Laminar Flame Computations Research Articles

Acceleration of premixed H[formula omitted]-Air-Steam flames when accounting for thermal radiation

Diluting hydrogen flames with steam can increase flame stability and reduce nitrogen oxide emissions. While the effects of dilution on the flame characteristics due to variations in chemistry or transport properties have been well investigated, this is not the case for those related to thermal radiation. Thermal radiation is indeed often neglected in free flame simulations. However, it can be critical in diluted flames in which participating species are present in the fresh gases. This study explores thermal radiation’s impact on H2-Air flames diluted with steam through 1D laminar flame computations. To this end, a reactive fluid solver is coupled with a semi-analytical thermal radiation code. Both the grey gas approximation, for a preliminary understanding, and the more accurate CK narrow-band model, for detailed simulations that account for spectral-dependent gas properties, are used. The combustion numerical setup is validated against literature results. Three main thermal radiation effects are highlighted: a preheating of the fresh gases, an increased laminar flame speed, and a decreased temperature in the burnt gases. It is found that domain length considerably affects this preheating and flame speed, which can then affects laminar burning velocity measurements. It is shown that thermal radiation and combustion can often be decoupled and that a preheated adiabatic flame can recover the main characteristics of the corresponding radiative flame. The increase of the laminar flame speed by thermal radiation is reported for a wide range of equivalence ratios, dilution levels, and pressures. The observed acceleration is especially significant for lean and very lean flames which can be up to 500% faster when considering thermal radiation.

Machine learning tabulation of thermochemistry in turbulent combustion: An approach based on hybrid flamelet/random data and multiple multilayer perceptrons

A new machine learning methodology is proposed for speeding up thermochemistry computations in simulations of turbulent combustion. The approach is suited to a range of methods including Direct Numerical Simulation (DNS), Probability Density Function (PDF) methods, unsteady flamelet, Conditional Moment Closure (CMC), Multiple Mapping Closure (MMC), Linear Eddy Model (LEM), Thickened Flame Model, the Partially Stirred Reactor (PaSR) method (as in OpenFOAM) and the computation of laminar flames. In these methods, the chemical source term must be evaluated at every time step, and is often the most expensive element of a simulation. The proposed methodology has two main objectives: to offer enhanced capacity for generalisation and to improve the accuracy of the ANN prediction. To accomplish the first objective, we propose a hybrid flamelet/random data (HFRD) method for generating the training set. The random element endows the resulting ANNs with increased capacity for generalisation. Regarding the second objective, a multiple multilayer perceptron (MMP) approach is developed where different multilayer perceptrons (MLPs) are trained to predict states that result in smaller or larger composition changes, as these states feature different dynamics. It is shown that the multiple MLP method can greatly reduce the prediction error, especially for states yielding small composition changes. The approach is used to simulate flamelets of varying strain rates, one-dimensional premixed flames with differential diffusion and varying equivalence ratio, and finally the Large Eddy Simulation (LES) of CH4/air piloted flames Sandia D, E and F, which feature different levels of local extinction. The simulation results show very good agreement with those obtained from direct integration, while the range of problems simulated indicates that the approach has great capacity for generalisation. Finally, a speed-up ratio of 12 is attained for the reaction step.

The computation of laminar flames

Hydrocarbon fuels will remain a major source of energy well into the second half of the 21st century and, despite dire warnings about their limited supply, known resources have actually increased over the past decade. Nevertheless, finite supplies and increasing demand will exert pressure on the efficient use of these fuels, especially if their price continues to climb. Specifically, electricity generation and propulsion will continue to rely heavily upon the burning of hydrocarbon fuels for many years to come. Although an understanding of combustion in practical combustors is essential to the goals of reducing pollution and increasing energy efficiency, three-dimensional models of these systems with detailed transportation fuel chemistry and complex transport are beyond our current computational capabilities. Instead, one can study flames with complex chemistry in simpler laminar configurations to provide insight into the chemical and physical processes occurring in many engineered systems. In this paper, we trace the development of mathematical models and computational methods for laminar flame problems, with a particular emphasis on numerical algorithms that enable their coupled solution. While most of the focus is on steady systems, we also discuss issues related to time-dependent flames.

Modelling Co Production In Vitiated Compartment Fires

Strategies for the modelling of CO production in compartment fires, based on laminar flamelet relationships, derived from computations of vitiated laminar diffusion flames, and on a simplified two-step eddy dissipation model, are identified and compared. A flamelet library is constructed which incorporates the parametric variation of ambient oxygen concentration and temperature. Expected departures from mixing controlled burning, arising in regions like hot ceiling layers, for example, are assessed with reference to homogeneous plug flow calculations, initiated from peak CO conditions drawn from the laminar flame computations. Implementation of the two approaches in CFD simulations of ventilation-controlled compartment fires using the SOFIE code suggest that both offer the prospect of reliable CO prediction, given a measure of calibration.

A new modeling approach for the autoignition of a non-premixed turbulent flame using DNS

Autoignition of turbulent non-premixed flames is encountered in practical systems such as diesel internalcombustion engines, but remains a challenge for modelers. The simplest models are generally, based on an ad hoc coupling between Arrhenius laws and the Magnussen eddy dissipation concept. Mastorakos et al. have shown, using direct numerical simulations with a single-step irreversible chemistry, that autoignition occurs for a given most reactive mixture fraction level and low scalar dissipation rates. They devised a model based on a conditional moment closure formulation. In this paper, an extended (by integrating over all the possible mixture fraction values) flame surface density concept is introduced and tested against two-dimensional direct numerical simulations including detailed chemistry and multicomponent transport phenomena, including thermal diffusion. The mean reaction rate to be modeled is split into two contributions: a flame generalized surface density, including a turbulence contribution, and mean reaction rate per unit of flame area that could be described from one-dimensional laminar flame computations First results appear to be very promising.

