【作者】 尹招琴；

【导师】 林建忠；

【作者基本信息】 浙江大学 ， 流体力学， 2008， 博士

【摘要】 PM_2.5（颗粒直径小于或等于2.5μm的可吸入颗粒物）是影响空气质量的重要污染物。已有研究表明,可吸入颗粒物浓度上升与呼吸系统疾病、心肺疾病的发病率、死亡率关系密切。汽车尾气排放的污染物是大气污染的主要来源,燃料中的硫在燃烧后形成SOx随着尾气被稀释和冷凝,会形成大量的PM_2.5。本文研究了汽车尾气排放时纳米颗粒的成核和凝并过程,分析了不同流场中颗粒物质的数密度和颗粒直径分布,论文由以下四部分组成。第一,采用热线风速仪对平行双射流流场进行测试,研究了雷诺数、双射流间距对流场特性的影响。研究结果表明,在射流喷口附近,沿射流方向,双射流是分离的;由于射流中间边界层的相互影响,双射流会逐渐混合,并最终形成单股射流;射流的平均速度、湍动能是对称的;双射流间的扰动随雷诺数的增加而增大,雷诺数越大,则雷诺应力和湍动能越大。双管间距越大,射流间的扰动就越小,但是射流宽度随着双管间距的增加而增加。减小双射流间距或者增加雷诺数,都能增加双射流混合距离。第二,用数值模拟方法对平行双射流场中颗粒的成核与凝聚过程进行了研究,给出了硫酸/水系统中经成核和凝聚过程形成的颗粒数密度和粒径的分布,说明成核过程会形成大量的纳米颗粒,在双管射流的中间和射流场周围,颗粒具有较高的浓度。在各种力的作用下,颗粒的凝聚过程使颗粒的数量减少,而直径变大。射流中硫的浓度对颗粒的浓度分布有影响,随着硫浓度的增加,流场中的颗粒数密度有明显增加,而成核形成的颗粒直径减小。环境湿度和雷诺数的增大,有利于成核过程,因而导致形成更多的颗粒。第三,通过大涡模拟方法计算双管冲击射流流场,获得流场流动特性;并计算了硫酸/水系统的成核和凝并过程,获得双管冲击射流流场中颗粒物质的数密度分布和浓度分布。计算结果表明,冲击射流分成自由射流区域、贴壁射流区域和上喷流区域,随着双射流间距的减小和射程的增加,双股射流间彼此的相互作用增加。颗粒尺度最大的区域发生在射流区域的周围。经过凝并过程后,颗粒数目大量减小,最大颗粒浓度在射流贴壁区域。双射流的间距影响射流中间颗粒数密度和尺度分布,双射流中间将形成更多的纳米颗粒,同时,颗粒物的尺度增加。射流冲击高度越大,靠近壁面处形成的颗粒数密度越小,总颗粒数也越小,所以增加射流射程对减小纳米颗粒的形成有益。在冲击高度较大的情况下,贴近壁面处形成的颗粒尺度较大。第四,研究了轿车不同行驶条件下流场的性质和颗粒物的分布。颗粒数密度最大区域在汽车尾部1米处;距离汽车尾部越远,颗粒物越少;成核形成的颗粒物最大尺度发生在流场边沿。纳米颗粒的数密度和尺度分布也受流场结构的影响,因而也受到汽车行驶速度和尾气排放速度的影响。汽车行驶速度较快时,尾气排放的H₂SO₄蒸气容易被稀释,有利于降低颗粒物质的形成。更多还原

