Hysteresis and transition in swirling nonpremixed flames

Strongly swirling nonpremixed flames are known to exhibit a hysteresis when transiting from an attached long, sooty, yellow flame to a short lifted blue flame, and vice versa. The upward transition (by increasing the air and fuel flow rates) corresponds to a vortex breakdown, i.e. an abrupt change from an attached swirling flame (unidirectional or with a weak bluff-body recirculation), to a lifted flame with a strong toroidal vortex occupying the bulk of the flame. Despite dramatic differences in their structures, mixing intensities and combustion performance, both flame types can be realised at identical flow rates, equivalence ratio and swirl intensity. We report here on comprehensive investigations of the two flame regimes at the same conditions in a well-controlled experiment in which the swirl was generated by the rotating outer pipe of the annular burner air passage. Fluid velocity measured with PIV (particle image velocimetry), the qualitative detection of reaction zones from OH PLIF (planar laser-induced fluorescence) and the temperature measured by CARS (coherent anti-Stokes Raman spectroscopy) revealed major differences in vortical structures, turbulence, mixing and reaction intensities in the two flames. We discuss the transition mechanism and arguments for the improved mixing, compact size and a broader stability range of the blue flame in comparison to the long yellow flame.     

Mixing and Recirculation Characteristics of A double COncentric Burner with Bluff—Body

The concentric bluff-body jet burner is widely used in industrial combustion systems. This kind of burner often generates a considerably complex recirculation zone behind the bluff body. As a result, the fuel often remains in the recirculation zone, achieving stability of flame. This study investigates, by means of experiments, the variations of the aerodynamics as the fluid is injected into a combustion chamber through a double concentric burner with a bluff-body. The observation and measurement of the aerodynamics in our experiment are conducted under a cold flow. The controlled parameters in our experiment are: variations in the blockage ratio of the center bluff body, the cone angle of the bluff body, and the velocity ratio (U _s/U_p) of the secondary jet and primary jet; the injection of helium bubbles into the primary and secondary jets to observe the recirculation zone behind the bluff body; using Tufts for observing the characteristics of corner recirculation zone in a combustion chamber, measuring the average velocity of each point within the aerodynamics by the 5-hole pitot tube; measuring the distribution of static pressure of the combustion chamber walls with a static pressure tap.     

Experimental study on velocity characteristics of recirculation zone in humid air non-premixed flame

To examine the effect of the flow field within the recirculation zone on flame structure, the characteristic velocity fields of methane/humid air flame in non-premixed combustion behind a disc bluff-body burner were experimentally studied by particle image velocimeter (PIV).The results show that two stagnation points exist on the centerline in the recirculation zone flame. However, the distance of the two stagnation points in humid air combustion shortens, and the minimal dimensionless velocity increases compared with the conventional non-humid air combustion. In addition, the positional curves of the minimal velocities can be partitioned into three phases representing three different flame patterns. The analysis of axial minimal velocities on the centerline and their positions under different co-flow air velocity conditions reveals that fuel-to-air velocity ratio is the crucial parameter that governs humid air combustion flame characteristics.     

Experimental study on the interactions for bluff-body and swirl in stabilized flame process

This paper focuses on investigating the interaction effects for swirl and bluff-body in stabilized flame process. Particle image velocimetry was used to measure velocity fields in three burners. First, the comparison of flames in bluff-body stabilized burners with and without swirl is presented. The results of the experiments present the variations of bluff-body stabilized flame when swirl is added into burner: the maximum reverse flow velocity and the maximum mean average radial velocity decrease; the maximum radial rootmean squared fluctuating (rms) velocity increases; the values of the axial velocity peak on the side of nozzle axis are lower, and the distance between the peak and centerline is bigger; the location of the maximum radial rms velocity moves to the outlet of annular air-flow from central recirculation zone (CRZ). Then, the comparison of flames in swirl burners with and without bluff-body is provided. The results of the experiments show the changes of swirling flame when bluff-body is added into swirl burner: the air vortex in the CRZ moves to the burner; the peak values of axial mean and rms velocity decrease; the distance between centerline and the mean axial and rms velocity peak increase; the peak of mean radial velocity decreases, and the peak of rms raidial velocity increase. The data from this experiment can also be established as benchmarks for the development and validation of combustion numerical simulations.     

