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Simultaneous detection of resonant and non-resonant femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (CARS) signals has been developed as a viable technique to provide in-situ referencing of the impulsive excitation efficiency for temperature assessments in flames. In the framework of CARS thermometry, the occurrence of both a resonant and a non-resonant contribution to the third-order susceptibility is well known. While the resonant part conceives the useful spectral information for deriving temperature and species concentrations in the probed volume, the non-resonant part is often disregarded. It nonetheless serves the CARS technique as an essential reference to map the finite bandwidth of the laser excitation fields and the transmission characteristics of the signal along the detection path. Hence, the standard protocols for CARS flame measurements include the time-averaged recording of the non-resonant signal, to be performed sequentially to the experiment. In the present work we present the successful single-shot recordings of both the resonant and non-resonant CARS signals, split on the same detector frame, realizing the in-situ referencing of the impulsive excitation efficiency. We demonstrate the use of this technique on one-dimensional CARS imaging spectra, acquired across the flame front of a laminar premixed methane/air flame. The effect of pulse dispersion on the laser excitation fields, while propagating in the participating medium, is proved to result, if not accounted for, in an ~1.3% systematic bias of the CARS-evaluated temperature in the oxidation region of the flame.

Nonlinear tomography can be combined with line-of-sight measurement techniques and take the full advantage of the nonlinear relationship between the target fields to be reconstructed and the measured projections. For example, it has been integrated with absorption spectroscopy for simultaneous imaging of temperature and species concentration, and the technique is referred to as nonlinear tomographic absorption spectroscopy (NTAS) which is especially suitable to applications where the optical access is extremely limited. However, the major drawback of nonlinear tomography is its high computational costs for the inversion process, which involves the solution of a large-scale nonlinear equation system. The situation becomes more severe when thousands of tomographic frames need to be processed. This limitation can be potentially overcome by applying a deep learning algorithm, which can build efficient mapping between the projections and the target fields for rapid reconstruction. Nevertheless, only preliminary study of convolutional neural networks was reported so far, and there are also other well-established methods which have not been investigated yet. In this work, we cite NTAS as an example and demonstrate the reconstruction of temperature field using deep brief network (DBN) and recurrent neural network (RNN) for the first time. This work also aims to provide systematic comparative studies between these representative algorithms. As expected, all the deep learning algorithms are very efficient in solving NTAS problems, and a typical reconstruction time is on the order of milliseconds. In addition, the results suggest that, in NTAS problem, RNN is superior to the other methods in terms of both accuracy and noise immunity, and it is more favorable for practical applications.

We report on an experimental demonstration of spatially-resolved detection of atomic hydrogen in flames using a single-ended configuration yielding 656-nm lasing in the backward direction upon 2-photon pumping with 205-nm femtosecond laser pulses. Spatial resolution is achieved by temporally-resolved detection of the backward lasing using a streak camera. The method is demonstrated in CH4/O2 flames; both in a setup consisting of two flames, with variable spacing between the flames, and in a single flame. Results from the two-flame experiment show that the backward lasing technique is able to determine changes in the separation between the flames as this distance was altered. By maximizing the temporal resolution of the streak camera, obtaining a highest spatial resolution of 1.65 mm, it is possible to resolve the hydrogen signal present in the two reaction zones in the single flame, where the separation between the reaction zones is ∼2 mm. The lasing signal is strong enough to allow single-shot measurements. Results obtained by backward lasing are compared with 2-photon planar laser-induced fluorescence (LIF) images recorded with detection perpendicular to the laser beam path and the results from the two methods qualitatively agree. Although further studies are needed in order to extract quantitative hydrogen concentrations, the present results indicate great potential for spatially resolved single-ended measurements, which would constitute a very valuable asset for combustion diagnostics in intractable geometries with limited optical access. It appears feasible to extend the technique to detection of any species for which resonant two-photon-excited lasing effect has been observed, such as O, N, C, CO and NH3.

