Dynamics and burning limits of near-limit hot,warm, and cool diffusion flames of dimethyl ether at elevated pressures

The near-limit diffusion flame regimes and extinction limits of dimethyl ether at elevated pressures and temperatures are examined numerically in the counterflow geometry with and without radiation at different oxygen concentrations. It is found that there are three different flame regimes—hot flame, warm flame, and cool flame—which exist, respectively, at high, intermediate, and low temperatures. Furthermore, they are governed by three distinct chain-branching reaction pathways. The results demonstrate that the warm flame has a double reaction zone structure and plays a critical role in the transition between cool and hot flames. It is also shown that the cool flame can be formed in several different ways: by either radiative extinction or stretch extinction of a hot flame or by stretch extinction of a warm flame. A warm flame can also be formed by radiative extinction of a hot flame or ignition of a cool flame. A general €-shaped flammability diagram showing the burning limits of all three flame regimes at different oxygen mole fractions is obtained. The results show that thermal radiation, reactant concentration, temperature, and pressure all have significant impacts on the flammable regions of the three flame regimes. Increases in oxidizer temperature, oxygen concentration, and pressure shift the cool flame regime to higher stretch rates and cause the warm flame to have two extinction limits. At elevated temperatures, it is found that there is a direct transition between the hot flame and warm flame at low stretch rates. The results also show that, unlike the hot flame, the cool flame structure cannot be scaled by using pressure-weighted stretch rates due to the its significant reactant leakage and strong dependence of reactivity on pressure. The present results advance the understanding of near-limit flame dynamics and provide guidance for experimental observation of different flame regimes.     

Memory effects of local flame dynamics in turbulent premixed flames

A premixed and thermo-diffusively unstable turbulent hydrogen-air flame-in-a-box case is simulated in conjunction with the flame particle tracking (FPT) method. The flame is located in the flamelet regime. The focus lies on the assessment of memory effects in local flame dynamics. By tracking flame particles on an iso-surface of the flame during flame-turbulence interaction, the time history of flame speed and flame stretch can be recorded for each point on the flame iso-surface in a Lagrangian reference frame. The results reveal a time delay between the local flame speed and flame stretch signal, showing that previous values of flame stretch affect currently observed values of flame speed. Furthermore, by choosing flame particles whose trajectories are dominated by single frequencies, the time delay can be quantified. While plotting instantaneous values of flame speed and flame stretch results in a large scattering for turbulent flames, a quasi-linear correlation can be achieved by shifting the time signal of flame stretch according to the time delay. The time delay itself depends on the local flow time scale, which is expressed as a local Damköhler number. There is, however, an important difference between consumption and displacement speed. While most analyses in the literature are limited to the flame displacement speed, the flame consumption speed is evaluated for each flame particle in this work as well, which shows a strong correlation with the local equivalence ratio even at unsteady conditions. As the flame particles move toward regions with more negative flame stretch, the consumption speed decreases as the flame locally extinguishes. At the same time, the diffusive component of the displacement speed increases, as the tangential component of the diffusive flux increases in regions with strong negative flame curvature.     

A numerical study on the formation of diffusion flame islands in a turbulent hydrogen jet lifted flame

This paper presents a numerical study on the formation of diffusion flame islands in a hydrogen jet lifted flame. A real size hydrogen jet lifted flame is numerically simulated by the DNS approach over a period of about 0.5 ms. The diameter of hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The lifted flame is composed of a stable leading edge flame, a vigorously turbulent inner rich premixed flame, and a number of outer diffusion flame islands. The relatively long-term observation makes it possible to understand in detail the time-dependent flame behavior in rather large time scales, which are as large as the time scale of the leading edge flame unsteadiness. From the observation, the following three findings are obtained concerning the formation of diffusion flame islands. (1) A thin oxygen diffusion layer is developed along the outer boundary of the lifted flame, where the diffusion flame islands burn in a rather flat shape. (2) When a diffusion flame island comes into contact with the fluctuating inner rich premixed flame, combustion is intensified due to an increase in the hydrogen supply by molecular diffusion. This process also works for the production of the diffusion flame islands in the oxygen diffusion layer. (3) When a large unburned gas volume penetrates into the leading edge flame, the structure of the leading edge flame changes. In this transformation process, a diffusion flame island comes near the leading edge flame. The local deficiency of oxygen plays an important role in this production process.     

