Unsteady Natural Convection in a Square Cavity by using Holographic Visualization

• Main Authors: Sun, Shi-Queen, 孫士欽
• Other Authors: Sheng-Mao Tieng
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/73202941782964893396

碩士 === 國立成功大學 === 航空太空工程學系 === 85 === The natural convection in a square cavity , arising from instantaneous temperature changes in both vertical side-walls of the cavity , is a very important phenomenon because of its wide application in both natural world and the industry. Most of its application is concerned with the unsteady state , so we will investigate the cause of the closed isotherms appearing in the unsteady natural convection in a square cavity by using holographic interferometry , particle tracing method , and dye tracing method. The appearance of closed isotherms in unsteady natural convection in a square cavity has never been reported in the recently studies until the research by S.M. Tieng and K.H. Sun in 1992.It will result in a particular inverse temperature gradient distribution in the cavity. So the investigation on the cause of closed isotherms will be helpful to better understanding about the detailed process of the unsteady natural convection in a square cavity . It is found that both 3-D effect and the rising time of the side-wall temperature are not the reasons of the closed isotherms in our initial study . Next , we examine some factors which may appear at the same time when boundary condition is changed , such as the vibration effect owing to the entrance of water from external reservoirs into water-gap of model and the deformation of the optical windows of model due to thermal conductivity. It is found that these are also not the causes of the closed isotherms . According to further tests such as the different temperature difference imposed on the vertical side-walls , and only single side heated , we come to two conclusions: firstly , closed isotherms respond to the temperature changes . Secondly , they occur only when the boundary condition changes . Additionally , closed isotherms are not found when the working medium is replaced by air . We can also conclude that the closed isotherm is a physical phenomenon belonging to the unsteady natural convection in a square cavity itself . Besides , it is also found in the experiment that the appearance of closed isotherms seems to be affected by the parameters like Ra , Pr , or A , which is worth to further study .