Study On The Flow Pattern And Airflow Characteristics Of Buoyancy-driven Ventilation Under Vertical Asymmetric Double Openings

As an important means of air conditioning,natural ventilation is widely used to improve indoor hot and humid environment and air quality account for its easy implementation,zero energy consumption and high comfort.Natural ventilation is mainly driven by the pressure difference,which can be caused by the ambient wind or temperature difference between indoor and outdoor.Due to the instability of the building wind environment,when the wind pressure formed by the external environment wind is small,natural ventilation is mainly dominated by thermal pressure.In addition,the classical theory of stack-driven ventilation is based on the fact that the vents are much smaller than the geometric dimensions of the room.For natural ventilation scenarios with large openings in real life,the application of this theoretical model has limitations.Therefore,it is of great significance to study the flow characteristics of natural ventilation driven by thermal pressure with large vent.This paper focuses on the study of the flow mechanism of natural ventilation driven by stack effect in a chamber with double openings,and limits the research object to quasi two-dimensional buoyancy driven flow.The influence of the size of the upper and lower openings on the flow pattern,neutral plane,interface position and volume flow was investigated.According to the energy equation and the principle of conservation of mass,the different flow patterns are analyzed theoretically,and a displacement ventilation model and a similar displacement ventilation model suitable for large openings are proposed.Based on the relationship between the neutral level,the interface and the upper edge of the lower opening,the displacement ventilation mode is subdivided into two models,I and II.Among them,the flow characteristic of displacement ventilation mode I is that the neutral level and the interface are higher than the upper edge of the lower opening;mode II is characterized by the neutral level is higher than the upper edge of the lower opening,the interface is lower than the upper edge of the lower opening,but the hot air never flows out from the lower opening.The theoretical models proposed in this paper shows that for the natural ventilation driven by stack effect,in the displacement and similar displacement ventilation modes,the position of the interface and the neutral level are independent of the buoyancy source,and only related to the geometry size of the ventilation room and the upper and lower openings.By numerically solving the theoretical models of different flow patterns,the critical conditions of the transition between the similar mixing ventilation(bidirectional flow at upper open),displacement ventilation mode I,displacement ventilation mode II,and similar displacement ventilation(bidirectional flow at lower open)are obtained,as well as the relationship between the parameters of neutral level,stratification interface and volume flow and the sizes of the upper and lower openings.The results of the solution show that the neutral level presents a V-shaped change trend of decreasing first and then increasing with the increase of the lower opening,while the stratification interface rises steeply first and then slowly increases after a smooth transition.with the increase of the lower opening.The density distribution of the internal flow field,the stratification interface and the flow pattern were studied by the brine water experiment.Firstly,for the fixed opening size,only the parameters of the buoyancy source are changed,and the corresponding interface position is investigated.The experimental results verifies that the stratification interface location is independent of the buoyancy flux under buoyancy-driven ventilation.Subsequently,the parameters of the buoyancy source were fixed,and by changing the size of the upper and lower openings,the change rule of the interface height with the lower opening was quantitatively studied,and the critical conditions of the upper and lower opening heights when the different flow patterns changed between each other were explored.The experimental results were in good agreement with the theoretical prediction.The volume flow rate through the upper and lower openings,and the position of the neutral level were studied by CFD numerical simulation.The results show that the height of the indoor neutral level is independent of the heat release rate,but the air exchange rate increases with the increase of the heat release rate.For a fixed upper opening,as the lower opening gradually increases,the ventilation mode changes from similar mixing ventilation,replacement ventilation to similar replacement ventilation.In this process,the volume flow inflow through the lower opening gradually increases,while the volume flow out through the upper opening increases first and then decreases.Meanwhile,the height of the neutral level decreases first and then increases with the increase of the lower opening.The results of CFD simulation are in good agreement with the prediction of theoretical model,which shows that the theoretical model is reasonable and has good applicability.