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ABSTRACT This work reports the S‐branch Raman linewidths of N2 broadened by H2O vapour at temperatures 870–1900 K determined using the picosecond time‐resolved pure rotational coherent anti‐Stokes Raman scattering (RCARS) approach. The coherent dephasing time constant of nitrogen in a binary mixture was determined from the time trace fitting of the RCARS signal. Raman linewidths of N2 for four different binary mixture compositions up to 20% H2O were obtained for each temperature and the N2‐H2O broadening coefficient was determined. The decay of nitrogen broadened by water vapour is faster than that of self‐broadened nitrogen for all temperatures leading to a higher Raman linewidth. Respectively, the S‐branch Raman linewidths of N2‐H2O deviate significantly in magnitude and slope from the Q‐branch Raman linewidth calculated by the modified energy gap (MEG) law. Therefore, taking into account the broadening effect of water molecules in nitrogen has the potential to significantly improve thermometry and species concentration determination. The determined S‐branch Raman linewidths and broadening coefficient will therefore find application in combustion and reactive flow diagnostics. [ABSTRACT FROM AUTHOR]

Linear tomographic absorption spectroscopy (LTAS) is a non-destructive diagnostic technique widely employed for gas sensing. The inverse problem of LTAS represents a classic example of an ill-posed problem. Linear iterative algorithms are commonly employed to address such problems, yielding generally poor reconstruction results due to the incapability to incorporate suitable prior conditions within the reconstruction process. Data-driven deep neural networks (DNN) have shown the potential to yield superior reconstruction results; however, they demand a substantial amount of measurement data that is challenging to acquire. To surmount this limitation, we proposed an untrained neural network (UNN) to tackle the inverse problem of LTAS. In conjunction with an early-stopping method based on running variance, UNN achieves improved reconstruction accuracy without supplementary training data. Numerical studies are conducted to explore the optimal network architecture of UNN and to assess the reliability of the early-stopping method. A comparison between UNN and superiorized ART (SUP-ART) substantiates the exceptional performance of UNN. [ABSTRACT FROM AUTHOR]

Velocity field quantification for high-speed flows is of fundamental importance to understand flow dynamics, turbulence, and flow–structure interactions. Optical velocimetry techniques commonly provide sparse information in the flows. Dense fields of velocity vectors with high spatial resolutions are indispensable for detailed analysis of complex motion patterns and accurate motion tracking within the field of view. In the present work, two-dimensional (2D) Rayleigh scattering imaging (RSI) at a rate of 10- to 100-kHz was utilized to quantify the high-speed flow velocity by employing deep learning-based optical flow analysis, along with density and temperature fields from Rayleigh scattering intensity profiles. High-speed Rayleigh scattering images are highly spatially resolved, have smooth gradients without intensity discontinuities, and precisely track key features of the flows. The deep learning-based optical flow method utilizes recurrent neural network architecture to extract the per-pixel features of both input images, calculate correlation from all pairs of the features, and get training by recurrently updating the optical flow. 2D instantaneous velocity fields of both nonreacting and reacting flows measured by RSI were obtained from deep learning-based optical flow analysis, thus extending RSI as a non-intrusive, nonseeded, and multiscalar measurement technique of high-speed nonreacting and reacting flows. [ABSTRACT FROM AUTHOR]

Snapshot multispectral imaging of chemical species in the flame is essential for improved understanding of the combustion process. In this article, we investigate the different configurations of a structured laser sheet-based multispectral imaging approach called the Frequency Recognition Algorithm for Multiple Exposures (FRAME). Using FRAME, a snapshot of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) excited by 283.5 nm laser and Laser-Induced Incandescence (LII) of soot particles excited by 532 nm laser are acquired simultaneously on a single FRAME image. A laminar diffusion flame of acetylene produced by a Gülder burner is used for the experiments. The standard FRAME approach is based on creating two spatially modulated laser sheets and arranging them in a cross-patterned configuration (X). However, the effect of using different configurations (angles) of the two laser sheets on the multispectral planar imaging of the flame has not yet been studied. Therefore, we have compared the FRAME approach in four different configurations while keeping the same flame conditions. First, we have compared the relation between laser fluence and LII signals with and without spatial modulation of the 532 nm laser sheet and found that both detections follow the same curve. When comparing the maps of flame species reconstructed from the standard FRAME configuration and other configurations, there are some dissimilarities. These differences are attributed to minor changes in the imaging plane, optical alignment, laser path length, different modulation frequencies of the laser sheet, laser extinction, laser fluence, etc. [ABSTRACT FROM AUTHOR]

