Kaoru Maruta

On the extinction limit and flammability limit of non-adiabatic stretched methane{air premixed flames

Extinction limits and the lean flammability limit of non-adiabatic stretched premixed methane-air flames are investigated numerically with detailed chemistry and two different Planck mean absorption coefficient models. Attention is paid to the combined effect of radiative heat loss and stretch at low stretch rate. It is found that for a mixture at an equivalence ratio lower than the standard lean flammability limit, a moderate stretch can strengthen the combustion and allow burning. The flame is extinguished at a high stretch rate due to stretch and is quenched at a low stretch rate due to radiation loss. A O-shaped curve of flame temperature versus stretch rate with two distinct extinction limits, a radiation extinction limit and a stretch extinction limit respectively on the left- and right-hand sides, is obtained. A C-shaped curve showing the flammability limit of the stretched methane-air flame is obtained by plotting these two extinction limits in the mixture strength coordinate. A good agreement is shown on comparing the predicted results with the experimental data. For equivalence ratio larger than a critical value, it is found that the O-shaped temperature curve opens up in the middle of the stable branch, so that the stable branch divides into two stable flame branches; a weak flame branch and a normal flame branch. The weak flame can survive between the radiation extinction limit and the opening point (jump limit) while the normal flame branch can survive from its stretch extinction limit to zero stretch rate. Finally, a G-shaped curve showing both extinction limits and jump limits of stretched methane-air flames is presented. It is found that the critical equivalence ratio for opening up corresponds to the standard flammability limit measured in microgravity. Furthermore, the results show that the flammability limit (inferior limit) of the stretched methane-air flame is lower than the standard flammability limit because flames are strengthened by a moderate stretch at Lewis number less than unity.

Burning velocity of turbulent premixed flames in a high-pressure environment

To explore the effects of ambient pressures on the turbulent burning velocity in a high-pressure environment, turbulent premixed flames of lean methane-air mixtures stabilized with a nozzle-type burner in a high-pressure chamber were investigated experimentally. Continuous combustion was investigated up to pressures of 3.0 MPa. Measurements of turbulent burning velocity were made using a mean-angle method based on a technique involving laser tomography and image processing. Results show that the effects of elevated pressure on turbulent burning velocity are significant and that the ratio of turbulent to laminar burning velocities S T / S L increases with both turbulence intensity u′ and pressure, reaching a value of 30 at 3.0 MPa under the present experimental conditions. The increases in S T / S L with increasing u ′/ S L , are rapid at high pressure, particularly for small u ′, that is, in the region of weak turbulence. An interesting similarity, of S T / S L variations was observed between the effect of pressure found in this experiment and the effect of a density jump as analyzed by Cambray and Joulin. Flame front instability theory based on Sivashinskys formulation was applied to flames in high-pressure environments: it was found that the region of wave numbers where the flame front is unstable extends to larger wave numbers with increasing pressure because the diffusive-thermal effect, which stabilizes the hydrodynamic instability, weakens. This suggests that hydrodynamic instability which enlarges the total flame area, plays an important role in the rapid increase of S T / S L with pressure in high-pressure environments.

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 and 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 C-shaped 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.

Extinction limits of catalytic combustion in microchannels

The limits to self-sustaining catalytic combustion in a microscale channel were studied computationally using a cylindrical tube reactor. The tube, 1 mm in diameter, 10 mm long, and coated with Pt catalyst, was assumed to be thermally thin, and the boundary condition on the wall was set to be either adiabatic or non-adiabatic with fixed heat transfer coefficient. Methane/air mixtures with average velocities of 0.0375–0.96 m/s (corresponding to Reynolds number, Re, ranging from 2.5 to 60) were used. When the wall boundary condition was adiabatic, the equivalence ratio at the extinction limit monotonically decreased with increasing Re. In contrast, for non-adiabatic conditions, the extinction curve exhibited U-shaped dual limit behavior, that is, the extinction limits increased/decreased with decreasing Re in smaller/larger Re regions, respectively. The former extinction limit is caused by heat loss through the wall, and the latter is a blow-off-type extinction due to insufficient residence time compared to the chemical timescale. These heat-losses and blow-off-type extinction limits are characterized by small/large surface coverage of Pt(s) and conversely large/small numbers of surface coverage of O(s). It was found that by diluting the mixture with N2 rather than air, the fuel concentration and peak temperatures at the limit decreased substantially for mixtures with fuel-to-oxygen ratios even slightly rich of stoichiometric because of a transition from O(s) coverage to CO(s) coverage. Analogous behavior was observed experimentally in a heat-recirculating “Swiss-roll” burner at low Re, suggesting that the phenomenon is commonplace in catalytic combustors near extinction. No corresponding behavior was found for non-catalytic combustion. These results suggest that exhaust-gas recirculation rather than lean mixtures are preferable for minimizing flame temperatures in catalytic microcombustors.

