Recomendados
Mais conteúdo relacionado
Destaque
Semelhante a Han Shen's Presentation
Semelhante a Han Shen's Presentation (20)
Han Shen's Presentation
- 1. Turbulence and Combustion Research Laboratory Turbulence and Combustion Research Laboratory Han Shen Department of Mechanical and Aerospace Engineering, The Ohio State University, Characterization of the Structure of Turbulent Nonpremixed Dimethyl Ether Jet Flames
- 2. Turbulence and Combustion Research Laboratory Outline Motivation Comparisons of CH4- and DME flame structure using “Equivalent” flames (1) Comparison at constant Reynolds number (2) Comparison at constant Damköhler number Comparison between Direct Numerical Simulation (DNS) and Experiments of DME Flames Conclusions and Recommendations for Future Work
- 3. Turbulence and Combustion Research Laboratory Motivation Dimethyl ether (DME) is a promising alternative to diesel fuel in compression-ignition engines DME results in high thermal efficiencies and favorable ignition properties due to its high cetane number DME can result in cleaner engine operation – decrease in particulate formation, nitrogen/sulfur oxides, and CO Little is known about the fundamental DME combustion processes, especially turbulence-chemistry interaction (TCI)
- 4. Turbulence and Combustion Research Laboratory Motivation (Cont.) DME is important in terms of “fundamental” research as well Detailed studies (large data sets) exist for simple fuels (e.g., hydrogen and methane) Larger and more complex fuel molecules are desired targets Experimentally, DME is chemically manageable, produces relatively low soot, and has sufficient vapor pressure to facilitate well-defined canonical turbulent flame experiments Chemically it is the simplest ether; kinetic models are manageable (< 500 reactions for “full” mechanism)
- 5. Turbulence and Combustion Research Laboratory Direct Comparisons of CH4- and DME- based Flame Structure
- 6. Turbulence and Combustion Research Laboratory Current DME-based flames are formulated to directly compare to well- characterized “DLR” (CH4/H2/N2) flames, which are target cases in TNF workshop Flame Configurations Fuel Mole Fractions Flame XCH4 XDME XH2 XN2 ReD xs Tad (K)a % Reblowout U (m/s) Da DLR A 0.221 - 0.332 0.447 15200 0.167 2122 65% 38.8 0.0619 DLR B 0.221 - 0.332 0.447 22800 0.167 2122 97% 58.3 0.0412 DME A - 0.221 0.332 0.447 15200 0.169 2199 47% 27.6 0.0899 DME B - 0.221 0.332 0.447 22800 0.169 2199 71% 41.4 0.0599 DME C - 0.221 0.332 0.447 31050 0.169 2199 97% 56.3 0.0440 a corresponds to f = 1 conditions DLR B DME B Fuel issues from a 0.8-cm diameter tube into a 0.3 m/s annular (30 cm x 30 cm) co-flow (same facility across fuels). Reynolds numbers, Damköhler number, and stoichiometric mixture fraction (xs) are matched across flames of different fuels.
- 7. Turbulence and Combustion Research Laboratory Previous Work Laminar flame calculations showed rapid DME pyrolysis and significant increases in CO, CH2O and other oxygenated hydrocarbons for DME flames CH2O PLIF imaging: signal much higher in DME flames; mean CH2O profiles peak in low temperature regions for DME flames (800-1000 K) and near peak temperature for DLR flames; CH2O is more actively consumed in low temperature regions for DME flames. CH2O CH2O Mie Mie x/d = 5 x/d = 10 x/d = 20 DLR A DME A 0 3000PLIF Signal (arb. Units) x200 x 1 40 mm
- 8. Turbulence and Combustion Research Laboratory Experimental Setup - OH PLIF OH PLIF was performed with 2 mJ/pulse, exciting the A-X (1,0) transition near 283 nm. OH PLIF was performed with 2.0 mJ/pulse, exciting the A-X (1,0) transition near 283 nm. OH emission from the A-X(1, 1), (0, 0) and B-X (0, 1) bands between 306 nm and 320 nm was detected by a lens-coupled ICCD camera system Field-of-view was 42 mm x 56 mm; ICCD gate was 100 ns
- 9. Turbulence and Combustion Research Laboratory Data Processing OH PLIF images were acquired at axial position of x/d = 7, 10, 20, 30, 40, and 60; Statistical analysis performed only on images at x/d=7, 10, and 20 800 OH PLIF images taken at each location to determine mean and RMS fluctuations Topographical flame characteristics were extracted: (1) OH layer curvature, (2) OH layer thickness, (3) OH layer orientation, (4) OH layer surface area, (5) “flame holes”, which were determined as a local discontinuity in the OH layer
