Turbulent Flames at High Reyolds Number a) Distributed Combustion - PowerPoint PPT Presentation

Turbulent Flames at High Reyolds Number a) Distributed Combustion = “Near Limit” b) Piloted Bunsen – Regime Diagram = not very Near c) JP-8 flames - maybe halfway ? Jim Driscoll Aaron Skiba, Tim Wabel, Cam Carter NSF Project CBET-1703543 Dr. Song-Charng Kong AFOSR Project FA9550-16-1-0028, Dr. Chiping Li 1

Distributed Turbulent Combustion ßà Auto ignition Tianfeng Lu, Jackie Chen, C.K. Law Air at 700 K Distributed Auto- 21% O 2 Combustion ? ignition “Diluted” oxidizer = 12% O 2, 1200 K High Products at Speed 2200 K Mixing, no flames Fuel H + O 2 + M à HO 2 + M DME + HO 2 à CH 3 OCH 2 +H 2 O 2 2

(a) Distributed combustion - not found for piloted Bunsen No Turbulence Distributed ? Distributed Intensity u’/S L Integral scale Integral scale Theory Experiments not preheated, not diluted, methane only 3

Distributed Combustion – how to find it ? Distributed if: Preheat Distributed Temp. Highly preheated - to 1200 K Combustion (K) Highly diluted with products N 2 , CO 2 H 2 O Temperatures rise only from Hi-Pilot 1200 K to 1700 K 21% Long residence time = recirculation, ß Dilution = % O 2 in reactants hot walls (Medwell, Dally) 4

Michigan DISTRIBU - burner Highly preheated to 1200 K Highly diluted – with N 2 , CO 2 or H 2 O – to 12% O 2 Use vitiator heater - so dilution fraction is well known Premix chamber - at high velocity to prevent flame formation Long residence time , strong mixing Hot walls 5 m = 15 feet O 2 injection T up to 700 K N 2 injection 5

Plan for next year 1. Complete Michigan DISTRIBU-burner, achieve distributed combustion 2. Apply our PLIF reaction rate diagnostics (formaldehyde-OH overlap, CH) 3. Measure boundaries of distributed combustion vs. flamelets Preheat Distributed Temp. Combustion (K) Hi-Pilot 21% ß Dilution = % O 2 in reactants 6

b) Piloted Bunsen = unheated, undiluted flame structure Fractal- like Broken densely (yes with Broadened packed poor back Preheat (yes) support) Layers (yes) Distributed (not unless add preheat, dilution, Thin swirl) Wrinkled flamelets 7 (yes)

Michigan Hi-Pilot Burner - methane, not heated, not diluted Mean velocity up to 100 m/s u’/S L up to 246 L x / δ L up to 215 Re T = up to 100,000 100 m/s 1. Reactedness structure (temperature) profiles Preheat thickness formald. & Rayleigh Reaction layer thickness CH and overlap Turbulence profiles, b.c.’s 2. Measured Regime Boundary Turbulent burning velocity (S T ) 3. “Events” at 20 kHz: pulse burst laser - OH + formald + stereo PIV 8

High Re METHANE Data Base = temperature, preheat zones, reaction zones, turbulence Temperature CH (Rayleigh) 10 kHz Formaldehyde OH turbulence (PIV) 20 kHz 20 kHz 20 kHz 9

Reactedness(Temperature) profiles from Rayleigh Large blobs of hot products convected upstream by turbulent diffusion similar to DNS of temperature J.H Chen 10

Preheat layer thickness from formaldehyde PLIF also from Rayleigh 11

Reaction Layers - from Overlap and CH Overlap CH PLIF method at 10 kHz 12

20 kHz Unsteady Events - simul / stereo-PIV / form.-OH Preheat (blue), Products (red) Simultaneous Eddies,Velocity Events: how do large eddies cause Damkohler diffusion of globs of hot gases upstream into reactants ? For what eddy size ? 13

Reaction layers CH PLIF at 10 kHz Hi-Pilot Burner Cam Carter’s Lab Tonghun Lee’s laser at AFRL What is the merging rate ? pocket formation rate ? extinction rate ? Why does area never exceed about five times the unwrinkled area ? 14

Events - measure the merging rate at high Re CH layers Measured flamelet Merging rate (M)

Predicted Regime Boundaries Motivation: Distributed Reactions When are models valid ? Thin flamelet model Thickened flamelet model Distributed reaction model 16

Flamelets not distributed above predicted “distributed” boundary Present Hi-Pilot data = not distributed 1000.0 Dunn, Barlow Alden = not Predicted u 0 ’/S L distributed “Distributed” Boundary 100.0 à is not valid for these expts. Thin 10.0 Flamelets (Gulder) 1.0 Laminar Wrinkled flames L x,0 / δ f 17