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Laser absorption spectroscopy tomography is an effective combustion diagnostic method for obtaining simultaneous two-dimensional distribution measurements of temperature and gas molar concentrations. For the reconstruction process of complex combustion flames, a new algorithm named 'multi-iterative adaptive optimization regularization' (MIARO) is proposed. This algorithm is a further development of another algorithm known as the 'modified adaptive algebraic reconstruction technique' (MAART) with the improvement of the initial value and adaptive regularization parameter selections. In MIARO, the problem of the MAART's initial value sensitivity is compensated for, and in addition, reconstruction parameters are also introduced into the regularization so that both the quality of reconstruction and the convergence of regularization are guaranteed. In butane burner experiments, an average relative error of 1.82% was achieved with MIARO, compared to 2.44% with MAART, which is a significant reduction of 25.1%. The simulation and experimental results clearly demonstrate that the MIARO algorithm can be used to reconstruct dynamic combustion fields and eliminate boundary artifacts with improved measurement accuracy and robustness. [ABSTRACT FROM AUTHOR]

Combustion diagnosis is crucial for quantifying exhaust gas and calculating combustion efficiency. Temperature, concentration, and velocity measurements allow for the accurate identification of combustion conditions. However, the current diagnostic equipment used to assess the combustion state has various issues, such as flow disturbance caused by probes and measurement delays caused by sampling. Furthermore, using multiple measurement devices can be particularly inconvenient. This study confirmed the potential application of an exhaust gas emission and engine operation monitoring system by simultaneously measuring temperature, concentration, and velocity using tunable diode laser absorption spectroscopy (TDLAS) technology. To determine the combustion chamber of methane combustion, multiple laser beams that are separated using a beam splitter were arranged in various forms to measure temperature, concentration, and velocity. The reconstructed temperature and concentration distribution results for each position of the measurement cell case showed similar trends at identical measurement positions. Additionally, experiments confirmed that the velocity measured by laser crossing at a 45-degree angle from the center increased as the supply gas flow rate increased. [ABSTRACT FROM AUTHOR]

Equivalence ratio (Φ) is one of the most important parameters in combustion diagnostics. In previous studies, flame color characteristics have been widely applied to model the Φ of premixed hydrocarbon flames. The flame spatial characteristics also change with the varying Φ. In this paper, a high-speed color camera was employed to capture the premixed propane flame images under different Φ conditions (Φ = 0.93 to 1.53). Then, the relationship between the spatial characteristics and the Φ variation was investigated. The area and height of propane premixed flames perform a strong sensitive response to the Φ variation. Based on the research above, the Φ measurement models were constructed using color and spatial characteristics. A comparison was made between the color characteristics (Color-Φ) model and the color-spatial characteristics (Multi-dimensional-Φ) model. Both models were applied to a set of color images of a premixed propane flame, and the result indicates that the Multi-dimensional-Φ model performs with higher accuracy.

Snapshot multispectral imaging of chemical species in the flame is essential for improved understanding of the combustion process. In this article, we investigate the different configurations of a structured laser sheet-based multispectral imaging approach called the Frequency Recognition Algorithm for Multiple Exposures (FRAME). Using FRAME, a snapshot of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) excited by 283.5 nm laser and Laser-Induced Incandescence (LII) of soot particles excited by 532 nm laser are acquired simultaneously on a single FRAME image. A laminar diffusion flame of acetylene produced by a Gülder burner is used for the experiments. The standard FRAME approach is based on creating two spatially modulated laser sheets and arranging them in a cross-patterned configuration (X). However, the effect of using different configurations (angles) of the two laser sheets on the multispectral planar imaging of the flame has not yet been studied. Therefore, we have compared the FRAME approach in four different configurations while keeping the same flame conditions. First, we have compared the relation between laser fluence and LII signals with and without spatial modulation of the 532 nm laser sheet and found that both detections follow the same curve. When comparing the maps of flame species reconstructed from the standard FRAME configuration and other configurations, there are some dissimilarities. These differences are attributed to minor changes in the imaging plane, optical alignment, laser path length, different modulation frequencies of the laser sheet, laser extinction, laser fluence, etc.

