Development of Lagrangian soot tracking method for the study of soot morphology in diesel spray combustion

Ong, Jiun Cai (2017) Development of Lagrangian soot tracking method for the study of soot morphology in diesel spray combustion. PhD thesis, University of Nottingham.

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The weakness of a conventional Eulerian soot model in capturing primary soot size and its inability to access individual soot information led to the development of a Lagrangian soot tracking (LST) model as reported in this thesis. The LST model aimed to access the history of individual soot particles and capture the soot concentration and primary soot size distribution in high pressure spray flames, under diesel-like conditions. The model was validated in a constant volume spray combustion chamber by comparing the predicted soot volume fraction (SVF), mean primary soot diameter and primary soot size distribution to the experimental data of n-heptane and n-dodecane spray combustion. The inception, surface growth and oxidation models were adopted and modified from the multistep Moss-Brookes (MB) soot model, which was used in this study as the representative of Eulerian soot model. Parametric studies were carried out to investigate the influence of soot surface ageing and oxidation rates on the overall soot formation. Following the parametric study, the developed LST model which incorporated surface ageing effect and higher oxidation rates was implemented to investigate the effect of ambient oxygen and density on soot morphology in n-heptane spray flame.

The LST model was shown to have better primary soot size prediction capability while still maintaining comparable performance in predicting SVF with respect to its Eulerian counterpart. The SVF distributions predicted by the LST model qualitatively correspond to the experimental results despite the peak soot location being predicted further downstream by 30 mm. The primary soot size distribution predicted by the LST model had the same order as the measured primary soot size distribution despite predicting larger soot size. The presence of surface ageing factor had a significant effect on the primary soot size distribution whereas only a slight effect on the SVF profile. A maximum soot size reduction of 48% was obtained when incorporating surface ageing effect. The consideration of surface ageing effect led to smaller primary soot size predicted and better agreement with the measured primary soot size distribution.

The peak and mean primary soot sizes increased with increasing ambient density, from 14.8 kg/m3 to 30 kg/m3, at the core of spray jet. Meanwhile, the decrease in oxygen level from 21% to 12% at an ambient density of 14.8 kg/m3 caused a non-monotonic effect on the primary soot sizes at the core of spray jet. Trivial differences were predicted when oxygen level decreased from 21% to 15%. However, a significantly smaller primary soot sizes were predicted when oxygen level decreased further to 12%. In addition to net growth rates, soot cloud span and soot age were also found to play an important role in evolution of primary soot size. An increase in ambient oxygen and density resulted in a more upstream first-soot location. The effect of ambient density on soot age was not significant, whereas a lower oxygen level resulted in a longer soot age. A maximum soot age of 0.50 ms was obtained for both 21% and 15% O2 cases at both density levels. As oxygen level decreased to 12%, the maximum soot age increased to 0.58 ms due to lower combustion temperature.

Overall, the LST model was shown to perform better in predicting primary soot size and can access information of individual soot particles which are both shortcomings of the Eulerian method. In addition, the LST model was also demonstrated to be able to predict soot age. Apart from playing a role in determining primary soot size, soot age can also serve as a useful parameter to answer various fundamental questions, such as when and where soot particles grow to a certain size, and help in the understanding of fundamental soot processes. Optimisation of the model and extension of its capability to capture soot aggregate structure, size and fractal dimension will be of interest in the future.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Ho, Jee-Hou
Ng, Hoon Kiat
Pang, Kar Mun
Keywords: soot formation, soot oxidation, primary soot particle size, Lagrangian soot tracking, LST, diesel engine
Subjects: T Technology > TJ Mechanical engineering and machinery
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 43024
Depositing User: ONG, JIUN CAI
Date Deposited: 12 Oct 2017 07:07
Last Modified: 14 Oct 2017 20:08

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