Measurement of the instantaneous flame front structure of syngas turbulent premixed flames at high pressure

Instantaneous flame front structure of syngas turbulent premixed flames including the local radius of curvature, the characteristic radius of curvature, the fractal inner cutoff scale and the local flame angle were derived from the experimental OH-PLIF images. The CO/H₂/CO₂/air flames as a model of syngas/air combustion were investigated at pressure of 0.5 MPa and compared to that of CH₄/air flames. The convex and concave structures of the flame front were detected and statistical analysis including the PDF and ADF of the local radius of curvature and local flame angle were conducted. Results show that the flame front of turbulent premixed flames at high pressure is a wrinkled flame front with small scale convex and concave structures superimposed with large scale flame branches. The convex structures are much more frequent than the concave ones on flame front which reflects a general characteristic of the turbulent premixed flames at high pressure. The syngas flames possess much wrinkled flame front with much smaller fine cusps structure compared to that of CH₄/air flames and the main difference is on the convex structure. The effect of turbulence on the general wrinkled scale of flame front is much weaker than that of the smallest wrinkled scale. The general wrinkled scale is mainly dominated by the turbulence vortex scale, while, the smallest wrinkled scale is strongly affected by the flame intrinsic instability. The effect of flame intrinsic instability on flame front of turbulent premixed flame is mainly on the formation of a large number of convex structure propagating to the unburned reactants and enlarge the effective contact surface between flame front and unburned reactants.     

The effect of chemical energy release on heat transfer from flames in small channels

Stable combustion in a heated tube, with a radius on the order of the flame thickness, is investigated experimentally and numerically. The downstream portion of the tube is heated by an external heat source resulting in a steady, axially varying temperature gradient along the tube wall. Strongly burning, axisymmetric methane/air flames are stabilized inside this wall temperature profile which are observed to be “flat” for sufficiently small tube dimensions. The position of these flames is dictated by a competition between the energy required to preheat the reactants, that released by combustion, and the heat lost to the wall. To model such flames, an extension to the standard 1-D, volumetric flame formulation is proposed to solve for wall/gas heat transfer by employing a thermal boundary layer. The boundary layer utilizes a non-linear, radially-varying heat source to account for combustion and captures the effect of enhanced interfacial heat transfer inside the reaction zone. The proposed numerical model gives improved quantitative predictions for flame stabilization position than approaches which neglect the effect of heat release by modeling heat transfer with Newton’s law of cooling and a local Nusselt number.     

Effects of ignition location on premixed hydrogen/air flame propagation in a closed combustion tube

The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker.     

The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames

Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner. This burner allows independent variation of the global stretch rate and overall flame curvature. In opposed-flow flames formed by 21.7% hydrogen diluted in carbon dioxide versus air, cells are formed near extinction with a low fuel Lewis number and a low initial mixture strength. Using an intensified CCD camera, the flame chemiluminescence is imaged to study cellular formation from the onset of cells to near extinction conditions. The experimental onset of cellular instability is found to be at or at a slightly lower Damköhler number than the numerically determined extinction limit based on a two-point boundary value solution of the tubular flame. For fuel Lewis numbers less than unity, concave curvature towards the fuel retards combustion and weakens the flame and convex curvature towards the fuel promotes combustion and strengthens the flame. In the cell formation process, the locally concave flame cell midsection is weakened and the locally convex flame cell ends are strengthened. With increasing stretch rate, the flame breaks into cells and the cell formation process continues until near-circular cells are formed with no concave midsection. Further increase in the stretch rate leads to cell extinction. With increasing stretch rate, the flame thickness at the cell midsection decreases similar to a planar opposed-flow flame while the flame thickness at the cell edges is unchanged and can even increase due to the strengthening effect of convex curvature at the flame edges toward the low Lewis number fuel. The results show the existence of cellular flames well beyond the two-point boundary value extinction limit and the importance of local flame curvature in the formation of flame cells.     

