- Email: [email protected]
Halographic size analysis of burning sprays
COMBUSTION A N D F L A M E 27, 395-397 (1976)
395
Halographic Size Analysis of Burning Sprays J. M. WEBSTER, R. P. WEIGHT and E. ARCHENHOLD Central Electricity Generating Board, Matchwood Engineering Laboratories, Marchwood, Southampton, England
INTRODUCTION It is well established that the droplet size distribution produced in the atomisation of fuel oils has a paramount influence on flame properties [1, 2]. In general, however, the various techniques which are available for droplet size analysis either cannot be used successfully in the presence of a flame or else they disturb the flame. An advantage of photographic techniques is that the spray and the flame are not disturbed but conventional photography has a resolution limited by lens characteristics and, especially when magnification is required, the droplets can only be observed within a very narrow depth of focus. Moreover, the luminosity of the flame presents serious problems. The whole concept of holography is that it reconstructs the image in its entirety enabling differential focussing through the various planes of droplets between the forward and back plane of the two graticules. Recent advances have offered an alternative technique for particle size analysis in an increasing number of applications [3]. Thus the sizes and positions of particles or droplets in three dimensions can be recorded in situ while the system under examination remains undisturbed. The minimum droplet size which can be measured is about 20 /2m, although droplets of only a few microns are visible. Furthermore, in studying burning sprays the flame luminosity can be eliminated by interposing a filter matched to pass only a few nanometers on either side of the laser wavelength. As the brightness of the laser far exceeds
that of the flame within this small bandwidth the flame luminosity is effectively suppressed [4].
EQUIPMENT AND TECHNIQUES In this investigation an ultrasonic atomiser was used to generate droplets of residual fuel oil of 10 to 100 /am diameter. The atomiser had been previously calibrated by a technique using liquid nitrogen [5]. A stable flame was required at a fixed position which was sufficiently reproducible for the laser system to be aligned and focused. Preliminary trials showed that the flame front could be stabilised at a distance of 20 mm from the atomiser tip in an air stream having a flow rate of 0.6 1 s- 1 (n.t.p.), an initial velocity of 5 m s - x and a fuel flow rate of 0.3 g s- 1 (Fig. la). A ruby laser fitted with a dye 'Q' switch was operated in a 'giant' single pulse mode of about 25 ns duration. The emergent beam was diverged by a 67 mm negative lens, folded by a mirror and passed through the flame to a shutter and an f 1.9 compound lens of 63 mm focal length. An interference filter of 20 nm bandwidth centred at 694 nm was placed over the compound lens to eliminate other wavelengths radiated by the flame. This system enabled the camera equipment to be placed at a safe distance from the flame (Fig. 1b). M1 recordings were made on Agfa 10E75 plates and the hologram was reconstructed with an argon ion laser (X = 514.5 nm); a helium-neon laser (X = 632.8 nm) would have been preferable because of its wavelength but a suitable one was not available. The magnification of the resulting hologram is Copyright © 1976 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.
396
J.M. WEBSTER, R. P. WEIGHT and E. ARCHENHOLD
OIL F E E D _ ~ . ~ (0.3~/s) ULTRASONIC ATOMISERHEAD
'T;~-,-'=--~-~
AIR suP~,v_L_
:d
-
-
5m/sSTREAMVELOCIT FLAMEFRONT~
G SPRAY
(a)
~
(a)
\
I
OIL SPRAY /
\ RUBY LASER
I
J"/
I PT
20rim BAND WIDTH
!
HOLOGRAPHIC CAMERA
(b) Fig. 1. (a) Flame apparatus; (b) Holographic optical system.
dependent on a number of factors [6], including divergence of the original recording laser beam and divergence of the reconstructing beam. Final magnification is also linearly dependent on the ratio of the wavelengths of the recording and reconstructing beams. For the purpose of this investigation magnification was measured by calibration graticules recorded in the front and rear of the field of view of the hologram but out of the flame region.
