Extinction limits of spreading flames over wires in microgravity

摘要

Tests with flames spreading over wires in microgravity were performed at external opposed flow conditions from 60 to 200 mm/s to examine the influence of flow velocity on the extinction limit. In the experiments, low density polyethylene insulated Nickel-chrome and Copper wire samples were used. The experiments were conducted both in normal gravity and microgravity attained by parabolic flights. The experimental results are discussed based on non-dimensional numbers developed by Takahashi et al. This study added the heat conduction through the wire insulation to the inner core to their model, which is considered as a heat loss term arising due to the presence of the inner core. Further the effect of curvature of the sample is taken into consideration. From the results of the experiments in normal gravity, there was a low oxygen limit for Copper wire at around 125 mm/s. However there was no low oxygen limit for Nickel-chrome wire; the limits of the oxygen concentration decreased monotonically with decreases in the external flow velocity. The non-dimensional number implies that the decrease in inner core temperature around the preheat zone occurring with Copper wire plays a role in the change in low oxygen limits. Further, in the external flow velocity range investigated here, there was no low oxygen limit in microgravity with the Copper wire. This might arise due to the changes in the flame shapes by gravity. The heat input from flame to the wire is supposed to increase in microgravity because the wire is surrounded by flame due to inhibited natural convection. This increased heat input might help the increase of the inner core temperature around the preheat zone. For this reason, there is a possibility that the low oxygen limit with Copper wire in microgravity exists at lower flow velocity than that in normal gravity.