Numerical simulation of dynamics of premixed flames: flame instability and vortex–flame interaction

The characteristics of cellular flames generated by intrinsic instability has been studied using two-dimensional (2-D) and three-dimensional (3-D) unsteady calculations of reactive flows, based on the compressible Navier–Stokes equation. Three basic types of phenomena, responsible for the intrinsic instability of premixed flames, are examined here, i.e. hydrodynamic, body-force and diffusive-thermal effects. Cellular flames are generated by these effects, and their characteristics—cell size, cell depth, flame-surface area, and flame velocity—depend on the adiabatic flame temperature, acceleration, and Lewis number. As intrinsic instability becomes stronger, the flame–surface area and flame velocity of cellular flames increase, and the behavior of cellular-flame fronts becomes unstable. The increment in the flame–surface area and the flame velocity of 3-D cellular flames is about twice that of 2-D cellular flames. This is due to the difference in the disposition of cells between 2-D and 3-D flames. Moreover, the flame velocity of cellular flames depends strongly on the length of computational domain in the direction tangential to the flame surface. As the length of computational domain becomes larger, the flame velocity increases. This is because the long-wavelength components of disturbances play an important role in the shape of cellular flames, i.e. the flame–surface area. Next, colliding interaction of a vortex pair with a premixed flame has been numerically studied in order to understand how the vortex affects the flame and how the flame affects the vortex. Three types of interacting behavior appear, depending on the ratio of the maximum circumferential velocity of the vortex to the burning velocity of the flame. The temporal evolution of the curvature, the strain rate, the stretch rate at the stagnation point, and that of the flame surface area and the global burning velocity are also analyzed, with different Lewis numbers and vortex strengths. Flame propagation along a vortex core, i.e. vortex bursting, is also numerically studied in order to understand how the vortex affects the flame propagation and how the flame affects the vortex. It is shown that flame evolution along a fine vortex tube is related to the formation of the Azimuthal component of vorticity, which is produced by convection and stretch effects, and that the density ratio of the flame and the Reynolds number of the vortex affect the propagation velocity. Flame propagation in a rotating cylinder has been also reviewed.     

Direct numerical simulations of NOx effect on multistage autoignition of DME/air mixture in the negative temperature coefficient regime for stratified HCCI engine conditions

Direct numerical simulations (DNSs), for a stratified flow in HCCI engine-like conditions, are performed to investigate the effects of exhaust gas recirculation (EGR) by NO_x and temperature/mixture stratification on autoignition of dimethyl ether (DME) in the negative temperature coefficient (NTC) region. Detailed chemistry for a DME/air mixture with NO_x addition is employed and solved by a hybrid multi-time scale (HMTS) algorithm. Three ignition stages are observed. The results show that adding (1000 ppm) NO enhances both low and intermediate temperature ignition delay times by the rapid OH radical pool formation (one to two orders of magnitude higher OH radicals concentrations are observed). In addition, NO from EGR was found to change the heat release rates differently at each ignition stage, where it mainly increases the low temperature ignition heat release rate with minimal effect on the ignition heat release rates at the second and third ignition stages. Sensitivity analysis is performed and the important reactions pathways for low temperature chemistry and ignition enhancement by NO addition are specified. The DNSs for stratified turbulent ignition show that the scales introduced by the mixture and thermal stratifications have a stronger effect on the second and third stage ignitions. Compared to homogenous ignition, stratified ignition shows a similar first autoignition delay time, but about 19% reduction in the second and third ignition delay times. Stratification, however, results in a lower averaged LTC ignition heat release rate and a higher averaged hot ignition heat release rate compared to homogenous ignition. The results also show that molecular transport plays an important role in stratified low temperature ignition, and that the scalar mixing time scale is strongly affected by local ignition. Two ignition-kernel propagation modes are observed: a wave-like, low-speed, deflagrative mode (the D-mode) and a spontaneous, high-speed, kinetically driven ignition mode (the S-mode). Three criteria are introduced to distinguish the two modes by different characteristic time scales and Damkhöler (Da) number using a progress variable conditioned by a proper ignition kernel indicator (IKI). The results show that the spontaneous ignition S-mode is characterized by low scalar dissipation rate, high displacement speed flame front, and high mixing Damkhöler number, while the D-mode is characterized by high scalar dissipation rate, low displacement speeds in the order of the laminar flame speed and a lower than unity Da number. The proposed criteria are applied at the different ignition stages.     

