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 Red, Grashof number Grd (or Archimedes number Ard), 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 Red and Grd (or Ard) are investigated systematically by varying one parameter at a time. Results show that ceiling average Nusselt number (Nud,av) increases with Red and decreases with Grd (or Ard). For a given Grd, the heat removal index (HRI) increases with Red up to Red ≈ 9000, then stays practically constant thereafter. For a given Red, HRI decreases with increasing Grd (or Ard). 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 Ard alone, irrespective of the particular values of Red and Grd, determines to a reasonable extent Nud,av and HRI when Ard is above its critical value. Streamlines and temperature contours are found to be consistent with other results and helped explain them.

► Performance of mixing air-distribution system in room is investigated under mixed convection conditions using CFD. ► Rigorous dimensional analysis is conducted to establish dimensionless parameters that govern performance of ventilation system. ► Effects of Red and Grd on ceiling average Nud, HRI, and ηe are investigated. ► Convection regimes in room ventilation are identified in terms of critical Archimedes number (Ard,cr). ► In mixed convection regime, performance of ventilation system is determined to a large extent by Ard alone.