Transient Grating Spectroscopy
Basic idea: The main aspects of the physical principle of TGS are depicted in figure 1. Two crossed coherent pulse-laser beams (green) with a very short pulse length of about 5 ns generate locally a refraction index grid in the flow. The grid, induced by electrostrictive effects, remains for a few hundred nanoseconds. During this time period, the grating expands with the local speed of sound in all directions. Due to the propagation in perpendicular direction of the grating fringes its contrast and thus ist visibility is modulated in time. The modulation frequency fm corresponds to the ratio between the local speed of sound (a) and the grating fringe spacing s:
where T is the absolute temperature, R the gas constant and k the isentropic exponent. R is inverse proportional to the averaged molar mass of the gas. A TGS-measurement can be considered as an instantaneous measurement because of the short signal acquisition time of about 200 ns. Therefore the technique is well suited for the application in unsteady flows and for resolving the temperature-time-distribution in a flow with periodic variations – as for example in rotation machines.
State of Development: TGS tests conducted at ambient temperature and pressure showed a standard deviation of only ± 1.22 % or ± 3.6 K, while at 3.5 bar overpressure this standard deviation reduced to ± 0.54 % or ± 1.6 K. This uncertainty is very low for an optical measurement technique and was obtained at DLR-facilities in 2004.
Similar to LIF and CARS, TGS was originally developed to be used for measurements in combustion chambers. The experiments described in literature mostly relate to this kind of use. It turned out that TGS was difficult to apply in these conditions because of the reduced density and, presumably, because of beam steering effects and distortions of the wave front due to the spatially inhomogeneous index of refraction caused by the heat releases of the flame.
The key concept for TGS within this proposal is to move away from the idea of using TGS in combustion because of the above mentioned problems, but to take advantage of its abilities, especially the very high accuracy, by using it for measurements in turbo machines and wind tunnels. This was not tried yet even though it might help to overcome unsolved problems.
Potential fields of application: Until today, the only temperature measurement devices applicable to compressors and turbines are temperature probes. Probe measurements are intrinsically intrusive and therefore not suitable for measurements between moving parts as for example rotating blades. Furthermore, due to the thermal inertia of the sensors, most of them are only capable of measuring averaged temperature values. Since the temperature field in the flow of a turbo machine is highly unsteady because of the rotating blades, probe measurements are only of limited use. Therefore, temperature measurements in the flow of turbines and compressors are an unsolved problem.
Further fields of operation are wind tunnels, especially low-temperature cryogenic ones, where the high density present improves the signal quality of TGS.
Scheduled research- and development tasks: Principally, the TGS technique needs optical access from two sides. Thus, the TGS optics should be arranged as shown in figure 3. The light beams cross the flow regime between rotor and stator. In this way the temperature distribution in the wake region of the upstream row of blades is measured. This control surface is necessary to determine the performance of the upstream part of the turbo machine. Since the required cone angle between the laser beams is only in the order of 3°, such a set-up will be applicable in most cases.
Even if the optical access is limited to only a single window, TGS may still be applicable, taking advantage of the fact that the output signal of TGS is a focused beam and not stray light. This signal beam might thus be reflected by a small mirror embedded in a non rotating component opposite of the measurement window. A TGS experiment in the wake region of a radial compressor (figure 4) is planned, where the signal beam will be monitored through the same window as the generating beams.
The measurement accuracy of TGS will be investigated in detail, since it is of crucial importance for applications in turbomachines. It will be enhanced by special lasers with the aim to reduce it as far as ± 1°C. To allow measurement campaigns on external test rigs, a fixed, rugged opto-mechanical system will be build.
Furthermore, it is projected to substitute the Ar+-laser by a long pulse YAG-laser with a pulse length of about 300 ns. This laser is a unique tool constructed and built by the DLR-Institute of Propulsion Technology. An tremendous increase of the signal intensity - about a factor of 10³ - is expected from this laser.
Work Plan:
⇒ Intermediate internal report, to be published if possible.