Experimental investigation of the morphology and stability of diffusion and edge flames in an opposed jet burner

The stability of methane/air and hydrogen/air flames in an axisymmetric counterflow burner was investigated experimentally for different burner geometries, degrees of fuel dilution, and combinations of flow velocities. Both planar diffusion flames and edge flames were observed, and the transitions between these flame types were studied. The experimental results confirmed previously published numerical predictions on diluted hydrogen/air flames: the existence of two distinct stable flame types; the possibility of switching between the two flame types by perturbing the flames, e.g., by suitably changing a flow velocity; and the strong hysteresis for the transition from one flame type to the other. Flame stability diagrams were compiled which delineate the range of fuel and air flow velocities for which the planar diffusion flame and the toroidal edge flame are stable. The lower boundary curve for the edge flame stability exhibits a characteristic minimum at a well-defined value of the fuel velocity. For fuel velocities lower than this value, the transition between the edge and the diffusion structure is reversible, and the flames exhibit bistable behavior. For higher fuel velocities, the decrease of air velocity leads to the extinction of the edge flame. An investigation of both the cold and the reactive flow field identified bistable behavior for the flow field as well. Except for very low flow rates, the stagnation plane stabilizes in two positions, close to either of the two nozzles. Detailed numerical simulations of hydrogen flames capture the essentials of this behavior. The observed flame extinction results from the interaction of the flame dynamics with the dynamics of the flow field.     

Direct numerical simulation of turbulent premixed ammonia and ammonia-hydrogen combustion under engine-relevant conditions

The combustion characteristics of ammonia and ammonia-hydrogen fuel blends under spark-ignited turbulent premixed engine-relevant conditions were investigated by means of direct numerical simulation and detailed chemistry. Several test cases were investigated for an outwardly expanding turbulent premixed flame configuration covering pure ammonia and ammonia-hydrogen fuel blends with 10% and 15% hydrogen content by volume for different equivalence ratio values of 0.9, 1.0 and 1.1. The results showed that the fuel-lean flames exhibit strong wrinkled structures at flame front compared to stoichiometric and fuel-rich flames. The heat release rate plots indicate that adding hydrogen into ammonia improves the reactivity of the flame and enhances the combustion process. The scatter plots of heat release rate versus local curvature coloured by NO formation, show that high heat release rate values occur in the concave structures and low heat release rate values occur in the convex structure, which is consistent with NO distribution. The highest turbulent burning velocity values were found for the fuel-lean cases due to more wrinkled flame front with lower effective Lewis number compared to fuel-rich cases. The results show a bending effect for the ratio between turbulent to laminar burning velocities with respect to hydrogen addition at all equivalence ratios with 10% hydrogen addition into ammonia exhibiting a highest value for the burning velocity ratio. Two distinct flame structures (concave and convex) were analysed in terms of local equivalence ratio based on the elements of N and O as well as H and O. They revealed an opposite distribution of NO formation normal to the flame front within concave and convex structures. Elementary chemical reactions involved in NO formation have shown that hydrogen addition into ammonia influences the reactivity of certain specific chemical reactions.     

Effects of the external turbulence on centrally-ignited spherical unstable Ch4/H2/air flames in the constant-volume combustion bomb

In the present study, we conducted experiments to investigate the effects of external turbulence on the development of spherical H₂/CH₄/air unstable flames developments at two different equivalence ratios associated with different turbulent intensities using a spherical constant-volume turbulent combustion bomb and high speed schlieren photography technology. Flame front morphology and acceleration process were recorded and different effects of weak external turbulent flow field and intrinsic flame instability on the unstable flame propagation were compared. Results showed the external turbulence has a great influence on the unstable flame propagation under rich fuel conditions. For fuel-lean premixed flames, however, the effects of external turbulence on the morphology of the cellular structure on the flame front was not that obvious. Critical radius decreased firstly and then kept almost unchanged with the augment of the turbulence intensity. This indicated the dominating inhibiting effect of flame stretch on the turbulent premixed flame at the initial stage of the flame front development. Beyond the critical radius, the acceleration exponent was found increasing with the enhancement of initial turbulence intensity for fuel-lean premixed flames. For fuel-rich conditions, however, the initial turbulence intensity had little effect on acceleration exponent. In order to evaluate the important impact of the intrinsic flame instability and external turbulent flow field for spherical propagating premixed flames, intrinsic flame instability scale and average diameter of vortex tube were calculated. Intrinsic flame instability scale decreased greatly and then stayed unchanged with the propagation of the flame front. The comparison between intrinsic flame instability scale and average diameter of vortex tube demonstrated that the external turbulent flow filed will be more important for the evolution of wrinkle structure in the final stage of the flame propagation, when the turbulence intensity was more than 0.404 m/s.     

