Paper Conference

Proceedings of BauSim Conference 2008: 2nd Conference of IBPSA-Germany and Austria


Geeignete Randbedingungen an vertikalen Flächen zur Kopplung von thermischer Gebäudesimulaltion mit CFD

Stefan Barp, Kögler René Kögler René

Abstract: It often makes sense to determine tbe local comfort parameter in a room by computational fluid dynamics (CFD). In a winter case the boundary conditions can be determined with a simple stationary energy balance. To take account of thermal storage effects and of the solar radiation in the summer case, often a thermal building simulation has to be performed for the determination of the boundary conditions. The boundary values of a certain time step are then used as input for the CFD simulation. This presentation shows that in case of using commercial CFD code with a common grid resolution (Low-Re models not feasible for practical applications), either the wall heat flux or the U value of the construction with the outside air temperature should be specified as boundary condition for vertical surfaces with natural convection. This way one has the energy balance of the room, as well as the resulting temperature level under control. By specifying the surface temperature, hence by coupling the CFD simulation and the thermal building simulation with the surface temperatures, one has neither the energy balance nor the resulting temperature level under control. The reason for that is, most commercial CFD codes use the same models for the simulation of natural as well as forced convection at vertical surfaces. However, this is not correct, since the near-wall profiles of velocity and temperature are completely different for forced and natural convection (Yuan, 1995). By specification of the surface temperature, the resulting wall heat flux and therefore the total heat exchange between the wall and the room are highly dependent on the first grid cell size (Kögler, 2007).
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