Numerical and Experimental Study on Flame Dynamics of the Premixed Methane-Air Mixture at Different Ignition Positions in a Two-Side 45° Branch Tube.

The combustion characteristics of premixed methane-air flames in a half-open tube with a two-sided 45° branch structure at different ignition positions were investigated by experiments and large eddy simulations. The numerical results were compared with the experimental results to verify the correctness of the model. The results show that the simulation results are highly consistent with the experiment. This study provides a basic understanding of the effects of the branch tube structure and the ignition position on flame dynamics. When the flame propagates to the branch interface, it forms a symmetrical vortex structure at the branch tube with the opposite rotation direction. When the ignition position is at IP0 and IP900, the maximum overpressures obtained in the experiment are 10.1 and 10.7 kPa, respectively, and 9.2 and 10.4 kPa in the simulation, respectively. At IP0, the Karlovitz number indicating the interaction intensity between the flame surface and the turbulence during flame propagation is a maximum of 9.2 and a minimum of 0.04. The premixed flame has a folded small flame, a corrugated small flame, and a thin reaction zone.

Flame Propagation Characteristics of Syngas-Air in the Hele-Shaw Duct with Different Equivalence Ratios and Ignition Positions.

In this paper, the effects of different ignition positions and equivalence ratios on the explosion characteristics of syngas in a half-open Hele-Shaw duct were investigated. The ignition points are set at distances of 0 and 500 mm from the closed end. Moreover, the research range of equivalence ratio is 0.8-1.2. The experimental results indicate that different ignition positions and equivalence ratios influence the flame front structure and the dynamic characteristics of flame propagation. When the ignition position is at the closed end, the flame front undergoes several typical propagation stages before eventually reaching the open end of the duct. The time required by the flame to reach the open end decreases as the equivalence ratio increases. Meanwhile, when the ignition is in the middle of the duct, the flame simultaneously spreads to the open and closed ends. The time required to reach both sides decreases with the increase in the equivalence ratio. The flame front structure and pressure are primarily affected by the ignition position and the equivalence ratio. At the same ignition position, flame propagation velocity and maximum overpressure increase with the equivalence ratio. The pressure oscillation becomes more intense when the ignition position is close to the open end. At IP500, when the equivalence ratio is 0.8, multiple finger-shaped flame fronts emerge, accompanied by high-frequency flame oscillations. This study can provide guidance for the study of the flame propagation characteristics of syngas in millimeter-scale burners.

Kinetic Analysis of Laminar Combustion Characteristics of a H2/Cl2 Mixture at CO2/N2 Dilution.

Garbage and biomass contain more chlorine, which reacts with H2 to form HCl gas during combustion or gasification, resulting in corrosion of metal walls. In this paper, based on the chlorine mechanism in Ansys Chemkin-Pro, the laminar combustion characteristics of H2/Cl2 are simulated with different diluents CO2/N2 under an initial temperature of 298 K, equivalence ratio range of 0.6-1.4, and initial pressure of 0.1-0.5 MPa. The results show that the laminar burning velocity of H2/Cl2 decreases significantly with the increase of dilution gas ratio, and the effect of diluent CO2 is more significant than that of N2. Due to the dilution effect, the fuel and oxidation components are reduced. Through sensitivity analysis, reaction R2: Cl + H2 = HCl + H is the main reaction of HCl formation. On improving the initial pressure, the laminar burning velocity is slightly lowered, and the thermal diffusivity of the fuel mixture increases with the increase of the initial pressure. According to the sensitivity analysis of the velocity, reactions R2, R9, and R10 are the main reactions that affect the laminar burning velocity, and the product HCl will be generated with a delay with the increase of the initial pressure.

Numerical Simulation of the Effect of CH4/CO Concentration on Combustion Characteristics of Low Calorific Value Syngas.

The composition of low calorific value synthesis gas varies greatly depending on the raw material and processing technology, which makes the combustion extremely complicated. The three mechanisms of the GRI-Mech 3.0, Li-Model, and FFCM-Mech are used to numerically simulate CH4/CO/H2/N2 air premixed combustion by using ANSYS CHEMKIN-PRO. The numerical simulation is the calculation of laminar flame velocity and adiabatic flame temperature at an initial temperature of 298 K, an equivalence ratio of 0.6-1.4, and an initial pressure of 0.1-0.5 MPa, discussing through thermodynamics and chemical kinetics. The formation of NO X , H, and OH radicals by fuel composition was analyzed. The result shows that the concentrations of H, O, and OH radicals have a positive effect on laminar flame velocity. The combustion reaction of H2 is higher than that of CH4 and CO; with the increase of N2 content, the priority is higher. The thermal diffusivity of flame under different equivalence ratios is affected by inert gas, which affects adiabatic combustion temperature and laminar combustion velocity. In thermal kinetics and chemical kinetics, CH4 has more influence on combustion temperature than CO, while laminar flame velocity is relatively low. Under the change of initial pressure, the laminar combustion flux increases to the initial pressure and the laminar combustion velocity decreases to the increase in pressure. Reactions H + O2 = O + OH, HO2 + H = 2OH, and CH3 + HO2 = OH + CH3O are mainly due to change in the concentration of O, H, and OH radicals.

Effect of the Inclination Angle on Premixed Flame Dynamics in Half-Open Ducts.

The propagation of premix methane/air flames in long half-open ducts with different inclination angles θ between the sidewall and the horizon was numerically investigated using the laminar model. The numerical result was compared with the experimental and theoretical ones to validate the numerical model. The results show that the numerical results are in good agreement with them. The investigation provides the basic understanding of the effects on changing the shape of the ducts to promote the premixed flame combustion. For methane/air, the position where the flame front begins to concave is pushed back with the increase in angle θ. The so-called "tulip" flame even disappeared, if the angle θ is bigger than one certain value. Moreover, the flame propagation speed and pressure are enhanced as the angle θ increases. In addition, the numerical simulation indicates that the burning gas creates an eddy near the tip of the flame, altering the flow field and causing the tulip flame to appear. However, with the angle θ increased, the flame propagation is restrained by the change in sidewalls, resulting in the different flow patterns to suppress the formation of tulip flames and promote flame combustion.

Inert nanoparticle suppression of gas explosion in the presence of obstacles.

The suppressing effects of inert nanoparticles on methane-air explosion, in an obstructed chamber with internal dimensions of 150 mm × 150 mm × 500 mm, were experimentally investigated. To this end, the flame behaviors in the presence of obstacles as well as overpressure transients during the explosions with and without nanoparticles were compared. Additionally, the effects of density, diameter, and material of nanoparticles on the suppressing behaviors were analyzed as well. The results showed that the methane-air deflagrating flame remains generally light blue if the nanoparticles are added. In particular, the flame obstacle interaction may enhance the suppression effect of the nanoparticles, and the flame acceleration rate and the peak overpressure decrease significantly. Increasing explosion suppression is seen up to about 100 g m-3 particle density, but further increase in particle density, up to 150 g m-3, yields no further increase in the explosion suppression ability. And as the particle size decreases, the suppressing effect is more evident. The experiments also showed that Al(OH)3, Mg(OH)2, and SiO2 all can be used to suppress the flame propagation and overpressure. However, the metal hydroxides suppress the methane explosions even more efficiently than SiO2 particles; Al(OH)3 particles have a slightly better inhibiting effect than Mg(OH)2. Mechanisms for the observed phenomena were discussed.