Yuko Saso

Flame-extinguishing concentrations and peak concentrations of N2, Ar, CO2 and their mixtures for hydrocarbon fuels

Fire-extinguishing efficiency of inert gas mixtures was investigated by measuring flame-extinguishing concentrations and peak concentrations for hydrocarbon fuels, because new fire-extinguishing agents composed of inert gas mixtures have been developed as halon alternatives. The flame-extinguishing concentrations of nitrogen, argon, carbon dioxide and their mixtures for n-heptane were measured with the FRI glass cup burner. The peak concentrations of the agents for methane-air and propane-air mixtures were also measured with the tubular flame burner. Chemical equilibrium calculations showed that the adiabatic flame temperatures of the cup burner flames at the extinction condition were almost constant for all the agents. The adiabatic flame temperatures at the flammability limit of the tubular flame for each fuel were also independent of the inert gas agent if the mixtures had the same equivalence ratio. The flame-extinguishing concentrations of the inert gas mixtures were predicted by a simple equation averaging over the flame-extinguishing concentrations of all component gases weighted by mole fraction. The equation has the same form as Le Chateliers law. For the flammability limits of the hydrocarbon-air mixtures, the same relation was also recognized in the effect of the mixed agents. The facts show clearly that this simple equation for the flame extinction concentrations is useful to estimate the fire suppression efficiency of any mixed agents of the inert gases. At the same time, it appears that the flame-extinguishing concentrations and the flammability limits reported in the paper are consistent.

Binary CF3Br- and CHF3–inert flame suppressants: effect of temperature on the flame inhibition effectiveness of CF3Br and CHF3

Abstract A numerical investigation with detailed chemistry and transport was conducted on the inhibition effectiveness of binary halogenated suppressant and inert gas mixtures. Computational results demonstrate that while positive synergism persists between CF 3 Br and inert gases, little or negative synergism exists between CHF 3 and inert gases. These synergistic effects are attributed to the sensitivity of flame inhibition effectiveness of the chemical suppressant, in that while the inhibition effectiveness of CF 3 Br is enhanced at lower flame temperatures, it remains relatively unchanged with CHF 3 over the examined temperature range. The temperature sensitivity of CF 3 Br inhibition effectiveness is a result of enhanced catalytic inhibition cycles as the flame temperature decreases. It is also demonstrated that water vapor tends to diminish the flame inhibition effectiveness of CHF 3 .

Extinction of counterfiow diffusion flames with halon replacements

Extinction of counter/low diffusion flames on liquid fuels was investigated, to confirm the superiority of the counterflow diffusion flame over the cup burner method for measuring flame extinguishing concentrations of fire suppressants, and to examine the fire suppression effects of halon replacements. The flame extinguishing concentration for the counterflow flame was less sensitive to the burner size than that for the cup burner method. Furthermore, the flow velocity of the fuel vapor had no change when the suppressant concentration in the oxidizer mixture of the counterflow diffusion flame was varied, whereas it changed remarkably in the case of the cup burner flame. The flame extinguishing concentrations of nitrogen, carbon dioxide, halon 1301 (CF3Br), and three kinds of hydrofluorocarbons (HFC) and perfluorocarbon (FC) for n-heptane or ethanol counterflow flames were measured at various strain rates. Adiabatic flame temperatures at the extinction concentrations were calculated using the flame extinguishing concentrations measured for counterflow flames, assuming various equivalence ratios. The results suggest that HFC-23 (CHF3) suppression exhibits a higher contribution to the chemical suppression effect than other HFC or FC.

