Deflagration to detonation transition (DDT) in jet ignited hydrogen-air mixtures: large scale experiments and flacs CFD predictions

As a result of a long history of model development and experimental validation, FLACS is established as a CFD-tool (computational fluid dynamics) for simulating hydrocarbon gas deflagrations with reasonable precision. FLACS is widely used in petrochemical industry and elsewhere for explosion predictions for input to risk assessments and design load specifications. In recent years the focus on predicting hydrogen explosions has increased. A dedicated project was carried out between 2001 and 2004 to improve the modelling and validation of hydrogen explosions wherein many small and largescale experiments were simulated. With the latest release of FLACS, the validation status for hydrogen explosions is therefore considered good. For hydrogen explosions, deflagration to detonation transition (DDT) can be a significant threat. Recently, FLACS has been extended to indicate the possibility of DDT in realistic situations. As a part of the study, four practical scenarios were simulated and the simulation results were found to compare well with experimental data. The model has now been developed further and used to simulate the experimental investigations performed by Fraunhofer Institute of Chemical Technology. These concerned the transition of a deflagration into a detonation in jet ignited hydrogen air mixtures within a partial confinement. The background for this project was the investigation of the potential hazards for a nuclear power plant, whose process heat is used for the operation of an adjacent chemical plant (e.g. for the gasification of coal), which should be located close to the nuclear plant to minimize heat losses. The test set up consisted of a rectangular container (3 m x 1.5 m x 1.5 m) with an opening on its front side. The container was followed by 2 parallel walls at a distance of 3 m with a length of 12 m and a height of 3 m. The whole volume was filled with a hydrogen air mixture, enclosed within a very thin PE-foil. The mixture was ignited at the rear side of the container. The experiments observed very high pressures and transition to detonation due to the high turbulence generated by a jet flame shooting into a large, reactive gas cloud followed by reflections of the high speed combustion front from the ground and the walls. The experiments observed DDT for 21 % hydrogen concentration, but not for mixtures less sensitive than that. The modeling results are able to capture the experimental observations, including pressure traces and locations of DDT, reasonably well. The possibility of DDT is indicated in terms of a spatial pressure gradient across the flame front. The effect of geometrical dimensions on the observation of DDT is also discussed by comparison with the detonation cell size. The flame speeds of the detonation front are somewhat lower than those observed in the experiments but the development of a shock ignition model is ongoing which is expected to resolve this difference.