Answers to questions on exam in laser-based combustion diagnostics on March 10, 2006
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1 Answers to questions on exam in laser-based combustion diagnostics on March 10, Examples of advantages and disadvantages with laser-based combustion diagnostic techniques: + Nonintrusive + High spatial resolution + High temporal resolution + Species-specific + In-situ measurements + Remote measurements + No upper temperature limit + Non-equilibrium can be probed - Optical access is needed - Complex experiments - High-level operator skill often needed - Interpretation of results may be advanced - Limited to small molecules - Often expensive 2. a) Spontaneous Raman scattering (Rotational CARS is also an alternative) b) Laser-induced fluorescence (LIF) c) CARS, preferably rotational CARS 3. Some issues that could be discussed in the answer: Combustion is often turbulent, and averaging over large time scales will give information of low quality. It is therefore important to probe the turbulent process with high temporal resolution. Thus, laser pulses of short duration are necessary. A pulse duration of 10 ns is much shorter than the typical time scale for a turbulent motion in flames. The property of short pulse duration described above can be provided by many commercially available pulsed lasers. The Nd:YAG laser cluster, which can deliver eight such short laser pulses with arbitrary time separation, in addition offers the possibility to follow the turbulent process. For example this system allows cycleresolved measurements in combustion engines.
2 4. a) Energy level diagram with all transitions/processes indicated: W 2i 2 P b 12 b 21 A 21 Q 21 1 where the designations have the following meanings: b 12 : absorption rate constant b 21 : stimulated emission rate constant A 21 : spontaneous emission rate constant Q 21 : collisional quenching rate constant W 2i : photoionization rate constant P: predissociation rate constant b) Quenching, which is indicated by Q 21 in the figure above, denotes non-radiative relaxation of the molecule back to the ground state. Upon absorption of a photon the molecule gets excited, i.e. a transition from the ground state (1) to a higher energy state (2) occur. The deexcitation, i.e. the transition from the higher energy state back to the ground state, may occur through spontaneous emission of a photon (A 21 ), i.e. emission of fluorescence, but it may also happen that the molecule loses its excess energy to neighboring molecules by collisions. Quenching thus corresponds to non-radiative transitions back to the ground state (Q 21 ). The LIF signal strength, from which species concentration is extracted, is dependent on the quenching, and thus knowledge of the quenching is needed for quantitative results. However, in most cases the quenching is not well known and it also depends on the conditions present in the measurement. For example the quenching is different for different species, pressures, temperatures, and compositions. Thus, the main difficulty associated to quenching is its variation with measurement conditions. In addition, quenching limits the LIF signal strength (I LIF A 21 /(A 21 +Q 21 )), i.e. it decreases the sensitivity of the measurement.
3 c) The LIF signal power as a function of laser irradiance may typically look like this: 1.0 Saturated regime (I ν >> I sat ): Fluorescence power Linear regime (I ν << I sat ): Laser irradiance (arb. unit) For low laser irradiances the fluorescence power depends linearly on the laser irradiance. This regime is hence termed the linear regime. As the laser irradiance is increased the fluorescence power starts to deviate from linearity because the absorption of molecules starts to get partially saturated, i.e. a significant fraction of the population in the ground state is transferred to the excited state and stimulated emission also starts to contribute. If the laser irradiance is increased even further even more molecules are transferred to the excited state and finally the absorption is fully saturated (i.e. the population in the excited state cannot get any larger because we have reached the limit where the absorption rate and stimulated emission rate are equal) and a plateau is reached, as shown in the diagram above. This regime is thus called the saturated regime. Saturated LIF is advantageous because the LIF signal power is independent on both the quenching (Q 21 ) and the laser irradiance in the saturated regime. Thus the quenching rate does not need to be known and we do not need to measure the laser irradiance. Saturation also maximizes the LIF signal power and thus leads to maximum species detectivity. Disadvantages with saturated LIF are that it is difficult to obtain complete saturation over the entire spatial and temporal profile of the laser pulse. The sample may be completely saturated in the center of the laser beam but not in the wings of a Gaussian laser profile. Also, high enough laser irradiance may be difficult to obtain at certain wavelengths and energy transfer processes during the time of the laser pulse may need to be modeled for quantitative results.