RESULTS AND DISCUSSION
Our results show that it is possible to record the entire cloud of droplets within the expanded laser beam. Although some magnification resulted from the divergent geometry of the reconstructing beam and differences in the wavelengths of the recording
.
I00~ m
(b) Fig. 2. Planes of focus taken from a reconstructed hologram.
and reconstructing systems, additional magnification of the (holographically) reconstructed image was necessary. A 25 mm microscope objective lens was therefore used to focus a selected plane of reconstructed droplet images on to a ground glass measuring screen. The image of a particular droplet could be related to a known point in the original flame within the resolution of the system (about 20/~m). Smaller droplets could not be measured accurately, although the hologram recorded them as resolution-limited images. The advantages of this technique may be summarized as follows: (1) Its application does not disturb the flame or combustion process. (2) A size analysis of droplets and their distribution in three dimensions can be obtained in a single record.
BRIEF COMMUNICATIONS To demonstrate the feasibility of the technqiue we traversed on the Z axis a cine camera linked to the microscope objective through the region of reconstructed droplets, thus recording all visible droplets in a very small area across the frame. This film is available for loan and demonstration purposes.
This work was carried out at Marchwood Engineering Laboratories and is published by permission o f the Central Electricity Generating Board.
397
REFERENCES 1. Eisenklam, P.,J. Inst. Fuel 36, 130-143 (1961). 2. Mellor, R., Chigier, N. A., and Beer, J. M., Combustion and Heat Transfer in Gas Turbine Systems, Pergamon Press, Oxford, 1971. 3. Thompson, B. J., J. Phys. E. Sci. lnstrum. 7,781788 (1974). 4. Quigley, M. B. C. and Webster, J. M., Weld J. 50, 11,461-466, (1971). 5. Street, P. J. and Danaford, V. E. J., J. Inst. Pet. 54, 241-242 (1968). 6. Webster,J. M.,J. Photo. ScL 19, 38-44, (1971). Received 3 January 1976.
395
Halographic Size Analysis of Burning Sprays J. M. WEBSTER, R. P. WEIGHT and E. ARCHENHOLD Central Electricity Generating Board, Matchwood Engineering Laboratories, Marchwood, Southampton, England
INTRODUCTION It is well established that the droplet size distribution produced in the atomisation of fuel oils has a paramount influence on flame properties [1, 2]. In general, however, the various techniques which are available for droplet size analysis either cannot be used successfully in the presence of a flame or else they disturb the flame. An advantage of photographic techniques is that the spray and the flame are not disturbed but conventional photography has a resolution limited by lens characteristics and, especially when magnification is required, the droplets can only be observed within a very narrow depth of focus. Moreover, the luminosity of the flame presents serious problems. The whole concept of holography is that it reconstructs the image in its entirety enabling differential focussing through the various planes of droplets between the forward and back plane of the two graticules. Recent advances have offered an alternative technique for particle size analysis in an increasing number of applications [3]. Thus the sizes and positions of particles or droplets in three dimensions can be recorded in situ while the system under examination remains undisturbed. The minimum droplet size which can be measured is about 20 /2m, although droplets of only a few microns are visible. Furthermore, in studying burning sprays the flame luminosity can be eliminated by interposing a filter matched to pass only a few nanometers on either side of the laser wavelength. As the brightness of the laser far exceeds
that of the flame within this small bandwidth the flame luminosity is effectively suppressed [4].
