holographic visualisation of a combustion flame · travels through a reference path, the other...

Optics and Lasers in Engineering 9 (1988) 85-100

Holographic Visualisation of a Combustion Flame

P. J. Bryanston-Cross & J. W. Gardner

Microengineering Centre, Department of Engineering, University of Warwick, Coventry CV4 7AL, UK

(Received 25 September 1987; revised version received and accepted 4 January 1988)

ABSTRACT

A series of experiments have been completed which use holographic interferometry to visualise a propane flame. The flame was produced by a O-1 m diameter combustor, which swirled the flow to maximise its burning efficiency. The interferometric fringes obtained show the temperature profile across the flame and the turbulent burning cell structure within it. The holograms were made in close proximity to the flame using a high power ruby double pulse laser combined with sharp optical filters and a fast large aperture shutter.

A semi-automatic system has been developed to digitise the complex interferometric fringe patterns generated. The system has been designed, using scanning optics held under computer control, to operate as an active microdensitometer. The microdensitometer has an optical resolution of 4000 x 4000 pixel points per picture and a potential resolution of 60 000 x 60 000 pixels.

1 INTRODUCTION

This experiment was completed at the EPFL (Ecole Polytechnique Federale de Lausanne) in conjunction with ETH (Eidgenossiche Technisce Hochscule Zurish), to investigate the application of holographic interferometry as an optical diagnostic tool for research in combustion.

Holographic interferometry was chosen as the most suitable tech- nique since it can provide the instantaneous capture of the whole flow

85

Optics and Lasers in Engineering 0143-8166/88/$03.50 0 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Northern Ireland

86 P. J. Bryanston-Cross & J. W. Gardner

field. Furthermore, it has a very high optical resolution (10 microns), and it can be used to provide a direct visualisation of the phase change generated by an optical beam passing through the medium of interest. Other methods considered such as laser anemometry,’ moire shadowgraphy’ or direct shadowgraphy3 and Schlieren methods4 were either too slow or lacked the resolution required to visualise the data. Holographic interferometry measures the absolute phase change of the medium, in this case created by temperature. Schlieren measures the first order differential of this phase change and shadowgraphy measures the second order.

Holographic interferometry is an extension to Mach-Zehnder interferometry, in which the amplitude of a plane wavefront is divided; half travels through a reference path, the other through the medium of interest. In the holographic case a similar comparison can be made between two holographic exposures made at different times. (See Appendix 1).

This interferometric approach used in this experiment was originally developed to visualise two dimensional transonic flows.’

2 COMBUSTION EXPERIMENT

2.1 Theoretical considerations

Before the combustion experiment was undertaken a series of computer simulations were developed to model and investigate theoretically the optical properties of the combustion flame. This was particularly important as the turbulent cell size of the flame was unknown. Calculations were made to determine whether the optical beam would encounter severe bending, due to sharp refractive index gradients; these could either be a source of error or if very large create caustic reflections as described by Jakeman et ~1.~ Examples of these simulations are shown in Figs 1 and 2.

2.2 Optical system

A diagram of the optical system is shown in Fig. 3. The output from the laser was first divided using a prism; 90% of the beam intensity being used to form the sample beam. The sample beam was then spatially filtered, collimated using a Schlieren mirror and passed through the combustion region of a O-1 m commercial combustor. The beam was

Holographic uisualisation of a combustion flame 87

PHASE DISTRIBUTION

I SOTHERMALS

Fig. la. Calculated phase distribution (top of figure) of a collimated laser beam having travelled through a simulated symmetrical combustion flame, (bottom).

PHASE DISTRIBUTION

Fig. lb. A second phase

ISOTHERMALS

distribution which has been calculated case.

for an asymmetric


EXIT LASER BEAM DEFLECTED BY

COLLIMATED LASER BEAM

‘ION

Fig. 2. An example of severe refractive index bending, creating caustics.

then imaged using a second Schlieren mirror onto a holographic plate through a daylight blocking ruby pass filter and gated shutter.

The undiverged reference beam was passed close to but outside the combustion region, spatially filtered and then recombined with the sample beam at an angle between the beams of approximately 30 degrees.