Flame combustion diagnostics is a technique that uses different methods to diagnose the flame combustion process and study its physical and chemical basis. As one of the most important parameters of the combustion process, the flame equivalence ratio has a significant influence on the entire flame combustion, especially on the combustion efficiency and the emission of pollutants. Therefore, the measurement of the flame equivalence ratio has a huge impact on efficient combustion and environment protection. In view of this, several effective measuring methods were proposed, which were based on the different characteristics of flames radicals such as spectral properties. With the rapid growth of machine learning, more and more scholars applied it in the combustion diagnostics due to the excellent ability to fit parameters. This paper presents a review of various measuring techniques of hydrocarbon flame equivalent ratio and the applications of machine learning in combustion diagnostics, finally making a brief comparison between different measuring methods.

Laser absorption spectroscopy tomography is an effective combustion diagnostic method for obtaining simultaneous two-dimensional distribution measurements of temperature and gas molar concentrations. For the reconstruction process of complex combustion flames, a new algorithm named ‘multi-iterative adaptive optimization regularization’ (MIARO) is proposed. This algorithm is a further development of another algorithm known as the ‘modified adaptive algebraic reconstruction technique’ (MAART) with the improvement of the initial value and adaptive regularization parameter selections. In MIARO, the problem of the MAART’s initial value sensitivity is compensated for, and in addition, reconstruction parameters are also introduced into the regularization so that both the quality of reconstruction and the convergence of regularization are guaranteed. In butane burner experiments, an average relative error of 1.82% was achieved with MIARO, compared to 2.44% with MAART, which is a significant reduction of 25.1%. The simulation and experimental results clearly demonstrate that the MIARO algorithm can be used to reconstruct dynamic combustion fields and eliminate boundary artifacts with improved measurement accuracy and robustness.

The present work combines background-oriented schlieren tomography (BOST) with computed tomography of chemiluminescence (CTC) using the TIMes technique to provide simultaneous three-dimensional fields of refractive index (i.e. , density) and CH* chemiluminescence (i.e. , flame luminescence) on a single-shot basis. The feasibility of the proposed non-intrusive technique is assessed based on simulations and experiments. Its accuracy with respect to the number of cameras is evaluated using phantoms of a realistic simulated turbulent flame. The technique is experimentally demonstrated to investigate turbulent flames issuing through a double-annular bluff-body burner (Cambridge-Sandia stratified swirl burner) under a variety of operating conditions. The simultaneous measurements are performed using a cost-effective setup consisting of 29 cameras split between BOST and CTC. The reconstructed instantaneous and averaged results reveal the three-dimensional fields of the refractive index (related to the temperature, pressure and species concentration) and CH* chemiluminescence (indicator of the flame front), showing their structure interaction. [ABSTRACT FROM AUTHOR]

Dual-comb spectroscopy (DCS) has emerged as an important new technique for high-resolution molecular spectroscopy. However, this powerful tool has not been widely utilized in combustion diagnostics due to the difficulty of maintaining the mutual coherence between the two comb sources. In this work, we demonstrated a free-running fiber-laser-based DCS for in situ and non-intrusive H2O and C2H2 measurements in laminar premixed flames. This picometer-resolution dual-comb spectroscopic system was developed by using a single erbium-doped fiber laser at 1.5 µm along with a multipass optical configuration for the enhanced absorption measurement. Because of the mutual coherence of the two comb pulse trains emitted from the same laser cavity, DCS could be performed without using any phase-locking systems. Absorption spectra of CH4/O2/air flames over the spectral range 6503 cm-1 - 6535 cm-1 were measured at different reactant flow rates and heights above the burner, showing a good agreement with the spectral simulation of high-temperature H2O using the HITEMP database. A noise equivalent (1s) absorption coefficient of 9.5 × 10-5 cm-1 was achieved using the current DCS system. Besides, the dual-comb measurement of a C2H4/air sooting flame was also performed at the equivalence ratio p = 2.94 and compared with the spectral simulation of high-temperature H2O and C2H2. All the spectral data were retrieved by 1500 averages of interferograms to increase the spectral signal-to-noise ratio (SNR), but a single interferogram could be acquired at 0.83 ms using the current freerunning dual-wavelength fiber laser. [ABSTRACT FROM AUTHOR]

