Deflagration to Detonation Transition Experiments with Hydrogen-Air Mixtures in Shock Tube and Obstacle Array Geometries The objectives of this work were 1. to provide integral experimental data for development of physical models and verification of numerical tools, 2. to investigate the statistical nature of DDT for weil deftned initial and boundary conditions, 3. to measure typical DDT Ioads in lean hydrogen-air mixtures. Different DDT mechanisms were investigated in three test

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... -idealised mode A (shock wave focussing in a 3-d reflector) -prototypic mode A (focussing of flame precursor wave in a 3-d reflector) -prototypic mode B (accelerating flame in obstacle array). For DDT by idealised mode A critical Mach numbers were determined for the incident shock as function of the H2 concentration. At this critical Mach number the ignition regime changes from slow/weak (= deflagration) to strang/fast (= detonation). Very good reproducibility of individual tests and consistence of the whole data base in terms of ignition modes was observed. The detailed processes in idealised mode A DDT tests are highly mechanistic and seem to be governed by the temperature dependent reaction kinetics. Prototypic DDT mode A from the precursor wave of an accelerating flame was detected in the FZK-tube at > 16.5% H2 in air, with a blockage ratio of 60%. At smaller hydrogen concentrations the precursor shock caused only a mild ignition (=deflagration) in the reflector. The tests have clearly proven the significance of DDT mode A in obstructed geometries: a flame which accelerates in a certain part of a complex installation to near sonic velocities emits pressure waves which can trigger a DDT in a distant part of the enclosure, especially if H 2 enrichments should be present in reflecting corners, e.g. caused by stratification. DDT mode B was observed in the fully obstructed tube for > 13.5% H 2 at 30% BR and for > 15% H 2 at 45% BR. With 60% BR no DDT occurred for ~ 15% He (po=1 bar). The occurrence of DDT could be clearly identified by three criteria, namely pressure amplitude, wave speed, and coupling of pressure and reaction front. Pressures developing due to DDT in lean H 2 -air mixtures were measured. The highest pressures are generated during the initial ignition phase in the reflector when the reaction starts from a highly pre-compressed state. The tests have resulted in new data on DDT mechanisms which can be used for model development and verification with increasing complexity. The simplest case are the idealised mode A experiments, which requires only simulation of the compressible flow and the H2-02 reaction but not of turbulence. Numerical models should be first verified on these tests.