**3.3 Concentration distributions of PAHs and soot**

PAHs are considered to be precursors of soot particles formed in a flame, because it bridges the mass gap between fuel molecules and soot particles (Hepp, 1995). Formation and growth of PAHs arise from chemical reactions beginning with pyrolysis of fuel, and then inception of soot particles occurs by coagulation of grown PAHs. To estimate the effect of fuel properties on the soot emission and the soot generation characteristics, concentration distributions of soot and PAHs was investigated.

Figure 7 shows planar images of LII and PAHs-LIF obtained from the test flames. Since the concentrations of fuel F was significantly lower than the other fuels, another color scale was supplied in the figure. Except for fuel F, high concentration of LII at outer edge of the flame reveals that the soot was presence cylindrically in the flame. Regarding the fuel F, although the presence of soot was cylindrically in the lower part of the flame, upper part was not such distribution. LII distributions indicate that soot particles quickly formed in the case of fuel C and D. Since these fuels contain relatively much two-ring and poly-aromatics, soot particles might be promptly formed from those aromatics.

In each flame, PAHs-LIF was detected just after the wick, and the intensity decreases with increasing the distance from the wick. Regarding the fuel F, PAHs-LIF appeared comparatively strong between *z*=10~20 mm. A very low PAHs-LIF intensity of fuel F implies a very low PAHs concentration. It was confirmed that PAHs-LIF intensity became stronger with the aromatic contents increases.

Figure 8 shows the intensity profiles of PAHs-LIF and LII on the flame axis. Note that the PAHs-LIF intensity of fuel F was indicated as multiply the original data by 5. The peak of

Effects of Fuel Properties on Diffusion Combustion and Deposit Accumulation 9

[10+4]

[×10+4]

0

0 0.4 0.8 1.2

E D F

*z* / *Lf*

(×5)

0

1

LII

LII intensity [a.u.]

2

3

[10+4]

[×10+4]

1

LII

LII intensity [a.u.]

0

2

4

PAHs–LIF intensity [a.u.]

0 5 10 15 20 25 <sup>0</sup>

Figure 9 shows the relationship between soot emission from the test flames and content of aromatics in the test fuels. Soot emission was derived from total LII intensity at *z*=65mm, where combustion reaction seemed to be finished. Soot emission linearly increased with

Content of aromatics [vol%]

6

PAHs–LIF

8 [10+5]

2

3

0 0.4 0.8 1.2

Fig. 8. Axial intensity profiles of PAHs-LIF and LII

1

Fig. 9. Relationship between soot emission and content of aromatics

2

3

Soot emission [a.u.]

increasing content of aromatics.

4

5 [10+5]

B A C

*z* / *Lf*

0

2

4

PAHs–LIF intensity [a.u.]

6

PAHs–LIF

8 [10+5]

the PAHs-LIF was located at just after the wick in any flames. Since fuel F did not contain any aromatics, the PAHs would be formed by pyrolysis of paraffin components and subsequent reactions within the wick. However, PAHs-LIF intensity of fuel F at just after the wick was much lower than that of the other fuels, thus the PAHs-LIF intensity detected at just after the wick of the other fuels might be derived mainly from the originally contained aromatics in the fuel. The PAHs-LIF intensity rapidly decreases with distance from the wick; this implies that the growth of PAHs occurred by condensation polymerization of PAHs and thereby the number of PAHs molecules decreases.

In the case of fuel F, increase of PAHs-LIF from *z*/*Lf* =0.15 suggested that the PAHs were newly formed within the flame. The same occurred within the other flames, but impact on the profile was small due to the relatively large LIF intensity of the originally contained PAHs. The disappearance location of the PAHs-LIF coincides with the location where the LII intensity rapidly increases. This figure clearly shows that the soot was formed via PAHs.

the PAHs-LIF was located at just after the wick in any flames. Since fuel F did not contain any aromatics, the PAHs would be formed by pyrolysis of paraffin components and subsequent reactions within the wick. However, PAHs-LIF intensity of fuel F at just after the wick was much lower than that of the other fuels, thus the PAHs-LIF intensity detected at just after the wick of the other fuels might be derived mainly from the originally contained aromatics in the fuel. The PAHs-LIF intensity rapidly decreases with distance from the wick; this implies that the growth of PAHs occurred by condensation polymerization of PAHs and

0

20 [mm]

0

F F

20 [mm]

0 16000

0 28000

[a.u.]

