PIV measurement and simulation of turbulent thermal free convection over a small heat source in a large enclosed cavity

Publication date: August 2015
Source:Building and Environment, Volume 90
Author(s): Xiwen Zhang , Genghui Su , Jiajia Yu , Zhaohui Yao , Feng He
To investigate the mechanism of natural convection in an enclosed cavity with a small horizontal heated plate (200 × 300 mm²) in the middle of the bottom and a cool plate at the top, an enclosed cavity (1960 × 980 × 800 mm³) is established. The working fluid is air at atmospheric pressure, and the Rayleigh number is of the order of 10⁹. Temperature measurements were obtained on the centerline over the plate using thermocouples (d = 0.1 mm), and velocity fields of the central plane over the plate were measured with PIV. Through utilizing some techniques, including the background-elimination technique and overlapping-joint technique in the experiment, accurate experiment data have been established. Three turbulence models are adopted and compared to the experimental data. The result shows that the numerical result of the RNG k-ε model best fits the experiment data at the centerline of the cavity. Analysis of simulation results enabled us to understand that the period of cycle swing is about 60s. Detailed information was obtained by illustrating the iso-surfaces and elucidating how streamlines associate with those surfaces. Because of the border effect of a small heat source, the streamlines moved to the center of the cavity from four main border directions, rose to the top, and then spread to four corners and moved downward, finally forming a complete cycle. Ultimately, the analysis of buoyancy and inertia force helps us to achieve a better understanding of the nature of flow.