Nascent soot particle size distributions down to 1 nm from a laminar premixed burner-stabilized stagnation ethylene flame

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Abstract

In this study, spatially resolved measurement of soot particle size distribution functions (PSDFs) down to ∼1 nm from a laminar premixed burner-stabilized stagnation ethylene flame was made by paralleling a commercial 3936 Scanning Mobility Particle Spectrometer (3936 SMPS) and a Diethylene Glycol (DEG) SMPS. While the 3936 SMPS may detect particles with a mobility diameter of 3–150 nm, DEG SMPS can be used to measure incipient soot particles of 1–10 nm. We found that the minimum diameter of the incipient soot particles appeared at ∼1.5 nm (though with some uncertainty caused by the classification device). A complete bimodality of the PSDFs was observed quantitatively when the burner-to-stagnation surface separation distance (Hp) was greater than 0.6 cm. Characterized by a lognormal distribution, the first peak appears to be relatively stable at different Hp, with the geometric standard deviation varying from 1.1 to 1.3 and the peak diameter ranging from 1.9 to 2.9 nm. The absolute number density of particles no bigger than the first peak diameter was found to be positively related to the first peak diameter and the geometric mean diameter of these particles.

Introduction

Soot generated from combustion is of considerable interest not only for its negative influence on environment and human health, but also for a number of applications, such as carbon black for automobile tires and pigment in laser-printer toners, all of which require a deep understanding of the mechanism of soot formation. One of the biggest challenges in this field of research is to understand the process of nucleation and mass growth, especially for nascent soot particles below 10 nm in flames.

Recent developments in experimental techniques have paved way for a few studies on nascent soot. The on-line dilution probe in conjunction with the scanning mobility particle sizer (SMPS), in particular, can follow the evolution of particle size distribution functions (PSDFs) from nucleation to mass growth for particles as small as 3 nm (if not specially mentioned, all diameters are referred to electrical mobility diameters). Based on this technique, previous studies [1], [2], [3], [4], [5], [6] have indicated that the premixed, lightly sooting ethylene flames are of bimodal characteristics at the region dominated by both soot nucleation and mass growth. However, the first peak was only partly observed because of the 3 nm cutoff [7] in commercial SMPS such as TSI 3936 used in these studies.

The 3 nm detection limit of commercial SMPS has caused several problems. First, without quantitative information of the sub-3 nm particles, the numerical simulations [1], [2], [6], [8], [9], [10] with a detailed gas-phase chemistry, nucleation and mass growth model could not be completely validated. Secondly, a previous study [10] indicated that the trough of PSDFs was very sensitive to the size of the nuclei, however a consensus about the size of the incipient soot particles has not been reached in various soot models. Mckinnon and Howard [11] made an operational definition for the size of a soot nucleus as 2000 amu and a projected area diameter of ∼1.5 nm based on off-line transmission electron microscopy observations. The smallest particle with a volume diameter of 0.87 nm was used in Ref. [2], while some nucleation models regarded the pyrene–pyrene dimer as the minimum-sized particle with a volume diameter of ∼0.8 nm [1]. Singh et al. [10], however, employed 224 carbon atoms, corresponding to spherical particles of 1.68 nm in volume diameter, to model the PSDFs, which matched well with experimental data. Thirdly, some experimental phenomena cannot be observed because of the limitation from experimental instruments. For example, the experimental results in Refs. [12], [13] indicate that the PSDFs will evolve into an apparent unimodal distribution in particle size larger than 3 nm in a high temperature flame, but it is unclear whether the observed unimodal distribution was affected by the instrument's detection limit.

While searching for solutions to the above-mentioned problems, the interests on studying particles smaller than 3 nm have continued over the years. Sgro et al. [14] have attempted to follow the gas to particle conversion process using a differential mobility analyzer (DMA) and a faraday cup electrometer (FCE). Based on their observations, there were three peaks in the PSDFs of the ethylene/air premixed flames at a sampling height of 10 mm. Particle diameters at the first and the second peaks of the PSDF were approximately 0.8 and 2 nm, respectively. According to their analysis, the first peak was attributable to carbon compounds or molecular clusters in the flame. Abid et al. [15] adopted a similar method to investigate the premixed flat ethylene flames and succeeded in extending the range of measurement down to ∼1.9 nm. Different from the results reported by Sgro et al. [14], minor drop-offs were observed around 2.1 nm, but they were attributed to inefficient non-steady state bipolar charging of the instrument. For super 3 nm soot particles, the measurement results obtained by FCE are comparable with those obtained by UCPC [15]. The common problem of these FCE measurements is that the results are interfered by charged molecular clusters (ions) when measuring particles smaller than 2.1 nm. In addition, FCE often suffers from high background noises. Although nascent soot particles smaller than 3 nm have been detected with FCE, it is difficult to obtain quantitative information due to these problems, and it remains unclear how small the particles could be in the fuel-rich premixed ethylene flames due to an ambiguous boundary between the nascent soot and the molecular clusters.