Measured Regime Diagram - broadening boundary Solid symbols = Measured Present Hi-Pilot data 1000.0 Dunn, Barlow Broadened Preheat, Alden data Thin Reaction layers u 0 ’/S L Open symbols = 100.0 Predicted broadening measured Boundary does not Thin agree w measurements Flamelets 10.0 (Gulder) Measured broadening boundary 1.0 Laminar D T = u’ L x = 180 D* L x,0 / δ f

CONCLUSIONS for METHANE fuel 1. First Hi Re data base - flame structure in “Extreme” turbulence u’/S L = 240, Re T = 100,000 - completed images of temperature, OH, formaldehyde, CH, velocity “intense” range = Re T = 5,000 = Dunn-Barlow, Alden (Lund) 2. Broadened preheat, thin reaction layers are observed Distributed or broken not observed for this expt. 3. Measured Regime Diagram – for piloted bunsen flames only 19

Two boundaries on the predicted Borghi do not match the Measured Regime Diagram Broadened preheat layers measured to occur when D T = u’ L x = 180 D* Broken reaction layers not observed in present work - for Karlovitz number up to five times the predicted value Dunn, Masri see some broken at Ka = ten times predicted à Flamelets occur over much wider range than predicted 20

Disclaimer - we study TWO out of SEVEN possible geometries Other geometries: 3. Premixed jet flame: is shear dominated 4. Bluff body 1. Afterburner 2. Gas Turbine LPP Combustor large mean velocity low mean velocity 5. Counter-flow flames small residence time large residence time not shear dominated 6. Spherical flame “High-Pilot’ “DISTRIBU-burner” 7. Well stirred reactors Piloted Bunsen burner distributed, flamelets favors flamelets and and extinction ? hard to extinguish 21

JP-8 Flames - in progress 100 m/s vaporized JP-8 heating tape JP-8 premixed fine spray Re T = 20,000 Delevan atomizer liquid JP-8 5 m = 15 feet T up to 700 K 22

Chemistry in High Re JP-8 Flames Why study JP-8 chemistry in turbulent flames ? à Only turbulent flames (at high Re) provide realistic residence times = time for fluid element to cross three layers Primary Reaction Zone Preheat Pyrolysis zone ? zone 23

Step 1: PLIF of preheated JP-8 flames at Michigan PLIF of formaldehyde, OH PLIF of JP-8 components: naphthalene, toluene, tri-methyl benzene Fluorescence naphthalene tri-methyl intensity C 10 H 8 benzene Gritsch ONERA 266 nm laser line 460 nm 540 nm wavelength Fluorescence toluene C 7 H 8 intensity Sick (Michigan) 266 nm laser line 460 nm 540 nm 24

STEP 2: Line CARS - of Jim Gord at AFRL primary reactions preheat pyrolysis Pump sheet Probe sheet 25

Profiles of 14 species in a vaporized JP-8 flame - can it be done ? 26

Chemistry in High Re JP-8 Flames– joint Michigan –AFRL pyrolysis zone high T reaction zone imaged with JP-8 PLIF imaged with OH PLIF preheat region imaged with formaldehyde PLIF CARS line imaging of temperature, species toluene methane H 2 CO X i X i OH OH ethylene CO 2 x x JP-8 components formaldehyde acetylene Fluorescence: 5 species (Michigan) CARS: 9 species (Gord, Meyer) 27

Chemistry data reduction Each laser shot (some at 10 Hz, some at kHz) a) Profile of one species and temperature (CARS or LIF line imaging) b) Simultaneous 2-D PLIF images of pyrolysis layer, preheat, reaction layer to measure residence time = thickness / normal velocity JP-8 For residence T X i time = 0.1 ms x, mm 28

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Events: extinction with poor back support 10 kHz CH-OH simultaneously reactants (black) no CH = extinction 30

Compare “Events” to DNS of Jackie Chen, John Bell “area increase” events: vortex passage, flame stretch, extinction, rollup, merging “Damkohler diffusion” events globs of hot gases convecting, broadening the preheat layer 31

Turbulent Burning Velocity (S T ) At high Reynolds number, was Damkohler prediction correct ? YES True measured burning velocity Wrinkled area / time averaged area low turbulence = linear variation of S T extreme turbulence = square root dependence previous data of Gulder 32

For more details: see our recent papers 36 th Comb. Symp. Seoul : “Measurements to Determine Regimes of Premixed Flames in Extreme Turbulence”, T. Wabel, A. Skiba, J. Temme, J. F. Driscoll “ Turbulent Burning Velocity Measurements: Extended to Extreme Levels of Turbulence” T. Wabel, A. Skiba, J. F. Driscoll “Reaction layer visualization: a comparison of two PLIF techniques and advantages of kHz- imaging” Skiba, T. Wabel, C. D. Carter, S. Hammack, J. Temme, T.H. Lee, J. F. Driscoll + two papers submitted for Journal publication 33