【Abstract】 PM_2.5 is a major component of air pollution in urban areas. Study shows that nanoparticles are able to penetrate into lung where they may enter interstitial tissue, causing severe respiratory inflammation and acute pulmonary toxicity. Automobile emission is a major source of air pollution. The sulfur in diesel fuel is oxidized to SO_X, that leads to the sulfuric acid and sulfates inthe PM_2.5.The purposes of this thesis are to study the flow structure and the formation of pollutantnanoparticles in a vehicular exhaust twin-jet plume with the nucleation and coagulation equations and to a better understand of the particle number concentration and size distribution. The thesis can be divided into fore parts.In the first part, experiments have been carried out with a hot-wire anemometer to investigate the flow field of the twin-jets. The velocities have been measured to assist in understanding the change in the flow. The effects of Reynolds number and the spacing between two nozzles on the flow field along the flow and lateral directions were examined. The conclusions are drawn as follows: In the flow direction, the twin-jets are clearly separated near the nozzle exits. Away from this region, the two jets interact, and then the two jets mix and merge to appear as a single jet. The velocity and turbulent energy profiles are fairly symmetrical about the center line of two jets for various Reynolds numbers and the spacing between two nozzles. The velocities between two jets change along the lateral direction more quickly than that of previous studies. The interference between two jets increases as Reynolds number increasing. The stronger the interference is, the larger the Reynolds shear stress and turbulent energy are.. The interference between two jets increases as the spacing between two nozzles decreasing. Furthermore, the width of the twin-jets spreads linearly downstream and grows with B. The merging length of two jets can be increased either by reducing B or increasing Reynolds number.In the second part, large eddy simulation method has been used to calculate the parallel twin-jet flow coupled with nucleation and coagulation equations. The nanoparticle number concentration distribution and size distribution produced by the pure nucleation and coagulation are shown in the paper . The binary homogeneous nucleation will produce many nanoparticle and it takes place mainly in the interval of the twin jets region and the circumambience of the jets. Coagulation modifies the particle size distribution since it reduces the overall number of particles and increases the mean size of them resulting from forces. Sulfur content effects on the nanoparticle number concentration distribution. The nucleation process will generate higher number concentration and smaller size of nanoparticles as sulfur content increases. High relative humidity and jet Reynolds number will increase the nucleation rate, which results in more nanoparticles.In the third part, large eddy simulation method has been used to calculate the flow and obtain the detailed flow structures of the impinging twin-jet for various conditions. The nucleation of sulphuric acid/water system and subsequent coagulation processes are simulated.The jets are clearly separated around the exits of nozzle, but significantly altered at the interacting side with their inner cone layers collided with each other at the middle plane between the two jets. After impinging on the plane, they are separated from the plane and forced to turn upward. There exist free jet region, adherent plane jet region and stagnation region in the flow. The interference between two jets increases with the decreasing of the space between two jets and the distance from the exit of nozzle to plane. The region of maximal particles size appears around the region of free jet. The number concentration decreases significantly after coagulation. The maximal number concentration produced by both nucleation and coagulation occurs in the region near the plane. The significant difference for various spaces between two jets is the number concentration and size distributions in the interface region of two jets. For the case with larger space, more nonoparticles are produced by nucleation and coagulation. There is also a wider size distribution for the case with larger space.The longer the distance from the exit of nozzle to plane is, the lower the number concentration is, and the fewer particles distribute near the plane. Increasing the distance from nozzle to plane is benefit to reduce the formation of nanoparticles. Though the number concentration is low for the case with longer distance from nozzle to plane, the size of particle in the region near the plane is large.In the forth part, large eddy simulation method has been used to calculate the flow and obtain the detailed flow structures of the moving car for various conditions. The maximal number concentration produced by both nucleation and coagulation occurs in 1m region behind the car. The longer the distance from the exhaust exits, the fewer particles distribute. The size of particleproduced by pure nucleation is larger on the edge of the gas-to-nanoparticle conversion regionThe nanoparticles number concentration and size distribution will be effected by the flowstructure which induced by the changing of the ratio of the ambient wind velocity to the exhaust gas velocity. Because a larger amount of H₂SO₄ vapor is diluted by high-ambient wind speed than by a low-ambient wind speed, the higher ambient wind speed is benefit to reduce the formation of nanoparticles and increase the particle size.更多还原