Soot suppression by acoustic oscillated combustion

ELSEVIER PII: SOO16-2361(97)00286-X

Fuel Vol. 77, No. 9/10, pp. 973-978, 1998 0 1998 Elsevier Science Ltd. All rights reserved

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Soot suppression by acoustic oscillated combustion

Masahiro Saitoar*, Masayuki Satob and Akira Nishimurab a Department of Mechanical System Engineering, Gunma University, I-5 1 Tenjin-cho, Kiryu, Gunma 376, Japan bDepartment of Biological and Chemical Engineering, Gunma University, l-5- 1 Tenjin-cho, Kiryu, Gunma 376, Japan (Received 25 July 1997)

The soot suppression of acetylene diffusion flames by applying acoustic oscillation was studied experimentally. The soot emission decreased by increasing the sound pressure level and lowering the frequency of the acoustic oscillation. The efficiency of soot suppression can be related with acoustic Reynolds number, Re*. The result showed that the efficiency of soot suppression exceeded 90% in a region of Re* > 3000. The lktme temperature increased by applying the acoustic oscillation. It is considered that the acoustic oscillation enhanced the mixing of the fuel gas and surrounding gas, consequently, the high flame temperature caused the re-oxidation of soot particles. 0 1998 Elsevier Science Ltd. All rights reserved

(Keywords: soot; acoustic oscillation; acoustic field; soot suppression)

NOMENCLATURE

D

-Q Re* SPL T u* W 6 P

P

Subscripts a f 0

inner diameter of glass tube (mm) frequency (Hz) flow rate of acetylene (ml mini’) acoustic Reynolds number (-) sound pressure level (dB) temperature (“C) air velocity at measuring position (m s-r) mass of soot (mg) fluctuation width of oscillating flame (mm) density of surrounding air (g cm-3) viscosity of surrounding air (g cm-’ s)

acoustically oscillated flame non-oscillated

INTRODUCTION

It is known that the acoustic field is effective for improving combustion and atomization characteristics’- . The authors536 reported that the evaporation/combustion rate coefficients were increased up to three and two times compared with non-oscillated evaporation and combustion at frequencies below 100 Hz and sound pressure levels above 100 dB. The objective of this study is to investigate the effects of acoustic oscillation on soot suppression.

Most previous studies on soot have been directed towards examination of the mechanism of soot formation and the measurement of soot concentration in flames using a laser light-scattering technique7-“. Gomez et ~1.~ studied the temperature and soot particles on the centerline of the

* Author to whom correspondence should be addressed.

axisymmetric laminar diffusion flames. They observed that soot onset on the centerline occurred when a temperature of 1080°C is encountered. Arai and Hiroyasu’* conducted the measurement of soot concentration in turbulent diffusion flames. They reported that the maximum soot concentration in the flame was increased with increasing the temperature at the soot formation zone in the flame. The soot concentration in the flame became maximum when the temperature in the soot formation zone in the flame was 1130°C.

A few studies on soot suppression have been published. Ohisa et ~1.‘~~‘~ studied the soot suppression using a corona discharge and reported the efficiency of soot suppression was maximum 97.5%. They suggested that the mechanism of the soot suppression by corona discharge was due to the aeration effect and the turbulence of the flame by ionic wind.

The authors15 investigated the soot emission by applying an electric field and concluded that the soot suppression was caused by high flame temperature due to the ionic wind which enhanced the mixing of the fuel gas and surrounding gas.

In the present study, the effects of frequency and sound pressure level of acoustic field on soot suppression are investigated for acetylene diffusion flames.

EXPERIMENTAL

The schematic diagram of experimental apparatus is shown in Figure 1. A sine wave was applied to a diffusion flame from two speakers (101.6 mm woofer, FOSTEX, FW-108) which were set at both ends of a Pyrex glass tube (O.D. 34 mm, I.D. 30 mm). The sound pressure level at the measuring position was measured by means of a sound level meter (Rion, NL-04). The frequency, f, and sound pressure

Fuel 1998 Volume 77 Number 9/10 973

soot suppression acoustic oscillated combustion: M. Saito et al.

Glass fiber filter

-.I Acetylene cylinder

Figure 1 Schematic diagram of the experimental apparatus

level, SPL, of the sine wave were changed in the range of f = 30-150 Hz and SPL = 40-l 10 dB, respectively.

Acetylene gas is used as the sample fuel in this experiment. The flow rate of acetylene gas ejected from the nozzle (stainless steel pipe, O.D. 2.0 mm, I.D. 1.0 mm) was kept constant at Q = 30,50,70 ml min-‘. Soot particles generated from the acetylene diffusion flame were sampled by means of a soot collector with a glass fiber filter. The glass fiber filter was dried for 60 min at a temperature of 170°C before soot sampling in order to prevent water within the filter. The sampling time for the soot particles was 1 min in each run. The weight of soot particles collected on the glass fiber filter was determined using a electronic balance with an accuracy of 0.01 mg.

