Soot formation in laminar counterflow flames

TLDR: A comprehensive review of the researches on various aspects of soot formation utilizing counterflow flames is provided in this paper, with focus on the most recent (post-2010) research progress.

About: This article is published in Progress in Energy and Combustion Science.The article was published on 2019-09-01 and is currently open access. It has received 276 citations till now. The article focuses on the topics: Diffusion flame & Soot.

Figures

Figure 41. Sooting structures of ethylene counterflow diffusion flames at elevated pressures. Data taken from Amin et al. [529].

Figure 21. Experimental peak PAH LIF signals detected at various wavelengths and peak soot LII signals in counterflow diffusion flames of 1-alkane fuels with 2-8 carbon atoms. Computed peak PAH concentrations were included. All data were normalized by the values for 1-butene flame. Reprinted from Wang et al. [635] with permission of Elsevier.

Figure 29. Correlation between threshold temperature for soot formation in SF flames and literature data of TSI. Reprinted from Wang and Chung [620] with permission of Elsevier.

Figure 39. Effects of pressure on the critical strain rate (a) and density-weighted critical strain rate (b) for soot formation in counterflow diffusion flames. Also shown is the linear correlation between ρ0Kcr and 𝑋Ó, ?/1P (c). Reprinted from Du et al. [573] with permission of Elsevier.

Figure 2. Major formation pathways of the first aromatic ring in flames of aliphatic fuels.

Figure 42. Axial profiles of temperature (a), SVF(b), and soot production rates (c) for ethylene CDFs with well-controlled convective time-temperature history. Adapted from Carbone et al. [489] with permission of Elsevier.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Soot formation in laminar counterflow flames" ?

In this paper, the authors defined soot as carbonaceous particles resulting from pyrolysis or incomplete combustion of hydrocarbon fuels. 

Q2. Why can soot particles be produced at locally rich regions?

But with the advent of gasoline direct injection (GDI) technology (intended mainly to improve fuel efficiency), soot particles can be produced at locally rich regions as a result of charge inhomogeneity. 

Q3. What methods were frequently used in investigations of sooting characteristics in high-pressure flames?

optical methods such as color pyrometry and laser light extinction / scattering were frequently applied in investigations of sooting characteristics in high-pressure flames. 

Q4. How did Gülder separate the effects of oxygen addition from thermal and dilution effects?

through reactant preheating to maintain adiabatic flame temperature and comparison against the N2-diluted flames, Gülder experimentally separated the chemical effects of oxygen addition from thermal and dilution effects. 

Q5. How can the authors study soot chemistry in counterflow flames?

In counterflow flames, residence time and stretch rate can be adjusted by varying fuel and/or oxidizer flow velocities, providing a unique way to study soot chemistry with variable residence time. 

Q6. What did Du and Guo find to offset the soot-promoting effects of higher flame?

The following were believed to offset soot-promoting effects of higher flame temperature from H2 addition: 1) In positively stretched flames, the preferential diffusion effects of H2 may cause precursor concentration reduction, decreasing the rate of soot formation; 

Q7. How can flames with different levels of dilution be compared?

In practice, flames with different levels of fuel stream dilution can be compared at a fixed peak temperature; and flames with different peak temperatures can also be compared at constant XO,0 and XF,0. 

Q8. How did they increase the range of oxygen addition in ethylene and propane flames?

They also extended the range of oxygen addition from an equivalence ratio of infinity (purely diffusion flame) to 3.0 and 2.6 for ethylene and propane flames, respectively. 

Q9. What is the effect of partial premixing on soot formation in methane flame?

In SFO flames, where the flame is located on the fuel side of the stagnation plane, fuel stream partial premixing was seen to monotonically decrease soot formation; while it was noted that such a decrease was consistent with the competition between soot inception, growth and oxidation, no detailed explanations were provided. 

Q10. What was the sticking coefficient of coagulation for the nascent organic carbon particle?

Through analysis of49the PSD data obtained from AFM and scattering measurements, D’Alessio et al. [324] found that the sticking coefficient of coagulation (i.e., the actual measured coagulation rate divided by the theoretical value obtained from gas kinetic theory) for the nascent nanoscale organic carbon particle (NOC, <3 nm) was orders of magnitude smaller than the larger soot particles. 

Q11. How does the dimensionality of counterflow flames affect the simulation of flames?

This becomes particularly relevant if models with detailed chemical kinetics and particle dynamics are to be employed for the simulation of these flames, as the decreased dimensionality of counterflow flames12significantly reduces the computational cost. 

Q12. What other methods were used to tackle the effects of pressure on soot properties?

The effects of pressure on soot properties other than SVF were also tackled, using light172scattering [69] and/or thermophoretic sampling [509]. 

Q13. How did they measure PAH and soot concentrations in ethylene-propane?

In a subsequent effort to identify the role of C3 chemistry in PAH growth beyond benzene, Lee et al. [582] measured PAH and soot concentrations in CDFs of ethylene-propane mixtures with a small amount of benzene addition (1.83%, molar concentration) to the fuel stream. 

Q14. Why is air sometimes intentionally co-injected with fuel in industrial combustion devices?

From a practical point of view, air is sometimes intentionally co-injected with fuel in industrial combustion devices, to improve fuel atomization.