Atomization of liquid fuels in an acoustic burner

Atomization burner K. M. Swamy,

of liquid fuels in an acoustic

K. L. Narayana

and J. S. Murty

Regional Research Laboratory, Bhubaneswar-751073, (Received 8 July 1988; revised 5 October 7988)

India

The acoustic burner based on a modified Hartmann principle in conjunction with a convergent divergent nozzle developed for atomization of high viscous fuels is used to study the atomization characteristics of high viscous fuel oil. The Sauter’s mean diameter values of the fuel oil droplets are evaluated by varying parameters such as atomizing air pressure; fuel oil pressure; and air/fuel ratio. The mean droplet sizes obtained varies between 12 and 60 pm in contrast to earlier studies which obtained droplet sizes between 55 and 90pm. A possible mechanism is proposed to explain the reasons for the finer atomization of fuel oil obtained using an acoustic burner. (Keywords: atomization;

fuel oils; combustion)

Acoustic generators have established their use in fuel combustion systems for improving their efficiency by the finer atomization of liquid fuels. Acoustic waves promote molecular agitation and turbulence in the gas, resulting in the continuous replacement of combustion products surrounding the fuel droplets by fresh oxidizing gas. Even though several types of efficient acoustic generators are reported I*’ their actual use is not as great as anticipated. This is because the outstanding features claimed in small scale devices could not be retained in larger units. Several investigators 3*4 have studied the atomization of liquid fuels using acoustic generators of Hartmann type and stem-jet type, and proposed relations to predict the mean droplet size of different fuels under varied experimental conditions. The dispersion characteristics of stem-jet acoustic atomizers using liquid paraffin have been reported ‘. The studies so far reported in the literature do not reflect the industrial in situ conditions, and suffer from certain drawbacks, such use of large quantity of compressed air for atomization and the impracticable method of fuel injection. The authors have developed an industrial acoustic device for atomization and combustion of residual fuel oil and furnace oils6. This paper reports the atomization characteristics of the acoustic atomization device. A probable mechanism for breaking-up of the fuels into line droplets in the acoustic field is proposed.

through the acoustic burner. The total oil flow is recorded. The fuel oil is preheated in an on-line preheater up to 100°C before atomization and combustion. The acoustic energy is produced by means of compressed air supplied to the burner through 12 mm air lines from the receiver of a reciprocating compressor. The air pressure is controlled by a regulating valve. The oil and air pressures are measured to an accuracy of fO.l kgcm-’ (10 kPa) by diaphragm and Bourdon pressure gauges, respectively. The oil spray was collected through the slits on a glass slide as shown in Figure 2. The liquid spray was captured (SAE 30 motor oil) for a fraction of a second when both

Figure 1 Schematic diagram of acoustic resonator; 3, paraboloid head; 4, compressed

burner: 1, nozzle; air; 5, furnace oil

2,

EXPERIMENTAL The acoustic burner based on a modified Hartmann principle in conjunction with convergent-divergent nozzle, has been developed for the atomization and combustion of heavy fuel oils (Figure 1). The properties of the fuel oil used are given in Table 1. The experimental set up used for collection of fuel oil droplets under varied conditions is shown in Figure 2. A $ h.p. rotary gear oil pump was used for supplying the oil from oil tank to the oil ports of the acoustic burner through an oil filter. Oil control valves are used to vary and regulate the oil flow 0016~2361/89/030387-04$3.OQ 0 1989 Butterworth & Co. (Publishers)

Ltd.

Table 1

Properties

of fuel oil used Temperature

Property

25°C

70°C

Density (kg m _ 3, Viscosity (m Pas) Surface tension (Nm _ ‘) Gross calorific value (kcal kg-‘)

953 1500 0.022 10000

930 180

“Value

at 4O”C, below which measurement

could not be made

FUEL, 1989, Vol 68, March

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Atomization

of liquid fuels in an acoustic burner: K. M. Swamy et al.

