Аннотация
Interest in natural convection in vertical channels has significantly increased over the last
several years because of its application in solar energy systems. In natural convective flows,
complex coherent structures develop whose the role in heat and mass transfer are not well
understood. An experimental and numerical investigation programme was therefore undertaken
to investigate the behaviour of such flows. The numerical study is based on Large-Eddy-
Simulations of a vertical channel with one side uniformly heated and subjected to random
velocity fluctuations at the inlet. Different stages of transitional flow development were identified
numerically with two characteristic frequency bands being observed in the flow, near the heated
wall. Spectral Proper Orthogonal Decomposition, a method derived from the Proper Orthogonal
Decomposition (POD) was also used and shown to be a powerful tool which allows the most
energetic modes to be separated accordingly to two characteristic frequency bands found
numerically. As result, the contribution of the two families of modes to the near wall turbulent
heat transfer and velocity-temperature correlation has been evaluated. Finally, the modes were
linked to coherent structures that are observed in instantaneous visualizations of the flow. POD
was also performed on experimental measurements showing similarities with the numerically
observed structures.
From past experimental studies of similar configurations it was found that large differences
in the experimental velocities often occurred for apparently the same conditions. In this work
variations of the external thermal stratification have been identified as one possible source of
these differences. The influence of external thermal gradients was investigated experimentally
and numerically. It is shown that the increase in the positive gradient of the external stratification
not only decreases the mass flow rate but also displaces the transition height to a lower location in
the channel. As a consequence, as the positive upwards external thermal stratification increases,
the flow evolves from a laminar flow to turbulent flow despite the reduction in mass flow rate.
Numerical simulations also allow the study of cases of weak and negative thermal stratifications
which are difficult to achieve in laboratories.
A theoretical model of the influence of the external thermal stratification on the mass flow
rate was also developed. There is an excellent agreement between the theoretical predictions
and the experimentally and numerically obtained mass flow rates. This clearly highlights
that external temperature dis
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