Anchoring of turbulent premixed hydrogen/air flames at externally heated walls

A joint experimental and numerical investigation of turbulent flame anchoring at externally heated walls is presented. The phenomenon has primarily been studied for laminar flames and micro-combustion while this study focuses on large-scale applications and elevated Reynolds number flows. Therefore, a novel burner design is developed and examined for a diverse set of operating conditions. Hydroxyl radical chemiluminescence measurements are employed to validate the numerical method. The numerical investigation evaluates the performance of various hydrogen/air kinetics, Reynolds-averaged turbulence models and the eddy dissipation concept (EDC) as a turbulence-chemistry interaction model. Simulation results show minor differences between detailed chemical mechanisms but pronounced deviations for a reduced kinetic. The baseline k-ω turbulence model is assessed to most accurately predict flame front position and shape. Universal applicability of EDC modeling constants is contradicted. Conclusively, the flame anchoring concept is considered a promising approach for pilot flames in continuous combustion devices.     

Extinction and near-extinction instability of non-premixed tubular flames

Tubular non-premixed flames are formed by an opposed tubular burner, a new tool to study the effects of curvature on extinction and flame instability of non-premixed flames. Extinction of the opposed tubular flames generated by burning diluted H₂, CH₄ or C₃H₈ with air is investigated for both concave and convex curvature. To examine the effects of curvature on extinction, the critical fuel dilution ratios at extinction are measured at various stretch rates, initial mixture strengths and flame curvature for fuels diluted in N₂, He, Ar or CO₂. In addition, the onset conditions of the cellular instability are mapped as a function of stretch rates, initial mixture strengths, and flame curvature. For fuel mixtures with Lewis numbers much less than unity, such as H₂/N₂, concave flame curvature towards the fuel suppresses cellular instabilities.     

The effects of heat loss on the burning velocity of cellular premixed flames generated by hydrodynamic and diffusive-thermal instabilities

The effects of heat loss on the burning velocity of cellular premixed flames are investigated by two-dimensional unsteady calculations of reactive flows based on the compressible Navier-Stokes equation and on the diffusive-thermal model equation. Hydrodynamic and diffusive-thermal instabilities are taken into account as contributing to the intrinsic instability of premixed flames. A sufficiently small disturbance is superimposed on a planar flame to obtain the relation between the growth rate and the wavenumber, i.e., the dispersion relation. As the heat loss becomes larger, the growth rate decreases and the unstable range narrows. This is because hydrodynamic instability caused by thermal expansion weakens for nonadiabatic flames. To investigate the characteristics of cellular flames, the disturbance with the linearly most unstable wavenumber, i.e., the critical wavenumber, is superimposed. As the superimposed disturbance evolves, the cellular-flame front forms due to the intrinsic instability. The lateral movement of cellular flames is observed at low Lewis numbers, and the behavior of cellular-flame fronts becomes more unstable for nonadiabatic flames. As the heat-loss parameter increases, the burning velocity of a cellular flame normalized by that of a planar flame increases at Lewis numbers lower than unity. By contrast, when the Lewis number is not less than unity, the burning-velocity increment decreases by increasing the heat loss. Diffusive-thermal instability thus has a pronounced influence on the unstable behavior and burning velocity of nonadiabatic cellular flames.     

