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Laser-Induced Grating Spectroscopy (LIGS) is applied to premixed CH 4 /air laminar flat flames under operating pressures of 1 to 6 bar. For the first time, temperature and water concentration have been acquired simultaneously in a reacting flow environment using LIGS. A 1064 nm pulsed laser is used as pump to generate a temporary stationary intensity grating in the probe volume. Water molecules in the flame products absorb the laser energy and generate a thermal grating if sufficiently high energies are delivered by the laser pulses, here more than 100 mJ per pulse. Such energies allow the electric field to polarize the dielectric medium, resulting in a detectable electrostrictive grating as well. This creates LIGS signals containing both the electrostrictive and the thermal contributions. The local speed of sound is derived from the oscillation frequency of LIGS signals, which can be accurately measured from the single shot power spectrum. Data show that the ratio between the electrostrictive and the thermal peak intensities is an indicator of the local water concentration. The measured values of speed of sound, temperature, and water concentration in the flames examined compare favorably with flame simulations with Chemkin, showing an estimated accuracy of 0.5 to 2.5% and a precision of 1.4–2%. These results confirm the potential for 1064 nm LIGS-based thermometry for high-precision temperature measurements of combustion processes. [ABSTRACT FROM AUTHOR]

Abstract Time-resolved three-dimensional diagnostics are experiencing rapid development recently due to the pressing need for the understanding of highly turbulent flows/flames that are usually encountered in next-generation gas-turbine engines or ramjets/scramjets. However, the volumetric measurements typically requires multiple high-speed cameras which are extremely expensive and are only affordable by a few groups in the world. In this work, we aim to develop an endoscopic tomographic system which relies on customized fiber bundles to simultaneously relay nine projections onto a single camera. The system was validated by reconstructing the 3D chemiluminescence distributions of flames produced by a swirl-stabilized burner. The dominant coherent structures of the flames were successfully extracted using dynamic mode decomposition (DMD) which was also adopted to predict the flame evolutions. We found that DMD can faithfully predict a periodically changing flames but the prediction of a non-periodical one is more difficult due to the high nonlinearity of flame dynamics. Although this technique is demonstrated under the context of flame chemiluminescence tomography, it can be easily adapted for other tomographic modalities such as fluorescence tomography used in biomedical imaging to reduce the experimental costs. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract We present the first demonstration of heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for in situ, non-intrusive and quantitative CO 2 concentration measurements in flames. Dispersion spectroscopy retrieves gas properties by measuring the refractive index in the vicinity of a molecular resonance. The HPSDS scheme features a significant diagnostic advantage of the intrinsic immunity to laser power fluctuations caused by beam steering, thermal radiation and soot scattering in combustion environments, and thus no extra calibration process is required. In this work, we described the spectroscopic fundamentals for measuring heterodyne phase signals in flames. As a proof of principle, we used a mid-infrared interband cascade laser (ICL) near 4183 nm to exploit the strong CO 2 transitions in the R-branch of the v 3 fundamental band. The HPSDS signals of four CO 2 lines, R(76), R(78), R(80) and R(82), were measured in CH 4 /air flames to obtain CO 2 concentrations at different equivalence ratios (Φ = 0.8–1.2), yielding a good agreement with the simultaneous laser absorption measurements using the same ICL. With its immunity to laser power fluctuations verified experimentally, the HPSDS sensor was successfully implemented to measure CO 2 concentrations in C 2 H 4 /air sooting flames (Φ = 1.78–2.38). Laser dispersion spectroscopy proves to be a promising and alternative diagnostic tool for combustion measurements. [ABSTRACT FROM AUTHOR]

Abstract 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 CH 4 /O 2 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 NH 3. [ABSTRACT FROM AUTHOR]

Abstract An experimental study was conducted based on the recently available computed tomography of chemiluminescence (CTC) technique with a swirl-stabilized gas turbine model combustor to obtain both time-resolved and three-dimensional (3D) dynamics of the turbulent flame in confined situation. The CTC system employed in the present study was designed in a cost-effective, but highly-efficient manner, which allows simultaneous recordings of six projections of the target flame with three cameras at a repetition rate of 1 kHz. Instantaneous flame topologies were successfully captured and highly-transient flame dynamics in the combustion chamber, such as local extinction and flame shape variations were also observed. A further analysis based on the instantaneous and phase-averaged 3D movements of the flame centroids suggested that the flame in the present combustor exhibited a helical but relatively stochastic propagation and a distinct azimuthal rotation around the combustor, which is in the direction of rotation imposed by the swirler to the flow. Results obtained in this work demonstrate the capability of the CTC technique to resolve the detailed spatiotemporal dynamics of turbulent flames in confined combustion systems, which is expected to be particularly helpful for the study of combustion instabilities, such as periodic combustion oscillations in fuel-lean gas turbine engines. Highlights • A cost-effective, intensity-efficient, time-resolved CTC system was proposed. • Time-resolved CTC measurements were first applied to a confined turbulent flame. • 3D dynamics of a turbulent flame were characterized by experimental approach. [ABSTRACT FROM AUTHOR]

Abstract Quantitative X-ray computed tomography (XCT) diagnostics for reacting flows are developed and demonstrated in application to premixed flames in open and optically inaccessible geometries. A laboratory X-ray scanner is employed to investigate methane/air flames that were diluted with krypton as an inert radiodense tracer gas. Effects of acquisition rate and tracer gas concentration on the signal-to-noise ratio are examined. It is shown that statistically converged three-dimensional attenuation measurements can be obtained with limited impact from the tracer gas and within an acceptable acquisition time. Specific aspects of the tomographic reconstruction and the experimental procedure are examined, with particular emphasis on the quantification of experimental uncertainties. A method is developed to determine density and temperature from the X-ray attenuation measurements. These experiments are complemented by one- and multi-dimensional calculations to quantify the influence of krypton on the flame behavior. To demonstrate the merit of XCT for optically inaccessible flames, measurements of a complex flame geometry in a tubular confinement are performed. The use of a coflow to provide a uniform tracer-gas concentration is shown to improve the quantitative temperature evaluation. These measurements demonstrate the viability of XCT for flame-structure analysis and multi-dimensional temperature measurements using laboratory X-ray systems. Further opportunities for improving this diagnostic are discussed. [ABSTRACT FROM AUTHOR]

