International journal of spray and combustion dynamics Volume .1 · Number . 2 . 2009 – pages 251 – 282 251
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV H. Konle1,∗, A. Rausch2, A. Fischer2, U. Doll2, C. E. Willert3, C. O. Paschereit1, I. Röhle2 1Institut
für Strömungsmechanik und Technische Akustik, Berlin Institute of Technology, Berlin, Germany Aerospace Center (DLR), Institute of Propulsion Technology, Engine Acoustics, Berlin, Germany 3German Aerospace Center (DLR), Institute of Propulsion Technology, Engine Measurement Systems, Cologne, Germany 2German
Received 13th August 2008; Revised Submission 21st November 2008; Accepted 24th November 2008
ABSTRACT Thermo-acoustic investigations require reliable measurement techniques in hot environments for pressure, density fluctuations with a high dynamic range and acoustic particle velocity. This paper presents recent developments of optical measurement techniques in combustion diagnostics. A fibre-optic microphone based on the interferometric detection of membrane deflections was designed to measure acoustic pressure oscillations. Due to the heat resistant design, the sensor has an upper temperature limitation of approximately 970 K. Rayleigh-Scattering measurements, using the density dependent intensity of scattered light were performed in an unconfined flame with approximately 1600 K to study amplitude and phase distribution of the flame pulsation. Acoustic particle velocity can be determined applying acoustic PIV (particle image velocimetry) technique. This paper shows a way to measure simultaneously the acoustic particle velocity and the locally resolved mean flow velocity of a turbulent flow. Together these non-invasive techniques are applicable to study thermo-acoustic processes and sound generation in combustion chambers or turbines. Key words: Fibre Optical Microphones, FOM, Rayleigh Scattering, time resolved density measurements, Acoustic PIV, acoustic particle velocity, acoustic in hot environments
1. INTRODUCTION In order to further reduce the NOx emissions of modern aero-engines and stationary gas turbines, one strategy is to apply lean or even lean-premixed combustion. This kind of combustion however tends to show very dangerous combustion oscillations that can cause severe damage and therefore need to be avoided in any circumstances. A lot of * Corresponding author:
[email protected]
252
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
research is carried out to better understand and predict these thermo-acoustic processes. On the experimental side better measurement techniques are required to get deeper insight in the complex coupling between flow mechanics, mixing, heat release, flame stabilization and acoustics. The most important measurement quantities are: •
• •
the amplitude of sound pressure and sound driven particle velocity of the acoustic waves propagating forward and backward through the components of a combustion system, in particular the flame, the position and heat release of the flame, the propagation of temperature and flow structure inhomogeneities with the mean flow.
It is a well accepted assumption that thermo-acoustic oscillations can be caused by the feedback of pressure fluctuations and sound waves originating from the flame, by the recirculation of flow inhomogeneities and by the acceleration of flow inhomogeneities at the combustor outlet nozzle, causing pressure changes that feedback to the combustion process. While some currently used measurement techniques for thermo-acoustic studies are quite mature, others have yet to be developed and qualified for this field of application. At this point there is a considerable need for methods capable of recovering data in the following three areas: • •
•
Acquisition of acoustic pressure fluctuations in hot flows of industrial development experiments of combustion systems, preferably in realistic operating conditions. Temporally and spatially resolved measurements of density fluctuations that are associated with the flow (entropy waves), in particular to correlate these with other time resolved data such as microphone measurements. Non-intrusive, spatially resolved measurements of the acoustic particle velocity.
In general, it is desirable to measure acoustic pressure fluctuations and the acoustic particle velocity simultaneously to derive local impedances. In particular, it is of great importance to measure directly the acoustic waves propagating towards and from the flame as this would allow the direct determination of transfer matrix of the flame front. This article describes recent work on three measurement techniques that have shown the potential of delivering the information mentioned above. Conventional microphones, such as condenser microphones, generally are used to measure sound pressure. The problem is that these devices are not heat resistant, since the pre-amplifying electronics need to be placed in the vicinity of the microphone membrane. To deal with this difficulty, one possibility is to use a so called microphone probe, which is basically a semi-infinite pipe, inserted in the hot environment and flushed with cold air (e.g. [1]). The microphone is placed at a T-junction in some distance from the hot environment. While reliable, the main disadvantage of these microphone probes is its stiffness along with the need to correct the recorded signals for the acoustic transfer function of the pipe. Further, continuous flushing with cooler air is necessary to prevent hot gases from entering the probe without damping the sound to be recorded at the same time. Alternative approaches towards acoustic pressure
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
253
measurements in hot environments are either microphones protected by a heat shield and water cooled housing or water cooled piezo-resistive pressure transducers. Apart from the obvious complexity of water cooling, piezo transducers show a higher noise level and accordingly a smaller dynamic range than microphones. In the presented work, it is the intention to build a heat resistant microphone by using light to measure the movement of the microphone membrane. The light will be supplied by an optical fibre, such that the entire microphone can be made of heat resistant materials (steel, silica glass and ceramics). With regard to the optical measurement of density gradients, Schlieren imaging, shadowgraphy and interference imaging have been used in the past. Due to the line-ofsight integrating nature, quantitative measurements, particularly of the absolute density levels, are difficult to obtain with these methods, especially in fully three-dimensional flow geometries present in nearly all flows of technical relevance. In flames density measurements are frequently acquired with spectroscopic methods such as Laser Induced Fluorescence (LIF), Raman scattering, and in some cases with Coherent Anti-Raman Spectroscopy (CARS). While these techniques are also capable of measuring temperature and chemical composition, their main drawback in thermo-acoustic investigations is their rather low acquisition rate, generally in the order of 10-20 Hz. This does not allow the acquisition of time series of density variations. Furthermore, the overall hardware complexity of the spectroscopic methods makes it difficult to apply them in industrial development experiments due to high pressures, limited optical access, strong absorption and disturbances due to fuel droplets. On this basis, the second part of this paper is devoted to density measurement on the basis of Rayleigh-Scattering, a technique that is well suited for the measurement of time-resolved density fluctuations. Compared to the spectroscopic methods this technique is much simpler to implement and can in particular be applied at high pressures. The sensitivity to reflections and Mie-Scattering can be reduced by using an iodine absorption cell that serves as spectral filter. Even though this filtering was not used in the presented experiments, its existence is of fundamental importance, since it will presumably enable to use Rayleigh-Scattering in confined environments, where significant laser induced background radiation has to be removed. Finally the acoustic particle velocity is generally determined by either Hot Wire Anemometry (HWA) or Laser Doppler Anemometry (LDA). HWA is restricted to relatively cold flows. As point measurement techniques, both LDA and HWA are quite time consuming but do yield a high precision. In order to acquire planar resolved measurements of the acoustic particle velocity field, the authors extended the Particle Imaging Velocity (PIV) method. The main challenge of this effort was to separate the rather low acoustic particle velocity (typ. in 5-10 mm/s range) from the much higher convection velocity of the flow (typ. 10’s of m/s). 2. A FIBRE-OPTIC MICROPHONE 2.1. Principle design Figure 1 outlines the design principle of a fibre optic microphone (FOM): An interferometer is used to measure the displacement of a membrane, which is excited by the acoustic field of interest. A single-mode optical fibre is used for both laser light
254
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
Interferometer Fiber
Membrane
Figure 1:
Principle of a FOM.