⇒ Evaluation of results by DLR-evaluation committee
⇒ Publication of results
⇒ Publication of results, documentation preferably as a PhD-thesis
Filters Rayleigh Scattering
Basic idea: Filtered Rayleigh scattering is based on the scattering of electromagnetic radiation (e.g. light) by atoms and molecules. Rayleigh scattering is elastic, which means that the scattered radiation has the same energy as the incident radiation. Since the scattered light intensity is proportional to the number of scattering objects, the density of the fluid can be determined by measuring the intensity of the Rayleigh stray light. This measurement principle is quite straightforward, but it requires two important prerequisites: Only a negligible amount of particles is allowed to be present in the flow, because they cause the relatively intensive Mie-Scattering, and only very little reflections of walls and windows. Since these requirements are hard to find in practical applications, the use of Rayleigh scattering for measurement purposes is not very common so far. However, to reduce the disturbing stray light, the technique of Filtered Rayleigh Scattering (FRS) was invented.
FRS employs an iodine cell that is placed in front of the detector serving as a narrow bandwidth filter (figure 5). An iodine cell is a closed glass tube filled with iodine vapour. This vapour has strong absorption lines which interfere for example with the 514 nm line of an Ar+-laser or the 532 nm line of a frequency doubled YAG-Laser. If the laser frequency is positioned in the absorption maximum of an iodine line, the light scattered by particles, walls and windows is blocked almost entirely. Contrary to this, a part of the Rayleigh scattered light is passing the iodine cell (figure 6). This is due to the broadening of scattered light because of the thermal motion of the molecules and due to acoustic effects. The acoustic part of the scattering process is called Brillouin scattering. By using an Iodine cell and a narrow bandwidth laser, Rayleigh- Brillouin scattered light can be separated from other – in that case disturbing - light. To derive the density by FRS, a normalisation is needed. The most common procedure is to calibrate the system in a gas of known temperature, pressure and chemical composition.
State of the Art: The lie shape and line intensity of Rayleigh-Brillouin scattering is a function of density, pressure and temperature. The centre frequency of a Rayleigh-Brillouin line is shifted by the Doppler Effect, if the gas in the probe area has a certain flow velocity. Since Rayleigh-Brillouin scattering as well as the line shape of the iodine can be calculated numerically, it is possible to derive density, pressure, temperature and flow velocity from FRS-experiments. An example of such a combined measurement in the flow of a supersonic jet is shown in figure 7. For this measurement, the physical quantities of the flow are derived from a series of FSR images taken at different laser frequencies. This remarkable result shows the potential of the FSR for flow investigations. However, so far nearly all publications about FSR were restricted to academic applications. The researchers tried to take as much advantage of the features of FSR as possible, such as planar and instantaneous measurements of several physical quantities. The lack of intensity of the Rayleigh-Brillouin scattering limited these applications to simple laboratory flows. The basic idea of this project is to apply the technique to industrial combustors by concentrating only on the point wise and line wise determination of density fluctuations.
Potential fields of application of FRS: To detect and to quantify temperature distributions, entropy waves and fluctuations of gas temperature in combustion processes, so far mostly thermo couples were used. They provide a constant temperature signal which in certain cases might be Fourier transformed. Unfortunately, they have severe disadvantages, because some quite critical corrections are necessary to compensate influences of the heat transfer rate, the energy exchange by radiation and the thermal inertia of the thermo couple. Furthermore, the technique is intrusive and the upper temperature is limited by the melting point of the material. Mechanical problems arise at high velocity because of the very small diameters of the thermo couples wires.
Other potential temperature measuring techniques as CARS and LIF offer only limited data rates of below 10Hz, restricted by the repetition rate of the pulse laser. Furthermore, these techniques are technically very demanding and tend to fail at the high pressures typical for modern combustors (>20 bar).
This shows the need for a simple non-intrusive technique which is applicable to combustion chambers at high pressures and temperatures, with the ability to determine the power spectrum of temperature and density changes up to frequencies of a few kHz.