Despite its key role for the study and modeling of nitrogen chemistry and NOx formation in combustion processes, HCN has only rarely been detected under high-temperature conditions. Here, we demonstrate quantitative detection of HCN behind incident and reflected shock waves using a novel sensitive single-tone mid-infrared frequency modulation (mid-IR-FM) detection scheme. The temperature-dependent pressure broadening of the P(26) line in the fundamental CH stretch vibration band was investigated in the temperature range 670K≤T≤1460K, yielding a pressure broadening coefficient for argon of 2γAr296K=(0.093±0.007)cm−1atm−1 and a temperature exponent of nAr=0.67±0.07. The sensitivity of the detection scheme was characterized by means of an Allan analysis, showing that HCN detection on the ppm mixing ratio level is possible at typical shock wave conditions. In order to demonstrate the capability of mid-IR-FM spectroscopy for future high-temperature reaction kinetic studies, we also report the first successful measurement of a reactive HCN decay profile induced by its reaction with oxygen atoms.

The influence of probe laser chirp on hybrid femtosecond/picosecond pure-rotational coherent anti-Stokes Raman scattering is studied theoretically and experimentally. Experiments of N2(hybrid fs/ps pure-rotational coherent anti-Stokes Raman scattering) are carried out using an in-house built second harmonic bandwidth compressor, results with different probe chirps of are reported. The experimental spectra are fitted with and without considering probe chirp. Including the chirp improves the fit to experimental spectra at all probe delays. The effect of probe pulse chirp is evaluated through a quantitative analysis of matching residuals.

We experimentally investigated clean optical emissions from multiple combustion intermediates including free radicals C2, CH, and CN at multiple wavelengths induced by ultrashort 1,030-nm laser pulses. We systematically study the evolution of the fluorescence emissions induced by the femtosecond laser filament in the combustion field with the parameters such as the laser pulse energy, pulse duration, and focal length. Compared with the previous work, we promote that the fluorescence emissions of the combustion product can be manipulated effectively by controlling the femtosecond laser characteristics including pulse energy, duration, and the focusing conditions. This process helps to optimize its signal-to-noise ratio, which provides a further application of the femtosecond laser pulses to sense the combustion intermediates.

Optical emission spectroscopy was used to measure rovibrationally resolved spectra of water formed during nitromethane combustion at elevated pressures in both oxidizing and inert atmospheres. Complementary kinetic models were used to predict flame temperatures, product distributions, and product formation rates, and results were compared with experimental observations. Experiments were carried out at pressures of 27.4 bar and 34.2 bar in inert atmospheres (referred to as monopropellant conditions) and air (referred to as bipropellant conditions). Dispersed emission shows many rovibrational transitions with the strongest occurring between 13,000 and 14,500 cm−1. These lines are primarily assigned to the relaxation from water's (3,0,1) vibrationally excited state to its vibrational ground state. Weaker progressions in this same region are assigned to water relaxation from the (1,0,3) and (2,2,1) vibrationally excited states. Data collected under mono- and bipropellant conditions showed very similar relative intensities of individual rovibrational lines. The spectra were fit using a simulated temperature of 2,500 K ± 500 K for both mono- and bipropellant conditions. Ex situ FTIR spectra of NM exhaust confirms the presence of H 2 O in both mono- and bipropellant combustion. Interestingly, analyses of these same spectra also show significant amounts of CO in monopropellant exhaust, but no detected CO in bipropellant exhaust. These latter findings show higher CO/CO 2 concentration ratios compared to simulations, motivating the need for refined models describing nitromethane monopropellant combustion. [ABSTRACT FROM AUTHOR]

