This was convenient for observation. Fig.5 illustrates such burning conditions of the target cribs when the fire fighting began.

#### 1.3.2 Condition of Fire Suppression

Comparison of experiments 1 and 2:

Because inhibitors penetrate better into carbonized layers, the quantities required were 742 liters of chemical solution as compared to 404 liters of water to extinguish the fire, the former exceeds the latter. This phenomenon was already known to us through laboratory tests as explained in Fig.2 $ ^{[5]} $ and was something anticipated. The target crib in the experiment consisted of half of raw wood and half of charred wood as Fig.5.

In these two experiments, strong flames blew into the lattice-work. The flames passed through the cross piles of carbonized layers to reach the wood(non-carbonized) layers. When the layers were heated for a while, the diagonal boundary of the two layers ignited as Fig.6(1). Experiments using chemicals showed that once the flames were gone, the weak flames in the boundary area slowed and disappeared. After repeated heating and reignition, the boundary gradually started to burn continuously. Also, the lower part of the crib close to the ground level and facing the burning area started to burn continuously and the fire gradually spread upwards and toward the non-carbonized layers as Fig.6(2), spreading in the end to the entire crib.

The upper layers that were exposed to stronger heat flux than the lower layers and caught fire from the earliest stage endured the heating well and rekindled lastly. This phenomenon was probably due to the higher degree of carbonization, that resulted in sufficient accumulation of inhibitors. These facts are important findings and suggest effective spraying method.

Experiments using water, on the other hand, showed continuous burning once the boundary caught fire. In the carbonized layers the fire recurred slowly but steadily, beginning at the area close to the burning area, as Fig.6(3).

The rate of fire spread is described in Table 1. They were obtained from the speed of fire spread at the center of the cribs at their 1/2 height as observed from the both sides. The figures do not reflect the 0.5 meters openings between the cribs and are thus not the average speed. In Table1, the marked suppressing effect of chemical inhibitor referred to water in charred layers is shown even when flames blew in and somehow the reverse effect was observed in non-carbonized raw wood layers.

Comparison of Experiments 3 and 4:

When two hoses were used to spray inhibitors, consumption of the chemical solution was 806 liters and the water totaled to 505 liters, the former again exceeds the latter. Inhibitors were sprayed from the top and sides with thicker concentration on the carbonized layers than the wood layers. The precise amount of inhibitors soaked on the layers in the hose experiment and in the fixed nozzle experiment were uncertain, but the amount is usually smaller in the former case although the density of accumulation may be more even in the layers.

The hose experiment consumed more inhibitors than the fixed nozzle experiment due to poor spraying efficiency. Since the wind blew in the

(1)

(2)

(3)

Fig.6 Aspects of flame spread: side view

(1) Ignition at boundary

(2) Ignition at bottom corner

(3) Ignition at both boundary and bottom corner