The behavior of the acoustic oscillated combustion was recorded using a CCD camera (SONY, TR-3) or 35 mm camera (Nikon F-801) with a motor drive. Also, the flame temperature was measured by means of a high- speed two-color pyrometer, which was described in detail elsewhere16.

RESULTS AND DISCUSSION

Burning behavior of acoustic oscillated combustion Figure 2 shows the typical photographs of acetylene

diffusion flames (Q = 30 ml min-‘) with increasing sound pressure level of acoustic oscillation under constant frequency off = 30 Hz. These photographs, except Figure 2(a), were taken by using a continuous mode (3.3 frames s-l) of motor drive with a shutter speed of l/8000 s. In these photographs, (b)-(f), nine frames of the oscillating flames were superposed on the same film.

In the case of non-oscillated combustion, Figure 2(a), a quiet envelope flame is formed normally. When mild oscillation of SPL = 47 dB is applied to the flame, the flame started to fluctuate transversely, as can be seen in Figure 2(b). As the SPL was raised, transverse oscillation of the flame became violent. At SPL > 52 dB, more violent oscillation was observed and burning soot particles appeared at the top of the flame Figure 2(f).

Effect of acoustic oscillation on soot suppression Figure 3 shows the effect of acoustic oscillation on soot

suppression. The vertical axis is defined as the ratio of mass of soot at acoustic oscillated combustion, W,, to that at non- oscillated combustion, WO. The horizontal axis is the sound pressure level and the parameter is frequency. In the case of f = 30 Hz, when the sound pressure level less than SPL = 50 dB (corresponding to Figure 2(b)) soot suppression due to acoustic oscillation was very low.

However, soot emission was suppressed rapidly at sound pressure level greater than 50 dB, and the efficiency of the soot suppression exceeds 90% in the region over SPL = 70 dB. Also, the value of SPL at rapid reduction of soot emission shifted to a greater sound pressure level as the frequencies increased f = 60, 90, 120 Hz stepwise.

Figure 4 shows the effect of flow rate of acetylene on soot suppression. As is evident from the figure, no effect of flow rate on the soot suppression was observed within the range of Q = 30-70 ml min-‘.

Flame jluctuation due to acoustic oscillation In order to investigate the frequency synchronization

between the flame fluctuation and the applied acoustic

974 Fuel 1998 Volume 77 Number 9/10

Soot suppression by acoustic oscillated combustion: M. Saito et al.

Figure 2 Burning behavior of acoustic oscillated combustion (Q = 30 ml mine’; f = 30 Hz)

oscillation, the oscillating flames were taken by using a CCD camera. The frequency of flame fluctuation was measured by playing back the frames of the picture. From the results, it was confirmed that the flame fluctuation was synchronized with the frequency of applied acoustic oscillation.

Also, the displacement and frequency of the surrounding air oscillating by acoustic wave was measured by means of a analogue sensor (upper portion of Figure I). The measurement of the displacement is based on the principle of the variation of eddy current between the sensor and a stainless steel foil (5 X 20 X 0.05 mm) placed at the measuring position. Figure 5 shows the oscillographs of the signal from the sensor for f = 30, 60, 90 Hz. From the results, the oscillation of the surrounding air at the

measuring position was synchronized with the applied acoustic oscillation.

Thus, it was found that the flame fluctuation due to acoustic oscillation was synchronized with the frequency of acoustic oscillation.

Relation between soot suppression and acoustic Reynolds number

Figure 6 shows the variation of the fluctuation width, 6, of a flame when acoustic oscillation was changed. 6 is the maximum of flame fluctuation as shown in Figure 7. The fluctuation width increased with increasing sound pressure level and with decreasing frequency. By using the measured fluctuation width, 6, air velocity at the measuring position,



Q = 30 dmin I

1 V f=120Hz 1 -1

40 50 60 70 80 90 100 110 120 130 140 150 Sound pressure level, SPL ( dB )

Figure 3 Effect of acoustic oscillation on soot suppression

\ ??4 \ Au \

f@

A f=60Hz,Q=SOmVmin

0 f=150Hz,Q=30mhin

A f=l5OHz,Q=5Oml/min

m f=150H~Q=70mllmm

01 ’ ’ ’ ,~I ’ I I I I

40 50 60 70 80 90 100 110 120 130 140 150 Sound pressure level, SPL ( dE8 )

Figure 4 Effect of flow rate of acetylene on soot suppression

u*, was determined as follows:

u * =f26. (1)

The direction of u* is reversed periodically because the flame is oscillating side by side. Thus, acoustic Reynolds number, Re*, is defined as

Re* = Du*plp (2)

where D is the inner diameter of the Pyrex glass tube, and p and ~1 are the density and viscosity of the surrounding air, respectively.

