A comparative study of H2-air premixed flame in micro combustors with different physical and boundary conditions

A numerical study of H₂-air premixed combustion in the micro channels with a detailed chemical reaction mechanism is performed by solving the two-dimensional fully elliptic governing equations of continuity, momentum, energy and species, coupled with the energy equation in the solid wall. A reference case is defined as the combustion in a cylindrical tube with 0.8 mm inner diameter and 8 mm length with a non-slip wall and a uniform velocity profile at the inlet plane. Different physical and boundary conditions have been applied in order to investigate their respective effects on the flame temperature. The conditions studied in the current paper include the combustor size and geometry, inlet velocity profile, axial heat conduction in the solid wall and slip-wall and temperature jump at the gas–solid interface. It is noted that effects of Knudsen number (slip-wall and temperature jump) on the thermal and fluid field are not very significant in a d = 0.4 mm micro combustor. Furthermore, the qualitative effects of Knudsen number on the flame temperature are analysed. The results of this paper indicate that these various boundary and physical conditions have effects on the flame temperature to different extent and should be carefully monitored when applied for different applications.     

Effects of subgrid scale and combustion modelling on flame structure of a turbulent premixed flame within LES and tabulated chemistry framework

The effects of combustion and SubGrid Scale (SGS) modelling on the overall flame characteristics of a turbulent premixed flame are investigated. This is achieved in terms of mean flow statistics, variances and flame surfaces. In particular, the chemical flame structure is analysed and compared. The Artificially Thickened Flame (ATF) approach coupled with the Flamelet Generated Manifolds (FGMs) and Filtered TAbulated Chemistry for LES (F-TACLES) approaches are used for this investigation. A Germano like procedure for dynamical calculation of SGS wrinkling is used which ensures the conservation of the total flame surface for both models. It turns out that using the dynamic SGS wrinkling model improves the results. Although the results of both combustion models in terms of statistics, mean and variances show very good agreement, the resolved flame surfaces hide different dynamic behaviour.     

Transient interactions between a premixed double flame and a vortex

The interaction between a laminar flame and a vortex is an important study for understanding the fundamentals of turbulent combustion. In the past, however, flame-vortex interactions have been investigated only for high-temperature flames. In this study, the impact of a vortex on a premixed double flame, which consists of a coupled cool flame and a hot flame, is examined experimentally and computationally using dimethyl ether/oxygen/ozone mixtures. The double flame is first shown to occur near the extinction limit of the hot flame. The differences between steady-state cool flames, double flames, and hot flames are explored in a one-dimensional counterflow configuration. The transient interactions between double flames and impinging vortices are then investigated experimentally using a micro-jet and numerically in two-dimensional transient modeling. It is seen that the vortex can extinguish the near-limit hot flame locally, resulting in a lone cool flame. At higher vortex intensities, the cool flame may also be extinguished after the extinction of the hot flame. It is found that there can be three different transient flame structures coexisting at the same time: an extinguished flame hole, a cool flame, and a double flame. Moreover, flame curvature is shown to play an important role in determining whether the vortex weakens or strengthens the cool flame and double flame.     

A theoretical study of premixed turbulent flame development

Flame development in a statistically stationary and uniform, planar, one-dimensional turbulent flow is theoretically studied. A generalized balance equation for the mean combustion progress variable, which includes turbulent diffusion and pressure-driven transport terms, as well as the mean rate of product creation, is introduced and analyzed by invoking the sole assumption of a self-similar flame structure, well-supported by numerous experiments. The assumption offers the opportunity to simplify the problem by splitting the aforementioned partial differential equation into two ordinary differential equations, which separately model spatial variations of the progress variable and time variations of flame speed and thickness. The self-similar profile of the progress variable, obtained in numerous experiments, is theoretically predicted. Closures of the normalized pressure-driven transport term and mean rate of product creation are obtained. The closed balance equation shows that turbulent diffusion dominates during the initial stage of flame development, followed by the transition to counter-gradient transport in a sufficiently developed flame. A criterion of the transition is derived. The transition is promoted by the heat release and pressure-driven transport. Fully developed mean flame brush thickness and speed are shown to decrease when either density ratio or pressure-driven transport increases. Solutions for the development of the thickness are obtained. The development is accelerated by the pressure-driven transport and heat release.     