[a.u.]

[a.u.]

PAHs fluorescence

A B D C E

0 56000

[a.u.]

In the case of fuel F, increase of PAHs-LIF from *z*/*Lf* =0.15 suggested that the PAHs were newly formed within the flame. The same occurred within the other flames, but impact on the profile was small due to the relatively large LIF intensity of the originally contained PAHs. The disappearance location of the PAHs-LIF coincides with the location where the LII intensity rapidly increases. This figure clearly shows that the soot was formed via PAHs.

0 740000

Fig. 7. Planar images of PAHs-LIF and LII

A F B D C E F

Laser-induced incandescence

thereby the number of PAHs molecules decreases.

Fig. 8. Axial intensity profiles of PAHs-LIF and LII

Fig. 9. Relationship between soot emission and content of aromatics

Figure 9 shows the relationship between soot emission from the test flames and content of aromatics in the test fuels. Soot emission was derived from total LII intensity at *z*=65mm, where combustion reaction seemed to be finished. Soot emission linearly increased with increasing content of aromatics.

Effects of Fuel Properties on Diffusion Combustion and Deposit Accumulation 11

Table 4 shows the results of the composition analyses of the test fuels. The results confirmed that fuel K consisted only of saturated hydrocarbons (i.e., paraffins and naphthenes). In contrast, other fuels contained between 7.6~19.5 vol% aromatic hydrocarbons. Most of the aromatic components of the fuels were one-ring aromatics; two-ring aromatics were very rare. In addition, the following low-stability components are also shown in Table 4: naphtheno benzenes, olefin hydrocarbons (bromine number), dienes (diene value), and organic peroxides (peroxide number). Naphtheno benzenes, compounds that consist of naphthene and aromatic rings, were contained in higher concentrations in fuel G (5.4 vol%); these molecules were not contained in fuel K. The bromine number was largest in fuel J (28.8 mgBr2/100g) and lowest in fuel K (4.4 mgBr2/100g). The diene values of fuels J and K indicated 0.05 gI2/100g and 0.01 gI2/100g, respectively; dienes were not detected in other fuels. The peroxide numbers

> Component G H I J K Aromatic HC [vol%] 19.5 17.2 16.9 7.6 0.0 Unsaturated HC [vol%] 0.1 0.0 0.0 0.0 0.0 Saturated HC [vol%] 80.4 82.8 83.1 92.4 100.

> 1-aromatics [vol%] 19.1 17.0 16.7 7.5 0.0 2-aromatics [vol%] 0.4 0.2 0.2 0.1 0.0 3+-aromatics [vol%] 0.0 0.0 0.0 0.0 0.0

> [vol%] 5.0 4.8 4.2 2.5 0.0

[vol%] 0.4 0.5 0.3 0.0 0.0

[vol%] 0.0 0.0 0.0 0.0 0.0

[mgBr2/100g] 9.4 9.0 10.1 28.8 4.4 Diene value [gI2/100g] 0.00 0.00 0.00 0.05 0.01

[mg/kg] 0.0 0.0 0.0 0.0 0.0

Tar-like deposits, formed by the thermal decomposition and polycondensation of the fuel, adhered to and accumulated on the upper part of the wick during combustion. To investigate the effects of fuel properties on tar-like deposit accumulation, tar-like deposits were accumulated on the wick by prolonged combustion. The deposit mass was obtained by subtracting the mass of unused wick from the mass of the wick that had accumulated deposits. In this process, if unburnt fuel remained in the wick with deposits, an accurate deposit mass could not be obtained. To eliminate remaining fuel, the wick was pulled up from the pool with a flame, and combustion was continued until all fuel was used. Obtained deposit was visually confirmed as tar-like deposit, but soot particle might have slightly

Table 4. Composition of test fuels used in the deposit accumulation experiment

**4.2 Relationship between fuel properties and deposit accumulation** 

0

demonstrate that none of the test fuels contained organic peroxides.

1-naphtheno benzenes

2-naphtheno benzenes

3+-naphtheno benzenes

Bromine number

Peroxide number

adhered on the wick.
