Elsevier

Fuel

Volume 279, 1 November 2020, 118511
Fuel

Full Length Article
Charge characteristics of sub-10 nm soot particles in premixed ethylene flames

https://doi.org/10.1016/j.fuel.2020.118511Get rights and content

Highlights

  • Particle size distribution and size-dependent charge fraction of 1–10 nm soot were investigated.

  • The negatively and positively charged particles as small as 1 nm were found.

  • At higher flame temperatures, more charged particles were found because more ions generated and coagulated to soot.

  • Ion-induced soot nucleation was not the main contributor to nucleation since the charge fraction was less than 10% when Tmax < 2000 K.

Abstract

In this work, the charge characteristics of sub-10 nm soot particles were studied in burner-stabilized-stagnation premixed ethylene flames, at an equivalent ratio of 2.0 and over the calculated maximum flame temperature (Tmax) range of 1665–1933 K, using micro-orifice probe sampling in tandem with neutralizer, half-mini differential mobility analyzer (Half-mini DMA) and electrometer. To obtain the charge fraction, the charged and total particle size distributions (PSDs) were measured with the neutralizer off and on, separately. Our results showed that both negatively charged particles (NCP) and positively charged particles (PCP) grew to bigger particles with the increase of height above burner (Hp). However, compared to PCP, NCP were relatively smaller in size, but higher in concentration, possibly because negative ions such as electrons diffused faster to coagulate with soot. Moreover, charge fraction decreased along Hp and newly nucleated soot particles were all neutral at higher Hp, which could be attributed to the higher temperature near the flame front, where the concentrations of ions were higher. Another interesting finding was the increase in charge fraction by two orders of magnitude (from 0.1% to 10%) as Tmax increased from 1665 K to 1933 K. All of these suggested the existence of particle-ions interaction close to the flame front, especially at high temperatures.

Introduction

Ions formed in high temperature flames may affect soot formation processes [1], [2], [3], [4], [5], [6]. On one hand, they can enhance particle coagulation by colliding with soot particles [7], [8], [9], [10]. On the other hand, they may serve as the initial soot precursor for generating incipient soot [11]. Since ions may play an important role in soot formation, they have received substantial attention for decades. Calcote et al. [11] suggested that precursor ions such as C3H3+ could react with neutral species, for example acetylene, to produce larger positive ions and charged particles. Lias et al. [12] showed that ion-molecule reaction rate coefficients were generally several orders of magnitude greater than those of neutral species, and ion-molecule reaction rates were fast in high-temperature sooting flames. Kraft and coworkers [13], [14] proposed that enhanced interactions between polar corannulene molecules and cations resulted in more rapid and abundant clustering and longer cluster life times, suggesting that interactions between curved PAHs and ions may play a significant role in soot formation. In contrast, Haynes et al. [15] argued that concentrations of ions were too low to play a major role in soot formation.

To investigate the effect of ions on nucleation and coagulation, it is necessary to study the charge characteristics of soot particles. Maricq [16] measured the charged particle size distributions (PSDs) and charge fractions of soot particles in premixed ethylene flames by nano-differential mobility analyzer (DMA) and condensation particle counter (CPC). The results showed that nanoparticles carried predominantly with a single charge and the charge fraction increased with the increase of size. Due to the low activation efficiency of CPC and high diffusion loss of nano-DMA for sub-3 nm particles [17], [18], measurements were performed only for particles larger than 3 nm. To shed light on sub-3 nm soot particles, Sgro et al. [19], [20] used a higher resolution TapCon DMA coupled with an electrometer to investigate charged particles. However, these works focused on the positively charged particles and no results of negatively charged particles were reported. Besides, the diffusion loss was still too high and the resolution was still too low to get accurate data for sub-3 nm particles. These limitations have hindered the discovery of charge characteristics for sub-3 nm particles.

To counteract the diffusion broadening of DMA transfer functions, de la Mora [21], [22] designed high resolution DMA (Half-mini DMA) with new configurations and with sheath flow rates over 100 lpm, which significantly reduced the residence time and diffusional loss of classified particles. By applying an electrometer downstream of the Half-mini DMA, Wang et al. [23], [24], [25] measured the concentrations of classified sub-3 nm particles successfully. Thus, the combination of Half-mini DMA coupled with an electrometer may serve as a powerful tool in studying the charge characteristics of soot particles below 3 nm.

In this work we investigated the charge characteristics of nascent soot (sub-10 nm) particles in burner-stabilized-stagnation premixed ethylene flames, using micro-orifice probe sampling in tandem with neutralizer, Half-mini DMA and electrometer. With the neutralizer on, total PSDs (including both charged and neutral particles) were obtained by measuring both positively charged particles (PCP) and negatively charged particles (NCP) coming out of the neutralizer. With the neutralizer off, the PSDs of natural positively and negatively charged particles from flames were measured. Considering that ions may be significantly affected by flame temperature, we studied the size-dependent charge characteristics of sub-10 nm soot particles at different flame temperatures.

Section snippets

Experimental

The experimental setup is shown in Fig. 1. It consists of burner-stabilized-stagnation flames, a high-dilution sampling system and sub-10 nm particle measurement instruments. The burner-stabilized-stagnation flames and the sampling probe are the same as those in our previous studies [26], [27], [28] while the combination of a Half-mini DMA and an electrometer is used to measure sub-10 nm particles.

Firstly, flames are generated by a commercial McKenna burner with a stainless outer layer and a

Results and discussion

Fig. 2 shows the PSDs in Flame A2 measured with a neutralizer. Soot particles passing through the neutralizer were assumed to follow the Fuchs’ steady-state charge distribution. The total PSDs were inferred from measurements of either positively charged particles or negatively charged particles, and they are termed as T-PCP and T-NCP, respectively. Each point in Fig. 2 is an average of at least three repeated experimental results. Due to the existence of negative ions below 1.6 nm and positive

Conclusion

In this study, we investigated the charge characteristics of sub-10 nm soot in premixed ethylene-oxygen–nitrogen flames (Φ = 2.0) with varying maximum flame temperatures from 1665 K to 1933 K. The charged and total particle size distributions were measured using a tandem equipment of neutralizer (off or on)-Half-mini DMA-electrometer. The size-dependent charge fractions of sub-10 nm soot at different Hp were analyzed. The charged PSDs showed a lognormal distribution with the peak diameter at

CRediT authorship contribution statement

Mengda Wang: Conceptualization, Methodology, Data curation, Investigation, Writing - original draft. Girish Sharma: Conceptualization, Methodology, Data curation, Investigation, Writing - review & editing. Huang Zhang: Data curation, Writing - review & editing. Xiaoqing You: Supervision, Resources, Writing - review & editing. Pratim Biswas: Supervision, Resources, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (51761125012) and a grant from the National Science Foundation, SusChEM: Ultrafine Particle Formation in Advanced Low Carbon Combustion Processes; CBET 1705864. Girish Sharma would like to thank McDonnell International Academy at Washington University in St Louis for their support.

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