高压下湍流火焰速度测量与拟合

利用连续燃烧本生灯结合OH-PLIF火焰结构激光诊断技术获得了高压下预混湍流火焰瞬时前锋面图像,通过图像处理,得到了高压下多种混合气的湍流火焰速度及其拟合关系式。结果表明:高压预混湍流火焰前锋面为高度褶皱凹凸结构,现象上表现为大尺度的凸向未燃气的凸结构和小尺度的凸向已燃气的凹结构。合成气火焰比CH_4/air更加褶皱,前锋面更加精细。本文所有实验点高压湍流火焰速度可以拟合为S_T/S_L与[(P/P_0)(u′/S_L)]~n的形式,n约为0.4左右,表明湍流涡与火焰面的相互作用遵循自相似的规律。     

预混湍流3D火焰面密度和燃料消耗率的估计

利用平面激光诱导荧光(OH-PLIF)技术测量了CH_4/air预混湍流火焰前锋面结构,通过图片处理得到了测量平面上的二维火焰面密度。基于不同的假设建立了三种不同模型,利用二维探测得到的火焰面信息来估计三维火焰面密度在测量面上的值,通过积分三维火焰面密度估计值得到燃烧系统的燃料消耗率。结果表明,预混湍流火焰前锋面为凹凸的褶皱结构,平面测量的二维火焰面密度明显低估了真实的火焰面密度,利用模型估计得到的3D火焰面密度对2D值有明显的改善。燃烧系统的燃料消耗率可以用来评估模型的可靠性,结果表明模型的误差都在40%以内。     

运用OH-PLIF方法探测预混火焰前锋结构

运用OH-PLIF方法探测了不同燃料系数和出口湍流度下甲烷/空气的锥形预混火焰的火焰前锋结构.实验结果表明,网格湍流是火焰根部产生皱折和不稳定性的重要因素,并且这种不稳定性会沿着火焰面向下游发展;而在火焰下游尤其是火焰顶部,流场中的低频拟序结构是形成大尺度皱折以及破碎火焰面的直接原因.相比于富燃料工况,贫预混火焰的火焰前锋更容易产生不稳定性,产生大尺度的皱折.导致这种皱折的不稳定性只与燃料系数有关,完全依赖于火焰自身的动力学而与来流特征无关.     

湍流预混火焰中的浮力效应

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。     

LIPS技术探测CH4/空气火焰中的OH自由基

论述了激光诱导偏振光谱技术的基本原理,实验方法及其对燃烧场的测量结果.利用光谱扫描方法获得了A2∑ -X2Ⅱ(0,0)跃迁带系列吸收谱线及P1(2)谱线的线型轮廓.测量了CH4/空气预混火焰不同化学配比、不同空间位置OH自由基的相对分布.实验中通过设计精密旋转调节装置有效提高了信噪比.     

信息光学 光学信息处理、图像识别

O4382006010288基于哈特曼波前探测层析重建三维温度场=Tomographicreconstruction of3-Dtemperature field based on Hart mannwavefront sensing[刊,中]/戴云(中科院光电所.四川,成都(610209)),张雨东…∥中国激光.—2005,32(10).—1406-1410基于哈特曼波前探测的流场层析测量技术结合了光学波前探测技术和计算机层析重建技术。系统由哈特曼传感器探测平行光束穿过流场后的投影波前,提取流场在多方向上的投影数据,采用计算机层析技术重建流场物理量的空间分布。为了验证系统的可行性和标定系统重建精度,对静态圆对称折射率场进行层析重…     

基于探测超声的新型生物组织光声层析成像

利用一束单频探测超声穿过由脉冲激光照射产生的光声激发区域，使之与光致声场产生相互作用，光声信号将耦合到探测束上，再解调探测超声来重建光吸收区域的图像，可以得到丰富的组织信息.该信息不仅反映了组织中光吸收的特性，也反映了组织的声学特性.实验中，通过旋转超声探测器进行数据采集，并利用滤波反投影算法重建图像，可得到高信噪比的光声层析图像.由于激光的窄光谱线宽，其吸收是特征分子选择性的，所以此方法既是一种无损伤的结构层析成像方法，也可用于组织的功能成像.     

