基于分形理论与碳分析的柴油机颗粒物特性

Characteristics of Diesel Particulates Based on Fractal Theory and Carbon Analysis

颗粒物外在形貌与内在含碳属性的表征已成为大气颗粒物源解析的重要手段.应用分形理论及热光碳分析法,研究高、低负荷工况下不同粒径级柴油机颗粒物形貌的盒维数及其碳组分特性.结果表明:颗粒物粒径级越小,其吸附的可溶性有机物(SOF)越多,由分形理论计算得到的盒维数数值增大.低负荷工况下的颗粒物聚集现象严重,从而导致其分形盒维数比高负荷的高;在较小粒径级或低负荷工况时,因细颗粒物的吸附效应,最易挥发的OC1组分增多.低负荷工况下,元素碳(EC)中的EC1(char-EC)大量生成,其m(OC)/m(EC)值大于10;而高负荷工况下,EC几乎主要由EC2(soot-EC)组成,其m(OC)/m(EC)值小于1.此外,颗粒物形貌的盒维数与其有机碳与无机碳组分含量具有一定的关联,颗粒物的m(OC)/m(EC)比值越大,其对应形貌图的盒维数越大,反之亦然.

Characterization of external morphology and intemal carbon fractions in diesel particulates has become an important method for identifying the particulates in atmosphere. Morphology box dimension and carbon fractions of diesel particulates with different sizes were analyzed by using fractal theory and thermal-optical analysis at various engine loads. Results show that with the decrease in particulate size, the amount of soluble organic frac- tions （SOF） absorbed by particulates increases, correspondingly, the box dimension value calculated by fractal theory augments. The box dimension values at low engine loads are larger than that at high loads due to the aggregation of particulates. While at low loads or in superfine particulates conditions, the most volatile OC1 component increases due to the adsorption effect. In addition, at low engine loads, a large amount of EC1 （char-EC）forms, and usually the m（OC）/m（EC） ratio is greater than 10. At high loads, however, the EC is mainly composed of EC2（soot- EC） and the m （OC）/m （EC） ratio is less than 1. In addition, the fractal box dimension of particulates appears regular correlation with their internal organic and elemental carbon fractions. As a rule, the greater m （OC）/m （EC） ratio in particulates corresponds to the higher fractal box dimension of particulates＇ morphology pattem, and vice versa.