delivery and return of collected scattered light to the detection electronics located externally. A collimating lens focuses the measurement beam to the backside of the membrane. Here a mechanically stable fixation ensures that only membrane vibrations and no additional setup vibrations are measured. A highly reflecting membrane surface is advantageous to ensure reliable signal detection. With optical arrangement a membrane movement results in a phase modulation of the laser light reflection and consequently in a modulation of the interference signal. A fast photo detector (e.g. photo multiplier, avalanche or PIN diode) detects the interference signal; the photo diode signal is finally used to calculate the membrane movement. Numerous different interferometer arrangements exist; for the FOM prototypes two different setups have been tested, both based on the amplitude splitting method. The optical configurations are described in section 2.2. The experimental results obtained with these interferometers are presented in section 2.3. 2.2. Realization of two different FOM setups 2.2.1. The optical setup based on a Mach-Zehnder interferometer The first interferometric configuration is the Mach-Zehnder (MZ) interferometer, which is also commonly used in commercially produced optical measurement devices. Figure 2 shows the required components to construct a fibre based, heterodyne setup. The laser (1) beam is split up via a first beam splitter (2) into the so called measurement and reference beam. The former one is coupled via a lens into a single-mode optical fibre (3), which guides the beam to the reflecting surface (6), whose vibrations have to be measured. The reflection from the surface travels back via the same fibre and interferes with the reference beam that is frequency shifted by means of an acousto-optic modulator (AOM) (4). The interference signal is detected by a photodiode (5). The movement of the reflector (6), the microphone membrane, modulates the carrier signal created by the AOM. Generally, this setup has two main disadvantages: (1) the temperature limitation of the polarization maintaining single-mode optical fibre and (2) the high vibration sensitivity of the setup including the fibre. Even small vibration amplitudes change the optical length of the fibre significantly and thus create high noise levels. This sensitivity to vibrations of both, the interferometer and the fibre, makes it
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
255
3 2
1
2
6 dx/dt
2
1 Laser 2 Beam splitter 3 Glass fiber 4 AOM 5 Diode 6 Reflector Figure 2:
4
5
Optical setup of the MZ interferometer.
difficult to perform accurate measurements on test rigs that show strong vibrations as it is often the case in medium and large scale combustion facilities. 2.2.2. The optical setup based on a Fabry-Pérot interferometer A very robust laser interferometer setup can be constructed using the etalon effect of a single-mode optical fibre that is supplied with light from a single-mode laser. This setup is also called Fabry-Pérot (FP) interferometer ([2, 3, 4]). The two interfering beams are the reflection of the fibre ending, caused by the interface between fibre core and the free air path, and the reflection of the object whose movement has to be measured. The acquired interference signal represents an interference pattern moving across the active area of the diode. A peak in the photo diode signal represents a passing area of constructive interference. The period between two such peaks is related to an object movement of half the wavelength λ of the used light source. Using a fringe counting procedure enables to estimate the amplitude of the object movement. Distinguishing between ‘peak high’ and ‘peak low’ results in a resolution of λ/4. Since diode signal oscillations can also occur due to intensity fluctuations of the laser light source used, it is necessary to monitor simultaneously the laser intensity to eliminate this source of error. The main advantages of this interferometric setup are the following: •
•
Non-polarization maintaining single-mode optical fibres can be used for this setup; i.e. commercial glass fibres with fibre claddings heat resistant up to 1000 K can be applied to create a high temperature resistant microphone. Apart from the fibre, the materials of the microphone are steel and fused glass only, such that the microphone becomes heat resistant up to 1000 K. This is sufficiently above the compressor outlet temperature of gas turbines (typically 750 - 850 K); thus the microphone can be cooled with the compressed air, which is also used to cool the liner of the combustion chamber. This results in a good applicability. Due to the fact that both interfering beams are guided in one fibre, the vibration sensitivity of this setup is of minor importance. In this way the application of
256
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
pol.
pol.
6 2
1 1 Laser 2 Pockels cell 3 Beam splitter 4 AOM 5 Diode 6 Non-polarizing singlemode fiber 7 Reflector
Figure 3:
•
3
3
7
3
5
dx/dt
4
Optical setup of the advanced FP interferometer.
FOMs in thermo-acoustic experiments and in particular in industrial development experiments becomes possible. The demand on the light source concerning the coherence length is very low, because of the short difference of the travel distances of the two interfering beams.
Since the resolution of this FP basic configuration is generally not sufficient for the detection of the movement of acoustically excited membranes, Figure 3 shows a possibility to increase the setup sensitivity by use of two additional optical devices: a Pockels cell and an acousto-optic modulator (AOM). By means of the frst mentioned device, acting with a frequency much higher than the acoustic frequencies of interest, the sensor detects two FP signals quasi simultaneously. For creating a constant phase difference of 90° between the two quasi simultaneous acquired FP signals, the AOM is used; the distance L between the fibre ending and the reflector has to be adapted to the frequency shift ∆ν of the AOM satisfying equation (1): ∆ν λ / 4 = ν0 2L
(1)
where ν0 stands for the frequency of the laser and λ for its wavelength. Figure 4 shows the photo diode signal of the basic FP setup observing a sinusoidal reflector movement; Figure 5 illustrates the diode signal of the advanced FP and how the signal has to be processed to get two quasi simultaneously acquired FP signals. Plotting the two FP signals against each other creates a circle due to their constant 90° phase difference. A closed circle in this Lissajous figure represents an object movement of λ/2, according to one passing period between two intensity peaks in the diode signal of the basic configuration. The movement of the object is recalculated by the arctangent of the two FP signals (see Figure 6). Intensity fluctuations influence both signals, i.e. the radius of the created circle, but not the orientation of the signal pair with respect to the origin. The possibility to analyze only segments of a circle enables the measurement of amplitudes
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
257
0.7
Diode signal / object movement [a.u.]