Proposed research program on FSR: The aim of the Helmholtz-University Young Investigators Group is to develop a FRS-system for measuring power spectra of density fluctuations in combustion chambers in the gas flow behind the primary zone of the combustor. Having left the lean primary zone, all combustion processes are thought to be finished, so the chemical composition of the gas stays unchanged.
Using the beam of a continuous wave single mode Ar+-laser, the Rayleigh-Brillouin scattering is detected by a Photo Multiplier placed behind an iodine cell (figure 8). The laser beam will be chopped by a pockels cell allowing lock-in detection of the scattered light signal to suppress all kind of AC-and DC-noise and to considerably improve the signal-to-noise ratio. Thus, a point measurement is performed allowing the detection of density fluctuations at high frequencies. To find entropy waves, which are generated by combustion oscillations, a second lock-in amplifier will be used. The pressure signals of a microphone or alternatively the OH-self luminescence, which are both indicators of the frequency and phase of a present combustion oscillation, will serve as reference signal.
In this set-up, the scattered light signal is filtered in three ways: through the iodine cell and two lock-in amplifiers. Additionally a laser line filter might be used as well to suppress distorting effects of the flame chemo-luminescence. This combination of filters should make the technique insensitive to disturbing bias effects and help to achieve a sufficient accuracy. Besides amplifying the amplitude, the lock-in amplifiers will also deliver the phase of the entropy waves, which is necessary to determine the propagation direction of the wave. The goal will be to achieve a measurement accuracy of better then 10 % in amplitude and 20° in phase resolution.
In a second step, high speed line cameras with a multi-channel plate will be used as a detector, to perform line measurements by shooting pictures of the laser beam. The lock-in amplification will be done numerically. This extension is primarily important to increase the data rate and therefore reduce the necessary measurement time on large test facilities with high operational costs. Lastly, a further increase in signal quality should be achieved by using a single mode high repetition rate pulsed YAG-laser with repetitions rate of 1 kHz or above instead of the Ar+-laser.
Combustion test will be performed on the new atmospheric 120 kW combustion test rig of the Hermann-Föttinger-Institute. If these tests are successful, next test objects will be the high pressure EDS-test rig at the DLR in Cologne and measurement campaigns in industry.
⇒ Intermediate internal report, if possible resulting in publication.
Year 3:
⇒ Evaluation of results by DLR-evaluation committee.
⇒ Publication of results, documentation preferably as a PhD-Thesis
Fibre Optic Microphones
Motivation: Acoustical investigations in combustion chambers are mostly done with microphones using electrical or electro-magnetical conversion of the sound pressure into voltage. For reasons of signal quality, electronic preamplifiers need to be placed in the vicinity of the microphone capsule, which incorporates the diaphragm and the capacitor or spool. Hence, these microphones are not heat resistant and can not be miniaturised: The microphone temperature is typically limited to 80 °C. The smallest diameters are typically ¼’’ or about 6 mm. Both facts prevent the direct application of conventional microphones in hot flows and in particular in combustion chambers.
An alternative method is to use probe microphones to spatially separate the sensitive microphones from the heat. These devices feature a long stainless steel tube that can be plugged in a small hole in the wall of the combustion chamber. A microphone is placed in a certain radial distance on a T-junction of the tube to measure the sound which propagates inside this waveguide. This set-up is often used for laboratory experiments. For industrial development experiments it is less convenient and common, mostly because the required access cannot be provided. This proves to be especially complex when sound waves in a full size gas turbine are monitored for example to set-up a control circuit.
Since no perfect way for measuring sound in combustion chambers exist, one goal of the Helmholtz Young Scientists Group will be to develop microphones on the basis of optical fibres. This concept was already tested by other researchers, but was not jet transferred to applications in combustors, even though it may help to overcome unsolved problems. Such a type of microphone has the potential to be more heat resistant at smaller mechanical dimensions that allow easy installation even at spots with limited access.