Porous media combustion is a promising combustion technology that enables operation over an extended range of conditions, with a variety of fuels and low pollutant emissions. However, the lack of understanding of the interstitial combustion processes remains a major obstacle towards the application of this technology and the development of predictive models. To understand these processes, this study presents an integrated experimental and computational effort to examine porous media combustion. Specifically, a new experimental diagnostic method is employed that simultaneously combines synchrotron X-ray micro-computed tomography (μ CT) and short-wavelength infrared thermometry. With this approach, the surface temperature and the 3D gas-phase temperature field are simultaneously measured inside a porous burner with micrometer spatial resolution. These measurements are then integrated into 3D pore-resolved simulations. This integrated approach enables a direct evaluation of current modeling capabilities, and a unique analysis of interstitial combustion in porous foams under realistic heating conditions. In light of these results, we discover that about 80% of the fuel consumption inside the matrix pores occurs far enough from the porous surface to be characteristic of a decoupled regime of preheated, but adiabatic flamelets. The remaining fuel consumption near the pore surface occurs in a non-adiabatic combustion mode. This insight and the proposed approach contribute to our fundamental understanding of porous media combustion, and suggest that multi-regime models could more accurately describe porous burners. [ABSTRACT FROM AUTHOR]

The increasing emphasis on low-carbon energy has heightened investigations into hydrogen combustion. This study focuses on the critical yet underexplored topic of thermoacoustic instabilities in micromix hydrogen combustors. We employ a newly designed micromix burner to explore the intricate relationships among acoustic modes, flame dynamics, and flow-vortex interactions. High-speed particle image velocimetry (PIV) and OH* chemiluminescence, supplemented by dynamic pressure measurements, are employed to capture dynamic behaviors and instability modes of the flame. Through thermoacoustic network analysis, we can identify two primary self-excited modes: a dominant low-frequency mode oscillating between 520 Hz and 565 Hz, and a secondary high-frequency mode surpassing 4400 Hz, originating from the hydrodynamic instability of the nozzle's jet flow. The low-frequency mode significantly influences the flame dynamics, generating a distinct longitudinal flame oscillation behavior. In particular, the inner premixed flame exhibits a periodic pattern of lifting and re-attachment phenomena. The outer diffusion flame, however, dynamically interacts with the outer shear layer (OSL) and the shed outer vortex rings (OVRs), which gives rise to notable deformations in the flame surface, manifesting as neck and swell structures propagating downstream with the OVRs. Furthermore, the dynamics of the OSL and OVRs are attributed to the fluctuations in the bulk flow velocity and local equivalence ratio at the nozzle exit, which are sustained by pressure oscillations induced by thermoacoustics. This presents important insights into the interplay between hydrodynamics and thermoacoustics in the formation of combustion instability in the micromix hydrogen combustor. [ABSTRACT FROM AUTHOR]

This study introduces flame image processing techniques to extract both the temperature and radiation parameters in the furnace. Additionally, a two-dimensional rectangular furnace system is established with emitting and reflecting walls and emitting and scattering spatial media. The radiation imaging model, developed through the distributions of ratios of energy scattered or reflected method, establishes a quantitative functional relationship between monochromatic radiation intensity images of the flame at two wavelengths and internal furnace temperature and radiation parameters. The Tikhonov regularization algorithm is used to reconstruct the radiation source terms within the furnace. An optimization algorithm is used to reconstruct the temperature and radiation parameters within the furnace, assuming uniform absorption and scattering coefficients. Despite the non-uniform distribution of internal radiation parameters, reconstructing the furnace temperature distribution using uniform radiation parameters remains feasible. The maximum relative error in temperature reconstruction is 2.28 %, which meets industrial temperature measurement requirements. Moreover, experimental studies are conducted on a coal-fired boiler to simultaneously detect both furnace cross-sectional temperature and radiation parameters. A single detector is used to obtain data sequentially from eight observation ports. During this process, flame images are captured under stable boiler operating conditions. These data are used to reconstruct the cross-sectional temperature distribution and radiation parameters in the burnout air zone of the boiler under different load conditions. Experimental results indicate that as the boiler load increases from 147 to 159 MW, the furnace temperature, absorption coefficient, and scattering coefficient all increase. Notably, the flame imaging processing method serves as a reliable method for monitoring the cross-sectional temperature field and radiation parameters in the large coal-fired boilers and is crucial for obtaining the data required for numerical simulations of combustion in large furnaces. [ABSTRACT FROM AUTHOR]

