Numerical sensitivity study of transient surface convection during night cooling

The energy performance of night cooling depends largely on the convective heat exchange between thermal mass and room air. This is assessed with BES models, although they have an inaccurate treatment of surface convection by using surface-averaged values for the convective heat transfer coefficient, assuming perfectly mixed air as well as simplified dynamical behavior. Therefore, a clear insight in the local and transient behavior of the air–mass system during night ventilation is essential. This is studied here through transient 2D CFD simulations including conjugate heat transfer. A discussion of the transient flow development and resulting surface convection of one case is combined with a sensitivity study. This study analyzes the influence of eight modeling choices, namely discretization, near wall treatment, simulation time step, turbulence model, conjugate heat transfer, radiation, inlet and outlet location and geometry. It was found that the convective surface heat flux during night cooling can vary greatly over time. This transient behavior is therefore of great importance for the simulation of buildings with night ventilation, as well as for the design of night ventilation systems. The results are nuanced by the influence of different modeling choices on flow development.

► Surface convection in a night ventilated room is simulated with 2D unsteady CFD.
► The flow development shows three phases from natural to forced convection flow.
► The surface convection at the ceiling varies strongly over the simulated time.
► Its sensitivity to modeling choices was investigated, resulting in 10–15%.
► Modeling of surface convection in BES models must be revised for night ventilation.