Two toughened glass shields were placed either side of the combustion flame to act as a thermal barrier for the optical components.

PULSE LASER

Fig. 3. Diagram of the Holographic Interferometer.

Holographic wkualisation of a combustion flame 89

Despite initial concern that these shields would deform (interfero- metrically) during the experiment, they were found not to add any spurious information.

Calculations were made as to the type of filter and shutter combina- tion required to block the light output of the flame but pass that of the ruby pulse laser. The shutter used was a O-1 m aperture electro- mechanical leaf opening shutter. The shutter took O-1 s to open at which time it was electronically gated to fire the laser. To prevent the transmission of movement it was placed in front of, but not rigidly connected to the holographic plate. Two filters were initially employed in front of the holographic plate, a daylight blocking filter and a far red blocking filter. It was found that due to the wavelength sensitivity of the holographic material used, only the daylight blocking filter was required.

The whole holographic set up, with the exception of the combustor, was mounted on a 2 m x 3 m optically stabilised table. The optical reference and object beams were kept as close together as possible, in order to minimise any phase shifts introduced by external vibration.

A JK ruby pulse laser (Lumonics Ltd, Rugby, UK) was used for the combustion project and the holographic plates were Agfa holotest 8E75HD (Belgium). Although the laser has a coherence length of 1 m, in order to maintain a good diffraction efficiency effort was made to path-match the distance travelled by the sample and reference beams so that they reached the plate at the same moment in time. The reference to object beam ratio was also kept close to unity.

The image plane method was used because the paraxial ray paths through the combustion area are highly correlated and thus produce very good fringe contrast and high diffraction efficiency interferograms.

The method also provides a ‘one to one’ ratio between the image to object plane. Optically, a ‘one to one’ image system has the advantage that the imperfection or distortions created can be minimised by imaging them back into themselves. This is of particular value in minimising the ray-path deflections caused by optical beams travelling through strong refractive index gradients.7

2.3 Holographic measurements

The absolute density field was measured by comparing the difference between a holographic exposure made before the combustor was ignited to that made during the combustion period (Fig. 4). The interferograms made show the absolute change in the refractive index of air due to the temperature rise in the gas.


Fig. 4. Absolute interferograms of propane flame.

Fig. 5. Absolute interferogram of propane flame with added finite fringes.

Holographic visualkation of a combustion flame 91

Fig. 6. Interferogram made with a pulse separation of 100 ps. (Note the changing turbulence scale with respect to distance from the combustion nozzle.)

In several of the holograms (Fig. 5) finite background fringes were added to the interferometric data in order to simplify the data extraction process. This was achieved by rotating the holographic plate by several mm between the two exposures. The rotation changes the path length of the beam between the two exposures creating a set of linear fringes which then add to those created by the flame.

The time between the holographic exposures in the absolute density interferograms were usually in the order of several minutes.

The unsteady component of the flame was visualised by making two holographic exposures of the plate using the double pulse separation of the laser. The pulse separation period can be varied from 1 ms to 1 ps. For the interferometric reconstruction shown in Fig. 6 the pulse separation was 100 ys. In this reconstruction the turbulent or unsteady component structure of the flame can clearly be seen. It is also possible to observe that the size of the turbulent cells increase with distance

from the combustion nozzle.

2.4 Quality of results

The interferometric data obtained is of high resolution, presenting a two dimensional integrated ray projection through the combustion field.


The fringes formed are sharp and well defined, making their interpretation as phase shifts relatively direct. The hologram formed maps onto the combustion process as a spatial snapshot of the frame. For the expedient interpretation of holographic data and as the nozzle is circular, it has been assumed that there is a large scale rotational symmetry along the central axis of the flame.

There are numerous ways in which similar information has been holographically obtained from combustion regions. Bachello and Houser, describe a Smartt interferometric method which, by the use of spatial filtering requires only one laser exposure during the combustion period. Trolinger’ has used multiple reference beams to enhance phase contrast and multiple exposures to time gate or correlate within combustion fields.

Nevertheless, the reconstructions shown in this paper show an image clarity, sharpness and detail not often presented in current literature.