不同预燃室结构对甲烷湍流射流点火燃烧的影响

基于光学定容燃烧弹试验平台,通过高速纹影摄像系统在相同甲烷燃料初始温度、压力及混合气浓度下,定量分析了不同结构预燃室湍流射流点火(turbulent jet ignition,TJI)的燃烧特性,包括火焰传播速度、火焰面积、火焰形态及燃烧压力等参数。研究结果表明,预燃室孔径越小,相同时间内火焰传播得越远,火焰传播速度和火焰面积增长速度越快,燃烧压力峰值越高。随着预燃室孔径减小,着火机理会由射流中带有火焰的火焰点火转变为火焰过孔时熄灭的喷射点火。喷射点火着火时刻延迟,初始火焰速度减慢,但燃烧压力峰值受影响不大。多级加速预燃室压力升高率与压力峰值与单孔预燃室相比变化不大。虽然火焰出口时速度较慢,但是火焰出口时刻提前且速度衰减较弱,因此多级加速预燃室火焰速度在短时间内超过单孔预燃室,并且压力和火焰面积也更早达到最大值。     

Flame-vortex interaction and mixing behaviors of turbulent non-premixed jet flames under acoustic forcing

This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO_x emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO_x is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO_x emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface.     

Autoignition and structure of nonpremixed CH4/H2 flames: Detailed and reduced kinetic models

The ignition dynamics and subsequent flame evolution of hydrogen-enriched methane mixtures are investigated numerically in a reacting vortex ring configuration. The CH₄/H₂ combustion is studied using a detailed reaction mechanism (GRI-Mech v3.0) and two augmented reduced mechanisms (11-step and 12-step). The main objective of this study is to identify the extent that the current reduced mechanisms can go in replicating the dynamics of the ignition process and flame structure in an unsteady nonpremixed configuration. The parameters of the numerical simulations are adjusted such that flame ignition occurs during either the formation or the postformation of the ring. The quasi-steady state assumption for O in the 12-step reduced kinetic model leads to shorter ignition delay times than those in the other kinetic models. For formation-phase ignition runs, the flame structure near the stoichiometric region is captured well by the 12-step model compared to GRI-Mech 3.0. For postformation ignition runs, the 12-step model predicts larger heat release rates and main species mole fractions compared to GRI-Mech 3.0. The 11-step model predicts well the ignition delay time. At later times the fuel-rich side of the flame predicted by this reduced mechanism exhibits differences from the detailed model. Counterflow diffusion flame results are used to further compare the fuel-rich chemistry for the detailed and augmented reduced kinetic models.     

Enclosure effects on flame spread over solid fuels in microgravity

Enclosure effects on the transition from a localized ignition to subsequent flame growth over a thermally thin solid fuel in microgravity are numerically investigated by solving the low Mach number time-dependent Navier-Stokes equations. The numerical model solves the two and three dimensional, time-dependent, convective/diffusive mass, and heat transport equations with a one-step global oxidation reaction in the gas phase coupled to a three-step global pyrolysis/oxidative reaction system in the solid phase. Cellulosic paper is used as the solid fuel and is placed in a slow imposed flow parallel to the surface. Ignition is initiated across the width of the sample or at a small circular area by an external thermal radiation source. Two cases are examined; an open configuration (i.e., without any enclosure) and the case with the test chamber used in our previous microgravity experiments. Numerical results show that the upstream centerline flame spread rate for the case with the enclosure is faster than that for the case without any enclosure. This is due to the confinement of the flow field and also thermal expansion initiated by heat and mass addition in the chamber. The confinement accelerates the flow in the chamber, which enhances oxygen transport into the flame. In the three-dimensional configuration with the spot ignition, the flame growth in the direction perpendicular to the flow is significantly enhanced by the confinement effects. The effect of the enclosure is most significant at the slowest flow condition investigated and the effect becomes less important with an increase in imposed flow velocity. The total heat release rate from the flame during a flame growth period increases significantly with the confinement and the enclosure effects should be accounted to avoid underestimating fire hazard in a spacecraft.     

Experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube

An experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube are measured by pressure transducers and light sensors. Results show that the pressure behind a shock wave first increases, and subsequently remains near constant value with an increase of the propagation distance. That is, a certain propagation distance is required to form a stable shock wave in the tube. In the front of the tube, the minimum value of pressure behind the shock wave (P_shock) required for spontaneous ignition decreases with the increase in axial distance to the diaphragm. However, the minimum P_shock remains nearly a constant value in the rear part of the tube. Moreover, the critical values of shock Mach number (M_S) for spontaneous ignition decrease with the increase in tube length. And the ignition delay time decreases with the increase of the M_S. As the ignition kernel grows in size to a flame, it propagates downstream along the tube with velocity greater than the theoretical flow velocity of the hydrogen-air contact surface. The flame propagation velocity relative to tube wall increases with M_S. When the self-sustained flame exits from the tube, a rapid non-premixed turbulent combustion is observed in the chamber. The combustion-wave overpressure increases with the increase of the M_S.     

Spherical flame initiation and propagation with thermally sensitive intermediate kinetics

Spherical flame initiation and propagation with thermally sensitive intermediate kinetics are studied analytically within the framework of large activation energy and quasi-steady assumptions. A correlation describing different flame regimes and transitions among the ignition kernels, flame balls, propagating spherical flames, and planar flames is derived. Based on this correlation, spherical flame propagation and initiation are then investigated. The flame propagation speed, Markstein length, and critical ignition power and radius are found to strongly depend on the Lewis numbers of fuel and radical and the heat of reaction. For spherical flame propagation, the trajectory is shown to change significantly with the fuel Lewis number and a C-shaped solution curve of flame propagation speed as a function of flame radius is observed for large fuel Lewis numbers. The Markstein length is shown to increase/decrease monotonically with the fuel/radical Lewis number. The influence of stretch on flame propagation (i.e. the absolute value of Markstein length) is found to decrease with the heat of reaction. For spherical flame initiation, the critical ignition power and radius are shown to increase with the fuel Lewis number and to decrease with the radical Lewis number and heat of reaction. Three different flame initiation regimes are observed and discussed. Furthermore, the validity of theoretical prediction is confirmed by transient numerical simulations including thermal expansion and detailed chemistry.     

Skeletal reaction models based on principal component analysis: Application to ethylene–air ignition, propagation, and extinction phenomena

The development of a skeletal reaction model based on Principal Component Analysis of local Sensitivity (PCAS) coefficients is reported. The analysis presented is comprehensive in the sense that it includes sensitivity coefficients from three distinct canonical reacting configurations, namely ignition, flame propagation, and flame extinction phenomena. To minimize the computational effort involved in constructing sensitivity coefficients, and with the objective of accurately predicting global features or target functions such as ignition delay, burning velocity and extinction strain rates, optimal temporal and spatial locations to perform the local sensitivity are identified. Furthermore, it is shown that the sensitivity coefficients of temperature and heat release, and/or global flame properties (or eigenvalues) associated with burning velocity and extinction strain rate, are sufficient to extract an accurate skeletal model to predict stated target functions. Application of the PCAS approach to a C₁–C₄ hydrocarbon kinetic model consisting of 111 species and 784 reversible reactions, with ethylene as the fuel of interest, is presented. The results clearly indicate that the smallest skeletal model that can be developed is dictated by non-premixed extinction phenomenon that has been neglected in previous analyses using various reduction approaches.     

Study on flame-vortex interaction in a spark ignition engine fueled with methane/carbon dioxide gases

A numerical study on the in-cylinder flame-vortex interaction of gaseous spark ignited engine fueled with methane/carbon dioxide is carried out by means of large-eddy method. Evolution of in-cylinder turbulence in charge phase and flame-vortex interaction during combustion process is analyzed in great detail. It's found out that the large scale coherent structures are transformed into homogeneous small scale vortexes during the intake and compression stroke. The strong vortex cores are generated by interaction between flame and in-cylinder background turbulence. Those generated vortex cores wrinkle flame surface and augment turbulent flame speed. The contra-rotation between the two vortexes of vortex-pair in the unburned area results in the appearance of large scale flame wrinkles, which is because the vortex-pair movement leads to the local entrainment and hence stretchs of the flame surface. With the increase of volume fraction of carbon dioxide in the gases, the turbulent flame speed is decreased, the effect of vortex pair on the flame structure is weakened, and the level of the flame wrinkling is decreased correspondingly.     

