用高空气球搭载微重力实验研究浮力对预混V形火焰的影响

在地面实验中观测到的燃烧现象,包含了浮力的影响。利用微重力实验在浮力消失后研究火焰,有助于深入理解燃烧过程。本文介绍了利用高空气球搭载微重力实验对甲烷－空气预混Ｖ形火焰的研究。实验提供了长时间微重力环境下火焰的动态图像。利用计算机图像处理方法对火焰图像的分析表明,在本实验的工况下,微重力下预混Ｖ形火焰锋面的张角比正常重力下变大,皱折和摆动加剧。这说明浮力确实影响预混燃烧过程。     

具有强剪切流等离子体中离子声波与涡旋的转化

研究了在具有剪切本底流的非磁化等离子体中离子声波的激发及其与涡旋的转化。从流体方程组出发，导出了描述低频静电扰动的微分方程组。采用非模方法，得到了扰动量的剪切傅里叶谐波分量随时间演化的方程组，并对方程组进行了数值分析。结果表明：在强剪切条件下，剪切旋涡可以转化为离子声波；同样地，离子声波也激发剪切旋涡。     

甲烷-空气预混V形火焰的CARS实验研究

本文将相干反斯托克斯(CARS)理论光谱计算和实验光谱分析的方法应用于预混V形火焰燃烧的温度测量实验,利用N2的Q支CARS谱线,使用单脉冲宽带方法获得了预混V形火焰的CARS信号光谱强度特性,测量了V形火焰水平方向和竖直方向上的温度分布特征,从中得出了火焰锋面的厚度,分析了火焰锋面的皱褶与摆动对CARS信号的影响。同时测量了不同燃料系数下V形火焰燃烧产物的温度,得出了温度随燃料系数的变化趋势,为进一步研究预混 V形火焰的结构提供了依据。     

低温化学影响下的火焰不稳定性研究

考虑详细化学反应及输运过程,对不可压N-S方程直接数值求解,研究了低温化学影响下的火焰水力学及胞状不稳定性。结果表明,预热后已发生低温化学反应的混合气,其火焰的扰动增长率明显大于未发生低温反应的混合气火焰。通过对预热后混合气火焰的关键参数分析,发现和未预热混气火焰相比其无量纲释热量、Lewis数及Zeldovich数都显著减小。依据已有理论可知,在Lewis数大于1的条件下,上述参数的减小将使得胞状不稳定性对扰动增长的抑制作用减弱。因此,预热后火焰的增长率明显增大。同时,在低温反应中,大量自由基和其它小分子的形成,使得预热后混合气火焰Lewis数减小,从而其火焰胞状结构的深度和高度都变小。     

预混气体燃烧火焰闪烁现象分析

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。     

基于模态分解的扩散火焰动态响应特性分析

针对扩散火焰燃烧不稳定性,以钝体扰流扩散火焰为对象,研究不同频率声波激励下火焰动态响应特性,通过傅里叶变换(FFT)与本征正交分解法(POD)分析火焰的图像的空间分布的频谱特性.结果 表明,火焰振荡的主要模态为火焰整体的横向振荡、尾流区的频闪振荡与纵向振荡,当外加声波激励频率与火焰自身不稳定频率接近时会增强火焰的频闪振荡与纵向振荡幅值.在高频声波激励条件下,火焰整体积分值的傅立叶变换分析无法获得激励频率的响应,而通过POD方法对火焰空间结构动态变化的分析可以得到激励频率的响应,对于分布式扩散火焰,POD方法能获得更多火焰动态变化特征.     

微重力环境中的蜡烛火焰

对蜡烛火焰动态特征的分析表明,从正常重力状态过渡到微重力状态,火焰的空气动力学特征比质量和能量的传输特征的变化快。通过一台差分干涉仪首次测量得到了微重力环境中蜡烛火焰的温度。结果表明,微重力蜡烛火焰的温度小于正常重力蜡烛火焰的温度。微重力蜡烛火焰之所以呈蓝色是因为其温度小于烟黑生成的阈值温度1300K。但当环境氧浓度足够高时,火焰温度大于烟黑生成的阈值温度,火焰中明显有烟黑生成,颜色为亮黄色。     

表面辐射热损失对微重力静止环境下火焰传播特性的影响

采用数值模拟方法研究了静止微重力环境中,表面辐射热损失对燃料表面火焰传播特性的影响以及表面辐射和压力对火焰传播特性的共同影响。结果表明,随着表面辐射增大,火焰传播速度减小,在考虑表面辐射后,随着压力的增大,火焰传播速度增大。采用无量纲参数分析了表面辐射对火焰传播速度的影响,进一步阐明了微重力环境下的火焰传播机理。     

微重力环境下蜡烛火焰与油滴沉降的实验研究

搭建了短时微重力实验系统,对微重力条件下蜡烛火焰与油滴沉降进行了实验观测,帮助学生理解微重力环境下的物理规律与实验现象.     

