Study of the Evolution of Soot Formation using Laser-induced

Incandescence

Lincoln Tolomelli e Tolomelli

, Luiz Gilberto Barreta

1,2

Pedro Teixeira Lacava

and Dermeval Carinhana Jr.

1,2

Instituto Tecnológico de Aeronáutica, Pr. Mal Eduardo Gomes, 50 – Vila das Acácias, São José dos Campos, Brazil

Instituto de Estudos Avançados, Trevo Cel Av José A. A. do Amarante, n

1 – Putim, São José dos Campos, Brazil

Keywords: Biodiesel, LII, Diagnostic, Non-intrusive.

Abstract: Soot particles usually cause respiratory diseases and other problems to human health. To prevent or at least

reduces soot emissions it is necessary to know its formation mechanism. Laser-Induced Incandescence (LII)

has been used to detect soot and its precursors, known as polycyclic aromatic hydrocarbons (PAHs), in

diffusion flames. In this work, several mixtures of diesel/biodiesel blends were investigated using two laser

wavelengths, at 532 nm, which excites both soot and PAHs, and at 1064 nm, which excites only soot. Thus,

the difference of intensity between both LII signals provides the proportion of soot/PAHs in the irradiated

regions of flames, and it can be associated to the evolution of soot formation along the flame.

1 INTRODUCTION

Diesel engines are currently the largest source of

power generation in the planet, with employments in

transportation, mining machinery and other areas.

However, Diesel cycle is the major source of

atmospheric pollutants, as soot and PAHs

(polycyclic aromatic hydrocarbons) (Braun et al.

2003). Due to their small size, soot particles can

penetrate into the alveoli of the lungs and may cause

respiratory diseases (Lacava et al. 2010). Besides

health problems, the presence of soot represents an

important loss of energy in the thermodynamic cycle

and its generation is related with the incomplete

combustion of hydrocarbon fuels. Thus, the main

interest in the soot reduction concerns both to human

health and to environmental applications (Frenklach,

2002).

Soot are particles with size less than 0.1 µm, and

they are formed in the flame front, i.e., the region in

contact with the atmosphere air where burner

reactions of hydrocarbon fuels take place and solid

nuclei are generated. In this region, the complete

oxidation of hydrocarbon fuels is also observed,

mainly in poor fuel and high temperature flame

conditions (Williams, 1976).

Alternative fuels, such as biodiesel, have been

developed to reduce the emission of soot of the

transportation sector. Biodiesel is made by the

esterification of long-chain fatty acids derived from

vegetable oils or animal fat (Mello et al, 2001).

Biodiesel and its blends diesel/biodiesel present a

high efficient combustion and can replace mineral

diesel in compression ignition engines without

significant changing of performance (Altin et al,

2001).

The soot detection can be carried out using

several intrusive and non-intrusive techniques. One

of the most efficient one is the Laser-Induced

Incandescence (LII). LII is based on the temperature

rise of soot particles above the background

temperature by a high power incident laser. In this

condition, the irradiated particles emit radiation like

a blackbody, and the intensity of the continuous

radiation is proportional to soot fraction of volume

(Melton, 1984). When the laser pulse irradiates a

soot particle, four physical-chemistry phenomena

are observed in the particle (Figure 1): the

absorption of the radiation by the particle; the heat

conduction; the sublimation and, finally; the

blackbody emission or incandescence. Since this

method was first described, LII has been applied in

studies concerned to the spatial and the temporal

qualitative distribution of soot, the quantitative

determination of the soot fraction volume (Shadix

and Smith, 1996) and in the size evaluation of

primary particles (Mewes and Seitzman, 1997).

102

Tolomelli e Tolomelli L., Gilberto Barreta L., Teixeira Lacava P. and Carinhana Jr. D..

Study of the Evolution of Soot Formation using Laser-induced Incandescence.

DOI: 10.5220/0004714501020106

In Proceedings of 2nd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2014), pages 102-106

ISBN: 978-989-758-008-6

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

Figure 1: Physical-chemistry processes observed when a

high power pulse irradiates a soot particle.

The aim of this work is to study the soot formation

in blends with several proportions of biodiesel/diesel

using LII as diagnostic non-intrusive technique. The

biodiesel flames were irradiated by laser

wavelengths of the 532 nm and 1064 nm. From the

comparison between both LII intensities, the

proportion between the productions of soot and

PAHs can be estimated (Vander Wal, 1996).

2 EXPERIMENT

2.1 Burner and Fuels

The burner used in this work is identical to described

in a previous work (Tran et al, 2012). A scheme of it

is shown in Figure 2. Several blends of

diesel/biodiesel were used as fuel, with an increment

of amounts (v/v) of biodiesel to the pure diesel,

identified in this work as B0, of 5% (B5), 10%

(B10), 20% (B20) and 50% (B50). The biodiesel

used in the experiments was provided by the

company Fertibom, and was made from bovine fat

(10%) and vegetable oil (90%).

2.2 LII Excitation and Detection

Basically, the LII diagnostic is divided in two steps:

the excitation of the soot and the detection of the

incandescence signal. The excitation process

depends of the wavelength and the fluency of the

incident laser. The radiation absorption is higher for

smaller laser wavelengths. However, for ultraviolet

lasers, the soot fluorescence, and mainly of the

PAHs fluorescence, can also take place as an

interference signal. Thus, the use of higher laser

wavelengths is recommended to avoid these spectral

interferences. However, the 532 nm laser also

produces a remarkable fluorescence in a large class

of PAHs. (Vander Wal, 2009)

Figure 2: Wick fed lamp: diffusion flames generator.