Fig. 9: Optical set-up of a vibrometer [copied from Polytec Cataloque] Physical principle of Fibre Optic Microphones: Basically, a Fibre Optic Microphone is the combination of a fibre vibrometer with a mirrored diaphragm. A draft set-up of a vibrometer is shown in figure 9. The light of a laser is split by a beam splitter into a reference beam and a main beam. The main beam is focused with a lens on the surface of an object. The light defused from this surface is collected by the same lens and mixed with the reference beam, so that interference occurs. The phase of the interference is proportional to the distance between the vibrometer and the object. If the object is moving in the direction of the vibrometer, a beat frequency occurs in the mixed signals proportional to the velocity of the object.
The resolution of commercially available vibrometers ranges down to velocities of 0.1 µm/s and displacements in the range of Nanometres and lower. Since optical fibres are used to carry the laser light to the focussing lens and also to take the reflected light back to the interferometer, the part of the Fibre Optic Microphone that is exposed to the sound wave and that needs to be introduced in the combustor consist of heat-resistant stainless steel, quartz glass and ceramics only. Therefore, Fibre Optic Microphones should be less affected by the heat than electrical ones. Since the sensor itself does not contain any electronic parts, it might also be miniaturised. Combining internal cooling and film cooling by air, the diaphragm should be sufficiently well protected to survive even in very hot environments.
Research program: Investigations are planned to start with commercially available diaphragms and a homodyne interferometer as well as a heterodyne interferometer. In first experiments, the movement of the diaphragms will be induced by an electrostatic actuator usually used to calibrate microphones. Alternatively a piezo translator will be employed. These tests aim to investigate and optimise the sensitivity of the technique. The goal should be to measure the actual position as well as the surface velocity of the diaphragm simultaneously. An interesting side aspect of this experiment could be to analyse the different modes of vibration of the diaphragm – which is important for the general comprehension of microphones.
A very important aspect will be the temperature influence on the elasticity of the membrane, which will first be tested in electrical ovens. For durability tests a so called “cooling channel” is available at the DLR-Institute of Propulsion Technology in Cologne, which allows exposing the Fibre Optic Microphones to combustion exhaust gases of temperatures of up to 2100 K and up to 35 atmospheres. The thermal capability of the microphones will certainly have to exceed 700 K, which is the typical outlet temperature of the compressor. It is most likely, that this will also be the temperature of the cooling air. The upper temperature limit is given by the temperature of the combustion chamber walls which goes up to approximately 1150 K. Therefore the goal is to reach a capability of 900 - 1000 K, assuming that the cooling of the microphone will be somehow more intensive than the wall cooling.
To collect the light that is reflected from the surface of the diaphragm, different set-ups will be tested. Different concepts are published in literature. The use of lenses will presumably increase the intensity of the collected light but will at the same time complicate the miniaturisation of the microphone. The miniaturisation of the microphone is planned to happen in co-operation with microphone producers. Their experience is important for being able to build diaphragms with diameters below ¼’’.
To process the interferometer signals, several concepts will be tested as for example LDA-data acquisition as well as direct digitalisation. The acquired data will be analysed with in-house developed state of the art data processing software for acoustics. For determining the transfer function of the microphones, a small acoustic duct wind tunnel for acoustic calibration purposes will be used. Heat transfer and cooling aspects can be investigated by optical flow visualisation techniques in a small wind tunnel. First tests in a combustion environment will be on the new 120 KW atmospheric combustor of the Hermann-Föttinger-Institute. If these tests are successful the next steps will be to use these microphones on the industrial high pressure test rigs of DLR in Cologne in co-operation with industrial partners.
With regards to a possible routine application of FOM, it will be important to have the possibility to acquire the signals of a larger number of microphones. This will be accomplished by an opto-mechanical multiplexer, that was developed at the Hermann-Föttinger-Institute some time ago and can be used for this purpose. This device will allow connecting a very large number of microphones to only one interferometer and analysing their signals sequentially.
Work plan:
Year 1 & 2: basic research period:
⇒ Publication of results, documentation preferably as a PhD-thesis.