Effective mechanisms to determine flame spread rate over ethylene-tetrafluoroethylene wire insulation: Discussion on dilution gas effect based on temperature measurements

Abstract

Rates of flame spreading for ethylene-tetrafluoroethylene (ETFE) insulated wires in microgravity were compared with downward rates of flame spreading in normal gravity. Three distinctive features were noted: (1) The rate of spreading in microgravity was faster than the rate of downward spreading in normal gravity (Vf0G/Vf1G>1) for almost all the tested conditions: (2) the increase in Vf0G/Vf1G was the largest with CO2 dilution: and (3) the ratio of Vf0G/Vf1G with CO2 dilution increased with decreases in O2 concentration, while the value for other dilution gases showed a peak value at a specific O2 concentration. The mechanism for these features is discussed with the detailed temperature distributions measured in microgravity. The preheat zone of microgravity flames was much thicker than with normal gravity. The decreases in flame temperatures by radiative heat losses in microgravity was smaller for wires because of a curvature effect. The thicker preheated zone and lower temperature decrease in microgravity caused the faster flame spreading in microgravity. The curvature effect is a basic difference from flat samples. Carbon dioxide reabsorbs the radiation heat, and the temperature of the preheated zone becomes higher than with the other dilution gases. The reabsorption effect caused the largest increase in rates of flame spreading in microgravity when compared with the situation under normal gravity. The reabsorption effect of CO2 has the potential to recover radiated heat while the other dilution gases lose this heat. This recovery effect apparently caused the increase in the Vf0G/Vf1G ratio at lower O2 concentrations.