Abstract
Following the previous study using sheets of paper, the downward smouldering propagation through rod-shaped materials held vertically in a chamber of given temperature was studied both experimentally and theoretically.
The material used were incense-sticks (senko) or cardboards. When a piece of an incense stick is pulverized and squeezed out from a glass tube after being kneaded with water, the rod-shaped paste, when dried, turns into a stick of any radius according to the tube. Rod-shaped samples were also made with paper. A sheet of moistened cardboard was tightly rolled into a circular or rectangular rod and then was bound with a thin thread. It was dried for some days in a furnace before being used.
The material of the rod turns a brownish color before burning and from this it is revealed that the isotherms near the front of smouldering is a plane perpendicular to the rod. As in the case of paper, we derived the following formulae considering the heat balance in the layer near the front.
υ2=8k/c2ρ2(q/(Ti-Ta)-h)(1/r+Δ/r2) for a circular rod,
υ2=8k/c2ρ2(q/(Ti-Ta)-h)(1/b+1/d+4Δ/bd) for a rectangular rod,
where υ = propagation velocity, c = specific heat, ρ = density, Ti = ignition temperature, Ta = ambient temperature, q = a chemical constant depending upon the material and Δ = thickness of the stagnant layer around the rod.
Experimental results showed very good agreement with these formulae and the values of Δ, q, h are all reasonable as shown in the following table.
circular rod rectangular rod
incense stick card board card board
Δ(mm) 2 2 1
q(cal/cm2·s) 0.11 0.13 0.13
h(cal/cm2·deg·s) 1.5×10-4 2.2×10-4 2.2×10-4
