Keßler A C Wassmer S Knapp V Weiser K Sachsenheimer A Raab G Langer N Eisenreich FraunhoferInstitut fuer Chemische Technologie ICT Pfinztal Germany ID: 1022392
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1. Effects of Radiation on the flame front of hydrogen air explosions Keßler A., C. Wassmer, S. Knapp, V. Weiser, K. Sachsenheimer, A. Raab, G. Langer, N. EisenreichFraunhofer-Institut fuer Chemische Technologie (ICT), Pfinztal, Germany, armin.kessler@ict.fraunhofer.deES - Energetic Systems
2. Unconfined Hydrogen/Air Gas Cloud ExplosionsOverpressures and flame speed increase on cloud sizeTurbulence induced by the expanding fire ballFlame instabilities, cellular structure of flame frontbut> Flame acceleration not fully explained> Mechanisms not fully understood
3. Unconfined Hydrogen/Air Gas Cloud ExplosionsRadiation of H2-Combustionbroad water bands in IR and NIR RangeOH-Radicals in the UV-Range Often understimated due to nearly non-visible flameRealistic industrial accidentsaccompanied by entrainment of contaminants inorganic and organic substancesmainly additional OH-Radicals in reaction zone
4. Unconfined Hydrogen/Air Gas Cloud ExplosionsApproach:Radiation from hot flame ball has to pass reaction zoneAbsorption of radiation by flame front speciesTurbulence could not be detected in earlier experiment by BOS imagingInvestigation of effects by image processing and spectroscopy
5. Experimental Setup0.5m³ premixed stoichiometric hydrogen / air mixturein plastics balloon ~1 m diameter.peeling of the plastic skin immediately before ignition (10ms delay)
6. Experiments – ballon rupture
7. Experiments – sparc ignited
8. Experiments – Image Sequence
9. Highspeed Camera Datatransmitted light (shadow images) from spot (8 x 5cm) at 16.000fps Spot shifted along the flame propagation for different experiments 0.625ms 0.94ms 1.25ms 1.56msFlame propagation in extracted section of 55 x 49mm20mm below ignitionFrames reflected to be compared to the data evaluation
10. Spot evaluated roughly in direction of flame propagation A B C DA originalB edge detectionC image brightness subtraction of subsequent framesD binarizedImage Processing of Frame at 1.56ms
11. Flame StructureContour plot A and inserts of circles B to mark flame front and salients.The salients to be sections of spheres15mm radius / 3 mm heigth in average at overall fire ball radius of 50mm> effective flame front area increased by10%
12. Flame Velocity Evaluationsections of 48mm width were included into analysisresulting in a straight line (constant velocity)A: y(x) = 23.2 + 16.5 x y1(x) = 21.75 + 15 xB: y(x) = 2.8 + 24.5 x y1(x) = -1.5 + 24.75 x Further evaluations gave results up to 25m/s at higher distances.Compacted images of the flame sections and straight lines to mark structures at the flame front: from B (A) and D (B) from two slides above A B
13. Radiation of water fireball produced by H2 combustionincreasing fire ball radius radiation intensity increases stronglyDepending on size emission bands overlap with absorption bands of reacting species in flame front.water bands close to maximum emission at relevant temperatures (1.3 and 1.9µm) overlap with bands from e.g. OH, HO2, H2O2 etc. -> Heat exchange
14. Time Resolved SpectroscopyUVfast scanning UV-spectrometer (Andor Newton) to analyse the OH band in the spectral range of 280 to 325 nm Andor SR500i UV/VIS-Spektrometer is an Imaging-Spektrometer 5ms/Spectrum is achieved with a vertical read-out of 12.9µs-> time resolution not sufficientNIRa NIR spectrometer (Avantes) to observe water bands between 1µm and 2.3 µm with time resolution of 2,000 spectra per second and data transfer in 1.0 ms -> adequate to resolve the emission.
15. UV-Spectroscopyrotational OH band from the 0-0 electronic/vibration transition at 306 - 315nmband tends to self-absorption which is difficult to take into account but relative comparison might work:> Temperatures seem to be higher for a distance of 15cm than those found at shorter distances Example of an OH 0-0 band and its fit (left) and the resulting temperatures (right), recorded at 100/s.(v11, v12 are 8cm and v15 and v17 are 23cm from ignition)
16. NIR SpectroscopyNIR spectra recorded with time intervals of 0.57ms and an exposure time 0.03ms more appropriate to the flame front resolution in time. The explosion emits no continuum radiation at all.strong bands occur in the NIR, using the evaluation by the BaM code of ICT An example of the NIR water bands above 1.2µm and its fit (left) > resulting temperature (right) between 2150 and 2250K.> Tendency to higher Temperatures for higher distances as well
17. Conclusionsstoichiometric hydrogen air explosions up to diameters of 1m show a cellular structure at the flame front,generated by emerging salients. Shape and size are similar for all experiments. A flame area increase by 1.1 is found.Time resolved temperature measurement gives values of over 2500K for the OH rotational bands,often found for OH bands in a reaction zone.
18. ConclusionsThe temperature obtained from the water bands within the spectral interval of 1.2 and 2.2µm are more realistic in the range of 2150K - 2250K. Although some evidence supports the suggestion of higher temperature depending on the distance from the point of ignition, further experiments will be required to verify this hypothesis in a convincing way.
19. Effects of Radiation on the flame front of hydrogen air explosions Keßler A., C. Wassmer, S. Knapp, V. Weiser, K. Sachsenheimer, A. Raab, G. Langer, N. EisenreichFraunhofer-Institut fuer Chemische Technologie (ICT), Pfinztal, Germany, armin.kessler@ict.fraunhofer.deES - Energetic SystemsThank you for your attention !