Figure 8 shows the relation between soot suppression and acoustic Reynolds number for all experimental data. In the region of Re* < 2000 (laminar flow region), the efficiency of soot suppression was less than 50%. On the contrary, the efficiency of soot suppression exceeds 90% in the region of Re* > 3000 (turbulent flow region).

Therefore, it is considered that soot suppression by acoustic oscillation was due to the mixing of the fuel gas and surrounding air.

Variation of flame temperature with acoustic oscillation Figure 9 shows histograms of the flame temperatures, Tf,

measured by using the high-speed two-color pyrometer. These data were obtained from about 100 runs under the same conditions of acoustic oscillation. As is evident from

20 ms/div., 50 mV/div.

Figure 5 Frequency characteristics of oscillating air at the measuring position

these histograms, in the case of non-oscillated combustion (a), the flame temperature was around Tf = 1500°C. On the other hand, in the case of acoustic oscillated combustion (b) and (c), high flame temperature appeared with increasing sound pressure level. It is concluded that the acoustic oscillation enhanced the mixing of the fuel gas and surrounding air. This results in the increase of flame temperature, and caused the re-oxidation of soot particles.



En/ /, , /, , , J 30 40 50 60 70 80 90 100 110 120

Figure 6 level

Figure 7

Sound pressure level, SPL ( dI3 )

Relation between flame fluctuation and sound pressure

(a) SPL : small (b) SPL : large

Illustration of flame fluctuation

* Non oscillation

0 Q=30mmin. f=3oHz

A Q=30,n!,min, MOHz

0 Q=30mlhin. f=9oHz

?? Q=SOmVmiq f=3OHz

A Q=5Odmin, f=mHz

W Q=SOm”m, PXHz

0 Q=‘lOm”mn, f=3OHz

V Q=7OmVmi~ f;6OHz

0 1000 2000 3000 4000 5000 6000 7000

Acoustic Reynold number, Re* ( - )

Figure 8 Relation between soot suppression and acoustic Reynolds number

CONCLUSIONS

The soot suppression of the acetylene diffusion flame by applying acoustic oscillation was studied. The following results were obtained.

(1)

(2)

In the laminar flow region (Re* < 2000), the efficiency of soot suppression was less than 50%, while the efficiency exceeds 90% in the turbulent flow region (Re* > 3000). The flame temperature for acoustic oscillated combustion was higher than that for non-oscillated combustion.

SOL,,,,,,,,,,,,,,,,,,

(a) Non oscillation

1425 1675 1925 2175

Flame temperature ( “c )

1325 1575 1825 2075

-I

o- -i

Flame temperature ( 9: )

1325 1475 1625 1775 1925 2075 2225

Flame temperature ( C )

Figure 9 Variation of flame temperature with acoustic oscillation: (a) non-oscillation, (b) SPL = 52 dB, (c) SPL = 66 dB

(3) It is considered that acoustic oscillation enhanced the mixing of the fuel gas and surrounding air, consequently the high flame temperature caused the re-oxidation of soot particles.

REFERENCES

Blaszczyk, J., Fuel, 1991, 70, 1023. Yavuzkurt, S., Ha, M. Y., Koopmann, G. M. and Scaroni, A. W., 1988 National Heat Transfer Conference, Vol. 106, 1989, p. 439. Ha, M. Y. and Yavuzkurt, S., Combustion and Flame, 1991, 86, 33. Hoover, D. V., Ryan, H. M., Pal, S., Merkle, C. L., Jacobs, H. R. and Santro, R. J., Heat and Mass Transfer in Spray Systems, Vol. 187. ASME, HTD, 1991, pp. 27-36. Saito, M., Sato, M. and Suzuki, I., Fuel, 1994, 73, 349. Saito, M., Hoshikawa, M. and Sato, M., Fuel, 1996,75,669. Gomez, A., Littman, M. G. and Glassman, I., Combustion and Flame, 1987,70, 225. Ayachi, N., Franz, I. and Brun, M., Proceedings of the Society of Automotive Engineers, 1995, 41. Kadota, T. and Hiroyasu, H., Combustion and Flame, 1984, 55, 195.



10 Hofeldt, D. L., SAP Paper 930079, March 1993. 14 Ohisa, H., Horisawa, H. and Kimura, I., Proceedings of the 11 Vander, R. L. and Weiland, K. J., Applied Physics B, 1994, 32th Japanese Symposium on Combustion, 1994, pp. 349-

59, 445. 351. 12 Arai, M. and Hiroyasu, H., Transactions of the Japan 15 Saito, M., Sato, M. and Sawada, K., Journal of Electro-

Society of Mechanical Engineers (in Japanese) (B), 1987, statics, 1997, 39, 305. 53, 2596. 16 Saito, M., Sadakata, M., Sato, M. and Sakai, T., Intema-

13 Ohisa, H., Kimura, I. and Horisawa, I., Proceedings of the tional Chemical Engineering, 1989, 29,494. 31th Japanese Symposium on Combustion, 1993, pp. 60- 62.


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Soot suppression by acoustic oscillated combustion