Experimental study on turbulent structure of humid air flame in a bluff-body burner

The main objective of the present experimental study is to analyze the turbulent structure in humid air non-premixed flame, and determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame. Particle Image Velocimetry (PIV) is used to capture the instantaneous appearance of vortex structures and obtain the quantitative velocity field. The distributions of Reynolds shear stress, mean and root-mean squared fluctuating (rms) velocities are examined to get insight into the effect of fuel-to-air velocity ratio on velocity flow field. The results show that with steam addition, the air-driven vortex in the bluff-body wake is thinner; the biggest peaks of rms velocity and Reynolds shear stress are lower; the distance between the peaks of rms velocity on the sides of centerline reduces. Besides these, the flame stability is affected. Both central fuel penetration limit and partially quenching limit reduce with steam addition.     

Experimental study on velocity characteristics of recirculation zone in humid air non-premixed flame

To examine the effect of the flow field within the recirculation zone on flame structure, the characteristic velocity fields of methane/humid air flame in non-premixed combustion behind a disc bluff-body burner were experimentally studied by particle image velocimeter (PIV).The results show that two stagnation points exist on the centerline in the recirculation zone flame. However, the distance of the two stagnation points in humid air combustion shortens, and the minimal dimensionless velocity increases compared with the conventional nonhumid air combustion. In addition, the positional curves of the minimal velocities can be partitioned into three phases representing three different flame patterns. The analysis of axial minimal velocities on the centerline and their positions under different co-flow air velocity conditions reveals that fuel-to-air velocity ratio is the crucial parameter that governs humid air combustion flame characteristics. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 357–360, 365 [译自: 上海交通大学学报]     

Active control of jet premixed flames in a model combustor with manipulation of large-scale vortical structures and mixing

Active control of a bluff-body stabilized methane/air jet flame issued from a coaxial nozzle is made by using miniature magnetic flap actuators attached to the outer nozzle. CH radical chemiluminescence, and CO and NO_x emissions are measured to assess the flame characteristics. By changing the flapping Strouhal number, the flame stability and CO emission are drastically improved under different equivalence ratio (?) conditions. At ? = 0.72, the optimum Strouhal number for stable combustion is unity, since methane/air mixing is enhanced by large-scale vortices synchronized with the flap motion. On the other hand, at a lower equivalence ratio of ? = 0.48, the optimum Strouhal number is much larger than unity; with small-scale vortices, the premixed combustion is stabilized by stratified mixing. In addition to acetone and OH-PLIF, a new two-line OH-PLIF is employed for flame temperature measurement. The longitudinal flame temperature distribution is obtained from conditional-averaged OH fluorescence intensities at the OH front taken with two different excitation lines. CO and NO_x emission characteristics of the controlled flame are discussed on the basis of the local fuel and OH distributions and the flame temperature.     

The anchoring mechanism of a bluff-body stabilized laminar premixed flame

The objective of this work is to investigate the mechanism of the laminar premixed flame anchoring near a heat-conducting bluff-body. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. No artificial flame anchoring boundary conditions were imposed. Simulations show a shear-layer stabilized flame just downstream of the bluff-body, with a recirculation zone formed by the products of combustion. A steel bluff-body resulted in a slightly larger recirculation zone than a ceramic bluff-body; the size of which grew as the equivalence ratio was decreased. A significant departure from the conventional two-zone flame-structure is shown in the anchoring region. In this region, the reaction zone is associated with a large negative energy convection (directed from products to reactants) resulting in a negative flame-displacement speed. It is shown that the premixed flame anchors at an immediate downstream location near the bluff-body where favorable ignition conditions are established; a region associated with (1) a sufficiently high temperature impacted by the conjugate heat exchange between the heat-conducting bluff-body and the hot reacting flow and (2) a locally maximum stoichiometry characterized by the preferential diffusion effects.     

