Air Distribution in a Room with Ceiling-Mounted Diffusers: Comparison with Wall-Mounted Diffuser, Vertical Ventilation and Displacement Ventilation

Peter V. Nielsen, Thomas Heby, Bertil Moeller-Jensen

Research output: Contribution to journalJournal articleResearchpeer-review

12 Citations (Scopus)


Experiments with air distribution in rooms generated by a radial ceiling-mounted diffuser and a diffuser generating flow with swirl are compared with the air distribution obtained by mixing ventilation from a wall-mounted diffuser, vertical ventilation and displacement ventilation. The air distribution generated by a radial diffuser is partly controlled by the momentum flow from the diffusers and partly from gravity forces where the thermal load and the temperature difference between room air and supply air deflect the radial wall jet down into the occupied zone. The ceiling diffuser with swirling flow generates a flow pattern in the room which is rather uninfluenced by the thermal load. The flow is highly mixed above the occupied zone and the air movement penetrates the occupied zone close to the walls.

All systems are tested in the same room with the same heat load consisting of two manikins, each sitting at a desk, two pc's and two desk lamps producing a total heat load of 480 W.

The design of the air distribution system is in all five cases based on flow elements from the diffuser, a maximum velocity assumption and a critical vertical temperature gradient in the room. The characteristics of the air distribution systems are addressed by analysing the acceptable conditions for the supply flow rate and the temperature difference for the different systems.

The paper shows that an air distribution system with ceiling-mounted air terminal units is able to generate comfortable velocity and temperature conditions at the same and at slightly higher loads as can be obtained by a vertical ventilation system, a mixing ventilation system with wall-mounted diffuser and a displacement ventilation system with a low velocity wall-mounted diffuser.

The comparison is extended by considering both the local discomfort caused by draught rating and the percentage of dissatisfied due to the temperature gradient when this is relevant to the systems. The draught rating is low for the ceiling-mounted diffusers as well as for the low impulse system (textile terminals) and the temperature gradient is also low because of the high level of room air mixing.

Original languageEnglish
JournalASHRAE Transactions
Issue number2
Pages (from-to)498-504
Number of pages8
Publication statusPublished - 2006

Bibliographical note

Peter V. Nielsen, PhD, ASHRAE Fellow


  • Ventilation
  • Mixing ventilation
  • Displacement ventilation
  • Vertical ventilation

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