An analysis of turbulent diffusion flame in axisymmetric jet

The kinetic theory of turbulent flow was employed to study the mixing limited combustion of hydrogen in axisymmetric jets. The integro-differential equations in two spatial and three velocity coordinates describing the combustion were reduced to a set of hyperbolic partial differential equations in the two spatial coordinates by a binodal approximation. The MacCormick's finite difference method was then employed for solution. The flame length was longer than that predicted by the flame-sheet analysis, and was found to be in general agreement with a recent experimental result. Increase of the turbulence energy and scale resulted in an enhancement of the combustion rate and, hence, in a shorter flame length. Details of the numerical method as well as of the physical findings are discussed

NOx Emissions Prediction from a Turbulent Diffusion Flame

This work studies the finite volume technique of numerical discretisation by considering a transport process of diffusion. The case investigated is the prediction of NOx emissions from a turbulent diffusion flame arising from the combustion of methane in a cylindrical furnace. A finite volume-based method in a commercial CFD code (Ansys Fluent) was used.This simulation study was done in two and three dimensions. The assumption of constant specific heat resulted to over-prediction of NOx   emissions from the combustion system while the use of variable mixture specific heat gave realistic results for the 2D and 3D simulations. The predicted average exit temperature and velocity of the flue gas were 1780 K and 3.12 m/s respectively for the 2D simulation, and 1800 K and 3.21 m/s respectively for the 3D simulation. The predicted mass fraction of NO pollutant (thermal and prompt) was 0.003311 and 0.004699 for the 2D and 3D simulations respectively.

Theoretical Analysis of Nitric Oxide Production in a Methane/Air Turbulent Diffusion Flame

The coherent flame model is applied to the methane-air turbulent diffusion flame with the objective of describing the production of nitric oxide. The example of a circular jet of methane discharging into a stationary air atmosphere is used to illustrate application of the model. In the model, the chemical reactions take place in laminar flame elements which are lengthened by the turbulent fluid motion and shortened when adjacent flame segments consume intervening reactant. The rates with which methane and air are consumed and nitric oxide generated in the strained laminar flame are computed numerically in an independent calculation. The model predicts nitric oxide levels of approximately 80 parts per million at the end of the flame generated by a 30.5 cm (1 foot) diameter jet of methane issuing at 3.05 x 10^3 cm/sec (100 ft/sec). The model also predicts that this level varies directly with the fuel jet diameter and inversely with the jet velocity. A possibly important nitric oxide production mechanism, neglected in the present analysis, can be treated in a proposed extension to the model

Experimental study of the thermal-acoustic efficiency in a long turbulent diffusion-flame burner

An acoustic source/propagation model is used to interpret measured noise spectra from a long turbulent burner. The acoustic model is based on the perturbation solution of the equations describing the unsteady one-dimensional flow of an inviscid ideal gas with a distributed heat source. The model assumes that the measured noise spectra are due uniquely to the unsteady component of combustion heat release. The model was applied to a long cylindrical hydrogen burner operating over a range of power levels between 4.5 kW and 22.3 kW. Acoustic impedances at the inlet to the burner and at the exit of the tube downstream of the burner were measured and are used as boundary conditions for the model. These measured impedances are also presented

Soot formation in a turbulent swirling flow

The qualitative understanding of soot formation in simple models of gas turbine primary-zone combustors is summarized. Soot formation in flame radiation and air pollution was investigated. Results are presented, namely: (1) if the fuel is premixed with air in approximately stoichiometric proportions, the sequence of states that a fluid element undergoes as it burns is quite different from the sequence when liquid or vapor fuel is injected into an air-flow; (2) swirling flows, as are typical or swirl-can combustors, when burning, can amplify small aerodynamic disturbances upstream of the swirl vanes; and (3) different fuels form significantly different amounts of soot. Each of these effects makes major changes in the amount of soot formed in a given combustor

The Coherent Flame Model for Turbulent Chemical Reactions

A description of the turbulent diffusion flame is proposed in which the flame structure is composed of a distribution of laminar diffusion flame elements, whose thickness is small in comparison with the large eddies. These elements retain their identity during the flame development; they are strained in their own plane by the gas motion, a process that not only extends their surface area, but also establishes the rate at which a flame element consumes the reactants. Where this flame stretching process has produced a high flame surface density, the flame area per unit volume, adjacent flame elements may consume the intervening reactant, thereby annihilating both flame segments. This is the flame shortening mechanism which, in balance with the flame stretching process, establishes the local level of the flame density. The consumption rate of reactant is then given simply by the product of the local flame density and the reactang consumption rate per unit area of flame surface. The proposed description permits a rather complete separation of the turbulent flow structure, on one hand, and the flame structure, on the other, and in this manner permits the treatment of reactions with complex chemistry with a minimum of added labor. The structure of the strained laminar diffusion flame may be determined by analysis, numerical computation, and by experiment without significant change to the model

