Nasser Darabiha

Laminar premixed hydrogen/air counterflow flame simulations using Flame Prolongation of ILDM with differential diffusion

The cost of including full kinetics in realistic computations remains extremely high. This has led many researchers to develop reduction techniques for the chemistry. These methods are generally valid only in a very limited range of equivalence ratio, pressure, or temperature and require extensive human time to develop the reduced schemes. Recently, an automatic method based on intrinsic low-dimensional manifolds (ILDM) has been proposed. Because of ILDM, the reduction of detailed reaction schemes is much simuplified, leading to the fast generation of look-up tables containing the information corresponding to the reduced chemical schemes. Nevertheless, the ILDM method is not well suited to describe the low-temperature domain, since the dimension and therefore the complexity of the databases increase tremendously in this zone. In this work, we propose a new version of the ILDM method, called flame prolongation of ILDM (FPI), which enables us to solve the problem at low temperatures. We used laminar premixed free flames to extend the manifolds generated with ILDM, thus leading to smooth and accurate evolutions of the species along all the flame front. In order to demonstrate the interest of FPI, we computed the response of a premixed flame to straining using the double-premixed counterflow flame configuration. We show that using FPI, the correct evolution is obtained for all species from almost unstrained flames up to flames near extinction. The computational times are tremendously reduced with FPI in comparison with full chemistry.

Modelling non-adiabatic partially premixed flames using flame-prolongation of ILDM

Many models are now available to describe chemistry at a low CPU cost, but only a few of them can be used to describe correctly premixed, partially premixed and diffusion combustion. One of them is the FPI model that uses two coordinates: the mixture fraction Z and the progress variable c. In this paper, we introduce a new evolution of the FPI method that can now handle heat losses. After a short review of kinetic models used in turbulent combustion, the main features of the new three-dimensional FPI method, in which we introduce a third coordinate for enthalpy h, are presented. First, a one-dimensional radiative premixed flame validation case is presented for a large range of radiative heat losses. Second, we present the results of simulations of two laminar burners. Both the fully and the partially premixed burner simulations give a good estimation of all the flame features such as the flame stabilization (driven by heat losses), the flame structure and the profile of major and minor species.

The influence of the temperature on extinction and ignition limits of strained hydrogen-air diffusion flames

Abstract Structures, extinction and ignition limits of strained diffusion flames are examined. The influence of the oxidizer stream temperature is specifically considered. Calculations are performed for diffusion flames formed by a counterflow of diluted hydrogen and air. The air temperature differs from the hydrogen temperature. The reactive flow equations are solved numerically by employing Newton iterations and adaptive continuation techniques. The model includes detailed transport and complex kinetics. Flame structures, extinction and ignition limits and characteristic flame response curves are determined for different values of the fuel-air ratio and for a set of air stream temperatures. It is shown that this last parameter strongly affects the conditions producing extinction. For an air stream temperature of 100OK, the critical extinction strain rate is ten times that found for an air stream at room temperature It is also found that a temperature exists beyond which combustion always takes place wha...

Extinction of Strained Premixed Propane-air Flames with Complex Chemistry

Abstract —The structure of a strained premixed laminar propane-air flame is examined. The flame is formed in the neighborhood of the stagnation point produced by the counterflow of fresh mixture an...

Experimental and numerical study of chemiluminescence in methane/air high-pressure flames for active control applications

Chemiluminescence of excited OH * , CH * , and C 2 * radicals was investigated as a tool for combustion control. A parametric study in premixed methane/air flames is presented regarding the effects of pressure (1 to 10 bar) and equivalence ratio (0.6 to 1.1). The experimental geometry corresponds to a Bunsen-type burner, with pilot flames to achieve steady combustion at very lean conditions. The burner was set in a pressurized vessel to control ambient pressure. The chemiluminescence was spatially measured using an intensified CCD camera with interference filters centered on the three radical emission bands. A monochromator and a low-resolution spectrometer were used to obtain spectrally resolved data. The three diagnostic techniques show good agreement. The experimental results show that the chemiluminescence of the radicals investigated has different dynamics for given pressure and equivalence ratio conditions. The OH * radical seems more suitable for lean flames, while CH * and C 2 * have a more monotonic behavior and stronger dynamics for richer flames. A numerical simulation with complex chemistry and transport modeling based on the PREMIX code was performed for two different kinetic schemes including OH * and CH * . A comparison is presented for integrated chemiluminescence (both spectrally and spatially), as well as for local excited radical concentration trends within the range of experimental conditions. Good qualitative agreement is found with the experimental results except for rich flames, where disagreements due to kinetic schemes are observed. As a conclusion, a new strategy for flame sensing using chemiluminescence over several wavelengths is proposed.

