An experimental and numerical analysis of forced mixing ventilation at transitional slot Reynolds numbers

Forced mixing ventilation is a commonly used ventilation principle in which air is forced into the upper part of the room at relatively high speed. Attachment of the wall jet to the ceiling, also known as the ‘Coanda effect’, is used to ensure that the air does not enter the occupant zone too early, thus preventing discomfort for the room occupants. Most mixing ventilation studies in the past have been conducted for wall jets with slot Reynolds numbers that are considered to be in the turbulent regime, while transitional flows can be present for lower slot Reynolds numbers. Previous 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 the first results of an experimental and numerical study of forced mixing ventilation at transitional slot Reynolds numbers, with focus on assessing the accuracy of steady RANS in combination with four frequently used turbulence models in Computational Fluid Dynamics (CFD) for indoor airflow. Visualisations with fluorescent dye seem to indicate that for a slot height (h/L) of 0.1 the flow is transitional for slot Reynolds numbers from about 800 to 2,900. 3D steady-state RANS simulations in combination with four turbulence models have been compared with time-averaged PIV measurements, for h/L = 0.1 and a slot Reynolds number of 1,500. In general, the SST k-¿ model shows the best agreement with the PIV measurements, whereas the standard, realizable and RNG k-e models show rather large deviations, at least in this particular case. Further validation studies should be made to analyse the accuracy of the CFD simulations for other slot heights and other slot Reynolds numbers.