The LII signal rises almost linearly with laser

energy. As energy increase, the system temperatures

also increase, and at about 4000 K, the vaporization

of irradiated species starts. Above this threshold, LII

signal remains quite constant. Figure 3 shows the

results of LII signal measurements from a pure

diesel flame (B0) versus the pulse energy of the both

532 and 1064 nm incident laser used in this work.

One can observes that for both lasers, LII signal

increases up to 14 mJ and, over this value, the

collected signal becomes slightly constant (Vander

Wal, 1994). This means that the LII signal does not

suffer interference of small variations from the

incident laser. All LII measurements in this work

were carried out above this laser energy to guarantee

that the LII measures were dependent only of the

soot fraction volume.

2.3 Experimental Arrangement

Figure 4 shows a scheme of the experimental

arrangement used in this work. A pulsed Nd:YAG

operating at first and second harmonic, 1064 and

532 nm, was used as energy source. These

wavelengths excite, respectively, the soot particles

and the soot plus the polycyclic aromatic

hydrocarbons existing on the soot surface. Laser

beams were concentrated in the flames by using a

lenses system, which increased the laser fluency.

The LII signal was collected by lenses into a

monochomator to avoid spectral interferences. The

LII signal was detected by photomultiplier tube and,

finally, recorded by a digital oscilloscope.

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Figure 3: The behavior of LII signal versus the incident

laser energy in pure diesel flames (B0).

Figure 4: Experimental set up of Laser-induced

incandescence.

2.4 Description of the Experiment

The LII signal was collected from two different

regions of the flames: at 80 mm and at 260 mm

above the top of burner. The first one corresponds,

according to the literature, to the region where starts

the soot formation, while the second one concerns to

the completion of this process (Gaydon, 1957). All

LII intensities were normalized using as reference

the values obtained at 260 mm for each flame,

because, in thesis, these values are the highest

possible of the LII signal. Thus, this procedure

allowed the comparison between the results of 1064

and 532 nm lasers and, as consequence, a qualitative

estimation of the proportion of soot/PHAs in the

flames. The LII signals showed in this work were

calculated from an average of at least 500

measurements. This applied statistics supported the

reliability of the acquired data.

3 RESULTS

3.1 Mapping of Soot/Pahs in the Flame

Figure 5 shows a picture of a B0 flame and a

representation of the investigated flame heights

(dashed lines). In diffusion flames like this, a strong

yellow emission is observed due to the presence of

soot (Gaydon, 1957). Beside the flame picture, one

can observed a graph of the LII intensities along the

horizontal axis of the flame for each height. At 0.8

mm, LII signal showed a distribution with two

maxima separated by a valley. Both ones

corresponds to the yellow soot region, while the

valley is due to the dark poor soot region localized

in the center of the flame. At 260 mm, however,

only one maximum is observed, and it is associated

to the strong yellow emission in the top of the flame.

3.2 Comparison between 1064 and 532

Nm Data

Figure 6 shows an overall view of the LII

distribution of the investigated flames. It can be

observed that the LII signal obtained using 532 nm

laser is higher than 1064 nm for all studied blends.

This behaviour can be atributed to the amount of

PAHs on the soot’s surface. According to the

literature (Gaydon, 1957), at the bottom of diffusion

flames, which corresponds to the height of 80 mm

above the burner in this work, the process of soot

particles formation is still in progress, and, therefore,

the existence of a certain amount of precursors on

the soot’s surface is expected.

Thus, the 532 nm LII intensity profile at the 80

mm corresponds to the convolution of the soot and

PHOTOPTICS2014-InternationalConferenceonPhotonics,OpticsandLaserTechnology

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Figure 5: The soot distribution in a diesel diffusion flame at heights of 80mm and 260 mm above the burner (dashed lines).

PAH signals, while the 1064 nm intensity is only

due to the soot presence. The results in Figure 6

showed that the difference between the LII

intensities of both laser wavelengths increases with

the rise of the biodiesel fraction. This rise is initially

small from B0 to B5 blends, but the increment ratio

systematically enlarges from the B5 to the B20.

However, the LII intensities of the B50 blend are

just slightly higher than observed for B20 blends.

This suggests that the adding of biodiesel to the pure

diesel fuel causes a delay in the soot formation

process. To confirm this inference, a more complete

mapping of the flames must be done.

4 CONCLUSIONS

The aim of this paper was to investigate the relative

amount of precursors on the soot’s surface in the

bottom of flames of diesel/biodiesel blends for

estimate the evolution of soot formation using the

technique of Laser-Induced Incandescence. The

results showed that the LII technique is efficient to

evaluate the spatial distribution of soot in a diffusion

flame. For all fuels investigated, B0, B5, B10, B20

and B50, the LII signal intensities obtained using

532 nm laser were higher than the obtained with

1064 nm laser. This behaviour is according to the

former studies of flame literature. At the bottom of

flames (80 mm), soot is still in formation process.

The results also showed the addition of biodiesel to

the diesel blends causes a delay in the soot formation

process.

ACKNOWLEDGEMENTS

The authors thanks to CAPES for the fellowships

and FINEP for financial support.

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StudyoftheEvolutionofSootFormationusingLaser-inducedIncandescence

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Figure 6: The LII signal intensities for the 532 nm and the 1064 nm lasers for all fuels investigated.

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