- 10. Turbulence and Combustion Research Laboratory Example OH PLIF Imaging (Constant Reynolds Number) DLR B DME B Re = 22800 x/d = 60 x/d = 40 x/d = 30 x/d = 20 x/d = 10 42 mm Similar OH PLIF signal levels in both sets of flames DLR flame appears more wrinkled with higher probability of local flame extinction (e.g., OH holes) OH layers in DME flame are more “laminar like, while OH layers are highly strained and segmented in DLR flame At equivalent Reynolds number, DME flames appear less affected by turbulence DLR B DME B
- 11. Turbulence and Combustion Research Laboratory Example OH PLIF Imaging (Constant Damköhler Number) Similar OH PLIF signal levels in all sets of flames DME C flames appear more wrinkled than the DME B flame, but not as wrinkled nor as contorted as the DLR B flames x/d=20 x/d=10 x/d=7 DLR B Re=22800 Da=0.041 At equivalent Damköhler numbers, DME flames appear less affected by turbulence DME B Re=22800 Da=0.059 DME C Re=31050 Da=0.044
- 12. Turbulence and Combustion Research Laboratory Mean and RMS Profiles (Constant Reynolds Number) DME flames have higher peak mean intensity of OH DLR flames show broader mean OH profiles DLR flames have higher relative RMS fluctuations For flame set A, results between the DLR and DME flames are similar. Results suggest that at the upstream positions (where strain is highest and TCI is most vigorous), DME flames are more robust; at downstream locations (strain is decreased), results are similar 0 0.25 0.5 0.75 1 1.25 1.5 x/d=7 DLR B DME B Average OH Profile 0 0.25 0.5 0.75 1 1.25 1.5 x/d=7 DLR B DME B RMS OH Profile 0 0.25 0.5 0.75 1 1.25 1.5 SignalIntensityinabs.unit x/d=10 DLR B DME B 0 0.25 0.5 0.75 1 1.25 1.5 SignalIntensityinabs.unit x/d=10 DLR B DME B -40 -20 0 20 40 0 0.25 0.5 0.75 1 1.25 1.5 Radial Position (mm) x/d=20 DLR B DME B -40 -20 0 20 40 0 0.25 0.5 0.75 1 1.25 1.5 Radial Position (mm) x/d=20 DLR B DME B
- 13. Turbulence and Combustion Research Laboratory Mean and RMS Profiles (Constant Damköhler Number) 0 0.25 0.5 0.75 1 1.25 1.5 x/d=7 DLR B DME C Average OH Profile 0 0.25 0.5 0.75 1 1.25 1.5 x/d=7 DLR B DME C RMS OH Profile 0 0.25 0.5 0.75 1 1.25 1.5 SignalIntensityinabs.unit x/d=10 DLR B DME C 0 0.25 0.5 0.75 1 1.25 1.5 SignalIntensityinabs.unit x/d=10 DLR B DME C -40 -20 0 20 40 0 0.25 0.5 0.75 1 1.25 1.5 Radial Position (mm) x/d=20 DLR B DME C -40 -20 0 20 40 0 0.25 0.5 0.75 1 1.25 1.5 Radial Position (mm) x/d=20 DLR B DME C DLR B peak OH intensity is approximately 75% of the mean OH intensity of DME C at all axial locations Relative RMS fluctuation of DLR B flames is ~ 80% of that of the DME C flames at all axial locations DLR B flames show broader RMS OH profiles Unlike the previous comparisons within flame set A and flame set B at a constant Reynolds number, the relative mean peak and RMS OH intensity between DLR and DME flames is independent of axial position
- 14. Turbulence and Combustion Research Laboratory Mean and RMS Profiles (Constant Damköhler Number) Mean OH intensity of DLR A increases relative to that of DME B flame with increasing axial position Mean OH intensity profile is not broader for DLR A flame in comparison to DME B flame RMS profiles are almost identical 0 0.25 0.5 0.75 1 1.25 1.5 x/d=7 DLR A DME B Average OH Profile 0 0.25 0.5 0.75 1 1.25 1.5 x/d=7 DLR A DME B RMS OH Profile 0 0.25 0.5 0.75 1 1.25 1.5 SignalIntensityinabs.unit x/d=10 DLR A DME B 0 0.25 0.5 0.75 1 1.25 1.5 SignalIntensityinabs.unit x/d=10 DLR A DME B -40 -20 0 20 40 0 0.25 0.5 0.75 1 1.25 1.5 Radial Position (mm) x/d=20 DLR A DME B -40 -20 0 20 40 0 0.25 0.5 0.75 1 1.25 1.5 Radial Position (mm) x/d=20 DLR A DME B
- 15. Turbulence and Combustion Research Laboratory OH Layer Thickness (Constant Reynolds Number) For the Re = 15,200 cases, the median of the distribution is ~ 0.4 mm for the DLR flame and ~ 0.6 mm for the DME flame DLR flames exhibit a higher probability of thin OH layers (<0.4 mm) and a slower decaying exponential tail to the PDF Indicates large influence of flow turbulence on the OH layer structure in the DLR flames “thin” OH layers indicate the presence of larger eddies which “stretch” (and thin) the layers thick OH layers indicates the interaction of smaller eddies which “thicken” the layers Results are qualitatively similar for the “B” flames 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR A DME A 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR A DME A 0 1 2 3 4 0 0.05 0.1 0.15 0.2 Thickness/Lamniar Flame Thickness Probability x/d=20 DLR A DME A 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME B 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME B 0 1 2 3 4 0 0.05 0.1 0.15 0.2 Thickness/Lamniar Flame Thickne Probability x/d=20 DLR B DME B 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME C 0 1 2 3 4 0 0.05 0.1 0.15 0.2 Thickness/Lamniar Flame Thicknes Probability x/d=20 DLR B DME C