Effects of heat loss and viscosity friction at walls on flame acceleration and deflagration to detonation transition

The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front.The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT.It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction.The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer.The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat.Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction.The transition to detonation depends non-monotonically on the channel width.     

Characteristic patterns of thermodiffusively unstable premixed lean hydrogen flames

Large-scale two-dimensional numerical simulations of thermodiffusively unstable, lean, premixed hydrogen flames have been performed using detailed finite rate chemistry to analyze flame intrinsic scales. The simulations feature a long integration time and large domain sizes to rule out effects of confinement on the dynamics of the flame front. For sufficiently large domain sizes, the total consumption speed of the flame is found to become independent of the domain size. An assessment of the characteristic scales of the flame front corrugation reveals the existence of a smallest and a largest flame intrinsic length scale. The smallest length manifests itself by local cusps, which lead to the formation of characteristic cells along the flame front. Their size is remarkably close to the most unstable wavelength predicted by a linear stability analysis of the flame front evolution in the linear regime. Independently of the domain size, a specific largest flame intrinsic structure, here referred to as flame finger, emerges from the interaction of multiple small-scale cusps. Thermodiffusively unstable flames are found to periodically form and destroy these flame fingers, but the formation of a global cusp that is known to emerge for purely hydrodynamically unstable flames is suppressed. The finite size of the largest scale fingers is explained by an instability in their movement. As they proceed towards the unburnt mixture, they tend to tilt and move laterally, thereby eventually being incorporated again by the rest of the flame. This behavior arises from the interaction of the flame fingers and the diverging velocity field ahead of them. Finally, the effect of equivalence ratio and unburnt gas temperature is investigated showing that flame fingers are found to develop only in case of a thermodiffusively unstable flame.     

Flame sheet dynamics of bluff-body stabilized flames during longitudinal acoustic forcing

Bluff-body stabilized flames are susceptible to combustion instabilities due to interactions between acoustics, vortical disturbances, and the flame. In order to elucidate these flow-flame interactions during an instability, an experimental and computational investigation of the flame-sheet dynamics of a harmonically excited flame was performed. It is shown that the flame dynamics are controlled by three key processes: excitation of shear layer instabilities by the axially oscillating flow, anchoring of the flame at the bluff body, and the kinematic response of the flame to this forcing. The near-field flame features are controlled by flame anchoring and the far-field by kinematic restoration. In the near-field, the flame response grows with downstream distance due to flame anchoring, which prevents significant flame movement near the attachment point. Theory predicts that this results in linear flame response characteristics as a function of perturbation amplitude, and a monotonic growth in magnitude of the flame-sheet fluctuations near the stabilization point, consistent with the experimental data. Farther downstream, the flame response reaches a maximum and then decays due to the dissipation of the vortical disturbances and action of flame propagation normal to itself, which acts to smooth out the wrinkles generated by the harmonic flow forcing. This behavior is strongly non-linear, resulting in significant variation in far-field flame-sheet response with perturbation amplitude.     

Experimental investigation on the self-ignition of pressurized hydrogen released by the failure of a rupture disk through tubes

Hydrogen is expected to be used as a clean energy carrier. However, when high-pressure hydrogen is suddenly released into the air through tubes, self-ignition can occur by a diffusion ignition mechanism. In this paper, the phenomena of self-ignition and flame propagation during the sudden release of high-pressure hydrogen were investigated experimentally. Experimental results show that self-ignition can occur when bursting pressure is sufficiently high in spite of the shortness of the tube. For example, self-ignition was observed at a bursting pressure as high as 23.5 MPa with 50 mm long tube. When self-ignition successfully occurs, a hydrogen jet flame is produced by the ignition. The flame is then stabilized at the tube outlet. From photodiode signals and flame images, the propagation of a flame inside the tube is confirmed and the flame is detected near the rupture disk as the bursting pressure increases. When the tube length is not long enough to produce self-ignition, a hydrogen flame is observed in the only boundary layer at the end of tube and it quenches after the flame exits the tube. Consequently, the formation of a complete flame across the tube is important to initiate self-ignition, which sustains a diffusion flame after jetting out of the tube into the air. Also, in order to establish a complete flame across the tube, it is necessary to have sufficient length such that the mixing region is generated by multi-dimensional shock–shock interactions.     