Cinematographic OH-PLIF measurements of two interacting turbulent premixed flames with and without acoustic forcing

This paper describes an experimental investigation into the interactions that occur between two lean turbulent premixed flames stabilised on conical bluff-bodies when they are moved closer together. Cinematographic OH-PLIF measurements were acquired to investigate adjacent flame front interactions as a function of flame separation distance (S). Flame surface density (FSD) and curvature were determined to characterise the unforced flames. Acoustic forcing was then applied to explore the amplitude dependent thermo-acoustic response. Phase-averaged FSD and global heat release measurements in the form of OH¹ chemiluminescence were obtained for a range of forcing frequencies (f) and amplitudes (A) as a function of S. As the flames were brought closer together the adjacent annular jets were found to merge into a single jet structure. This caused adjacent flame fronts to merge above the wake region between the two flames at a location determined by the jet efflux (flame angle) and S. This region of flame–flame interaction we refer to as ‘interacting region’. In the unforced flames, a trend of increasingly negative curvature for decreasing S produced a small net increase in flame surface area via cusp formation. When subjected to acoustic forcing, S-dependent regimes were found in the global heat release response as a function A. The overall trend showed that the occurrence of jet/flame merging reduces the value of A at which non-linear response occurs. In support of previous findings for flames stabilised along shear layers, the phase-averaged FSD showed that the flame dynamics that drive the thermo-acoustic response result from the roll-up of vortices which generate large-scale vortex–flame interactions. Compared with axisymmetric flames, the occurrence of jet merging alters the vortex–flame interactions resulting in an asymmetric contribution to the heat release between the wall and interacting regions. The majority of the heat release was found to occur in the interacting region through the rapid production and destruction of flame surface area. The occurrence of jet merging and large-scale interactions between adjacent flames result in different physical mechanisms that drive the thermo-acoustic response compared with single axisymmetric flames.     

Effect of NO on extinction and re-ignition of vortex-perturbed hydrogen flames

The catalytic effect of nitric oxide (NO) on the dynamics of extinction and re-ignition of a vortex-perturbed non-premixed hydrogen–air flame is studied in a counterflow burner. A diffusion flame is established with counterflowing streams of nitrogen-diluted hydrogen at ambient temperature and air heated to a range of temperatures that brackets the auto-ignition temperature. Localized extinction is induced by impulsively driving a fuel-side toroidal vortex into the steady flame, and the recovery of the extinguished region is monitored by planar laser-induced fluorescence (PLIF) of the hydroxyl radical (OH). The dynamics of flame recovery depend on the air temperature and fuel concentration, and four different recovery modes are identified. These modes involve combinations of edge-flame propagation and the expansion of an auto-ignition kernel that forms within the extinguished region. The addition of a small amount of NO significantly alters the re-ignition process by shifting the balance between chain-termination and chain-propagation reactions to enhance auto-ignition. The ignition enhancement by this catalytic effect causes a shift in the conditions that govern the recovery modes. In addition, the effects of NO concentration and vortex strength on the flame recovery are examined. Direct numerical simulations of the flame–vortex interaction with and without NO doping show how the small amount of OH produced by NO-catalyzed reactions has a significant impact on the development of an auto-ignition kernel. This joint experimental and numerical study provides detailed insight into the interaction between transient flows and ignition processes.     

On cycle-to-cycle heat release variations in a simulated spark ignition heat engine

The cycle-by-cycle variations in heat release are analyzed by means of a quasi-dimensional computer simulation and a turbulent combustion model. The influence of some basic combustion parameters with a clear physical meaning is investigated: the characteristic length of the unburned eddies entrained within the flame front, a characteristic turbulent speed, and the location of the ignition kernel. The evolution of the simulated time series with the fuel–air equivalence ratio, ?, from lean mixtures to over stoichiometric conditions, is examined and compared with previous experiments. Fluctuations on the characteristic length of unburned eddies are found to be essential to simulate the cycle-to-cycle heat release variations and recover experimental results. A non-linear analysis of the system is performed. It is remarkable that at equivalence ratios around ? ? 0.65, embedding and surrogate procedures show that the dimensionality of the system is small.     