Laminar Burning Velocities of Trifluoromethane-Methane Mixtures--Experiment and Numerical Simulation

Abstract The laminar burning velocities of trifluoromethane(CHF3)–methane(CH4)–oxygen(O2)–diluent mixtures were determined over extensive fuel concentration ranges using the counterflow flame technique. Numerical simulation was performed by employing a detailed kinetic model compiled on the bases of the GRI-Mech for methane combustion and a recent CHF3 reaction kinetic model. Comparisons between the experimental data and numerical results indicate that while the qualitative experimental trends are well predicted by the model, there exist significant numerical disagreements between model and experiment. Through sensitivity and flux analyses, we identified several rate parameters which are influential to burning velocity predictions, and proposed reasonable adjustments to these parameters either based on new experimental measurements or by considering their associated uncertainties. The effect of CHF3 addition on the reduction of CH4 burning velocities was also experimentally and numerically examined. By substituting the inert gas in the unburned CH4–O2–inert mixture with CHF3 while maintaining a constant adiabatic flame temperature, both burning velocities and mass burning rates decrease with an increase in CHF3 substitution, thus demonstrating positively the combined kinetic and transport effects of CHF3 on burning velocity reduction and in flame inhibition.

Scale Effect of Cup Burner on Flame-Extinguishing Concentrations

In an attempt to reduce the amount of chemical used in fire suppression testing of halon alternative agents, smaller cup burner systems than that of ISO were constructed, and the flame-extinguishing concentrations of Halon 1301 for three liquid fuels were measured. Reducing the cup diameter or the chimney diameter of the cup burner changed significantly not only the flame-extinguishing concentrations but also the flame behavior near extinction. Nevertheless, the reduced size cup burners with which the flame-extinguishing concentration was equal to that of the ISO cup burner were found. One cup burner can reduce the amount of extinguishing agent used in testing by 90%.

Flammability limits of combustible gases and vapors measured by a tubular flame method

A tubular flame burner method was employed as a new test method for flammability limits, and repeatable and reliable flammability limits were measured. The data were obtained under well-defined conditions. New data on the flammable regions of mixtures of several hydrocarbons and ethanol with inert gases and halogenated fire suppressants in air are reported and compared with the literature data.

Response of counterflow diffusion flame stabilized on a methanol pool to suppressant doping

Response of a counterflow diffusion flame over a methanol pool to suppressant doping was investigated to understand inhibition processes. Experimentally, flame temperatures, locations, and fuel gasification rates were measured as a function of nitrogen concentration added to air, while flame-extinguishing concentrations of nitrogen, CHF 3 , and C 3 HF 7 were measured as a function of oxidizer velocity, with two different thermal conditions of the fuel pool. Computations were performed using a counterflow flame model with fuel gasification and heat gain/loss at the pool surface and with detailed chemistry and transport in the gas phase. Flame responses to nitrogen, CO 2 , CHF 3 , C 3 HF 7 , and CF 3 Br addition were computed at constant oxidizer velocities. The model well predicts the measured flame-extinguishing concentrations. The maximum flame temperature was reduced monotonically with the addition of nitrogen in both the experiment and computation, while no remarkable reduction was computed with CHF 3 , C 3 HF 7 , or CF 3 Br doping, resulting in significant difference in the flame temperature at extinction. Nevertheless, the condition of the vaporizing pool was insensitive to the suppressant variation. The computation further shows that the addition of CHF 3 and C 3 HF 7 promotes dominant chain branching reaction on the oxidizer side of the flame, while simultaneously inhibiting it in the fuel oxidation region. None of local flame parameters such as peak H, O, or OH concentration reflects universally the suppressant performance in the flame, while the total integrated amount of these flame radicals is reduced linearly over the entire range of the suppressant concentrations and shows excellent agreement at the extinction turning points. A concept of limit integrated amount of the reactive flame radicals is suggested. Difference between the suppressants in the sensitivity of the flame-extinguishing concentrations to the heat balance at the fuel pool is also demonstrated and interpreted based on the flame temperature effect on the suppressant performance.

