Steady RANS modelling of transitional ventilation room air flow

Forced mixing ventilation is a commonly used ventilation principle in which air is forced into the upper part of the room at relatively high speeds. Attachment of the wall jet to the ceiling, also known as the 'Coanda effect', is used to ensure the air does not enter the occupant zone too early, thus preventing discomfort of the room occupants. Most mixing ventilation studies in the past have been conducted for wall jets with slot Reynolds numbers (Reslot) that are considered to be in the turbulent regime, while a transitional flow regime can be present for low Reslot values. However, previous Computational Fluid Dynamics (CFD) studies have indicated possible deficiencies of the commonly used Reynolds-Averaged Navier-Stokes (RANS) equations in combination with a turbulence model to provide closure, when applied for transitional flows. This paper presents a numerical analysis of forced mixing ventilation at a transitional slot Reynolds number (Reslot ≈ 1,000). CFD simulations of transitional wall jets in a confined space are conducted, using steady-state RANS modelling with difference turbulence models to provide closure. The results of the RANS simulations are compared with Particle Image Velocimetry (PIV) measurements in a reduced-scale model to assess the capability of the models to predict the transitional flow pattern. In addition, the ventilation efficiency is determined, demonstrating differences up to 36% in the calculated air exchange efficiency using different turbulence models.