Numerical modelling of surface condensation on diffuser for cold air distribution systems.

Condensation on the surfaces of diffuser and cold air dumping are the two major concerns in the application of cold air distribution brought about by the high temperature difference between supply air and room air. Condensation will form if the surface temperature of the diffuser is lower than the dew point temperature of ambient air. The presence of surface condensation can promote growth of unhealthy and smelly mold, and produce unwelcome damage of a structural and/or aesthetic nature. Cold air dumping is a major factor that detracts from thermal comfort in an airconditioned room. In this paper, a computational study of the airflow, temperature and vapour concentration for a multi-cone circular ceiling diffuser is presented. The risk of surface condensation is investigated by combining the heat and moist transfer of room air. The results show that ( 1 ) the surface condensation is most likely to happen on the inner cone of the multicone diffuser, (2) the surface condensation risk increases as the supply flow rate increases, (3) the surface condensation risk can be greatly reduced by providing an opening (central hole) in the inner cone to enable sufficient cold supply air to mix with the entrained room air, (4) the existence of a lip on the cones has a very significant effect on cold air dumping.