EQUIPMENT AND TECHNIQUES In this investigation an ultrasonic atomiser was used to generate droplets of residual fuel oil of 10 to 100 /am diameter. The atomiser had been previously calibrated by a technique using liquid nitrogen [5]. A stable flame was required at a fixed position which was sufficiently reproducible for the laser system to be aligned and focused. Preliminary trials showed that the flame front could be stabilised at a distance of 20 mm from the atomiser tip in an air stream having a flow rate of 0.6 1 s- 1 (n.t.p.), an initial velocity of 5 m s - x and a fuel flow rate of 0.3 g s- 1 (Fig. la). A ruby laser fitted with a dye 'Q' switch was operated in a 'giant' single pulse mode of about 25 ns duration. The emergent beam was diverged by a 67 mm negative lens, folded by a mirror and passed through the flame to a shutter and an f 1.9 compound lens of 63 mm focal length. An interference filter of 20 nm bandwidth centred at 694 nm was placed over the compound lens to eliminate other wavelengths radiated by the flame. This system enabled the camera equipment to be placed at a safe distance from the flame (Fig. 1b). M1 recordings were made on Agfa 10E75 plates and the hologram was reconstructed with an argon ion laser (X = 514.5 nm); a helium-neon laser (X = 632.8 nm) would have been preferable because of its wavelength but a suitable one was not available. The magnification of the resulting hologram is Copyright © 1976 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.
396
J.M. WEBSTER, R. P. WEIGHT and E. ARCHENHOLD
OIL F E E D _ ~ . ~ (0.3~/s) ULTRASONIC ATOMISERHEAD
'T;~-,-'=--~-~
AIR suP~,v_L_
:d
-
-
5m/sSTREAMVELOCIT FLAMEFRONT~
G SPRAY
(a)
~
(a)
\
I
OIL SPRAY /
\ RUBY LASER
I
J"/
I PT
20rim BAND WIDTH
!
HOLOGRAPHIC CAMERA
(b) Fig. 1. (a) Flame apparatus; (b) Holographic optical system.
dependent on a number of factors [6], including divergence of the original recording laser beam and divergence of the reconstructing beam. Final magnification is also linearly dependent on the ratio of the wavelengths of the recording and reconstructing beams. For the purpose of this investigation magnification was measured by calibration graticules recorded in the front and rear of the field of view of the hologram but out of the flame region.
RESULTS AND DISCUSSION
Our results show that it is possible to record the entire cloud of droplets within the expanded laser beam. Although some magnification resulted from the divergent geometry of the reconstructing beam and differences in the wavelengths of the recording
.
I00~ m
(b) Fig. 2. Planes of focus taken from a reconstructed hologram.
and reconstructing systems, additional magnification of the (holographically) reconstructed image was necessary. A 25 mm microscope objective lens was therefore used to focus a selected plane of reconstructed droplet images on to a ground glass measuring screen. The image of a particular droplet could be related to a known point in the original flame within the resolution of the system (about 20/~m). Smaller droplets could not be measured accurately, although the hologram recorded them as resolution-limited images. The advantages of this technique may be summarized as follows: (1) Its application does not disturb the flame or combustion process. (2) A size analysis of droplets and their distribution in three dimensions can be obtained in a single record.
BRIEF COMMUNICATIONS To demonstrate the feasibility of the technqiue we traversed on the Z axis a cine camera linked to the microscope objective through the region of reconstructed droplets, thus recording all visible droplets in a very small area across the frame. This film is available for loan and demonstration purposes.
This work was carried out at Marchwood Engineering Laboratories and is published by permission o f the Central Electricity Generating Board.
397
REFERENCES 1. Eisenklam, P.,J. Inst. Fuel 36, 130-143 (1961). 2. Mellor, R., Chigier, N. A., and Beer, J. M., Combustion and Heat Transfer in Gas Turbine Systems, Pergamon Press, Oxford, 1971. 3. Thompson, B. J., J. Phys. E. Sci. lnstrum. 7,781788 (1974). 4. Quigley, M. B. C. and Webster, J. M., Weld J. 50, 11,461-466, (1971). 5. Street, P. J. and Danaford, V. E. J., J. Inst. Pet. 54, 241-242 (1968). 6. Webster,J. M.,J. Photo. ScL 19, 38-44, (1971). Received 3 January 1976.