Nonlinear interaction between a precessing vortex core and acoustic oscillations in a turbulent swirling flame

The interaction of a helical mode with acoustic oscillations is studied experimentally in a turbulent swirl-stabilized premixed flame. In addition to a precessing vortex core (PVC), the helical mode features perturbations in the outer shear layer of the burner flow. Measurements of the acoustic pressure, unsteady velocity field and flame emission are made in different regimes including self-sustained combustion oscillations and stable regimes with and without acoustic forcing. The acoustic oscillation and the helical mode create a pronounced rotating heat release rate perturbation at a frequency corresponding to the difference of the frequencies of the two individual mechanisms. Measurements over a wide range of operating conditions for different flow rates and equivalence ratios show that while the helical mode is always present, with a constant Strouhal number, self-excited thermoacoustic oscillations exist only in a narrow region. The interaction can be observed also in cases of thermoacoustically stable conditions when external acoustic modulation is applied to the system. The evolution of the helical mode with the forcing amplitude is examined. High-speed imaging from the downstream side of the combustor demonstrates that the heat release rate perturbation associated with the nonlinear interaction of the helical mode and the acoustic oscillations produces a ”yin and yang” -type pattern rotating with the interaction frequency in the direction of the mean swirl. At unstable conditions, the oscillation amplitude associated with the interaction is found to be significantly stronger in the heat release rate than in the velocity signal, indicating that the nonlinear interaction primarily occurs in the flame response and not in the aerodynamic field. The latter is, however, generally possible as is demonstrated under non-reacting conditions with acoustic forcing. Based on a second-order analysis of the G-equation, it is shown that the nonlinear flame dynamics necessarily generate the observed interaction component if the flame is simultaneously perturbed by a helical mode and acoustic oscillations.     

Experimental study of vortex-flame interaction in a gas turbine model combustor

The interaction of a helical precessing vortex core (PVC) with turbulent swirl flames in a gas turbine model combustor is studied experimentally. The combustor is operated with air and methane at atmospheric pressure and thermal powers from 10 to 35 kW. The flow field is measured using particle image velocimetry (PIV), and the dominant unsteady vortex structures are determined using proper orthogonal decomposition. For all operating conditions, a PVC is detected in the shear layer of the inner recirculation zone (IRZ). In addition, a co-rotating helical vortex in the outer shear layer (OSL) and a central vortex originating in the exhaust tube are found. OH chemiluminescence (CL) images show that the flames are mainly stabilized in the inner shear layer (ISL), where also the PVC is located. Phase-averaged images of OH-CL show that for all conditions, a major part of heat release takes place in a helical zone that is coupled to the PVC. The mechanisms of the interaction between PVC and flame are then studied for the case P = 10 kW using simultaneous PIV and OH-PLIF measurements with a repetition rate of 5 kHz. The measurements show that the PVC causes a regular sequence of flame roll-up, mixing of burned and unburned gas, and subsequent ignition of the mixture in the ISL. These effects are directly linked to the periodic vortex motions. A phase-averaged analysis of the flow field further shows that the PVC induces an unsteady lower stagnation point that is not present in the average flow field. The motion of the stagnation point is linked to the periodic precession of the PVC. Near this point burned and unburned gas collide frontally and a significant amount of heat release takes place. The flame dynamics near this point is also coupled to the PVC. In this way, a part of the reaction zone is periodically drawn from the stagnation point into the ISL, and thus serves as an ignition source for the reactions in this layer. In total, the effects in the ISL and at the stagnation point showed that the PVC plays an essential role in the stabilization mechanism of the turbulent swirl flames. In contrast to the PVC, the vortices in the OSL and near the exhaust tube have no direct effect on the flame since they are located outside the flame zone.     

