【作者】 孙德帅；

【导师】 郭庆杰；

【作者基本信息】 青岛科技大学 ， 化学工程， 2009， 博士

【摘要】 可吸入颗粒物是指通过口鼻进入人体呼吸道的颗粒物总称,通常指粒径小于10μm的颗粒物。该颗粒物对人类和环境的危害极大,而传统的除尘器捕集该类颗粒的效率却很低。利用可吸入颗粒的粘附特性,施加外场作用,使颗粒物发生团聚,可吸入颗粒平均粒径增大后容易被电除尘器等常规设备从携带气体中清除。因此可吸入颗粒物团聚作为一种预处理方法具有操作简单、效率较高、成本低廉,有望获得工业上的应用。本文利用流体力学软件FLUENT6.3对颗粒在流场中的运动轨迹进行模拟,分析颗粒在流场中的分布。建立了可吸入颗粒声场与射流耦合团聚实验设备,系统研究可吸入颗粒在声场、射流及声场与射流耦合作用下的冷态团聚特性。采用颗粒群动力学方程对颗粒碰撞团聚演变过程进行模拟计算,并与实验结果相比较。采用颗粒轨道模型模拟颗粒在流场中的运动轨迹,首次通过动量方程将声场引入模型中,声场作用使颗粒在团聚室内分布不均匀性增加,某些区域产生聚集。声场对小颗粒夹带速度高于对大颗粒的夹带速度。湍流射流团聚过程中,颗粒与湍流射流气体相互作用,不同粒径颗粒从湍流射流流场中获得能量不同,导致颗粒运动速度随颗粒粒径增大而减小。声场与射流耦合团聚过程中,射流与声场同时强化颗粒的运动。声场对颗粒的夹带作用导致可吸入颗粒碰撞团聚,在声场频率为1416 Hz时可吸入颗粒团聚清除效率达到最大值;声压级越高越有利于颗粒的团聚清除。团聚室内相对湿度为40-50%时,颗粒在声场中的临界停留时间为9.0-11.0 s。声场作用下不同粒径颗粒的团聚清除效率呈U型分布,较小颗粒(<1μm)与较大颗粒(5-10μm)的质量清除效率高于中间粒径颗粒(1-5μm)的清除效率。在可吸入颗粒团聚室中引入湍流气体射流,能够促进可吸入颗粒团聚。增大射流出口雷诺数和射流与主气流的气速比,可吸入颗粒的质量与数量清除效率均增大。当射流垂直主气流注入团聚室,可吸入颗粒的清除效率最高;射流与主气流逆向倾斜入射时,可吸入颗粒清除效率高于射流与主气流同向倾斜入射。声场与湍流射流耦合作用下颗粒团聚实验发现,可吸入颗粒清除效率随声场声压级增大而增加;在声场频率为1416 Hz时达到最大值。增大射流与主气流气速比能够提高颗粒清除效率。射流垂直入射,可吸入颗粒清除效率最高;但同向倾斜射流与声场耦合作用对颗粒的团聚促进效率高于逆向倾斜射流与声场耦合作用。颗粒清除效率随射流出口雷诺数增大而增加,但过高雷诺数的射流与声场的耦合作用反而使颗粒清除效率减小。可吸入颗粒在声场团聚、射流团聚及声场与射流耦合团聚过程中随飞灰颗粒质量浓度增加,颗粒质量与数量清除效率均降低。团聚室内气体相对湿度同样显著影响可吸入颗粒团聚清除效率,相对湿度为约40%时,可吸入颗粒清除效率达到最大值;相对湿度过高或过低,颗粒清除效率均降低。团聚后颗粒的质量中位径由2-3μm增大至5-7μm。对颗粒群动力学方程采用区域算法求解计算颗粒数目浓度的演化过程。计算过程中团聚核函数采用声场同向团聚核函数、经典湍流团聚核函数,将颗粒团聚清除效率的数值计算结果和实验结果进行了比较,两者基本吻合。对声场与湍流射流耦合作用下可吸入颗粒团聚核函数采用声场团聚核函数与湍流团聚核函数线性相加,对颗粒团聚动力学方程求解结果表明耦合作用下亚微米级颗粒数目浓度计算值略高于实验值;粒径较大微米级颗粒计算结果则与实验结果吻合良好。更多还原

