Methane-air Jet Diffusion Flame Research Articles

Blowoff mechanism of airburst blast extinguishment of a methane air jet diffusion flame with micro explosive

In the study, experiments of airburst blast extinguishment of a methane-air jet diffusion flame have been conducted with 20 mg silver azide pellets, and the extinguishing processes have been observed using a high-speed camera and schlieren method. From the experimental results, we have examined the extinguishing mechanisms. When the blast wave interacts with the diffusion flame, vortex pair is generated in the flame due to the baroclinic effect which is caused by the misalignment between the pressure gradient of the blast wave and the density gradient in the combustion region. The vortical motion leads to the local extinction, and finally, the whole flame is blown off. In airburst, a reflected shock wave occurs on the ground surface. Depending on the height of burst and the distance between the explosion location and the extinguishing target, the reflected blast wave merges with the incident blast wave and the Mach stem is formed before the blast wave interferes with the diffusion flame. As a result, it is found that the extinguishing effectiveness is increased by the presence of the Mach Stem. Moreover, the optimal height of burst in the airburst blast extinguishment is clarified.

Examination of Extinguishment Method with Liquid Nitrogen Packed in a Spherical Ice Capsule

Liquid nitrogen which is a cryogenic fluid can be used as a fire extinguishing agent. This is because liquid nitrogen vaporizes rapidly under atmospheric condition, which results in cooling of burning materials and its surroundings in firefighting, and also the volume of the vaporized nitrogen becomes approximately 700 times larger than that of its liquid state, which results in reduction of oxygen concentration in air or fuel concentration in combustion zone. Moreover, liquid nitrogen has no water damage and can extinguish fires more cleanly than dry chemical extinguishing agents. However, it is difficult to delivery liquid nitrogen over long distance from the extinguishing equipment to fires through the surrounding air because of its rapid vaporization. If liquid nitrogen is filled into a capsule and the capsule wall suppresses heat transfer from the surroundings to the liquid nitrogen, it will be easy to transport liquid nitrogen to the targeted fire area without rapid vaporization. Moreover, by using the capsule, it may be possible to increase the extinguishing effectiveness of liquid nitrogen. In the present study, in order to clarify the fundamental characteristics of flame extinguishment by using the capsule filled with liquid nitrogen, extinguishing experiments of a methane-air jet diffusion flame have been performed. The spherical hollow ball made of ice was used as the extinguishing capsule and the ice capsule was formed by rotating casting machine. The wall thickness and the outer diameter were 2 mm and 20 mm, respectively. The filling volume of liquid nitrogen was 20 cm3. The ice capsule was dropped freely from the height of 800 mm and impacted on the aluminum plate, in which the round burner to form a jet diffusion flame was embedded. The extinguishing probability was measured by varying the distance from the impact point of the ice capsule and the flame. The extinguishing processes were recorded with high-speed camera. Moreover, for comparison, the same extinguishing experiments have been performed using the ice capsule filled with water. As a result, liquid nitrogen shows the higher extinguishing effectiveness than water because liquid nitrogen droplets released from the ice capsule to the collided horizontal plate can travel over the longer distance due to the Leidenfrost effect and liquid nitrogen can be vaporized rapidly under room temperature condition. The extinguishable range of the ice capsule filled with liquid nitrogen is less than 200 mm.

ODT Closure with Extinction and Reignition in Piloted Methane-Air Jet Diffusion Flames

A novel tabulation procedure for reactive scalar statistics based on the one-dimensional turbulence (ODT) is implemented to study extinction and reignition in piloted methane-air jet diffusion flames. The formulation is based on constructing the scalar statistics from stand-alone temporal jet simulations using ODT. The statistics are correlated in terms of two parameters based on a single transported variable: the mean mixture fraction, which measures the extent of mixing between the fuel and oxidizer streams, and the centerline mixture fraction, which measures the extent of entrainment into the fuel jet or the jet evolution downstream. The evolution of momentum and passive scalars is computed using a Reynolds-Averaged Navier-Stokes (RANS) formulation, which uses the 2D table for look-up of the mean density. Other reactive scalars' profiles are obtained from the 2D table and the computed momentum and scalar fields from RANS. Comparison of the computed and the experimental statistics for momentum and scalars shows that the tabulation scheme along with the RANS model yields reasonable predictions of the processes of extinction and reignition in piloted jet diffusion flames.