This paper proposes a new algorithm that can simultaneously measure the two-dimensional temperature distribution and absorption coefficient distribution of a pulverized-coal-fired (PCF) boiler furnace. On the base of optimal estimation of the uniform absorption and scattering coefficients as well as 2-D temperature distribution from the color radiation images of the flame by the Tikhonov regularization algorithm, the product of the absorption coefficient and the absolute blackbody emission intensity forms a new unknown distribution of emission source, which is also reconstructed from the color radiation images of the flame by the Tikhonov method. Then the distribution of absorption coefficient can be obtained from the new distribution of emission source as the 2-D temperature distribution has been known. Experimental investigation has been conducted on a 660 MWe four corner, tangentially firing, PCF boiler. In the flame detectors, a three-band pass filter is added in the CCD industrial camera to improve the monochromaticity of the original flame image. The temperature and absorption coefficient distributions of the cross-sections at three detection layers under a load of 330 MW and at the highest layer under a load of 500 MW are obtained. In order to verify the reliability of the algorithm, the flame radiation intensity images calculated from the 2-D distributions of temperature and absorption coefficient reconstructed at the highest layer under the load of 330 MW are taken as measurement data, the algorithm is repeated and the results show that the 2-D temperature distribution reconstructed is similar to the input one, with relative error less than 1% between their mean temperatures, and relative error less than 10% between the reconstructed and input mean values of absorption and scattering coefficients. It demonstrates that the algorithm and technology proposed in this paper have important application prospects in the monitoring, diagnosis and control of PCF combustion processes. [ABSTRACT FROM AUTHOR]

This work reports an innovative method for three-dimensional (3D) temperature measurements of turbulent flames using tomographic cesium atomic emission. For this effort, the cesium chloride is seeded into the combusting flow, generating bright cesium atomic emission at two transitions (62 s 1/2 → 62 p 3/2,1/2). Ten cameras with bandpass spectral filters (850±20 nm) capture signals from the 62 s 1/2 → 62 p 3/2 transition. Another ten cameras with filters (890±20 nm) capture signals from the 62 s 1/2 → 62 p 1/2 transition. Those images from ten perspectives captured simultaneously from each of the two transitions are used to reconstruct overlapping cesium fields with different Boltzmann fractions and map the 3D temperature field. The technique is validated both numerically using the simulated flame phantom and experimentally on a stable laminar flame. Last, we demonstrate the application of the method in 3D temperature measurements of a turbulent swirl flame. The proposed technique offers great opportunity for spectroscopic temperature measurement in turbulent flames without the requirement of laser excitation or image intensifiers. [ABSTRACT FROM AUTHOR]

The volumetric tomography reconstruction technique (VTRT) can calculate and restore the three-dimensional physical field based on the multi-view projection information of the flame and similar flow fields, which plays an important role in the research of volumetric targets. However, the reconstruction of three-dimensional scalar/vector fields representing the physical information of the flow field (temperature, density, chemical composition, etc.) requires a significant amount of computational resources and time, especially in the turbulent flame flow field, which requires high spatial and temporal resolutions. Based on the said background, in this work, we proposed a Fast Neural Fluid Reconstruction Technique (Fast-NFRT) based on deep learning. In this investigation, we first test the reconstruction accuracy, speed, and noise robustness of Fast-NFRT using numerically simulated flames. Then Fast-NFRT is used to reconstruct the experimental turbulent jet flame under two different conditions. Finally, with the camera settings preserved, the Fast-NFRT model was tested for transfer learning between the numerical simulation flame and the two experimental jet flames to examine the generalization performance of the reconstruction. It is found that the proposed Fast-NFRT model can achieve a temporal resolution of 50-500 fps with a similar reconstruction fidelity to traditional algebraic reconstruction methods, which demonstrates the capacity of the Fast-NFRT model and its potential in real-time reconstruction applications and dynamic analysis for complex flow dynamics. [ABSTRACT FROM AUTHOR]

1-methylnaphthalene (1-MN) is a widely used laser-induced fluorescence (LIF) tracer for planar imaging of mixture formation and temperature distributions in internal combustion (IC) engines. As the LIF measurement results can be biased by partial tracer oxidation, the conversion of 1-MN and the base fuel isooctane is analyzed in a calibration cell. First, measurements using supercontinuum laser absorption spectroscopy (SCLAS) are presented in order to quantify the conversion by detection of the produced H2O mole fraction. A single mode fiber (SMF) coupled setup is presented, with the fiber core acting as entrance slit of a Czerny-Turner spectrometer. Dependencies on residence time and global air-fuel ratio are presented at pressures up to 1.5 MPa and temperatures up to 900 K, at which distinct tracer and fuel consumption is observed. Signal loss due to intense beam steering was partially compensated using a self-stabilizing double-pass setup with a retroreflector.

Abstract Crank angle-resolved temperatures have been measured using laser induced grating spectroscopy (LIGS) in a motored reciprocating compression machine to simulate diesel engine operating conditions. A portable LIGS system based on a pulsed Nd:YAG laser, fundamental emission at 1064 nm and the fourth harmonic at 266 nm, was used with a c.w. diode-pumped solid state laser as probe at 660 nm. Laser induced thermal grating scattering (LITGS) using resonant absorption by 1-methylnaphthalene, as a substitute fuel, of the 266 nm pump-radiation was used for temperature measurements during non-combusting cycles. Laser induced electrostrictive grating scattering (LIEGS) using 1064 nm pump-radiation was used to measure temperatures in both combusting and non-combusting cycles with good agreement with the results of LITGS measurements which had a single-shot precision of ± 15 K and standard error of ± 1.5 K. The accuracy was estimated to be ± 3 K based on the uncertainty involved in the modified equation of state used in the derivation from the LIGS measurements of sound speed in the gas. Differences in the in-cylinder bulk gas temperature between combusting and non-combusting cycles were unambiguously resolved and temperatures of 2300 ± 100 K, typical of flames, were recorded in individual cycles. The results confirm the potential for LIGS-based thermometry for high-precision thermometry of combustion under compression-ignition conditions. [ABSTRACT FROM AUTHOR]

