Analysis of the interfacial mixing in the gravity-driven counterflow through a large vertical opening using Large Eddy Simulation

The study of natural convection flows in multizone enclosures is a topic of great importance due to its direct influence on room air circulation patterns, distribution of indoor air contaminants, and thermal comfort inside buildings. In this research, ANSYS Fluent is used to investigate the density-driven bidirectional flow through a large vertical opening connecting two isothermal reservoirs, the so-called bulk flow regime. Many research works on the natural convection flow through a large vertical opening between two enclosures have been done. However, they paid less attention to the unsteady flow structures generated by the sophisticated bidirectional flow, especially in the middle of the doorway. Large Eddy Simulation (LES) results show the development of unsteady flow structures in the middle of the doorway, a phenomenon called “interfacial mixing”. This phenomenon has been hardly documented in the literature, only using experiments. Even though unsteady flow structures develop, they do not significantly affect bulk quantities such as the discharge coefficient (Cd). Introduction The behavior of natural convection in confined spaces has attracted a lot of attention originating from its application in buildings air conditioning, contaminant spread and electronic equipment cooling. The vast majority of these spaces include internal partitions that the flow can traverse partially. In most situations, the presence of internal partitions with an opening lead to an intrinsically three-dimensional and transient flow field. The characteristics of such complicated flow are of strong interest to many communities and researchers. Several parameters can have an important role in driving this flow such as temperature differences, door motion, occupant motion or wind dynamic pressure. In past investigations, Computational Fluid Dynamics (CFD) simulations of the natural convection flow through a large vertical opening between two enclosures have been performed, but mostly using RANS, and/or two-dimensional assumption, and/or what can be considered today as very coarse grids. This made the unsteady flow structures impossible to be captured by these CFD simulations. From the experimental side, several past studies reported on the flows in differently heated enclosures separated by a partition wall. While these studies analyzed the flow inside these enclosures, the flow within the doorway itself was measured for a limited number of locations. However, the CFD standards of today enable to better investigate the unsteady nature of the bidirectional flow inside large vertical openings. Two separate mechanisms for natural convection flow between a hot and a cold zone were investigated in the experimental study of Scott et al. [1]: the boundary layer regime and the bulk density-driven regime. The transition between both regimes was established as a function of the aperture size located in the middle of the partition wall. In another similar work, the effect of internal partitions on the convective heat transfer across an enclosure was described by Neymark et al. [2]. They developed a Nusselt-Rayleigh correlation using the aperture width and a resistance model. Georges et al. [3] conducted RANS simulations using the RNG k − ε model and ANSYS Fluent for two rooms with an open door in the partition wall. They proved that the thermal radiation could affect the transition between the two aforementioned regimes and opposed to previous reports claiming that the aperture size was the main driving parameter causing such transition. Favarolo and Manz [4] employed the LVEL k − ε turbulence model [5] in the FLOVENT commercial CFD software to analyze the influence of different parameters such as temperature difference between indoor and outdoor air on the airflow rate through a large open window. The vertical position and the horizontal distance of the opening from the wall were diagnosed to have the greatest and minor impact on the discharge coefficient (Cd), respectively. Based on their literature review, the value of Cd varies between 0.3 and 0.8, and the origin of these large variations is not clear to the authors. Allard and Utsumi [6] mentioned that this phenomenological coefficient takes into consideration the contraction of the flow while it passes through the opening. They concluded that the definition of Cd is still ambiguous and requires more precision. Pelletret et al. [7] investigate various approaches to determine the Cd as a function of opening height or temperature difference. They also show the difficulty in determining the Cd and point out that a variety of definitions for this coefficient has been introduced in the literature. Hence, despite the significant amount of numerical and experimental studies conducted to date, no clear conclusion can be drawn from previous studies. BuildSim-Nordic 2020