Experimental and numerical study of the influence of the plenum box on the airflow pattern generated by a swirl air diffuser

When CFD is employed to design a ventilation system, one of the most delicate aspects is the modeling of the diffuser. The designer has different choices, which range from simulating the detailed geometry of the diffuser, including the plenum box, to use special boundary conditions to reproduce the velocity profile in each slot of the diffuser. This study is carried out to examine the influence of the plenum box on the turbulent airflow pattern generated by a swirl air diffuser from the experimental and numerical points of view. First, we perform experiments to evaluate such influence. Then, these experiments are contrasted with two numerical approximations to the problem using the Fluent 6.3.26 commercial CFD software. We first perform a simulation of the complete diffuser geometry - including the plenum box - and later a simplified simulation of the diffuser, where the whole geometry is simplified to a plane in which the velocity components and the slots positioning are specified via a particular boundary condition. The main findings of this study reveal that the simplified simulation is good enough at close range, while in the remote flow field the differences between both approaches practically disappear.