3D Flame Research Articles

Burning velocity and OH concentration in premixed combustion

In the present paper, we have simulated laminar premixed flames in several types of flow configuration. The counter-flow flame, 1D flame in uniform flow, and 2D flame on a slot burner have been considered. The fuel is propane, and detailed chemistry has been used. We have obtained burning velocity in these flames to examine the response of peak OH concentration in flame zone to burning velocity (SL). In case of counter-flow flame, peak OH concentration is decreased at the larger stretch rate. At the same time, the burning velocity is decreased. The peak OH concentration of 1D flame in uniform flow is decreased at lower equivalence ratio, with smaller burning velocity. It is interesting to note that, for both flames as well as 2D flame, the same linear relationship is observed between two parameters of peak OH and SL. Thus, consistent with earlier findings of turbulent flames, peak OH concentration can be a good measure of burning velocity. In the experiments, we have estimated local burning velocity in turbulent combustion with a cyclone-jet combustor. The slot burner has been used for calibration. It is found that the peak OH concentration in the flame becomes smaller with an increase of turbulence. The local burning velocity is monotonically decreased at higher turbulence. In some cases, its value is almost zero, which could be local extinction. When the fuel is propane, Markstein number is positive for lean mixture. In that case, the burning velocity is decreased due to the stretch effect, which is well in accordance with the fact that the turbulence always suppresses the burning velocity in our measurements.

Increasing the efficiency of postprocessing for turbulent reacting flows

Turbulent combustion of fossil fuels is nowadays still by far the most important process to cover the worldwide energy needs. Furthermore, turbulent flames are widely used in a wealth of practical applications. Nevertheless, fundamental aspects of turbulent combustion are still poorly understood due to the complexity of the underlying, individual physical processes and due to their complex nonlinear coupling. In order to improve combustion processes, two complementary means are classically used: experimental investigations and numerical simulations. Both lead nowadays to a huge quantity of raw data. In order to extract all useful information from these data, a library containing essential postprocessing methods has been developed: it contains numerous tools to analyze and visualize 2D and 3D flames and flow fields, to investigate geometry and structure of flames both locally and globally, to quantify the interaction between flow fields and flames, and to determine statistics and correlations of all variables that are essential for model development. This library has been coded using scripts implemented into the MatLab platform. Available tools and illustrative examples are described in this paper in order to demonstrate the interest of this approach.

Reconstructed 3D flame structures in noise-controlled swirl-stabilized combustor

Flame structures of turbulent premixed flames in a noise-controlled, swirl-stabilized combustor are investigated to clarify the mechanism of combustion noise reduction by the secondary fuel injection. Planar laser-induced fluorescence (PLIF) is conducted for several cases with different secondary fuel injection, and 3D flame structure is reconstructed from PLIF results on multiple planes. The secondary fuel injection suppresses the fluctuation of high-temperature gas in the recirculation zone and reduces Reynolds stress and entropy terms in the acoustic sound source. In the flame zone, effects of the injection frequency are discussed by introducing mean progress variable. The flame brush is very wide for the no control case, whereas it becomes thin and is confined to a narrow space for the secondary fuel injection cases. The investigated combustor gives minimum sound level at a relevant fuel injection frequency, which is very low compared with the natural acoustic mode of the combustor. The flame brush becomes very thin, and self-induced oscillations of the flame brush disappear at this relevant frequency. The oscillation of the flame brush represents large-scale fluctuation of the mean heat release rate. The relations between characteristics of flame brush and combustion noise are discussed by introducing instantaneous and dynamical effects of flame front on the entropy term of the sound source. The secondary fuel injection works for the control of the entropy term in the sound source because the thin flame brush represents suppression of the instantaneous and dynamical effects.

A218 OH PLIF計測に基づく乱流予混合火炎の三次元火炎構造の再構築

Planar laser induced fluorescence (PLIF) of OH radical was conducted to clarify the relation between the flame structure and noise characteristics of the swirl-stabilized combustor controlled by secondary fuel injection. Three dimensional (3D) flame structures of turbulent premixed flames were reconstructed from the PLIF results on multiple planes. The 3D structures reveal that secondary fuel injection reduces the fluctuation of high temperature gas in the recirculation zone and that of flame front in the flame zone, which results in the reduction of the sound source.