Experimental and numerical study of a conical turbulent partially premixed flame

The structure and dynamics of a turbulent partially premixed methane/air flame in a conical burner were investigated using laser diagnostics and large-eddy simulations (LES). The flame structure inside the cone was characterized in detail using LES based on a two-scalar flamelet model, with the mixture fraction for the mixing field and level-set G-function for the partially premixed flame front propagation. In addition, planar laser induced florescence (PLIF) of CH and chemiluminescence imaging with high speed video were performed through a glass cone. CH and CH₂O PLIF were also used to examine the flame structures above the cone. It is shown that in the entire flame the CH layer remains very thin, whereas the CH₂O layer is rather thick. The flame is stabilized inside the cone a short distance above the nozzle. The stabilization of the flame can be simulated by the triple-flame model but not the flamelet-quenching model. The results show that flame stabilization in the cone is a result of premixed flame front propagation and flow reversal near the wall of the cone which is deemed to be dependent on the cone angle. Flamelet based LES is shown to capture the measured CH structures whereas the predicted CH₂O structure is somewhat thinner than the experiments.     

Temperature field measurement of an array of laminar premixed slot flame Jets using Mach-Zehnder interferometry

An experimental study was performed to investigate the influence of Reynolds number (Re) and non-dimensional jet-to-jet spacing (S/D_h) on flame shape, structure and temperature field of an array of laminar premixed slot flame jets. Mach-Zehnder interferometry technique is used to obtain an insight to the overall temperature field between single, twin and triple slot flame jets. The slot jets with large aspect ratio (L/W), length of L=60 mm and width of W=6 mm were used to eliminate the three-dimensional effect of temperature field. The effect of jet-to-jet spacing was investigated on flame characteristics under the test conditions of 200≤Re≤400 and equivalence ratio (φ) of unity. The present measurement reveals that the variation of maximum flame temperature with increment of Reynolds number is mainly due to heat transfer effects and is negligible while the flame height is increased. For the cases of twin and triple flame jets by increasing Reynolds number and decreasing non-dimensional jet-to-jet spacing (S/D_h), the interferences between the jets are increased and the jets attracted each other. Strong interference was observed at S/D_h=1.15. For the case of triple jets at this S/D_h, the central jet was suppressed while the side jets deflected towards the inner jet. The interference between jets was found to reduce the heat flux in the jet-to-jet interacting zone due to incomplete combustion. Also the optimum jet-to-jet spacing of triple flame jets is obtained at each Reynolds number to enhance the heat transfer performance of the jets.     

Effects of confinement on premixed turbulent swirling flame using large Eddy simulation

This paper utilises large eddy simulation (LES) to study swirling reacting flows by comparison with experimental observations. The purpose is to provide further insights in engineering designs, as well as to improve modelling. A reduced-scale swirl burner has been developed for the experiments. Comparison of particle image velocimetry (PIV) measurements with LES results using finite rate chemistry shows that LES captures all the salient features of an unconfined flame including velocity and temperature distributions. However, when the flame is confined within a cylindrical combustor, the simulated flame shape is initially not consistent with experimental observation. Investigations show that the discrepancy is caused by the often practised assumption of adiabatic wall temperature. With the use of an assumed wall temperature distribution guided by laboratory observation, results of LES are consistent with experiments. Although the latter LES approach requires more computational resources, the improvement is found to be justified.     

An experimental and kinetic modeling study of a premixed nitromethane flame at low pressure

A premixed nitromethane/oxygen/argon flame at low pressure (4.67 kPa) has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 30 flame species including hydrocarbons, oxygenated and nitrogenous intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species have been determined by scanning burner position at some selected photon energies. The results indicate that N₂ and NO are the major nitrogenous products in the nitromethane flame. Compared with previous studies on nitromethane combustion, a number of unreported intermediates, including C₃H₄, C₄H₆, C₄H₈, C₂H₂O, C₂H₄O, CH₃CN, H₂CNHO, C₃H₃N and C₃H₇N, are observed in this work. Based on our experimental results and previous modeling studies, a detailed oxidation mechanism including 69 species and 314 reactions has been developed to simulate the flame structure. Despite some small discrepancies, the predictions by the modeling study are in reasonable agreement with the experimental results.     

Numerical study of transient evolution of lifted jet flames: partially premixed flame propagation and influence of physical dimensions

Three-dimensional (3D) unsteady Reynolds-averaged Navier–Stokes simulations of a spark-ignited turbulent methane/air jet flame evolving from ignition to stabilisation are conducted for different jet velocities. A partially premixed combustion model is used involving a correlated joint probability density function and both premixed and non-premixed combustion mode contributions. The 3D simulation results for the temporal evolution of the flame's leading edge are compared with previous two-dimensional (2D) results and experimental data. The comparison shows that the final stabilised flame lift-off height is well predicted by both 2D and 3D computations. However, the transient evolution of the flame's leading edge computed from 3D simulation agrees reasonably well with experiment, whereas evident discrepancies were found in the previous 2D study. This difference suggests that the third physical dimension plays an important role during the flame transient evolution process. The flame brush's leading edge displacement speed resulting from reaction, normal and tangential diffusion processes are studied at different typical stages after ignition in order to understand the effect of the third physical dimension further. Substantial differences are found for the reaction and normal diffusion components between 2D and 3D simulations especially in the initial propagation stage. The evolution of reaction progress variable scalar gradients and its interaction with the flow and mixing field in the 3D physical space have an important effect on the flame's leading edge propagation.     