Rijke管热声非线性不稳定增长过程的研究

本文分析了Ｒｉｊｋｅ管热声不稳定性的非线性增长过程，通过流体力学中的三个基本守衡方程，得出了管内的声学量之间的关系，并通过采用热声相互作用的非线性关系式和管口声波的非线性反射条件，发展了一种可以计算出管内声波从小扰动增长到大振幅振动直到由于非线性效应的影响而停止增长的全过程的方法，最后实验验证了理论。     

Detailed measurements of local scalar front structures in stagnation-type turbulent premixed flames

Local scalar front structures of OH mole fraction, reaction progress variable, and its three-dimensional gradient have been measured in stagnation-type turbulent premixed flames. The reaction progress variable front is observed to change with increasing turbulence from parallel iso-scalar contours but reduced progress variable gradients, called the lamella-like front, to disrupted non-parallel iso-contours that deviate substantially from those of wrinkled laminar flamelets, called the non-flamelet front. This transition is attributed to the different scales of interaction between the flame internal structure and a spectrum of turbulence extending from the integral scale to the Kolmogorov scale. The lamella-like front pattern occurs when the length scales of interaction are smaller than the laminar flame thickness but the time scales are greater than the flame residence time. The non-flamelet front pattern occurs when the length scales of interaction are greater than the laminar flame thickness but the time scales are smaller than the flame residence time. This difference corresponds to the change of combustion regime from complex-strain flame front to turbulent flame front on a revised regime diagram. A correlation is also proposed for the turbulent flame brush thickness as a function of turbulent Reynolds number and heat release parameter. The heat release parameter is considered to arise from the non-passive effects of flame-surface wrinkling.     

Turbulent propagation of premixed flames in the presence of Darrieus–Landau instability

We investigate the role played by hydrodynamic instability in the wrinkled flamelet regime of turbulent combustion, where the intensity of turbulence is small compared to the laminar flame speed and the scale large compared to the flame thickness. To this end the Michelson–Sivashinsky (MS) equation for flame front propagation in one and two spatial dimensions is studied in the presence of uncorrelated and correlated noise representing a turbulent flow field. The combined effect of turbulence intensity, integral scale, and an instability parameter related to the Markstein length are examined and turbulent propagation speed monitored for both stable planar flames and corrugated flames for which the planar conformation is unstable. For planar flames a particularly simple scaling law emerges, involving quadratic dependence on intensity and a linear dependence on the degree of instability. For corrugated flames we find the dependence on intensity to be substantially weaker than quadratic, revealing that corrugated flames are more resilient to turbulence than planar flames. The existence of a threshold turbulence intensity is also observed, below which the corrugated flame in the presence of turbulence behaves like a laminar flame. We also analyze the conformation of the flame surface in the presence of turbulence, revealing primary, large-scale wrinkles of a size comparable to the main corrugation. When the integral scale is much smaller than the characteristic corrugation length we observe, in addition to primary wrinkles, secondary small-scale wrinkles contaminating the surface. The flame then acquires a multi-scale, self-similar conformation, with a fractal dimension, for one-dimensional flames, plateauing at 1.23 for large intensities. The existence of an intermediate integral scale is also found at which the turbulent speed is maximized. When two-dimensional flames are subject to turbulence, the primary wrinkling patterns give rise to polyhedral-cellular structures which bear a very close resemblance to those observed in experiments on hydrodynamically unstable expanding spherical flames.     

H2/air旋流预混燃烧的流场特性和火焰结构分析

本文采用完全可压缩的N-S方程,对当量比为1.0的H₂/air旋流预混火焰进行了直接数值模拟研究。氢气和空气的化学反应采用9种组分19步的详细机理。模拟结果表明,强旋流流场中存在回流区,碗形旋流火焰稳定在回流区的外围。在火焰面上沿火焰法向提取了局部火焰结构,将局部湍流火焰结构与层流预混火焰的火焰结构进行了比较,发现局部湍流火焰比层流预混火焰更薄,燃烧强度更高。     

Analytical predictions of shapes of laminar diffusion flames in microgravity and earth gravity

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.     