The Effect of Strain Rate on a Premixed Laminar Flame

The structure of a strained premixed laminar flame is examined. The flame is formed in the vicinity of a stagnation point established by the counterflow of fresh mixture and hot combustion products. This ideal configuration analyzed by Libby and Williams [18] with activation energy asymptotics is here examined numerically. This allows an exact description of flame and flow structure and a calculation of the mass rate of reaction per unit flame area for the whole range of strain rates. Previous results obtained for intermediate and high strain rates are confirmed. However, for low strain rates the mass rate of reaction per unit flame area differs from that determined with large activation energy asymptotics. The present calculations also provide the exact value of the strain rate (or Damkiihler number) for which the partial extinction regime appears. If the strain rate is increased beyond that value the flame front develops on the hot side of the stagnation point. The reactive front first moves away from the stagnation point and then moves back toward that point for the very large values of the strain rate.

Mass transfer and combustion in transcritical non-premixed counterflows

The problem of transcritical mass transfer and combustion is considered in the counterflow geometry established at ambient pressures exceeding the critical pressure of the fluids but for an injection temperature (or injection temperatures) below critical. Real gas effects taking place under these conditions are treated with suitable models for thermodynamics and transport properties. A set of routines designated as ‘TransChem’ is constructed to extend the standard ‘Chemkin’ package to the transcritical regime. Three configurations are investigated. In the first case a low temperature oxygen stream impinges on a high temperature supercritical stream of the same substance and the ambient pressure exceeds the critical pressure. The structure of this flow is determined numerically and the mass transfer taking place between the dense oxygen and the hot stream is evaluated. It is shown that this rate can be correlated in terms of a transfer number which depends on the hot and cold temperatures and on the critical temperature. The mass transfer rate is also found to evolve like the square root of the strain rate. In the second case a flame is formed in the counterflow of a stream of cold oxygen injected at a temperature which is below critical and impinges on a supercritical stream of methane. In this flow, the flame is established in the light gas region adjacent to a sharp density gradient. The flame structure is close to that found in a gaseous situation except for the very large density change on the oxygen side which fixes the oxygen mass transfer rate to the flame. In the third case, the two streams are transcritical with temperatures below the respective critical values. The flow features two sharp gradients on the oxygen and methane sides and the flame is established between these two layers in the low density gaseous region.

Study of radiative effects on laminar counterflow H2/O2N2 diffusion flames

Abstract: The effects of radiative transfer on the structure and extinction limits of counterflow H-2/O-2/N-2 diffusion flames are studied numerically using detailed kinetics and transport properties. The radiative properties of the main emitting species, H2O and OH in these flames, are represented using a statistical narrow-band model. The radiative transfer equation and flow governing equations are solved in a coupled manner. The model is first validated by comparing numerical results with Rayleigh temperature and total flame radiative emission measurements. It is then applied to the numerical study of radiative effects on flame structure and properties. These effects, i.e., a decrease in flame temperature, flame width and production of minor species, are found to be the most important for high values of inlet H-2 mass fraction and for low strain rates. Quantitative values of radiative low strain rate extinction limits are given. The limits of validity and the discrepancies due to the optically thin medium approximation are also investigated.

Laminar counterflow spray diffusion flames: A comparison between experimental results and complex chemistry calculations

Abstract Experimental and numerical studies of laminar flames formed by the counterflow of a monodisperse fuel spray with an air stream are reported in this article. In this simple configuration it is possible to analyze the influence of the phase transfer terms on the flame structure. The experimental setup used to produce such laminar spray diffusion flames is first described. A set of experiments are carried with liquid heptane fuel sprays. The flame is characterized with a laser sheet imaging system and with a particle sizing apparatus based on laser light diffraction. Results of a numerical study are then presented. The two phase-reacting flow equations are solved through Newton iterations and adaptative gridding using detailed transport and complex chemistry. An iterative procedure is devised to solve the gas- and liquid-phase balance equations. Comparison between experimental and numerical values of the diameter are found to be in good agreement.

Progress in Numerical Combustion

Abstract This article begins with a synthetic presentation of key issues in the numerical description of combustion phenomena. Different levels of combustion modeling are identified and characterized. It is indicated how these modeling levels may be used to deal with fundamental questions or technological applications. Important advances have been made in detailed numerical modeling of complex flames and in direct simulation of flame/turbulence and flame/flow interactions. Results obtained in these areas have been employed to improve physical modeling methods which are currently used to calculate reactive flowfields in practical combustors operating in the turbulent regime. As physical modeling relies on average Navier-Stokes equations it requires closure rules for turbulent fluxes and for mean reaction rates. Considerable effort has been expanded to devise novel closure schemes or improve current models. Progress has been accomplished in the development of probabilistic methods in which the probability d...