Fractal structures of hydrodynamically unstable and diffusive-thermally unstable flames

This paper discusses the fractal structure of a hydrodynamically unstable flame with the background of the risk assessment of an explosion hazard. An accidental gas explosion usually occurs in a large-scale quiescent combustible mixture. A spherical flame outwardly propagates from the ignition point, and the flame accelerates owing to hydrodynamic instability. From the viewpoint of risk assessment, it is essential to consider such an increase in flame speed because the damage of an explosion is significantly influenced by the flame speed. Because hydrodynamically unstable flames have fractal structures and the flame area (and hence the flame speed) can be estimated using the fractal dimension, it is important to know the fractal dimension of the flame under the condition of a potential accidental explosion. Three methods (a box-counting method, a Fourier analysis, and a method based on the scale dependence of the flame speed) are tested to calculate the fractal dimension of a purely hydrodynamically unstable flame that is neutral in terms of diffusive-thermal instability. These methods are applied to the numerical solution of the Sivashinsky equation, but they can be also used to the result of an ordinary CFD calculation. The fractal structure of a purely diffusive-thermally unstable flame, which is neutral in terms of hydrodynamic instability, is also studied for comparison. The results show that all the three methods yield consistent fractal dimensions for the hydrodynamically unstable flame, whereas the diffusive-thermally unstable flame does not exhibit fractal characters. This is because the former flame has a hierarchical structure, whereas wrinkles of a specific wavelength mainly grow in the latter flame. The dependence of the fractal dimension on the thermal expansion ratio is also discussed.     

Effect of hydrogen and helium addition to fuel on soot formation in an axisymmetric coflow laminar methane/air diffusion flame

A detailed numerical study was conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane/air diffusion flame. Detailed gas-phase chemistry and thermal and transport properties were employed in the numerical calculations. Soot was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Numerical results were compared with available experimental data. Both experimental and numerical results show that helium addition is more effective than hydrogen addition in reducing soot loading in the methane/air diffusion flame. These results are different from the previous investigations in ethylene/air diffusion flames. Hydrogen chemically enhances soot formation when added to methane. The different chemical effects of hydrogen addition to ethylene and methane on soot formation are explained in terms of the different effects of hydrogen addition on propargyl, benzene, and pyrene formation low in the flames.     

Numerical study of opposed non-premixed jet flames of methane in a coaxial narrow air tube

In a previous study, an opposed non-premixed jet flame (ONPJF) in a coaxial narrow air tube was described, and various flame structures and their flame extinction limits were evaluated experimentally. In this study, flame structures and flow variation near the ONPJF of methane in a narrow air tube were numerically investigated using a one-step reaction model. Boundary conditions were examined for various tube sizes: cold or adiabatic walls and no-slip or slip walls. The flame extinction limits were numerically evaluated and classified into three modes: a higher air limit (HA-limit), a lower fuel limit (LF-limit), and a lower air limit (LA-limit). The HA-limits were determined by flame stretch, while the LF-limits and LA-limits were determined by the thermal quenching effect. The extinction mechanism at the LA-limit was investigated in detail. The structural transition was observed, and an enclosed edge flame structure was observed at the LA-limits. These results will be helpful in understanding the overall behavior of opposed non-premixed flames in narrow spaces, and in designing small-scale combustors with better stabilization performance.     

Analysis of exergy loss vs heat transfer rate for Rayleigh–Bénard convection of various fluids in enclosures with curved walls

The investigation of entropy generation is highly desirable for the optimization of the thermal systems to avoid larger energy wastage and ensure higher heat transfer rate. The numerical investigation of natural convection within enclosures with the concave and convex horizontal walls involving the Rayleigh–Bénard heating is performed via entropy generation approach. The spatial distributions of the temperature (θ), fluid flow (ψ), entropy generation due to heat transfer and fluid friction (S_θ and S_ψ) are discussed extensively for various Rayleigh numbers and Prandtl numbers involving various wall curvatures. A number of complex patterns of spatial distributions of fluid flow and temperature for cavities with concave or convex isothermal walls (top and bottom) have been obtained. The zones of high entropy generation for temperature and fluid flow are detected within cavities with concave and convex horizontal walls. The optimal situation involves the high heat transfer rate with moderate or low entropy generation. Overall, case 3 (highly concave) is found to be optimal over cases 1 and 2 (concave) and cases 1–3 (convex) for all Pr and Ra.     

