Structure Of Laminar Diffusion Flames Research Articles

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO2 concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO2 dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.

Asymptotic structure of laminar diffusion flames at high pressure

We analyze the structure of laminar diffusion flames at high pressure in the limit of large activation energy for the particular configuration of a steady flame in counterflow. We consider a dense fluid in which normal Fickian diffusion of the fuel is limited, and thermal diffusion, i.e., the Soret effect, is the dominant mechanism for fuel mass transport. Temperature and species profiles, as well as flame temperature and location, are determined as a function of Damköhler number and Soret diffusion coefficient. In particular, we find that oxidant is entirely consumed by the flame, while some fuel leaks through. For light fuels, the fuel profile is found to have a local peak on the oxidant side as a result of thermal diffusion. Our analysis includes a description of extinction phenomena, including explicit criteria in terms of the Soret diffusion coefficient, ratio of temperature of the two streams, and the Damköhler number at extinction.

A numerical study of quasi-steady burning characteristics of a condensed fuel: effect of angular orientation of fuel surface

A numerical study of laminar diffusion flames established over a condensed fuel surface, inclined at several angular orientations in the range of –90°⩽θ⩽+90° with respect to the vertical axis, under atmospheric pressure and normal gravity environment, is presented. Methanol is employed as the fuel. A numerical model, which solves transient gas-phase, two-dimensional governing conservation equations, with a single-step global reaction for methanol–air oxidation and an optically thin radiation sub-model, has been employed in the present investigation. Numerical results have been validated against the experimental data from the present study. Thereafter, the model is used to investigate the influence of angular orientation of fuel surface on its quasi-steady burning characteristics. Results in terms of fuel mass burning rate, flame stand-off distances, temperature field, velocity profiles and oxygen contours have been presented and discussed in detail. It is observed that orientation angles in the range of –45°⩽θ⩽ –30° (fuel surface facing upwards), yield the maximum mass burning rates. The flame anchoring location near the leading edge of the fuel surface, normal gradient of fuel vapor mass fraction at the surface and oxygen contours have been used to explore this unique behavior. Based on the numerical results, a theoretical correlation to predict the mass burning rate as a function of fuel surface orientation is also proposed. Furthermore, a discussion on the differences in the structure of laminar diffusion flame established over fuel surface as a function of its angular orientation is included.

Influence of boundary conditions on the structure of laminar boundary layer with hydrogen combustion on a permeable surface

A numerical study was performed to examine how thermal and diffusion boundary conditions affect the structure of laminar diffusion flame in air flow with porous blowing and combustion of hydrogen. Boundary conditions of two types were compared, with lengthwise-constant porous-wall temperature ( T W = const throughout the whole range of blowing ratios), and with lengthwise-constant temperature of the fuel supplied into the main flow ( T ′ = const ). With a liquid fuel having constant evaporating-surface temperature, we deal with boundary conditions of the first type. With a gas fuel, as a rule, we encounter the regime with T ′ = const . It is shown that, in spite of the significant difference in velocity and temperature profiles, in both cases the surface friction coefficients have close values. Also, it is found that the wall heat flux exhibits a maximum if considered as a function of fuel supply intensity. Nonetheless, the function of relative heat transfer monotonically decreases with blowing intensity, much like it does in non-reactive flow.

Cars measurements and computations of the structure of laminar stagnation-point methane-air counterflow diffusion flames

Information on the structure of laminar diffusion flames under the influence of the strain of an inhomogeneous velocity field is essential for the understanding of tubulent diffusion flames. Coherent Anti-Stokes Raman Scattering (CARS) is used to determine temperature and concentration profiles in an atmospheric pressure CH 4 /air counterflow diffusion flame. First a comparison between thermocouple and CARS measurements is done in a high temperature furnace up to 2000 K. The temperature dependent CARS spectra of N 2 are evaluated by a simulation program. Agreement between both techniques is obtained within 40 K over the complete temperature range. The experimental data from the counterflow diffusion flame are compared with calculations where the corresponding governing equations for enthalpy, momentum, and chemical species are used in their two dimensional form. By boundary layer approximations and a coordinate transformation the equations are transformed into a one dimensional form, which is integrated with a finite differences method. Transport is described by a simplified multicomponent model. The detailed reaction mechanism consists of more than 250 elementary steps and 39 chemical species. The comparison between the experimental and simulated temperature profiles shows satisfactory agreement on the fuel rich side but deviates significantly on the fuel lean side of the mixing layer. This discrepancy is ascribed to shortcomings in the model underlying the numerical simulation. Further calculations should therefore replace the use of boundary layer approximations by true two-dimensional representations of the flow field.

