Design masterclass 3 Air movement
Now let’s look at laminar flows. When a heat source is
introduced into a room, we observe natural convection as air around the source is warmed, becomes buoyant and rises. Air is drawn in from the surroundings to replace the air displaced by convection. This creates a laminar plume which, following the same laws for the conservation of momentum, accelerates as it rises and contracts, but does not involve any turbulent mixing with the room air (Figure 2). This is the source of room air movement in a displacement ventilation system. However, the method of introducing the supply air
is critical to ensure that the displacement effect works correctly. In many instances the grilles chosen for displacement systems are too small, and thus introduce the air with a pressure difference creating a turbulent
The method of introducing
the supply air is critical to ensure that the displacement effect works correctly
jet. Similarly, if supply air is too cold in relation to the room air, it is possible to create a buoyant plume, again creating turbulent mixing and upsetting the laminar flows. The choice between mixing and displacement
ventilation and the location of supply air grilles can depend on a large number of variables. So having a brief understanding of the nature of turbulent and laminar flows allows us to quickly assess the likely room conditions from a proposed ventilation solution. This saves us having to know the final grille selections in order to use manufacturer’s nomograms; nor do we need to spend time on complex computational fluid dynamics (CFD) analysis in the early stages of a design. l © Doug King 2010
Doug King is principal of King Shaw Associates and Visiting Professor of Building Physics at Bath University
Figure 3: A jet at a different temperature to the room air is subject to gravity: its path is determined by its buoyancy. The ceiling effect is often used to overcome the negative buoyancy of cold supply air
DESIGN MASTERCLASSES IN CIBSE JOURNAL The first two Design Masterclasses in this series were published in the June and August editions of CIBSE Journal and can be accessed online at
www.cibsejournal.com
CMR AIR MANAGEMENT SYSTEMS
Sizes 80 - 100 - 160 - 200 - 250 -315 - 400 Operating Pressure from 50 to 750 Pa Accuracy and repeatability within 3% Response time instantly
Field adjustable set point or motorised Air dampening shock absorber standard No hunting of the damper action
Fast acting constant volume valves
Rubber seal push in fitting for easy installation PRECISION COMPONENTS FOR VENTILATION AND PROCESS AIR CONTROL
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www.cibsejournal.com October 2010 CIBSE Journal 65
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