OpenFOAM for LES of premixed combustion and mixing processes Hannes Kröger, Steffen Jahnke, Nikolai Kornev, Egon Hassel 1
Introduction LTT Rostock: OpenFOAM is used for LES in different projects LES of premixed combustion: Combustion Induced Vortex Breakdown LES of mixing processes: Jet mixer Development of the inflow generator Dimpled surfaces in heat exchangers 2
Contents LES of combustion Combustion Induced Vortex Breakdown (CIVB) Introduction, Motivation Computations, Results Current developments LES of mixing processes Jet mixer Introduction Influence of mesh, SGS model and inflow conditions 3
Contents LES of combustion Combustion Induced Vortex Breakdown (CIVB) Introduction, Motivation Computations, Results Current developments LES of mixing processes Jet mixer Introduction Influence of mesh, SGS model and inflow conditions 4
Vortex Breakdown recirculation bubble is formed in vortical flows if swirl strength exceeds critical threshold flow Picture: Lim / National University of Singapore known from delta wings at high angles of attack strong vortical flows Picture:University of Liverpool flow 5
Vortex Breakdown utilized in premixed gas turbine combustion chambers for aerodynamical flame stabilization combustion chamber mixing pipe Uax flow st Uax >> st Picture: Fritz / PhD Thesis, Technical University Munich stable flame vortex breakdown is enforced by jump of cross sectional area stable flame position flame burns in recirculation bubble 6
Combustion Induced Vortex Breakdown aerodynamical flame stabilization can get unstable Investigations at TU Munich: CIVB is responsible mixing pipe under certain conditions (high load) flame propagation into mixing pipe can be observed reason is interaction of flame and vortex breakdown Picture: Fritz / PhD Thesis, Technical University Munich 7
Model experiment: CIVB in free vortices Objective: Flame flashback in a free rotating jet Targets: understanding the physical mechanism influence of parameters quantitative prediction of CIVB phenomenon 8
CIVB in free vortices Experimental observations Cold flow with critical swirl: Axial velocity 180 <u> [m/s] 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 0.25 160 140 z [mm] 120 100 80 60 40 20 0 40 20 0 y [mm] 20 Flamespeed estimated from film: v f 2.5 m / s Estimated turbulent flamespeed: st s L u ' =0.17m / s 1m/ s=1.17m /s 9
Numerical simulations Step 1: Calculation of isothermal flow through swirl generator device to get mean velocity profiles PIV LES Geometry of swirl generator 10
Numerical simulations Step 2: Calculation of the reacting flow (partially premixed) Solver: Xoodles (Weller b model) inhomogeneousmixture with transport equations for Su and Xi Resolution: 1 million cells (cell edge length 1.5mm at flame) SGS model: oneeqeddy Boundary conditions: mean velocity from precursor simulation turbulent fluctuations from inflow generator (turbulent spot method) 11
Numerical simulations qualitative reproduction of phenomenon successful 12
Results instantaneous velocity field: recirculation bubble in front of flame tip confirmation after averaging in flame fixed coordinate system x x=0 Uax 13
Further analysis Vortex breakdown: explanation via vorticity Vorticity is turned Circumferentially aligned vorticity induces deceleration on vortex axis positive feedback: vortex breakdown Strain effect in incompressible flows vorticity production t u = u 1 volume expansion 2 p u baroclinic production strain stretch torus vorticity induced velocity only present in variable density flows flow 14
Further analysis Which production mechanism contributes most? Vorticity induces velocity uind according to: (Biot Savart Law) An induced acceleration can be obtained: Integration was carried out with a postprocessing program using the OpenFOAM library 15
Further analysis induced accelerations averaged in flame fixed CS: strain/stretch: largest contribution to deceleration of axial flow x x=0 large fluctuations Uax 16
Current developments Implementation of PDF methods for combustion simulation presumed PDF's: beta PDF, clipped gaussian,... Flame Surface Density PDF (L. Vervisch & P. Domingo) Partial PDF (A. Mura & R. Borghi) usage of ILDM chemistry tables 17
Implementation new library: libcompositionpdf MultidimensionalLookupTable<T> chemistrytable beta clippedgaussian... compositionpdf<pdf> presumedpdfthermo<pdf> presumedpdfoodles MultidimensionalLookupTable: presumedpdfthermo: lookup table with arbitrary number of progress variables thermo class for presumed PDF method 18
Application depending on selected (and implemented) PDF selected progress variable generated lookup table the implementation can be used for non premixed and premixed combustion currently computation of test cases 19
Test cases premixed ORACLES burner non premixed Sydney swirl flame SM1 Isosurface T=1100K work in progress, no quantitative results yet 20
Contents LES of combustion Combustion Induced Vortex Breakdown (CIVB) Introduction, Motivation Computations, Results Current developments LES of mixing processes Jet mixer Introduction Influence of mesh, SGS model and inflow conditions 21
Jet mixer Jet mixer in chemical industry: chemical reactor Project targets: Investigation of micro mixing in liquids computational domain 22
Jet mixer Investigations Numerical Experimental RANS (CFX 5) LIF (mixing) LES (inhouse code) LDA (velocities) LES (OpenFOAM) 23
Jet mixer Influence of mesh type experiment 1. block structured hex 2. unstructured hex 3. tetrahedra 1. 2. 3. x/d computational domain 24
Jet mixer influence of SGS model 1. 2. experiment 1. locdynsmagorinsky 2. locdynoneeqeddy 3. dynoneeqeddy 4. DMM 3. 4. x/d computational domain 25
Jet mixer influence of inflow BC's Comparison of random inflow quasi periodic inflow random random (5x turb. intensity) quasi periodic Isosurfaces 8th June 2007 2= 5000 26
Future Works Complete implementation of presumed PDF methods G equation / WENO scheme for unstructured tetrahedral meshes 27
Thank you for your attention! We gratefully acknowledge the support of the OpenFOAM developer community DFG (Deutsche Forschungsgemeinschaft) HLRN (Norddeutscher Verbund für Hoch und Höchstleistungsrechnen) 28