4 5. a) Vibrational CARS Rotational CARS ω R ω CARS v + 1 ω R1 ω R2 ω CARS v J + 2 J b) Vibrational CARS and rotational CARS have roughly the same experimental complexity. Both methods are very accurate for temperature measurements. Rotational CARS is better at low and moderate temperatures ( K) whereas vibrational CARS is better at high temperatures (T > 1200 K). At elevated pressures rotational CARS is a better alternative than vibrational CARS due to the collisional narrowing effect present in vibrational CARS spectra at high pressures. Rotatational CARS can be used for simultaneous measurement of multiple species concentrations (major species) whereas vibrational CARS normally is restricted to single species concentration measurement. Dual-broadband rotational CARS has a higher precision for single-shot temperature measurement than vibrational CARS. Problems with interfering stray light at 532 nm is common for rotational CARS measurements in practical combustion devices, e.g. combustion engines. Stray light interference is usually not a problem in vibrational CARS since the CARS signal is here located spectrally well separated from the wavelengths of all primary laser beams. c) In order for a rotational CARS signal to be generated the molecule must be rotational Raman active, i.e. the polarizability of the molecule must change upon molecular rotation. For a completely symmetric molecule like methane (CH 4 ) the polarizability of the molecule does not change as the molecule rotates, and it is thus not rotational Raman active. In other words; the molecule s charge distribution seen by the electromagnetic wave incident onto the molecule, i.e. the laser radiation, looks the same regardless of the molecular rotation. In order for a vibrational CARS signal to be generated the molecule must be vibrational Raman active, i.e. the polarizability of the molecule must change upon molecular vibration. The C-H stretching vibrations in methane are strongly vibrational Raman active and hence results in strong vibrational CARS signals.
5 6. Some major issues that could be discussed in the answer: Take a look at the figure. If the peak at 309 nm is due to Raman scattering one would expect a much stronger peak at 283 nm due to Rayleigh scattering. Since this is not the case the signal cannot be due to Raman scattering. Tune the wavelength slightly. If it is a Raman signal the peak will move. If it is a LIF signal it will disappear. Using the tunable pico-second laser and a fast PMT, study the signal time-resolved. A Raman signal is instantaneous while a LIF signal has a life time associated to it (typically in the nanosecond range). So if it is a LIF signal you will see a fast rise time followed by an exponential decay curve on the oscilloscope screen. Study the polarization of the signal. A Raman signal is often polarized while a LIF signal is not polarized. Vary the laser intensity. A Raman signal is linearly dependent on the laser intensity while a LIF signal will saturate at high laser intensities. Vary the pressure. A Raman signal is linearly dependent on the pressure while a LIF signal usually suffers from more quenching with increasing pressure. Measure the total signal strength. Maybe the signal is too high to be a Raman signal. In addition it can be mentioned that one can also measure absolute wavelengths and try to find corresponding transitions between states in the molecules. One may improve the spectral resolution and try to identify unresolved features etc. OH has many absorption lines in the vicinity of 283 nm and the corresponding fluorescence appears around 309 nm, i.e. the LIF signal is from OH. 7. a) The LII signal increases strongly with increasing laser fluence for fluences up to 0.15 J/cm 2 since the soot particles are getting hotter with increasing laser fluence and hence the Planck radiation emitted (the incandescence) increases with increasing laser fluence. The strong rise of the signal is because the total LII-signal increases as a function of T 4 and in addition there is a shift in the emission to shorter wavelengths according to the Wien displacement law. The LII signal then reaches a plateau at around 0.2 J/cm 2, at which the soot particles reaches a maximum temperature of slightly more than 4000 K. The explanation for this plateau is that for an increase in laser fluence in this range, the loss of LII signal from the center of the laser beam due to vaporization of soot is compensated by an increase in LII signal from the edges of the beam. b) Soot at flame temperatures radiate strongly and gives rise to natural background luminosity, also described by Planck s law and the emissivity expression. This background luminosity thus interferes with the LII signal to be measured. The blackbody radiation at 2000 K decreases strongly with decreasing wavelength from the visible to the UV regime. Thus in terms of efficient suppression of interfering background luminosity it is beneficial to use a short detection wavelength, i.e. below 450 nm.
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