frequency Cumulative % , %, % 8, % 6, % 4, % 2, %, %

16 16 Concentration measurement points 9 below the secondary air jets 18 above the secondary air jets side front 3D top

17 17 Concentration Profiles (8 Nozzle) Concentration profile 38 millimeters below secondary air feeding CO concentration, 38 below CO2 concentration, 38 below CO [%] 4 3 CO2 [%] THC concentration, 38 below NO concentration, 38 below THC [ppm] 2 NO [ppm] 6 4 O 2 ~ 1 2

18 18 Concentration Profiles (8 Nozzle) Concentration profile 68 millimeters above secondary air feeding CO concentration, 68 above CO2 concentration, 68 above CO [ppm] 15 CO2 [%] O2 concentration, 68 above NO concentration, 68 above ,5 3 2,5 8 O2 [%] 2 NO [ppm] 6 THC ~ 1,5 1,

19 19 Concentration Profiles (8 Nozzle) Concentration profile 188 millimeters above secondary air feeding CO concentration, 188 above CO2 concentration, 188 above CO [ppm] 15 CO2 [%] O2 concentration, 188 above NO concentration, 188 above 4, 3,5 12 3, O2 [%] 2,5 2, 1,5 NO [ppm] 8 6 THC ~ 1, 4 266,5 2,

20 2 Concentration Profiles (8 Nozzle) Concentration profile 23 millimeters above secondary air feeding CO concentration, 23 above CO2 concentration, 32 above CO [ppm] 15 CO2 [%] O2 concentration, 32 above NO concentration, 32 above ,5 3 2,5 8 THC ~ O2 [%] 2 NO [ppm] 6 1,5 1 4,5 2

21 21 Concentration Profiles (8 Nozzle) Concentration profile 38 millimeters above secondary air feeding CO concentration, 38 above CO2 concentration, 38 above CO [ppm] 15 CO2 [%] O2 concentration, 38 above NO concentration, 38 above ,5 3 2,5 8 O2 [%] 2 NO [ppm] 6 THC ~ 1,5 1,

22 22 Concentration Profiles (8 Nozzle) Concentration profile 72 millimeters above secondary air feeding CO2 concentration, 72 above O2 concentration, 72 above NO concentration, 72 above CO2 [%] O2 [%] 3,5 3 2,5 2 1,5 1 NO [ppm] ,5 2 THC ~ CO ~

23 23 Computations

24 24 What Is LES? In LES technique, Large (energy-containing) turbulent scales solved while the smallest scales modeled Large scales geometry-dependent while smallest scales universal good approximation log(e(k)) modeled in RANS solved in DNS solved in LES modeled in LES solved in LES modeled in LES inertial sub-range, slope ~ -5/3 energy containing integral scales filter length ~ grid size large scales viscous sub-range log(k) Turbulence energy in wave number space dx Schematic of scale separation in LES

25 25 Why to Use LES in Reactive Flows? Accurate turbulence modeling needed describing interaction between turbulence and chemical reactions Large scale mixing fully solved LES able to outperform RANS turbulence models that can employ more sophisticated chemistry models Chemical reactions occur mostly in the smallest scales LES scale separation approximation acceptable Thermal NO formation slow but highly temperature sensitive process and hence strongly influenced by flow field

26 26 Turbulent Non-Premixed Models Real propane-air flame consists of several hundred species and thousands of reactions How to model it properly? Reaction rate approach comprehensive but unpractical in LES approach limited to one- or two-step reactions Conserved scalar (mixture fraction) based methods usually chosen in LES computations Decouples chemistry (mixture fraction) and mixing (scalar dissipation) Solutions can be based on several methods: flameletbased presumed PDF, conditional moment closure, transport equation PDF

27 Non-Premixed Flamelet-Based PDF Method 27 Turbulent flame composed of sheet of laminar flames called flamelets where solutions pre-computed from 2D flames Model applicable if turbulent scales larger than flame thickness satisfied in most industrial applications Instantaneous stoichiometric turbulent flame front Fuel Instantaneous flamelet 2D laminar stagnation point flame Oxidizer temperature Fuel Oxidizer PDF look-up table scalar dissipation mixture fraction Laminar flame solutions

28 28 LES Flamelet PDF Solver LES solver Pressure Velocities Mixture fraction Density Viscosity (Temperature) (Species mass fractions) p(z) p(χ) z Presumed PDF for mixture fraction from flamelet solutions Presumed PDF of stoichiometric scalar dissipation from flamelet solutions z Model for scalar dissipation rate Model for stoichiometric scalar dissipation rate

29 29 Computational Tools Experimental cases simulated using CFD using LES technique and steady-rans LES simulations performed with structured solver by Charles Pierce (Stanford University, USA) Restrictions for grid size and time step apply only center section of pilot reactor simulated Steady-RANS simulations performed with coercial ANSYS FLUENT 6.2/6.3 solver Laminar flamelets pre-computed with FlameMaster (by Heinz Pitsch) using elemental reaction mechanism files

30 LES and RANS Computation Geometries 3 Side view of pilot reactor Top view of pilot reactor RANS simulations LES simulations Earlier LES cold flow investigation by Kari Koskinen (4)

31 31 Steady-RANS Simulations Block-structured mesh with several Chimera (nonconformal) grid interfaces Best compromise between grid size (~3M) and accuracy Computations being performed with coercial FLUENT 6.2/6.3 software Non-premixed Flamelet-based PDF Realizable k-e turbulence model Non-conformal grid interfaces Closer look on secondary air grid interface

32 32 Progress on LES Computations Özcan & Larsen (1) single jet-in-crossflow Propane-air flame test case Pilot reactor

33 PDF Chemistry Table Tabulation Process 33 Table optimization Second mixture fraction tabulation temperature PDF look-up table Chemistry look-up table LES computations Flamelet Library Interface to FLUENT RANS simulations scalar dissipation mixture fraction Chemical mechanisms: C 3 H 8 + NO Aonia + NO Radiation Lewis number effect: Le=1, Le 1, Le unequal

34 34 Flamelet Computation Results Stoichiometric temperature [K] 1 / Stoichiometric Scalar Dissipation Rate [s]

35 35 Pilot Reactor Run Dimensionalized by primary air flow conditions: Re=367, L ref =.1 m, U ref =5.45 m/s, t=.1t, d jet =8 Axial velocity on centerline (z=)

36 36 Conclusions Measurements with 8 secondary air jet performed and preliminary LDA and concentration results are satisfactory Flow field complex due to instabilities (jets interlaced) CO and TCH ~ in the highest concentration measurement points NO x level raise 2ppm 8ppm near the secondary air feeding and remain unchanged With 12 secondary air nozzle, burner power reduced 16 kw 9 kw due to too high temperatures (>1 K)

37 37 Conclusions (continues ) Only small differences found between propane elementary reaction mechanisms Differences between different chemical species need to be studied Pilot reactor computations started in November and will be performed till end of March 7 Results will be reported on project home page: