Combustion Regimes of Hydrogen-air-steam Mixtures

In the case of a severe nuclear power plant accident, hydrogen gas formation may occur, from the core degradation, and cooling water evaporation and subsequent oxidation of zircaloy. These phenomena increase the risk of hazardous combustion events in the reactor, especially when combined with an ignition source. If not handled carefully, these types of accidents can cause severe damage to the reactor building with potential radioactive effects on the environment. Although hydrogen-air combustion has been investigated before, hydrogen-air-steam mixtures remain unstudied under reactor-like conditions. Thus, this study investigated such mixtures’ combustion regimes. A closed tube of 318 liters (7.65m tall and 0.23m inner diameter) measures the flame speed, flame propagation and shock wave behaviors for 11-15 %vol hydrogen mixtures combined with 0, 20 or 30 %vol steam, and air. Thus, both the effect of steam and hydrogen content was investigated and compared. The experimental setup combined photomultiplier tubes, pressure sensors, and shock detectors to give a full view of the different combustion regimes. A number of obstacles changed the in-chamber turbulence, during flame propagation, to provide further reactor-like environments. This changed turbulence affected the combustion regimes and enhanced the flame speed for some cases. The results showed varying combustion behaviors depending on the water vapor concentration, where a higher concentration meant a lower flame speed, reduced pressure load, and sometimes combustion extinction. At 0 %vol steam dilution, the flame speed remained supersonic for all H2 concentrations, while at 30 %vol steam dilution, the flame speed remained subsonic for all H2 concentrations. Thus, with high levels of steam dilution, the risk for shock waves, leading to potential reactor building destruction, decreases."