Abstract Combustion instabilities are often manifested in modern fuel-lean gas turbine combustors. Investigating the mechanisms and developing control strategies for combustion instabilities in such systems are of practical importance and give rise to interesting scientific issues as large-amplitude pressure and heat release perturbations can lead to catastrophic and irreversible consequences on costly gas turbine hardware. In recent years, tremendous efforts have been made to achieve a deeper understanding of the periodic combustion oscillations in gas turbine engines with both advanced numerical simulations and experimental diagnostics. In the latter case, state-of-the-art, non-intrusive diagnostic techniques have been well adopted to conduct fundamental studies on combustion instabilities in gas turbine model combustors. For example, simultaneous time-resolved measurements with planar laser-induced fluorescence (PLIF) for characterizing flame structure and particle image velocimetry (PIV) for imaging flow field significantly contribute to the understanding of the role of flame–flow–acoustics coupling in the events of combustion instabilities, and to the development and validation of advanced numerical models. However, planar measurements can be restrictive when flames are not axisymmetric or exhibit complex large-scale three-dimensional (3D) dynamics, which are commonly encountered in practical combustion system when combustion instabilities occur. Therefore, more recently, new volumetric imaging techniques for combustion diagnostics have attracted considerable research efforts. This paper categorizes different advanced non-intrusive combustion diagnostic techniques, including their basic principles and especially applications for the study of combustion instability. Some of the recent progresses in the diagnostic techniques, such as computed tomography of chemiluminescence (CTC), volumetric laser induced fluorescence (VLIF), rainbow-PIV, etc., are also discussed. These volumetric combustion diagnostic techniques offer the advantage to measure both spatial and temporal characteristics of the flame/flow of interest, which will enable deeper insights into the nature of unsteady combustion in the future. This brief review is intended to be useful for both researchers and engineers to design and conduct further fundamental experiments on combustion instabilities in gas turbine engines. [ABSTRACT FROM AUTHOR]

Femtosecond laser electronic excitation tagging (FLEET) is a molecular tagging velocimetry technique that can be applied in combustion flow fields, although detailed studies of its application in combustion are still needed. We report the applicability of FLEET in premixed CH4–air flames. We found that FLEET can be applied in all of the combustion areas (e.g., the unburned region, the burned region and the reaction zone). The FLEET signal in the unburned region is significantly higher than that in the burned region. This technique is suitable for both lean and rich CH4–air combustion flow fields and its performance in lean flames is better than that in rich flames. [ABSTRACT FROM AUTHOR]

Abstract This study focused on the synthesis of aluminium nitride (AlN) by combusting aluminium nanopowder in air. To investigate the combustion of aluminium nanopowder, a copper bromide laser monitor was used. The optical system equipped with brightness amplification allowed the elimination of the background lighting effect and enabled the high time-resolved recording of the process. In particular, the laser monitor enabled us to detect changes in the morphology and optical properties of the surface of the aluminium nanopowder sample as well as to observe the propagation of the combustion waves in spite of the intense background lighting during combustion. The main time parameters of the combustion of aluminium nanopowder in air were determined. To improve and facilitate the processing of laser monitored high-speed video recordings, we proposed to analyse the time dependence of the intensity of the output signal of the laser monitor. The dependence was used to successfully detect the occurrences of all combustion waves and describe their dynamics. The time dependence also favourably represented the evolution of the reflection coefficient of the combustion products of aluminium nanopowder. This is the first time that this property of aluminium nanopowder has been investigated. The reflection coefficient evolution coupled with video recordings of the sample surface development during the combustion of nanopowder could be used to control the combustion process. [ABSTRACT FROM AUTHOR]

Flame temperature, which is an important parameter in combustion diagnostics, provides a better understanding of flame radiative characteristics and can be deduced from the flame emission spectrum. However, in a combustion process with a large-scale flame that requires field measurement, considerable attenuation of the signature from the flame source occurs owing to the absorption of cold molecules in the atmosphere, which produces uncertainties while estimating the flame temperature. We analyze the effect of atmospheric absorption on the flame temperature estimation uncertainty with the optical emission spectroscopic method. We determine the flame temperature of IR (infrared) countermeasure flare combustion by fitting the reference spectrum model applying atmospheric absorption and without applying atmospheric absorption. Line-by-line modeling is conducted using a high-temperature molecular spectroscopic database. The instrument function of the spectrometer is applied, and the atmospheric transmission information is obtained by using the moderate-resolution atmospheric transmission database. In addition, we calculate the estimated flame temperature uncertainty without applying atmospheric absorption when the atmospheric path length, molecular concentration, and flame temperature are varied. Through the analysis, we suggest the wavelength range where the atmospheric absorption is negligible so that the uncertainty is minimized without applying it. Finally, we determine the flame temperature of the IR flare detected with an absolutely calibrated radiometric spectrometer by considering atmospheric absorption. All the fitting results show good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Midwave infrared (MWIR) measurements of a well-characterized JetCat turbine engine were made using a Telops infrared Fourier transform spectrometer (IFTS) hyperspectral imager (HSI), which utilizes a 320 #215; 256 indium-antimonide (InSb) focal plane array, calibrated to give a spectral distribution of radiance for each pixel within the image. Measurements were also made using a Santa Barbara focal plane IR imager (InSb, 320 #215; 256 array) that reported integrated radiance values in the MWIR. These measurements were made at varying engine RPM and different nozzle orientations to determine temperature and concentrations of the flow. The different orientations allow for the determination of the degree of asymmetry in the flow in two dimensions. The spectral information from these measurements was used to determine temperature and species concentrations of carbon dioxide and water vapor produced by the combustion of the Jet-A fuel. This was done utilizing the hyperspectral tomography (HT) process, which combines multiple measurement paths and high-resolution spectral data to determine the temperatures and concentrations of CO2 and H2O throughout the volume. Corresponding local temperature and pressure readings were taken in order to corroborate the radiometric data. Solution sets on a grid of 0.15-in. spacing based on measurements made at 2 cm-1 spectral resolution were generated for six exhaust planes throughout the exhaust flow of a 4 in: × 1 in: rectangular exhaust nozzle. The HT process generated core plume temperature values within ±35 K of measured thermocouple data for exhaust planes near the nozzle exit plane. These results show the successful application of the HT algorithms to IFTS HSI data for temperature and species concentration reconstruction. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) distributions of temperature and species concentration are crucial information to understand PAH inception and soot formation in diffusion flames. We report a tunable diode laser absorption spectroscopy (TDLAS) measurement to reconstruct the temperature and carbon dioxide concentration profiles in axisymmetric methane co-flow diffusion flame. The TDLAS sensor selected the v 3 bandhead region of CO 2 molecule at 4.2 um to ensure high temperature sensitivity and avoid ambient background absorption. A Fourier analysis Abel inversion transform algorithm was used to reconstruct 2D distribution of attenuation coefficient and showed higher inversion accuracy compared with other commonly used inversion methods. Evaluation of the reconstruction accuracy of the present Fourier analysis and traditional Onion Peeling algorithm was also performed using the simulated temperature and CO 2 concentration profiles from CFD calculation. The experimental reconstructed temperature and CO 2 volume fraction profiles were obtained and compared with thermocouple measurement and CFD simulation. The results show that the temperature from TDLAS measurement agrees well with CFD simulation. Flame temperature measured by thermocouple is lower compared with TDLAS and CFD because of the perturbation introduced by insertion of the thermocouple, especially at the edge of the flame. [ABSTRACT FROM AUTHOR]