A visualization study on the effect of forcing amplitude on tone-excited isothermal jets and jet diffusion flames

An experimental study was conducted to investigate the effects of axial forcing on the flow structures near the nozzle exit in coaxial isothermal jets and jet diffusion flames. The jet was excited by adding a periodic velocity fluctuation ranging from 0 to 400% of the mean jet velocity at the tube resonating frequency. The phase-averaged axial velocity fluctuation at the jet centre was measured with a one-component LDV and phase-locked visualization using a light chopper and a phase-conditioning circuit was performed. The changes of large-scale structures in the near field of the jet are described from the visualization of horizontal and vertical cross-cut Mie scattering images. The flow structures of the forced isothermal jet are classified into three regions on the basis of the emergence of azimuthal structures and the periodic behaviour of vortex structures. The jittering of azimuthal structures was characterized by a forcing amplitude ratio and the velocity difference between the jet and the co-flowing fluid. In case of the forced reacting jet, flame heights were measured from video tape recordings of the sooting images of the flame. The dependence of flame height on the forcing amplitude ratio shows the existence of a flame-length elongation and reduction region. The flame elongation is found to be related to the suppression of the flame flickering by forcing. From the Mie scattering images and flame-length measurements, it is suggested that the intense mixing observed in the fully forced laminar jet and the reduction of the flame length is closely related to the development of azimuthal structures. © 1998 John Wiley & Sons, Ltd.     

Control of flames by tangential jet actuators in oxy-fuel burners

The active control of oxy-fuel flames from burners with separated jets is investigated. The control system consists of four small jet actuators, placed tangential to the exit of the main jets to generate a swirling flow. These actuators are able to modify the flow structure and to act on mixing between the reactants and consequently on the flame behavior. The burner (25 kW) is composed of separated jets, one jet of natural gas and one or two jets of pure oxygen. Experiments are conducted with three burner configurations, according to the number of jets, the jet exit velocities, and the separation distance between the jets. OH chemiluminescence measurements, particle image velocimetry, and measurements of NO_x emissions are used to characterize the flow and the flame structure. Results show that the small jet actuators have a significant influence on the behavior of jets and the flame characteristics, particularly in the stabilization zone. It is shown that the control leads to a decrease in lift-off heights and to better stability of the flame. The use of jet actuators induces high jet spreading and an increase in turbulence intensity, which improves the mixing between the reactants and the surrounding fluid. Pollutant measurements show important results in terms of NO_x reductions (up to 60%), in particular for low swirl intensity. The burner parameters, such as the number of jets and the spacing between the jets, also impact the flame behavior and NO_x formation.     

The behaviour of lifted oxy-fuel flames in burners with separated jets

Oxy-fuel combustion in separated-jet burners has been proven to increase thermal efficiency and to have a potential for NOx emission reduction. This paper presents an investigation into confined, turbulent, oxy-flames generated by a burner consisting of a central natural gas jet surrounded by two oxygen jets. The study is focused on the identifying the influence of burner parameters on the flame characteristics and topology, namely stability, lift-off height and flame length. The effects of the natural gas and oxygen jet exit velocities, the distance separating the jets and the deflection of oxygen jets towards the natural gas jet are examined. The OH chemiluminescence. Results show that the lift-off heights increase when jet exit velocities and the distance separating the jets are increased. The deflection of oxygen jets decreases the lift-off height and increases the volume of flame in the transversal plane. The flame length increases principally with the oxygen exit velocity and the separation distance, and decreases considerably when the angle of oxygen jets is increased.     

Non-premixed flame characteristics and exhaust gas concentrations behind rifled bluff-body cones

Four bluff-body cones with/without rifled v-grooves were installed behind a non-premixed traditional combustion nozzle to intensify the bluff-body effects and swirl flow. The spiral rifles transformed axial momentum (or axial velocity) into tangential momentum (or tangential velocity). The interaction between the fuel tangential component and axial air flow increased turbulence intensity (T.I.). The Schlieren photography was utilized to visualize the flame structures and classify three flame patterns—jet flame, recirculation flame, and turbulence flame. The jet flame occurs when fuel-jet velocity is high and air-jet velocity (u_a) is low. However, the turbulence flame exists at the high air-jet velocity. The flame lengths were measured using the direct photography scheme. The flame length at high u_a is significantly shorter than that at low u_a. Furthermore, the increase of rifle number (i.e., increasing T.I.) induces the high maximum temperature and low nitric-oxide concentration.     

