柴油发动机尾气纳米颗粒演变的TEMOM-LES模型

 中国雾霾主要成因是包括柴油发动机在内的污染源排放导致的二次颗粒物, 本文针对柴油发动机尾气排放的颗粒物进行研究, 主要关注极大数目微纳米颗粒的综合效应导致的颗粒参数的变化, 次了解根据不同柴油含硫标准下产生的颗粒物的分布情况。通过把针对尺度谱演变问题的泰勒展开矩方法(TEMOM)和大涡模拟方法(LES)技术进行结合, 研究了排气管附近柴油二次颗粒物的演变过程, 并在相同流动工况条件下, 对加拿大、新加坡、欧盟、美国、俄罗斯、中国和日本不同国家和地区不同柴油含硫标准所导致的二次颗粒物的总颗粒数浓度、总体积浓度和颗粒大小进行了对比研究。结果显示, 颗粒物在近排气管处主要分布在射流剪切层, 颗粒数浓度和颗粒物直径的最大值在距离喷嘴0.6 m 附近产生, 在射流下游, 射流与下游空气混合稀释效应明显, 颗粒数浓度和颗粒直径趋于一稳定值。通过研究时间平均颗粒场的分布发现, 低燃硫量柴油尾气产生的颗粒数浓度比高燃硫量产生的颗粒数浓度低4 个数量级, 产生的颗粒体积浓度最大相差6 个数量级。

Evolution of nano-particles within a diesel car exhaust plume via TEMOM-LES method

A study on the evolution of secondary nanoparticles in a diesel engine exhaust is presented, coupling the large eddy simulation (LES) and the Taylor expansion method of moments (TEMOM), with the aim to reveal the differences in total particle number concentration, volume concentration and geometric mean diameter of diesel engines from Canada, Singapore, European, USA, Russia, China and Japan. The investigated aerosol system involved nanoparticle diffusion, nucleation, coagulation and condensation. The nucleation model is the binary homogeneous nucleation of water-acid system, which is valid for the engine exhaust dilution conditions. Considering the particle diameter range in the particle evolution, the free molecule regime coagulation formula is used. The particle condensation growth rate is obtained by calculating the arrival and loss of acid molecules at the entire particle surface. Five different diesel fuel sulfur contents, 10, 15, 30, 50, 350 mg/kg, are chosen as the initial conditions to investigate the particle evolution. It is shown that particles mainly form in the exhaust shear layer while the maximum values of particle number concentration and particle diameter appear in the downstream 0.6 m from the tailpipe. It is obvious that the fuel sulfur content has a significant effect on the formation of secondary nanoparticles. The total particle number concentration and total volume concentration are about four and six orders of magnitude smaller in the lowest fuel sulfur standard than those in the highest fuel sulfur standard, respectively.