Quenching distance measurement of highly to mildly flammable compounds

•Test method of quenching distance measurement is modified to evaluate dq of highly to mildly flammable compounds.•A good correlation between dq and burning velocity was obtained.•Emin for mildly flammable compounds were estimated to be more than one order of magnitude higher than that of propane.

Quenching distance measurements were carried out for 11 highly to only mildly flammable gases (which include alkanes, fluorinated alkanes and alkenes, and ammonia) to elucidate the ignition and quenching characteristics of low-GWP (global warming potential) alternative materials. For buoyant flames of mildly flammable compounds, conventional 25 mm diameter parallel plates in the vertical position provided significantly smaller quenching distance (dq) than 100 mm diameter plates in the horizontal position. A good correlation was obtained between the quenching distance (dq,h in mm) measured by the latter test apparatus and the maximum burning velocity (Su0,max in cm s−1) for these compounds: dq,h=58.12(ρuSu0,max)−0.926, where ρu is the unburned gas density. The mildly flammable compounds that have Su0,max below 10 cm s−1 have a dq more than three times larger than that of propane. Initial development of the schlieren flame radius was observed for mildly flammable CH2F2/air mixture using thin electrodes and a variety of spark energies. It was confirmed that the parallel plate quenching distance was essentially equal to the minimum flame diameter in a free space. By applying the measured dq,h and Su0,max in the simplified heat loss theory, the minimum ignition energy (Emin in mJ) was expressed by Emin=0.0712dq,h2.97. The results showed that the mildly flammable compounds have Emin that is more than an order of magnitude greater than that of propane.

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