二甲醚湍流射流推举火焰的燃烧机理研究

对长、宽、高为650 mm×400 mm×12 mm的半闭口狭窄矩形通道（海伦-肖装置）内的甲烷/空气层流预混火焰传播过程进行了实验研究,探究当量比φ在0.6~1.2范围内、火焰传播角度ω在垂直向下-90°至垂直向上90°区间对火焰前锋轮廓发展及非标准层流火焰速度的影响。结果表明：火焰在通道内的传播分为热膨胀、准稳态传播和端壁效应3个阶段,每个阶段具有各自不同的前锋轮廓特征。由于瑞利-泰勒不稳定性机制的作用,所有当量比工况下向上传播的火焰均在准稳态传播阶段中呈现出明显的锋面褶皱与胞状结构;对向下传播的火焰而言,其在贫燃工况（φ为0.6,0.8）下的胞状不稳定性得到了有效抑制,而在当量比φ=1.0及富燃工况（φ=1.2）下,该稳定性效应并不显著。火焰瞬时速度与标准层流速度的比值Ui/UL,在φ=0.6的极贫燃工况与其他当量比工况下展现出明显不同的发展特性,极贫燃工况火焰向上传播时（ω=90°）,Ui/UL随着传播过程的进行一直增大,直到火焰触碰壁面末端熄灭,整个过程Ui/UL与火焰传播方向呈正相关关系;而对于其他当量比工况,Ui/UL在传播过程中均先升高后下降,火焰触碰壁面末端熄灭前其值趋于稳定,其平均速度与标准层流速度的比值Ua/UL在水平传播（ω=0°）时达到最大值。     

Visualization of the unburned gas flow field ahead of an accelerating flame in an obstructed square channel

The effect of blockage ratio on the early phase of the flame acceleration process was investigated in an obstructed square cross-section channel. Flame acceleration was promoted by an array of top-and bottom-surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. It was determined that flame acceleration is more pronounced for higher blockage obstacles during the initial stage of flame acceleration up to a flame velocity below the speed of sound of the reactants. The progression of the flame shape and flame area was determined by constructing a series of three-dimensional flame surface models using synchronized orthogonal schlieren images. A novel schlieren based photographic technique was used to visualize the unburned gas flow field ahead of the flame front. A small amount of helium gas is injected into the channel before ignition, and the evolution of the helium diluted unburned gas pocket is tracked simultaneously with the flame front. Using this technique the formation of a vortex downstream of each obstacle was observed. The size of the vortex increases with time until it reaches the channel wall and completely spans the distance between adjacent obstacles. A shear layer develops separating the core flow from the recirculation zone between the obstacles. The evolution of oscillations in centerline flame velocity is discussed in the context of the development of these flow structures in the unburned gas.     

Spatially distributed flame transfer functions for predicting combustion dynamics in lean premixed gas turbine combustors

The present paper describes a methodology to improve the accuracy of prediction of the eigenfrequencies and growth rates of self-induced instabilities and demonstrates its application to a laboratory-scale, swirl-stabilized, lean-premixed, gas turbine combustor. The influence of the spatial heat release distribution is accounted for using local flame transfer function (FTF) measurements. The two-microphone technique and CH^∗ chemiluminescence intensity measurements are used to determine the input (inlet velocity perturbation) and the output functions (heat release oscillation), respectively, for the local flame transfer functions. The experimentally determined local flame transfer functions are superposed using the flame transfer function superposition principle, and the result is incorporated into an analytic thermoacoustic model, in order to predict the linear stability characteristics of a given system. Results show that when the flame length is not acoustically compact the model prediction calculated using the local flame transfer functions is better than the prediction made using the global flame transfer function. In the case of a flame in the compact flame regime, accurate predictions of eigenfrequencies and growth rates can be obtained using the global flame transfer function. It was also found that the general response characteristics of the local FTF (gain and phase) are qualitatively the same as those of the global FTF.     

Ignition of polyoxymethylene

An experimental opposed flow diffusion flame system has been developed with which polymer ignition may be carefully investigated. The effects of ignition source intensity, oxidizer flow rate, and oxidizer composition on the ignition of polyoxymethylene have been studied. Comparisons with available theoretical models have been made where possible. It is concluded that the polymer surface temperature at ignition is independent of ignition source intensity, oxidizer composition, and oxidizer flow rate; that the ignition delay time is independent of oxidizer velocity over the range of velocities for which a flat flame can be established in the opposed flow diffusion flame system; that gas phase absorption of radiation may affect the ignition process even for relatively small optical depths and low source intensities; that the ignition delay time decreases linearly with increasing oxygen concentration in $\text{oxygen}\text{nitrogen}$ mixtures for low flame stretch rates; that ignition occurs during the transition stage when the free stream oxygen mass fraction is less than the oxygen mass fraction in the solid, and for higher free stream oxygen mass fractions it appears that oxygen diffusion to the surface is enhancing the gasification rate during the transition stage and/or decreasing the period of the transition stage; and that the various processes occurring in the gas phase during the ignition event appear to result in a net liberation of energy.