Japan's research on gaseous flames

Although research studies on gaseous flames in microgravity in Japan have not been one-sided, they have been limited, for the most part, to comparatively fundamental studies. At present it is only possible to achieve a microgravity field by the use of drop towers, as far as gaseous flames are concerned. Compared with experiments on droplets, including droplet arrays, which have been vigorously performed in Japan, studies on gaseous flames have just begun. Experiments on ignition of gaseous fuel, flammability limits, flame stability, effect of magnetic field on flames, and carbon formation from gaseous flames are currently being carried out in microgravity. Seven subjects related to these topics are introduced and discussed herein

Numerical Study on NOx Emission from an Industrial Furnace Utilizing Regenerative Technology

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Effects of radiative heat loss on the extinction of counterflow premixed H2-air flames

Radiation heat loss has an important impact on near-limit flames. It has been shown that radiation heat loss can make stretched CH4-air flames extinguish at a lower stretch rate. Numerical calculations of counterflow premixed H2-air flames were conducted using an accurate description of the chemical kinetics and transport properties. Radiation heat loss was considered. The results show that in addition to the stretch extinction limit, radiation heat loss also allows the lower and middle equivalence ratio counterflow premixed H2-air flames to extinguish at a lower stretch rate. For middle equivalence ratio counterflow H2-air flames, the closed temperature profiles become distorted O-shaped curves due to the lower Lewis number, being different from those of CH4-air flames. For higher equivalence ratio counterflow premixed H2-air flames, there are two stable flame branches\u2014a normal flame branch and a weak flame branch. When the equivalence ratio is greater than a critical value, the closed temperature profile curve of every equivalence ratio flame opens and the normal flame curve can be extended to zero stretch rate. The calculation of the concentration limit of one-dimensional planar premixed H2-air flames was also conducted. The results show that the critical equivalence ratio corresponds to the concentration limit of the one-dimensional planar premixed H2-air flame. The extension of the flammable region due to the stretch is amplified for the counterflow H2-air flame because of its much lower Lewis number than that of the counterflow CH4-air flame.NRC publication: Ye

Experimental study of methane-air premixed flame extinction at small stretch rates in microgravity

Extinction experiments on counterflow methane-air premixed flame at small stretch rates were conducted under microgravity conditions to measure the fuel concentration of the mixture at extinction over a range of very low stretch rates and to observe extinction characteristics of small stretch-rate flames in which radiative heat loss may be dominant. By employing a low-speed counterflow system and microgravity, it is possible to remove the effects of buoyancy, conductive heat loss to the burner, and flame curvature on the flame extinction simultaneously. Consequently, stable counterflow twin flames at low stretch rates can be established, and hence excellent conditions for extinction measurements and observations can be realized. Stationary counterflow premixed flames at stagnation velocity gradients ranging from 4 to 20 s ‫1מ‬ were successfully established and the influence of the stagnation velocity gradient on extinction was examined under conditions of a microgravity field of 10 ‫4מ‬ to 10 ‫5מ‬ g with a duration of 10 s. This field is generated by the 490-m drop shaft of the Japan Microgravity Center (JAMIC) in Hokkaido, Japan. Results show that extinction limits strongly depend on the stagnation velocity gradient even in the range of low stretch rates, and there is a turning point on the left portion of the extinction limit curve. A recent numerical calculation involving radiative heat loss indicated the existence of a radiation extinction limit at a certain low stretch rate, in addition to the stretch extinction limit at a large stretch rate. This suggests that the existence of the turning point observed in the present experiment may be the result of radiative heat loss

Flame Bifurcations and Flammable Regions of Radiative Counterflow Premixed Flames with General Lewis Numbers

Flame bifurcations, extinction, and flammable regions of radiative counterflow premixed flames and the planar propagating flames are investigated numerically with general Lewis numbers. New insights and phenomena are presented by examining the interaction of radiation heat loss, Lewis number effect, and stretch. At a low Lewis number, the results show that for equivalence ratio lower than the standard flammability limit, radiation extinction limit and stretch extinction limit exist, respectively, at low and high stretch rates. For a higher equivalence ratio, this flame isola opens up and yields two stable flame branches, a normal flame branch, and a weak flame branch. A G-shaped curve showing the limits and flammable regions of these two flame regimes is obtained. The results show that the inferior limit of counterflow flame is lower than the standard limit. At unity Lewis number, the results show that, for equivalence ratio lower than the standard limit, there is also a flame isola with two distinct extinction limits at low and high stretch rates. For equivalence ratio larger than the standard limit, a new flame branch with flame standing far from the stagnation plane emerges on the low stretch rate side of the flame isola. This flame is related to the standard limit with the decrease of fuel concentration and merges with the flame isola when the fuel concentration is larger than a critical value. The extinction curve is also a G-shaped curve and the inferior limit is lower than the standard limit. At Lewis number larger than a critical value, for equivalence ratio near the standard limit, there is only the weakly stretched flame branch with a single stretch extinction limit. With an increase of fuel concentration, a weak flame isola emerges. As the fuel concentration further increases, this weak flame isola merges with the weakly stretched flame branch yielding three kinds of flame regimes. The resulting extinction curve is a K-shaped curve and the inferior limit is equal to the standard limit. The present results show that flame bifurcations and the existence of weak flames are physically intrinsic phenomena of radiative counterflow premixed flames. It is found that the normal flame and the weak flame can jump to each other at their limits. The obtained G and K-shaped curves show a clear relation between the counterflow flame and the planar propagating flame. The occurrence of G\u2013K transition with the increase of Lewis number also gives a good explanation to the physics of the experimental results.NRC publication: Ye

Extinction of low-stretched diffusion flame in microgravity

Extinction of counterflow diffusion flames of air and methane diluted with nitrogen is studied by drop tower experiments and numerical calculation using detailed chemistry transport properties. Radiative heat loss from the flame zone is taken into consideration. Experimental results identified two kinds of extinction at the same fuel concentration, that is, in addition to the widely known stretch extinction, another type of extinction is observed when the stretch rate is sufficiently low. Consequently, plots of stretch rates versus fuel concentration limits exhibit a \u201cC-shaped\u201d extinction curve. Numerical calculation including radiative heat loss from the flame zone qualitatively agreed with the experimental results and indicated that the mechanism of counterflow diffusion flame extinction at low stretch rates was radiative heat loss.NRC publication: Ye