Fast and reliable solution of the Navier–Stokes equations including chemistry

In this paper, we describe recent developments in the design and implementation of Navier-Stokes solvers based on finite element discretization. The most important ingredients are residual driven a posteriori mesh refinement, fully coupled defect-correction iteration for linearization, and optimal multigrid preconditioning. These techniques were systematically developed for computing incompressible viscous flows in general domains. Recently they have been extended to compressible low-Mach flows involving chemical reactions. The potential of automatic mesh adaptation together with multilevel techniques is illustrated by several examples, (1) the accurate prediction of drag and lift coefficients, (2) the determination of CARS-signals of species concentration in flow reactors, (3) the computation of laminar flames.

Instantaneous and Mean Compositional Structure of Bluff-Body Stabilized Nonpremixed Flames

Turbulent nonpremixed flames stabilized on an axisymmetric bluff-body burner are studied with fuels ranging from simple H 2/CO to complex H 2/CH 4 and gaseous methanol. The fuel-jet velocity is varied to investigate the Damköhler number effects on gas emissions, localized extinction (LE) in the neck zone, and the structure of the recirculation zone dependency on the flow field. Simultaneous, single-point measurements of temperature, major species, OH, and NO are made using the Raman/Rayleigh/Laser induced fluorescence (LIF) technique. The data are collected at different axial and radial locations along the full length of most flames and are presented in the form of ensemble means, root-mean-square (rms) fluctuations, scatter plots, and probability density functions (PDF). It is found that up to three mixing layers may exist in the recirculation zone, one on the air side of the outer vortex, one between the inner and the outer vortices, and one between the fuel jet and the inner vortex. With increasing jet momentum flux, the average mixture in the outer vortex loses its strength and the stoichiometric contour shifts closer to the fuel jet. The decay rate of the mixture fraction on the centerline exhibits similar trends to the ordinary jet flame downstream of the recirculation zone whereas different trends are found inside the recirculation zone. The laminar flame computations with constant mass diffusivities and Lewis number (Le) = 1 are found to be better guides for the measured temperature and stable species mass fraction in the turbulent flames. The measured peak mass fractions of CO and H 2 are similar to those reported earlier for pilot-stabilized flames of similar fuels. Compared with laminar flame compositions with equal diffusivities and Le = 1.0, measured CO may be in superflamelet concentration. Hydroxyl radical and H 2 are found not to be in superflamelet levels contrary to earlier findings in piloted flames. The start of LE and the bimodality of the conditional PDF are consistent with those reported earlier for piloted flames of similar fuels.

Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.

Simultaneous Imaging of Temperature and OH Number Density in Turbulent Diffusion Flames

A single laser source and a single intensified CCD camera have been used to image temperature and the hydroxyl radical, OH in turbulent nonpremixed flames. Images are simultaneous and quantitative. Temperature is derived from Rayleigh scattering. Laser Induced Fluorescence (LlF) is used to image OH. Correction of the LIF image for quenching effects is done using a combination of laminar flame calculations and the temperature measurements. Images for flames of the following fuels are presented: CH3OH, C2H5OH, CNG (91% CH4 by volume) and CNG-H2(31% CNG, 69% H2 by volume). The OH profile is used as a measure of reaction zone thickness. Number densities of OH are found to be in superflamelet levels for all flames investigated here. Temperatures agree well with laminar flame computations. Flames show evidence of partial or full premixing of the fluid in the reignition zone at high velocities downstream of regions of high local extinction. The OH profiles are seen to increase in width and this is consistant with an increase in calculated reaction zone width and an increase in stoichiometric mixture fraction. An increase in Reynolds number alone does not change the OH profile thickness.

A mixedness-reactedness flamelet model for turbulent diffusion flames

A novel, non-equilibrium, combustion model is proposed, to simulate turbulent diffusion flames, based on a laminar flamelet approach that was originally developed for premixed combustion. The model embodies a mixedness factor to describe the mixture strength and a reactedness parameter to describe the degree of completeness of combustion. The effect of straining is accounted for in prior laminar flame computations and the imposition of a known distribution of flame straining on the flamelets. Profiles of laminar heat release rate against temperature and the assumed independence of fluctuations in mixedness, reactedness and flame straining enable a simple evaluation of turbulent mean heat release rate to be made. This approach is used to predict numerically the field solutions for lifted, turbulent jet, methane-air, diffusion flames, in conjunction with the k −e turbulence model. The computations predict an approximately linear relationship between lift-off height and fuel jet velocity, in good agreement with available experimental data. Results suggest that the mechanism of flame stabilization is associated with the complex interactions between convection, turbulent mixing, heat release rate under strain and thermal expansion, rather than with a single phenomenological parameter such as, for example, the flame strain extinction limit.

Flame computations by a Chebyshev multidomain method

AbstractA Chebyshev collocation method is proposed for the computation of laminar flame propagation in a two‐dimensional gaseous medium. The method is based on a domain decomposition technique associated with co‐ordinate transforms to map the infinite physical subdomains into finite computational ones. The influence matrix method is used to handle the patching conditions at the interfaces. This technique is particularly efficient since at each time step only matrix products have to be performed. The method is tested first on an elliptic model problem; it is then applied to laminar flame computations, including calculations of cellular instabilities of flame fronts.