0.6 0.5 0.4 0.3 0.2 0.1 0 −0.1 Fabry-Pérot signal
−0.2
Object movement 0.35
Figure 4:
0.355
0.36 Time [sec]
0.365
0.37
Object movement and resulting intensity modulation due to interference in the Fabry-Pérot resonator of fibre ending and membrane.
0.8 Diode signal Signal 1 0.7
Signal 2 Pockels signal
0.6
0.5
0.4
0.3
0.2
0.1
Figure 5:
0.356
0.357
0.358 Time [sec]
0.359
0.36
Two Fabry-Pérot signals of two light waves with a frequency shift that results in a 90° phase shift of the interference signals.
258
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
1
0.5 0.4
0.5 Calculated displacement [µm]
0.3
Signal 2 [−]
0
−0.5
−1
0.2 0.1 0 −0.1 −0.2 −0.3 −0.4
−1.5
−0.5 −2
Figure 6:
−1
−0.5
0 0.5 Signal 1 [−]
1
0.14
0.15 0.16 Time [sec]
Lissajous Figure of two 90° phase-shifted Fabry-Pérot signals and object movement reconstructed from the absolute phase angle that can be derived from the Lissajous plot.
far smaller than λ/4; thus, the sensitivity enhancement is achieved, which is necessary for acoustic measurements. For preliminary tests a 532 nm single-mode laser, a Pockels cell with a maximum switching frequency of 100 kHz, and a piezo-electric translator that simulated the object movement were used. This study showed that for object movements with an amplitude of approximate 500 nm the switching frequency of the Pockels cell has to be at least 50 times the frequency of the moving object in order to ensure that the whole evolution of the Fabry-Pérot signal is detected, i.e. the Pockels cell frequency has to be adapted to the maximum interference-event-frequency (influenced by both frequency and amplitude of the object movement) that has to be measured. 2.2.3. The sensor head of the designed FOM In case of the FOM the reflector for the interferometer setup is the backside of a membrane. Since this assembly has to fulfil the requirements for a high temperature application as well, it has to be designed accordingly. The membrane assembly of the first prototype is a hollow cylinder, which holds the lens to focus the laser beam on the membrane. On one side the glass fibre is mounted, while the other ending is closed with the reflecting membrane. For the fixation of the membrane onto the hollow cylinder two
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
259
Fiber position Membrane
Lens
Figure 7:
FOM sensor head.
possibilities were successfully tested: the use of adhesive in combination with aluminium foils with thickness of approx. 2µm, whereby the temperature limitation of the adhesive enabled only measurements at ambient temperatures, and the clamping of stainless steel foils with thickness of approx. 12.5µm by means of a retaining ring as shown in Figure 7. It should be stressed that this clamping process represents a very simple and economic way to manufacture and repair sensor heads. The stainless steel membrane is very easy to fabricate and easy to handle, in contrast to membranes of conventional condenser microphones that can only be produced by special companies and break easily, for example when touched by human fingers. The effective membrane diameter for this sensor head design is approx. 6.35 mm (1/4”). The dynamic range of the designed FOM depends on the resolution of the interferometer as well as on the physical properties of the membrane (e.g. thickness, diameter, stiffness). Thus these parameters can be used to adapt the sensor to the favoured application. 2.3. Experimental results 2.3.1. Application of the Fabry-Pérot setup for cold measurements First acoustic measurements with the FOM based on the Fabry-Pérot setup were performed in an acoustic test rig at ambient temperatures. A loudspeaker was used to create a sweep excitation in the test section where the FOM was installed. A conventional condenser microphone was installed at the same axial position and used in parallel. Since the frequency of excitation was varied only between 200 and 1000 Hz and the cut-on frequency of the test channel is approximately 2144 Hz at ambient temperature, it was ensured that the acoustic field of only plane waves was identical for both sensors during the experiment. Figure 8 shows the spectra calculated from the acquired sensor signals. The microphone spectrum is given in [dB] calculated using a one-point calibration with a pistonphone. The spectrum of the FOM is given in [µm]. Since deflection and pressure are linearly proportional their spectral content can be compared directly. This experiment shows the good correlation between the FOM and the microphone signals and validates our new sensor technique versus an established
260
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
condenser microphone 0.1 Amplitude microphone [dB]
100
50 200
300
400
500
Amplitude FOM [ µ m]
FOM FP
600
700
800
900
1000
1100
Frequency [Hz]
Figure 8:
Spectra of the FOM (right axis) and the condenser microphone (left axis).
measurement technique for acoustics in cold conditions. The characteristic shape of the spectra is caused by the transfer function of the test rig. 2.3.2. Measurements obtained with the Mach-Zehnder interferometer: Calibration of the sensor for cold measurements This FOM configuration was first calibrated by use of the one-point-calibration procedure applying a pistonphone to create a well-defined acoustic pressure at an appointed frequency (i.e. 124 dB at 251.4 Hz). Thus, sound pressure values can be calculated from interferometrically measured displacements. These [Pa] values can be compared directly to the data acquired by established measurement techniques for sound pressures. Figure 9 shows the spectra of the calibrated FOM in comparison to the test results of a condenser microphone for an experiment where both sensors were wallflushed mounted in a cold acoustic test rig with acoustic excitation between 200 and 4000 Hz. Once again the characteristic shape is defined by the transfer function of the setup. The FOM shows a high correlation to the established measurement technique even above the first cut-on frequency of 2144 Hz. A further possibility to validate the performance of the FOM is the calculation of the transfer function between the FOM and the condenser microphone installed in parallel and its comparison to the transfer function calculated between two simultaneously installed condenser microphones. After calibration of all sensors the resulting transfer functions should have a value of 1 for the amplitude and a constant value of 0 for the
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
261
120 FOM Condenser microphone
115
Sound pressure [dB]
110 105 100 95 90 85 80 75
Figure 9:
500
1000
1500
2000 2500 Frequency [Hz]
3000
3500
4000
Comparison between FOM and condenser microphone after calibration.
phase up to the first cut-on frequency of the test rig. Slight deviations from the ideal values beyond this frequency may arise from the appearance of sound waves that propagate in higher modes. Figures 10 and 11 present the result of this experiment. Figure 10 shows the amplitude of the two transfer functions “FOM - condenser microphone” and “condenser microphone - condenser microphone”, Figure 11 the phase of the transfer functions. Both sensor combinations indicate the expected trends, whereby especially the transfer function of the 2 condenser microphones shows the characteristic influence of the cut-on frequencies marked as vertical, dashed lines. The result obtained with 2 condenser microphones is used to compare the new measurement technique once again against an established one, and it shows the high correspondence between the sensors. The irregularities at 500 and 700-800 Hz are not fully understood at this point and therefore are still under investigation. The calibration of the FOM allows also the determination of the minimum detectable sound pressure, i.e. the lower limit for acoustic measurements due to background noise (thermal and electronic noise) created by the FOM components. The prototype based on the Mach-Zehnder interferometer shows a minimum detectable sound pressure of approximately 50 dB; the value for conventional condenser microphones lies in the range of 20 dB, while for piezo-resistive pressure transducers the detection limit lies above 90 dB (depending on the model). Thus, the lower detection limit of the FOM is at least 40 dB below the limit of piezo-resistive pressure transducers, which currently is one of the most frequently used measurement techniques for acoustic measurements in hot environments.
