Lower Flammability Limit of Difluoromethane and Percolation Theory

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Measurements of flammability limits by the ASTM E 681 method are believed to be affected by vessel size. In order to investigate the effect of volume on measured lower flammability limits in air, experimental measurements of the lower flammability limit of difluoromethane (R-32) are made at 21°C using 3, 5, 12, and 20 L vessels. A spark ignition source is used with the voltage adjusted to the value where dielectric breakdown just begins to occur (approximately 7–12 kV). The results demonstrated a higher concentration for the lower flammability limit for the vessels smaller than 5 L suggesting that wall effects quench the flame propagation. The lower flammability limit for vessels with a 5 L or greater volume are in good agreement with those obtained in earlier investigations. For 5 L and larger vessels the lower flammability limits systematically shift to higher concentrations of difluoromethane with increasing vessel size which is consistent with a percolation model for spatial flame propagation. Extrapolation of the data to infinite vessel size using a percolation model yielded a lower flammability limit of 15.2 vol.%. The results are consistent with the work of Richard for refrigerant mixtures, R134a/R152a and R125/R152a, in large volume vessels (200 L) where effects of vessel walls are negligible.

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