【Abstract】 Particles smaller than 10μm were defined as inhalable particles, which could penetrate deeply into the respiratory system. These particles cause serious healthy hazards due to being captured difficultyly by conventional filters such as electrostatic precipitators and cyclone separators. Inhalable particles were characterized by great adhesive forces due to very small size in diameter. External forces can accelerate the particles to collide and agglomerate together so that a significant shift of particle size distribution from small to large sizes. Large groups were convenient to be captured by conventional filters. Agglomeration process was an efficient preconditioning method for inhalble particles, which was easy in operation with low cost.FLUENT6.3, a CFD simulation software, was used to simulate the particle tracks of in flow field in order to evaluate the probability of collision. A macro-agglomeration setup was used for study the removal efficiency of inhalable particles in acoustic field, turbulent jet and the coupling effes of sound field and turbulent jet. The aerosol dynamics equation was solved by sectional methods to calculate the size distribution of inhalable particles during the agglomeration process.The discrete phase model in FLUENT6.3 was employed to calculate the tracks in flow field. Modified momentum equation containing acoustic field was first developed. Inhalable particles suspended in a gas flow were distributed honuniformly and even concentrated in local area due to the entrainment of acoustic field. The velocities of small particles were higher than those of large ones. For turbulent agglomeration process, inhalable particles could quickly acquire energy from turbulent jet to improve the particles velocity when particle moved to the core space of turbulent jet. The energy exchange between the fluid and particle varied with the particle size, which makes small particles have higher velocities.Each particle was entrained by the acoustic field in the propagation direction of the applied wave, and this entrainment served as the direct driving force leading to collision between the particles. Acoustic agglomeration showed that the maximum removal efficiency reached at the frequency of 1416 Hz. And the agglomeration of inhalable particles increased with increasing sound pressure level. The critic residence time of particles ranged from 9.0 s to11.0 s in acoustic field at 40-50% relative humidity. For inhalable particles, the small particles (<1μm) and large ones (5-10μm) were easy to remove, but mid-size particles (1-5μm) were difficult to be captured.A turbulent gas jet was introduced into the particle agglomeration chamber to generate the local turbulent field. Experimental findings indicated the mass and number removal efficiency increased with increasing Reynolds number in jet exit and velocity ratio of jet-to-crossflow. The gas jet at the injection angle of 90o could achieve high removal efficiency. Gas jet at the obtuse angle injection (120o or 140 o) favors the removal of mid-size particles compared with ones at the acute inclined injection (40o or 60o).The coupling effects of gas jet and acoustic field were employed to intensify the agglomeration process of inhalable particles. During the acoustic agglomeration and the coupling agglomeration process, the removal efficiency increased with an increase in sound pressure level, and reached to the maximum value at the frequency of 1416 Hz. During the jet agglomeration and the coupling agglomeration, the removal efficiency increased with increasing velocity ratio of jet-crossflow. The optimal agglomeraton was observed while gas jet was ejected vertical into acoustic agglomeration chamber. For jet agglomeration, increasing the Reynolds number in jet exit could enhance the removal of particles, whereas high Reynolds number in jet exit would decrease the removal value for the coupling agglomeration process.In agglomeration process the increased initial concentration of fly ash results in decreasing of removal efficiency of inhalable particles. When relative humidity in chamber approached 40-50%, the maximum removal efficiency was acquired, and higher or lower humidity could reduce the efficiency. After agglomeration, the mass mean median diameter of the aggregate group increased from 2-3μm to 5-7μm.Aerosol dynamics equation was solved by sectional method to simulate the evolution of particles number concentration. Classical orthokinetic coefficient and classical turbulent coefficient were choosen to calculate the agglomeration processes. The numerical removal efficiency in acoustic agglomeration or turbulent jet agglomeration process agreed well with experimental results and they fit well. In coupling agglomeration process, a linear combination of the acoustic kernel and turbulent kernel was used in aerosol dynamics equation. The simulation particle number concentration was slightly greater than the experimental ones for submicron particles, but they fit well for micron particles.更多还原