EXPLOSION HAZARD OF HYDROGEN-AIR MIXTURES IN THE LARGE VOLUMES

1. JIHT of RAS EXPLOSIONHAZARD OF HYDROGEN-AIR MIXTURES IN THE LARGE VOLUMES V.A. Petukhov, I.M. Naboko, and V.E. Fortov Joint Institute for High Temperatures of Russian Academy of Sciences, Institute for High Energy Densities Izhorskaya 13/19, Moscow, 127512, Russia petukhov@ihed.ras.ru

2. JIHT of RAS • Intention - investigation of non-stationary combustion of hydrogen-air mixtures that is important to the problems of safety • The conditions of occurrence and development of non-stationary combustion of hydrogen-air mixtures were studied: • in the tubes • in the conic element • in the spherical 12-m diameter chamber • On formation of non-stationary combustion in conic element the pressure in the cone top can reach 1000 atm • Investigations showed that in large closed volumes the non-stationary combustion of hydrogen-air mixtures can develop from a small energy source and pressure can exceed the Chapman-Jouguet pressure

3. JIHT of RAS Scheme of experimental conic setup Total volume of experimental setup is 190 l. Along the cone generatrix, 5 pressure sensors (1 – 5; M113A33 PCB) are located and one more (6; M109B11 PCB) is placed at the cone top. Window-slot for high-speed photography is located along the generatrix. The mixture was kept for about an hour. Its composition was monitored by gas analyzerIVA-IV (Chimavtomatika, Russia). The mixture initiation was caused by blasting a RDX charge with the mass from 0.4 to 4.2 g. The charge was blasted by exploding copper wire 0.1mm in diameter.

4. JIHT of RAS Experimental conic setup Metal cone The processes of combustion investigated on the installation can be conventionally divided into 3 types depending on initiation energy and mixture composition Window-slot Rubber envelope

5. JIHT of RAS Typical of the first regime results for combustion of stoichiometric H2+air mixture initiated by 3.5 g RDX Streak photograph of combustion process in the Cone Explosion luminescence in the cone top The front of primary combustion comes to the cone top after the explosion and registration of the maximum pressure in the cone top. The front of secondary combustion interacts with the front of primary combustion near the cone top. direction along the axis of the cone Front of primary combustion Initiation of combustion time

6. JIHT of RAS Typical of the first regime results for combustion of stoichiometric H2+air mixture initiated by 3.5 g RDX Maximum pressure at the cone top (P) and the time lapse between moments of initiation and registration of maximum pressure (t) The values of pressure in the cone top exceeding 1000 atmospheres result from the explosion caused by the cumulative collapse of disturbances, which were spread before the front of the primary combustion.

7. JIHT of RAS Typical of the second regime results for combustion of H2+air mixture in the Cone With weaker initiation (under 2 g of RDX for stoichiometric H2+air mixture ) as well as for poorer mixtures the second regime is realized. In this regime, ignition takes place in the cone top, and the secondary flame front moves downwards. The interaction with the primary front happens at the cone base. Streak photograph of combustion direction along the axis of the cone Initiation of combustion time Front of primary combustion Front of secondary combustion

8. JIHT of RAS Maximum pressure at the cone top (P) and the time lapse between the moment of initiation and the moment of registration of maximum pressure (t) Typical of the second regime results for combustion of H2+air mixture in the Cone This regime is characterized by smaller values of pressure and slower process developmentas compared with the first regime

9. JIHT of RAS direction along the axis of the cone time Initiation of combustion Front of primarycombustion Typical of the third regime results for combustion of H2+air mixture in the Cone With even weaker initiation and poorer mixtures the third regime takes place. In this regime no ignition in the focusing area is registered, no explosion takes place and no luminescence occurs in the cone top at the maximum pressures. Streak photograph of combustion

10. JIHT of RAS Maximum pressure at the cone top (P) and the time lapse between the moment of initiation and the moment of registration of maximum pressure (t) Typical of the third regime results for combustion of H2+air mixture in the Cone Maximum pressure values in cone top are 5-10 times higher than the pressure of focusing in inert gas at the same charge of initiation

11. JIHT of RAS 3.0 2.0 1.0 Maximum pressure recorded in the cone top in relation to the concentration of hydrogen and mass of initiating RDX charge 1.5 The results for all range of the investigated parameters were processed by the least-squares method

12. JIHT of RAS Explosive chamber Inside diameter– 12 m; wall thickness– 100 mm; weight – 470 t Chamber is designed for explosion up to 1000 kgTNT and has been tested for static pressure of 150 atm

13. JIHT of RAS Experimental scheme upper manhole kI-kIV – crusher membrane transducers i1 – i4 – ionization sensors lattice made from wood for turbulization of gas streams bottom manhole all dimensions are in mm

14. JIHT of RAS The goal of the experiment was to obtain the maximum possible values of pressure in large closed volumes at combustion of hydrogen-air mixtures with weak initiation. 29% H2 + 71% air Initial pressure – 1.4 atm The mixture was held in the chamber during of 100 hours. Measurements of mixture composition were carried out repeatedly with gas analyzer GT-201 (Gas Tech, Inc., USA). The initiation of the reaction in the mixture was performed by exploding copper wire in the chamber centre; the discharged energy was about 6 J. Inside view of explosive spherical camber before experiment

15. JIHT of RAS Results of measurement of primary flame front velocity by ionizing sensors S - distance between the sensor and the point of ignition t - time lapse between flame front arrival to the sensor and the moment of ignition U(0-i)av. - average velocity of flame front propagation from the initiation point to the location of i-th sensor U(i-i+1)av.- average velocity of flame front propagation between sensors, Uimax- maximal calculated value of the flame front velocity in the vicinityof i-th sensor under the assumption that motion is uniformly accelerated

16. JIHT of RAS Data ofcrusher membrane transducers Before the installation crusher transducers were calibrated by means of blasting various spherical charges TNT and RDX.At calibrating the pressure was determined by calculation and by means of piezoelectric transducers.

17. JIHT of RAS Explosive chamberafter experiment…

18. JIHT of RAS … and now

19. JIHT of RAS Conclusions • The results presented are of interest for hydrogen safety. • The results obtained in the Cone are of interest as there is a cumulating device in almost each room (room angles). • It is demonstrated that in large closed volumes non-stationary combustion can develop from a source of small energy. Thepressure can exceed the Chapman-Jouguet pressure. • In our opinion such a high pressure in wall area could be the result of explosion when intense shock waves were reflected from the wall. • When the mixture is initiated in the center of the chamber weak shock waves are formed. We suppose there are two mechanisms of the intensification of the waves: first – due to disturbances caused by the primary combustion front; second – due to shock waves passage through flame front. • In our experiment original shock waves could interact with the flame front three times. Estimations showed that for our conditions it was possible to appear such shock waves which reflected from the wall before the front of primary combustion reached the wall and were characterized by the intensity sufficient for initiation an explosion similar to that observed in the shock tube and cone.