- 16. Turbulence and Combustion Research Laboratory OH Layer Thickness (Constant Damköhler Number) DLR A flames still appear to display a slightly slower decaying tail to the pdf indicating larger probability of thicker OH layers. DLR B and DME C flames show similar behavior with no obvious differences 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR A DME B 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR A DME B 0 1 2 3 4 0 0.05 0.1 0.15 0.2 Thickness/Lamniar Flame Thickness Probability x/d=20 DLR A DME B 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME C 0 1 2 3 4 0 0.05 0.1 0.15 0.2 Thickness/Lamniar Flame Thickne Probability x/d=20 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME C 0 1 2 3 4 0 0.05 0.1 0.15 0.2 Thickness/Lamniar Flame Thickness Probability x/d=20 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME C 0 1 2 3 4 0 0.05 0.1 0.15 0.2 Thickness/Lamniar Flame Thicknes Probability x/d=20 DLR B DME C
- 17. Turbulence and Combustion Research Laboratory OH Layer Curvature (Constant Reynolds Number) DME flame A has a higher probability of small absolute curvatures (near zero) and a narrower distribution of curvature DME flame B is qualitatively similar Near-zero curvature is consistent with a laminar flame; that is, OH layers oriented parallel to axial flow Similar to instantaneous images, the DME flames are statistically more “laminar like” as compared to the DLR flames for a given Reynolds number. 0 0.1 0.2 0.3 0.4 Probability x/d=7 DLR A DME A 0 0.1 0.2 0.3 0.4 Probability x/d=10 DLR A DME A 0 1 2 3 4 0 0.1 0.2 0.3 0.4 Curvature (1/mm) Probability x/d=20 DLR A DME A 0 0.1 0.2 0.3 0.4 Probability x/d=7 DLR B DME B 0 0.1 0.2 0.3 0.4 Probability x/d=10 DLR B DME B 0 1 2 3 4 0 0.1 0.2 0.3 0.4 Curvature (1/mm) Probability x/d=20 DLR B DME B 0 0.1 0.2 0.3 0.4 Probability x/d=7 DLR B DME C 0 0.1 0.2 0.3 0.4 Probability x/d=10 DLR B DME C 0 1 2 3 4 0 0.1 0.2 0.3 0.4 Curvature (1/mm) Probability x/d=20 DLR B DME C
- 18. Turbulence and Combustion Research Laboratory OH Layer Curvature (Constant Damköhler Number) Constant Damköhler number conditions show little difference between DLR and DME flames At the upstream axial positions of x/D = 7 and x/D = 10, DME flames exhibit less curvature The less wrinkled or more “laminar- like” nature of the DME flames appears to support the notion that DME flames are less affected by the local turbulent flow field. 0 0.1 0.2 0.3 0.4 Probability x/d=7 DLR B DME C 0 0.1 0.2 0.3 0.4 Probability x/d=10 DLR B DME C 0 1 2 3 4 0 0.1 0.2 0.3 0.4 Curvature (1/mm) Probability x/d=20 DLR B DME C 0 0.1 0.2 0.3 0.4 Probability x/d=7 DLR B DME C 0 0.1 0.2 0.3 0.4 Probability x/d=10 DLR B DME C 0 1 2 3 4 0 0.1 0.2 0.3 0.4 Curvature (1/mm) Probability x/d=20 DLR B DME C 0 0.1 0.2 0.3 0.4 Probability x/d=7 DLR A DME B 0 0.1 0.2 0.3 0.4 Probability x/d=10 DLR A DME B 0 1 2 3 4 0 0.1 0.2 0.3 0.4 Curvature (1/mm) Probability x/d=20 DLR A DME B
- 19. Turbulence and Combustion Research Laboratory OH Layer Orientation (Constant Reynolds Number) Centered at 90 degrees (parallel to the flow direction) DME A has a narrower distribution around 90 degrees, especially at the upstream axial locations of x/d=7 and 10 At x/d=20, DLR A and DME A are similar B case is qualitatively similar to A case At x/d=20, DLR B and DME B are almost identical 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR A DME A 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR A DME A 0 50 100 150 200 0 0.05 0.1 0.15 0.2 Orientation (degree) Probability x/d=20 DLR A DME A 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME B 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME B 0 50 100 150 200 0 0.05 0.1 0.15 0.2 Orientation (degree) Probability x/d=20 DLR B DME B