Water Droplets Behavior in Extinguishing the Methane-Air Counterflow Diffusion Flame

The behavior of fine water droplets was investigated in a methane-air counterflow diffusion flame, specifically when the water droplets were added to the inlet air feed and used to extinguish the diffusion flame. The water droplets studied had a relatively broad size distribution (from 1 to 60 m), with the number mean diameter of 15 m and Sauter mean diameter of 25 m. With an increase in the velocity gradient, the flame approaches towards the stagnation plane and flame thickness decreases. The flame properties, such as flame location and flame thickness are insensitive to the water mass loading. Along the stagnation stream line, the velocity decreases towards a local minimum just before the flame front. In contrast, the velocity increases in the flame zone due to the thermal expansion, and then decreases towards the stagnation point. In the decelerating zone, the droplet mass flux decreases on approach to the flame zone due to a non-equilibrium in velocity of large droplets between liquid and gas phases. Furthermore, as the droplet-laden air approaches the flame front, the flow stream begins to diverge in the counterflow field. Since the small droplets move away from the burner axis, the divergence of the air flow acts to reduce the water droplet mass flux along the stagnation streamline. Large droplets moving faster than the air flow „catch up‟ to slower ones just before the flame front, and the mass flux of the droplets tends to increase, resulting in the droplets accumulating just before the flame zone. Measurements of the velocities of individual droplets show that large droplets move faster than the small droplets which are in equilibrium (in velocity) between liquid and gas phases. Droplet behavior was found to be controlled by the Stokes number. Equilibrium in velocity is re-established just in front of the flame zone. Closer to the flame front, evaporation occurs and the mass flux of the droplets decreases drastically. On the other hand, thermophoresis acts on extremely small droplets to move them along the steep temperature gradient and these droplets are forced from the high to low temperature regions against the convection velocity. The thermophoretic velocity was estimated in the present study to be compared with the measured convection velocity and it was concluded that the thermophoretic effect is negligibly small over all sizes of droplets.

Fire Extinguishing Effect Of Mixed Agents Of Halon 1301 And Inert Gases

Multiplier effect of mixed agents of trifluorobromomethane (halon 1301) and inert gases on flame extinction has been investigated, for the purpose of minimizing the use of halons. The inert gases examined are argon, nitrogen and carbon dioxide. The fire-extinguishing efficiency was determined by flame-extinguishing concentrations for n-heptane measured with a cupburner. An additive-property equation was derived by thermodynamic considerations to predict the fire-extinguishng efficiency of mixed agents and used as a measure of multiplier-effect test. The mixed agents of halon 1301 and inert gases showed multiplier effect. There was the optimized mixing ratios of the most effective agents. Flame-extinction temperatures were calculated for the n-heptane flames with the flameextinguishing concentrations of agents. The temperatures suggested that the chemical effect of halon 1301 was enhanced with decreasing the flame temperature by the addition of inert gases. The flame temperature and the concentration of halon 1301 determined strength of the multiplier effect of halon 1301 and inert gas.

Effect of Oxygen Concentration on the Carbon Monoxide Yields from Methane and Methanol Flames

Reduced oxygen concentration environment by diluting with nitrogen (N2) is expected as one of the alternative fire prevention and extinguishing technologies. To ensure the fire safety under reduced oxygen concentration, it is necessary to explore the carbon monoxide (CO) production because CO is known as a major cause of fatalities in compartment fire. To reveal the CO production in gaseous and liquid combustion under reduced oxygen concentration, the CO yield under oxygen-reduced condition was investigated experimentally using a cup burner. CH4 and CH3OH were used as fuels and supplied through the inner cup. Mixtures consisting of oxidizer (O2) and fire extinguishing agent (N2) were issued through the outer chimney. The relative percentages of O2/N2 were varied, maintaining the total flow rate of O2 + N2 constant. The CO concentration in the post-combustion mixture was measured as a function of the oxygen concentration. In the case of CH4 flame, the CO concentration was increased with decreasing O2 concentration. In contrast, in the case of CH3OH, the CO concentration was decreased with decreasing O2 concentration. The results show that the influence of the oxygen concentration on the CO production yield in liquid combustion is apparently different from that of gaseous combustion.