Infrared molecular tagging velocimetry
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摘要: 分子标记测速法(MTV)和粒子成像测速法(PIV)常被用于流动显示和流场成像测量，但在示踪粒子跟随性差、示踪粒子分布不均匀时，示踪粒子的引入会给PIV带来速度测量系统误差，而不需引入示踪粒子的MTV因荧光寿命长度限制，主要应用在高速和超声速流动测量中。为了发展无需示踪粒子、可适用于低速气流场的二维流速成像方法，本文介绍了一种基于激光诱导红外荧光的新型分子标记测速法，并在二氧化碳气体轴对称湍流射流中进行了速度测量与验证。在红外分子标记测速法中，通过红外脉冲激光选择性激发气体小分子的共振振动能级跃迁实现分子的标记，随后通过红外相机对不同时刻下跟随流场流动的激发态分子记录其发射荧光分布，进而处理得到流动速度场信息。通过考虑分子振动能量传递过程模型、有限荧光寿命、横向速度分量和分子扩散运动对荧光分布的影响，实现从荧光分布图像定量获取速度场分布。将该方法应用于5～51 m/s速度的二氧化碳湍流射流中，得到了射流轴向速度的径向分布，速度测量的相对不确定度优于8%，径向空间分辨率达到107 μm，且该速度分布与湍流射流理论结果及前人实验测量结果符合较好。利用该方法分辨了射流在不同轴向位置的径向速度分布，观察到了射流从势核区到充分发展区的演变。利用该方法可以获得低速气流场的高分辨瞬时速度成像，后续通过提高红外激光的脉冲功率、激发效率和重复频率，可以进一步提高该方法的测量精度、空间分辨率和时间分辨率，从而有潜力在近壁面流动、微尺度流动和大梯度流动等粒子示踪不适合用场景中提供定量流场测量的有效途径。Abstract: Molecular Tagging Velocimetry (MTV) and Particle Imaging Velocimetry (PIV) are often used for flow visualization and velocity field imaging. However, the requirement for tracer particles can bring systematic errors to the velocity measurement of PIV method when the tracer particles have poor followability and uneven distribution. In the case of MTV, although particle seeding is not required, the finite fluorescence lifetime of the tracer molecules typically constrains its use only in high-speed and supersonic flows. To develop a velocity field imaging method with no requirements of tracer particles and suitable for low-speed flows, a novel MTV method based on infrared (IR) laser-induced fluorescence is developed and verified in axisymmetric turbulent jet of carbon dioxide. Resonant vibrational transition of the small gas molecule is selectively excited by an infrared pulsed laser to achieve molecular tagging, and the fluorescence distributions of the excited molecules at different instants are then imaged by an infrared camera, from which the velocity distributions are deduced. The effects of the molecular vibrational energy transfer process model, finite fluorescence lifetime, lateral velocity component and molecular diffusion motion on the fluorescence distribution are analyzed to improve the accuracy of the velocity measurement. The proposed method has been successfully verified in carbon dioxide turbulent jets with velocities ranging from 5 m/s to 51 m/s, and the radial distribution of the axial velocity in the main region of the jet is measured. The radial spatial resolution can reach 107 microns, and the velocity distribution is consistent with the theoretical calculation of turbulent jets and previous experimental results. The relative uncertainty of velocity measurement is better than 8%. This method can be used to obtain high-resolution instantaneous velocity imaging of low-speed flow field. Subsequently, by improving the pulse power, excitation efficiency and repetition frequency of the infrared laser, the measurement accuracy, spatial resolution and temporal resolution of this method can be further improved. Therefore, the proposed method bears great potential to provide a quantitative velocity field imaging method in the near-wall flow, micro-scale flow and large gradient flow where it is difficult to introduce tracer particles.
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图 1 红外MTV中振动能量跃迁过程示意图
Figure 1. A schematic diagram of vibrational energy transition in infrared MTV
图 2 不同时刻的CO2荧光分布随流体运动的变化
Figure 2. The evolution of CO2 fluorescence distribution with fluid motion at different times
图 3 红外MTV测量实验装置与射流喷嘴装置
Figure 3. The experimental setup of infrared MTV and jet nozzle
图 4 激发光、相机时序图与曝光窗口示意图
Figure 4. Sequence diagram of excitation light and camera, and exposure window diagram
图 5 激光通过N2和CO2气体池后的透射光强
Figure 5. Transmitted laser intensity through N2 and CO2 gas cell
图 7 Re = 2500射流中不同轴向位置的红外MTV标记位置测量
Figure 7. Infrared MTV marked position measurement at different axial positions in jet at Re = 2500
图 8 Re = 2500射流中不同轴向位置的轴向速度径向分布测量结果
Figure 8. Measured radial distribution of axial velocity at different axial positions in jet at Re = 2500
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