An experimental study of the rich premixed ethylbenzene flame at low pressure

A slightly sooting premixed ethylbenzene flame with an equivalence ratio of 1.90 was investigated at low pressure (4.0 kPa) using molecular-beam mass spectrometry (MBMS) and tunable synchrotron vacuum ultraviolet (VUV) photoionization. Basing on the ionization threshold measurements of photoionization efficiency (PIE) spectra, combustion intermediates up to C₁₉H₁₂ were identified, including a number of radicals and isomeric species. Mole fraction profiles of observed flame species were evaluated from the measurements of burner scan at the photon energies near ionization thresholds. Besides, the flame temperature profile was measured by a Pt/Pt-13%Rh thermocouple. From the intermediate identification and mole fraction measurements, the degradation of ethylbenzene, as well as the formation of some interested polycyclic aromatic hydrocarbons (PAHs), was discussed in detail. It is suggested that the formation of most typical PAHs observed in this work can be related to the H-abstraction/C₂H₂-addition (HACA) mechanism. Furthermore, the high concentration levels of intermediates in this flame is ascribed to the weak C-C bonds in the sidechain of ethylbenzene, which provides a potential explanation of the high sooting tendencies of ethylbenzene and other monocyclic aromatic fuels with complex sidechain structure. This study is anticipated to be constructive for combustion investigations of aromatic fuels, and the discussion is hoped to be helpful for further modeling studies concerning PAHs formation in combustion process.     

Dynamic behavior of a freely-propagating turbulent premixed flame under global stretch-rate oscillations

Dynamic features of a freely propagating turbulent premixed flame under global stretch rate oscillations were investigated by utilizing a jet-type low-swirl burner equipped with a high-speed valve on the swirl jet line. The bulk flow velocity, equivalence ratio and the nominal mean swirl number were 5 m/s, 0.80 and 1.23, respectively. Seven velocity forcing amplitudes, from 0.09 to 0.55, were examined with a single forcing frequency of 50 Hz. Three kinds of optical measurements, OH-PLIF, OH^* chemiluminescence and PIV, were conducted. All the data were measured or post-processed in a phase-locked manner to obtain phase-resolved information. The global transverse stretch rate showed in-phase oscillations centering around 60 (1/s). The oscillation amplitude of the stretch rate grew with the increment of the forcing amplitude. The turbulent flame structure in the core flow region varied largely in axial direction in response to the flowfield oscillations. The flame brush thickness and the flame surface area oscillated with a phase shift to the stretch rate oscillations. These two properties showed a maximum and minimum values in the increasing and decreasing stretch periods, respectively, for all the forcing amplitudes. Despite large variations in flame brush thickness at different phase angles, the normalized profiles collapse onto a consistent curve. This suggests that the self-similarity sustains in this dynamic flame. The global OH^* fluctuation response (i.e. response of global heat-release rate fluctuation) showed a linear dependency to the forcing velocity oscillation amplitudes. The flame surface area fluctuation response showed a linear tendency as well with a slope similar to that of the global OH^* fluctuation. This indicated that the flame surface area variations play a critical role in the global flame response.     

Triple flame: Inherent asymmetries and pentasectional character

A two-dimensional triple-flame numerical model of a laminar combustion process reflects flame asymmetric structural features that other analytical models do not generate. It reveals the pentasectional character of the triple flame, composed of the central pure diffusion-flame branch and the fuel-rich and fuel-lean branches, each of which is divided into two sections: a near-stoichiometric section and a previously unreported near-flammability-limits section with combined diffusion and premixed character. Results include propagation velocity, fuel and oxidiser mass fractions, temperature and reaction rates. Realistic stoichiometric ratios and reaction orders match experimental planar flame characteristics. Constant density, a one-step reaction, and a mixture fraction gradient at the inlet as the simulation parameter are imposed. The upstream equivalence ratio or the upstream reactant mass fractions are linear or hyperbolic functions of the transverse coordinate. The use here of experimental kinetics data differs from previous analytical works and results in flame asymmetry and different flammability limits. Upstream mixture composition gradient affects propagation velocity, flame curvature, diffusion flame reaction rate, and flammability limits. Flammability limits extend beyond those of a planar flame due to transverse heat and mass diffusion causing the pentasectional character.     

