Evolution Characteristics of Indoor Pollutant Concentration in Buoyancy-Driven Natural Ventilation
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摘要: 为探究热压自然通风向稳态发展过程室内气态污染物的迁移规律，对不同高度处污染物浓度演化进行了分析. 首先结合预测热分层瞬时变化的非均匀三层模型，针对室内不同的下层污染物混合特性，建立了均匀混合和纯置换两种热压自然通风室内污染物输送模型；其次，采用4阶龙格-库塔（Runge-Kutta）方法迭代求解，得到了通风过程室内污染物层高与浓度演变特性；最后，分析了有效通风面积和浮力组合系数对污染物浓度变化的影响. 结果表明：垂直速度为0的界面与新鲜空气层界面是两个不同的分界面；有效通风面积越大，两者高度差的峰值越大，到达稳态所需时间越短；浮力组合系数越大，任意时刻两者高度差越大，且随有效通风面积的增大越发明显；下层污染物混合特性会影响污染物分层以及演变特性，但不改变稳定时刻污染物浓度分布，通风过程室内上层污染物浓度均要大于下层；纯置换模型下原有冷空气层污染物浓度随时间不断降低至定值，下层污染物浓度保持恒定，而上层污染物浓度在初始阶段急剧增大，后缓慢减低；均匀混合模型的上下层污染物浓度会缓慢衰减至定值；此外，增大有效通风面积可缩短污染物衰减时间，提高排污效率，而浮力组合系数对污染物浓度变化的影响则可忽略.Abstract: Evolution of pollutant concentration at different heights was analyzed to explore the evolution rule of indoor gaseous pollutants during a period from buoyacy-driven natural ventilation to steady state. Firstly, according to the non-uniform three-layer model used to examine the transient flows driven by buoyancy force, two theoretical models that correspond to homogeneous mix and pure displacement were developed for the different mixing characteristics of the inoor lower-layer pollutants. Then, the fourth-order Runge-Kutta method was adopted to find a iterative solution, and the height and concentration variation characteristics of the indoor pollutants during the ventilation process were obtained. Finally, the effects of the effective ventilation area and buoyancy combination coefficient on pollutant concentration was discussed. The results show that, the thermal stratification interface with zero vertical velocity greatly differs from the fresh air layer interface, and the height difference between thermal stratification interfaces will have a larger peak value and a shorter time to reach steady state when the dimensionless effective ventilation area becomes larger. The greater buoyancy combination coefficient is, the greater the height difference between thermal stratification interfaces at any time is, which becomes more obvious with the increase of the dimensionless effective ventilation area. The mixing characteristics of lower-layer pollutants affect the stratification and time evolution, but do not change the concentration distribution in steady state, and the upper layer has the highest pollutant concentration for both models. For the pure-displacement model, the pollutant concentration of the original layer decreases until it reaches stability, and the lower-layer pollutant concentration remains constant, while that of the upper layer increases sharply in the initial stage before decaying. Meanwhile, the average pollutant concentration of the upper and lower layers both slowly decay to a stable value for the homogeneous mix model. Furthermore, a larger dimensionless effective ventilation area can result in a faster decay of the dimensionless pollutant on the pollutant concentration can be almost ignored.
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图 1 热压自然通风瞬时流动示意
Figure 1. Schematic of transient flow in buoyancy-driven natural ventilation
图 2 模型Ⅰ下通风过程典型界面垂直速度
Figure 2. Vertical velocities of typical section during ventilation progress for model Ⅰ
图 3 模型Ⅱ下通风过程典型界面垂直速度
Figure 3. Vertical velocities of typical section during ventilation progress for model Ⅱ
图 5 a = 0.2，λ = 0.5时污染物浓度的瞬时变化
Figure 5. Transient change of pollutant concentration for a = 0.2 and λ = 0.5
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