共查询到18条相似文献,搜索用时 125 毫秒
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预混气体燃烧火焰闪烁现象分析 总被引:1,自引:0,他引:1
在低速射流的预混火焰和扩散火焰中都存在火焰闪烁现象。对扩散火焰,其机理已比较明确,是由于浮力诱导引起的一种水力学不稳定性。而对预混火焰闪烁现象则存在水力学不稳定性和热驱动不稳定性两种观点。本文根据水力学不不稳定性观点,把预混火焰的闪烁现象看成是包围火焰锋面的已燃混气层中内、外区间在垂直方向上的相对脉动,应用Kelvin-Helmholtz不稳定性机理进行了分析,获得了火焰闪烁频率与重力和压力的关系式,并与已有的结果作了对比。 相似文献
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采用高速摄像仪对稠密颗粒射流倾斜撞击形成的类液体颗粒膜特征进行实验研究,考察了颗粒粒径、射流速度以及射流含固率等因素对颗粒膜形态及动态特征的影响.结果表明:随着颗粒粒径增大,稠密颗粒倾斜撞击流由颗粒膜向散射模式转变;随着射流速度增加,气固不稳定增强,射流流量出现脉动,正面与侧面分别表现为颗粒膜的非轴对称振荡和表面波纹结构;颗粒膜非轴对称振荡的振荡频率和振荡幅度随射流速度的增大而增大;表面波纹速度和波长沿传播方向增大,波纹间存在叠加现象.颗粒膜出现非轴对称振荡主要是因为喷嘴出口由气固不稳定性引起的射流流量脉动,射流流量脉动频率与撞击面振荡频率基本相当. 相似文献
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本文利用数值模拟研究了浮力对湍流预混V形火焰平均速度场的影响,发现浮力效应主要体现在远场区域,而在火焰刷附近非常有限;利用落塔和 OH-PLIF 方法在正常重力和微重力下观测了火焰皱褶,发现浮力压制火焰皱褶的程度与湍流强度密切相关。分析表明斜压机理是浮力影响火焰皱褶的重要原因。 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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《Combustion Theory and Modelling》2013,17(4):499-515
The structure and propagation properties of diffusion neutral triple flames subject to buoyancy effects are studied numerically using a high-accuracy scheme. A wide range of gravity conditions, heat release, and mixing widths for a scalar mixing layer are computed for downward-propagating (in the same direction as the gravity vector) and upward-propagating (in the opposite direction to the gravity vector) triple flames. These results are used to identify non-dimensional quantities, which parametrize the triple flame responses. Results show that buoyancy acts primarily to modify the overall span of the premixed branches in response to gas acceleration across the triple flame. The impact of buoyancy on the structure of triple flame is less pronounced than its impact on the topology of the branches. The trailing diffusion branch is affected by buoyancy primarily as a result of the changes in the overall flame size, which consequently modifies the rates of diffusion of excess fuel and oxidizer from the premixed branches to the diffusion branch. A simple analytical model for the triple flame speed, which accounts for both buoyancy and heat release is developed. Comparisons of the proposed model with the numerical results for a wide range of gravity, heat release and mixing width conditions, yield very good agreement. The analysis shows that under neutral diffusion, downward propagation reduces the triple flame speed, while upward propagation enhances it. For the former condition, a critical Froude number may be evaluated, which corresponds to a vanishing triple flame speed. 相似文献
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S. S. Krishnan J. M. Abshire P. B. Sunderland Z.-G. Yuan J. P. Gore 《Combustion Theory and Modelling》2013,17(4):605-620
Flame shape is an important observed characteristic of flames that can be used to scale flame properties such as heat release rates and radiation. Flame shape is affected by fuel type, oxygen levels in the oxidiser, inverse burning and gravity. The objective of this study is to understand the effect of high oxygen concentrations, inverse burning, and gravity on the predictions of flame shapes. Flame shapes are obtained from recent analytical models and compared with experimental data for a number of inverse and normal ethane flame configurations with varying oxygen concentrations in the oxidiser and under earth gravity and microgravity conditions. The Roper flame shape model was extended to predict the complete flame shapes of laminar gas jet normal and inverse diffusion flames on round burners. The Spalding model was extended to inverse diffusion flames. The results show that the extended Roper model results in reasonable predictions for all microgravity and earth gravity flames except for enhanced oxygen normal diffusion flames under earth gravity conditions. The results also show trends towards cooler flames in microgravity that are in line with past experimental observations. Some key characteristics of the predicted flame shapes and parameters needed to describe the flame shape using the extended Roper model are discussed. 相似文献
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S.H. Won J. Kim K.J. Hong M.S. Cha S.H. Chung 《Proceedings of the Combustion Institute》2005,30(1):339-347
The stabilization mechanism of lifted flames in the near field of coflow jets has been investigated experimentally and numerically for methane fuel diluted with nitrogen. The lifted flames were observed only in the near field of coflow jets until blowout occurred in the normal gravity condition. To elucidate the stabilization mechanism for the stationary lifted flames of methane having the Schmidt number smaller than unity, the behavior of the flame in the buoyancy-free condition, and unsteady propagation characteristics after ignition were investigated numerically at various conditions of jet velocity. It has been found that buoyancy plays an important role for flame stabilization of lifted flames under normal gravity, such that the flame becomes attached to the nozzle in microgravity. The stabilization mechanism is found to be due to the variation of the propagation speed of the lifted flame edge with axial distance from the nozzle in the near field of the coflow as compared to the local flow velocity variation at the edge. 相似文献
