| Summary: | 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 .
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