Effects of ambient CO2 and H2O on soot processes in n-dodecane spray combustion using large eddy simulation

Min Zhang,Jiun Cai Ong,Kar Mun Pang,Xue-Song Bai,Jens H Walther

doi:10.1016/j.fuel.2021.122700

Abstract

In this study, large eddy simulations, coupled a two-equation soot model, are performed to investigate the effects of ambient carbon dioxide (CO2) and water (H2O) additions on the soot formation and oxidation processes in an n-dodecane spray flame. In the soot model, acetylene (C2H2) is soot precursor and surface growth species, while hydroxyl radical (OH) and oxygen (O2) are soot oxidizers. The effect of ambient CO2 and H2O additions on soot formation/oxidation can be separated into thermal and chemical effects. For the thermal effects, the ambient CO2 and H2O additions increase C2H2 but reduce OH formation by lowering the flame temperature. This leads to a higher soot mass formed. On the contrary to the thermal effects, the ambient CO2 and H2O additions reduce the soot formation due to their chemical effects. The reaction CH2∗+CO2↔CH2O+CO is found to be responsible for reducing C2H2 formation. The ambient H2O addition results in a higher OH but lower the C2H2 mass formed owing to the reverse reactions H2+OH↔H2O+H and OH+OH↔H2O+O. Furthermore, the chemical effects is more significant than the thermal effects under the tested conditions. This leads to a lower soot mass formed when adding ambient CO2 and H2O.

Highlights

Exhaust gas recirculation (EGR) has been developed to reduce the nitrogen oxides in diesel engines for the last decades [1,2]
The liquid penetration length (LPL) is defined as the maximum axial distance from the nozzle to the downstream location with the liquid fuel mass reaching 95% of its total instantaneous value [41], while the vapor penetration length (VPL) is defined as the axial distance from the nozzle to the downstream location where 0.1% fuel mass fraction is observed [42]
It should be noted that the measured data is the quasi-steady value ensemble-averaged from 12 repeatable experimental tests [19], while the simulated data is from a single realization

Summary

Introduction

Exhaust gas recirculation (EGR) has been developed to reduce the nitrogen oxides in diesel engines for the last decades [1,2]. Recent researches have shown that the components of EGR, i.e., carbon dioxide (CO2) and water (H2O), have significant influence on the soot formation processes [3,4]. It is fundamentally and practically important to further understand the effects of these EGR components on soot formation processes. A numerical study with respect to the effects of ambient CO2 addition on the soot formation in a ethylene (C2H4) diffusion flame was conducted by Guo et al [6], in which they proposed that the soot suppression effects due to the ambient CO2 addition is mainly attributed by three mechanisms: dilution, thermal, and chemical. There has been a consensus that these three mechanisms are responsible for the effects of ambient CO2 addition on soot formation processes [4,5, 13,14]. Liu et al [5] and Guo et al [6]

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