引力波背景下的电磁波

本文首先介绍了激光干涉引力波探测实验原理教学的时空图方法,紧接着对引力波背景下一种具有简单然而非平庸时空轮廓的电磁场的波动方程进行了化简并求出了其解析解,最后利用所得解对激光干涉引力波探测实验中测地光子近似的有效性进行了定量的评估和分析。我们的结果对激光干涉引力波探测原理和弯曲时空中麦克斯韦方程组及电磁波理论的教学有积极的探讨价值。     

激光扫描轮廓曲面的动态测量

本文提出一种新颖的轮廓曲面动态，实时，非接触光学测量方法，对飞机机翼，汽轮机叶片以及坦克，轿车等的轮廓曲面提供了一种有效的测量方法，该方法是利用激光测头扫描探测，ＣＣＤ（电荷耦合器件）或ＰＳＤ作为光电接收器件，利用微机控制的闭环测量方法，文中还给出了一种ＣＣＤ的新探测驱动方法。     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

网格湍流对剪切层中预混火焰结构的影响

本文研究网格湍流对射流剪切层以及建立在其中的预混火焰的影响。利用热线风速仪测量射流的速度场,发现网格湍流使剪切层内湍流强度明显降低,抑制了低频速度脉动,同时增加了湍动能在小尺度脉动上的分配,使湍流更趋于各向同性,这表明网格湍流抑制了剪切层内的大涡和拟序结构。用细丝热电偶测量了火焰温度,结果显示网格湍流使火焰前峰的低频大幅摆动减少,小尺度皱褶增加,火焰区平均温度更高,说明网格湍流有利于剪切层中预混火焰的强化和稳定。     

ps-LIF measurements of minor species concentration in a counterflow diffusion flame interacting with a vortex

Interactions of vortices and flame fronts may be considered as the basic structural elements of turbulent combustion. Additionally, they play an important role in flame instabilities as well as extinction and ignition processes. An ideal geometry to study these interactions is the counterflow diffusion burner with an additional actuator-driven nozzle for the generation of a vortex ring. This burner has already been well-characterized by other methods including CARS, LDA and PLIF. We present first quantitative measurements of minor species concentration in this flame using a short-pulse laser and time- and spatially resolved fluorescence detection with a streak camera. Quench-free OH concentrations are obtained by analysis of the time-resolved profiles. The high power density of the laser pulses allowed linewise detection of hydrogen using a three-photon excitation scheme. Simultaneously, shape and position of the vortex was monitored using two-dimensional detection of flame emissions. Spatially resolved concentration profiles of H and OH are presented for different interaction heights and times in the vortex. For steady flames, comparisons with model calculations are shown. Received: 19 July 2000 / Revised version: 13 December 2000 / Published online: 21 February 2001     

Detection of atomic oxygen in flames by absorption spectroscopy

The absolute concentration of atomic oxygen in an atmospheric pressure hydrogen/air flame has been measured using Intracavity Laser Spectroscopy (ICLS) based on a dye laser pumped by an argon-ion laser. Absorptions at the highly forbidden transitions at 630.030 nm and 636.380 nm were observed at an equivalent optical length of up to 10 km. The relatively low intensity of the dye laser avoids photochemical interferences that are inherent to some other methods for detecting atomic oxygen. The detection sensitivity is about 6 × 10¹⁴ atom/cm³ and can be improved with better flame and laser stabilization.     

Temporal evolution of flame stretch due to turbulence and the hydrodynamic instability

The temporal evolution of the strain rate on a turbulent premixed flame was measured experimentally using cinema-stereoscopic particle image velocimetry. Turbulence strains a flame due to velocity gradients associated both directly with the turbulence and those caused by the hydrodynamic instability, which are initiated by the turbulence. The development of flame wrinkles caused by both of these mechanisms was observed. Wrinkles generated by the turbulence formed around vortical structures, which passed through the flame and were attenuated. After the turbulent structures had passed, the hydrodynamic instability flow pattern developed and caused additional strain. The hydrodynamic instability also caused the growth of small flame front perturbations into large wrinkles. In the moderately turbulent flame investigated, it was found that the evolution of the strain rate caused by turbulence–flame interactions followed a common pattern involving three temporal regimes. In the first, the turbulence exerted extensive (positive) strain on the flame, creating a wrinkle that had negative curvature (concave towards the reactants). This was followed by a transition period, leading into the third regime in which the flow pattern and strain rate were dominated by the hydrodynamic instability mechanism. It was also found that the magnitudes of the strain rate in the first and third regimes were similar. Hence, the hydrodynamic instability mechanism caused significant strain on a flame and should be included in turbulent combustion models.     

