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Abstract
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 distributions are key driving factors and their influence is accurately quantified in this work.