A-

r-7

‘8

6

9 010 2

0

c I,

Experimental set up for collection of atomized fuel oil droplets: 1, compressor; 2, pressure gauge; 3, air pressure regulating valve; 4, air flow meter; 5, acoustic burner; 6, fuel oil tank; 7, oil tilter; 8, gear pump;9, oil pre-beater; 10, oil totalizer; Ii, slits; 12, glass slide

Figure 2

the slides were colinear with the burner tip and glass slide. The spray was intercepted and collected on a sample collector (glass slide), 25 cm downstream of the atomizer outlet (burner tip). The slit shutters were made of 3 mm thick aluminium sheets, and connected through a rope and a pulley arrangement as shown in Figure 2. The slits were circular with a 10mm diameter. The size distribution of droplets was determined using an optical microscope at a magnification of 1250 following Misutani et at.‘s procedure’ and Poger et al.‘. The measuring accuracy of droplet sizes for less than 20 pm is about 5 %, and the same is 2 ‘A for droplets greater than 20 pm size. A representative area was observed on the glass slide and the number of individual sizes of the droplets were counted. The SMD (Sauter’s mean diameter) was computed from these measurements using the relation SMD=CNd3/CndZ where ‘n’ denotes the number of droplets of diameter ‘d’. The acoustic frequency was measured by feeding the output through a sensitive microphone to an oscilloscope and a frequency counter. The sound intensity was measured with a precision impulse sound level meter, keeping the microphone in the axis of the burner at a distance of 25 cm.

levels as a function of primary air pressure are presented in Figure 3. It can be seen that the frequency and sound level show an increasing trend with an increase in atomizing air pressure. In Figure 4, the variation of SMD is shown as a function of air to fuel ratio at different fuel flow rates. At any specific oil flow rate, the droplet size decreased nonlinearly with an increase in air/fuel ratio. It is also observed that for a given air/fuel ratio, the SMD values decrease with increasing fuel Row rate up to a certain level, 5.59 g s-l, and for a further increase in fuel flow rate the SMD values also increased (Table 2). The decrease in SMD values with increase in fuel flow rate is contrary to the observations made in the case of any plain jet air blast atomizer. This anomaly can be explained from computing the acoustic power applied for atomizing 1 g oil. From the last column in Table 2, it can be seen that the acoustic power applied for atomizing 1 g oil increased up to the flow rate of 5.59 and thereafter decreased for further increases in fuel flow rate. The specific acoustic power consumed per gram of oil is shown in Figure 5. When the specific acoustic power consumption is constant (e.g. 6.5 w g- ’ of fuel), the SMD values are found to increase with increase in fuel oil flow rate as in the case of the air blast atomizer. When the air/fuel ratio was varied from 1 to 6 for a constant oil flow rate, the SMD values decreased from 59 to 12 pm (Figure 4). For comparison, in the case of highly Table 2

SMD values of fuel oil obtained for different oil flow rates

Fuel oil (KPa)

Fuel flow rate (gs-‘)

Air Atomizing flow air pressure rate (g s-‘) (K Pa)

Air/fuel ratio

SMD value

Specific acoustic power of fuel VP-‘)

1

2

3

4

5

6

7

50 100 150 190

2.26 3.99 5.59 11.18

50 100 200 200

6.09 7.75 12.18 12.18

2.69 1.94 2.17 1.09

46 44 18 25

0 5.38 12.35

210

16.77

350

16.61

0.99

49

r

RESULTS AND DISCUSSION The fuel oil was used at 70°C. Parameters such as oil pressure, oil flow, air pressure are adjusted to desired values, and droplet sampling is carried out only after stabilizing the conditions for at least 15 min. The droplet size distributions were measured under the microscope and the SMD values were evaluated. The variation of the mean droplet sizes was found for different primary air and oil pressures. The acoustic power delivered by the burner W,, is computed using the following equation’

