In fire spread through RDF (Refuse Derived Fuel) or wood-pellet packed bed, the color of layer gradually changes by charring or combustion. A cross section which fixed layer in gypsum was observed in order to clarify the structure of combustion zone in the fire spread (smoldering). Temperature profiles, aspects of discoloration and charring were examined experimentally. In the method by gypsum solidification, color (brightness) image processing is desirable for RDF and wood pellet, because change of color is more sensitive than volume for heating. Wood (Western hemlock) cut in cylindrical of 15 mm (D) and 37 mm (L) was used as a sample. The direction of fire spread was made to be upward and downward. As a result of the experiment, it was found that the thickness of discoloration region was respectively 23 mm and 33 mm in the upward fire spread and the downward fire spread. Though it is shown that the method of this study is useful, it is necessary to measure temperature of gas phase and solid phase in a packed bed simultaneously in order to clarify the structure of combustion zone.
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