The stand-off measurements of temperature and pressure are important for a variety of physical and engineering processes. Filtered Rayleigh scattering (FRS) configurations that enhance and suppress sensitivity to temperature are modeled and demonstrated using premixed, atmospheric methane-air Hencken burner generating temperatures up to 2100 K. For a characteristic flame gas mixture, modeling of the FRS signal using a molecular iodine filter shows that conditions exist for which the signal has a greater dynamic range or does not change significantly with respect to high gas temperatures. Laser Rayleigh scattering (LRS) measurements coupled with nonadiabatic flame simulations validate the temperatures, while FRS measurements are compared with the model predictions for different observation angles and iodine cell conditions. Results are discussed in the context of making direct measurements of gas pressure at high temperatures and within turbulent flames. [ABSTRACT FROM AUTHOR]

Combustion diagnostics play an essential role in energy engineering, transportation, and aerospace industries, which has great potential in combustion efficiency improvement and polluting emission control. The three-dimensional (3D) visualization of the combustion field and the measurement of key physical parameters such as temperature, species concentration, and velocity during the combustion process are important topics in the field of combustion diagnostics. Benefiting from the non-contact and non-intrusive advantages of the optical detection method as well as the advantages of the 3D fullfield measurement of the measured field by computational tomography, flame chemiluminescence tomography (FCT) has the ability to realize non-intrusive and instantaneous 3D quantitative measurement and 3D full-field visualization of key physical parameters in the combustion process, which has crucial research significance in combustion diagnostics. In this study, we review the progress of FCT technique. First, we provide an extensive review of practical applications of FCT in state-of-the-art combustion diagnostics and research. Then, the basic concepts and mathematical theory of FCT are elaborated. Finally, we introduce the conventional reconstruction algorithm and proceed to more popular artificial intelligence-based algorithms. [ABSTRACT FROM AUTHOR]

A group contribution equation is proposed for the prediction of the cool flame and main auto-ignition delay time for linear and slightly branched paraffinic fuels. The experiments were carried out in a constant volume combustion bomb where pressure and temperature were modified for five linear paraffinic hydrocarbons (n-C10, n-C12, n-C14, n-C16 and n-C18) and three C16 isomers with different degree of branching (2-methylpentadecane; 2,6,10-trimethyltridecane and 2,2,4,4,6,8,8-heptamethylnonane). An Arrhenius-type correlation is proposed, with extended parameters to the number of groups CH3, CH2 and CH and the equivalence ratio. The proposed equation has enough accuracy to estimate the autoignition delay times of paraffinic fuels (from n-dodecane to n-octadecane) for temperature ranging from 535 to 650ºC and pressure from 11 to 21 bar. [ABSTRACT FROM AUTHOR]

• L p -norm relaxation approach combined with non-negative constraint is proposed. • Impact of sampling characteristics on ill-posedness of inverse problem is analyzed. • Robustness and validity of L p -norm are investigated under measurement noise. • Experiment system is built to implement premixed flame 3D temperature measurement. Optical imaging technology based on flame spontaneous radiation is a promising tool for combustion diagnosis. However, the existing radiation thermometry technology faces the problems of low angular resolution and low efficiency in solving radiative transfer. Therefore, a novel optical imaging model is proposed, which combines light field imaging theory with equation-solving integral equation method based on radiation distribution factor to effectively decouple the radiative properties related to transfer process and the temperature-related source terms. Aiming at the problem that traditional method may cause over-smooth effect, L p -norm regularization based on non-negative constraint is established. Results show that L p -norm can improve reconstruction quality and robustness, but p value needs to be reasonably selected according to noise level and flame inhomogeneity. Experimental study of ethylene/air-premixed flames shows that the methodology can realize 3D measurement (900 ∼ 1550 K). Compared with laser absorption spectroscopy and thermocouple, error is better than 3 % at height above burner 15 mm. [ABSTRACT FROM AUTHOR]

