Abstr:Air flows in an open plan air conditioned office
Application Area 4: Civil Construction & HVAC
Application Challenge AC4-03
This application challenge concerns the assessment of a ventilation system for an open plan office. The office is situated next to the docks at Cardiff Bay and was designed in 1994-1997 by Arup. The system is designed to maintain air temperatures within the office within specified limits. There are a variety of possible heating, ventilation and cooling systems that can be applied. In the specific case described here the system injects cooler air through diffusers mounted in the floor. The cool air is then mixed efficiently, to achieve uniform cool temperatures across the space, and yet generates low air speeds, so as to avoid drafts. The Cardiff Bay system supplies air as low level jets through swirl diffusers. The aim of the simulation is to establish how the temperature and flow velocities within the office vary with the supplied flow rate.
At the design stage, 3D CFD simulations were performed using temperature boundary conditions derived from Arup software, described below. Secondly, the air flow and temperature distributions were measured in the completed office. Thirdly, further 3D simulations were performed using the measured surface temperatures as boundary conditions.
Expectations and standards for internal environment of buildings continue to increase. In addition, there is pressure to evaluate the performance of buildings at the design stage. Whilst many heating and ventilation systems can be designed using established engineering methods, novel heating and ventilation systems, and systems within high-prestige buildings require more careful analysis at the design stage. Here CFD is becoming a tool of increasing importance.
There are two primary generic types of ventilation system. Firstly, mixing ventilation systems inject air into the space, usually above head height, with relatively high momentum Secondly, displacement ventilation systems supply air at low level and with low momentum and then the natural sources of heat generate buoyant plumes, which carry heat and pollutants to high level and out through vents in the ceiling. This approach leads to temperature stratification within the space. The case chosen for this application challenge is challenging because it is a combination of the two approaches in that the velocity of the supply air is higher than in a typical displacement system.
The competency of the CFD in this Application Challenge is judged by the ability to model the temperature within the occupied zone (usually defined as up to 1.8m above floor level) given the value of the supply flow rate. Other important design and assessment parameters include air speed, air quality (which is usually judged through age of air), radiant temperature and air moisture content; these quantities are not considered by this study.
Contributors: Isabelle Lavedrine; Darren Woolf; Stephen Belcher - Arup