3 COMPARISON BETWEEN MANUAL AND AUTOMATIC DATA COLLECTION METHODS

3.1 Requirement of the data collection system

A typical photographic reconstruction of an absolute interferogram is shown in Fig. 5. For the purposes of evaluating the data several ‘manual’ traces along the fringes have been made. In these cases a fringe would normally travel in one direction unless ‘deflected’ by a change in the refractive index/temperature caused by the flame. The amount of phase change is calculated by the number of fringe shifts it experiences. It can clearly be seen there is a high degree of turbulence in the flame and a large number of whole fringe shifts in the data obtained.

The problem of producing an automatic or semi-automatic method of extracting the fringe data has been examined. One of the first requirements has been to construct an imaging system capable of resolving the fringe pattern itself. The minimum number of pixels thought necessary to represent the field was 4000 x 4000 and because the image quality was also of variable intensity there was a need for a 64 grey level representation. This requirement exceeds most conventional CCD camera imaging techniques which tend to have a limited resolution of either 512 x 512 pixels or 1000 x 1000 pixels. The width of the interferometric combustion fringes were also found to vary from 2 to 50 pixels at an effective screen resolution of 4000 X 4000 pixels.

Holographic visualisation of a combustion flame 93

The alternative to the use of a conventional camera has been to develop an active scanning microdensitometer. The philosophy of this device is based upon a laser scanning camera developed by Adrain et al *, lo for producing high resolution images of nuclear fuel rods.

3.2 Description of the active microdensitometer

The active scanning microdensitometer, as shown in Fig. 7 consists of a 5 mW He/Ne laser light source, the beam from which is expanded by two negative lenses, converged through a final F3 positive lens and then reflected via two mirrors held under computer control. The two mirrors allow the laser beam to be scanned in the X and Y directions with speeds up to 3 kHz and with a 12 bit resolution.

In the case of the combustion data the initial photographic data has been enlarged some 8 times to a size of 0.5 m and a copy negative has been scanned. The laser beam is focused at a distance of 1 m from the scanning head to a diameter size which can be varied from 20 to 250 pm onto the surface of the photographic negative. The emergent beam is then collected using a low quality fresnel lens and imaged onto the face of a photodetector. Light intensity variations are monitored by the detector and fed back into the host computer via a further 12 bit A/D conversion card.

The microdensitometer has been configured to operate in several modes, the first of which scans an area equivalent to 512 x 512 pixels which is then displayed by a digital frame-store, also linked to computer. Currently this process takes approximately 100 s to and gives a direct indication of the quality of the data being

COMPUTER

cl

PHOTODETECTOR

Fig. 7. Active microdensitometer.

the host perform scanned

94 P. .I. Bryanston-Cross & J. W. Gardner

Fig. 8. Displayed analogue picture obtained by scanning a laser spot over a photographic negative of the absolute interferogram shown in Fig. 5. The laser beam width is approximately 0.5 mm and the typical width and interferometric fringe in the

negative is 3 mm.

Fig. 9. Displaye :d analogue picture obtained by scanning a laser spot. the laser spot has been reduced to 0.05 mm.

The diameter of

Fig. 10. Displ

Holographic vkualisation of a combustion flame 95

lg be :en a applied.

out. Figures 8 and 9 are examples of the resolution possible from the device for different spot sizes. It is also possible to see directly the effect of various filters. Examples in the stages of binary thresholding are shown in Fig. 10.

This first mode of operation can be seen as an evaluation for studying the quality of the data captured and a first step to defining the nature of the image processing algorithm to be applied.

A second stage can then be applied, that of actively preprocessing the data during the scanning stage, before digital storage. This reduces the amount of data to be stored. As an example the system has been used to perform a simple active line following algorithm also shown in Fig.

10.

3.3 Performance of the microdensitometer

Various algorithms for data reduction are now in the process of evaluation. In the case of simple line following operations the device has performed well. There is, however, difficulty in dealing with some of the more complex problems shown, such as connectivity and fringe


merging. The simplest method is to scan part of the field, display the digital image and correct the points of difficulty manually.

It is noted when infinite interferometric fringe data is used then the order of the fringe is not critical, merely its deflection from a predetermined starting point. It is, however, necessary to be able to track along any single fringe. The problems created in tracking a fringe may also be overcome by applying a phase heterodyne approach as described by Decker.”

