Figure 3 Flame appearances of R123 (a) and R125 (b) recorded by using a high-speed camera.
For clearly observing the process of fire extinguising, the flame behavior in the last 0.3 s before the fire was extinguished was recorded (in Figure 3). R123 and R125 extinguished the cup-burner flame both through the process of oscillation-separation-extinction. As the concentration reached the extinction limit, the flame oscillated frequently, and the flame root began leaving the rim of the cup-burner. Due to the sudden increase in the distance between the root of the flame and the burner at a certain point, the flame cannot return to the edge of burner, the two agents eventually cause the flame to blow out.
The addition of two extinguishing agents weakened the adhesion of the flame with the rim of cup, causing the flame to vibrate continuously. However, it was also found that R123 (Figure 3a) made the flame oscillation more obvious than R125 (Figure 3b) before extinguishing. For co-flow diffusion flame in the cup burner, the reaction kernel provides a continuing ignition source in the flame base, which can consume the new incoming reactants fast and maintain combustion, thereby the trailing diffusion flame in the flow can keep holding in succession[28, 29]. However, when the flame root starts appearing the separation from the rim of cup, the reaction kernel which was at the flame base was weakened by the addition of extinguishing agents, thus the phenomenon of flame oscillation was occurred. It is indicated that the influnce of R123 on the reaction kernel is greater than R125, the addition of R123 may make the phenomenon of flame oscillation more obvious.