Volumetric Lagrangian Particle Velocity And Temperature Measurements In Large Aspect Ratio Rayleigh-Bénard Convection

Abstract

Rayleigh-Bénard convection (RBC) is a well-known model experiment for temperature-driven flows. Even though RBC has been studied for decades, many open questions remain. Temporally and spatially resolved temperature and velocity data are crucial for a better understanding of the underlying physics. This paper proposes a new approach that combines Lagrangian Particle Tracking (LPT) with Particle Image Thermometry (PIT). The combined measurement techniques should be utilized to investigate the flow inside a large aspect ratio RBC cell with a lateral size of w = 700 mm and a height h = 28 mm, which corresponds to an aspect ratio of 25. Due to the large aspect ratio, the cameras observe the flow through the transparent cooling plate. We show that LPT of thermal convection flows within the setup can be performed successfully. Subsequently, we describe the temperature measurement technique PIT based on Thermochromic Liquid Crystals (TLCs) and discuss and quantify the additional challenge arising from color imaging of small particles. Namely, the sparse color sampling of a color filter array (CFA) camera and chromatic dispersion resulting from the oblique viewing angle of the color camera. To circumvent the sparse sampling of the CFA camera, we use a three-chip camera that resolves each color (red, green, blue) at full sensor resolution. We quantify the influence of the chromatic dispersion by cross-correlating the color channels. We then apply the resulting disparity maps in a correcting step to reduce the disparities by shifting the red and blue towards the green channel. The visual comparison of the corrected and uncorrected images shows a reduction in the color dispersion. Beyond that, the remaining disparities between the colors of the corrected images are minimal. This indicates that the correction step is useful to increase the quality of the particle images for temperature estimation.