The methods of extracting information using this system from flexible photogrametric data is both flexible and inexpensive. The scanning camera has been built on a low budget (&lOk) and has been used to develop a number of functions, the fringe following work shown in this paper is one of them. A further extension of this work would be to use more sensitive (16 bit) acquisition cards, a faster processor and quicker scanning mirrors. This would allow a field size of 60 000 x 60 000 x 16 pixels to be resolved at Direct Memory Addressing (DMA) transfer speeds.

4 INTERPRETATION OF INTERFEROMETRIC RESULTS

The disadvantage of the image plane approach is that interferometric fringes formed represent an integrated ray-path through the combustion flame.

Several mathematical techniques have been used for solving the problem of three dimensional refractive index fields. When the problem was first investigated in the early seventies by Sweeney and Vest’* and Bracewell and Werneckle,13 the major difficulties were in the size and speed of the computers then available. Recently the problem has been re-evaluated by Tan et al., I4 using an ART approach, and Hesselink,ls using a Fourier transform method.

In the case of the ETH combustion flame, to simplify the solution a centre of symmetry has been assumed. The first approach was to make a direct inversion of a series of simultaneous equations which repre- sented the ray-paths taken through the combustion field. This solution proved unstable and an alternative method was developed. In this case a cosine function was generated and integrated numerically to simulate the optical rays passing through the refractive index field. The resultant calculated phase change was then evaluated against the phase change measured in the interferogram.

It was found that a function of the general form:

A=A,cos3(8+c)

Holographic visualisation of a combustion j7ame 97

- v 700

” ?! 600 3 e z 500

E 3 400

; 0 300 E

‘Ei 200 ._ 0 a 100

axis of rotational symmetry

0 1 I I I *

100 50 ’ -50 -100 Distance from centre (mm)

Position of C om bustor

Fig. 11. Comparison between the flame temperature measured using a thermocouple and that interpreted from holographic interferometry.

could be used to represent the flame where A, is the maximum density value, 8 is evaluated from 0 to z/2 and c is a constant, and was varied until a ‘fit’ with experimental data was achieved. Finally the cosine function, which represents the density profile through the flame was converted into a temperature using the ideal gas equation and compared with thermocouple measurements made in the same region of the flame, as shown in Fig. 11.

The approach discussed has to date only been evaluated for this particular symmetric case.

CONCLUSIONS

This experiment has demonstrated that a propane flame can be visualised with very high resolution using holographic interferometry. By using the two laser pulses with a 100 ps separation it has also been possible to visualise the turbulent structure within the combustion region.


The initial difficulties found in transferring high resolution interferometric information into a digitally stored form have been overcome with the construction of an active scanning microdensitometer. It has been demonstrated that this device, which is linked via a microcompu- ter to a digital frame-store, can be used to store and display very high resolution data, well beyond the capabilities possible with conventional video capture systems.

An evaluation has been made of the problems associated with deconvolving tomographically the interferometric data. In the example case shown this has been achieved by assuming a centre of symmetry in the propane flame. The method developed has been to compare a generalised function with the measured phase information.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the ETH Zurich (Eidgenossiche Technisce Hochscule) for its support of this work. In particular Prof. P. Sutter and Dr J. Gass (ETH) for the design of the combustor, Mr Breretta-Piccoli (EPFL) (Ecole Polytechnique Federal de Lausanne) for his assistance in the construction of the optical table and Prof. A. Bolts (EPFL) for providing the pulse laser system and making available the laboratory support essential for the completion of this work. They also wish to acknowledge Mr Dragan Andonov for his work on the fringe thinning and presentation routines.

REFERENCES

1. Strazisar, A. Laser fringe anemometry for aero engine components, Agard Conference Proceedings No. 399, 67th Symposium, Philadelphia, Pen- nsylvania, 19-23 May 1986, pp. 6.1 to 6.32.

2. Kaffri, 0. Moire grid techniques for flow visualisation. Applied Optics, 5 (1981) 555.

3. Bryanston-Cross, P. J., Edwards, J. & Squire, L. Measurements in an unsteady two dimensional shock/boundary layer interaction. Presented at the IUTAM Unsteady Aerodynamics Conference, Jesus College Cam- brige, 1984.

