共查询到19条相似文献,搜索用时 171 毫秒
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利用微重力条件下向外传播的球形火焰,对贫燃极限附近甲烷/空气预混火焰的层流燃烧速度进行了测量,得到当量比从0.512(本文微重力实验中测定的可燃极限)到0.601范围内的零拉伸层流燃烧速度,并与前人实验数据和使用3种化学反应动力学模型的计算结果进行了比较.本文实验结果与已有的微重力实验数据非常接近,而其他研究者在常重力... 相似文献
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航空煤油火焰传播特性对航空动力装置的研发与设计均具有重要意义。本文在液体燃料对冲火焰实验台上,使用相位多普勒粒子分析仪(PDPA)在较宽的当量比范围内,测量了三种煤油表征燃料与空气掺混气的层流火焰传播速度。在标准大气压下,初温378 K时正癸烷、甲基环己烷和初温388 K时甲苯与空气预混气燃烧时能够达到的最大火焰传播速度为64.2 cm/s、58.3 cm/s和52.4 cm/s。在实验数据的基础上,进一步采用Chemkin软件对预混火焰进行了动力学分析,探讨了造成三种燃料火焰传播性质差异的动力学原因。 相似文献
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地面常重力(1g)条件下,丙烷/空气预混火焰向上传播的富燃极限为9.2%C_3H_8,而向下传播时的富燃极限仅为6.3%C_3H_8,二者之间存在明显差距。利用微重力条件下的实验,对燃料浓度从6.5%到8.6%(微重力实验中测定的可燃极限)范围内的丙烷/空气预混火焰特性进行了研究。实验发现,重力对近极限丙烷/空气火焰的传播有显著影响,影响程度随着当量比的增加而增大。微重力下丙烷/空气的富燃极限为8.6%C_3H_8(φ=2.24),明显高于1g条件下向下传播火焰的可燃极限,略低于向上传播火焰的可燃极限。随着当量比的增大,根据压力变化曲线计算的火焰层流燃烧速度从8.5cm/s逐渐减小到2.7 cm/s,可燃极限处的层流燃烧速度与前人实验数据一致。 相似文献
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本文通过理论分析和数值模拟系统地研究了辐射热损失对球形火焰传播速度的影响。研究结果表明辐射热损失的影响分为直接影响和间接影响;直接影响指辐射热损失会降低火焰温度,从而降低火焰传播速度;间接影响指辐射热损失导致的冷却会引起逆向火焰传播的流动,从而降低火焰传播速度。对近可燃极限预混气体,直接影响起主导作用;对高辐射强度预混气体,间接影响起主导作用。本文研究的结果对球形火焰法测量火焰传播速度有着重要的指导意义。 相似文献
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《工程热物理学报》2016,(6)
为揭示障碍物对火焰的加速特性,本文运用高速摄影技术和压力传感器分别对空管道和置障管道内甲烷/空气预混火焰的形状变化、火焰传播速度及升压特性进行了实验研究,利用Power-law火焰褶皱模型对管道内流场结构进行了大涡模拟分析。结果表明:空管道和置障管道内甲烷/空气预混火焰传播过程都经历了层流火焰向湍流的转变,且置障管道内的湍流强度明显高于空管道的;障碍物未对前期火焰传播过程产生影响,23ms之前,空管道和置障管道内火焰形态完全一致;障碍物能显著提高管道内的压力上升速率,相比于空管道,障碍物对管道内火焰传播速度提升501%;空管道内涡旋出现在管道的近壁侧,置障管道内涡旋出现在障碍物的背风侧,管道内出现的涡旋结构是火焰形态不断变化的根本原因;空管道湍流燃烧机制始终处于薄火焰区域,而置障管道内湍流火焰机制由薄火焰区域转变到破碎火焰区域;基于大涡模拟的Power-law火焰褶皱模型成功再现了实验中观察到的火焰形状、火焰前锋速度及流场结构,说明该模型适用于置障管道内预混火焰传播特性的研究. 相似文献
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A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures. 相似文献
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Charles Turquand d’Auzay Nilanjan Chakraborty 《Proceedings of the Combustion Institute》2021,38(2):2775-2782
The localised forced ignition and the early stages of the subsequent flame propagation in a planar turbulent methane/air jet in ambient air have been simulated using Direct Numerical Simulation (DNS) and a two-step chemical mechanism. Sixteen identical energy depositions events were simulated for four independent flow realisations at four different locations. The successful ignition and subsequent flame propagation have been found to be well correlated to the mean mixture fraction and flammability factor values of the energy deposition location. Furthermore, similarly to what has been observed in experiments, the early stages of flame development from the ignition kernel involved initial downstream convection of the kernel, followed by simultaneous radial expansion and downstream propagation and finally the upstream propagation of the flame base indicating the onset of flame stabilisation. The mixture composition and the scalar dissipation rate (SDR) values in the immediate vicinity of the ignitor have been identified to play key roles in determining the outcome of the external energy deposition, while the development of an edge flame structure propagating along the stoichiometric mixture fraction iso-surface was found to be necessary but not sufficient for the flame to propagate upstream. It has also been found that in the case of successful self-sustained burning, the edge flame was developing in low SDR regions, and that the most probable edge flame speed remains close to the theoretical laminar value irrespective of the flame development history. Finally, the mean flame speed of the edge flame elements propagating towards the nozzle exit has been found to be considerably greater than the unstrained laminar burning velocity. Thus, the edge flame, depending on its orientation with respect to the flow, is able to propagate upstream and initiate the onset of flame stabilisation. 相似文献
