Design masterclass 3 Air movement Heady mix Masterclass Professor Doug King
In his third masterclass, Professor Doug King looks at some of the mechanics of room air movement
W
hen we are designing ventilation or air conditioning systems, it is important to find an efficient means of delivering fresh or conditioned air to the room, and,
ensuring even distribution without causing discomfort due to draughts or temperature fluctuations. There are two principal means of air delivery: mixing ventilation and displacement ventilation. Mixing ventilation, as its name suggests, aims to
achieve a mixed condition so that the air quality and temperature is uniform throughout the room. This is accomplished by introducing the supply air with sufficient momentum to stir up the air in the room. The supply air becomes thoroughly mixed with the room air within a short distance from the supply grille, which means that we can use greater temperature differentials in order to deliver heating or cooling into the space. Displacement ventilation uses the natural convection
generated by heat sources within the room to create the air movement without mixing. The supply air is introduced at close to the desired room condition, directly to the occupied zone, whilst overheated air convects away for extraction at ceiling level. It is worth understanding the mechanics of room
air movement, as this allows us to quickly assess the likely success of ventilation systems without needing to resort to in-depth analysis. Essentially, we need to know whether our air movement is turbulent, creating mixing, or laminar, when simple convection will dominate. When air is introduced through an orifice from a
high-pressure zone into a low-pressure zone, as in any mechanical ventilation system, it forms a characteristic flow pattern, known as a jet (see Figure 1). At the edges of the jet, the moving air interacts with the stationary room air, creating a ring of vortices around the laminar
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core of the jet. In this turbulent boundary some of the room air is entrained into the jet and dragged along with it. This causes the jet to expand conically but, as the mass of moving air increases, the velocity decreases in order to conserve momentum. After some distance the turbulence penetrates the core of the jet and the jet velocity decays rapidly with further distance. Empirically it has been determined that the angle of expansion of a jet is constant at 11.8C. Now, due to the conservation of momentum, the mean jet velocity at a distance from the orifice is inversely proportional to the mass of air it contains, which is proportional to its cross-sectional area.
Turbulent flows dominate most ventilation scenarios. Here a buoyant plume is created when air rising by stack effect expands into a wider space above the balcony rail. The smoke reveals the turbulence at the boundary layer
> October 2010 CIBSE Journal 63
All images and diagrams courtesty of Doug King
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