Nonlinear tomographic absorption spectroscopy (NTAS) is an emerging gas sensing technique for reactive flows that has been proven to be capable of simultaneously imaging temperature and concentration of absorbing gas. However, the nonlinear tomographic problems are typically solved with an optimization algorithm such as simulated annealing which suffers from high computational cost. This problem becomes more severe when thousands of tomographic data needs to be processed for the temporal resolution of turbulent flames. To overcome this limitation, in this work we propose a reconstruction method based on convolutional neural networks (CNN) which can take full advantage of the large amount tomographic data to build an efficient neural networks to rapidly predict the reconstruction by feeding the sinograms to it. Simulative studies were performed to investigate how the parameters will affect the performance of neural networks. The results show that CNN can effectively reduce the computational cost and at the same time achieve a similar accuracy level as SA. The successful demonstration CNN in this work indicates possible applications of other sophisticated deep neural networks such as deep belief networks (DBN) and generative adversarial networks (GAN) to nonlinear tomography. © 2018 Elsevier Ltd. [ABSTRACT FROM AUTHOR]

A novel algorithm to fast deconvolve a molecular transition for its Gaussian and Lorentzian broadening was reported, and was further used to simultaneously measure multiple parameters, i.e. species concentration, temperature, and pressure, for combustion diagnostics. The proposed method was demonstrated by theory simulation and laboratory verification with combustion experiment. Direct absorption spectroscopy of H 2 O transition at 7185.6 cm −1 in premixed methane/air flames is used to verify the accuracy. The measurement temperature is agreement with that by two-line thermometry and thermocouple. The method is simple and feasible for non-intrusive simultaneous measurements of gas temperature, concentration, and pressure without calibration or other contact sensors. With an appropriately selected spectral line for analysis, the method can be applied to combustion diagnostics and industrial process control applications. [ABSTRACT FROM AUTHOR]

Lowering greenhouse gas emissions is one of the most challenging demands of today's society. Especially, the automotive industry struggles with the development of more efficient internal combustion (IC) engines. As an alternative to conventional fuels, methane has the potential for a significant emission reduction. In methane fuelled engines, the process of mixture formation, which determines the properties of combustion after ignition, differs significantly from gasoline and diesel engines and needs to be understood and controlled in order to develop engines with high efficiency. This work demonstrates the development of a gas sensing system that can serve as a diagnostic tool for measuring crank-angle resolved relative air–fuel ratios in methane-fuelled near-production IC engines. By application of non-dispersive infrared absorption spectroscopy at two distinct spectral regions in the ν 3 absorption band of methane around 3.3 µm, the system is able to determine fuel density and temperature simultaneously. A modified spark plug probe allows for straightforward application at engine test stations. Here, the application of the detection system in a rapid compression machine is presented, which enables validation and characterization of the system on well-defined gas mixtures under engine-like dynamic conditions. In extension to a recent proof-of-principle study, a refined data analysis procedure is introduced that allows the correction of artefacts originating from mechanical distortions of the sensor probe. In addition, the measured temperatures are compared to data obtained with a commercially available system based on the spectrally resolved detection of water absorption in the near infrared. [ABSTRACT FROM AUTHOR]

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For flame chemiluminescence tomography (FCT) based on lens imaging, the projection model is essential because it formulates the mathematical relation between the flame projections captured by cameras and the chemiluminescence field, and, through this relation, the field is reconstructed. This work proposed the blurry-spot (BS) model, which takes more universal assumptions and has higher accuracy than the widely applied line-of-sight model. By combining the geometrical camera model and the thin-lens equation, the BS model takes into account perspective effect of the camera lens; by combining ray-tracing technique and Monte Carlo simulation, it also considers inhomogeneous distribution of captured radiance on the image plane. Performance of these two models in FCT was numerically compared, and results showed that using the BS model could lead to better reconstruction quality in wider application ranges. [ABSTRACT FROM AUTHOR]