燃烧法合成碳纳米管的实验方案设计

碳纳米管是一种新型的碳材料,其合成方法多种多样。燃烧法是一种新兴的合成方法,燃烧过程提供用于碳纳米管生长的高温环境,同时也提供足够的烃原料。目前,用于合成碳纳米管的原料包括气体燃料和液体燃料,火焰类型主要有层流扩散火焰、逆流扩散火焰和预混火焰等。影响炭纳米管火焰合成的因素主要有气体成分,温度,催化剂,燃氧比和采样条件。我们采用甲烷扩散火焰用于实验研究炭纳米管的合成条件。实验系统包括扩散火焰喷嘴,混和段,质量流量计,取样探针和基板,气源。内径5 mm的喷嘴与内径100 mm的钢筒同轴。实验测得在气量为0．20 SLM时火焰高度为 3．5 cm。涂覆有催化剂的基板水平朝下置于火焰中采样,并将采集的样品进行电镜分析。本文还对燃烧法合成碳纳米管的机理进行了分析。     

Magnetic field influence on coflow laminar diffusion flames

The behavior of a laminar methane air flame with a central methane jet and a surrounding air coflow is analyzed in a large range of fuel and air flow rates. Different regimes of flame stability are described from an anchored flame to a stable lifted flame which is destabilized before extinction. Influence of an upward increasing magnetic field generated by an electromagnet is then studied. Experimental measurements at different values of methane and air flow rates show that the flame lift-off height is decreased by the magnetic gradient. These effects are attributed to the magnetic force which develops on air via its action on the paramagnetic oxygen molecules. The magnetic force interacts with the air jet structures upstream of the flame and then influences the flame stability.     

Experimental analysis and theoretical lift-off criterion for H2/air flames stabilized on a dual swirl injector

Stabilization mechanisms of partially premixed H₂/air flames on a coaxial dual swirl injector are investigated at atmospheric conditions. Hydrogen is injected through a central duct, and the air by the outer annular channel. Both channels are swirled and two stabilization modes are observed depending on the geometrical configuration of the injector and on the operating conditions. In certain regimes, the H₂/air flame stabilizes on the injector lips as a diffusion flame. For other operating conditions, the flame is lifted from the injector and burns mainly in partially premixed regime leading to limited NOx emissions. PIV measurements in cold flow conditions and direct observations of the flame indicate that the flame stabilization mode is mainly controlled by the inner hydrogen swirl level, the injector recess and the hydrogen velocity. For a given air flowrate, a minimum hydrogen velocity to lift the flame is determined for each combination of inner swirl level and injector recess. Assuming the flame close to the injector lips behaves like an edge flame, a model for flame stabilization based on the triple flame speed and the location of the stoichiometric mixture fraction line is built. According to this model, the flame is anchored to the injector if the triple flame can propagate to the inner injector lips, i.e., if the velocity along the stoichiometric line is lower than the triple flame speed. The model is tested using hydrogen diluted with argon and air diluted with nitrogen. Two cases producing predicted opposite trends are verified. First, the stoichiometric line is moved in the direction of lower velocity zone keeping the triple flame speed constant in order to anchor a lifted flame. Next, the stoichiometric line is kept constant and the triple flame speed is reduced in order to lift an anchored flame. The mechanisms driving flame stabilization are discussed.     