Diesel flame lift-off stabilization in the presence of laser-ignition: a numerical study

Diesel flame lift-off and stabilization in the presence of laser-ignition were numerically investigated with the method of Eulerian stochastic fields. The aim was to scrutinise the interaction between the lifted diesel flame and an ignition kernel upstream of the lifted flame. The numerical simulation was carried out in a constant-volume combustion vessel with n-heptane as fuel. The process was studied previously in an experiment employing Diesel #2 as the fuel in the same combustion vessel. In the experiment a lifted flame was first established at a position downstream of the nozzle. An ignition kernel was then initiated using a high-energy pulse laser at a position upstream of the natural lift-off position of the diesel flame. The laser-ignition kernel was modelled using a high-temperature (～2000 K) hot spot. In both experiment and simulations the upstream front of the ignition kernel was shown to remain around the initial laser ignition site for a substantially long period of time, while the downstream front of the ignition kernel propagates rapidly towards the natural lift-off position downstream of the laser ignition site. The lift-off position oscillated before the final stabilization at the natural lift-off position. The structures and the propagation speed of the reaction fronts in the laser-ignition kernel and the main flame were analysed. Two different stabilization mechanisms, the auto-ignition mechanism and the flame propagation mechanism, were identified for the naturally lifted flame and the laser-induced reaction front, respectively. A mechanism was proposed to explain the oscillation of the lift-off position.     

Simultaneous development of time resolved laser tomography and PIV for flames propagation studies

An understanding of flame propagation needs the knowledge of the flame location and of the velocity field near the front. Time resolved laser Tomography coupled with high resolution cross correlation PIV has been developed. The method is based on the use of a copper vapor laser and a highspeed camera. The seeding level used for tomography provides an excellent resolution of the flame location and of the velocity vectors in the fresh gases. Experiment is conducted in a simulated engine where turbulence is controlled by means of a perforated grid. Interest is shown for the determination of laminar burning velocity and for the study of interaction between the flame front and the turbulent structures. Perspectives of stereoscopic PIV are also started on. It is shown that the existence of a normal component to the sheet plane of the velocity can lead to an erroneous flame velocity determination.     

Simultaneous in-situ measurements of gas temperature and pyrolysis of biomass smoldering via X-ray computed tomography

In-situ X-ray computed tomography (XCT) imaging is employed to investigate the smoldering dynamics of biomass at the sub-millimeter scale. This technique provides simultaneous and spatially-resolved information about the gas temperature and the biomass density, thereby enabling tracking of the pyrolysis and char oxidation fronts. To achieve well-controlled heating and flow conditioning, oak biomass samples are instrumented above a diffusion flame inside a tube, with total oxygen concentrations of 6% and 11% per volume. Experiments are performed on a laboratory XCT system. The flow is diluted with Kr to increase X-ray attenuation in the gas phase thus allowing for simultaneous 3D measurements of sample density and surrounding temperature. XCT scans are acquired every 90 s at a spatial resolution of 135 µm. The high spatial resolution enables the volumetric visualization of the smoldering process that is associated with pyrolysis and char oxidation. These measurements show how the grain structure affects flame stabilization and induces fingering of the pyrolysis front, while crack formation accelerates the char oxidation locally. Evaluations of the sample mass via XCT are compared with load cell measurements, showing good agreement. A low-order model is developed to evaluate the propagation speeds of pyrolysis and oxidation fronts from the X-ray data over time, and comparisons are made with the surface recess speed.     

Analysis of the flame dynamics in methane/hydrogen fuel blends at elevated pressures

We investigate the Flame Transfer Function (FTF) of a lean-premixed, laminar slit flame numerically. Based on the reference case at atmospheric pressure, we investigate four different scenarios: (i) varying the hydrogen content in the fuel at constant equivalence ratio (ER) (resulting in an increase of the laminar flame speed); (ii) varying the hydrogen content in the fuel at varying ER (resulting in a constant laminar flame speed); (iii) varying the operating pressures from 1 to 5 bar (resulting in a decrease of the laminar flame speed); and (iv) combining a hydrogen-enriched flame at an elevated pressure of 3 bar (resulting in the same flame speed as the reference case). We identify in this case that the laminar flame speed and the flame thickness impact the FTF independently. We show that the low-pass behavior of the flame is shifted towards higher frequencies when the operating pressure increases, and demonstrate that wrinkles along the flame front preserve in contrast to the atmospheric operating pressure configurations. These results are in line with past studies, that relate the dampening of flame front wrinkling to a decreasing Markstein length. We therefore conclude that a decreasing flame thickness, due to increasing operating pressure, causes a decreasing Markstein length and therefore less pronounced dampening of flame front wrinkles.     

Resonance of a turbulent flame in a high-frequency acoustic wave

Resonance of a weakly turbulent flame in a high-frequency acoustic wave is obtained. Because of the resonance, an acoustic wave may increase noticeably the amplitude of flame wrinkles, and the respective increase in propagation velocity of the turbulent flame front becomes larger by a factor of 10-20. The effect of resonance is especially important for turbulent flames with realistic thermal expansion propagating in a closed burning chamber, which may account for considerable scattering of experimental results on turbulent flame velocity.