Investigations On The Evolution Of Indoor Air Pollution In Natural Displacement Ventilation Rooms

Natural ventilation is an important strategy for improving the indoor air environment. The buoyancy-driven natural displacement ventilation is one of the major types of natural ventilation and the ventilation flow would reach the steady state after a period of development. However, the timescale of the transient natural displacement ventilation to its steady state may be comparable to the time of occupancy, and more attention should be paid on the study of the effects of the transient flow on the indoor thermal environment and indoor air pollution.Three modified theoretical models were developed on the basis of previous models to examine the transient natural displacement ventilation in an enclosure which is initially of uniform temperature, equal to the exterior (ΔT0=0). The main difference between modified models and Kaye&Hunt’s model is that the buoyant layer is regarded as composed of a middle layer and a near-ceiling layer rather than being well-mixed. Different assumptions on the buoyancies of the near-ceiling layer and the outflow through the upper opening were made in different modified models. Comparisons were made between the predictions of Kaye&Hunt’s model and three modified models and experimental results reported in the literature. Three modified models were found to perform better than Kaye&Hunt’s model. Meanwhile, the predictions of the Modified Model3seemed to agree slightly better with the experimental data than those of the other two modified models.The Modified Model3was then employed to investigate the time evolution of the thermal stratification interface height, thermal buoyancy and flow rate during the transient process in the case of ΔT0=0. The results indicated that the extent to which the warm air layer depth overshoots its steady-state depth is more significant when the dimensionless effective vent area, a, is smaller. However, the overshoot scale is very small relative to the enclosure height even for a near zero. The near-ceiling layer depth becomes smaller gradually and the middle warm layer depth increases rapidly to its maximum during the early stage and then varies slightly. It is shown that the dimensionless time taken for the thermal stratification interface to reach three characteristic positions is longer for smaller dimensionless effective vent area. The variations of the warm layer buoyancy and ventilation flow rate with time are both dependent on the source buoyancy flux (Bo), the floor area (S), the effective vent area (A*) and the enclosure height (H). The larger buoyancy may be obtained for larger B0or smaller S, A*or H. The larger ventilation flow rate may be achieved by decreasing S or increasing Bo. A*or H.The gaseous pollutant transport model during transient natural ventilation was presented on the basic of the airflow characteristic to analyze the time variation of indoor pollutant concentration in the case of ΔT0=0. It is demonstrated that the gaseous pollutant concentrations of the warm layer and the original layer first increase evidently in the early stage of ventilation and then decrease continuously. The elevation phase of indoor concentration is longer and the transient pollutant concentration indoors is higher for higher outdoor concentration. The pollutant flushing effect of ventilation flow can be improved by increasing B0or decreasing S. The variation of indoor concentration is more rapid and the peak concentration is higher for larger.A*or smaller H.Four modes of the transient natural displacement ventilation in a pre-heated room (ΔT0>0) were analyzed by using the theoretical model proposed by Fitzgerald&Woods, and three critical values of initial buoyancy of indoor air, δ0c, were then presented in Mode1, Mode3and Mode4respectively to distinguish different cases of the three modes. The cases or phases in every ventilation mode were then analyzed in detail and some improved models, such as four-layer model and penetration without entrainment model, were developed.The theoretical analysis of the transient natural ventilation in a pre-heated room show that the thermal stratification interface may ascend or descend across the steady-state interface during the transient process of three modes except for Mode2. As for Mode1, the upper interface of the original layer would descend across the steady-state interface if the initial buoyancy of indoor air is less than its critical value. The extent to which the depth of the upper warm layer overshoots its steady-state depth is more significant for smaller dimensionless effective vent area or smaller initial buoyancy. In Mode3and Mode4, the thermal stratification interface would ascend across the steady-state interface during the evolution. The extent to which the depth of the lower layer overshoots its steady-state depth is more significant when the initial buoyancy of indoor air, δ0, is larger. As for the transient natural ventilation in the case of ΔT0>0, the initial flow rate depends on the effective vent area (A*), the enclosure height (H), the initial indoor temperature (To) and outdoor temperature (Ta), and is proportional to the first three parameters but is inversely proportional to Ta. The steady-state flow rate is determined by A*. H and B0and is proportional to these three parameters. The initial flow rate is larger than the steady-state flow rate in three ventilation modes except for Mode1. The larger flow rate during transient ventilation may be achieved for larger Bo, A*or H in all ventilation modes. Increasing the floor area, S, tends to obtain larger flow rate during the transient process of three modes except for Mode1. Furthermore, the transient flow rate increases as the initial temperature of indoor air, To, increases in Mode1, Mode2and Mode4. As for Mode3, the flow rate decreases more rapidly and the transient flow rate is smaller after a few minutes of evolution for higher T0.The conservation equations of indoor gaseous pollutant for ΔT0>0were presented on the basic of the thermal stratification characteristic during the transient ventilation. The time evolution of indoor gaseous pollutant concentration can be obtained by solving the simultaneous equations including the pollutant equations and the transient ventilation models. For the transient pollutant flushing of natural displacement ventilation from a pre-heated room, the indoor pollutant concentration would decrease more rapidly by increasing A*or decreasing S or H. Increasing B0is beneficial to reduce the pollutant concentration in Mode1, Mode2and Mode4, and has little influence on the pollutant flushing in Mode3. The pollutant removal efficiency can be enhanced by increasing the initial temperature of indoor air in three ventilation modes with the exception of Mode4.Full-scale experiments were conducted to measure the time variation of temperature distribution and CO2concentration in a chamber during the transient natural displacement ventilation, and to investigate the effects of the initial temperature difference between interior and exterior, the vent characteristic and the heat source characteristic on the transient process. The results indicated that the initial temperature difference (ΔT0) plays an important role on the thermal stratification, the pollutant concentration stratification, the flow rate variation and the pollutant flushing of airflow. The area and shape of the vent have little effect on the maximum temperature difference along the vertical direction and the temperature difference between head and foot. But the vent area would affect the thermal stratification characteristic to different extent for different initial temperature difference. The transient flow rate is larger and hence is good to the removal of indoor pollutant for larger vent. The effect of the vent shape on the pollutant removal is comparatively complex. The experimental results merely suggested that the pollutant flushing of ventilation flow is more efficient if the vent height is much greater than its width.Experiments showed that increasing the heat source strength will increase the maximum temperature difference along the vertical direction and the temperature difference between head and foot. But the change of heat source strength has no evident influence on the thermal stratification indoors. The larger ventilation flow rate can be achieved by increasing the heat source strength when ΔT0≥0, which is consistent with the theoretical analysis above. The removal of indoor pollutant can therefore be speeded up by increasing Bo in the case of ΔT0≥0. However, increasing B0is not helpful to reduce the pollutant concentration indoors when ΔT0<0. The vertical position of the heat source has great effect on the vertical temperature distribution. The vertical temperature gradients in the upper zone and lower zone are more evident and smaller respectively as the heat source is located in higher position. However, the effects of the vertical position of heat source on the transient flow rate and pollutant removal efficiency when ΔT0≥0are both unclear. The pollutant flushing effect of ventilation flow can be improved by elevating the heat source if the room is initially colder than the exterior. The effects of the horizontal position of heat source on the vertical temperature difference, the thermal stratification, the ventilation flow rate and the concentration decay of indoor pollutant are all negligible.