Experimental Study Of Salt Water Model In Atrium With Buoyancy-driven Ventilation

An atrium is an important construction form to many naturally ventilated buildings,which is used to enhance the ventilation in storeys.The natural ventilation in atrium building and the determination of discharge coefficient through opening are studied by using simplified mathematical model with salt water experiment.Based on the equation of the stratification and the flow rate along each flowpath and the theoretical analysis of the turbulent plume,the flow rate of each vent and the thermal stratification position in the storey and the atrium are predicted,then compared with the experimental results.For the influence of the density difference on discharge coefficient,the experiments of displacement ventilation with transient discharge flow and thermal stratification caused by local buoyancy source in space were carried out,so as to determine the relationship between discharge coefficient and discharge plume parameter.The theoretical model of natural ventilation is established,and the effect of discharge coefficient on natural ventilation is also revealed.The motional equations governing the flow in Boussinesq and non-Boussinesq plume and the conservational equations for the buoyant layer in a box are combined together to provide the detailed evolution of the height of thermal stratification interface,density deficit and outflow vertical velocity with time.The research work of this paper provides theoretical guidance for the ventilation design of atrium enhanced storey,and determines the value of discharge coefficient in ventilation engineering application.Research indicates:（1）Through the theoretical analysis of the atrium,the criterion for enhancement of ventilation flow rate in the storey is determined.When E>E_isolate,the ventilation in the storey is enhanced.However,when E≤E_isolate,the ventilation is restricted.Ventilation enhancement parameter E is determined by the atrium height,the solar heat gains and the opening area of the upper atrium.The storey ventilation parameter E_isolate is a function of the effective opening area of the storey alone.（2）The criterion for enhancement of ventilation flow rate in the storey is related to the area of upper atrium opening A*_upperH²,but not to the lower atrium opening A*_lowerH².The atrium height M/H and solar buoyancy flux B_solar/B play similar roles in enhancing ventilation parameter E.Increasing any of these parameters will increase the storey air flow,and increase the height of atrium and storey stratification interface.However,due to the different expression forms of composition parameter E,there are obvious differences on whether to enhance ventilation.Given the storey parameters and atrium height,when upper atrium opening A*_upperH² greater than the critical value,no matter what the B_solar/B value is,the E>E_isolate condition is satisfied,and the atrium enhance the ventilation in storeys.But there is a critical value of M/H,when M/H is too small,it will reduce the effect of the ventilation in storeys.（3）The displacement ventilation experiment was carried out by using salt water model.The experimental results show that the discharge coefficient is only related to discharge plume parameterΓ_d alone,but not to buoyancy flux.WhenΓ_d≤4.56,C_dcan be a constant value of 0.60,whileΓ_d>4.56,C_d decreases significantly with the increasing ofΓ_d.（4）When the displacement flow is at a steady state,the thickness of the buoyant layer is only a function of the discharge plume parameterΓ_d,and is independent of the source buoyant flux and of the geometry.When the dimensionless interface heightζ>0.76,the position ofζcan be accurately predicted by the fitting curve of the discharge coefficient as a function ofΓ_d.When the dimensionless interface heightζ>0.49,the plume at the vent will be lazy,then forced at lower locations.For lazy plume,a neck can occur as soon asΓ_d exceeds 1.5 or 2.5,and the corresponding interface position is 0.53≤ζ_ss≤0.67.WhenΓ_d>4.56,theζ_ss>0.76,0.76 is the critical dimensionless interface height of variable discharge coefficient.