Three-dimensional temperature and velocity measurements in fluids using thermographic phosphor tracer particles

湍流本质上是三维的。在过去的十年中，层析粒子图像测速仪(PIV)的发展使得三维速度测量成为可能，从而促进了对湍流结构的理解的巨大进步。在许多湍流传热过程中，无论是自然发生的(例如海洋、大气或地幔中的自然对流)，还是为了提高设备(例如燃气轮机和电子电路)的效率或可靠性而诱导的，仅有速度场的知识不足以明确地描述流动，同时进行温度测量是非常必要的。该项目提出了一种新的概念来同时测量三维温度和速度，该概念是基于热成像荧光粉颗粒和基于三维颗粒的测速技术相结合的。与基于从多个视角重建体积信号的三维标量测量概念不同，这里探测的是单个微米尺寸的热成像荧光粉颗粒的温度。无论是三角剖分还是层析-PIV重建，都可以准确地确定粒子的位置，从而获得三维的温度场。这一概念允许高空间分辨率，只需增加两个成像传感器和一个紫外光激光器来激发粒子并形成其发光的光谱过滤图像，以进行基于比率的测温。此外，这一附加视图具有较短的景深，可用于通过减少鬼粒子的数量来提高速度测量的质量。在这个项目中，我们将使用申请者实验室已有的激光和摄像设备设置一个6摄像头系统，并结合厚光片(~7-10 mm)。初始测量将在湍流加热射流中进行。由于该标准测试用例具有明确定义的等温区，因此可以用它来评估温度精度方面的测量性能，并检测潜在的定位误差。首先，将开发用于低粒子图像密度(每像素0.005个粒子)的成像工具，使用三角测量进行粒子定位，并使用简单的针孔投影来分配发光信号。基于层析重建算法的更高粒子图像密度的测量方法，以及更精确的成像模型将被开发出来。作为演示，这种三维温度和速度诊断将应用于测量加热圆柱体后面的尾迹，提供等温和等涡度表面的同时可视化，并展示这种测量对于理解复杂的三维传热现象的重要性。这类研究对于从根本上理解自然对流或改进工业冷却设备都是至关重要的。

Turbulent flows are inherently 3-dimensional. Over the past decade, the development of Tomographic Particle Image Velocimetry (PIV) has enabled three-dimensional velocity measurements, thereby facilitating tremendous progress in the understanding of turbulent flow structures. In many turbulent heat transfer processes, whether naturally occurring (e.g., natural convection in the ocean, atmosphere or mantle) or induced to improve the efficiency or reliability of devices (e.g., gas turbines and electronic circuits), knowledge of the velocity field alone is insufficient to unambiguously describe the flow, and simultaneous temperature measurements are highly desirable. This project proposes a novel concept for simultaneous three-dimensional temperature and velocity measurements based on combining thermographic phosphor particles with 3-dimensional particle-based velocimetry techniques. Unlike the three-dimensional scalar measurement concept based on tomographic reconstruction of volumetric signals from multiple views, here the temperature of individual micron-size thermographic phosphor particles is probed. Particle locations can be accurately determined from eithertriangulation or Tomographic-PIV reconstruction so that a 3-dimensional temperature field will be obtained. This concept allows high spatial resolution and only requires the addition of two imaging sensors and a UV laser to excite the particles and form spectrally filtered images of their luminescence for ratio-based thermometry. Furthermore, this additional view has a short depth of field that can be used to enhance the quality of the velocity measurements by decreasing the amount of ghost particles. In this project we will set-up a 6-camera system in combination with thick light sheets (~7-10 mm) using laser and camera equipment already available at the applicants lab. Initial measurements will be performed in a turbulent heated jet. Since this standard test case has well-defined isothermal regions, it can be used to assess the measurement performance in terms of temperature precision and detect potential positioning errors. First, imaging tools for low particle image densities (0.005 particles per pixels) will be developed using triangulation for particle positioning, and simple pinhole projections for luminescence signal assignment. Methods for measurements at higher particle image densities based on tomographic reconstruction algorithm, and more accurate imaging models will then be developed. As a demonstration, this 3D temperature and velocity diagnostic will then be applied to measure the wake behind a heated cylinder, providing the simultaneous visualisation of isothermal and iso-vorticity surfaces, and demonstrating the importance of such measurements for the understanding of complex 3-dimensional heat transfer phenomena. Such investigations are crucial, e.g. for the fundamental understanding of natural convection, or to the improvement of industrial cooling devices.