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Accueil du site > Thèmes de recherche > Combustion & systèmes réactifs > 1.6 Dynamics of chemical explosions > 1.6.2 Strongly accelerated flames and transition to detonation

1.6.2 Strongly accelerated flames and transition to detonation Criteria for flame acceleration
For more than a decade, ICARE has been involved in different programs with industrial partners (EDF, AREVA) as well as organisms (CEA, IRSN) concerned about the safety of nuclear power plants. In order to address the case of a severe accident involving hydrogen explosions, a highly instrumented facility (ENACCEF) has been developed in order to study the risk of flame acceleration and to propose a criteria that can identify potentially destructive mixtures. The criterion that has been proposed is based on different properties of the mixture (expansion factor, Lewis and Zeldovich numbers). The effect of hydrogen concentration gradients on this criterion has been studied (TH26 ; ACTI24-25). Moreover, in the framework of the OECD committee on the safety of nuclear installations, this study has been chosen as a test case for more than 15 countries in the framework of International Standard Problem (ISP-49). Within the ANR project PAN-H Hydromel aiming at evaluating explosion hazards related to the transport of hydrogen using the existing network for natural gas, one solution we explored was to mitigate its explosive properties by adding methane. For this purpose, the inhibitor effect of methane for strongly reactive mixtures has been investigated (COMM28), using also the fundamental explosion parameters acquired in §2.2.1. Flame / water droplet interactions
In the case of a severe accident in a nuclear power plant, the safety procedures include the use of water sprays in order to abate the radioactive aerosols and reduce the pressure inside the reactor building. A two phase mixture of H2/air/steam/water droplets is then formed which could be inside the flammability limits. In addition, the presence of water can create a short electric circuit which can act as an ignition source. It is therefore crucial to understand whether 22 the water spray will extinguish the flame or on the contrary accelerate it. This work has been conducted in collaboration with IRSN and AREVA (TH26 ; ACL176 ; ACTI44, 79). The behavior of the flame of H2-air-H2Ovap mixtures near the flammability limits is investigated as a function of the initial temperature (ACTI61, 78). Detonation properties of liquid fuels
In order to select a liquid fuel that is easily stored and capable of achieving a transition from deflagration to detonation in a very short distance, for pulsed detonation engines applications, the sensitivity to detonation of heptane when mixed with oxygen or air was studied (see also Thematic Research Group IV § A detailed kinetic mechanism has been proposed to describe its explosive properties (ACL44). During this study, a novel technique of enhancement of DDT have been proposed by coupling a Schelkin spiral and hot gas jet injection as well as the construction of an electro-spray device capable of producing droplets with an average diameter below 0.5 μm. In this case, detonation transition was successfully obtained (TH23, ACTI45).