1.1. Laboratory Scale Experiments
MEC is the key parameter to evaluate the efficiency of the fire suppressants and to design the fire extinguishing system. Due to the large amount of preparatory work, high cost, poor repeatability, safety issues and complexity, it is difficult to obtain accurate or even meaningful results of MEC through full-scale experiments [
52,
53]. However, the laboratory scale tests overcome these disadvantages and are widely used to test the MEC of various fire suppressants with different fuels.
The cup burner method is widely used to measure the MEC of gaseous fire suppressants. The laminar co-flow diffusion flame of the cup burner resembles real fire. Furthermore, the flame is more stable compared to real fire, leading to the higher MEC measured by the cup burner method. This method has been adopted as the standard procedure listed in ISO 14520 and NFPA 2001, since the first systematic introduction by Hirst [
52]. However, the cup burner method is generally used to test the MEC of gaseous agents at room temperature. As for C
6F
12O with the high boiling temperature of 49 °C (1 atm), the method is modified so that the liquid agent is measured after vaporization by pre-heating.
Table 1 shows the MEC of C
6F
12O tested by different researchers through the modified method, from which it can be concluded that the test results of MEC are highly consistent, and the extinguishing efficiency of C
6F
12O is also very high. In terms of the synergistic effect of C
6F
12O with nitrogen, carbon dioxide and Halon 1301, the extinguishing efficiency shows a negative synergistic effect of mutual inhibition, while when C
6F
12O was combined with HFC 125, it shows a positive synergistic effect [
54]. It could be speculated that the fire extinguishing mechanism of C
6F
12O might be similar with HFC 125.
Table 1. Measurement of critical fire-extinguishing concentration of C6F12O based on cup burner.
Researcher |
Fuel |
Test Result of MEC |
Note |
Carnazza et al. [55] |
n-heptane, alcohol and other liquid fuels |
n-heptane 4.5%, alcohol 5.6% |
|
Andersson et al. [24,56] |
propane |
6.4% |
lower than HFC 125 and HFC 227ea under the same experimental conditions, higher than Halon 1301 |
Rivers et al. [14] |
propane |
3.5% |
lower than Halon 1301 and Halon 1211 under the same experimental conditions but the required mass for the same fire extinguishing efficiency is relatively high |
Takahashi et al. [57] |
propane |
4.17% |
the calculated MEC is 4.12% |
Li [54] |
n-heptane |
4.5–5% |
under different gasification heating temperature, air temperature, heating coil and environment temperature |
The cup burner method is easily influenced by fuel type, fuel level, burner size, agent temperature, air and agent flow rate and pre-burn time [
58], and the turbulence state in practical fire development is also neglected, leading to consistency problems between the laboratory scale and lager scale experiments [
59]. In order to comprehensively evaluate the fire extinguishing concentration of the fire suppressants, researchers [
60,
61] have proposed the tubular burner method, in which the fuel and fire extinguishing mediums are mixed in a hot bath in advance before the fuel burns, and the flow rate of fire suppressants is adjusted until the flame is extinguished. This method determines the amount of needed fire suppressants by measuring the required extinguishing medium portion (REMP) value, which is defined as the ratio of the mass flow rate of the fire suppressant to the mass flow rate of the fuel. Andersson et al. [
24] used this method to measure the REMP value of C
6F
12O. The results showed that the REMP value of C
6F
12O (15) was much higher than that of Halon 1301 (1.5), HFC 125 (5.6) and HFC 227ea (6.8), which also verified that the fire extinguishing volume fraction of C
6F
12O is low, while the required mass concentration is high.
Although there are some differences between the two methods mentioned above in terms of the supply mode of the fire extinguishing agent and fuel and the calculation method of the fire extinguishing concentration and flame combustion state, these two methods have a common character in that when they are applied to measure C
6F
12O concentration, the fire suppressant vaporizes before reaching the flame, and what they measure is the fire extinguishing concentration of the agent in a gaseous state. For the high boiling point extinguishing agent, C
6F
12O, partial evaporation occurs in the transport pipe of the fire extinguishing system, and the remaining part of the agent is sprayed in droplets. When the droplets approach the flame, phase change will happen under the high temperature of the fire, which will absorb the heat of the flame and reduce the temperature of the fire. The evaporative heat of this part has effects on the fire extinguishing, especially for water and polar molecules, while these two methods cannot take into account the extinguishing contribution of liquid phase transition of C
6F
12O [
62].
Yang [
63,
64] proposed a dispersed liquid agent fire suppression screen apparatus (DLAFSS), a kind of counterflow cylindrical burner which can form a stable two-dimensional laminar non-premixed flame. It can accurately measure the fire extinguishing efficiency of solid, liquid and gaseous fire extinguishing mediums, and has been applied in when testing the fire extinguishing efficiency of some liquid suppressants [
65]. However, research on the fire extinguishing efficiency of the high boiling point agent C
6F
12O is still lacking.
C
6F
12O can be used in a total flooding fire extinguishment system. 3M researchers [
55] tested the minimum inert concentrations for methane and propane air mixtures according to ISO14520 standard and obtained minimum inert concentrations for methane and propane of 8–9%. Andersson et al. [
56] also obtained similar experimental results. The inert concentration of C
6F
12O (7–9%) is similar to that of Halon 1301 (7.5–8.7%) and lower than that of HFC 125 (14–16%) and HFC 227ea (11–12%). However, in the FAA’s aerosol can test (FAA-ACT) [
66,
67,
68], aiming at examining the feasibility of applying halon substitutes to aircraft, several halon substitutes including C
6F
12O were tested with concentrations lower than the minimum inert concentration. The results showed that these halon substitutes all lead to a pressure rise in the can to some degree, and with the addition of a low concentration of C
6F
12O (4.2%), the pressure increases nearly three times, indicating that C
6F
12O and other halon substitutes enhance flame combustion under some certain conditions.