B13 乱流燃焼のDNSと複合レーザ計測

With the developments of computer technologies, three-dimensional direct numerical simulations (DNS) of turbulent combustion have been realized with a detailed or reduced kinetic mechanism. The 3D DNS gives detailed information about turbulent flames, while there are few experimental techniques which have high accuracy enough to compare with DNS. In this paper, after showing summary of recent DNS of turbulent premixed flames, newly-developed laser diagnostics are presented. Simultaneous CH-OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) are used to investigate the local flame structure of the turbulent premixed flames. From CH-OH PLIF and PIV measurements, flame fronts are identified, and the curvature of the flame front and the tangential strain rate at the flame front are evaluated. The experimental results are compared with 3D DNS of hydrogen-air and methane-air turbulent premixed flames. The flame displacement speeds in turbulent premixed flames have been measured directly by the CH double-pulsed PLIF. Since the time interval of the successive CH PLIF can be selected arbitrarily, both of the large scale dynamics and local displacement of the flame front can be obtained. As an application of laser diagnostics for development of high-efficient and low-emission combustors, reconstruction of 3D flame structure is shown by using multiple-plane OH PLIF.

Modelling and control of the flame temperature distribution using probability density function shaping

This paper presents three control algorithms for the output probability density function (PDF) control of the 2D and 3D flame distribution systems. For the 2D flame distribution systems, control methods for both static and dynamic flame systems are presented, where at first the temperature distribution of the gas jet flames along the cross-section is approximated. Then the flame energy distribution (FED) is obtained as the output to be controlled by using a B-spline expansion technique. The general static output PDF control algorithm is used in the 2D static flame system, where the dynamic system consists of a static temperature model of gas jet flames and a second-order actuator. This leads to a second-order closed-loop system, where a singular state space model is used to describe the dynamics with the weights of the B-spline functions as the state variables. Finally, a predictive control algorithm is designed for such an output PDF system. For the 3D flame distribution systems, all the temperature values of the flames are firstly mapped into one temperature plane, and the shape of the temperature distribution on this plane can then be controlled by the 3D flame control method proposed in this paper. Three cases are studied for the proposed control methods and desired simulation results have been obtained.

Modeling Low Mach Number Reacting Flow with Detailed Chemistry and Transport

An efficient projection scheme is developed for the simulation of reacting flow with detailed kinetics and transport. The scheme is based on a zero-Mach-number formulation of the compressible conservation equations for an ideal gas mixture. It relies on Strang splitting of the discrete evolution equations, where diffusion is integrated in two half steps that are symmetrically distributed around a single stiff step for the reaction source terms. The diffusive half-step is integrated using an explicit single-step, multistage, Runge---Kutta---Chebyshev (RKC) method. The resulting construction is second-order convergent, and has superior efficiency due to the extended real-stability region of the RKC scheme. Two additional efficiency-enhancements are also explored, based on an extrapolation procedure for the transport coefficients and on the use of approximate Jacobian data evaluated on a coarse mesh. We demonstrate the construction in 1D and 2D flames, and examine consequences of splitting errors. By including the above enhancements, performance tests using 2D computations with a detailed C1C2 methane-air mechanism and a mixture-averaged transport model indicate that speedup factors of about 15 are achieved over the starting split-stiff scheme

Structure and dynamic response of radiative diffusion flames

Nitrogen-diluted hydrogen burning in air is modeled numerically using a constant density and one-step reaction model in a plane two-dimensional counterflow configuration. An optically thin assumption is used to investigate the effects of radiation on the dynamics, structure, and extinction of diffusion flames. While there exist dual steady-state extinction limits for the 1D radiative flame response, it is found that as the 1D radiative extinction point is approached the 1D low-stretch diffusion flame exhibits oscillatory response, even with sub-unity Lewis number fuel. These radiation-induced limit cycle oscillations are found to have increasing amplitude and decreasing frequency as the stretch rate is reduced. Flame oscillation eventually leads to permanent extinction at the stretch rate which is larger than the steady-state radiative extinction value. Along the 1D radiative response curve, the transition from 1D flame to 2D structure and the differences in the resulting 2D flame patterns are also examined using a variety of initial profiles, with special emphasis on the comparison of using the initial profiles with and without a flame edge. Similar to the previous studies on the high-stretch adiabatic edge flames using the same configuration, the high-stretch radiative flames are found to resist 1D blow-off quenching through various 2D structures, including propagating front and steady cellular flames for initial profiles with and without flame edges. For all initial profiles studied, the low-stretch radiative flames are also found to exhibit different 2D flame phenomena near the 1D radiative extinction limit, such as transient cellular structures, steady cellular structures, and pulsating ignition fronts. Although the results demonstrate the presence of low-stretch and high-stretch 2D bifurcation branches close to the corresponding 1D extinction limits irrespective of the initial profile used, particular 2D flame structures in certain stretch rate range are initial profile dependent. The existence of two-dimensional flame structures beyond the 1D steady-state radiative extinction limit suggests that the flammable range is expanded as compared to that predicted by the 1D model. Hence, multi-dimensional flame patterns need to be accounted for when determining the flammability limits for a given system.

