Martino Schiavetti , Marco Carcassi

1. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagrations Experimental results Martino Schiavetti, Marco Carcassi Department of Civil and Industrial Engineering (DICI) University of Pisa

2. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Presentation overview: • CVE Test facility • Major peaks in a vented deflagration • Experimental campaigns • Results (data analysis) • Presentation of a model for the generation of the acoustic oscillation • Conclusions International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

3. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results CVE Test Facility (Chambre View Explosion) Design pressure 35 kPa Internal volume ~25 m3 Vent area ~1 m2 Vent opening pressure 1.5-2.5 kPa CVE Vent International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

4. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Major peaks attained during vented deflagrations Overpressure’s peak overview P1: Pressure peak associated with the pressure drop following the removal of the vent panel and subsequent venting of the unburned gases P2: Occurs when the flame front reaches the vent area (external explosion?) When the burned gases start to flow out of the vent opening, their density being the density of the unburned gases divided by a factor E, the outflow rate suddenly increase by a factor of E1/2 P3: Caused by the decrease of combustion products generation following the reduction of the flame front area when it reaches the walls (In the test performed in the CVE facility this peak has been observed only for H2 concentration lower than 9%vol.) P4: Oscillatory pressure peak attributed to the coupling of the pressure waves generated by the acoustic resonances within the room with the flame front International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

5. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Experimental campaigns (Test configurations) Lean hydrogen concentrations 6-13% vol. Homogeneous concentrations Despite of the use of the fans the concentrations measured in the upper and lower sampling points differ approximately of 1,5% vol. in every test Mixture ignited in the center line of the wall opposite the vent at 1m height from the floor International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

6. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Experimental campaigns (Obstacles configurations) 2 Empty 4 2 4 3 6 5 8 7 7 International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

7. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Experimental campaigns (Number of tests analyzed ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

8. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Results (Maximum overpressure vs. conc. for all the tests) R.K. Kumar Vented combustion of hydrogen-air mixtures In a large rectangular volume Maximum peak overpressure for concentrations below 10% is limited by the vent opening pressure irrespective of the obstacle configuration peak overpressure at 11.2% higher than the one at 12% International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

9. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Results (Overpressure time history at different concentrations) The flow of the burned mixture towards the vent produces the slight depression measured by the transducer placed on the wall opposite the vent area. The slight difference between the two pressure transducer, some kPa, has been observed at the end of the explosion in every test. For tests performed at higher concentration, where pressure oscillation are involved, only one pressure reading is reported to provide a more clear view. After the flame front reaches the vent, in the rest of the volume the flame area continues to increase for a period of time due to the low burning velocity of the mixture in direction perpendicular to the vent, the reduction of the flame front area towards the end of the explosion, generates the third peak (P3), caused by the decrease of the combustion products generation. When the flame front is not close to the vent area during the deployment, Helmholtz oscillation had been observed after the vent deployment, oscillations that eventually decay as the flame continues to expand. International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

10. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Results (Overpressure time history at different concentrations) Flame-acoustic interaction is higher when the flame approaches the walls (specially for lean H2 mixtures) The volumetric flow rate through the vent area has a sudden increase when the unburned gases start to flow out of the enclosure, this discontinuity may be responsible of generating the physical response of the flammable envelope that changes the burning behavior of the expanding flame front making it more susceptible to be influenced by acoustic oscillations generated inside the vented volume. Sui International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

11. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Results (Overpressure time history at different concentrations) N.B. Pictures are not in the same scale! From the literature: higher flame acoustic interaction when the flame is closer to the walls At increasing concentration the two peaks are closer one another since they overlap at concentration between 11 and 11.5%. International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

12. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Phenomena generated by the flame reaching the vent area The increasing of the flow out of the vent area is responsible for the production of the second peak (lean hydrogen deflagration) Harris formula The flow of the burned gases out of the vent generates an abrupt change in the burning behavior of the flame The abrupt increase of the flow out of the vent area is responsible for the discontinuity that generates acoustic oscillation International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

13. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Phenomena generated by the flame reaching the vent area The flow of the burned gases out of the vent generates an abrupt change in the burning behavior of the flame Burning velocity affected by the expansion ratio (E) which provide a constant acceleration in direction perpendicular to the flame front – the acceleration of the flame front towards the vent is enhanced by the flow field generated during the deflagration Burning velocity drops back to values close to the laminar burning velocity, not being any more affected by the acceleration of the expanding burned products the flame front becomes more susceptible to the oscillating acceleration of the acoustic response International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

14. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Phenomena generated by the flame reaching the vent area The abrupt increase of flow out of the vent area generates the acoustic response of the chamber (ignition opposite the vent) >12% The flame reaches the vent when the flame already touched the walls in direction perpendicular to the venting flow 11 % >H2 conc.>11.5% The flame reaches the vent when the flame is close to the walls International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

15. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Frequency analysis of the pressure oscillation (Fourier discrete transform applied to the overall signal) Frequency interval that can be related to the acoustic response of the chamber International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

16. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Results (Maximum overpressure vs. concentrations all the tests) In configuration 8 the behavior in the range of concentration under investigation is monotonic because the flame, accelerated by the second set of obstacles reaches the vent earlier In some obstacle configuration the presence of the obstacles seemed to limit the effect of the flame acoustic interaction International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

17. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results Conclusions • Vented deflagration were performed in a 25 m3 volume facility, having an approximate vent area 1 m2, with a homogeneous hydrogen concentration ranging from 6% to 13%. • The maximum overpressure vs. concentration results graph showed a non-monotonic behavior with a peak at 11% concentration. • Comparison between the pressure time history of the tests showed that between 11% and 11.5% average H2 concentrations the two peaks P2 and P4 as described in the introduction were overlapping • While at lower concentration the flame is still far from the walls when the flame front reaches the vent area, at 11.3% vol. the flame surface very close to the walls at the same time that the flame front reached the vent area, while at 12% the flame already touched the walls when the flame front reached the vent area • The Fourier transform of the pressure time history at 11.3% shows strong pressure oscillation at frequency characteristic of the acoustic response of the chamber, while pressure time history at average concentration higher than 12% do not supporting the previous statement • The flame front reaching the vent area has been claimed to be responsible of making the flame front more susceptible to be affected by acoustic oscillation and at the same time of provoking the acoustic response of the chamber International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015

18. Non-monotonic overpressure vs. H2 concentration behavior during vented deflagration. Experimental results TANK YOU FOR YOUR ATTENTION International Conference on Hydrogen Safety, Yokohama (Japan), 19-21 October 2015