6.37 6.53

I

ll.O-

10.5al

I" Y

w

-140

G >

W,=295 d&,/s

where d, is the nozzle diameter (in cm); P, is the atomizing air pressure in kg cmm2. From this value of W,, the specific acoustic power utilized (i.e. the acoustic power used for atomizing 1 g of fuel) is computed and recorded in the last column of Table 2. In the same table, for different oil flow rates, the SMD values obtained at the lowest air/fuel ratio has been presented. The frequency of the acoustic waves and the sound

388

FUEL,

1989, Vol 68, March

‘w

-130

=: 3 E:

ATOMIZING

AIR PRESSURE,

KPa

The variation of frequency and sound level as a function of atomizing air pressure

Figure 3

Atomization

20

1

w

0

I

I

I

--

2

3

c

5

AIR/ Figure 4 The variation fuel flow rates

FUEL

6

7

RATIO

of SMD values with air/fuel

ratio at different

of liquid fuels in an acoustic

et al.

burner: K. M. Swamy

20pm are obtained in the present study. The beneficial effect resulting in the fine atomization of fuel oil is attributed to the injection of fuel under pressure into the acoustic region. The dependence of an average droplet size with different liquid fuel properties (surface tension, viscosity, density) and various acoustic properties (frequency, sound intensity or amplitude) reported earlier4 was under a gravity feed of fuel. The variation of SMD values as a function of oil flow rate is shown in Figure 5. Figure 5 shows that the SMD values decrease with increase of oil flow rate, up to a flow rateof8gs -l and thereafter the SMD values are found to increase with further increase in oil flow rate. This result confirms that at an oil flow rate of 8 g s-l (corresponding to 30 kg h-l), liner atomization is obtained experimentally. The acoustic burner is designed to operate at 30 kg h-l capacity, and optimum results are obtained at that fuel flow rate. It further establishes that in a fuel range of 2.5g s-l to 16g s-’ (corresponding to 9.363 kg h-l) the atomization is still fine and uniform, giving SMD values of less than 50pm. Therefore, it can be concluded that the acoustic burner has a turn down ratio of 1:7 when compared with 1:3, which is obtained in the case of conventional burners. The average droplet sizes obtained by various investigators for different liquids for the highest liquid flow rate under different air/fuel ratios are presented in Table 3 along with the authors’ results. From Tables 2 and 3, one can conclude that at a fuel flow rate of 60 kg h-’ and an air/fuel ratio of 1, SMD values of around 50 pm are obtained in the present study. Possible mechanism for the degrudation of fuel oil

In any gas-jet generator, two types of acoustic waves are produced : pulsating-type compression shock waves; and intense sound waves generated as the oscillation of compression shock waves. The first type of waves are due to the interaction of the forward flow and the periodic reverse flow, and the second type of waves are due to a relaxation mechanism. In general, the gas-jet atomizers generate acoustic waves of low frequencies (< 20 kHz). Due to the lower frequencies, the acoustic beam is wide and the wave length is considerably larger, as shown in

-14 -12

-10 -8

Figure 6.

-6 -4

1

01 150 I 0

5

OIL FLOW Figure 5

The variation

I

I

10

15

RATE,

_.

20

The spherical droplets formed in the convergent nozzle due to the kinetic energy of the atomizing air, are broken into ligments by the pulsating shock waves in zone 1 of Figure 6. The droplets travelling in the acoustic field flatten and elongate in the pulsating compression area. This process repeats several times before the ligment is

g/s

of SMD values with oil flow rate

viscous oils such as furnace oil, the Jasuja’,” correlation for a plain jet air blast atomizer was used (Figure 4). The results indicate that SMD values of 85pm are obtained with conventional air blast type burners. In contrast, SMD values of around 30pm are obtained using the acoustic burner. It is interesting to compare the recent work on acoustic atomization of furnace oil. To gain a clear picture, the results of earlier investigators have also been plotted in Figure 4. For an air/fuel ratio of 3 to 6, the SMD values reported are 64 to 56pm whereas 40 to