G0601-6-4 噴流拡散火炎のシャボン玉消火過程の可視化(熱工学(6)燃焼(1))

Basically, the filling gas in a soap bubble dose not diffuse into the surrounding air, and the soap bubble can keep the filling gas concentration constant until its bursting. If the soap bubble is filled with an extinguishing gas, it can be used as a fire extinguishing ball. In order to clarify the extinguishment characteristics and mechanisms, we have performed blowout experiments of a methane-air jet diffusion flame by using the nitrogen-filled soap bubble and visualized the blowout processes by schlieren method. As a result, when the nitrogen-filled soap bubble contacts at the thermal boundary of the flame base of the jet diffusion flame, the soap liquid film ruptures and the extinguishing gas is supplied to the flame. Then the local extinction occurs at the flame base, and the rest flame lifts off. Finally, the whole flame is blown out.

G0601-6-3 渦輪を利用した噴流拡散火炎の消火特性(熱工学(6)燃焼(1))

A vortex ring is easily produced by pushing fluid through a circular orifice. If the vortex ring interacts with a diffusion flame, a local extinction occurs. In order to use a vortex ring for fire fighting, we have investigated the characteristics of the air vortex ring extinguishment of a methane-air jet diffusion flame. From the blowout experiments, we calculated probability of the vortex ring blowout and measured displacement speed of the vortex ring. As a result, when the air vortex ring impacts with the flame base zone of the jet diffusion flame at the displacement speed enough to generate a local extinction, the whole flame can be extinguished.

Conditional Moment Closure for Large Eddy Simulations

A conditional moment closure (CMC) based combustion model for large-eddy simulations (LES) of turbulent reacting flow is proposed and evaluated. Transport equations for the conditionally filtered species are derived that are consistent with the LES formulation and closures are suggested for the modelling of the conditional velocity, conditional scalar dissipation and the fluctuations around the conditional mean. A conventional β-probability density distribution of the scalar is used together with dynamic modelling for the sub-grid fluxes. The model is validated by comparison of simulations with measurements of a piloted, turbulent methane-air jet diffusion flame.

Numerical Investigations Of CO2 As Fire Suppressing Agent

CF3Br (Halon 1301) and inert gases is useful for their efficient use and for developing new agents. Because of the similarities between unsteady jet diffusion flames formed over the cup burner and uncontrolled fires, it is believed that studies of fire-suppressing agents in the former system could provide valuable information on the behavior of such agents in actual fires. In the present study, suppression characteristics of CO² were investigated in two flame systems: 1) a periodically oscillating, methane-air jet diffusion flame formed over a cup burner, and 2) a steady-state planar flame formed between opposing jets of fuel and air. A detailed chemical-kinetics model having 31 species and 346 elementary-reaction steps was used. Calculations made for the cup burner yielded a flame-flicker frequency of about 10 Hz. The suppression mechanisms promoted by CO² were investigated by adding CO² to the airflow, while maintaining the total flow rate constant, for both the cup-burner and opposed-jet flames. In the cup-burner flame, the addition of CO² reduced the flame temperature to ~1620 K at suppression. Addition of CO² destabilized the flame base, which then moved downstream in search of a new stabilization location. For CO² volume fractions greater than 14.5 %, the flame base moved out of the computational area, as it could not find a stabilization point within this domain. The unsteady flickering motion of the flame and higher concentrations of CO² accelerated this quenching process through blowout. Even for very high concentrations of CO², the calculations did not yield simultaneous quenching of the entire cup-burner flame. On the other hand, the opposed-jet flame was extinguished through the global extinction of flame chemistry. The low-strain (30 s-1) opposed-jet flame extinguished for CO² volume fractions > 16.4 %, while the moderately strained (90 s-1) flame extinguished for volume fractions > 10.4 %. Both the opposed-jet flames extinguished nearly at the same flame temperature (~1580 K), indicating that the extinction limits in these flames are primarily controlled by chemical kinetics.

Advanced Fluid Information. Effect of presumed PDF selection on the numerical result for turbulent diffusion flames.

In the present work, the effect of probability density function (PDF) selection and intermittency on the result of the numerical simulation are examined by the simulation of a turbulent methane-air jet diffusion flame, where air is preheated to 1073K. As to the PDFs, beta-function and clipped Gaussian are considered. It is found that intermittency has a large impact on the numerical result, while the choice of PDF has a minor influence. When intermittency is considered, the flame height becomes larger and maximum temperature decreases. Comparison of radial profiles with experimental data shows that it is necessary to consider intermittency also for high temperature air diffusion flames.

