Hydrogen:Air:Steam Flammability Limits and Combustion Characteristics in the FITS Vessel (NUREG/CR-3468, SAND84-0383)
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Manuscript Completed: August 1986
Date Published: December 1986
Billy W. Marshall, Jr.
Sandia National Laboratories
Albuquerque, New Mexico 87185
Operated by Sandia Corporation
for the U.S. Department of Energy
Under Contract No. DE-AC04-76DP00789
Division of Reactor System Safety
Accident Evaluation Branch
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
Under Memorandum of Understanding DOE 40-550-75
NRC FIN A-1246
For the past few years, the United States Nuclear Regulatory Commission has sponsored research at Sandia National Laboratories addressing the combustion characteristics and flammability limits of combustible atmospheres that might occur inside containment during a loss-of-coolant accident inside a pressurized water reactor (PWR). Combustion of certain hydrogen:air:steam atmospheres could, at least hypothetically, threaten the integrity of the containment structure. To assist in the resolution of these issues, a series of 239 hydrogen:air:steam combustion experiments was performed in a 5.6 m3 vessel.
Experimentally observed flammability limits of hydrogen:air:steam mixtures in both turbulent and quiescent environments were measured and a correlation developed that describes the three-component flammability limit. The newly developed correlation can be used to estimate the flammability of a mixture at these scales and larger scales to obtain approximate ignition conditions.
Transient combustion pressures of hydrogen:air mixtures were found to increase with increasing hydrogen concentrations up to ~30%, at which point a decrease was observed with further increases in the hydrogen concentration. More severe combustion environments occurred for tests initially at ambient temperature (~300 K) than for those initially at elevated temperatures (~385 K) due to decreases in the bulk gas density with increases in the gas temperature. The transient combustion-pressure data measured for the hydrogen:air:steam tests indicate that the addition of steam reduces the normalized peak combustion pressure (Pmax/Po) as compared to equivalent hydrogen:air burns. Furthermore, turbulence was found to affect the extent of combustion and other combustion characteristics of the lean hydrogen burns (i.e., 10% hydrogen by volume) where buoyancy governs flame propagation. However, burns containing richer hydrogen concentrations were not appreciably affect by turbulence.
The experimentally measured pressure decays were used to infer the "global" total, radiative, and convective heat transfer characteristics during the postcombustion cooling phase. Convection was found to dominate the time integrated heat transfer (i.e., energy deposition) of the leaner (<10%) hydrogen:air burns, accounting for 50 to 70% of the post combustion heat transfer. In contrast, radiation was slightly more prevalent than convection for the hydrogen:air burns near stoichiometry. When moderate quantities of steam were added to the environment, radiation became the dominant postcombustion (or time-integrated) cooling mechanism due to the increase in bulk gas emittance. If richer steam concentrations (i.e., >~30% by volume) were added to the environment, radiation and convection appear to be equally important heat transfer mechanisms.