The article of record as published may be found at http://dx.doi.org/10.1016/j.proci.2018.05.021 A novel single-ended mid-infrared laser-absorption sensor for time-resolved measurements of water mole fraction and temperature was developed and deployed within the annulus of a hydrogen/air-fed rotating detonation engine (RDE). The sensor transmitted two laser beams targeting mid-infrared water transitions through a single optical port on the outer wall of the cylindrical RDE annulus and measured the backscattered radiation from the RDE inner surface using a photodetector for a round-trip path of 1.52 cm. Optimizing the sensor's optical arrangement using numerical ray tracing to minimize interference from optical emission, beam steering, and scattered laser light from window surfaces was essential to sensor performance. Scanned-wavelength-modulation spectroscopy with second-harmonic detection and first-harmonic normalization was implemented to allow for frequency-domain multiplexing of the two lasers and to suppress non-absorbing interference sources such as beam-steering and emission. Tunable diode lasers near 2551 and 2482 nm were modulated at 100 and 122 kHz, respectively, and sinusoidally scanned across the peaks of their respective water transitions at 10 kHz to provide a measurement rate of 20 kHz and detection limit of 0.5% water by mole. Experimentally derived spectroscopic parameters enabled water and temperature sensing with respective uncertainties of 7.3% and 5.3% relative to the measured values. Time-resolved and time-averaged sensor measurements of gas temperature and water vapor mole fraction allow quantitative evaluation of the combustion progress at the measurement location and thus provide a design tool for RDE optimization. Broadly, this single-ended laser sensor should find applications in other combustion systems where optical access is limited. Office of Naval Research Innovative Scientific Solutions Incorporated (ISSI) National Defense Science and Engineering Graduate (NDSEG) Fellowship Grant No. N00014-15-P-1121 DoD, Air Force Office of Scientific Research and Army Research Office

Laser ignition was applied on a cryogenic rocket combustor. The induced plasma breakdown was analyzed using spectroscopic (LIBS) and other optical methods. Hence, plasma temperature, localized equivalence ratio and the exact ignition time were determined.

Based on tunable diode laser absorption spectroscopy (TDLAS), two-dimensional (2D) distribution reconstructions of gas concentration and temperature are realized using an algebraic reconstruction technique (ART). The influence of the beam distribution and grid size on combustion field reconstruction is investigated to attain optimal reconstruction results with a limited number of beams. Under limited optical-path numbers, it shows that a better spatial resolution is attainable only when the laser beam paths are vertical and parallel to the symmetry axis of the combustion field. Furthermore, experiments with 16 beam paths using one and two flat flame combustion fields are carried out in different fuel-air equivalence ratios under room temperature. The results are in agreement with the simulation results, and the time resolution is less than 1 s.

In multiangle elastic light scattering (MAELS) experiments, the morphology of aerosolized particles is inferred by shining collimated radiation through the aerosol and then measuring the scattered light intensity over a set of angles. In the case of soot-laden-aerosols MAELS can, in principle, be used to recover the size distribution of soot aggregates, although this involves solving an ill-posed inverse problem. This paper presents a design-of-experiment methodology for identifying the set of angles that maximizes the information content of the angular scattering measurements, thereby minimizing the ill-posedness of the underlying inverse problem. While the optimized angles highlight the physical significance of the scattering regimes, they do not improve the accuracy of size distributions reconstructed from simulated experimental data.

International audience; Planar laser-induced fluorescence on atomic iron is investigated in this paper, and a measurement strategy is proposed to monitor the fluorescence of iron atoms with good sensitivity. A model is proposed to fit the experimental fluorescence spectra, and good agreement is found between simulated and experimental spectra. Emission and laser-induced fluorescence measurements are performed in the flames of ammonium perchlorate composite propellants containing iron-based catalysts. A fluorescence signal from iron atoms after excitation at 248 nm is observed for the first time in propellant flames. Images of the spatial distribution of iron atoms are recorded in the flame in which turbulent structures are generated. Iron fluorescence is detected up to 1.0 MPa, which opens the way to application in propellant combustion.; L'imagerie de fluorescence induite par laser est appliquée aux atomes de fer et une stratégie pour leurs mesurer avec une bonne sensibilité est présentée dans cet article. Un modèle est développé pour calculer les spectres de fluorescence et un bon accord est trouvé entre les spectres calculés et expérimentaux. Les mesures en émission et en fluorescence induite par laser sont effectuées dans des flammes de propergols composites à base de perchlorate d'ammonium dopés avec des catalyseurs ferriques.Le signal de fluorescence induit par l'excitation laser à 248 nm des atomes de fer est observé pour la première fois dans les flammes de propergol. La répartition spatiale des atomes de fer est visualisée en flamme dont les structures turbulentes sont forcées. La fluorescence du fer est détectée jusqu'à 1,0 MPa, ce qui ouvre des perspectives de visualisation d'écoulements de gaz de combustion de propergols solides.

