Experimental analysis of natural convection in open joint ventilated façades with 2D PIV

The main particularity of Open Joint Ventilated Facades (OJVF) is that they have an exterior opaque coating separated from the mass wall by a ventilated air cavity. The exterior coating material is arranged in slabs separated by open joints that enable exterior air to enter and leave the cavity all along the wall. Under radiation conditions (and negligible wind velocity) the natural thermal convection produces a chimney effect that forces external air to circulate along the air cavity at an unknown rate. As a consequence of this mass exchange through the openings, the heat transfer problem turns more complex: air motion and thermal field are strongly coupled and therefore highly dependent on geometric characteristics of the wall. This article reports the application of Particle Image Velocimetry technique (PIV) to measure the velocity field inside the air cavity of an OJVF model in laboratory conditions. Measurements were performed for the vertical central plane of the cavity, for three different heating conditions corresponding to Ra = 5.92 × 108, Ra = 9.19 × 108 and Ra = 1.35 × 109, based in the channel height, and with a Re about 104. Detailed information of the flow behaviour inside the air cavity are presented and discussed. Special attention is paid to the ventilation effect through the joints.