Development and validation of radiation-coupled multimodal heat flux boundary condition for incompressible buoyant internal flows in OpenFOAM

Abstract

This work deals with the benchmarking of newly implemented thermal boundary conditions for the incompressible Boussinesq heat transfer solver in OpenFOAM. Internal fluid flows for building physics purposes can be accurately modelled with the incompressible Boussinesq approach. In order to accurately model thermal heat transfer phenomena using CFD more flexible Neumann and Robin thermal boundary condition are required which do not exist in the standard package of OpenFOAM for incompressible Boussinesq solvers. A new boundary condition is derived from other compressible solvers in OpenFOAM and adapted for use with the Boussinesq approximation. This new boundary condition is benchmarked against the commercial software ANSYS CFX by means of an assessment of the mean air temperature and the mean velocities as well as the mean radiant temperature. A simple L-shaped room case is used to compare results between OpenFOAM and ANSYS CFX. The mesh used is for both softwares identical. Six different scenarios are investigated by varying the adjacent wall temperatures (Robin boundary condition). For one scenario, the Neumann boundary condition is also applied. Furthermore, radiation coupling is integrated into the new boundary condition and included in the model. The radiation models view factors (viewFactor) and discrete ordinate method (fvDOM) in OpenFOAM are compared. The comparison the two softwares show very good agreement for air and mean radiant temperatures with a maximum of 0.25 K deviation. However, the CFX results show consistently a more pronounced fluid flow within the room. The results between fvDOM and viewFactor within OpenFOAM are quite similar to each other and furthermore match well with the results obtained in CFX.