Tomographic absorption spectroscopy (TAS) is a promising temperature and concentration monitoring technology because it has fewer optical access requirements and species selectivity. However, the inhomogeneous distribution of temperature and species concentration in a practical reaction flow can cause laser deflection, which may affect the imaging quality. Thus, this study aimed to develop a novel tomographic absorption deflection spectroscopy (TADS) technology for in-situ, quantitative, and simultaneous measurements of temperature and H 2 O concentration. The proposed system is based on the gas mixing equation and ray tracing, considering the inhomogeneous gradient refractive index and laser bending. Moreover, multi-prior information, including smooth regularization, non-negative constraints, and topological shape constraints, is coupled to mitigate the ill-posedness of inverse problems, thereby improving the accuracy and robustness of multiparameter tomography. Numerical verification and a comparison of algorithms for multimodal flames indicate that the proposed TADS technology exhibits excellent anti-noise performance and can significantly improve reconstruction quality. Additionally, a proof-of-concept based on experimental data of a steady methane diffusion flame is performed to demonstrate the feasibility of TADS as a practical combustion diagnostic. The reconstruction results of instantaneous temperature and H 2 O concentration on six different cross-sections show that the topological structure, dynamics, and parameter distribution can be quantitatively demonstrated. This technology is expected to enhance the practicability of laser absorption spectroscopy in the in-situ measurement of complex combustion reaction flows with high temperature, high pressure, and strong distortion. [ABSTRACT FROM AUTHOR]

This work discusses the optical systems with brightness amplification based on two copper bromide brightness amplifiers for studying the combustion of aluminum-based nanopowders. It is proposed to use a system with brightness amplification and independent illumination–bistatic laser monitor to increase the brightness and sharpness of images. Independent illumination is realized using a second copper bromide laser and makes it possible to increase the exposure of the object under investigation in comparison with the conventional laser monitor with one brightness amplifier. In addition to independent illumination, a two-channel laser monitor is proposed to examine the surface of burning materials with different magnifications in one area of the sample, as well as simultaneously in two areas of the sample. The delay between the lasing pulses of the brightness amplifiers can be adjusted with high accuracy so that the radiation from one amplifier does not affect the images of the other. The proposed experimental technique makes it possible to investigate the surface of powder materials during high-temperature combustion, accompanied by intense glowing and scattering of combustion products. This article presents the results of real-time visualization of the surface of burning metal nanopowders and mixtures using bistatic and two-channel laser monitors. [ABSTRACT FROM AUTHOR]

Spontaneous Raman scattering from nitrogen is well-suited for temperature measurements in combustion experiments, especially at high pressure which increases the Raman signal because of the higher density in the measurement volume. In this work we investigate high density effects on the anisotropic tensor component of ro-vibrational spontaneous Raman scattering, which must be understood to obtain accurate thermometry in high pressure gases using high-fidelity Raman simulations. We measure the collision broadening for the anisotropic component of spontaneous Raman scattering from room temperature nitrogen over the pressure range 10–70 atm for three gas compositions: pure nitrogen, air, and nitrogen in argon. Line broadening coefficients inferred from these measurements were found to be 14 ± 5% larger than the corresponding broadening coefficients of the isotropic Q branch. Air broadening coefficients were found to be very similar and about 2.5% smaller than nitrogen self-broadening coefficients. Argon broadening coefficients were 25% smaller at rotational quantum number 7 and about 50% smaller at rotational quantum number 21. Additionally, we found that our unmixed line model for the O and S branches gave good fits for all pressures, which indicates that line mixing effects are not significant in the O and S branches over this range of pressures. Using indirect experimental evidence we infer that line mixing effects in the anisotropic component of the Q branch were below the threshold set by our experimental spectral resolution at pressures up to 70 atm at room temperature. Assuming that the anisotropic Q branch lines mix like the isotropic lines results in a small systematic error in the inferred temperature at flame temperatures, with the error increasing slowly with pressure. The bias can be easily removed by modeling the anisotropic spectrum separately from the isotropic spectrum. Line mixing effects should be included in the model of the isotropic part of the Raman spectrum, but can probably be neglected in the anisotropic part of the spontaneous Raman spectrum of N 2 for pressures below 400 atm at flame temperatures. [ABSTRACT FROM AUTHOR]