Dynamic behavior of diffusion flame interacting with a large-scale vortex by laser imaging techniques

Instantaneous and simultaneous measurements of two-dimensional temperature and OH-LIF profiles by combining Rayleigh scattering with laser induced fluorescence (LIF) were demonstrated in a nitrogen-diluted hydrogen (H₂ 30% + N₂ 70%) laminar normal diffusion flame interacting with a large-scale vortex by oscillating central fuel flow or in an inverse diffusion flame by oscillating central airflow. The dynamic behavior of the diffusion flame extinction and reignition during the flame–vortex interaction processes was investigated. The results obtained are described as follows. (1) The width of the reaction zone decreases remarkably, and a decrease in flame temperature and OH-LIF is seen with increasing central airflow in an inverse diffusion flame. OH-LIF increases, and temperature does not change with increasing central fuel flow in a normal diffusion flame. The computations predict the experimental results well, and it is revealed that flame temperature characteristics result from the preferential diffusion of heat and species, which induces excess enthalpy or on enthalpy deficit, and an increase or decrease in H₂ mole fraction in the flame. (2) When a large velocity fluctuation is given to the central flow, the temperature and the OH-LIF at the reaction zone become thin at the convex and circumferential part of the vortex where a high temperature layer exists, and the temperature at the reaction zone is lowered in the inverse flame and the normal flame. (3) The width and temperature of the reaction zone interacting with the vortex recover quickly to that of the laminar steady flame after the vortex passing in the normal flame, but the recovery to that of the steady flame after the vortex passing is delayed in the inverse flame. (4) When a remarkably large velocity fluctuation is given to the central airflow in the inverse flame, thinning of temperature and reaction zone starts at the convex and circumferential part of the vortex, resulting in a and flame extinction completely occurs at the tail part of the vortex and makes the pair of edge flames. The outside edge flame reignites and connects with the upstream reaction zone. The inside edge flame finally extinguishes as the supply of fuel is interrupted by the outside edge flame.     

Methane flames in a jet impinging onto a wall

Traditionally, research has focused on positive stretch in the stagnation flow and negative stretch along the Bunsen flame. Only a very limited amount of research has been devoted to studying the behavior of a conical Bunsen flame established in a stagnation flow, which is significantly affected by the combined effects of the curvature stretch and the aerodynamic straining. This investigation is aimed at studying the characteristics of laminar conical premixed flames in an impinging jet flow experimentally and theoretically. First, we analyze the transport processes of a nonreactive impinging jet flow numerically. For lower burner-to-plate distance, the potential core becomes concave at the top. Hence, a conical Bunsen flame established in such a flow field may suffer positive flow stretch. The predicted flame shapes using a simple model incorporated with the numerical results agree well with the experimental observations. Flame shapes exhibit double-solution characteristics in a certain range of methane concentrations. Experimentally, by following different paths of adjusting methane concentration (decreasing from rich to lean or increasing from lean to rich), two different flame configurations (planar or conical flame) may exist at the same flow conditions, namely burner-to-plate distance, inlet velocity, and methane concentration. At the higher (or lower) critical methane concentration, the transition from a flat flame to a conical flame (or from a conical flame to a flat flame) occurs. The calculation of stretch and measurement of flame temperature for the low inlet velocity, 0.8 m/s, show that the stretch of a conical flame established in a stagnation flow is negative (dominated by the flame curvature). However, it is important to emphasize that at high velocity, e.g., U_in = 1.6 m/s, a negatively stretched flame tip can suffer positive flow stretch. This significant finding has been verified in the experiment since the conical flame tip is higher than the positively stretched flat flame.     

Flame stabilization regimes for premixed flames anchored behind cylindrical flame holders

The objective of this study is to construct a regime diagram for laminar flames stabilized behind flame holders with respect to the presence of a recirculation zone (RZ), trend of heat loss to the burner, and flow strain and flame curvature effects. This is achieved by varying the radius of the cylindrical flame holder and the mixture velocity between the flashback limit and the blow-off limit at a fixed equivalence ratio. It is found that for all flame holders, a RZ vortex is not present near the flashback limit. At flashback, flow strain is almost zero and the flame curvature is found to be the main contributor to flame stretch. With increasing mixture velocity, the heat loss to the flame holder decreases for smaller radii and a RZ does not appear till blow-off occurs. For flame holders with radii greater than twice the flame thickness, the heat loss to the flame holder first decreases with increasing mixture velocity without a RZ. A further increase in the mixture velocity does not result in blow-off but instead, a RZ appears behind the flame holder reversing the heat loss trend. In this scenario, flow strain is found to increase significantly and becomes the major contributor to flame stretch, although curvature effects are still present. With the RZ present, the blow-off limits are significantly extended and the stabilization mechanism is altered. The RZ vortex shields the flame base from intense pre-heating resulting from the increase in heat loss to the flame-holder while it provides support to the flame leading edge by recirculation of hot products. The results obtained from this study are used to construct a regime diagram, which offers a broader view of the whole flame stabilization process and its mechanisms.     

