Experimental study was made to investigate the flame which propagates through a methane/air mixture flow with periodic concentration fluctuation by using an original stagnation flow type burner. The burner was devised so as to fluctuate the mixture concentration (equivalence ratio) only in the direction of flow without varying the velocity field. Two kinds of fluctuation, (ϕm, a)=(0.85, 0.06) and (1.07, 0.17), where ϕm is the mean equivalence ratio and a is the fluctuation amplitude, were examined for the frequency f ranging from 3 Hz to 50 Hz, and following results were obtained: (1) In a certain range of f, the fluctuation widths of the burning velocity, the temperature and the CO concentration of the burnt gas were larger than those expected from the equivalence ratio fluctuation of the mixture at the burner exit. (2) For the lean side fluctuation with ϕm=0.85 and a=0.06, the time average NOx concentration of the burnt gas for the dynamic flame is higher than that for the static flame in the low frequency range, but this relation is reversed in the high frequency range. (3) For the fluctuation with ϕm=1.07 and a=0.17, which includes the stoichiometric condition, the NOx concentration of the dynamic flame is always lower than that of the static flame, and it lowers with an increase in f and has the minimum at a certain value of f. These characteristics of the dynamic flame can be related with the periodic change of the direction of flame movement; when the flame moves against (along) the mixture flow, the inflow mass flux of the deficient reactant at the flame front increases (decreases) as compared with the steady or stationary flame.
A new concept of the droplet array generation technique was proposed to realize a high quality and high reliability performance of microgravity experiments of multiple-droplet combustion. Each fuel droplet was formed on the intersection of the X-shape fine SiC fibers by supplying the liquid fuel through a fine glass tube. Several sets of the X-shape fibers and corresponding fine glass tubes were aligned at an interval to make a droplet array. All the droplets in array were simultaneously generated in a short time. In flame spread experiments, an end-droplet was ignited by a hot-wire igniter to initiate the flame spread along the array. Microgravity experiments of droplet array combustion were demonstrated with the new droplet array generation technique using a drop experiment facility MGLAB in Japan. In microgravity, large droplets, which were often failed to be generated in normal gravity, were successfully generated through this method. The present technique is also effective in droplet array combustion experiments using a high-volatility fuel, whose prevaporization is not negligible. The flame spread rate and the flame spread limit of linear droplet arrays were compared with existing experimental results. Effects of the suspending fiber on the flame spread were also discussed.
Nitrogen gas can be an extinguisher or a mitigating material in the case of sodium leak and fire accident in an air atmosphere, which may occur at a liquid metal cooled nuclear power plant. However sodium combustion residuum sometimes reignites in the air atmosphere even at room temperature when it was produced by nitrogen gas injection to the burning sodium. In this study we have been investigating the cause of reignition and prevention measures. Experiments were carried out with small type test equipment, which can handle 1 g order sodium fire and extinguishment. Sodium combustion residua, which were made by our equipment and sampled, were analyzed by X-ray diffraction and chemical analyses. The chemical analysis of reignitable residua showed that the residuum contained metallic sodium of about 40 wt-% (61 mol-%) to 60-wt% (76 mol-%) and most of the rest was sodium-monoxide (Na2O). Sodium-peroxide (Na2O2) was also included in less than 1 wt-% of the residuum. Sodium or Na2O cannot ignite by itself in the air atmosphere at room temperature in a few minutes. Therefore the reignition seems to be due to increase in the local temperature that is caused by oxidizing heat of Na and by adiabatic effect of Na2O. It is important to deactivate this dispersed sodium on oxygen for prevention of the residuum reignition, hence it is considered as a rational measure to change the sodium to sodium-carbonate. Our experiments showed that the dispersed sodium on the exterior of residuum could be changed to carbonate by a mixture of carbon-dioxide (CO2) gas (2 to 8vol-%), humidity (0.6 to 3vol-%) and nitrogen gas. The deactivated residuum did not reignite in the air atmosphere below 473 K.