- 20. Turbulence and Combustion Research Laboratory OH Layer Orientation (Constant Damköhler Number) Centered at 90 degrees (parallel to the flow direction) DME has a narrower distribution around 90 degrees at x/d=7 but differs less compared to equivalent Reynolds number cases At x/d=10 and 20, DLR and DME flames are very similar Consistent with the mean and RMS profiles 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME C 0 50 100 150 200 0 0.05 0.1 0.15 0.2 Orientation (degree)Probability x/d=20 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR A DME B 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR A DME B 0 50 100 150 200 0 0.05 0.1 0.15 0.2 Orientation (degree) Probability x/d=20 DLR A DME B 0 0.05 0.1 0.15 0.2 Probability x/d=7 DLR B DME C 0 0.05 0.1 0.15 0.2 Probability x/d=10 DLR B DME C 0 50 100 150 200 0 0.05 0.1 0.15 0.2 Orientation (degree) Probability x/d=20 DLR B DME C
- 21. Turbulence and Combustion Research Laboratory OH Layer Surface Area (Constant Reynolds Number) DME A flame exhibits a narrower distribution at x/d=7 Further downstream, DLR A flame peaks at a larger surface area with a slower decaying tail Same pattern is observed in B cases Consistent with the previous results for OH layer thickness and OH layer curvature 0 0.05 0.1 0.15 Probability x/d=7 DLR B DME B 0 0.05 0.1 0.15 Probability x/d=10 DLR B DME B 0 100 200 300 0 0.05 0.1 0.15 Surface Area (mm 2 ) Probability x/d=20 DLR B DME B 0 0.05 0.1 0.15 Probability x/d=7 DLR A DME A 0 0.05 0.1 0.15 Probability x/d=10 DLR A DME A 0 100 200 300 0 0.05 0.1 0.15 Surface Area (mm 2 ) Probability x/d=20 DLR A DME A 0 0.05 0.1 0.15 Probability x/d=7 DLR B DME B 0 0.05 0.1 0.15 Probability x/d=10 DLR B DME B 0 100 200 300 0 0.05 0.1 0.15 Surface Area (mm 2 ) Probability x/d=20 DLR B DME B 0 0.05 0.1 0.15 Probability x/d=7 DLR B DME B 0 0.05 0.1 0.15 Probability x/d=10 DLR B DME B 0 100 200 300 0 0.05 0.1 0.15 Surface Area (mm 2 ) Probability x/d=20 DLR B DME B
- 22. Turbulence and Combustion Research Laboratory OH Layer Surface Area (Constant Damköhler Number) No obvious differences 0 0.05 0.1 0.15 Probability x/d=7 DLR A DME B 0 0.05 0.1 0.15 Probability x/d=10 DLR A DME B 0 100 200 300 0 0.05 0.1 0.15 Surface Area (mm 2 ) Probability x/d=20 DLR A DME B 0 0.05 0.1 0.15 Probability x/d=7 DLR B DME C 0 0.05 0.1 0.15 Probability x/d=10 DLR B DME C 0 100 200 300 0 0.05 0.1 0.15 Surface Area (mm 2 ) Probability x/d=20 DLR B DME C 0 0.05 0.1 0.15 Probability x/d=7 DLR B DME B 0 0.05 0.1 0.15 Probability x/d=10 DLR B DME B 0 100 200 300 0 0.05 0.1 0.15 Surface Area (mm 2 ) Probability x/d=20 DLR B DME B
- 23. Turbulence and Combustion Research Laboratory OH Layer Holes (Constant Reynolds Number) For DME flame A, there is little probability of observing a OH discontinuity at x/d = 7 and 10; average “holes”/image ~2 for DLR flame A. For “A” flames there is an increasing probability of finding an OH layer hole with increasing axial distance; rate increases significantly for DLR flame A. For DLR B flame, the distribution of holes remains constant for increasing axial position; for DME B, the number increases with increasing axial position 0 0.25 0.5 0.75 1 x/d=7 Probability DLR A 0 0.25 0.5 0.75 1 x/d=7 Probability DME A 0 0.25 0.5 0.75 1 x/d=10 Probability 0 0.25 0.5 0.75 1 x/d=10 Probability 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 0.25 0.5 0.75 1 x/d=7 Probability DLR B 0 0.25 0.5 0.75 1 x/d=7 Probability DME B 0 0.25 0.5 0.75 1 x/d=10 Probability 0 0.25 0.5 0.75 1 x/d=10 Probability 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture
- 24. Turbulence and Combustion Research Laboratory OH Layer Holes (Constant Damköhler Number) At equivalent Damköhler numbers, DME flames differ significantly as compared to DLR flames despite the similarities in all previously-examined parameters The most probable number of OH layer holes of DME C is between 1 and 2 per image, while it is between 4 and 5 for DLR B flames DME C has increasing number of OH layer holes with increasing downstream position 0 0.25 0.5 0.75 1 x/d=7 Probability DLR A 0 0.25 0.5 0.75 1 x/d=7 Probability DME A 0 0.25 0.5 0.75 1 x/d=10 Probability 0 0.25 0.5 0.75 1 x/d=10 Probability 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 0.25 0.5 0.75 1 x/d=7 Probability DLR B 0 0.25 0.5 0.75 1 x/d=7 Probability DME B 0 0.25 0.5 0.75 1 x/d=10 Probability 0 0.25 0.5 0.75 1 x/d=10 Probability 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 0.25 0.5 0.75 1 x/d=7 Probability DLR B 0 0.25 0.5 0.75 1 x/d=7 Probability DME C 0 0.25 0.5 0.75 1 x/d=10 Probability 0 0.25 0.5 0.75 1 x/d=10 Probability 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture 0 5 10 0 0.25 0.5 0.75 1 x/d=20 Probability Number of Holes per Picture