A reaction kernel hypothesis for the stability limit of methane jet diffusion flames

The lifting limit of an axisymmetric, laminar, co-flow methane-air jet diffusion flame under normal earth gravity has been successfully predicted. Computations of the time-dependent full Navier-Stokes equations with buoyancy were performed using an implicit, third-order accurate numerical scheme and a detailed C 2 chemistry model. A one-step global chemistry model was also used to reveal its deficiencies and to demonstrate the need for “tuning” its kinetic parameters for the studies on flame lifting. The detailed chemistry model resulted in the standoff distance of the flame from the burner rim in good agreement with that measured previously. As the mean co-flow air velocity was increased, at a fixed fuel jet velocity under the near-limit condition, the calculated reaction kernel (peak reactivity spot) in the flame base broadened and rapidly shifted away downstream. As a result, a higher reactivity (heat-release rate, oxygen consumption rate, etc.) at the reaction kernel could be obtained to sustain combustion against a higher incoming flow velocity, or a shorter residence time. The reactivity augmentation is due to a “blowing” effect, which caused enhanced convective and diffusive fluxes of oxygen into the relatively low-temperature (∼1550 K) fuel-lean (equivalence ratio ≈0.55) reaction kernel. Based on these new findings, a reaction kernel hypothesis is proposed for the diffusion flame stability, namely, that a subtle balance between the residence time and reaction time in the reaction kernel is maintained by its continuous movement in the downstream direction in response to the destabilizing effect caused by an increase in the co-flow air velocity, and the overall reaction time eventually exceeds the available residence time at the stability limit. If a secondary stabilizing point is obtained as a result of the transition to a turbulent flame base downstream, the flame lifts off, otherwise it blows off.

The effect of coflow velocity on a lifted methane-air jet diffusion flame

The effect of coflow velocity on flame liftoff is studied using numerical simulations of methane-air diffusion flames. The numerical model solves the time-dependent, axisymmetric (2-D), Navier-Stokes equations coupled to submodels for chemical reaction and heat release, soot formation, and radiation transport. The computations predict a flame structure similar to that observed experimentally. The flame stabilization point is located on the stoichiometric surface in the inner shear layer. Animations of the simulations show that the radial and axial location of the stabilization point varies in time by 1–2 cm, as the flow field is distorted by passing vortices in the inner shear layer. Flame liftoff heights compare well with those observed experimentally. The computations show that liftoff height increases with jet exit velocity and with the air coflow velocity. For higher coflow velocities, the inner-shear-layer vortices begin to form farther downstream, and the jet spreads more slowly. The scalar dissipation rate along the stoichiometric contour decreases sharply at the liftoff height. All of the liftoff data for a wide range of jet (20–50 m/s) and coflow (10–1500 cm/s) velocities collapse onto one curve when liftoff height is plotted against an effective velocity. These data indicate that the momentum of the coflowing stream acts in combination with the jet momentum to push the base of the flame farther away. This suggests that the momentum at the flame base is a strong factor in determining flame liftoff height.

Primitive Variable Modeling of Multidimensional Laminar Flames

Abstract We employ an accurate numerical algorithm to simulate two model flames—an unconfined lifted and a confined, coflowing, methane-air jet diffusion flame using detailed chemistry and complex transport models. The algorithm employs Newton's method to obtain the primitive variable solution of the large system of strongly coupled elliptic governing equations. The Newton equations are solved by a block-line tridiagonal method. We employ a global grid refinement technique which equidistributes meshes according to the gradients and curvatures of the solution obtained on the previous mesh and bounds the ratio of adjacent grid step size. The algorithm can be applied to problems ranging from non-reacting flows to reacting flows in two- or three-dimensional configurations. In the unconfined case, the lifted flame and the “triple flame” are both predicted in the numerical solution. The computed solutions agree well with the experimental results. The comparison of the present solutions with the previously repor...

Stabilization Heights in Lifted Methane-Air Jet Diffusion Flames Diluted with Nitrogen

Abstract Abstract– Measurements of stabilization heights in lifted turbulent methane-air jet diffusion flames are performed in order to test a theory on local quenching of dilfusion flamelets. The global residence time (d/u0 the nozzle diameter, u0, the nozzle exit velocity) is scaled with Zq the instantaneous scalar dissipation rate at quenching. This quantity had been identified before to be charncteristic for the extinction of diffusion flamelets. Values for Zg, are deduced from measurements of laminar diffusion flames in a counter flow geometry. The scaling is performed for constant values of Zq the conserved scalar variable at sioichiometric conditions. Both the fuel and the air stream are diluted to obtain constant values of Zq. Scaling with Zq makes the curves for different residence times collapse into one curve with however, considerable experimental scatter. From this is concluded that Zq, is the essential kinetic parameter describing quenching effects in turbulent diffusion flames.