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M.K. Kim 《Proceedings of the Combustion Institute》2007,31(1):901-908
The effect of velocity gradient on the propagation speed of tribrachial flame edge has been investigated experimentally in laminar coflow jets for propane fuel. It was observed that the propagation speed of tribrachial flame showed appreciable deviations at various jet velocities in high mixture fraction gradient regime. From the similarity solutions, it was demonstrated that the velocity gradient varied significantly during the flame propagation. To examine the effect of velocity gradient, detail structures of tribrachial flames were investigated from OH LIF images and Abel transformed images of flame luminosity. It was revealed that the tribrachial point was located on the slanted surface of the premixed wing, and this slanted angle was correlated with the velocity gradient along the stoichiometric contour. The temperature field was visualized qualitatively by the Rayleigh scattering image. The propagation speed of tribrachial flame was corrected by considering the direction of flame propagation with the slanted angle and effective heat conduction to upstream. The corrected propagation speed of tribrachial flame was correlated well. Thus, the mixture fraction gradient together with the velocity gradient affected the propagation speed. 相似文献
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《Proceedings of the Combustion Institute》2023,39(2):2269-2278
Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, revealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length. 相似文献
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Mahdi Faghih 《Proceedings of the Combustion Institute》2019,37(4):4673-4680
Usually premixed flame propagation and laminar burning velocity are studied for mixtures at normal or elevated temperatures and pressures, under which the ignition delay time of the premixture is much larger than the flame resistance time. However, in spark-ignition engines and spark-assisted compression ignition engines, the end-gas in the front of premixed flame is at the state that autoignition might happen before the mixture is consumed by the premixed flame. In this study, laminar premixed flames propagating into an autoigniting dimethyl ether/air mixture are simulated considering detailed chemistry and transport. The emphasis is on the laminar burning velocity of autoigniting mixtures under engine-relevant conditions. Two types of premixed flames are considered: one is the premixed planar flame propagating into an autoigniting DME/air without confinement; and the other is premixed spherical flame propagating inside a closed chamber, for which four stages are identified. Due to the confinement, the unburned mixture is compressed to high temperature and pressure close to or under engine-relevant conditions. The laminar burning velocity is determined from the constant-volume propagating spherical flame method as well as PREMIX. The laminar burning velocities of autoigniting DME/air mixture at different temperatures, pressures, and autoignition progresses are obtained. It is shown that the first-stage and second-stage autoignition can significantly accelerate the flame propagation and thereby greatly increase the laminar burning velocity. When the first-stage autoignition occurs in the unburned mixture, the isentropic compression assumption does not hold and thereby the traditional method cannot be used to calculate the laminar burning velocity. A modified method without using the isentropic compression assumption is proposed. It is shown to work well for autoigniting mixtures. Besides, a power law correlation is obtained based on all the laminar burning velocity data. It works well for mixtures before autoignition while improvement is still needed for mixtures after autoignition. 相似文献