- 25. Turbulence and Combustion Research Laboratory Comparison between Direct Numerical Simulation and Experiments of DME Flames
- 26. Turbulence and Combustion Research Laboratory Numerical and Experimental Flame Characteristics The work is a research collaboration with Sandia National Laboratories The goals is to achieve understanding of DME combustion at near-extinction conditions The DNS configuration is a 3D, non-premixed, temporally-evolving slot jet flame The experiment is a spatially- evolving, axisymmetric jet flame The fuel (12% DME, 18% H2 and 70% N2) and oxidizer streams (31% O2 and 69% N2) are matched between DNS and experiment Parameter DNS Expt Jet Reynolds number, Rejet 13050 13050 Jet Width, H (mm) 2.3 N/A Hydraulic diameter, D (mm) 4.1 3.43 Jet Velocity, ΔU (m/s) 98 57 Fuel core width, HZ (mm) 3.6 N/A Pressure, P (atm) 1.0 1.0 Damkohler number, Da 0.08 0.074 Stoichiometric mixture fraction, Zst 0.375 0.375 Unburnt temperature, Tu (K) 450 298 Burnt temperature, Tb (K) 2380 2299 Extinction dissipation rate, Χq (s-1) 1950 1210
- 27. Turbulence and Combustion Research Laboratory Parametric Study of Nozzle Geometry DDNS is the hydraulic dynamic used in the DNS studies, ν is the kinematic viscosity of the fuel, and T is the temperature of the fuel 13600 Vary tube thickness to find the optimized tube design Tube inner diameter is 3.43 mm with a wall thickness of 3.05 mm Achieved equivalent Re and Da
- 28. Turbulence and Combustion Research Laboratory Simultaneous OH and CH2O PLIF Experiment Layout
- 29. Turbulence and Combustion Research Laboratory Instantaneous realizations of simultaneous OH/CH2O measurements OH exists within high- temperature, “burning” regions CH2O exists in low-temperature regions The OH/CH2O region is a good surrogate for peak heat release rate The spatial locations were chosen to correspond to the three phases identified in the DNS, i.e. burning, partially-extinguished and reigniting. Qualitatively, these locations compare well with representative realizations from the DNS
- 30. Turbulence and Combustion Research Laboratory Joint PDF of OH and CH2O Two species that play a key role in the extinction re-ignition process are OH and CH2O Both joint PDFs exhibit the same functional form, i.e. there exists a strong anti-correlation between OH and CH2O OH is located primarily in burning regions, CH2O is found primarily in non- burning regions
- 31. Turbulence and Combustion Research Laboratory PDF of Alignment Index of CH2O and OH Alignment index: PDF peaks at −1 at all time instances in the DNS and spatial locations in the experiment, indicating that the gradients of OH and CH2O are opposed.
- 32. Turbulence and Combustion Research Laboratory Conclusions I Statistical descriptions of the OH layer thickness, OH layer curvature, OH layer orientation, OH layer surface area, and OH layer “holes” showed differences in the flame structure between the DLR and DME flames At the constant Reynolds numbers o The CH4-based DLR flames displayed higher probability of both thinner and thicker OH layer distribution, presumably due to higher levels of interaction with local turbulent flowfield o DLR flames display higher probability of larger curvature, surface area, OH layer orientation and number of OH layer holes per image o DME-based flames appear to be less affected by local turbulence than DLR flames
- 33. Turbulence and Combustion Research Laboratory Conclusions II At the constant Damköhler numbers o The DME and DLR flames exhibit very similar statistics in terms of OH layer thickness, OH layer curvature, OH layer orientation, and OH layer surface area o The DME flames displayed much less discontinuation and individual OH layer breaks than DLR flames even at near-extinction condition o The exact reason for the DME flames exhibiting more robustness (in terms of local extinction) is not known, but presumably is due to rich low-temperature chemistry and/or accelerated re-ignition processes in the DME flames as compared to the methane-based DLR flames.
