The effect of sample size on the characteristics (spread rate, temperature) of fire spread upward or downward in a packed bed of cylindrical pellet was experimentally examined. Ten kinds of wood (Western hemlock) pellets of which the dimension differed were used : aspect ratio 1 and 2.5, diameter 6.5~30 mm, length 6.5~75 mm. The sample size was evaluated in equivalent diameter, for example, volume equivalent diameter D
ve and hydraulic equivalent diameter. It was found that the fire spread characteristics were strongly influenced in sample diameter and opening (or void fraction). Spread rate of fire V
b increased to D
ve=14~16 mm linearly with sample diameter, then V
b was constant or decreased within D
ve =30~35 mm regardless of the spread direction of fire. However, V
b increased in downward fire spread of the bottom side opened, when the sample diameter increased further.
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