Flame structure and flame spread rate over a solid fuel in partially premixed atmospheres

We have investigated the downward flame spread over a thin solid fuel. Hydrogen, methane, or propane, included in the gaseous product of pyrolysis reaction, is added in the ambient air. The fuel concentration is kept below the lean flammability limit to observe the partially premixing effect. Both experimental and numerical studies have been conducted. Results show that, in partially premixed atmospheres, both blue flame and luminous flame regions are enlarged, and the flame spread rate is increased. Based on the flame index, a so-called triple flame is observed. The heat release rate ahead of the original diffusion flame is increased by adding the fuel, and its profile is moved upstream. Here, we focus on the heat input by adding the fuel in the opposed air, which could be a direct factor to intensify the combustion reaction. The dependence of the flame spread rate on the heat input is almost the same for methane and propane/air mixtures, but larger effect is observed for hydrogen/air mixture. Since the deficient reactant in lean mixture is fuel, the larger effect of hydrogen could be explained based on the Lewis number consideration. That is, the combustion is surely intensified for all cases, but this effect is larger for lean hydrogen/air mixture (Le < 1), because more fuel diffuses toward the lean premixed flame ahead of the original diffusion flame. Resultantly, the pyrolysis reaction is promoted to support the higher flame spread rate.     

DNS assessment of relation between mean reaction and scalar dissipation rates in the flamelet regime of premixed turbulent combustion

The linear relation between the mean rate of product creation and the mean scalar dissipation rate, derived in the seminal paper by K.N.C. Bray [‘The interaction between turbulence and combustion’, Proceedings of the Combustion Institute, Vol. 17 (1979), pp. 223–233], is the cornerstone for models of premixed turbulent combustion that deal with the dissipation rate in order to close the reaction rate. In the present work, this linear relation is straightforwardly validated by analysing data computed earlier in the 3D Direct Numerical Simulation (DNS) of three statistically stationary, 1D, planar turbulent flames associated with the flamelet regime of premixed combustion. Although the linear relation does not hold at the leading and trailing edges of the mean flame brush, such a result is expected within the framework of Bray's theory. However, the present DNS yields substantially larger (smaller) values of an input parameter c_m (or K₂ = 1/(2c_m ? 1)), involved by the studied linear relation, when compared to the commonly used value of c_m = 0.7 (or K₂ = 2.5). To gain further insight into the issue and into the eventual dependence of c_m on mixture composition, the DNS data are combined with the results of numerical simulations of stationary, 1D, planar laminar methane–air flames with complex chemistry, with the results being reported in terms of differently defined combustion progress variables c, i.e. the normalised temperature, density, or mole fraction of CH₄, O₂, CO₂ or H₂O. Such a study indicates the dependence of c_m both on the definition of c and on the equivalence ratio. Nevertheless, K₂ and c_m can be estimated by processing the results of simulations of counterpart laminar premixed flames. Similar conclusions were also drawn by skipping the DNS data, but invoking a presumed beta probability density function in order to evaluate c_m for the differently defined c's and various equivalence ratios.     

Large-eddy simulation of a bluff-body stabilised turbulent premixed flame using the transported flame surface density approach

A premixed propane–air flame stabilised on a triangular bluff body in a model jet-engine afterburner configuration is investigated using large-eddy simulation (LES). The reaction rate source term for turbulent premixed combustion is closed using the transported flame surface density (TFSD) model. In this approach, there is no need to assume local equilibrium between the generation and destruction of subgrid FSD, as commonly done in simple algebraic closure models. Instead, the key processes that create and destroy FSD are accounted for explicitly. This allows the model to capture large-scale unsteady flame propagation in the presence of combustion instabilities, or in situations where the flame encounters progressive wrinkling with time. In this study, comprehensive validation of the numerical method is carried out. For the non-reacting flow, good agreement for both the time-averaged and root-mean-square velocity fields are obtained, and the Karman type vortex shedding behaviour seen in the experiment is well represented. For the reacting flow, two mesh configurations are used to investigate the sensitivity of the LES results to the numerical resolution. Profiles for the velocity and temperature fields exhibit good agreement with the experimental data for both the coarse and dense mesh. This demonstrates the capability of LES coupled with the TFSD approach in representing the highly unsteady premixed combustion observed in this configuration. The instantaneous flow pattern and turbulent flame behaviour are discussed, and the differences between the non-reacting and reacting flow are described through visualisation of vortical structures and their interaction with the flame. Lastly, the generation and destruction of FSD are evaluated by examining the individual terms in the FSD transport equation. Localised regions where straining, curvature and propagation are each dominant are observed, highlighting the importance of non-equilibrium effects of FSD generation and destruction in the model afterburner.     