- 34. Turbulence and Combustion Research Laboratory Conclusions III and Future Work Simultaneous OH and CH2O imaging displayed similar extinction and re-ignition phenomena as observed in the DNS Joint statistics of OH and CH2O exhibit a strong anti-correlation Probability density function (PDF) of the alignment index peaked at -1 at all time (DNS) and spatial (experiments) instances, indicating that the gradients of OH and CH2O are opposed. OH and CH2O PLIF images overlap only in very small spatial regions, presumably demarcating the regions between fuel oxidation and primary heat release. Future work includes simultaneous OH PLIF, CH2O PLIF and velocity measurements via PIV to investigate relationship between the local turbulent flow field (vorticity and strain rate) and the DME flame chemistry
- 35. Turbulence and Combustion Research Laboratory Acknowledgements Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research as well as the Combustion Energy Frontier Research Center funded by the US department of Energy, Office of Science.
Notas do Editor
- Good afternoon! My name is Han Shen. Today, my topic is about the characterization of the structure of turbulent nonpremixed dimethyl ether jet flame
- Here is an outline for my presentation. First of all, I will discuss the motivation of this work. Then I will talk about the first part of my thesis which is comarisons of CH4- and DME flame structure using equivalent flames. I will explain the definition and motivation for the development of equivalent flame later. The comparisons are conducted into two categories, first is the comparison of the CH4 and DME flames at constant Reynolds number and the second is the comparison at constant Damkohler numbers. The second part of my thesis is the comparison between direct numerical simulation and experiments of DME flames which was a part of a collaboration with Dr. Jackie Chen at Sandia National Laboratory. I will only present the experimental work which was done at OSU as well as comparisons to the DNS. At last, I will give some conclusions from the current work and recommendations for future studies.
- So the first question is obviously, why are we interested in DME. Well let me explain this a little bit. So far, researchers have found that (1,2) Nowadays, fossil fuel combustion on environmental impact has become a more and more serious topic, comparing with other fuels, 3 Back to laboratory, turbulent combustion which results in complex relationships between unsteady fluid dynamics, species transport, finite-rate chemical kinetics, and non-equilibrium thermodynamics over a broad range of spatial and temporal scales is known little. While each of these processes has received considerable attention over the last few decades, our knowledge of the effects of their complex coupling (even in laboratory-scale turbulent flames with simple fuels) remains incomplete.
- Previously, there has been a substantial effort on simple fuels such as methane to obtain detailed measurements such as velocity, temperature, and species concentrations. In recent years, there has been a push to extend the benchmark data sets to include larger and more complex fuel molecules such as DME. 4, 5, 6. Such reaction mechanisms are tractable for turbulent combustion simulations
- Here is the first part of my thesis, direct comparisons of CH4- and DME-based flame structure
- Starting with the well-known DLR jet flames, (22.1%CH4/33.2%H2, 44.7% N2), the methane was directly replaced by DME, resulting in a second flame system with composition of 22.1% DME, 33.2% H2, and 44.7% N2. It is noted that this fuel composition results in the same stoichiometric mixture fraction (s = 0.17) as the DLR flames and when operated at the same Reynolds numbers and damkohler number, the two flame configurations represent a unique set of target flames for “direct” comparisons and turbulence chemistry interaction investigations. This is what “equivalent” flames means Here is the table of five different flame cases, we can see that for the constant Reynolds number comparison, DLR A/DME A and DLR B/DME B form two pairs for comparison, while for the constant Damköhler number comparison, DLR A/DME B and DLR B/DME C form two pairs for comparison. In our lab, 2
- In a previous study by our research group, we reported laminar flame calculations, flame blow out conditions, CH2O PLIF and Mie scattering imaging The laminar flame calculations showed significant differences between the two flame systems in terms of fuel decomposition and consumption reactions including very rapid pyrolysis of DME which led to large levels of CH2O and H2 under richer mixture fraction conditions. Flame blow out studies showed that the DME flames were more robust under high Reynolds number conditions and blew out at Reynolds numbers that were approximately 40% higher than the DLR flames. CH2O PLIF results showed that DME-based flames yielded CH2O signals that were approximately two orders of magnitude higher than the corresponding DLR flames. Furthermore, it was shown that CH2O was consumed more rapidly within the lower-temperature regions in the DME flames indicating more active low-temperature oxidation processes than the DLR flame.