钝体驻定湍流扩散火焰局部熄火的PDF模拟

用标量联合的概率密度函数(PDF)方法对钝体驻定的湍流射流扩散Sydney火焰HM3进行数值模拟,速度场用一个修正的LRR-IP雷诺应力模型求解.采用3种不同层次的甲烷反应动力学机理对火焰的宏观结构和熄火特征进行比较研究,并结合当地自适应建表方法加速化学反应计算.结果表明,计算值和实验数据吻合较好,在回流区内,不同反应机理的预测值差别不大,但在"颈部"区域,C2机理相对C1机理可以更准确地模拟标量场的变化和局部熄火现象.     

Effects of preferential transport in turbulent bluff-body-stabilized lean premixed CH4/air flames

Preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames, and differential diffusion of heat and mass can have significant effects on both local flame structure and global flame parameters, such as turbulent flame speed. However, models for turbulent premixed combustion normally assume that atomic mass fractions are conserved from reactants to fully burnt products. Experiments reported here indicate that this basic assumption may be incorrect for an important class of turbulent flames. Measurements of major species and temperature in the near field of turbulent, bluff-body stabilized, lean premixed methane–air flames (Le = 0.98) reveal significant departures from expected conditional mean compositional structure in the combustion products as well as within the flame. Net increases exceeding 10% in the equivalence ratio and the carbon-to-hydrogen atom ratio are observed across the turbulent flame brush. Corresponding measurements across an unstrained laminar flame at similar equivalence ratio are in close agreement with calculations performed using Chemkin with the GRI 3.0 mechanism and multi-component transport, confirming accuracy of experimental techniques. Results suggest that the large effects observed in the turbulent bluff-body burner are cause by preferential transport of H₂ and H₂O through the preheat zone ahead of CO₂ and CO, followed by convective transport downstream and away from the local flame brush. This preferential transport effect increases with increasing velocity of reactants past the bluff body and is apparently amplified by the presence of a strong recirculation zone where excess CO₂ is accumulated.     

A modified piloted burner for stabilizing turbulent flames of inhomogeneous mixtures

A modification of the well-known jet piloted burner is introduced to enable the stabilization of partially premixed flames with varying degrees of inhomogeneity in mixture fraction or equivalence ratio. A second tube is added within the pilot annulus which now surrounds two concentric pipes, one carrying fuel and the other air. The central pipe can also be recessed within the annulus upstream of the burner’s exit plane. Two flow configurations are tested: FJ which refers to fuel issuing from the central pipe while air issues from the annulus, and FA where the reverse is true. A key feature of the FJ configuration is that when the central tube is slightly recessed, the fuel partially premixes with air from the annulus inducing inhomogeneity, the extent of which depends on the recession distance.It is found that flame stability is significantly improved due to this inhomogeneity such that, for intermediate recession distances in the range 50–100 mm, and for the same air/fuel ratio, the blow-off limits for the FJ cases are more than 50% higher than those of the FA counterparts where fuel is injected in the annulus. Detailed stability limits for both the FJ and FA configurations are presented here along with measurements of velocity and mixing fields at the jet exit plane. Rayleigh scattering is used to image mixture fraction in non-reacting jets while measurements of velocity and turbulence fields are made using standard Laser Doppler Velocimetry. It is shown that, at intermediate recession distances, significant differences in the mean and rms fluctuations of the velocity and mixture fraction profiles exist between the FA and FJ cases. An indicator of stratification, extracted from the mixture fraction images at the exit plane of non-reacting jets, confirms that a high degree of inhomogeneity correlates well with improved flame stability.     