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S. Rouvreau P. Cordeiro J.L. Torero P. Joulain 《Proceedings of the Combustion Institute》2005,30(1):519-526
A numerical study was conducted to analyze the effect of g-jitter on micro-gravity flames. A boundary layer laminar diffusion flame was used as a test case. This configuration is commonly used to study flame spread in microgravity, thus it is essential to understand the role of g-jitter in these flames. Furthermore, the role of buoyancy increases with the stream-wise coordinate permitting a systematic study of the impact of acceleration perturbations with a reduced number of experimental results. The evolution of experimental stand-off distances defined during parabolic flights compared well, in a qualitative manner, with numerical simulations, validating the aerodynamic aspects of the model. A systematic study using a sinusoidal function showed that perturbations characterized by high frequencies (>1 Hz) do not affect the flame stand-off distance. This is independent of the amplitude within the range of typical perturbations observed during parabolic flights. Perturbations occurring at lower frequencies significantly affected the flame geometry. Averaging over time through periods much longer than the perturbation cycle did not eventually reveal departure from purely zero-gravity flames. Fuel and oxidizer velocities have opposite effects on the sensitivity of the flames to gravity fluctuations. An increase in oxidizer velocity results in a sensitivity decrease. The influence of the multiple parameters of the problem can qualitatively be combined within a previously reported non-dimensional group. Nevertheless, it cannot account for the influence of frequency. 相似文献
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《Proceedings of the Combustion Institute》2019,37(2):2005-2012
Stabilization of laminar lifted coflow jet flames of nitrogen-diluted methane was investigated experimentally and numerically. As the fuel jet velocity was increased, two distinct behaviors in liftoff height were observed depending on the initial fuel mole fraction; a monotonically increasing trend and a decreasing and then increasing trend (U-shaped behavior). The former was observed in the jet-developing region and the latter in the jet-developed region. Because the decreasing behavior of liftoff height with jet velocity has not been observed at ambient temperature, the present study focuses on decreasing liftoff height behavior. To elucidate the physical mechanism underlying the U-shaped behavior, numerical simulations of reacting jets were conducted by adopting a skeletal mechanism. The U-shaped behavior was related to the buoyancy. At small jet velocities, the relative importance of the buoyancy over convection was strong and the flow field was accelerated in the downstream region to stabilize the lifted flame. As the jet velocity increased, the relative importance of buoyancy decreased and the liftoff height decreased. As the jet velocity further increased, the flame stabilization was controlled by jet momentum and the liftoff height increased. 相似文献
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Kevin T. Walsh Joseph Fielding Mitchell D. Smooke Marshall B. Long Amable Lin 《Proceedings of the Combustion Institute》2005,30(1):357-365
As a sensitive marker of changes in flame structure, the number densities of excited-state CH (denoted CH*), and excited-state OH (denoted OH*) are imaged in coflow laminar diffusion flames. Measurements are made both in normal gravity and on the NASA KC-135 reduced-gravity aircraft. The spatial distribution of these radicals provides information about flame structure and lift-off heights that can be directly compared with computational predictions. Measurements and computations are compared over a range of buoyancy and fuel dilution levels. Results indicate that the lift-off heights and flame shapes predicted by the computations are in excellent agreement with measurement for both normal gravity (1g) and reduced gravity flames at low dilution levels. As the fuel mixture is increasingly diluted, however, the 1g lift-off heights become underpredicted. This trend continues until the computations predict stable flames at highly dilute fuel mixtures beyond the 1g experimental blow-off limit. To better understand this behavior, an analysis was performed, which indicates that the lift-off height is sensitive to the laminar flame speed of the corresponding premixed mixture at the flame edge. By varying the rates of two key “flame speed” controlling reactions, we were able to modify the predicted lift-off heights so as to be in closer agreement with the experiments. The results indicate that reaction sets that work well in low dilution systems may need to be modified to accommodate high dilution flames. 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