Numerical simulation of flame dynamics associated with negative velocity induced by deformed flame shape

In this study, the influence of the negative velocity field formed ahead of an abruptly deformed flame tip on the propagation behaviour of a laminar premixed flame is numerically investigated. A strong deformation in the flame front is induced by imposing a very narrow, in-line pre-heating zone in the unburned region. The simulation is performed under low Mach number approximation by using a multi-scale multi-physics Computational Fluid Dynamics (CFD) solver FrontFlow/Red with one-step finite rate chemistry in order to track the time-dependent flame dynamics. The computed results unveil that the flame front is deformed significantly within a short time due to the narrow in-line pre-heating effect. The flame deformation induces a strong negative velocity field ahead of the deformed flame tip, acting in the direction of propagation, which gives rise to a strong pair vortex. This strong pair vortex interacts with the flame tip and then slides down along the flame surface in the upstream direction during propagation. This flame-vortex interaction causes further deformation in the flame surface in the upstream direction, and consequently, the flame exhibits a wave-like surface, which enhances the flame propagation speed. The auto-generation of a strong pair vortex ahead of the flame front due to the localised thermal input could be applied as one of the methods to control the combustion externally. It is also expected that the results obtained in this study could have a significant impact on the detailed understanding of the local thermo-fluid dynamical interaction process of turbulent combustion in practical combustors.     

Effect of heat release distribution on combustion oscillation

This paper describes the effect of flame position and its spatial variation on prediction accuracy of combustion oscillation in a dry low emission (DLE) combustor. A one-dimensional linear model has been developed. The flame is usually treated as fixed and located at a single axial plane in conventional analysis. However, in practice, the flame position varies during operation. A new flame model considering this variation by a spatial distribution function has been developed. Variation of flame position is empirically measured by using UV images of OH radicals in the oscillating flame. A triangular distribution function is introduced into the flame model because it is similar to the experimentally obtained distribution function. A ‘top-hat’ distribution is also considered to test the influence of distribution shape on the result. Numerical results are compared with experimental data. The triangular flame model shows better prediction of the stability boundaries of combustion oscillation compared with the simple flame sheet model. The results of the top-hat flame model differ from those of the experiment. It is found that consideration of the spatial distribution yields good results for the DLE combustor.     

Prediction of the steady flame spread rate by the principle of minimal entropy production

The propagation of a two-dimensional diffusive flame over a combustible material is studied by solving steady-state conservation equations written in the coordinate system fixed on the flame front. The analysis of a two-dimensional problem featuring flame spread rate as an eigenvalue has shown that there is no unique solution relative to the flame spread rate unless some additional condition is involved. A novel approach to the prediction of flame spread rate is proposed using the principles of irreversible thermodynamics. The steady flame propagation is considered as a stationary non-equilibrium thermodynamic state, which can be characterized, according to the formulation of Prigogine, by minimal entropy production. A numerical algorithm for the prediction of flame spread rate has been developed and tested on the investigation of downward flame spread over thin sheets of paper. The adequate physical background of the proposed approach and satisfactory agreement with experimental data have been shown.     

Impact of turbulence on flame brush development of acoustically excited rod-stabilized flames

Coherent structures, such as those arising from hydrodynamic instabilities or excited by thermoacoustic oscillations, can significantly impact flame structure and, consequently, the nature of heat release. The focus of this work is to study how coherent oscillations of varying amplitudes can impact the growth of the flame brush in a bluff-body stabilized flame and how this impact is influenced by the free stream turbulence intensity of the flow approaching the bluff body. We do this by providing external acoustic excitation at the natural frequency of vortex shedding to simulate a highly-coupled thermoacoustic instability, and we vary the in-flow turbulence intensity using perforated plates upstream of the flame. We use high-speed stereoscopic particle image velocimetry to obtain the three-component velocity field and we use the Mie-scattering images to quantify the behavior of the flame edge. Our results show that in the low-turbulence conditions, presence of high-amplitude acoustic excitation can cause the flame brush to exhibit a step-function growth, indicating that the presence of strong vortical structures close to the flame can suppress flame brush growth. This impact is strongly dependent on the in-flow turbulence intensity and the flame brush development in conditions with higher levels of in-flow turbulence are minimally impacted by increasing amplitudes of acoustic excitation. These findings suggest that the sensitivity of the flow and flame to high-amplitude coherent oscillations is a strong function of the in-flow turbulence intensity.     

