Soot formation and the growth process of incipient soot particle were investigated by Laser Induced Incandescence (LII method), time resolved LII (TIRE-LII), high resolution electron microscopy (HRTEM) and elemental analyzer in normal and inverse diffusion flames.
LII and TIRE-LII methods were used to measure the distribution of two dimensional soot volume fraction and primary particle size. The soot was directly sampled by thermophoretic method and its diameter was examined by TEM photography. Two suitable delay times of TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signals. And the error bound of this method was also determined through the numerical analysis on LII process of soot particle.
The evolution characteristics of physical properties of soot were examined with these carefully calibrated optical methods in normal diffusion flames. From these results we can characterize the aging process of soot into three parts which are inception region, growth region and oxidation region. The effects of oxygen and carbon dioxide as diluents in oxidizer side were investigated in each region by using an $O_2 + [CO_2, N_2 and (Ar + CO_2)]$ mixture to isolate thermal effect of $CO_2$ systematically. The number concentration of primary particle and soot volume fraction abruptly decreased due to the addition of $CO_2$ in co-flow side. This suppression is resulted from the reduced residence time in inception regime because the nucleation point recedes from the nozzle and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of $CO_2$ including thermal dissociation. In inception regime, the decrease of number density results in the decrease of total surface area and it cause the soot volume fraction to decrease because the specific surface growth rate is almost same value in $CO_2$ dilution case. The increase of oxygen concentration in a co-flow caused an enhancement of soot nucleation and thus the residence time in an inception region increase but the growth distance decrease due to the active soot oxidation by the increased temperature and oxygen concentration. In this condition the specific growth rate is almost same value. Therefore finally, this result suggests that in growth region, the specific growth rate has a weak dependency on the co-flow conditions in non-premixed co-flowing flames.
It is difficult to examine the evolution of incipient particles and the chemical property of soot with spatial resolution in normal diffusion flames due to the inherent flame structure which consists of the flame sheet, PAH and soot particle zones. That is, in normal diffusion flame, we can not sample soot particles without the interference of flame because sooting zone is covered with flame which causes the structure and morphology of incipient particles to change. Therefore in this study, the crystallinity of tar-like material, nano-structure of incipient particle and carbonaceous process were extensively examined in inverse diffusion flames. TEM micrographs reveal a condensed phase of semitransparent tar-like material transforming from the abrupt coalescence of pyrolysis products (PAH). The unstructured tar-like materials are transformed into precursor particles with relatively distinct boundary and crystalline near the flame. It is noticeable that the precursor particle has the regular crystalline looks like layer structures. The change of C/H ratio to the residence time can be divided into two parts as explained in previous paragraph with the results of TEM photographs. One is very slowly increasing regime where tar-like materials are transformed into precursor particles (inception process) and the other is fast increasing region where surface growth affects the increase of C/H ratio dominantly (surface growth region). The C/H ratio at the beginning of the yellow flame can be estimated in the slowly changing region about C/H ratio= 1.73 (0.05) in both cases which gives valuable information about the transition of gas phase to solid particle. These results provide a clear picture of a transition to mature soot from precursor materials.
본 연구는 레이저를 이용한 광학적 접근 방법과 직접 포집에 의한 방법을 이용하여 확산화염에서의 매연의 생성과 성장과정을 조사하였다. 매연의 생성과 성장영역에서 매연의 물리적인 특징들의 변화를 일반적 형태의 확산화염에서 관찰하였고, 초기 매연 입자의 생성과 고온 영역에서의 체재시간에 따른 매연 입자의 C/H ratio의 변화를 역 확산화염에서 직접 포집에 의한 방법을 이용하여 관찰하였다.