Comparison of CARS Measurements and Calculations of the Structure of Laminar Methane‐Air Counterflow Diffusion Flames

AbstractInformation on the structure of laminar diffusion flames under the influence of the strain of an inhomogeneous velocity field is essential for the understanding of turbulent diffusion flames.—To simulate the structure of the stagnation‐point flame the corresponding governing equations for enthalpy, momentum, and chemical species are to be solved. By boundary layer approximations and a coordinate transformation these equations are transformed into a one dimensional form, which is integrated with a finite differences method. Transport is described by a simplified multicomponent model. The detailed reaction mechanism consists of more than 250 elementary steps and 39 chemical species.—For comparison with the numerical results Coherent Anti‐Stokes Raman Scattering (CARS) is used to determine temperature and concentration profiles in an atmospheric pressure CH4/air counterflow diffusion flame. First a comparison between thermocouple and CARS measurements is done in a high temperature furnace up to 2000 K. The temperature dependent CARS spectra of N2 and O2 are evaluated by a computer program. Agreement between both techniques is obtained within 40 K over the complete temperature range. The same procedure is done with oxygen CARS spectra giving a temperature uncertainty of ± 80 K.—The comparison between the experimental and simulated temperature profiles shows satisfactory agreement on the fuel rich side but deviates significantly on the fuel lean side of the mixing layer. This discrepancy is ascribed to shortcomings in the model underlying the numerical simulation. Further ejaculations should therefore replace the use of boundary layer approximations by true two‐dimensional representations of the flow field.

The study of the structure of laminar diffusion flames by optical methods

Stabilization of flat diffusion flames - optical methods for analysis of structure at velocities up to blow-out

A principle of mixing similarity and its application to inhomogeneous combustion

We have endeavored to demonstrate theoretically and experimentally that, in the inhomogeneous combustion of hydrocarbon-air systems, a high degrce of similarity prevails in the distribution of the initial fuel (H and C) and oxidant (O and N) elements, respectively—subject to the limitation that free soot does not escape from the flame. The same data were used to establish the effective mean diffusion coefficient of the elements in ethane-air flames, ( D 1 cm 2 /sec=0.23±10 per cent) and butane-air flames ( D 1 cm 2 /sec=0.18±10 per cent). There is good reason to believe that this same range of values holds for the inhomogenous combustion (with air) of any hydrocarbon, regardless of its molecular weight. The data were treated in terms of analytical expressions which, theoretically, are valid in the presence of gradients and variations in the mean molecular weight of the gases. The butane-air flame analysis strongly supports this theoretical prediction. On the basis of mixing similarity, a picture was developed which is in accord with a wide variety of observations on both the gross and detailed structure of laminar diffusion flames, and which provides a common physical basis for the work of Burke and Schumann, of Jost, and of Wolfhard and Parker. Mixing similarity clearly delineates the domains of diffusional, chemical kinetic, thermo Supplementary information (Tables similar to 1A and 2A, appendix, for all flames) has been deposited as Document Number 4981 with the ADI Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the Document number and by remitting $2.50 for photoprints or $1.75 for 35-mm microfilm. Advance payment is required. Make checks or money orders payable to: Chief, Photoduplication Service, Library of Congress. dynamic and fluid dynamic phenomena. Although, for reasons of mathematical simplicity and illustrative clarity, we have dealt entirely with laminar diffusion flames, the general physicochemical principles are applicable to any flow regime.