4. Davies, M. R., Determination of turbulent flow properties by a penetrat- ing beam dynamic shadowgraph method. J. Phys. E: Sci. Instrum., 20 (1987) 1271-7.

5. Bryanston-Cross, P. J., Lang, T., Oldfield, M. & Norton, R. Inter- ferometric measurements in a turbine cascade using image plane holography. J. Engineering for Power, (January 1981) 124-31.

6.

7.

8.

9.

10.

11.

12.

13.

14.

1.5.

Holographic vkualisation of a combustion flame 99

Jakeman, E., Klewe, R., Richards, P. & Walker, J. Applications of non-Gaussian scattering of laser light to measurements in a propane flame. J. Phys. D: Appl. Phys., 17 (1984) 1941-52. Vest, C. Holographic Interferometry, John Wiley & Sons, New York, 1981 pp. 324-9. Bachello, W. & Houser, M. Optical interferometry in fluid dyamics research. NASA Conference Publication 2477 Automated Reduction of Data from Images and Holograms, Moffett Field Research Center, California, lo-11 Jan 1985, pp. 261-95. Trolinger, J. Diagnostics of turbulence by holography. SPIE Vol 125 Advances in Laser Technology for the Atmospheric Sciences 1977, pp. 105-113. Adrain, S., Armour, I., Lichnowski, A. & Jagger, A novel laser scanning camera, Optics and Laser Technology, (April 1983) 77-82. Decker, A. Beam modulation methods in quantitative and flow visualisation holographic interferometry, Agard Conference Proceedings No. 399, Advanced Instrumentation for Aero Engine Components, 67th Sympo- sium, Philadelphia, Pennsylvania, 19-23 May 1986. Sweeney, D. & Vest, C. Reconstruction of a three dimensional refractive index fields for multi-directional interferometric information, Applied Optics, 12 (1977) 2649-64. Bracewell, R. & Werneckle, Image reconstruction over a finite field, J. Optical Society of America, 65 (1976) 2276-90. Tan, H. & Modarress, D. A new ART code for tomographic interferometry, NASA Conference Publication 2477, Automated Reduction of Data from Images and Holograms, Moffitt Field, California, lo-11 January 1985. Hesselink, L. Quantitative three dimensional flow visualisation, Presented at the 3rd Symposium on Flow Visualisation, Ann Arbor, Michigan, September 1983, pp. 375-80.

APPENDIX 1

Interferometric fringes are produced by the phase change of a ray passing through a fluid. Changes in refractive index can be related to density for gases by the Dale Gladston expression.

(p - 1)/p = Dale Gl a s d t on constant C = 2.24 x lop4 m/kg (for air)

where p is the density of air and p is the refractive index of air. The number of fringes can be calculated by considering the change in

the refractive index over a defined pathlength.

nA = PAp

where IZ is the number of fringes, P is the pathlength, A is the wavelength of light.

In the case of a three dimensional distribution the problem can be


ay Path

Temperature Field

Fig. Al.

simplified by dividing the volume into a number of finite boxes each of unit length and its own specific value of refractive index (Fig. Al). The number of fringe shifts then becomes the line integral of the ray pathlength through the volume. In two dimensions this can be shown as:

Ap = ds = I@x)~ + (dy)*

where j-ids = S, the pathlength line integral through the combustion flame; pXY, value of the gas density as a function of position in the combustion flame.

A problem arises when the number of fringe shifts is known and the spatial value of refractive index needs to be determined.

One method is to solve the field as a set of simultaneous equations, building in redundancy by over sampling the field and taking a large number of different ray projections through it as shown in Fig. Al. This direction inversion approach tends to be unstable.

A second approach used in this paper has been to generate a function known to be similar to that measured and they perform the ray path integral through it. The calculated fringe field can then be evaluated directly against measurement as shown in Fig. 11. In the symmetrical combustion case, the solution was simple to determine. If a non- symmetric case were considered a more complicated function could be used and solved by interacting between several ray paths.

holographic visualisation of a combustion flame · travels through a reference path, the other...

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