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

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015

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), 19-21 October 2015