Combustion Simulation Using the Lattice Boltzmann Method

Even though laser diagnostics have significantly improved and can obtain an instantaneous 2D flame image of the velocity field, it is still difficult to obtain data such as scalar flux or reaction rates experimentally. It is also essential to understand 3D flame structures in turbulent combustion. Chemically non-reacting turbulent flows are complex and chemical reactions make the problem more complicated. Due to practical limitations of computational costs, conventional numerical methods are very expensive for carrying out 3D numerical simulations at high Reynolds numbers with detailed chemical reactions. In this study, we have used the lattice Boltzmann method (LBM) to simulate a combustion field. The LBM is an efficient alternative for the numerical simulation of this type of flow. To confirm the validity of the LBM, a flame in simple flow geometry is simulated and the laminar burning velocity is obtained. Both 2D and 3D simulations have been completed. A jet flame has been also simulated to demonstrate the LBM capability of simulating unsteady flames with vortices. The scheme with detailed chemistry has been tested for simulation of a counter-flow flame.

Whatman plc, UK

Developing a strong, flame resistant, and thermally insulating structural material is essential for reducing energy consumption and improving safety in buildings. Wood is a pervasive structural material with natural thermally insulating properties. However, its flammability and insufficient mechanical strength compromises its safety, particularly in mid-rise and high-rise buildings that have higher safety and structural requirements. Herein, we demonstrate a strong, flame-retardant, and thermally insulating wood laminate featuring a compact structure that we synthesized by infiltrating delignified basswood with bentonite nanosheets, followed by hot-pressing to densify the material. Bentonite nanosheets can be infiltrated into the microchannel of the delignified wood to achieve a good compatible and 3D flame resistant protective surface. Flame retardant tests show that the peak heat release rate of this wood laminate is reduced by more than 50% compared to natural basswood. Additionally, the wood laminate does not start to burn until after 7 min of direct exposure to an open flame, and then self-extinguishing when removed from the igniting flame, suggesting the excellent flame resistance of the composite. The wood laminate also exhibits a high tensile strength of 330 MPa that is more than 8-times higher than the starting material. In addition, the wood laminate exhibits lower thermal conductivities of 0.2 and 0.18 W m−1 K−1 in the axial and radial direction, respectively. The demonstrated wood laminate shows great promise as a high-performance structural material with strong flame-retardant, thermally insulating, and mechanical robust properties that are needed for the construction of energy efficient buildings.

Numerical simulation of the reactive two-phase flow in a kerosene/air tubular combustor

Numerical simulation plays today an important role in the design of gas turbine combustors. It requires the understanding of basic physical phenomena encountered in turbulent and reactive flows. In most industrial applications concerning airbreathing propulsion, the fuel is injected in the liquid phase which often consists of a spray of fine droplets. This paper presents numerical simulations of the flowfield, inside a realistic tubular-type combustion chamber geometry, taking into account the interaction between the liquid droplets and the gaseous phase. The in-house ONERA's numerical code MSD was used to compute the turbulent reactive two-phase flow inside this 3D flame tube. In order to promote the liquid spray dispersion in the flow, the kerosene/air mixture is injected through a swirling injection system. A Lagrangian approach has been implemented in order to describe with good accuracy the droplet behaviour. This Lagrangian approach is numerically coupled with the Eulerian formulation describing the gaseous flow. Droplet dispersion induced by the turbulence of the flow is governed by probabilistic models taking into account the local turbulence conditions controlled by a two-equation k– ε model. Results are presented showing the influence of the prescribed swirl intensity on the global flow features. This investigation eventually helped us in finding the most appropriate conditions for future experimental testings.

