Air flows in an open plan air conditioned office

Application Challenge 4-03 © copyright ERCOFTAC 2004

Introduction

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.

Relevance to Industrial Sector

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.

Design or Assessment Parameters

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.

Figure 1: Geometry and dimensions of office space

Figure 1 shows the geometry and dimensions of the office space. The coordinate system is marked at the origin. (Below, when the CFD results are presented, details of the grid are also described, from which digital data for the geometry can be extracted.) The structural design comprises a steel frame with pre-cast concrete vaults. The external envelope is traditional brick, cavity and block construction. The building footprint is “L” shaped: it is 36m long, between 14 and 25m wide and 3.1m high. The radius of curvature of the front wall is 42m. The office is open plan with cellular work spaces. A notional corridor goes through the centre and the work spaces are either side of the corridor. Figure 2 shows the position of the inlets in the floor and the outlet slot in the roof. The location of the 46 inlets is defined in the file inlet_pos.dat (which lists the x and y coordinates of the centre of each inlet in metres). The extract was situated on the ceiling and was 34.2m long starting at x=0.9m, and 0.512m wide starting at y=6.4m. The total surface area of the inlets was 0.828m², and of the extract 17.5m². Furniture was not represented. Since the cooling system needs to be effective for a range of possible furniture layouts and the air measurement is predominantly upwards, the detailed positioning of furniture should not be important.

Figure 2. Position of inlet swirl diffusers marked in yellow and outlet slot marked in green

Flow Physics and Fluid Dynamics Data

The air within the space is stably stratified. The flow is driven by

• Mechanically forced flow through the floor-mounted swirl diffusers, which lead to momentum jets that mix with the surrounding stably stratified air.

• Buoyancy driven flow arising from the space heating due to internal loads, such as occupants and computers, which are distributed through the lowest 1.2m above the floor. This heated volume then mixes convectively with the stably stratified layer aloft.

The basic fluid dynamics parameter of the flow is the air change rate, ACR, defined to be the number of times the flow rate into the space replaces the volume of air within the space per hour. Hence, if Q is the volume flow rate through all the swirl diffusers (measured in $m^{3}s^{-1}$ , and V is the volume of the space, then ACR = Q * 60 * 60 / V. There are 46 swirl diffusers in the space and the volume of the space is approximately $550m^{3}$ . Simulations were run for ACR = 1.5, 2.0 and 3.0 per hour.

Across most of the room space, including the occupied zone, the mean velocities and turbulence levels are low, although they are higher near the swirl diffusers. The equation used to calculate the Reynolds number is ${\mbox{Re}}=UL/\nu$ . The relevant Reynolds number is based on the length and velocity scales of the swirl diffusers. With the ACR values given above, each swirl diffuser emits a jet of width of order 0.2m with velocity of order $2ms^{-1}$ . This yields Re of the order of 4000 when ACR = 2.0. Hence the swirl diffusers emit fully turbulent jets.