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Coherent structures and impact of the external thermal stratification in a transitional natural convection vertical channel

Abstract : In natural convective flows, complex coherent structures develop whose the role in heat and mass transfer are not well understood. A numerical study, based on Large-Eddy-Simulations of a vertical channel with one side uniformly was therefore carried out. Different stages of transitional flow development were identified numerically with two characteristic frequency bands being observed in the flow, near the heated wall. Methods derived from the Proper Orthogonal Decomposition (POD) was also used and 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. POD was also performed on experimental measurements showing similarities with the numerically observed structures.In this work, variations of the external thermal stratification have been identified as one possible source of these differences and its influence was therefore 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. 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
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  • HAL Id : tel-02006828, version 1

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Martin Thebault. Coherent structures and impact of the external thermal stratification in a transitional natural convection vertical channel. Thermics [physics.class-ph]. Université de Lyon; University of New South Wales, 2018. English. ⟨NNT : 2018LYSE1327⟩. ⟨tel-02006828⟩

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