Case Study – Swirl Diffuser CFD Model

Swirl diffusers in floor of lecture theatre

Swirl diffusers in the floor of the lecture theatre

Waterloo WFO swirl diffuser

Computational domain of the swirl diffuser CFD model

The challenge

Atkinson Science is creating a CFD model to calculate the temperature in a banked lecture theatre. The theatre will be ventilated by Waterloo WFO swirl diffusers mounted in the floor beneath the seats. The lower boundary of the model will include the floor and the exit plane of the diffusers. To calculate the flow generated by the diffusers, we must specify all three components of the air velocity in the exit plane of the diffusers.

The solution

By choosing floor mounted swirl diffusers, the building services engineer has decided to use displacement ventilation to create a comfortable environment in the theatre. In this type of ventilation, air enters the occupied space through terminals in the floor and spreads across the floor to form a reservoir of fresh, cool air. Any heat sources in the space will generate a plume of warm air rising upwards, and the fresh, cool air will flow upwards to replace the warm air. The warm air rises to a high level where it is removed. The cool air does not mix with the warm air but simply replaces it.

To provide the velocity components needed as boundary conditions in the theatre model, Atkinson Science created a separate CFD model of the Waterloo WFO swirl diffuser. A picture of the diffuser is shown opposite. The swirl plate lies flush with the carpeting on the theatre floor and most of the assembly is hidden beneath the floor. One can see that the pattern of slots in the swirl plate repeats itself every 30°. Consequently, we only need to model a 30° sector of the diffuser assembly and to ensure that the flow entering one side of the sector is the same as the flow leaving the other side by imposing periodic boundary conditions. The computational domain of the model is shown opposite. Solid surfaces are shown in blue, the boundary across which air enters the domain is shown in yellow, and the boundary across which air leaves the domain is shown in red. All other boundaries have periodicity enforced on them.

To show how the air spreads outwards from the swirl plate, we seeded the air entering the model with smoke. The smoke pattern shown below was created by duplicating the CFD solution eleven times and adding all twelve solutions together. One can see that the smoke, and thus the air from the diffuser, spreads out along the floor like the spokes in a bicycle wheel. The CFD model gave us the three components of air velocity in the exit plane of the diffuser, which we then specified at the locations of the swirl diffusers in the theatre model.

The benefits

By creating a separate CFD model of a swirl diffuser, Atkinson Science were able to specify accurate boundary conditions for the swirl diffusers in the CFD model of the banked lecture theatre.

Read the case study of the banked lecture theatre.