الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Cold air systems have proven to offer economical and environmental
benefits, and are found suitable for many applications. However, these systems
did not spread because of the fear from cold draft formation leading to
unaccepted indoor air conditions. The current study aims to evaluate the
performance of cold air systems when used in enclosed spaces using
Computational Fluid Dynamics (CFD).
A brief introduction is presented explaining theory, advantages, and
disadvantages of cold air systems. Also, different factors and design
considerations affecting human comfort are explained. A brief literature review
of previous work in the field of cold air distribution systems and indoor flow
simulations is presented as well.
The basic theory and governing equations of CFD are explained, and
different turbulence modeling techniques are presented. The CFD code used in
the current study is validated by comparing its results with experimental results
found in the literature. Three validation cases are conducted to assess the
accuracy of the CFD model and to evaluate several turbulence models when
used for indoor simulations.
The flow of cold air was studied in a 2D room at different supply
temperatures and velocities and at different sensible heat loads. The velocity
field and the temperature distribution were analyzed and used to calculate the
effective draft temperature. And hence, the comfort level was assessed using the
air diffusion performance index, ADPI. The effect of each factor was studied
independently and it was found that the supply temperature and velocity have
minimum effect on the room conditions. The room thermal environment was
uniform and satisfactory even at low supply temperature due to the good mixing
of air inside the room. Reduction in supply temperature and reduction in supply
velocity each causes limited enhancement on the ADPI. The main source of
drafts inside the room was the heat flux from the floor. The velocities induced
V
by the floor heat flux were much higher than those caused by the air jet. At
lower thermal loads, the induced air velocity was weaker, and hence the ADPI
values increased.
It was observed that air circulates in the room under the effect of natural
convection with no air supplied to the room. As the rate of heat transferred by
air increased, the air circulation and velocity increased as well. Air circulation
and velocity were also enhanced when the percent of the thermal load
concentrated in the floor was increased in the air conditioned room. Also, it was
found that separating the air jet from the ceiling does not affect the jet
attachment to the ceiling. Air jet when supplied far from the ceiling was still
pushed upwards by the air circulating in the room causing it to attach to the
ceiling.
Cold air system was also studied in a 3D room similar to a typical office
room with a standard vertical supply diffuser. A comparison was made between
the performance of a conventional air conditioning system (supply air at 14ºC)
and a cold air system (supply air at 10ºC) in the same room space. The
performance of the conventional system was convenient since air velocities and
temperatures throughout the room were within acceptable limits. The cold air
system on the other hand had a poor performance due to high-velocity lowtemperature
air falling directly to room space which reduced comfort level in
the room. It was found that changing the air supply angle from the diffuser from
45° to 30° has noticeably improved the performance of the cold air system and
increased the room’s comfort level. This improvement was due to the nearly
horizontal supply air which caused the air jet to attach to the room’s ceiling and
hence eliminating the direct effect of cold air on the room.
Finally, a summary of the work carried out in this thesis along with
general conclusions obtained from the study and recommendations for future
studies in the same field are presented.