Heat transfer characteristics in air-conditioned rooms using mixing air-distribution system under mixed convection conditions

摘要

The present paper deals with CFD study of flow and thermal characteristics in air-conditioned rooms under turbulent mixed convection conditions. Air enters room from a sidewall slot flush with isothermal ceiling and leaves through a slot in opposite wall flush with floor. Velocity and temperature distributions are determined by a CFD model that has been previously validated by comparison with experimental data. Rigorous dimensional analysis shows that performance of the air-distribution system depends on: supply Reynolds number Re_d, Grashof number Gr_d (or Archimedes number Ar_d), room aspect ratio L/H, dimensionless heights of supply and return ports (d/H and t/H), where H is room height. For a room with fixed geometry, the effects of Re_d and Gr_d (or Ar_d) are investigated systematically by varying one parameter at a time. Results show that ceiling average Nusselt number (Nu_(d,av)) increases with Re_d and decreases with Gr_d (or Ar_d). For a given Gr_d, the heat removal index (HRI) increases with Re_d up to Re_d ≈ 9000, then stays practically constant thereafter. For a given Re_d, HRI decreases with increasing Gr_d (or Ar_d). A criterion for distinction between forced and mixed convection situations is developed, resulting in a critical Archimedes number of 0.11. Results further indicate that Ar_d alone, irrespective of the particular values of Re_d and Gr_d, determines to a reasonable extent Nu_(d,av) and HRI when Ar_d is above its critical value. Streamlines and temperature contours are found to be consistent with other results and helped explain them.