Effects of heat conduction and radical quenching on premixed stagnation flame stabilised by a wall

The premixed stagnation flame stabilised by a wall is analysed theoretically considering thermally sensitive intermediate kinetics. We consider the limit case of infinitely large activation energy of the chain-branching reaction, in which the radical is produced infinitely fast once the cross-over temperature is reached. Under the assumptions of potential flow field and constant density, the correlation for flame position and stretch rate of the premixed stagnation flame is derived. Based on this correlation, the effects of heat conduction and radical quenching on the wall surface are examined. The wall temperature is shown to have great impact on flame bifurcation and extinction, especially when the flame is close to the wall. Different flame structures are observed for near-wall normal flame, weak flame, and critically quenched flame. The fuel and radical Lewis numbers are found to have opposite effects on the extinction stretch rate. Moreover, it is also demonstrated that only when the flame is close to the wall does the radical quenching strongly influence the flame bifurcation and extinction. The extinction stretch rate is shown to decrease with the amount of radical quenching for different fuel and radical Lewis numbers. Besides, the coupling between the wall heat conduction and radical quenching is found to greatly influence the bifurcation and extinction of the premixed stagnation flame.     

Nonlinear hydrodynamic and thermoacoustic oscillations of a bluff-body stabilised turbulent premixed flame

Turbulent premixed flames often experience thermoacoustic instabilities when the combustion heat release rate is in phase with acoustic pressure fluctuations. Linear methods often assume a priori that oscillations are periodic and occur at a dominant frequency with a fixed amplitude. Such assumptions are not made when using nonlinear analysis. When an oscillation is fully saturated, nonlinear analysis can serve as a useful avenue to reveal flame behaviour far more elaborate than period-one limit cycles, including quasi-periodicity and chaos in hydrodynamically or thermoacoustically self-excited system. In this paper, the behaviour of a bluff-body stabilised turbulent premixed propane/air flame in a model jet-engine afterburner configuration is investigated using computational fluid dynamics. For the frequencies of interest in this investigation, an unsteady Reynolds-averaged Navier–Stokes approach is found to be appropriate. Combustion is represented using a modified laminar flamelet approach with an algebraic closure for the flame surface density. The results are validated by comparison with existing experimental data and with large eddy simulation, and the observed self-excited oscillations in pressure and heat release are studied using methods derived from dynamical systems theory. A systematic analysis is carried out by increasing the equivalence ratio of the reactant stream supplied to the premixed flame. A strong variation in the global flame structure is observed. The flame exhibits a self-excited hydrodynamic oscillation at low equivalence ratios, becomes steady as the equivalence ratio is increased to intermediate values, and again exhibits a self-excited thermoacoustic oscillation at higher equivalence ratios. Rich nonlinear behaviour is observed and the investigation demonstrates that turbulent premixed flames can exhibit complex dynamical behaviour including quasiperiodicity, limit cycles and period-two limit cycles due to the interactions of various physical mechanisms. This has implications in selecting the operating conditions for such flames and for devising proper control strategies for the avoidance of thermoacoustic instability.     

Hydrodynamic instability of premixed flame propagating in narrow planar channel in the presence of gas flow

The paper analyses the hydrodynamic instability of a flame propagating in the space between two parallel plates in the presence of gas flow. The linear analysis was performed in the framework of a two-dimensional model that describes the averaged gas flow in the space between the plates and the perturbations development of two-dimensional combustion wave. The model includes the parametric dependences of the flame front propagation velocity on its local curvature and on the combustible gas velocity averaged along the height of the channel. It is assumed that the viscous gas flow changes the surface area of the flame front and thereby affects the propagation velocity of the two-dimensional combustion wave. In the absence of the influence of the channel walls on the gas flow, the model transforms into the Darrieus–Landau model of flame hydrodynamic instability. The dependences of the instability growth rate on the wave vector of disturbances, the velocity of the unperturbed gas flow, the viscous friction coefficients and other parameters of the problem are obtained. It is shown that the viscous gas flow in the channel can lead, in some cases, to a significant increase in instability compared with a flame propagating in free space. In particular, the instability increment depends on the direction of the gas flow with respect direction of the flame propagation. In the case when the gas flow moves in the opposite direction to the direction of the flame propagation, the pulsating instability can appear.