Two-color, planar laser-induced fluorescence (PLIF)-based two-dimensional (2D) thermometry techniques for reacting flows, which are typically developed in the laboratory conditions, face a stiff challenge in their practical implementation in harsh environments such as combustion rigs. In addition to limited optical access, the critical experimental conditions (i.e., uncontrolled humidity, vibration, and large thermal gradients) often restrict sensitive laser system operation and cause difficulties maintaining beam-overlap. Thus, an all fiber-coupled, two-color OH-PLIF system has been developed, employing two long optical fibers allowing isolation of the laser and signal-collection systems. Two OH-excitation laser beams (∼283 nm and ∼286 nm) are delivered through a common 6 m long, 400 µm core, deep ultraviolet (UV)-enhanced multimode fiber. The fluorescence signal (∼310 nm) is collected by a 3 m long, UV-grade imaging fiber. Proof-of-principle temperature measurements are demonstrated in atmospheric pressure, near adiabatic, CH4/O2/N2 jet flames. The effects of the excitation pulse interval on fiber transmission are investigated. The proof-of-principle measurements show significant promise for thermometry in harsh environments such as gas turbine engine tests. [ABSTRACT FROM AUTHOR]

A new algorithm for the evaluation of the integral line intensity for inferring the correct value for the temperature of a hot zone in the diagnostic of combustion by absorption spectroscopy with diode lasers is proposed. The algorithm is based not on the fitting of the baseline (BL) but on the expansion of the experimental and simulated spectra in a series of orthogonal polynomials, subtracting of the first three components of the expansion from both the experimental and simulated spectra, and fitting the spectra thus modified. The algorithm is tested in the numerical experiment by the simulation of the absorption spectra using a spectroscopic database, the addition of white noise, and the parabolic BL. Such constructed absorption spectra are treated as experimental in further calculations. The theoretical absorption spectra were simulated with the parameters (temperature, total pressure, concentration of water vapor) close to the parameters used for simulation of the experimental data. Then, spectra were expanded in the series of orthogonal polynomials and first components were subtracted from both spectra. The value of the correct integral line intensities and hence the correct temperature evaluation were obtained by fitting of the thus modified experimental and simulated spectra. The dependence of the mean and standard deviation of the evaluation of the integral line intensity on the linewidth and the number of subtracted components (first two or three) were examined. The proposed algorithm provides a correct estimation of temperature with standard deviation better than 60 K (for T = 1000 K) for the line half-width up to 0.6 cm−1. The proposed algorithm allows for obtaining the parameters of a hot zone without the fitting of usually unknown BL. [ABSTRACT FROM AUTHOR]

Tomographic absorption spectroscopy (TAS) has attracted increased research efforts recently due to the development in both hardware and new imaging concepts such as nonlinear tomography and compressed sensing. Nonlinear TAS is one of the emerging modality that bases on the concept of nonlinear tomography and has been successfully demonstrated both numerically and experimentally. However, all the previous demonstrations were realized using only two orthogonal projections simply for ease of implementation. In this work, we examine the performance of nonlinear TAS using other beam arrangements and test the effectiveness of the beam optimization technique that has been developed for linear TAS. In addition, so far only smoothness prior has been adopted and applied in nonlinear TAS. Nevertheless, there are also other useful priors such as sparseness and model-based prior which have not been investigated yet. This work aims to show how these priors can be implemented and included in the reconstruction process. Regularization through Bayesian formulation will be introduced specifically for this purpose, and a method for the determination of a proper regularization factor will be proposed. The comparative studies performed with different beam arrangements and regularization schemes on a few representative phantoms suggest that the beam optimization method developed for linear TAS also works for the nonlinear counterpart and the regularization scheme should be selected properly according to the available a priori information under specific application scenarios so as to achieve the best reconstruction fidelity. Though this work is conducted under the context of nonlinear TAS, it can also provide useful insights for other tomographic modalities. © 2017 Elsevier Ltd. [ABSTRACT FROM AUTHOR]

• Refraction and reflection are integrated into the calibration and imaging model for volumetric measurement. • Synthetic calibration is designed to show the significance of refraction. • Simulation and experiment are conducted to prove the superiority of the proposed method. Flame emission tomography is a powerful tool for spatially-resolved combustion diagnostics. Modalities for measuring both open and confined flames have been developed and found in extensive applications. However, the available methods for confined space are restricted to cylindrical enclosures. There is another common confinement, i.e., rectangular enclosure. The key characteristic of this confinement is two pairs of parallel transparent plates. The light will be refracted when passing through the plates, causing projection distortion. Another key point is the reflection of the plates, which will induce secondary projections. However, reflection is seldomly discussed in the literature. The existing methods for rectangular enclosure are all based on Zhang's model [Seventh IEEE International Conference on Computer Vision (ICCV) (IEEE, 1999), Vol. 661, pp. 666–673], which ignores the refraction and reflection of the enclosure. As a result, this work proposes a modified calibration model, i.e., refraction-reflection model for volumetric tomography, which for the first time incorporates both the refraction and reflection processes. The synthetic calibration shows refraction of the plate can not only shift the projection but also rotate it. The simulation and experiment show that the imaging model based on Zhang's model will not only wrongly estimate the position of the target, but also have a low accuracy. Nevertheless, the imaging model based on the refraction-reflection model can successfully resolve the above problems and show its superiority to the imaging model based on Zhang's model. [ABSTRACT FROM AUTHOR]

In the past decades, combustion chemistry research grew rapidly due to the development of combustion diagnostic methods, quantum chemistry methods, kinetic theory, and computational techniques. A lot of kinetic models have been developed for fuels from hydrogen to transportation fuel surrogates. Besides, multi-scale research method has been widely adopted to develop comprehensive models, which are expected to cover combustion conditions in real combustion devices. However, critical gaps still remain between the laboratory research and real engine application due to the insufficient research work on high pressure and low temperature combustion chemistry. Besides, there is also a great need of predictive pollutant formation model. Further development of combustion chemistry research depends on a closer interaction of combustion diagnostics, theoretical calculation and kinetic model development. This paper summarizes the recent progress in combustion chemistry research briefly and outlines the challenges and perspectives. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