262
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
4 3.5
FOM—Condenser microphone 2 Condenser microphones
Amplitude [−]
3 2.5 2 1.5 1 0.5 0
Figure 10:
150
500
1000
1500
2000 2500 Frequency [Hz]
3000
3500
4000
3000
3500
4000
Amplitude of the transfer function.
FOM—condenser microphone 2 condenser microphones
100
Phase [°]
50
0 −50 −100 −150 500
1000
1500
2000
2500
Frequency [Hz]
Figure 11:
Phase of the transfer function.
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
263
Amplitude microphone [Pa] / FOM [µm]
102 Probe microphone FOM 101 100 10−1 10−2 10−3 10−4
Figure 12:
100 200 300 400 500 600 700 800 900 1000 Frequency [Hz]
Spectra of a probe microphone and the FOM.
2.3.3. Application of the MZ FOM for acoustic measurements in the hot exhaust gas pipe flow of a combustion system In further tests the novel FOM was successfully applied at elevated temperatures in a combustion oscillation test rig of the German Aerospace Center (DLR). To compare the signals of the FOM to the signals of an established measurement technique, in-house designed probe microphones (e.g. [1]) were used. Since the design of these sensors does not allow flush-mounting on the wall, a calibration procedure has to be performed in advance to calculate amplitude- and phase-correction coefficients. The test rig was operated at a thermal power of 7 kW. Figure 12 shows the two spectra of the applied sensors. The signal of the probe microphone shows the pressure oscillation amplitude in [Pa], the signal of the FOM the amplitude of the membrane displacement in [µm]. Both spectra show a good similarity. The same was true for the phase difference in the frequency interval from 1-2 kHz (not shown here), while some deviations occurred for frequencies below 1 kHz that are still under investigation. These deviations presumably arise from the different sensor positions, from the accuracy of the correction coefficients of the probe sensors and in particular from the relatively high vibration sensitivity of the polarization maintaining fibre. 3. DENSITY MEASUREMENTS USING RAYLEIGH-SCATTERING 3.1. Principle of Measurement Rayleigh-Scattering is an elastic diffusion process that results from the interaction of light with atoms or molecules. The amplitude of Rayleigh-Scattering is directly proportional to the scattering cross section and to the number density of gas molecules. Therefore it can be utilized to determine gas density, as long as the chemical
264
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
EMCCD Lens system Ar-laser
Beam dump
Burner
Figure 13:
Measurement setup.
composition does not change. In the past this technique has been demonstrated for free jets [5, 6]. Its applicability was also demonstrated in more challenging environments, such as in combustion chambers [7]. Correlations of Rayleigh-Scattering measurements and acoustic measurements were used to identify noise sources in free jets [8]. To date, only either 2D static density maps or time resolved point measurements via photomultipliers could be recorded. Recent advances in camera technology now make it possible to record time resolved 2D (planar) density maps. Thus frequency and phase resolved data can be acquired. 3.2. Setup A continuous wave argon ion laser operating at 514 nm with a power of 1.0 W and a beam diameter of 1.5 mm was used as light source. The measurement setup is shown in Figure 13. The laser beam passed a flame and was blocked by a beam dump. The scattered light was collected by a lens system and detected by an electron-multiplying charge coupled device camera (EMCCD). It was possible to record a large number of pixels at data rates fulfilling the Nyquist criterion in respect to the oscillation frequency of the flame. An optical bandpass filter with 9 nm FWHM and centre wavelength of 514 nm was used to reduce background illumination. The laser beam was traversed through the flame to record a 2D time resolved density map of the flame. In comparison to measurements obtained in ambient air the Rayleigh-Scattering images of the flame exhibited a roughly 30 percent decrease in the standard deviation of the measured pixel intensities. In part this was due to the practical absence of dust in the flame because it was burned. Furthermore Rayleigh signal contributions from soot was believed to be of minor importance since the burner was operated in a lean (stoichiometrically balanced) condition in which very little - if any - soot is produced. To compare the Rayleigh-Scattering measurements to a conventional measurement technique the flame was scanned by a fast thermocouple as well (diameter 25 µm). A second fast thermocouple was placed in a fixed position where the flame showed strong oscillations. The purpose of this second thermocouple was to serve as a reference to derive phase information.
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
Burner plate.
Normed fouriertransformation
Figure 14:
265
1 0.8 0.6 0.4 0.2 0
0
5
10 Frequency [Hz]
Figure 15:
Spectrum of the flame pulsation.
Figure 16:
Spectrum of the flame pulsation in 60˚ steps.
