PLIF技术测量浓度场及其二维数字校正

推导了ＰＬＩＦ技术测量浓度场时液地内荧光强度的分布，提出了ＰＬＩＦ技术的二维数字校正方程和校正方法来消除激光片光的高斯分布和沿光程衰减的影响．在此基础上，对圆形紊动射流的浓度场进行了测量和校正．此外，还将测量结果与前人的半经验理论进行了分析和比较。     

Subgrid-scale dynamics and model test in a turbulent stratified jet with coexistence of stable and unstable stratification

The subgrid-scale dynamics of stratified flows is studied in a horizontally introduced turbulent jet with coexistence of stable and unstable stratification of a low Richardson number case and a high Richardson number case. The positive production of subgrid-scale kinetic energy and the production of scalar variance suggest the forward energy cascade. The subgrid-scale buoyant destruction plays a role as a sink of subgrid-scale kinetic energy in the stable stratification while holds a role of turbulent generation in the unstable stratification. The role-switch of buoyant destruction in the stable stratification of high-Ri case implies the occurrence of a destabilising process triggered by the coupled instability mechanisms. The energy balance assumption related to the production of and the dissipation of subgrid-scale kinetic energy as well as the subgrid-scale buoyant destruction may fail. The a-priori test suggests the scale-invariant dynamic and standard Smagorinsky models not to work properly here, while the scale-dependent dynamic model gives a decent performance but with restrictions of the ratio between two testing filter scales.     

Measurement of mixing processes with combined digital particle image velocimetry and planar laser induced fluorescence

A combined digital particle image velocimetry (DPIV) and planar laser induced fluorescence (PLIF) approach was developed to measure both the time mean and turbulent mass transport in mixing processes. The system couples the two well-known techniques to enable synchronized planar measurements of flow velocities and concentrations in a study area. The potential interference effect between the seeding particles for DPIV and the fluorescent dye excitation for PLIF was carefully investigated. The performance of the system was verified with the experimental results of a turbulent round jet discharging into a stagnant environment. Comparison between the measurements obtained in the present study with the large body of existing information on pure jets is satisfactory. The key advantage of the shorter duration required with this approach compared to point-based techniques is highlighted.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.     

PLIF法定量测量甲烷-空气火焰二维温度场分布

利用平面激光诱导荧光(PLIF)技术，通过选择适合的OH自由基激励线，定量测量了甲烷-空气燃烧火焰的二维温度场分布。给出炉面中心上方火焰温度随离炉面高度的变化和距炉面12 mm高处沿炉面水平方向变化的实验测量结果并进行了讨论与分析。与利用相干反斯托克斯喇曼散射（CARS）技术进行测温的实验结果相比，该测量的相对不确定度优于5％。     

Simultaneous high speed PIV and CH PLIF using R-branch excitation in the C2Σ+-X2Π (0,0) band

Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690?nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which allows for high-framerate imaging with low laser pulse energy. Most importantly, the simultaneous imaging of both CH PLIF and PIV is enabled by the use of a custom edge filter, which blocks scattering at the laser wavelength (below ～311?nm) while efficiently transmitting fluorescence at longer wavelengths. The Hi-Pilot Bunsen burner operated with a turbulent Reynolds number of 7900 was used to demonstrate simultaneous PIV and CH PLIF utilizing this filtered detection scheme. Instances where pockets of products were observed well upstream of the mean flame brush are found to be the result of out-of-plane motion of the flame sheet. Such instances can lead to ambiguous results when interpreting the thickness of reaction layers. However, the temporally resolved nature of the present diagnostics facilitate the identification and proper treatment of such situations. The strategy demonstrated here can yield important information in the study of turbulent flames by providing temporally resolved flame dynamics in terms of flame sheet visualization and velocity fields.     

Understanding in-cylinder soot reduction in the use of high pressure fuel injection in a small-bore diesel engine

This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.     

激波与椭圆形重气柱相互作用的PLIF实验

在水平激波管中,采用平面激光诱发荧光(planar laser-induced fluorescence, PLIF)方法对椭圆形重气柱界面的Richtmyer-Meshkov不稳定性进行实验。气柱由SF₆混入一定比例的丙酮蒸气构成,环境气体为空气。通过改变椭圆形气柱的长短轴比值,得到了激波马赫数为1.25时,3种初始界面的演化形态。通过相对体积分数标定,得到了界面失稳演化过程中的相对体积分数分布,观察到了激波作用后界面气体聚集、转移、消散等现象。实验结果发现,对于流向轴长与展向轴长之比较大的气柱界面,初始界面产生的涡量更大且分布更广,其界面不稳定性发展得越迅速和剧烈。失稳发展迅速的界面甚至出现涡对碰撞并产生尾部射流结构的现象。初始界面直接决定了失稳发展初期形成的涡对强度和间距,并对后期演化有重要影响。     

The effect of total reflection in PLIF imaging of annular thin films

Planar laser-induced fluorescence (PLIF) imaging techniques are increasingly used to study wavy annular flows in vertical and horizontal pipes. We present a ray-tracing study that analyzes the imaging properties of the PLIF technique for uniform and wavy annular films. Part of the emitted fluorescence will hit the water–air interface at angles greater than the critical angle of total reflection. As a consequence, PLIF imaging probes the film twice: From bottom to top and then again from top to bottom. Deflected fluorescence widens PLIF images of uniform films by about 30%. Surface waves further increase the contribution of deflected rays to the overall signal. At the steep slopes of large amplitude axial waves, total reflection might widen PLIF signals by up to a factor of two.     

扩散型气柱界面R-M失稳中混合率的实验研究

Richtmyer-Meshkov(R-M)不稳定性普遍存在于众多工程问题中,激波管实验是研究R-M失稳问题的主要手段.高精度的平面激光诱导荧光(planar laser-induced fluorescence,PLIF)技术具有分子量级的示踪能力,可获得界面气体浓度(摩尔分数)分布,为研究界面失稳混合问题提供了有力工具.在弱激波(Ma=1.25)冲击扩散型气柱界面实验中,采用PLIF技术对R-M失稳引起的SF6-Air界面混合问题进行了研究.通过改变椭圆形初始界面的长短轴比,得到了3种扩散型初始界面失稳演化过程中气体摩尔分数,观察到了斜压机制下界面的简单拉伸、二次不稳定性、挤压射流等现象.利用浓度分布进一步得到了界面的瞬时混合率,通过瞬时混合率、界面整体平均混合率以及混合率的概率密度分布,分析了界面在不同演化阶段的界面混合特征,初步讨论了界面失稳混合的机制.演化初期,界面在斜压涡的作用下发生拉伸卷曲,通过增大浓度梯度来促进界面的混合.当演化进一步发展,二次不稳定性出现后,界面通过小尺度对流的方式达到湍流混合状态,而浓度梯度驱使的分子间混合逐渐减弱.由浓度梯度引起的扩散与由二次不稳定性引起的对流存在着"竞争"关系,二者共同主导了界面的混合.