Particle size is a crucial parameter in nanopowder use and production. A method to obtain information about this during the particle synthesis would be very valuable in order to optimize the process. In the field of nanoparticles flame synthesis, an extension of the techniques used in soot diagnostics sounds rather obvious, but it is far from being easily accomplished. In this paper investigations on the application of the laser induced incandescence to TiO 2 nanoparticles in a flame reactor are reported. The work basically concerns the effects of laser fluence on the spectral structure of the laser-excited radiation emitted from the particles. It will be shown that in many cases such a radiation cannot be plainly assumed as an incandescence signal. Measurements of the signal time decay are reported and interpreted in the light of the spectral indications. Finally, the dependence of such a time decay on the particle size is experimentally demonstrated.

The development process for gas turbine combustion systems includes single-burner high-pressure combustion tests as an important validation step. In these tests the performance of a combustor is investigated at realistic gas turbine conditions. Measurement techniques that are typically used in these tests include mass flow meters, thermocouples, pressure transducers, and probes for exhaust-gas composition measurements. These measurement techniques, however, do not provide direct information of the flame behavior. Chemiluminescence measurements have proven to being a valuable and robust technique to close this gap [1,2]. This paper summarizes the results of chemiluminescence measurements performed at Siemens full-scale high-pressure single-burner combustion test rigs at the German Aerospace Center (DLR) in Cologne, Germany. To minimize the impact of the measurement system on the experiment, the optical access to the test rigs was provided by a water-cooled endoscopic probe. The probe was located in a side-wall downstream of the burner, viewing upstream towards the burner outlet. The probe was successfully operated up to full engine pressure and flame temperatures of approximately 1900 K. For the detection of the chemiluminescence signal different approaches were applied: • Spectral analyses of the chemiluminescence signal were done by using an USB spectrometer. • For flame imaging up to two intensified CCD cameras were applied. In front of the cameras various combinations of optical filters were installed to selectively record the respective chemiluminescent species (OH*, CH*, CO2*). • For studies with special focus on combustion dynamics an intensified high-speed CMOS camera was used. High-repetition-rate measurements were used for identifying the shapes of flame modes. • Acoustic pressure oscillations inside the combustion chamber were recorded by pressure transducers simultaneously to the camera images. This allows the pressure oscillations to be correlated with flame fluctuations during post-processing [3,4]. Generally, the robustness of endoscopic chemiluminescence measurements was successfully demonstrated in numerous tests at realistic gas turbine conditions. The applied imaging setups provided new information about the connection between the flame position and NOx emissions as well as the correlation of flame fluctuations and pressure oscillations. Hence, they have become a valuable experimental tool to improve the evaluation and understanding of the combustor performance. Future work will focus on further improvement of quantitative evaluations by compensation of line-of-sight image integration, reabsorption of OH* by OH, and beam steering.

This chapter provides an overview on the application of particle image velocimetry (PIV) and Doppler global velocimetry (DGV) in combustion test facilities that are operated at pressures of up to 10 bar. Emphasis is placed on the experimental aspects of each application rather than the interpretation of the acquired flow-field data because many of the encountered problems and chosen solution strategies are unique to this area of velocimetry application. In particular, imaging configurations, seeding techniques, data-acquisition strategies as well as pre- and postprocessing methodologies are outlined.

An optimal system for temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) in turbulent flames and flows is presented. In addition to a single-mode pump laser and a modeless dye laser, an echelle spectrometer with a cross disperser is used. This system permits simultaneous measurement of the N2 CARS spectrum and the broadband dye laser profile. A procedure is developed to use software to transform this profile into the excitation profile by which the spectrum is referenced. Simultaneous shot-to-shot referencing is compared to sequential averaged referencing for data obtained in flat flames and in room air. At flame temperatures, the resultant 1.5% imprecision is limited by flame fluctuations, indicating that the system may have a single-shot imprecision below 1%. At room temperature, the 3.8% single-shot imprecision is of the same order as the best values reported for dual-broadband pure-rotational CARS. Using the unique shot-to-shot excitation profiles, simultaneous referencing eliminates systematic errors. At 2000 and 300 K, the 95% confidence intervals are estimated to be +/- 20 and +/- 10 K, respectively.