The ‘family tree’ of air distribution systems
Department of Mechanical and Aerospace Engineering Seminar
Friday, Oct. 19, 2:15-3:10, Watson Theatre, Watson Hall
by Peter Vilhelm Hielsen
Aalborg University, Department of Civil Engineering, Sohngaardsholmsvej, Denmark
The interconnection between all air distributions principles are addressed in this presentation. It is known from the similarity principles that the air distribution in a room with fully developed flow can be described by the Archimedes number, the geometry of the room, the geometry of the supply and return openings and other boundary conditions as heat and cooling loads and surface temperatures. The Archimedes number can be considered as the ratio between gravity forces and momentum forces, so a full, and simplified, description of the air distribution in a room can be expressed by
where ∆To is temperature difference between return and supply, uo is supply velocity and qo is flow rate to the room.
The ratio between the total area ao of the supply openings and the wall area A (in which the supply openings are located) ao/A is an important parameter for the air distribution in the room. The momentum flow from the openings controls the air movement if ao/A is very small, while the plumes from the heat sources control the air movement
if ao/A is large. It is possible to obtain a unidirectional flow in the room if ao/A is equal to 1.0. A complete description can be given by:
• Cooling mode or heating mode
• Archimedes number ∆To/qo2, or flow rate of air supplied to the
room, qo and temperature difference between return and supply air,
• The ratio between the total area of the supply openings and the
wall area, ao/A
• Location, high or low, of the air supply opening(s)
The whole family of air distribution patterns can, in the case of cooling, be described in two three-dimensional charts, “family trees”: one for a high location of the supply opening and one for a low location of the air supply opening. The presentation will address the interconnection between the different systems; indicate the systems with possibility for new development. The discussion will be based on fluid dynamics principles, measurements made in the laboratory during a number of years, and CFD predictions.
The lecture will be finalized with a complete description of all systems in one scheme.
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