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，层析粒子图像测速技术（PIV）的发展使三维速度测量成为可能，从而促进了对湍流结构的理解取得了巨大进展。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）还是为了提高设备的效率或可靠性而诱导的（例如，燃气轮机和电子电路），仅了解速度场不足以明确地描述流动，同时进行温度测量是非常可取的。本课题提出了一种基于热成像荧光粉粒子与基于三维粒子的测速技术相结合的三维温度和速度同步测量的新概念。不同于基于多视角体积信号层析重建的三维标量测量概念，这里探测的是单个微米尺寸热成像荧光粉颗粒的温度。通过三角剖分或层析- piv重建，可以精确地确定粒子位置，从而获得三维温度场。这个概念允许高空间分辨率，只需要增加两个成像传感器和一个紫外激光器来激发粒子，并形成基于比例的测温光谱过滤图像。此外，这种额外的视图具有较短的景深，可以通过减少幽灵粒子的数量来提高速度测量的质量。在这个项目中，我们将使用申请人实验室已有的激光和相机设备，建立一个6摄像头系统，结合厚光片（~7-10毫米）。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域，因此可用于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进行发光信号分配。基于层析重建算法的更高粒子图像密度测量方法，以及更精确的成像模型将被开发出来。作为演示，这种3D温度和速度诊断将应用于测量加热圆柱体后面的尾迹，同时提供等温和等涡量表面的可视化，并展示这种测量对于理解复杂的三维传热现象的重要性。这样的研究是至关重要的，例如，对自然对流的基本理解，或对工业冷却装置的改进。

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.