1. Analysis of acoustic pressure oscillation during vented deflagrations
Martino Schiavetti, Marco Carcassi
Department of Civil and Industrial Engineering (DICI)
University of Pisa

2. Presentation overview:
Analysis of acoustic pressure oscillation during vented deflagrations
Presentation overview:
CVE Test facility
Experimental campaigns
Presentation of a model for the generation
of the acoustic oscillation
Data analysis in confirmation of the
proposed model
Possible explanation of the flame-acoustic
interaction method
Conclusions
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

3. CVE Test Facility (Chambre View Explosion)
Analysis of acoustic pressure oscillation during vented deflagrations
CVE Test Facility (Chambre View Explosion)
Design pressure 35 kPa
Internal volume ~25 m3
Vent area ~1 m2
Vent opening pressure ~ 2.4 kPa
CVE Vent
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

4. Experimental campaigns (Obstacles configurations)
Analysis of acoustic pressure oscillation during vented deflagrations
Experimental campaigns (Obstacles configurations)
Empty 2 4 2 3 4 5 6 7 8 7
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

5. Experimental campaigns (Most interesting test configuration)
Analysis of acoustic pressure oscillation during vented deflagrations
Experimental campaigns (Most interesting test configuration)
Lean hydrogen concentrations 6-13% vol.
One pressure transducer inside the CVE test facility
One pressure transducer inside the RED_CVE box
RED_CVE box Internal volume 0,6814 m3
Three different vent dimensions
were investigated for the internal box
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
Homogeneous concentrations
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

6. Phenomena generated by the flame reaching the vent area
Analysis of acoustic pressure oscillation during vented deflagrations
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), October 2015

7. Phenomena generated by the flame reaching the vent area
Analysis of acoustic pressure oscillation during vented deflagrations
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 production and expansion of burning products “pushes” the flame front towards the unburned mixture stabilizing the flame
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), October 2015

8. Phenomena generated by the flame reaching the vent area
Analysis of acoustic pressure oscillation during vented deflagrations
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
The initial amplitude of the acoustic oscillation depend on
The net overpressure inside the facility when the flame reaches the vent (P2 Peak)
The initial frequency of the acoustic oscillation depend on
H2 concentration (concentration in the unburned mixture as well as affecting the temperature of the combustion product
Flame position with respect to the walls
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

9. Ignition opposite the vent area (Ignition 1)
Analysis of acoustic pressure oscillation during vented deflagrations
Ignition opposite the vent area (Ignition 1)
The pressure transducer place on the lateral wall records pressure oscillation earlier compared to the one placed opposite the vent area
Pressure time history inside the CVE test facility
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

10. Ignition inside the box (Ignition 2)
Analysis of acoustic pressure oscillation during vented deflagrations
Ignition inside the box (Ignition 2)
f = 320 –> 390 Hz
As soon as the flame front reached the vent in the small box the oscillations increased their amplitude meaning that the interaction between the acoustic waves and the flame front started to take place
At low frequency Helmholtz oscillation other were superimposed, which frequency could be related to the resonant mode of the enclosure, these oscillations tended to be dumped out and did not really interact with the flame front in the early stage of the deflagration
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

11. Ignition inside the box (Ignition 2)
Analysis of acoustic pressure oscillation during vented deflagrations
Ignition inside the box (Ignition 2)
Detailed analysis shows that subsequent peaks are reached at subsequent time steps
(frequency
Is increasing)
Higher H2 conc. – Higher sound speed – higher frequencies
Discrete Fourier transform of the overall signal
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

12. Approximate estimation of the acoustic response of the chambers
Analysis of acoustic pressure oscillation during vented deflagrations
Approximate estimation of the acoustic response of the chambers
Estimating the sound speed using a generic method for calculating the properties of combustion products
Acoustic modes of the RED-CVE box
Acoustic modes of the CVE test facility
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

13. Analysis of acoustic pressure oscillation during vented deflagrations
Pressure drop corresponds to a change in the balance between combustion products generated inside and exiting through the vent that provoke the resonant response of the chamber
Flame front outside the box, pressure time history In the box follows the behavior attained in the CVE facility to which Helmholtz oscillation are superimposed
Flame front already affected by pressure oscillation when entering the box – frequency and amplitude increasing while progressing inside
9.5%
10.1%
Frequency of the oscillation abruptly increase to values characteristic of the resonant response of the smaller chamber
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

14. Analysis of acoustic pressure oscillation during vented deflagrations
Ignition inside the CVE test facility in front of the vent (Ignition 3)
Frequency analysis of the overpressure inside the CVE Test facility
Discrete Fourier transform of the overall signal
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

15. Ignition inside RED_CVE box (Ignition 2) RED-CVE vent dimensions
Analysis of acoustic pressure oscillation during vented deflagrations
Ignition inside RED_CVE box (Ignition 2)
H2 Concentration 11.8%
Results for three different vent dimensions
RED-CVE vent dimensions
Area [m2]
Vent 1
0,027675
Vent 2
0,042025
Vent 3
0,05535
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

16. Possible explanation of the flame acoustic interaction method
Analysis of acoustic pressure oscillation during vented deflagrations
Possible explanation of the flame acoustic interaction method
The mechanism of interaction between the flame front and the acoustic oscillation may be due to the acoustic accelerations
The flame front react to the imposed acceleration field as well as it does to gravity
Burned gases
ρb < ρu
ab > au
Unburned gases
The flame front can be seen as an interface separating two fluids, when the oscillating acceleration is directed towards the unburnt gases the amplitude of the reaction zone will tend to increase, while in the opposite case will tend to decrease
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

17. Analysis of acoustic pressure oscillation during vented deflagrations
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%, for tests involving a second vented volume inside the first 3 ignition position were investigated, far vent ignition, ignition inside the second volume and ignition close to the vent of the main facility.
Results for tests with ignition located on the wall opposite the vent showed that the interaction between the flame front and the acoustic oscillation were prompted after the flame reached the vent area
The abrupt increase of discharge rate when the burned products reached the vent area may be claim to be responsible of triggering the acoustic response of the chamber
At the same time the flow of the burned products out of the vent area leaves the flame more prone to be affected by the acoustic perturbation
The mechanism of interaction between the acoustic oscillation and the flame front may be due to the acoustic acceleration
In the bigger environment the frequency analysis of the acquired signal showed much more “noise” if compare with the case were the smaller volume was under investigation
The vent opening can also trigger the resonant response of the chamber, nevertheless in case of low strength vent the generated acoustic acceleration are weak and the flame is still expanding far from the vent area accelerated towards the unburned gases by the expansion of the combustion products
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015

18. TANK YOU FOR YOUR ATTENTION
Analysis of acoustic pressure oscillation during vented deflagrations
TANK YOU FOR YOUR ATTENTION
International Conference on Hydrogen Safety, Yokohama (Japan), October 2015