Turbulent flame topology and the wrinkled structure characteristics of high pressure syngas flames up to 1.0 MPa

The turbulent flame topology characteristics of the model syngas with two different hydrogen ratios were statistically investigated, namely CO/H₂ ratio at 65/35 and 80/20, at equivalence ratio of 0.7. The combustion pressure was kept at 0.5 MPa and 1.0 MPa, to simulate the engine-like condition. The model syngas was diluted with CO₂ with a mole fraction of 0.3 which mimics the flue gas recycle in the turbulent combustion. CH₄/air flame with equivalence ratio of 1.0 was also tested for comparison. The flame was anchored on a premixed type Bunsen burner, which can generate a controllable turbulent flow. Flame front, which is represented by the sharp increased interface of the OH radical distribution, was measured with OH-PLIF technique. Flame front parameters were obtained through image processing to interpret the flame topology characteristics. Results showed that the turbulent flames possess a wrinkled character with smaller scale concave/convex structure superimposed on a larger scale convex structure under high pressure. The wrinkled structure of syngas flame is much finer and more corrugated than hydrocarbon fuel flames. The main reason is that scale of wrinkled structure is smaller for syngas flame, resulting from the unstable physics. Hydrogen in syngas can increase the intensity of the finer structure. Moreover, the model syngas flames have larger flame surface density than CH₄/air flame, and hydrogen ratio in syngas can increase flame surface density. This would be mainly attributed to the fact that the syngas flames have smaller flame intrinsic instability scale l_i than CH₄/air flame. S_T/S_L of the model syngas tested in this study is higher than CH₄/air flames for both pressures, due to the high diffusivity and fast burning property of H₂. This is mainly due to smaller L_M and l_i. V_f of the two model syngas is much smaller than CH₄/air flames, which suggests that syngas flame would lead to a larger possibility to occur combustion oscillation.     

发射CT法同时测量含烟黑火焰的温度与烟黑浓度分布的实验研究

基于烟黑辐射特性,利用烟黑单色辐射强度图像信息,采用CT算法同时重建含烟黑火焰温度与烟黑浓度分布,对蜡烛火焰与煤油火焰的温度与烟黑体积分数进行了测量．测量结果表明在两种火焰中,较大烟黑浓度都位于较高火焰温度之内,即在火焰外环的反应区内．另外,由于煤油火焰的燃料量大,因而会增大火焰中的烟黑浓度,辐射损失增大,降低火焰温度．这与有关实验结论是一致的．     

Heat release and flame structure measurements of self-excited acoustically-driven premixed methane flames

An open-open organ pipe burner (Rijke tube) with a bluff-body ring was used to create a self-excited, acoustically-driven, premixed methane-air conical flame, with equivalence ratios ranging from 0.85 to 1.05. The feed tube velocities corresponded to Re = 1780-4450. Coupled oscillations in pressure, velocity, and heat release from the flame are naturally encouraged at resonant frequencies in the Rijke tube combustor. This coupling creates sustainable self-exited oscillations in flame front area and shape. The period of the oscillations occur at the resonant frequency of the combustion chamber when the flame is placed ∼¼ of the distance from the bottom of the tube. In this investigation, the shape of these acoustically-driven flames is measured by employing both OH planar laser-induced fluorescence (PLIF) and chemiluminescence imaging and the images are correlated to simultaneously measured pressure in the combustor. Past research on acoustically perturbed flames has focused on qualitative flame area and heat release relationships under imposed velocity perturbations at imposed frequencies. This study reports quantitative empirical fits with respect to pressure or phase angle in a self-generated pressure oscillation. The OH-PLIF images were single temporal shots and the chemiluminescence images were phase averaged on chip, such that 15 exposures were used to create one image. Thus, both measurements were time resolved during the flame oscillation. Phase-resolved area and heat release variations throughout the pressure oscillation were computed. A relation between flame area and the phase angle before the pressure maximum was derived for all flames in order to quantitatively show that the Rayleigh criterion was satisfied in the combustor. Qualitative trends in oscillating flame area were found with respect to feed tube flow rates. A logarithmic relation was found between the RMS pressure and both the normalized average area and heat release rate for all flames.     

