Figure 4 Flame (a) and appearances (b) at various concentration of R123 and flame appearances height (c) and appearances (d) at various concentration of R125.
Figure 4 shows the evolution of the flame height after adding different volume concentrations of two extinguishing agents. With the addition of R123 (Figure 4a and b), the flame height occurred the process of rapid increase (0-2.21 %, 8 to 13.3 cm), slow increase (2.21-6.33 %, 13.3 to 17.2 cm) and slight decrease (7.31 %, 17.2 to 16.9 cm). There is a reason for the increased height that R123 can increase the O2 demand of combustion system and make unfired gas buoyancy to burn with O2[6, 30]. The density of the agent and its decomposition products, and their heat-release all effect the buoyancy forces, then may affect the flame height variations and flame oscillations[9]. Furthermore, after the R123 concentration exceeded 6.33 %, the flame height decreased from 17.2 to 16.9 cm. From Figure 4b, as the concentration of R123 above 6.33 %, flame oscillation become more evident. The decreased height and oscillation of flame mainly result from two theory. On one hand, abundant fluorine-containing or chlorine-containing groups were generated with plenty of R123 inhibiting combustion reactions, further effectively inhibiting the combustion chain reactions[31]. On the other hand, R123 and oxidizers was flowing on both sides of the nozzle, and they entered the reaction zone from the root of flame, thus the reaction kernel would be affected and weakened. Therefore, the reaction kernel is too weak to maintain the flame stable, and the balance location of the reaction time and the residence time becomes difficult to obtain. Eventually, the flame oscillated violently, then blown out.
In comparison with R123, R125 made the flame height increase continuously (0-8.91 %, 8 to 20 cm). Apart from the different changes of flame height, the flame width did not have obvious change with the addition of R125 (Figure 4c and d), but the flame was widened with the addition of R123 (Figure 4a and b). For the co-flow diffusion flame in cup burner, the flame width was wider, the combustion reaction was slower[32], thus the addition of R123 made total combustion reactions slow, which may accelerate the process of the fire extinction[33]. Consistent with the results of the minimum fire extinguishing concentration calculated earlier, the extinguishing performance of R123 is better than that of R125.