重力对扩散射流火焰动态特性的影响

本文探讨重力对扩散射流火焰动态特性的影响规律。结果表明,火焰闪烁现象是一种浮力诱导不稳定性,在浮力消失或反向重力场中,不存在这种不稳定性现象,闪烁频率与燃料射流速度无直接关系,但涡的大小随燃料射流速度的增大而增大。存在触发火焰闪烁的临界高度,闪烁频率与重力成平方根关系式。反向重力情况下,也存在浮力稳定型平面火焰,它反映了浮力与火焰的耦合作用。     

On nonlinear model equations for the response of premixed flames to acoustic like accelerations

The response of a dynamical flame model to imposed acoustic accelerations is studied analytically and numerically. Through linear stability analyses, two analytical approximations for the primary and the parametric stability boundaries are found. The approximation for the primary instability boundary is accurate for any periodic accelerations, in the limit of large acoustic frequencies. The critical acoustic amplitude u _a for Landau–Darrieus instability suppression is identified and found to depend only on the density contrast and the shape of the periodic acoustic stimuli. The proposed model evolution equation is next integrated numerically with various imposed acoustic accelerations; the primary and parametric flame responses are identified. It is shown analytically and numerically that in the presence of a fully developed, yet weakened by acoustics, Landau–Darrieus (or primary) instability the wrinkle amplitude and the mean flame speed oscillate at the same frequency as the acoustic stimuli; the threshold for suppression of primary instability by acoustic forcing is determined exactly. Increasing the acoustic amplitude allows the flame to respond parametrically to the acoustics. This response is characterised by troughs and crests interchanging their roles while the mean flame speed again oscillates with the same frequency as the acoustic stimuli and at twice that of wrinkle amplitude oscillations.     

Violent folding of a flame front in a flame-acoustic resonance

The first direct numerical simulations of violent flame folding because of the flame-acoustic resonance are performed. Flame propagates in a tube from an open end to a closed one. Acoustic amplitude becomes extremely large when the acoustic mode between the flame and the closed tube end comes in resonance with intrinsic flame oscillations. The acoustic oscillations produce an effective acceleration field at the flame front leading to a strong Rayleigh-Taylor instability during every second half period of the oscillations. The Rayleigh-Taylor instability makes the flame front strongly corrugated with elongated jets of heavy fuel mixture penetrating the burnt gas and even with pockets of unburned matter separated from the flame front.     

Effect of Le on criteria of transition to secondary acoustic instability of downward-propagating flame in a tube with controlled curvature induced by external laser

An experimental study is conducted to investigate the effect of Le on the transition to secondary acoustic instability when the curvature of the flame front in a tube is induced and controlled by using external laser irradiation. Once a downward-propagating flame in the primary acoustic instability region is exposed to a specific laser irradiation condition, the flame is transferred to the secondary acoustic instability region. The transition limit is decreased, that is, transition occurs is an easier manner, with increasing laser power input. While the flame propagates with increasing laser irradiation, the flame first exhibits a convex curvature owing to laser irradiation and then a concave structure is formed owing to buoyancy-induced flow. Two types of transition behavior caused by the concave structure and the convex structure are observed. The conflicting thermal-diffusive effect depending on Le leads to the differing transition behaviors. Based on an evaluation of the flame stretch effect attributed to the flame front curvature, it is confirmed that the Lewis number effect influences the transition criteria.     

Exploring a critical diameter for thermo-acoustic instability of downward propagating flames in tubes

Thermo-acoustic oscillations are observed when a flame ignited at open end of a tube propagates towards the closed end due to interaction between unsteady heat release rate fluctuations from flame and acoustic fluctuations. In our past work, it was found that thermo-acoustic instability increases with decreasing diameter from 7.0 cm to 3.0 cm. A recent study in flame propagation in Hele–Shaw cells showed that thermo-acoustic instability is not observed for plate separation less than or equal to 0.4 cm. Thermoacoustic instabilities cannot be observed in very narrow tubes due to excessive damping from the wall. This opens up the possibility of a critical diameter where thermo-acoustic instability would be maximum. In this work we perform flame propagation experiments with diameter of combustion tube in the range 0.5 cm to 3 cm for a fixed length of 70.2 cm. It was found that thermo-acoustic parametric instability begins at lowest laminar burning velocity when the diameter is around 1.0 cm. This diameter is termed as critical diameter. Critical diameter is found to be independent of Lewis number of mixtures. Existence of a critical diameter is thus proved experimentally. Growth rates of primary instability increase with decreasing diameter and show a maximum around the critical diameter and decrease with further decrease in tube diameter. But, growth rates of secondary instability as well as maximum pressure fluctuation amplitude decreases continuously with decreasing diameter. Mechanisms responsible for these observations and existence of a critical diameter are clarified.