Laminar burning velocities and transition to unstable flames in H2/O2/N2 and C3H8/O2/N2 mixtures

Effects of positive flame stretch on laminar burning velocities, and conditions for transition to unstable flames, were studied experimentally for freely propagating spherical flames at both stable and unstable preferential-diffusion conditions. The data base involved new measurements for H₂/O₂/N₂ mixtures at values of flame stretch up to 7600 s⁻¹, and existing measurements for C₃H₈/O₂/N₂ mixtures at values of flame stretch up to 900 s⁻¹. Laminar burning velocities varied linearly with increasing Karlovitz numbers—either decreasing or increasing at stable or unstable preferential-diffusion conditions—yielding Markstein numbers that primarily varied with the fuel-equivalence ratio. Neutral preferential-diffusion conditions, however, were shifted toward the unstable side of the maximum laminar burning velocity condition that the simplest preferential-diffusion theories associate with neutral stability. All flames exhibited transition to unstable flames: unstable preferential-diffusion coditions yielded early transition to irregular flame surfaces, and stable preferential-diffusion conditions yielded delayed transition to cellular flames by hydrodynamic instability. Conditions for hydrodynamic instability transitions for H₂/O₂/N₂ mixtures were consistent with an earlier correlation due to Groff for propane/air flames, based on the predictions of Istratov and Librovich.     

Sensitivity analysis of transfer functions of laminar flames

The sensitivity of laminar premixed methane/air flames responses to acoustic forcing is investigated using direct numerical simulation to determine which parameters control their flame transfer function. Five parameters are varied: (1) the flame speed s_L, (2) the expansion angle of the burnt gases α, (3) the inlet air temperature T_a, (4) the inlet duct temperature T_d and (5) the combustor wall temperature T_w. The delay of the flame transfer function is computed for the axisymetric flames of Boudy et al. [1] and the slot flames of Kornilov et al. [2]. Stationary flames are first computed and compared to experimental data in terms of flame shape and velocity fields. The flames are then forced at different frequencies. Direct numerical simulations reproduce the flame transfer functions correctly. The sensitivity analysis of the flame transfer function is done by changing parameters one by one and measuring their effect on the delay. This analysis reveals that the flame speed s_L and the inlet duct temperature T_d are the two parameters controlling the flame delay and that any precise computation of the flame transfer function delay must first have proper models for these two quantities.     

Propagation speeds and stretch rates measured along wrinkled flames to assess the theory of flame stretch

Local propagation speeds and stretch rates were measured along a premixed flame that undergoes unsteady wrinkling in order to see if these two quantities correlate in the manner that is predicted by the theory of flame stretch. The Markstein number, which relates these two quantities, also was measured. Previous studies had considered the simple geometries of counterflow or spherical flames, but in this case a complex geometry was generated by interacting a flame with a vortex, such that both the strain and curvature components of the stretch rate are present. The diagnostics used were shadowgraph movies and simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence. The overall conclusion is that the theory of flame stretch remains valid for these unsteady complex conditions, because the measured trends are found to be in agreement with trends predicted by the theory. That is, propagation speeds decrease at locations where positive stretch is applied to stable (lean propane-air and rich methane-air) flames. Conversely, propagation speeds increase where positive stretch was applied to unstable (lean methane-air) flames. The shape of the profiles of propagation speed along stable flames is opposite to that of unstable flames, as is predicted by the theory. However, values of Markstein number show large variations and are much larger than that of an outwardly propagating spherical flame. Negative strain regions are of particular interest because they previously had not been studied experimentally; these regions yield the largest propagation speeds for the stable cases and some negative speeds for the unstable cases.