Abstract
Upward flame spread over paper in an air stream flowing vertically downward has been studied. The experiments were conducted in a vertical duct of 10 cm × 10 cm cross section and 22 cm long, which was placed underneath a converging nozzle of a wind tunnel. The paper used for the experiments was a sheet of filter paper of 10 cm × 23 cm surface area and 0.026 cm thick.
Following detailed observations of the behavior of flame spread, the effects of the air stream on gas-temperature profiles near the preheat zone and streams passing through the preheat zone were examined using particle tracer techniques, fine-wire thermocouples, and schlieren photography.
Three different types of flame spread were observed. In Region I, representing the range of air-stream velocities from 0 to 85 cm/sec, flame spread was accelerative, although the acceleration decreased with the increase of the air-stream velocity. In Region II, representing the range of air-stream velocities from 85 cm/sec to 125 cm/sec, local flame spread rate fluctuated greatly, and flame spread had intermediate characteristics between those of accelerative flame spread and steady flame spread. In Region III, representing the range of air-stream velocities from 125 cm/sec to 190 cm/sec, the behavior of flame spread was almost steady and resembled that of downward flame spread.
In Region I, the unburned material far preceding the pyrolysis front was preheated by a hot gas stream. In Region II, the unburned material preceding the pyrolysis front was preheated periodically. In this case, the heat transfer from one side of the paper was different from that from the other side. In Region III, the preheating of the unburned material was confined in a narrow region ahead of the pyrolysis front.
The most noticeable difference between the flow fields near the leading edges of spreading flames observed in Region I and Region III was the difference of gas-stream directions. In Region I, the gas near the leading flame edge flowed upward, while in Region III it flowed downward. In Region II, it flowed upward and downward alternately.
It was inferred that the flame spread phenomena in different directions resembled each other if the external air streams canceled the difference of the buoyancy effects caused by the density difference between the gas near the flame front and ambient air and caused similar flow fields near the leading flame edges.
The difference of the momentums of the external air streams for the cases when similar stable flames spread vertically upward and downward was estimated to be about 0.2 kg · sec/(m2 · sec).
