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

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 approximate to 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. (C) 2012 Elsevier Masson SAS. All rights reserved.