15
20
266
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
A premixed propane/butane unconfined flame was investigated. Figure 14 shows the burner plate with an diameter of 3.8 cm. The region above several concentric rings of holes in the burner plate exhibited a multitude of small flame structures (about 4 mm high) that together comprised the primary combustion zone. Above this region a second combustion zone with slightly bluish colour extends to about 40 mm above the burner plate. 3.3. Measurements in an unconfined flame The frequency spectrum of the flame pulsation taken with the EMCCD in the upper part of the flame (Figure 15) shows a pulsation frequency of approximately 12 Hz. Figure 16 provides an impression of the flame luminescence in the visible spectral range at different phase angles of the oscillation process. In addition 400 thermocouple measurements were carried out in an area of 100 mm × 22 mm. The positioning error of the thermocouple was half a millimetre, thus the dimension of one measurement point on the right half of the following pictures is one millimetre. On the left half of the thermocouple pictures, an interpolation of the measurement data is shown, like it is usual to get a general idea of the data. In the current configuration the Rayleigh-Scattering measurement provides line data, thus only 100 measurements were necessary to obtain a dense grid of the investigation area. The error of positioning in the vertical direction was approximately half a millimetre; the spatial resolution in horizontal direction was 0.2 mm (237 measurement points (pixels) in 56 mm). The Rayleigh measurements were calibrated via the scattered laser intensity at ambient temperature and density. For comparison with the thermocouple measurements, the Rayleigh data were converted to temperature data using the ideal gas law and assuming room temperature in the lower left and right corner of the image. As it can be seen from Figure 17, the Rayleigh results compare in general fairly well with the thermocouple data. The hottest area of the flame, that is between 50 mm and 60 mm vertical position is found 10 mm higher in the thermocouple data field. Also the hot area at large is higher in the thermocouple measurements. The main reason for this discrepancy could be the manual adjustment of the mass flow rate of the burner between the two measurements which were not obtained coincidentally. Also the limited number of measurement points of thermocouple data result in uncertainties in the interpolated temperature field. The second discrepancy between Rayleigh-Scattering and thermocouple measurements is found in the inner flame region above the burner plate between 10 mm and about 40 mm vertical position. As mentioned in the setup section this is the area of flame glow and the secondary combustion zone. Since the level of scattered intensity is directly proportional to the gas density and also to the scattering cross section, the RayleighScattering data in this area has much higher uncertainties. In literature [9, 10] correction factors for C3H8 flames are given. Nevertheless the principal structure of the flame in this zone is reliable, because the flame is premixed. The main combustion process occurred between 0 mm and 10 mm above the burner and more
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
267
Temperatur [K] 100
100
90
90
80
80
70
70
60
60
Vertical position [mm]
Vertical position [mm]
1400
50 40 30
1200
1000 50 40 800 30
20
20
10
10
0
0
600
400 Burner 0
10 20 30 40 Horizontal position [mm]
Figure 17:
Burner 10 20 30 40 50 Horizontal position [mm]
Mean flame temperature (right: Rayleigh-Scattering measurements, left: thermocouple measurements and their spline interpolation).
than 70% of the gas is nitrogen which does not contribute to the combustion. The rest of the Rayleigh-Scattering data matches well to the thermocouple data. The amplitude of fluctuation, that is, the maximum of the FFT at the pulsation frequency is shown in Figure 18, together with the maximum pulsation measured by the fast thermocouple. The thermocouple data needed to be normalized, because even the low oscillation frequencies of about 12 Hz are above the cut-off frequency of the fast thermocouple and frequency response compensation was not applied. The centre of the flame shows almost no pulsation. Even though a laser bandpass filter was used, remaining background flare was still detectable in the areas with flame glow (flame glow is visualised in Figure 16). Since the flame luminosity also oscillates with the frequency of the flame pulsation neither conventional background subtraction nor frequency filtering of the acquired time-resolved camera signal can be used to remove
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
100
100
90
90
80
80
70
70 Vertical position [mm]
Vertical position [mm]
268
60 50 40 30
0.5 40 0.4 30
10
10 20 30 40 Horizontal position [mm]
Figure 18:
0.7
50
10
0
0.8
0.6
20
Burner
0.9
60
20
0
Temperature Fluctuation in % 1
0
0.3 0.2 0.1 Burner 10 20 30 40 50 Horizontal position [mm]
0
Amplitude of the flame pulsation (right: Rayleigh-Scattering measurements, left: thermocouple measurements and their spline interpolation).
this error source. Depending on the position in the measuring area (what means depending on flame temperature) the ratio of scattered laser light and background light varies between 0.2 and 2. In an effort to overcome this severe and very fundamental problem, a simple and robust post processing procedure was developed: The camera also observed smaller regions on both sides of the laser beam (Figure 19). These regions were spatially far enough from the beam to be free of laser light induced Rayleigh-Scattering but close enough to show the same level of flame glow including time dependant fluctuations. At a given position in the direction of the laser beam, the remaining flame glow was measured on both sides of the beam, averaged and then subtracted from the light intensity measured inside the volume of the beam. It should be stressed, that in this way also the oscillating components of the background flare can be eliminated, in contrast to conventional background subtraction that only eliminates constant components. The effect of this special procedure can be seen in Figure 20. The right hand side shows the (magnitude squared) coherence between the thermocouple and the Rayleigh measurement with, and the left hand side without this
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
Rayleigh scattering caused by laser light
Flame luminescence
Pixels used to determine the background level
Figure 19:
269
Pixel that needs to be corrected for background radiation
Reduction of the influence of background radiation on the Rayleigh scattering intensity.
90
90
0.9
80
80
0.8
70
70 Vertical position [mm]
100
Vertical position [mm]
100
60 50 40 30
60
0.5 40 0.4 30 20
10
10
0.3 0.2 0.1
0 Burner 10 20 30 40 50 Horizontal position [mm]
Figure 20:
0.6
50
20
0
0.7
Burner 10 20 30 40 50 Horizontal position [mm]
0
Coherence between the time and spatially resolved Rayleigh signal and a fast reference thermocouple. The right image is corrected for the influence of oscillating flame glow by a special procedure, while the left image is not.
270
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
Phase in° 100
100
90
90
80
80
70
70
350
300
Vertical position [mm]
Vertical position [mm]
250 60 50 40 30
60 200
50 40
150 30
20
20
10
10
0
0
100
50 Burner 0
Figure 21:
10 20 30 40 Horizontal position [mm]
Burner 10 20 30 40 50 Horizontal position [mm]
Phase shift to reference thermocouple (right: Rayleigh-Scattering measurements, left: thermocouple measurements and corresponding spline interpolation).
correction. In the triangular region above the burner, the correlation is far too high in the uncorrected case (left), since the temperature in this region does not oscillate (according to the thermocouple measurement). This can be explained by the remaining influences of the oscillating flame glow. In this regard it should be observed that the flame luminosity is a volume effect, that is, it does not originate from the middle plane where the laser beam is located but is an integral quantity along the line of sight. This disturbing influence can significantly be reduced, as it becomes obvious in the corrected image (right), where the correlation in the triangular region above the burner is strongly reduced. Figure 21 shows the phase shift between the Rayleigh measurement and the signal of the reference thermocouple. The phase shift was computed by cross correlation. Measurement points with coherence lower than 70% (Figure 20) were filtered out and, whenever justifiable interpolated. Larger areas with low coherence were marked black. The image shows nicely the entropy waves (cold and hot spots [11, 12]) that are generated by the oscillating combustion and which propagate with the flow. This propagation process is also shown by a vertical profile of the phase distribution taken at
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
271
400 300
Rayleigh—Scattering Linear fit to RS Thermocouple
Phase shift in°
200 100 0 −100 −200 −300 −400 −500
Figure 22:
0
20
40 60 Vertical position in mm
80
100
Vertical phase shift to reference thermocouple at 14 mm horizontal position.