Development of simplified simulation for turbulent diffusion flame by using model of overall one step reaction formula

本研究は、燃焼現象の簡易数値解析の指針を得るために、まず手始めとして総括一段反応燃焼モデルを用いたメタン乱流拡散火炎のフレームホルダー付近での温度分布、火炎高さおよび燃料出口温度等のシミュレーションを行うことにより、簡易型数値解析シミュレーションの妥当性についての検討を行った。Numerical Simulations of turbulent diffusion flame for methane in air have been carried out to examine the behavior of temperature, concentration of fuel and distribution of velocity near the flame holder by using model of overall one step reaction formula. Initial conditions for turbulent diffusion flame for methane correspond to 300 K and 0.1 MPa and a mixture of 79% nitrogen and 21% oxygen by volume is used as a substitute for air.The research results show that 1) The highest calculated value of temperature near the flame holder is of the order of 2000 K 2) The model of overall one step reaction formula is fairly useful for determining the combustion characteristics of turbulent diffusion flame

Laboratory measurements in a turbulent, swirling flow

Measurements of soot inside a flame-tube burner using a special water-flushed probe are discussed. The soot is measured at a series of points at each burner, and upon occasion gaseous constitutents NO, CO, hydrocarbons, etc., were also measured. Four geometries of flame-tube burners were studied, as well as a variety of different fuels. The role of upstream geometry on the downstream pollutant formation was studied. It was found that the amount of soot formed in particularly sensitive to how aerodynamically clean the configuration of the burner is upstream of the injector swirl vanes. The effect of pressure on soot formation was also studied. It was found that beyond a certain Reynolds number, the peak amount of soot formed in the burner is constant

Large eddy simulation of a turbulent diffusion flame: some aspects of subgrid modelling consistency

This is a copy of the author 's final draft version of an article published in the journal Flow turbulence and combustion. The final publication is available at Springer via http://dx.doi.org/10.1007/s10494-017-9813-2In the context of Large Eddy Simulation (LES) solely for the momentum transport equation there may be found several models for the turbulent subgrid fluxes. Furthermore, among those relying on the eddy diffusivity approach, each model may be based on different invariants of the strain rate. Besides, when heat and mass transfer are also considered, closures for the subgrid turbulent scalar fluxes are also required. Hence, different model combinations may be considered. Additionally, when other physical phenomena are included, such as combustion, further subgrid modelling is involved. Therefore, in the present study a LES simulation of a turbulent diffusion flame is performed and different combination of subgrid models are used in order to analyse the numerical effects in the simulations. Several models for the turbulent momentum subgrid fluxes are considered, both constant and dynamically evaluated Schmidt numbers. Regarding combustion, in the context of the Flamelet/Progress-Variable (FPV) model, with an assumed probability density function for the turbulent-chemistry interactions and four different closures for the subgrid mixture fraction variance are considered. Hence, a large number of model combinations are possible. The present study highlights the need for a consistent closure of subgrid effects. It is shown that, in the context of an FPV modelling, incorrect capture of subgrid mixing results in a flame lift-off for the studied flame (DLR A diffusion flame), even though experimentally an attached flame was reported. It is found that a consistent formulation is required, that is, all subgrid closures should become active in the same regions of the domain to avoid modelling inconsistencies. In contrast, when the classical flamelet approach is used, no lift-off is observed. The reason is that the classical flamelet includes only a limited subset of the possible flame states, i.e. only includes burning flamelets and extinguished flamelets for scalar dissipation rates past the extinction one.The present work has been financially supported by the Ministerio de Economía y Competitividad of the Spanish government through project ENE2014-60577-R. We would also like to thank the reviewers for their helpful comments which have led to an improved article. The authors declare that they have no conflict of interest.Peer ReviewedPostprint (author's final draft

Combustion/particle sizing experiments at the Naval Postgraduate School Combustion Research Laboratory

Particle behavior in combustion processes is an active research area at NPS. Currently, four research efforts are being conducted: (1) There is a long standing need to better understand the soot production and combustion processes in gas turbine combustors, both from a concern for improved engine life and to minimize exhaust particulates. Soot emissions are strongly effected by fuel composition and additives; (2) A more recent need for particle sizing/behavior measurements is in the combustor of a solid fuel ramjet which uses a metallized fuel. High speed motion pictures are being used to study rather large burning particles; (3) In solid propellant rocket motors, metals are used to improve specific impulse and/or to provide damping for combustion pressure oscillations. Particle sizing experiments are being conducted using diode arrays to measure the light intensity as a function of scattering angle; (4) Once a good quality hologram is attained, a need exists for obtaining the particle distributions from hologram in a short period of time. A Quantimet 720 Image Analyzer is being used to reconstruct images