Investigation of thermal boundary layer dynamics in turbulent liquid metal convection by ultrasound localization microscopy of near-wall velocity fields and temperature measurements

通过近壁速度场和温度测量的超声定位显微镜研究湍流液态金属对流中的热边界层动力学

• 批准号: 512483557
• 负责人: Dr. Lars Büttner
• 金额: --
• 依托单位: Professur für Mess- und Sensorsystemtechnik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/512483557?language=en
• 关键词: Investigation; thermal; boundary; layer; dynamics

The dynamics and interaction of thermal and viscous boundary layers (BL’s) will be studied experimentally in highly turbulent liquid metal convection at small Prandtl numbers (Pr) using the ternary alloy GaInSn (Pr = 0.03). Rayleigh-Bénard convection at large Rayleigh numbers (Ra) of up to Ra ≅ 5x10e9 is characterized by a fully turbulent flow field, with the temperature field exhibiting significantly more coherence than the velocity field due to the high thermal diffusivity. A crucial role for heat transport in turbulent convection is played by the BL’s. Here, a special feature of liquid metals becomes apparent, which has hardly been researched so far: The much thinner viscous boundary layer is embedded in the thermal BL. Therefore, the thermal BL and thus the convective heat transport are strongly influenced by the turbulent large-scale convection (LSC). The parameter range considered here has so far been inaccessible by direct numerical simulations. To investigate the interaction between BL’s and LSC in detail for the first time in liquid metal laboratory experiments, the study involves dedicated investigations on novel measurement methods. Near-wall velocities shall be measured by means of Ultrasound Localization Microscopy (ULM). To apply this super-resolution method, investigations on adaptive nonlinear beamforming techniques to locate particle movements in the micrometer-range will be conducted. Furthermore, a compound imaging by two opposing phased array probes to maintain sufficient spatial resolution throughout the full measurement depth will be investigated. Both methods are novel applications in ultrasound imaging. For obtaining spatially resolved temperature data in the melt, fiber optic sensor measurements will be applied. The application of fiber optic bragg grating is a novel method to obtain temperature fields in liquid metal flows. The experiments, in which near-wall temperatures and flow velocities are measured in turbulent convection with high resolution, set a new milestone for the understanding of convective transport processes in fluids at small Pr with their numerous applications in geo- and astrophysical flows as well as in engineering systems.

本文将利用三元合金GaInSn （Pr = 0.03）实验研究小普朗特数（Pr）下高湍流液态金属对流中热边界层和粘性边界层的动力学和相互作用。在Ra = 5 × 10e9的大瑞利数（Ra）下，对流具有完全紊流的特征，由于热扩散率高，温度场比速度场具有明显的相干性。边界层在湍流对流的热传递中起着至关重要的作用。在这里，液态金属的一个特殊特征变得明显，这是迄今为止很少研究的：热边界层中嵌入了更薄的粘性边界层，因此热边界层以及对流热输运受到湍流大尺度对流（LSC）的强烈影响。这里考虑的参数范围到目前为止还无法通过直接的数值模拟得到。为了首次在液态金属实验室实验中详细研究BL和LSC之间的相互作用，本研究包括对新的测量方法的专门研究。近壁速度应通过超声定位显微镜（ULM）测量。为了应用这种超分辨率方法，将研究自适应非线性波束形成技术，以定位微米范围内的粒子运动。此外，将研究两个相对相控阵探头在整个测量深度保持足够的空间分辨率的复合成像。这两种方法都是超声成像的新应用。为了获得熔体中空间分辨的温度数据，将采用光纤传感器测量。光纤光栅的应用是一种获取液态金属流动温度场的新方法。这些实验以高分辨率测量了湍流对流中的近壁温度和流速，为理解小Pr下流体中的对流输运过程树立了新的里程碑，并在地球和天体物理流动以及工程系统中得到了广泛应用。