The lower detection limit of commercial SMPS (∼3 nm) mentioned above is due to one of its components, ultrafine condensation particle counter (UCPC, TSI 3025 or 3776), which uses butanol as the working fluid [7]. It can hardly activate particles smaller than 3 nm because of the significant Kelvin effect of these particles. Recent studies [16], [17] suggested that the working fluids with high surface tension and low vapor pressure may overcome this problem without causing homogenous nucleation. Diethylene glycol (DEG) meets with these criteria. UCPC using DEG as the working fluid has been proved to be able to activate particles down to ∼1 nm [16], [18], [19], [20]. Compared with FCE, DEG UCPC has low background noise and can be tuned to become insensitive to selected charged molecular clusters (ions) by adjusting the vapor saturation ratio inside the activation/growth unit [18]. Based on the new UCPC, DEG SMPS has been developed as a size spectrometer for sub-3 nm particles, which has been successfully deployed in measuring newly formed particles during atmospheric nucleation events [18], [19], [21]. This new spectrometer has not been used in any previous soot formation studies.

The purpose of this study is to expand the soot particle detection limit down to 1 nm, obtain detailed PSDFs in a burner-stabilized stagnation ethylene flame using the burner-stabilized stagnation probe together with the prototype DEG SMPS and a commercial TSI 3936 SMPS, and analyze the evolution of PSDFs, especially for sub-3 nm soot particles.

Section snippets

Experimental

The experimental setup is presented in Fig. 1. Similar to what was described in our previous work [22], [23], a laminar premixed flat ethylene flame with an unburned composition of 14.4% (mol) ethylene, 21.6% (mol) oxygen and 64% (mol) argon (equivalence ratio, φ = 2) was generated by a commercial McKenna burner with a stainless outer layer and a 6 cm-diameter-bronze water-cooled porous sintered plug. The cold gas velocity was 7 cm/s (STP), which was controlled by a sonic nozzle and calibrated by a

Results and discussion

We measured the temperature profiles at several burner-to-stagnation surface separations and made radiation corrections using the method described above. As shown in Fig. 2, the vertical error bars represent the uncertainty of emissivity of the coated thermocouple, which generates an approximate ± 90 K uncertainty in peak temperatures. The horizontal error bars are due to the position uncertainty. The model predicted temperature profiles agree well with the radiation-corrected measured

Conclusion

By expanding the lower detection limit down to ∼1 nm using a DEG SMPS, we observed a complete bimodality in the size distribution of nascent soot in a laminar burner-stabilized stagnation premixed ethylene flame. The smallest particles detected by the DEG SMPS in flame at different separation distances appeared at ∼1.5 nm in diameter, although only particles with a diameter larger than 1.67 nm were sure to exist in the flame with the nanoDMA. The diameter at the first peak was ∼2.4 nm, which was

Acknowledgment

Financially support from the National Key Basic Research and Development Program of China (2013CB228502 and 2013CB228505), the National Key Foundation for Exploring Scientific Instrument of China (20121318549), the National Natural Science Foundation of China (91541122, 21221004, 41227805, and 21422703), and “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB05000000) are acknowledged.

References (35)

  • J. Appel et al.

    Chemosphere

    (2001)
  • M. Balthasar et al.

    Combust. Flame

    (2003)
  • M.M. Maricq et al.

    Combust. Flame

    (2003)
  • B. Öktem et al.

    Combust. Flame

    (2005)
  • R. Lindstedt et al.

    Proc. Combust. Inst.

    (2013)
  • WangH. et al.

    Combust. Flame

    (1997)
  • M. Balthasar et al.

    Proc. Combust. Inst.

    (2005)
  • J. Singh et al.

    Combust. Flame

    (2006)
  • ZhaoB. et al.

    Proc. Combust. Inst.

    (2005)
  • A.D. Abid et al.

    Combust. Flame

    (2008)
  • L.A. Sgro et al.

    Proc. Combust. Inst.

    (2007)
  • A. Abid et al.

    Proc. Combust. Inst.

    (2009)
  • TangQ. et al.

    Combust. Flame

    (2016)
  • J. Camacho et al.

    Combust. Flame

    (2015)
  • R. Peterson et al.

    Combust. Flame

    (1985)
  • C. Saggese et al.

    Combust. Flame

    (2016)
  • M. Commodo et al.

    Proc. Combust. Inst.

    (2015)
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    1

    Quanxi Tang and Runlong Cai contributed equally to this work and should be considered as co-first authors.

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