Edge flames stabilized in a non-premixed microcombustor

The dynamics of an edge flame confined in a non-premixed microcombustor model is studied numerically within the context of a diffusive-thermal model. Fuel and oxidizer, separated upstream by a thin plate, flow through a channel with a prescribed velocity. At the tip of the plate, the fuel and oxidizer mix and, when ignited, an edge flame is sustained at some distance from the plate. The objective in this work is to consider the effects of confinement, differential diffusion, and heat loss on the dynamics of an edge flame in a narrow channel. We consider a wide range of channel widths and allow for changing Lewis numbers, and both adiabatic conditions and heat losses along the channel walls. The results illustrate how the flame shape and standoff distance are affected by the channel width, by mixture composition through variations in Lewis numbers and by heat losses. Conditions for flame stabilization, flame oscillations and flame extinction or blowoff are predicted.     

A novel apparatus for flame spread study

One of the challenges in the experimental study of flame spread is that, even if the flame spreads at a steady rate, the propagating flame creates an unsteady phenomenon with respect to the laboratory frame of reference. As a result, only a few studies have been done where the detailed flame structure has been experimentally measured along with the spread rates. In this work, we demonstrate the feasibility of a new flame spread apparatus that moves the fuel in the opposite direction of the flame so as to keep the leading edge of the flame stationary with respect to the laboratory. A thermocouple, fixed to the laboratory frame of reference, in front of the leading edge of the flame, senses the presence of the flame and a PID controller keeps the set point temperature constant by moving the sample holder, driven by a stepper motor, in the opposite direction at the velocity of the spread. Unlike conventional studies, this apparatus, called the flame stabilizer, produces real time spread rate with a time resolution of 0.3 s. In this paper, instantaneous flame spread rate and the visible flame structure are compared between a downward spreading flame and the corresponding stabilized flame for spread over ashless filter paper. The results indicate that the difference between the two configurations are within experimental uncertainties and the stabilized flame can represent a spreading flame adequately, including variability of flame spread rate and the flame geometry, for further observations.     

Experimental research on flame revolution and precession of fire whirls

This paper presents the first experimental effort to explore the large scale 3-D flame instabilities of fire whirls, including the inclined flame revolution during the transition from a general pool fire to fire whirl, and the swirling flame precession in a quasi-steady fire whirl. The experimental medium-scale fire whirls were produced by a fixed-frame facility. Experimental observations indicate that flame revolution is an important flame instability during the formation of fire whirl, showing that the entire flame is inclined and revolves around the geometrical axis of symmetry with increasing angular velocity until the critical point, without the self-rotation of the flame. It is found that the inlet velocity fluctuates synchronously with the flame revolution. As soon as the fire whirl forms, the erect swirling flame starts to precess around the geometrical axis of symmetry. Analysis indicates that during flame precession the periodic fluctuations of inlet velocity disappear and a local annular external recirculation zone (ERZ) is produced outside the flame (vortex core), while the flow is upward inside. It is found that the inlet velocities are nearly constant within the continuous flame in order to maintain a stable generating eddy. A good linear correlation exists between the average inlet velocities and average ambient circulations for all fuel pan sizes. The precession frequency is relatively stable during one test. The frequencies of flame revolution and precession are both proportional to the average inlet velocity, and the corresponding Strouhal numbers are constants of 0.42 and 0.80, respectively. The flame revolves and precesses in the same direction as the self-rotation of the fire whirl flame in all tests. The flame revolution is related to the periodical fluctuations of inlet flow, while the flame precession is considered to be linked to the occurrence of ERZ in fire whirls.