a horizontal position of 14 mm between 10 and 100 mm vertical position (Figure 22). The profile shows a total phase shift of approximately 500°, thus the density waves propagate with approximately 0.8 m/s. Expectedly there is a phase shift of 180° between the density measurements done by Rayleigh-Scattering and the temperature measurements done by the thermocouple. Figure 21 also shows that directly above the burner in the region of the outer flame front, the density oscillation is axially symmetric, while at the top of the flame, the density oscillation shows almost an inversion between left and right hand side. That leads to the assumption, that in the lower region, the outer flame front shows breathing-like movement, while the cone end of the flame performs a rotation around the vertical symmetry axis, which results in a nutation like behaviour of the upper end of the flame. This is further confirmed by the spectrum given in Figure 15 which shows an additional smaller peak at 15.5 Hz. This spectrum was taken at a point in the upper area of the flame, as mentioned above. This second peak does not show up in the lower region closer to the burner. It should be noted, that instead of a fast thermocouple, it is also possible to use a microphone or another Rayleigh signal as a reference. In particular it is possible to correlate Rayleigh signals along the laser beam, for example to derive a spatial coherence length. 4. ACOUSTIC PIV 4.1. Principle of acoustic PIV Applying PIV to measure the acoustic particle velocity was shown in literature by Hann and Greated [13], who developed a correlation method which allows the measurement of the amplitude oscillation of a sinusoidal sound field by using the spectrum of multiple exposed images. Acoustic particle velocity measurements in an impedance tube
272
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
were realised by Blackshire [14] and Humphreys et al. [15] for a variety of pure tone sound fields. Parallel measurements of acoustic streaming and particle velocities are demonstrated by Nabavi et al. [16]. Berson et al. [17] have applied PIV in the stack of a thermo-acoustic refrigerator to measure acoustic velocities also in presence of standing waves. The present work mainly focuses on measurements of the acoustic particle velocity and the flow field in the absence of standing waves, so this method is applicable for flow configurations where a superposition of the flow and the acoustic field exists. PIV measures the velocity field of particles in a laser induced light sheet and derives a velocity vector expression of the flow field by dividing the particle displacement of two consecutive PIV images by the pulse distance. When a sound wave is propagating in a homogeneous gas medium it creates small local disturbances in density, pressure and velocity. All fluctuations, depending on spatial location and time, are considered to be small compared to the mean values. The velocity fluctuations can be interpreted as the acoustic particle velocity. It is the perturbation velocity of a particle moving back and forth in the direction of the sound wave propagation. It is not the velocity of the wave propagation itself which is called speed of sound. Usually an additional non-negligible turbulence term exists, thus the measured velocity utotal consist of three components and can be described as: ′ utotal = umean + uturbulence + uacoustic .
(2)
Whereas u–mean represents the mean flow component, uturbuloence the turbulence term of the flow field, and u′acoustic describes the acoustic velocity that varies depending on the phase of excitation. As usual, a PIV velocity map is derived from two exposed single images. Figure 23 visualise the principle of recording two sets of PIV images. In order to extract the acoustic particle velocity, a synchronised image acquisition and appropriate post processing is necessary. The image acquisition is based on a circuit device that admits an 180° phase shift between two following vector images with respect to the acoustic excitation. Therefore, the frequency of the investigated phenomenon needs to be known and requires previous microphone measurements. The first exposed raw image of the first set (see Figure 23 (left)) is acquired when the particle displacement due to the excitation frequency is deflected regarding the zero-crossing - here exemplary at 90° - the maximum positive displacement. The second PIV image is taken at the second maximum displacement at 270° located at the negative side of the zero-crossing. Using both raw images a vector field can be calculated. The second set of raw images is acquired in the same way, but switched in the image order (see Figure 23 (right)). Since the maxima of the second set are inversed, both vector fields consist of an opposed acoustic particle velocity vector which can be described by Eqn.2. Additional an arrow representation of the total measured velocity is included below the graphs in both images. A subtraction of both equations representing vector field one and two ends up in: ′ utotal − utotal = uturbulence − uturbulence + 2uacoustic . 2
1
2
1
(3)
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
Images for vector field 2
90° 1. Image
u'a1
0
270° 2. Image
Particle displacement
Particle displacement
Images for vector field 1
90° 2. Image u'a2
0
270° 1. Image
Time + utotal1 = u--mean +
Figure 23:
Time +
uturb1
273
+
+ u'a1
utotal2 = u--mean +
+ uturb2
+
u'a2
First set of two PIV images (left) and second set of two PIV images having 180° phase shift to the first set (right) and the vector representation of the total measured velocity.
Thereby the constant velocity fraction u–mean vanishes and the calculated acoustic particle velocity only depends on the turbulence term uturbulence2 — uturbulence1. This turbulence term is a stochastic term, thus there is nearly no correlation neither in time nor in spatial location between these two successive vector maps. Due to the randomness of the turbulence this term tends to disappear in a large number of measurements. Now it is possible to calculate the acoustic particle velocity by subtracting and averaging a sufficient number of images. If 2N represents the number of images Eqn. 3 can be written as: 1 N →∞ N lim
2N ∑ u ′ − utotal = 2uacoustic . i =1 total2i 2 i −1
(4)
To obtain the best results the phase shift between the two raw images of one vector map has to be taken into consideration. Further the optimum pulse distance is infuenced by the magnitude of the mean flow because the conditions need to be suitable for the image cross correlation. 4.2. Experimental procedure for acoustic PIV A squared test section made of acrylic glass allows an optical access into the region of interest, as shown in Figure 24. The dimensions of this test section are 155 mm in length and 140 mm in height, and 140 mm in depth. On one side a loudspeaker is connected via a speaker horn, which is conical inside allowing a smoother transition from the loudspeaker cross sectional area into the test section. The excitation signal emitted by the loudspeaker is generated by a frequency generator and can be adjusted via an amplifier up to a sound pressure of 104 dB inside
274
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
PC cooling fan
Microphone
Loudspeaker Seeding pipe supplier
Figure 24:
Sketch of the test section.