Table 3 Comparison of average investigators _____~_

droplets

Average droplet diameter

Reference

Liquid atomized

1

2

1 5

Molten Molten

4 Present study

Furnace

oil

Residual

furnace

paraffin paraffin

oil

size obtained

by different

(pm)

Air/liquid ratio

Liquid now studied (kg h-‘1

3

4

5

100 13%150 8@100 5664

0.2 to 0.3 0.11 to 0.15 0.38 to 0.61 3.0 to 6.2

50 512 142-207 - I1

lo-50

1.0 to 2.7

FUEL, 1989, Vol 68, March

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Atomization

of liquid fuels in an acoustic

burner: K. M. Swamy

et al.

evaluated by measuring the droplet sizes. The results are summarized as follows: Acoustic atomization yields an average droplet size of 30pm, which is found to be far superior to that of conventional air blast atomization, which gives 85 ,um. The atomization is fairly good in the fuel flow range 2.5-16 g s-i, giving rise to a turn down ratio of 1:7 in contrast to 1:3 in conventional burners. The probable mechanism for atomization of fuel oils is proposed. The finer atomization is attributed to the combined action of pneumatic atomization, caused by gas flows. Further disintegraton occurs due to acoustic waves generated in the resonant area.

OB

L-k Figure 6

FUBL DRopLETs PULSATING

coYPRFssIow

S”cxx

ACKNOWLEDGEMENTS

WAYFS.

Fine fuel droplets formation using gas-jet generator

formed. These ligments travel further and disintegrate or fragment, as shown in Figure 6, due to the high intensity sound waves in zone II. These waves produce alternate adiabatic compressions and rarefactions, together with corresponding changes in density and temperature, resulting in an increase and decrease of the pressure in the medium. In the negative pressure phase, it is possible to reach a point where one can overcome the natural cohesive forces of the fuel ligments. During the transmission of the intense sound waves (acoustic energy) from the air to the fuel ligment, some energy is reflected (causing disturbances at the ligment surface), while the rest of the energy transmits into the ligment (causing the formation ofcapillary waves on the inner side of the ligment surface), resulting in a fine fog of droplets as shown in Figure 6.

The authors are grateful to the Director, Regional Research Laboratory Bhubaneswar for permission to publish this paper, and also to the referees who have made valuable suggestions for improvement of this paper. REFERENCES

CONCLUSIONS The characteristics of acoustic atomization have been studied using atomizers of the modified Hartmann type applicable to industrial oil burners. The mean diameters obtained under varied experimental conditions have been

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FUEL, 1989, Vol 68, March

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Pashkovaskii, B. S. and Tebenkov, B. P. Son. Phys. Acoust. 1975, 20 (6), 538 Rozenberg, L. D. in ‘Sources of High Intensity Ultrasound’, Plenum Press, New York, 1969, p. 66 Khandwawala, A. I., Natarajan, R. and Gupta, M. C. Fuel 1974, 58,268 Ramesh, N. R., Sridhara, K. and Natarajan, R. Fuel 1985,64, 1677 Rudakov, Ya. D., Geller,Z. I.,Goponenko, A. M. and Rudakov, G. Ya. Thermal Engg. 1972, 19 (lo), 118 Swamy, K. M., Narayana, K. L. and Murty, J. S. ‘An improved liquid fuel burner used in oil fired furnaces’, Indian Patent No. 158837, Gazette of India, Part III, Sec. 2, on 31.1.1987 Misutani, Y., Uga, Y. and Nishimoto, T. Bulletin ofthe JSME 1972,83,620 Poger, M. A. and Eknadiosyants. Soo. Phys. Acoust. 1975,20 (4), 361 Jasuja, A. K. ‘Atomization of Crude and Residual Fuel Oils’, ASME Paper 78/G.T./83, presented at ASME Gas Turbine Conference, London, April 1978 Jasuja, A. K. ‘Plain-Jet-Air-Blast Atomization of Alternative Liquid Petroleum Fuels Under High Ambient Air Pressure Conditions’, ASME Paper 82-GT-32, 1982