Turbulence measurements and observations of turbulent premixed flames at elevated pressures up to 3.0 MPa

Abstract In order to explore the characteristics of turbulence and turbulent premixed flames in a high-pressure environment, a nozzle-type burner with a turbulence generator was installed in a high-pressure chamber. Turbulence measurements and combustion experiments were conducted with the chamber pressure up to 3.0 MPa. Methane-air mixtures were used for the combustion experiments and confirmed that the turbulent premixed flames were successfully stabilized. Flame observations were made using instantaneous Schlieren photographs and high-speed laser tomography. Turbulence measurements were conducted using a hot-wire anemometer installed in the high-pressure chamber. It was found that the scales of turbulence generated by perforated plates at elevated pressure are smaller than those at atmospheric pressure. From flame observations, the following features of the flames at elevated pressure were found: (1) wrinkled structures of the flames become very fine and complex, and the cusps become sharp as pressure rises;(2) the flamelet breaks at many points of the flames and the scales of broken flamelets become small; (3) small-scale parts of the flame front convex to the unburned mixture frequently occur and move quickly to the unburned side. The effects of ambient pressure on turbulence characteristics and possible mechanisms which produce the wrinkled structure of the fine scales and generate flame front disturbances in the high-pressure environment are discussed.

Experimental study on general correlation of turbulent burning velocity at high pressure

Turbulent burning velocities for lean C2H4-air and C3H8-air mixtures were measured for Bunsen-type turbulent premixed flames stabilized in a high-pressure chamber keeping the chamber pressure constant. The experiments were performed for a pressure range up to 1.0 MPa and for u′/SL range up to 25. The effects of equivalence ratio, Lewis number, and pressure exponent of laminar burning velocity were examined. Results show that for both C2H4-air and C3H8-air flames, ST/SL increases rapidly with increasing u′/SL, particularly for weak turbulence (u′/SL The verifications of existing correlations between ST/SL, u′/SL, and turbulence Reynolds number, R1, were made to elucidate effects of elevated pressure on turbulent burning velocity and the relationship between these parameters. Results show that the data for our experiments had a similar tendency to the general correlation proposed by Abdel-Gayed et al. on turbulent burning velocity but had a larger ST/SL than the correlation. A 1/4 power law of R1 was seen for data in u′/SL>1.0, consistent with the fractal flamelet model of turbulent burning velocity proposed by Gouldin. The best correlation between these parameters derived from the experimental data was ST/SL ∝ [(P/P0)(u′/SL)]n for the whole range of u′/SL and the exponent, n, is close to 0.4.

Nonlinear dynamics of flame in a narrow channel with a temperature gradient

The non-stationary behaviour of near-limit premixed flame propagating in a microchannel with temperature gradient was theoretically investigated. A one-dimensional (1D) nonlinear evolutionary equation of the flame front was obtained. The nonlinear model outlined the flame stabilization, nonlinear flame oscillations and flames with repetitive extinction and ignition processes that were observed in experiments.

Radiation extinction limit of counterflow premixed lean methane-air flames

Abstract The application of the laminar flamelet concept to turbulent flame modeling requires a detailed understanding of stretched laminar flames. In this study, we used numerical methods, including are-length continuation, to simulate the extinction characteristics of counterflow premixed fuel-lean, methane-air flames. Attention was primarily paid to the effect of radiative heat loss on the extinction characteristics of these flames. The results show that at medium to low values of the stretch rate, the radiative heat loss has a particularly strong impact on the counterflow premixed fuel-lean, methane-air flames. It was also found that, in addition to the stretch extinction limit at a high stretch rate, there exits a radiation extinction limit at a low stretch rate. Furthermore, the relationship between these two extinction limits and the equivalence ratio is obtained.

Effects of the Lewis number and radiative heat loss on the bifurcation and extinction of CH 4 /O 2 -N 2 -He flames

Eects of the Lewis number and radiative heat loss on flame bifurcations and extinc- tion of CH 4/O2-N2-He flames are investigated numerically with detailed chemistry. Attention is paid to the interaction between radiation heat loss and the Lewis number eect. The Planck mean absorption coecients of CO, CO2, and H2O are calculated using the statistical narrow-band model and compared with the data given by Tien. The use of Tiens Planck mean absorption coecients overpredicts radiative heat loss by nearly 30 % in a counterflow conguration. The new Planck mean absorption coecients are then used to calculate the extinction limits of the planar propagating flame and the counterflow flame when the Lewis number changes from 0.967 to 1.8. The interaction between radiation heat loss and the Lewis number eect greatly enriches the phenomenon of flame bifurcation. The existence of multiple flames is shown to be a physically intrinsic phenomenon of radiating counterflow flames. Eight kinds of typical patterns of flame bifurcation are identied. The competition between radiation heat loss and the Lewis number eect results in two distinct phenomena, depending on if the Lewis number is greater or less than a critical value. Comparisons between the standard limits of the unstrained flames and the flammability limits of the counterflow flames indicate that the flammability limit of the counterflow flame is lower than the standard limit when the Lewis number is less than the critical value and is equal to the standard limit when the Lewis number is higher than this critical value. Finally, a G-shaped curve and a K-shaped curve which respectively represent the flammable regions of the multiple flames for Lewis numbers lower and higher than the critical value are obtained. The G- and K-shaped curves show a clear relationship between the stretched counterflow flame and the unstrained planar flame. The present results provide a good explanation of the physics revealed experimentally in microgravity.

EXPERIMENTAL STUDY ON 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 11 were successfully established and the influence of the stagnation velocity gradient on extinction was examined under conditions of a microgravity field of 10 14 to 10 15 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.