- In today’s presentation, I mainly focus on the OH PLIF imaging results from the two flame configurations to examine differences in the turbulent flame structure. OH regions in the high-strain rate, nearfield of non-premixed flames provide a suitable marker for the high-temperature reaction zone. In this manner, the instantaneous OH PLIF images provide a means for statistical analysis of the effects of turbulence on flame structure including flame (OH layer) curvature, OH layer thickness, OH layer orientation, OH layer surface area and local flame (OH layer) extinction. This is my experimental setup. (talk about the setup) The Nd:Yag laser at 532nm was used to pump the dye laser which emits laser at 566nm. After the doubling crystal, 283nm wavelength laser which has about 2 mj/pulse energy was first expanded and then formed into a sheet. The OH emission was detected by a lens-coupled ICCD camera system. The field of view was 42mm * 56 mm, and ICCD gate was 100 ns
- 1,2 An average background image was first removed from the images, followed by a correction for spatial non-uniformities in intensity of the laser sheet. 3,4 (First, the OH PLIF images were converted to a binary image (0 or 1) by defining a threshold as 10% of the peak signal intensity. In this manner, all pixels with counts less than 10% of the peak intensity were defined as zero and all other pixels were defined as one. Next, the edges of OH layer were extracted for calculating curvature and OH layer thickness. ) The local curvature of the OH layer was calculated by fitting a second order polynomial function to twenty sequential points. The OH layer thickness was calculated from the two outside contours that defined the local OH layer. A local normal vector was determined at each point along the OH layer, and the distance between the projection points along the normal vector to the opposite side of the OH layer contours were defined as the local OH layer thickness. The orientation is defined as the angle between the local slope of the point on the OH layer edge and the horizontal axis. For example, the point of interest is defined as the origin; the angle is defined as 0° to 180° from left to right. The OH layer surface area is a parameter that is complementary to the previous statistics involving OH layer thickness and curvature. It describes the net effect of flame thinning or thickening and wrinkling due to turbulence. In this work, the surface area is calculated as OH layer surface area in the physical domain in unit of mm2 In order to estimate local extinction effects in each flame, the number of two-dimensional OH layer holes were determined in each image and analyzed statistically. A “hole” was defined as a region where two continuous OH layers were completely disconnected from one another
- Here is some example OH plif images at constant damkohler number at about 0.04, I put the DME B flame there just for comparisons to see the differences at constant reynolds number and constant damkohler number. 1,2 Even though, DLR B and DME C are at the same percentage of the blowout reynolds number which is 96% and at the equibalent damkohler number, dme flames appear less affected by turbulence
- This figure shows the mean and RMS OH PLIF intensity profiles for both DLR and DME flames B at x/d = 7, 10 and 20. For the mean profiles, all results have been normalized by peak intensity of the mean profile for DME flame. The RMS intensities have been normalized by the peak of the mean profiles for each flame, respectively. First, it is noted that the peak mean intensity of OH is significantly higher in the DME flames as compared to the DLR flames. For example, the peak of the OH profiles for the DLR flames are approximately 70% of the peak OH profiles for the DME flames at x/d = 7. however, it is noted that peak values in the instantaneous images were quite similar between the two flame configurations. This indicates that the DLR flames are more subject to spatial fluctuation due to more intense interactions with the local turbulent flow field. This is consistent with the “broader” mean OH profiles of the DLR flames and the higher relative RMS fluctuations (Bottom paragraph)
- Damkohler number 0.04
- Here is the mean and RMS profiles for DLR and DME flames at constant damkohler number of 0.06. 1,Unlike the DLR B flame in comparison to the DME C flame, the mean OH intensity of DLR A increases relative to that of the DME B flame with increasing axial position. 2, In addition, the OH mean intensity profile is not broader for the DLR A flame in comparison to the DME B flame, whereas the DLR A flame OH intensity distribution was broader as compared to the DME A flame. 3, For the OH PLIF RMS profiles, the DLR A and DME B flames are nearly identical which differs from the constant Reynolds number comparison between DLR A and DME A and the constant Damköhler number comparison between DLR B and DME C.
- The OH layer thickness was calculated from the two outside contours that defined the local OH layer. A local normal vector was determined at each point along the OH layer, and the distance between the projection points along the normal vector to the opposite side of the OH layer contours were defined as the local OH layer thickness. Fro, the image we see that the DLR flames exhibit a higher probability of smaller OH layer thicknesses (< 0.4 mm) and a longer exponential tail that decays slower than that of the DME flames. Both of these factors are quite indicative of the larger influence of the flow field turbulence on the OH layer structure in the DLR flames as compared to the DME flames. The higher probability of “thin” OH layer thicknesses in the DLR flames indicates the presence of larger eddies interacting with the OH layers, acting to “stretch” and thin the layers. This is consistent with the instantaneous images, which show higher levels of wrinkling in the DLR flames The higher probability of “thick” OH layers in the DLR flames (as compared to the DME flames) indicates the interaction of smaller eddies that act to enhance local mixing to “thicken” the observed OH layers (an indication of high-temperature regions) or may indicate some probability of small eddies penetrating the OH zone to directly thicken them.