The influence of hydrodynamic instability on the structure of cellular flames

The influence of hydrodynamic instability on the structure of two-dimensional (2D) and three-dimensional (3D) cellular flames is numerically investigated. The equation used is the compressible Navier–Stokes equation including a one-step irreversible chemical reaction. We superimpose an infinitesimal disturbance on the stationary plane flame and calculate the evolution of the disturbed flame front to obtain the relation between the growth rate and the wave number, i.e., the dispersion relation. With an increase in flame temperature, the growth rate increases since hydrodynamic instability becomes stronger. The unstable range normalized by the preheat zone thickness hardly changes, even though the flame temperature increases. The critical wave number, which corresponds to the maximum growth rate, is almost constant. Therefore, the normalized spacing between cells of the cellular flame is independent of the flame temperature. Moreover, we superimpose the disturbance with the critical wave number to investigate the structure of cellular flames. The stationary cellular flame is obtained when the inlet-flow velocity is set to the flame velocity of the cellular flame. The higher the flame temperature, the deeper the cell and the broader the flame surface. In addition, the cell depth and the surface area of the 3D flame are larger than those of the 2D flame. This is caused by the difference in the disposition of cells.

The Influence of Hydrodynamic Instability on the Structure of Cellular Flames.

The influence of hydrodynamic instability on the structure of two-dimensional (2D) and three-dimensional (3D) cellular flames is numerically investigated. The equation used is the compressible Navier–Stokes equation including a one-step irreversible chemical reaction. We superimpose an infinitesimal disturbance on the stationary plane flame and calculate the evolution of the disturbed flame front to obtain the relation between the growth rate and the wave number, i.e., the dispersion relation. With an increase in flame temperature, the growth rate increases since hydrodynamic instability becomes stronger. The unstable range normalized by the preheat zone thickness hardly changes, even though the flame temperature increases. The critical wave number, which corresponds to the maximum growth rate, is almost constant. Therefore, the normalized spacing between cells of the cellular flame is independent of the flame temperature. Moreover, we superimpose the disturbance with the critical wave number to investigate the structure of cellular flames. The stationary cellular flame is obtained when the inlet-flow velocity is set to the flame velocity of the cellular flame. The higher the flame temperature, the deeper the cell and the broader the flame surface. In addition, the cell depth and the surface area of the 3D flame are larger than those of the 2D flame. This is caused by the difference in the disposition of cells.

Instabilities in smooth 2D and 3D isothermal flames

Chemical reactions exhibiting autocatalytic feedback support constant-form, constant-velocity reaction-diffusion wave fronts. The dimensionless velocity c of planar fronts depends on the ratio δ of the diffusion coefficients for the reactant A and autocatalytic species B and an approximate expression for c( δ) for δ1 ( but 1 − δ −1 ⪡ 1) is presented. The implications of this, along with previous results for c appropriate to other ranges of δ, in terms of the dependence of the actual wave velocity dx dt as a function of the species diffusion coefficients D A and D B are discussed. An eikonal equation is then presented for the effect of curvature on the wave velocity for smooth circular or spherical waves. The coefficient that appears in the curvature term depends on δ: for δ less than some ‘critical’ value δ∗ ≈ 2.3, the coefficient is negative, indicating that the wave velocity increases towards the planar wave velocity c( δ) as the curvature decreases; for δδ∗, however, the coefficient is positive, indicating that the wave velocity decreases as the curvature decreases. This change in behaviour suggests that systems with δδ∗ will not exhibit a ‘critical radius’ below which wave propagation fails. The change in response to curvature also underlies the loss of stability of smooth waves to spatial perturbation transverse to the direction of propagation. For planar waves, the requirement for instability is δδ∗ with an additional condition on the wave number of the imposed perturbation (or, equivalently, on the width of the reaction zone). For spherical or circular waves, this latter condition is translated as a requirement on the radius of the smooth wave at the moment the perturbation is applied.

On Non-Linear Instabilities of Cellular Premixed Flames

Abstract A study of finite amplitude instabilities occurring in cellular premixed flames is undertaken, both in the case of 2D and 3D flames. We use two different models: the complete equations of isobaric flames with one global exothermic reaction and a modified Michelson-Sivashinsky equation (including gravity effects). Different instabilities are found that can be explained in terms of Eckhaus type instabilities of cellular patterns. An oscillatory instability also exists, which produces self turbulizing flames