X-ray Computed Tomography (XCT) measurements are performed to characterize the internal volumetric flame structure within a Porous Media Burner (PMB). X-ray attenuation measurements are obtained using a multi-zone silicon carbide PMB combusting a radiodense Kr/O 2 /N 2 /CH 4 mixture designed to enhance X-ray contrast. Time-averaged tomographic reconstructions of the X-ray attenuation field are used to assess internal PMB flame structure. Several key internal physical phenomena are observed, including heat recirculation within the combustion region, spatial inhomogeneity within the reaction zone, and preheating of gas in the upstream porous section. Further, the theory of XCT measurements applied to combustion systems is developed and implemented to arrive at 3D implied temperature field measurements at high spatial resolution. Implied temperature results are shown to exhibit quantitative agreement with those of a known 1D volume-averaged model for porous media combustion as well as with standard thermocouple measurements. These results shed light on internal thermophysical processes within PMBs while demonstrating the significant potential of XCT in obtaining quantitative, spatially resolved field data within optically inaccessible combustion environments. [ABSTRACT FROM AUTHOR]

Flame chemiluminescence tomography (FCT), which combines computerized tomography theory and multidirectional chemiluminescence emission measurements, can realize instantaneous three-dimensional (3-D) diagnostics for flames with high spatial and temporal resolutions. One critical step of FCT is to record the projections by multiple cameras from different view angles. For high accuracy reconstructions, it requires that extrinsic parameters (the positions and orientations) and intrinsic parameters (especially the image distances) of cameras be accurately calibrated first. Taking the focus effect of the camera into account, a modified camera calibration method was presented for FCT, and a 3-D calibration pattern was designed to solve the parameters. The precision of the method was evaluated by reprojections of feature points to cameras with the calibration results. The maximum root mean square error of the feature points' position is 1.42 pixels and 0.0064 mm for the image distance. An FCT system with 12 cameras was calibrated by the proposed method and the 3-D CH* intensity of a propane flame was measured. The results showed that the FCT system provides reasonable reconstruction accuracy using the camera's calibration results. [ABSTRACT FROM AUTHOR]

Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics. [ABSTRACT FROM AUTHOR]

We present a system for accurate tomographic reconstruction of the combustion temperature and H 2 O vapor concentration of a flame based on laser absorption measurements, in combination with an innovative two-step algebraic reconstruction technique. A total of 11 collimated laser beams generated from outputs of fiber-coupled diode lasers formed a two-dimensional 5 × 6 orthogonal beam grids and measured at two H 2 O absorption transitions (7154.354/7154.353 cm −1 and 7467.769 cm −1 ). The measurement system was designed on a rotation platform to achieve a two-folder improvement in spatial resolution. Numerical simulation showed that the proposed two-step algebraic reconstruction technique for temperature and concentration, respectively, greatly improved the reconstruction accuracy of species concentration when compared with a traditional calculation. Experimental results demonstrated the good performances of the measurement system and the two-step reconstruction technique for applications such as flame monitoring and combustion diagnosis. [ABSTRACT FROM AUTHOR]

Hydrogen atoms are key species in combustion of hydrogen/hydrocarbon fuels. Interference-free detection of hydrogen atoms natively generated in flames using femtosecond laser-induced fluorescence (LIF) was investigated employing two colors, i.e., 243 nm and 486 nm, as excitation source: two-photon excitation followed by a relay one-photon excitation. This strategy was compared with another commonly adopted two-photon LIF strategy using 205 nm for excitation. The potential interferences were investigated, and a direct verification method was proposed to prove this strategy be interference-free, and imaging of hydrogen atoms natively generated in methane/air flames was achieved. [ABSTRACT FROM AUTHOR]

• Eliminating the influence of carrier phase on measurement in the time domain. • An improved simulated annealing algorithm is proposed to obtain the optimal temperature and concentration. • It is the first time to obtain temperature and concentration simultaneously through a single spectral line in laser dispersion spectroscopy. • The anti-noise ability is verified by simulation, and the measurement accuracy and repeatability are verified by experiment. • It provides a new idea (time domain analysis) for the solution of laser dispersion spectroscopy. Quantitative measurement of temperature and concentration is of wide interest in combustion diagnostics. In this paper, simultaneous temperature and water concentration measurements in a combustion flame were realized by using a novel laser dispersion spectrum extraction method immune to carrier phase variation. The laser dispersion phase signal obtained in the frequency domain was converted into the molecular free induction decay (FID) signal in the time domain by inverse Fourier transform. Then the FID signals of the dispersion phase and the carrier phase were separated in the time domain. The optimal temperature and concentration values were efficiently searched by an improved simulated annealing algorithm through the FID signal in the window function. Numerical simulations verified the noise immunity of the proposed method at different noise levels. Experiments were implemented on a methane-air laminar premixed flame, and the results showed that the proposed method performed better in measurement stability and accuracy than laser absorption spectrometry, especially in the presence of incident laser power fluctuations. This method is proven promising in the field of combustion diagnosis. [ABSTRACT FROM AUTHOR]