- For the same damkohler number comparison, we see that 1 2
- The local curvature of the OH layer was calculated by fitting a second order polynomial function to twenty sequential points. After we know the function, then it is easy to calculate the curvature. Consistent with the previous results, DME flame A has a higher probability of small absolute curvatures (near zero) and a narrower distribution of curvature as compared to the DLR flame A results. 2,3,4
- The orientation is defined as the angle between the local slope of the point on the OH layer edge and the horizontal axis. For example, the point of interest is defined as the origin; the angle is defined as 0° to 180° from left to right. From the plot, we see that both DLR and DME flames’ OH layer Orientation 1
- The OH layer surface area is a parameter that is complementary to the previous statistics involving OH layer thickness and curvature. It describes the net effect of flame thinning or thickening and wrinkling due to turbulence. In this work, the surface area is calculated as OH layer surface area in the physical domain in unit of mm2
- In order to estimate local extinction effects in each flame, the number of two-dimensional OH layer holes were determined in each image and analyzed statistically. A “hole” was defined as a region where two continuous OH layers were completely disconnected from one another
- Considering the results shown here, it is clear that the turbulence-chemistry interaction is different for the two flame systems. . This is particular true when comparing the DLR B and DME C cases, which are both at 96% of the blowout velocity (or Reynolds number). Even at this conditions, the DME C flame exhibits little indication of local extinction events while the DLR B flame exhibits a much higher degree of local flame extinction. The reason for this behavior still remains unknown, but there are two probable explanations. First, it is noted that the DME flames have more active low-temperature chemistry. For this reason, the pyrolysis and low-temperature oxidation of DME forms more H2 and CO at high mixture fractions that enhances the flame and makes the flame very robust. The statistical results also could be explained by re-ignition mechanisms. Since the OH PLIF measurements are acquired at 10Hz, dynamic phenomena such as re-ignition cannot be captured. However, one possible mechanism is that the when the DME flames are subjected to equivalent turbulence levels as the DLR flames, a similar number of OH layer holes form, but re-ignition is much more efficient due to the larger CH2O and CO mole fractions which can react with the higher levels of H2 and H. These two explanations are purely assertions at this point as further investigations are necessary to understand the physical mechanisms behind this.
- Here is the second part of my thesis, which is…..
- This is a research collaboration with Sandia National Lab, they perform the numerical calculation while I did the experimental work. Today I will only show the experimental work result and the comparison between numerical and experimental works. 2; The focus of the current collaborative effort is a comparison of scalar statistics between simulation and experiment under similar operating conditions. 4; 3; While the specific DNS and experimental configurations are different , the jet Reynolds and Damköhler numbers of the experiment were designed to match the DNS as shown in Table, Matching across experiment and DNS is critical and was achieved by selecting a particular nozzle diameter such that these criteria were satisfied. To make sure that DNS and Experiment flame are under the similar operating conditions, reynolds number ,damkohler number, flame stochimetric mixture fraction need to be the same, which leaves us jet nozzle diameter the only variable to be played with
- To find the right size tube diameter, we calculated it by this equation 1; Using a small diameter nozzle along with a heavily-diluted fuel leads to blowout at lower Reynolds numbers, thus a blowout study was performed to find the optimized tube design by varying the tube wall thickness. The “optimized” designed was determined as the smallest wall thickness that could support a flame with Reynolds number equal to 13050. Thicker-walled tubing leads to flame stabilization via re-circulation on the rim, which changes the physics as compared to the DNS. It is clear that increasing tip thickness increases the flame stability and results in an increase in the blowout Reynolds number
- Talk about experimental setup. The upper portion is the same as the OH plif experiment in the first part of my thesis. The lower portion is the CH2O imaging system. A seeded nd:yag laser emits at 355nm, after a set of sheet forming optics, it combined with the 283nm laser sheet. The CH2O signal was acquired by a CCD camera with proper filter and laser timing to block the OH signal.
- Yellow is CH2O signals, red is OH signal, while the green is their overlap. 1,2,3,4,5
- 1 Here is a log plot of the joint PDF of OH and CH2O derived from their respective mole fractions in the simulation and the signal intensities observed in the experiment. The PDF is conditioned such that only points in the overlap region are considered, since this is the region of interest from the point of view of predicting heat release. This was taken to correspond to values between 1% and 50% of the maximum respective signal intensity for the experiment and for the DNS. It should be noted that the joint PDFs should not be expected to be identical due to the different geometries of the DNS and experiment. From the plot, we see that 2
- To further quantify the degree of anti-correlation between OH and CH2O species, an alignment index is defined between gradients of OH and CH2O as 1 The pdf of this alignment index for different time in DNS and different axial locations in experiment is shown here. It is clear that 2
- Here I would like to thank donors …..