Equivalence Ratio Flame Research Articles

Effects of Karlovitz Number on Flame Surface Wrinkling in Turbulent Lean Premixed Methane-Air Flames

ABSTRACTDirect numerical simulations are carried out to study the effects of the Karlovitz number (Ka) on the surface wrinkling of lean methane-air flames of equivalence ratio 0.5 and non-dimensional turbulence intensities (urms/SL) of 2–25. The mixture pressure is 20 bar and temperature is 810 K to simulate approximate conditions in lean-burn natural gas engines. The Karlovitz number varies from 1.1–49.4, and the Damköhler (Da) number from 0.26–3.2, corresponding to turbulent premixed combustion in the thin reaction zone regime. With increasing Ka, there is increased probability of forming highly curved surfaces locally. The distribution of the shape factor of the local surface does not vary with Ka. The surface-averaged tangential strain rate is proportional to the Ka for flames in the thin reaction zone regime. By examining the flame displacement speed on the flame surface, it is found that on a highly curved surface, irrespective of whether it is convex or concave, flame stretch becomes negative due to the negative propagation term. The functional dependence of the conditional mean flame displacement speed on curvature is only weakly dependent on Ka and the shape factor. The physical insights derived from the analysis are applied to the modeling of the flame surface density equation.

Study on the flame propagation and gas explosion in propane/air mixtures

The characteristics of flame propagations and gas explosions in propane/air mixtures were studied in a 1.16m3 explosion chamber with two quartz glass windows of 250mm diameter. The outward propagations of the spherical flame were recorded by the Schlieren system and high speed motion analysis system. The pressure and temperature histories of the explosion process were measured by pressure sensors and platinum–rhodium thermal couples of 0.02mm diameter mounted on the wall of the experimental chamber. The instabilities of the propane air flame with different equivalence ratio flame were observed and the critical radii of flame instability onset were obtained. The laminar burning velocities of propane air mixture were obtained by analyzing the experimental results of Schlieren system by using outwardly propagating spherical method. The variation of laminar burning velocity with equivalence ratio was studied. Pressure and temperature responses of the gas explosion were obtained and analyzed. The variations of the maximum overpressure and the maximum temperature with equivalence ratio were obtained and discussed. In propane/air mixture with an equivalence ratio near 1.8, two explosions take places.

Premixed flame front structure in intense turbulence

Six methane/air flames of equivalence ratio 0.7, Ka =1–17 stabilized on a intense turbulence low-swirlburner of were investigated by Rayleigh laser sheet measurements of the gas density within the preheat zone (approximately 400–1000 K). Lean premixed turbulent flames with Karlovitz numbers Ka , greater than unity are becoming of increasing practical importance but only limited detailed experimental data are available. This regime is often called the distributed reaction zone and a new approach to analyzing flame front broadening was developed to address issues of flame three dimensionality. Instantaneous progress variable, c , contour sets constitute the basic data and a statistical analysis of three aspects of this data are presented: (1) c contour spacing (2) flame front curvature, and (3) flame surface density. A detailed investigation revealed little change of these parameters with turbulence, even at the highest Ka numbers: the contour spacings were similar to those derived from laminar flame calculations and neither the flame front curvature nor the flame surface density varied significantly with c . Geometric effects of turbulence on flame front structure still predominate at these turbulence levels. The primary effect of increasing turbulence is increasingly to convolute the flame front and so increase the flame surface density, reduce the scalar length scales, and increase the burning rate. This finding is also reflected in the significant broadening of the flame front curvature distributions with increasing u′/S L . The data presented here indicate that the diagrams often used to delimit the various regimes of premixed turbulent combustion and hence guide modeling are based on parameters which need to be carefully investigated to determine their appropriateness. The results are consistent with an interpretation of combustion/turbulence interactions which sees the reaction zone thickness as the significant scalar length scale.

Mechanism of Soot Formation in Premixed C2/H2/O2Flames

Abstract Neutral species (molecules, atoms and radicals) present in rich C2H2/O2 premixed flat flame stabilized under reduced pressure (20 torrs) have been analyzed by the coupled molecular beam-mass spectrometry technique. To investigate whether particular variations are observed at the sooting limit (φ = 2.1) six flames of equivalence ratio respectively equal to 1.8, 2.0, 2.1, 2.2, 2.6 and 3.0 have been studied. Results have been mainly used to compare the role played by polyacetylenic hydrocarbons and polycyclic aromatic hydrocarbons (PAH) in the mechanism of soot formation. Main features of the profiles of mole fraction and mass fraction flux versus distance to the burner show that polyacetylenics cannot be considered as active intermediates. On the other hand a strong reactivity is observed at the boundary between oxidation and soot formation zones for PAH. If PAH play an active role in soot formation, it is worth knowing the nature of the species leading to the production of the first aromatic rings. For (his purpose variation of the profiles of mass fraction flux for species containing one to six carbon atoms has been examined as well as the role played by deshydrogenation reactions. Results suggest that C4H4 may play a major role in the first step of the mechanism of soot formation.