Combustion diagnostic techniques based on laser-induced plasmas have recently demonstrated their capabilities in the simultaneous measurement of mixture fraction and temperature. Their successful applications rely on the interpretation of energy deposition behaviors during the plasma formation process. In this work, the energy deposition behaviors of laser-induced breakdown were analyzed quantitatively with high temporal resolution in methane-air mixture, heated air, and after-burn gas. Using a nanosecond laser with multiple longitudinal modes, a novel method was introduced to determine the breakdown delay with 0.1 ns resolution, by matching the random temporal features of incident and transmitted laser pulses shot-to-shot. The laser irradiance at the moment of breakdown and the pre-breakdown energy can then be precisely measured. The influence of gas temperature and laser energy on the breakdown delay and pre-breakdown energy was experimentally investigated, demonstrating that the controlling factor of breakdown initiation is energy threshold instead of irradiance threshold under our experimental conditions. The difference between pre-breakdown energy and breakdown threshold was observed and found to be increasing with gas temperature. Based on the theoretical analysis on electron growth and loss, the mechanisms behind such observations were analyzed and explained. The physical connections between monitored quantities in recently developed thermometry methods, i.e., breakdown threshold, breakdown delay, and deposited energy were clarified. The contributions from elongated breakdown delay in the pre-breakdown region and reduced absorption efficiency in the post-breakdown region to the total deposited energy were also discussed. Utilizing temporally resolved methods, this work constitutes a fundamental study of energy deposition behaviors, focusing on the breakdown initiation in typical combustion environments, which will further promote the successful implementation of combustion diagnostic approaches based on laser-induced plasmas. [ABSTRACT FROM AUTHOR]

Photofragmentation laser-induced fluorescence (PFLIF) is for the first time performed based on picosecond laser pulses for detection of hydroperoxyl radicals (HO 2 ) in a stoichiometric laminar methane/air flame. Photofragmentation is performed with a pump laser pulse of 80 ps duration and a wavelength of 266 nm, whereupon the produced OH photofragments are detected by a second picosecond probe laser pulse, inducing fluorescence via excitation in the A 2 Σ + ( v = 1) ← X 2 Π( v = 0) band of OH near 283 nm. Excitation spectra of the OH photofragments formed in the reaction zone were recorded for pump-probe delays ranging from 0 to 5 ns. The spectra suggest that the population distribution of the nascent OH fragments is rotationally cold and that it takes on the order of 5 ns for the nascent non-equilibrium rotational distribution to relax into a thermal distribution. The radial OH-fragment distribution was extracted from spectral images (radial position versus emission wavelength) recorded at six different pump-probe delays. Photochemical OH production was observed both in the reaction zone and the product zone. Comparison with a kinetic model for OH production suggests that more than 20% of the oxygen fragments produced by photolysis in the reaction zone are formed in the excited 1 D state, explaining a very rapid initial signal growth. The OH-production model was also compared with previous reaction-zone data, acquired with nanosecond laser pulses in the same flame, indicating that no O( 1 D), but only O( 3 P), is formed. A plausible explanation of the discrepancy between the two results is that the picosecond pulses, having more than two-orders of magnitude higher irradiance than the nanosecond pulses used in the previous study, might cause 2-photon photodissociation, allowing production of O( 1 D). In terms of flame diagnostics with PFLIF, it is concluded that a setup based on nanosecond laser pulses, rather than picosecond pulses, appears preferable since photochemical OH production in the reaction zone can be avoided while for short delay times the ratio between the photofragment signal and the photochemical interference in the product zone, stemming from CO 2 photolysis, is sufficiently large to clearly visualize the photofragments. [ABSTRACT FROM AUTHOR]

We report collision-independent detection of gas-phase nitric oxide (NO) via a two-pulse, femtosecond (fs) time-resolved parametric four-wave-mixing (PFWM) optical-probe scheme. This approach exploits the broadband two-photon excitation of rovibrational manifolds associated with resonant electronic molecular transitions, allowing species-specific detection. The observed time-domain fs-PFWM spectral signature associated with this broadband electronic two-photon excitation process is shown to exhibit negligible dependence on colliding-partner concentrations at short (<10 ps) time delays, even in the presence of species that strongly quench excited-state fluorescence. Furthermore, by employing a hybrid fs/ps PFWM scheme, the broadband two-photon absorption spectrum of the molecular species of interest is demonstrated to be resolved in the wavenumber domain without need to scan the probe-pulse delay. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Laser-induced breakdown spectroscopy (LIBS) is used for the mapping of local structures (i.e., reactants and products zones) and for the determination of fuel distribution by means of the local equivalence ratio φ in laminar, premixed air-hydrocarbon flames. The determination of laser threshold energy to induce breakdown in the different zones of flames is employed for the identification and demarcation of the local structures of a premixed laminar flame, while complementary results about fuel concentration were obtained from measurements of the cyanogen (CN) band B2Σ+-B²Σ+, (Δν) at 388.3 nm and the ratio of the atomic lines of hydrogen (Hα) and oxygen (O(I)), Hα/O. The combination of these LIBS-based methods provides a relatively simple to use, rapid, and accurate tool for online and in situ combustion diagnostics, providing valuable information about the fuel distribution and the spatial variations of the local structures of a flame. [ABSTRACT FROM AUTHOR]

Chirped probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS) thermometry was performed at 5 kHz in a hydrogen jet diffusion flame with an air co-flow. Measurements were performed at different heights and radial locations within the jet diffusion flame, up to 16 nozzle exit diameters downstream ( x/ d = 16). The near-nozzle measurements were characterized by large, organized, buoyancy-driven instabilities that become more chaotic at the downstream locations x/ d ≥ 4. The diffusion flame results highlight temperature fluctuations characteristic of the buoyancy-driven Kelvin-Helmholtz-type instability and provide new insights into the transient structure of these flames. At some measurement locations, the time-varying temperatures ranged from 300 K to nearly 2400 K. The CPP fs-CARS signal intensity is a factor of approximately 1000 times lower at 2400 K compared with 300 K. A dual-channel detection system was used to increase the dynamic range of the CARS measurements. The determination of temperature from the single shot spectra is discussed in detail. Laser and detection system parameters were determined from CPP fs-CARS spectra obtained from a near-adiabatic laminar calibration flame apparatus. The temperature precision of the system was determined from these calibration measurements and was found to be better than 2.0% at 2200 K. The influence of an instrument response function on spectral fitting parameters is systematically assessed. Published 2015. This article is a U.S. Government work and is in the public domain in the USA. [ABSTRACT FROM AUTHOR]