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J. Beeckmann R. Hesse J. Schaback H. Pitsch E. Varea N. Chaumeix 《Proceedings of the Combustion Institute》2019,37(2):1521-1528
Laminar burning velocities are of great importance in many combustion models as well as for validation and improvement of chemical kinetic schemes. Determining laminar burning velocities with high accuracy is quite challenging and different approaches exist. Hence, a comparison of existing methods measuring and evaluating laminar burning velocities is of interest. Here, two optical diagnostics, high speed tomography and Schlieren cinematography, are simultaneously set up to investigate methods for evaluating laminar flame speed in a spherical flame configuration. The hypothesis to obtain the same flame propagation radii over time with the two different techniques is addressed. Another important aspect is the estimation of flame properties, such as the unstretched flame propagation speed and Markstein length in the burnt gas phase and if these are estimated satisfactorily by common experimental approaches. Thorough evaluation of the data with several extrapolation techniques is undertaken. A systematic extrapolation approach is presented to give more confidence into results generated experimentally. The significance of the linear extrapolation routine is highlighted in this context. Measurements of spherically expanding flames are carried out in two high-pressure, high-temperature, constant-volume vessels at RWTH in Aachen, Germany and at ICARE in Orleans, France. For the discussion of the systematic extrapolation approach, flame speed measurements of methane / air mixtures with mixture Lewis numbers moderately away from unity are used. Conditions were varied from lean to rich mixtures, at temperatures of 298–373 K, and pressures of 1 atm and 5 bar. 相似文献
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本文使用定容圆柱形燃烧弹,在初始温度373 K和初始压力1、2、5、10 atm的条件下,对当量比从0.7到1.5的1-庚烯/空气混合物的层流火焰传播进行了研究.利用记录的纹影图像处理得到层流火焰传播速度和马克斯坦长度.基于先前报道的1-己烯燃烧反应动力学模型,发展了1-庚烯的模型.该模型验证了本工作测量的1-庚烯层流火焰传播速度数据及文献中的1-庚烯着火延迟时间数据.通过开展敏感性分析和路径分析,帮助理解了1-庚烯在不同压力下的高温化学及其对层流火焰传播的影响.另外,比较了1-庚烯/空气和先前报道的正庚烷/空气的层流火焰传播.由于更强的放热性及反应活性,1-庚烯/空气的层流火焰传播速度在绝大多数条件下均快于正庚烷/空气的结果. 相似文献
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Direct numerical simulations with a C3-chemistry model have been performed to investigate the transient behavior and internal structure of flames propagating in an axisymmetric fuel jet of methane, ethane, ethylene, acetylene, or propane in normal earth gravity (1g) and zero gravity (0g). The fuel issued from a 3-mm-i.d. tube into quasi-quiescent air for a fixed mixing time of 0.3 s before it was ignited along the centerline where the fuel–air mixture was at stoichiometry. The edge of the flame formed a vigorously burning peak reactivity spot, i.e., reaction kernel, and propagated through a flammable mixture layer, leaving behind a trailing diffusion flame. The reaction kernel broadened laterally across the flammable mixture layer and possessed characteristics of premixed flames in the direction of propagation and unique flame structure in the transverse direction. The reaction kernel grew wings on both fuel and air sides to form a triple-flame-like structure, particularly for ethylene and acetylene, whereas for alkanes, the fuel-rich wing tended to merge with the main diffusion flame zone, particularly methane. The topology of edge diffusion flames depend on the properties of fuels, particularly the rich flammability limit, and the mechanistic oxidation pathways. The transit velocity of edge diffusion flames, determined from a time series of calculated temperature field, equaled to the measured laminar flame speed of the stoichiometric fuel–air mixtures, available in the literature, independent of the gravity level. 相似文献