Effect Of Indoor Air Stability On Flow Characteristics And Pollutant Diffusion In A Cavity

The concept of indoor air stability is derived from atmospheric stability, which stands for the ability to resist vertical motion and can be determined by vertical temperature lapse rate (γ= ??T /?Y ). From the point view of heat transfer, the concept can be further explained as the intensity and direction of conduction indoor, which also reflects the convection intensity. In order to investigate the flow characteristics of a cavity by a combination of indoor air stability and ventilation, the criteria Ri = Gr /Re2 are introduced in this paper. Two physical models CaseⅠand CaseⅡare established to investigate the flow characteristics in a cavity under stable and unstable conditions. After that, three different examples (A, B and C) are developed with different models and boundary conditions, in which A and B has different Gr numbers, while B and C are in different models. By the use of CFD simulation, three different inlet velocities ( Re = 100, Re = 316and Re = 3162) are used to calculate the air distribution and contaminant diffusion in the cavity.It is found that when Ri >> 1 the heat in the cavity is mainly transferred by conduction and the convection effect is concentrated in only small areas. When Ri < 1, the heat transfer is dominated by convection and the effect of convection is not obvious. Conduction and convection in the same direction will increase the total heat transfer rate, while the two in opposite directions will result in offset between each other. The temperature distribution in the cavity can be explained by the total heat transfer rate combined with the streamline distributions. When Gr number increases at constant Re values, the heat transfer intensity of mixed convection will increase mainly caused by the increase of natural convection.When the air jet from the inlet is changed into CO 2, the relative size of indoor air stability and the velocity of pollutant diffusion can also be evaluated by Ri = Gr /Re2. If Ri >> 1, the influences of indoor air stability on pollutant diffusion and streamline distribution are obvious. The stable conditions inhibit the motion of the fluid, while the unstable conditions promote fluid flow. If Ri < 1, the indoor air stability has almost no effect on indoor air flow due to high inlet velocity of the pollutant.As to the streamline distribution, when Ri >> 1 the streamline profiles are changing with the propagation of the pollutant. Moreover, the streamline profiles of the stable and unstable conditions vary with larger vortex areas of instability. When Ri < 1 the shape of streamline is the same with the one of air jet and doesn’t change with time.As to the pollutant diffusion, when Ri >> 1 the unstable conditions are with larger vortex regions in which the contaminants and the indoor air are mixed well and then removed from the export with higher speeds than those of the stable conditions. However, in stable conditions, the contaminants are easy to accumulate in the cavity and difficult to discharge outdoors. Regardless of the indoor air stability, the propagation speed of the pollutant will increase with the inlet velocity.This research will provide theoretical basis for the evaluation of indoor air quality and ventilation rate.