Diffusion flame

About: Diffusion flame is a research topic. Over the lifetime, 9266 publications have been published within this topic receiving 233522 citations.

Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations.

Abstract: In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a Re-T,f(0.5) scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given Re-T,Re-f, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by Re-T,M(0.5) irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames.

Local flame structure in the well-stirred reactor regime

Abstract: Direct numerical simulations of hydrogen/air turbulent premixed flame progagating in the three-dimensional turbulence are conducted to investigate local flame structures in the well-stirred reactor regime. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the H 2 /air reaction in turbulence. Although the flame condition is classified into the well-stirred reactor regime, the geometry of the regions with high heat release rate shows thin sheetlike structure. The fluctuation of the heat release rate along the flame surface is relatively high, and the maximum heat release rate reaches up to 1,3 times the corresponding laminar flame. The heat release rate tends to be high in the regions convex toward the burned side. The flame structure in the case of the well-stirred reactor regime shows a double-layered feature. One may conclude that reaction zone becomes thick in the well-stirred reactor regimes only from temperature, H, and OH distributions, while the heat release rate, mass fraction of O atoms, and reaction rates of O atoms and OH radicals are fluctuating significantly in that region in fact. Specifically, reaction rates of O atoms and OH radicals show characteristic behaviors in the burned side due to their chemical characteristics. In the preheat zone, mass fraction and reaction rate of HO 2 show quite thin and smooth distributions compared to other properties such as the heat release rate. The distribution of H 2 O 2 reaction rate reflects the double-layered feature of the well-stirred reactor regime very well. It is shown that the double-layered feature can be explained by discussing the balances of the elementary reactions in detail. As the flame front can be defined even in the well-stirred reactor regime, statistics of the local flame elements are also discussed.

Examining the Relationship Between Black Carbon and Soot in Flames and Engine Exhaust

Abstract: This article investigates the black carbon (BC) content of soot formed in premixed and diffusion flames and emitted by light duty gasoline and diesel vehicles. BC is measured photoacoustically and compared with particulate mass collected by filter and calculated from particle size distributions. The BC fraction of soot from rich premixed ethylene flames increases with height above the burner, but can remain well below unity in modestly sooting flames. The BC fraction produced by a propane diffusion flame soot generator (combustion aerosol standard, CAST) falls as the fuel is diluted with nitrogen, the principal means used to adjust the desired particle size. Thermally treating the soot to remove possible condensed semivolatile species does little to change these trends. Transmission electron microscopy (TEM) images show that despite low BC content, these particles display the characteristic fractal-like agglomerate morphology of soot. Particle mass spectra reveal polycyclic aromatic hydrocarbon (PAH) and ...

Mass, Mobility, Volatility, and Morphology of Soot Particles Generated by a McKenna and Inverted Burner

Abstract: Particle mass, mobility, volatile mass fraction, effective density, mass concentration, mass–mobility exponent, and particle morphology were measured from soot generated from a premixed flame (McKenna burner) and an inverted diffusion flame over a range of equivalence ratios. It was found that the mass fraction of volatile material on the soot from the McKenna burner could be up to 0.83 at a high equivalence ratio, but there was no measurable volatile material on the soot from the inverted burner. The inverted burner can produce soot at different mass–mobility exponents, ranging from 2.23 to 2.54, over a range of global equivalence ratios of 0.53–0.67, while the mass–mobility exponent ranges from 2.19 to 2.99 for fresh soot and 2.19 to 2.81 for denuded soot for the McKenna burner at equivalence ratios of 2.0–3.75. Transmission electron microscopy analysis of inverted burner soot shows that a range of particle morphologies is present at a given global equivalence ratio, likely due to different local equiva...