The scaling of turbulent flame acceleration and detonation transition for hydrogen–air mixtures in the RUT facility

To model the combustion stage of Loss of Coolant Accident (LOCA) in the containment of a nuclear power plant (NPP), a series of large-scale benchmark experiments have been conducted in the RUT facility. Flame propagation regimes for very lean hydrogen-air mixtures from 10 to 14 vol.% H2 have been investigated in the RUT set-up. The facility consists of three parts: 34-m channel, 10.5-m canyon, 20-m second channel. The total volume of the mixture was about 480 m3. 30 and 60% of the channel cross-section was blocked by concrete blocks. Slow and sonic deflagrations in the channel have been established for hydrogen concentrations up to 12.5 vol.% H2, as a detonation transition at 14 vol.% of hydrogen was registered. In a canyon of a bigger dimension, the detonation was observed even at 12.5 vol.% H2 due to shock reflection at the far corner of the canyon. Such detonable concentrations for channel and canyon geometries confirm the validity of the 7A criterion for detonation onset, recommended in the NEA SOAR report and significantly extend the conventional concentration detonability limits of 18-59 vol.% H2 (NASA STD 8719.16 - Safety Standards) commonly used in non-nuclear safety guidelines. ... mehrCurrent experimental and numerical data demonstrate a very high risk of detonation even for very lean hydrogen-air mixtures. The scaling down of flame acceleration and detonation transition was investigated for the same geometry in a MINIRUT facility of 50 times smaller size than the original RUT facility confirms the validity of the 7A criterion in two orders of magnitude smaller scale. The detonation in the channel and in the canyon of a 50 times smaller scale has occurred for hydrogen-air mixtures with a detonation cell size of 50-57 times smaller than for a real-scale RUT facility. The video monitoring confirmed the shock reflection and shock-flame interaction mechanisms for deflagration to detonation transition (DDT). Beyond the detonation cases for leaner hydrogen-air mixtures, the flame can propagate as subsonic or sonic deflagration. The criterion for sonic deflagration will be the critical expansion ratio a = 3.75, which corresponds to 10.5-11 vol.% H2. The experiments also confirmed that the flame dynamics for the deflagration mode do not scale down as the DDT process. Only CFD modeling can predict the turbulent flame propagation at different scales.