Effect of fuel jet arrangement on the mixing rate inside trapezoidal cavity flame holder at supersonic flow

Fuel mixing inside the supersonic combustion chamber is a significant process for development of modern scramjets. In this article, computational fluid dynamic (CFD) approach is applied to investigate the effect of various fuel injections on the mixing rate inside the supersonic combustion chamber. The mixing of hydrogen jets with four different arrangements inside the cavity flame holder is comprehensively studied. In order to examine the effect of multi jets within a cavity flameholder, a three-dimensional model is established and Navier-stocks equations are solved to simulate the flow and mixing zone inside a cavity region. Obtained results show that the injection of hydrogen jet from the bottom of cavity flame holder considerable enhances the ignition zone within the cavity. Moreover, the backward fuel injection is more superior to forward fuel injection since low-pressure vortex could significantly distribute the fuel and enlarge the mixing zone inside the cavity flame holder.     

Analysis of Cold Flowfield of Multi—Annular Opposed Jets

The technique of the use of multi-annular opposed jets as different from using swirl and bluff body createsan excellent recirculation zone with desired size in a large space.The size of recirculation,the magnitude ofreverse velocity and turbulence intensity are much greater than those formed by bluff body.Factors affectingthe flowfield include the velocity ratio of the opposed jets to the primary air J,the diameter and constructionof the opposed jet ring,secondary air velocity and configuration,and confined or unconfined flow condition andso on.This method is a promising way for flame stabilization in combustion technology.     

基于二阶矩封闭湍流模型的非预混湍流火焰的数值模拟

应用二阶矩封闭湍流模型进行了湍流非预混钝体稳定火焰数值模拟的研究.应用LRR-IP模型,JM模型,SSG模型以及两个修正后的LRR-IP模型等二阶矩封闭湍流模型,进行了钝体稳定火焰数值模拟的研究.对于复杂的钝体稳定火焰,一些模型无法给出令人满意的结果,而且不同模型的结果差异很大.在研究中,湍流燃烧模型采用了化学平衡模型和假设PDF模型.研究结果表明,对于钝体稳定火焰,SSG模型以及两个修正后的LRR-IP模型要优于其他几个二阶矩封闭湍流模型.     

Counter-gradient transport in the combustion of a premixed CH4/air annular jet by combined PIV/OH-LIF

A combination of PIV/OH laser induced fluorescence technique is used to measure the conditional - burned and unburned - gas velocity in a turbulent premixed CH₄/air annular bluff-body stabilized burner. By changing the equivalence ratio from lean to almost stoichiometric, the energy budget of the recirculating region anchoring the flame is altered in such a way to increasingly lift the flame away from the jet exit. The overall turbulence intensity interacting with each flame is thus systematically varied in a significant range, allowing for a parametric study of its effect on turbulent scalar transport under well controlled conditions, always well within the flamelet regime. The component of the flux normal to the average front is found to reverse its direction, confirming the Bray number as a good indicator of gradient/counter-gradient behavior, once the actual incoming turbulence level felt locally by the flame is assumed as the proper control parameter.     

Effects of nozzle length on flame and emission behaviors of multi-fuel-jet inverse diffusion flame burner

An experimental study was performed to investigate the effects of the nozzle length on the air-pollutant-emission and noise-radiation behaviors of a burner utilizing a multi-fuel-jet inverse diffusion flame (MIDF). Comparison of the experimental results obtained from two MIDF burners, one with a long nozzle and the other with a short nozzle, operating under the same air/fuel supply conditions (Re_air and Ф) shows rather significant differences in the flame appearance, flame centerline temperature, CO/CO₂ concentrations and the noise radiation. The nozzle length influences development of the jets and hence interaction between the air/fuel jets including their mixing process. The short nozzle produces a flame with a shorter base height and a smaller potential core due to the enhanced air/fuel mixing. It also leads to faster and more complete combustion at the inner reaction cone of the flame due to the stronger and faster air/fuel mixing. The nozzle length affects the CO and CO₂ concentrations, and higher peak values are obtained with the short-nozzle flame. Flame noise of the MIDF is defined as the noise radiation at different flame heights, which is of varying strength but of the same dominant frequency in the range of 250–700 Hz. The noise radiation from the inner reaction cone of the flame is stronger than that from the lower and upper parts of the flame, and the maximum noise radiation occurs when the total amounts of air and fuel in the combustion zone are at the stoichiometric air/fuel ratio. For all the experiments conducted in the present study, the MIDF produced by the long nozzle is always noisier than its counterpart and it is due to the increase of the low-frequency noise components.