the test section. The amplitude of excitation is measured by a wall-flush mounted 1/4” microphone inside the upper wall of the test section. A PC cooling fan installed at the upper wall is used to additionally generate a turbulent flow field that depends on the rotation speed level of the fan. The side of the test section opposite to the speaker is terminated by acoustic foam, which reduces the reflections of the sound wave and minimises the seeding leakage. The inlet for the oil based liquid aerosol seeding is located at the lower side wall of the test section. The seeding particles with nearly 1 µm in diameter are illuminated by a conventional PIV laser system: a dual-cavity, pulsed Nd-YAG laser system operating at 532 nm wavelength. The two laser beams are focused and expanded by a set of four lenses to span the beam into a laser sheet of 2 mm thickness inside the horizontal plane of middle section. The CCD camera with a lens of 85 mm focal length is mounted perpendicular to the laser sheet. A region of 40×30 mm is recorded at a resolution of 1600×1200 pixels. Synchronisation between laser and camera is realised by a multichannel pulse generator commonly used for PIV. A trigger input on the pulse generator is used to the laser-camera system to the excitation signal of the loudspeaker. A self-made electronic circuit is used to alternately change the phase of the trigger signal before it is feed into the synchroniser. This circuit basically consists of a binary counter that reduces the frequency of the “camera output signal” by a factor of 2 and an “exclusive OR gate” which changes the phase of the trigger signal by 180° in case the binary counter output is set to “low” (see Figure 25). Since the trigger input frequency is much higher than the camera frequency, the synchroniser box has enough time to adjust to the new time base if a phase-switch occurs. To adjust the phase within one period the trigger box can be additionally changed in phase, which allows moving in 0.5° steps. 4.3. Results for cold flow conditions For investigations of the amplitude sensitivity the excitation of the loudspeaker was set to a single constant frequency varying in amplitude inside the test section. A successful implementation of this acquisition and data processing technique is shown in Figure 26 for an excitation of 104 dB at 211 Hz.
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
275
Synchronous binary counter with dual clock
Camera output
×
2Hz
1 Hz TTL
1 2
Phase shift Inputs b
Output
=1 a Exclusive OR gate
Excitation signal 211Hz
Figure 25:
Output to trigger box 211Hz (180° phase shifted each 1Hz)
Scheme of the 180° phase shift circuit device.
u'_acoustic :
15 20 25 30 35 40 45 50
25
Height [mm]
20
15
10
5
−5
Figure 26:
0
5
10
15 Length [mm]
20
25
30
Calculated acoustic particle velocity for sinusoidal excitation (211 Hz, 104 dB) in empty test section.
276
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
Mean acoustic particle velocity (mm/s)
100
10
1
Ambient noise in the lab louder than sound field in the test section 0.1 60
Figure 27:
70
80 90 Sound pressure level (dB)
211Hz 100
110
Mean acoustic particle velocity versus the sound pressure level of the excitation.
The acoustic field propagates from right to the left with higher acoustic particle velocity near the horn outlet. Obviously, a planar wave is leaving the horn and the transition to a spherical wave is also clearly visible. The result of the complete variation in amplitude, ranging from 65 to 104 dB can be seen in Figure 27. The low frequency of 211 Hz has been selected because at this frequency the acoustic velocity is low but the displacement is high at equal amplitudes. As expected higher acoustic particle velocities have been measured by increasing the amplitude of the loudspeaker. The number of averaged image sets decreases from 100 at lower amplitude levels to 20 image sets at higher amplitudes. In the shown double logarithmic scale, a linear trend between increasing sound pressure level and acoustic velocity can be identified. Measuring the particle velocities below 80 dB was difficult, since the background noise from the laboratory, in particular from the laser power supplies dominated the loudspeaker signal. It should be noted that these uncertainties are not caused by the small displacements in the PIV recordings. In order to investigate the turbulence sensitivity a PC cooling fan was installed inside the test section for measuring the acoustic particle velocity in presence of turbulent flow. Figure 28 and Figure 29 demonstrate the infuence of the flow turbulence. Figure 28 shows a typical flow image with superposed acoustic particle velocity and turbulence field. In a series of 160 PIV acquisitions, an average turbulence level of nearly 5 times higher than the maximum acoustic particle velocity was measured. Assuming a maximum particle velocity of 100 mm/s the RMS value can be calculated to 500 mm/s. Figure 29 shows the particle velocity that was derived from these 160 acquisitions by averaging. This image has to be compared to Figure 28. In particular in the regions with lower acoustic particle velocity (left hand side of the image), the influence of the turbulent flow remains visible even after averaging the 160 vector fields. In the right
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
u'_acoustic [mm/s]:
20
30
40
50
60
70
80
90
277
100 110 120
25
Height [mm]
20
15
10
5
−5
Figure 28:
0
5
10
15 Length [mm]
20
25
30
A single PIV image at 211 Hz and 104 dB with 40 percent rotation speed to give an idea of the turbulence level which is present.
part of the image with higher acoustic amplitudes the particle velocities seem to be almost correct. Indeed, averaging 160 measurements reduces the standard deviation by a factor of the square root of 160 that is nearly 13 resulting in a remaining new RMS value for the averaged image of nearly 40 mm/s. In the right part of the Figure 29 with higher acoustic amplitudes, this new RMS value is dominated by the approximately 80 to 100 mm/s of acoustic particle velocity. The left hand side is still in the same magnitude of this RMS value. This example should simply demonstrate the capability of acoustic PIV. If it is carried out in the way described above a prediction by simple statistical approximations can be done. Of course the vector plot in Figure 29 is still noisy, but it was tested that by increasing the number of acquired PIV measurements, the result can be improved using statistics. Comparing Figure 26 and Figure 29 shows differences in the absolute measured acoustic particle velocity. These differences are results of different camera positions with respect to the test section. In Figure 29 the camera position was moved by about 15 mm towards the speaker. This measurement technique can be transferred to combustion conditions. Instead of a loudspeaker and a signal generator, the acoustic
278
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
u'_acoustic [mm/s]:
20 25 30 35 40 45 50 55 60 65 70 75 80
25
Height [mm]
20
15
10
5
−5
Figure 29:
0
5
10
15 Length [mm]
20
25
30
The acoustic particle velocity, derived from 160 acquisitions at 211 Hz and 104 dB with 40 percent of rotation speed of the PC fan.
excitation will originate from the thermo-acoustic oscillation and the reference signal will be taken from a heat resistant microphone or a FOM for example. 5. CONCLUSION AND OUTLOOK A fibre optic microphone was presented which is based upon the interferometric detection of the movement of an acoustically excited membrane. Two interferometers were successfully applied: • •
An advanced Fabry-Pérot interferometer was applied for acoustic measurements at ambient temperature. A Mach-Zehnder interferometer was used to create a calibrated sensor that was applied for diverse measurements at ambient temperature and for first measurements of acoustic spectra in the exhaust gas pipe of an atmospheric combustion test rig.
