Non-Boussinesq experiments on natural ventilation in a 2D semi-confined enclosure

This paper presents an experimental study which aimed to quantify the transition between displacement and blocked regimes in a box naturally ventilated through a lower and an upper opening. It complements and extends the results obtained by Paranthoën & Gonzalez (IJHFF, 2010) with heated air in a similar box. The ventilation was here driven by a buoyant air/helium mixture which was continuously released through a slot at the bottom of the box. The slot and the openings were the same width as the box, so that the experiments can be considered as quasi-2D. The Richardson number Ri of the buoyant source (determined by the ratio between buoyancy and momentum) varied over two orders of magnitude (between 0.1 and 10) in our experiments. An intermediate regime defined by the occurrence of bidirectional flow at the base opening, with incoming fresh air and outgoing buoyant fluid, was observed. This intermediate regime can be predicted by a simple correlation based on the Richardson number of the source and the dimensions of opening areas. The full blocked regime (with no more incoming fresh air) was only obtained for large flow rates and/or weakly buoyant releases corresponding to Richardson numbers lower than about 2−3. We found a good agreement with the classical relation published by Woods, Caulfield and Phillips (JFM, 2003) provided that the density differences involved were not too high.