1. Effects of Radiation on the flame front of hydrogen air explosions
ES - Energetic Systems
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. Eisenreich
Fraunhofer-Institut fuer Chemische Technologie (ICT), Pfinztal, Germany,

2. Unconfined Hydrogen/Air Gas Cloud Explosions
Overpressures and flame speed increase on cloud size
Turbulence induced by the expanding fire ball
Flame instabilities, cellular structure of flame front
but
> Flame acceleration not fully explained
> Mechanisms not fully understood

3. Unconfined Hydrogen/Air Gas Cloud Explosions
Radiation of H2-Combustion
broad water bands in IR and NIR Range
OH-Radicals in the UV-Range
Often understimated due to nearly non-visible flame
Realistic industrial accidents
accompanied by entrainment of contaminants inorganic and organic substances
mainly additional OH-Radicals in reaction zone

4. Unconfined Hydrogen/Air Gas Cloud Explosions
Approach:
Radiation from hot flame ball has to pass reaction zone
Absorption of radiation by flame front species
Turbulence could not be detected in earlier experiment by BOS imaging
Investigation of effects by image processing and spectroscopy

5. Experimental Setup
0.5m³ premixed stoichiometric hydrogen / air mixture
in 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 Data
transmitted light (shadow images) from spot (8 x 5cm) at fps
Spot shifted along the flame propagation for different experiments
0.625ms ms ms ms
Flame propagation in extracted section of 55 x 49mm 20mm below ignition
Frames reflected to be compared to the data evaluation

10. Image Processing of Frame at 1.56ms
Spot evaluated roughly in direction of flame propagation
A B C D
A original B edge detection C image brightness subtraction of subsequent frames D binarized

11. Flame Structure
Contour plot A and inserts of circles B to mark flame front and salients.
The salients to be sections of spheres 15mm radius / 3 mm heigth in average at overall fire ball radius of 50mm
> effective flame front area increased by10%

12. Flame Velocity Evaluation
Compacted images of the flame sections and straight lines to mark structures at the flame front:
𝑻 𝒌,𝒋 = 𝒊=𝟏 𝑵𝑹 𝒈 𝒌,𝒊,𝒋
from B (A) and D (B) from two slides above
A B
sections of 48mm width were included into analysis
resulting in a straight line (constant velocity)
A: y(x) = x y1(x) = x
B: y(x) = x y1(x) = x
Further evaluations gave results up to 25m/s at higher distances.

13. Radiation of water fireball produced by H2 combustion
increasing fire ball radius  radiation intensity increases strongly
Depending 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 SpectroscopyUV
fast 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 sufficient
NIR
a 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-Spectroscopy
rotational OH band from the 0-0 electronic/vibration transition at nm band 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 Spectroscopy
NIR 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. Conclusions
stoichiometric 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. Conclusions
The temperature obtained from the water bands within the spectral interval of 1.2 and 2.2µm are more realistic in the range of 2150K K.
 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
Thank you for your attention !
ES - Energetic Systems
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. Eisenreich
Fraunhofer-Institut fuer Chemische Technologie (ICT), Pfinztal, Germany,