It is important to note that the fabrication and handling of membranes is a lot easier for FOMs than in the case of conventional condenser microphones. It is also important to note that the Fabry-Pérot interferometer features a minor sensitivity regarding harsh
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
279
environments (e.g. high temperatures, test rig vibrations, elevated pressures). Therefore this design will be investigated and further optimised in future. The optical installation is currently being redesigned to mobilise the setup and simplify its application. The mentioned atmospheric combustion test rig will be used to study the heat resistance of the sensor increasing the cooling air temperature up to the temperature limitation of 970 K, so that measurements in combustion chambers become possible, where the cooling air temperature normally does not exceed 850 K. Rayleigh-Scattering measurements were carried out to investigate the mean density, amplitude and phase of density pulsations in an unconfined, premixed propane / butane flame. The generation and propagation of so called entropy waves (hot and cold spots), became quite visible. A procedure was developed that allows eliminating the influence of oscillating flame luminescence. This is an important improvement to conventional background subtraction techniques that only eliminate DC offsets due to background radiation. In a further step, an iodine cell and a single mode frequency stabilized laser will be used to reduce stray light from walls and windows. Using the strong and narrow absorption lines of iodine allows the blockage of non-Rayleigh stray light [18] such as Mie scattering or reflections from windows and other surfaces. Finally this article demonstrated a synchronised PIV recording technique combined with an unique data post processing that is able to determine simultaneously the acoustic particle velocity and the mean flow velocity of a flow field respectively. In this context a circuit device was developed that allows the recording of raw images in the required order at the desired phase angle. Thus the proposed post processing can be done straightforwardly. Applying this technique reasonable results for the acoustic field are achieved for excitation amplitudes larger than 80 dB. To further extend the applicability of this method, multi-sine wave excitations are aimed for studying the presence of overlaid excitation frequencies in an effort of developing post processing algorithms that allow a more precise distinction between flow structures and acoustic particle velocity [19, 20]. 6. ACKNOWLEDGEMENT The authors acknowledge gratefully the financial support by the Helmholtz Association (HGF). Thanks are also extended to Henrik Birke, Alexandre Buffet, Emilie Sauvage, and Mirko Spitalny for their supporting work. REFERENCES [1] F. Bake, U. Michel, and I. Röhle. Investigation of entropy noise in aero-engine combustors. J. Eng. Gas Turbines Power, 129:370–376, April 2007. [2] P.G. Davis, I.J. Bush, and G.S. Maurer. Fiber optic displacement sensor. Fourth Pacific Northwest Fiber Workshop Proc. SPIE, 3489, May 1998. [3] A.C. Lewin, A.D. Kersey, and D.A. Jackson. Non-contact surface vibration analysis using a monomode fiber optic interferometer incorporating an open air path. J.Phys. E: Sci., 18:604–608, 1985.
280
Development of optical measurement techniques for thermo-acoustic diagnostics: Fibre-Optic Microphone, Rayleigh-Scattering, and Acoustic PIV
[4]
M. Schmidt, N. Fürstenau, W. Bock, and W. Urbanczyk. Fiber-optic polarimetric strain sensor with three-wavelength digital phase demodulation. Optics Letters, 25:1334–1336, 2000. R.B. Miles, W.R. Lempert, and J.N. Forkey. Laser rayleigh scattering. Meas. Sci. Technol, 12:33–51, 2001. R.W. Dibble and M.B. Long. Investigation of differential diffusion in turbulent jet flows using planar laser rayleigh scattering. Combustion and Flame, 143:644-649, December 2005. P.R. Medwell, P. Kalt, and B.B. Dally. Simultaneous imaging of oh, formaldehyde, and temperature of turbulent nonpremixed jet flames in a heated and diluted coflow. Combustion and Flame, 148:48–61, January 2007. J. Panda and R.G. Seasholtz. Experimental investigation of density fluctuations in high-speed jets and correlation with generated noise. J. Fluid Mech., 450:97–130, January 2002. I. Namer and R.W. Schefer. Error estimates for rayleigh scattering density and temperature measurements in premixed flames. Exp. Fluids, 3:1-9, January 1985. G. Sutton, A. Levick, G. Edwards, and D. Greenhalgh. A combustion temperature and species standard for the calibration of laser diagnostic techniques. Combustion and Flame, 147:39–48, October 2006. W. Polifke, C.O. Paschereit, and K. Döbbeling. Constructive and destructive interference of acoustic and entropy waves in a premixed combustor with a choked exit. Int. J. Acoust. Vib, 6:135–146, 2001. F. Bake, N. Kings, and I. Röhle. Fundamental mechanism of entropy noise in aero-engines: Experimental investigation. J. Eng. Gas Turbines Power, 130:011202–1 – 011202–6, January 2008. D.B. Hann and C.A. Greated. The measurement of flow velocity and acoustic particle velocity using particle image velocimetry. Meas. Sci. Technol., 8:1517–1522, December 1997. J.L. Blackshire. Analysis of particle image velocimetry (piv) data for acoustic particle velocity measurements. In NASA Contractor Report 201664. NASA, January 1997. W. M. Jr. Humphreys, S. M. Bartram, T. L. Parrott, and M. G. Jones. Digital piv measurements of acoustic particle displacements in a normal incidence impedance tube. In 20th AIAA Advanced Measurment and Ground Testing Technology Conference, number 1998–2611, Albuquerque, NM, June 1998. AIAA. M. Nabavi, K.H.K. Siddiqui, and J. Dargahi. Simultaneous measurement of acoustic and streaming velocities using synchronized piv technique. Meas. Sci. Technol., 18(7):1811–1817, May 2007. A. Berson, M. Michard, and P. Blanc-Benon. Measurements of acoustic velocity in the stack of a thermoacoustic refrigerator using particle image velocimetry. Heat and Mass Transfer, 44:1015–1023, June 2008.
[5] [6]
[7]
[8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
International journal of spray and combustion dynamics · Volume . 1 · Number . 2 . 2009
281
[18] R.B. Miles, A.P. Yalin, Z. Tang, S.H. Zaidi, and J.N. Forkey. Flow field imaging through sharp-edged atomic and molecular ‘notch’ filters. Meas. Sci. Technol, 12:442–151, 2001. [19] K. Polthier and E. PreuSS. Variational approach to vector field decomposition, scientific visualization. In Proc. of Eurographics Workshop on Scientific Visualization, pages 147–156, Amsterdam, 2000. Springer Verlag. [20] Y. Tong, S. Lombeyda, A.N. Hirani, and M. Desbrun. Discrete multiscale vector field decomposition. ACM Transactions on Graphics (TOG), 22:445–452, July 2003.