Understanding the dynamics of fire events in ground vehicles is critical to improving crew survivability. To advance our understanding, four dimensional (4D) measurements are sorely needed to resolve both the temporal and spatial dynamics of fire events. However, there are several key challenges toward such measurements, including equipment requirements and optical access. 4D measurements, especially with sufficient temporal resolution, can be equipment intensive. Such equipment requirements are further compounded by the relatively hostile environments encountered in vehicular testbeds. Moreover, there is often very limited optical access available for obtaining such measurements within vehicular environments. This work describes the design and implementation of a customized fiber-based endoscope (FBE) setup to overcome these challenges in order to enable 4D flame measurements in a ground vehicle testbed located at the Army Research Laboratory, Aberdeen Proving Ground. Using a customized 9-to-1 FBE bundle, 4D imaging of relatively large-scale fire events was demonstrated at 500 Hz in a volume of 35 cm (width) × 35 cm (depth) × 29 cm (height), with a single camera located at a safe distance outside the vehicle. The measurements were then processed by volumetric tomography techniques to resolve the temporal dynamics and spatial structures of the target flame within the vehicle testbed. [ABSTRACT FROM AUTHOR]

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.

Volumetric imaging represents one of the major development trends of flow diagnostics due to both the advancement in hardware and the requirement for more information to further understand complicated turbulent and/or reactive flows. Background-oriented Schlieren tomography (BOST) has become increasingly popular due to its experimental simplicity. It has been demonstrated to be capable of simultaneously recovering the distributions of refractive index, density, and temperature of flows. However, its capability in thermometry has only been demonstrated under the axisymmetric assumption, which greatly limits its applicability. In this work, we dedicated to developing a cost-effective BOST system for the simultaneous retrieval of refractive index, density, and temperature distributions for the asymmetric flame. A few representative tomographic inversion algorithms were assessed as well. Both numerical and experimental demonstrations were conducted and the results show that our implemented BOST can successfully reconstruct the three-dimensional temperature distribution with a satisfactory accuracy. [ABSTRACT FROM AUTHOR]

New combustion concepts and engine designs are being currently investigated in order to comply with upcoming pollutant regulations and reduce fuel consumption. In this context, two-stroke architectures appear as a promising solution for the implementation of some combustion concepts. However, scavenging processes in a two-stroke engine are much more challenging than for a four-stroke engine, and the residual mass of burnt gases retained inside the cylinder needs to be properly determined in order to keep control over the in-cylinder composition, hence over the combustion conditions and pollutant emissions. In this study, a new methodology for the estimation of the internal residual gas fraction is introduced, which is based on the thermodynamic processes occurring in the engine investigated and makes use of basic engine instrumentation and measurement equipment usually available in a conventional test cell. Several versions of the estimator were developed so that different requirements could be met, such as those of real-time estimation on an engine test bench but with reduced precision or, on the contrary, highly precise but time-consuming computations for post-processing purposes and combustion diagnosis. The consistency of the internal residual gas estimator was then validated through its application to real engine tests at different operating points. [ABSTRACT FROM AUTHOR]

For the purpose of employing laser-induced breakdown spectroscopy (LIBS), nanosecond laser pulses (6 ns FWHM) from a standard Q-switched Nd:YAG laser (2nd harmonic) are modulated or chopped —using a novel method utilizing a variable air-pressure optical cell—to limit the inverse-Bremsstrahlung photon absorption process occurring in the breakdown plasma. The resulting plasma does not ignite flammable mixtures, but plasma emission is sufficiently strong to enable the characterization of reactants without strong perturbation (i.e., no ignition or shock wave results from the breakdown plasma). Two strong atomic emission lines, H (656 nm) and N II (568 nm), are chosen to find correlations between the plasma emission spectrum and the fuel concentration; plasma emission is captured after a delay of 20 ns with a gate width of 60 ns, the time over which there are well defined emission peaks. The chopped laser pulse is created within the pressurized optical cell with an initial breakdown plasma, by focusing the beam in high-pressure air. The pulse width of the laser beam transmitted through the cell is dependent on the cell pressure, and for this work, a pulse duration of approximately 600 ps was derived from the cell operated at a pressure of 10 bar. The chopped pulses were used for LIBS, and a 2D concentration distribution of a stoichiometric methane-air flow in ambient air were recorded without combustion reactions initiated by the laser-induced plasma. [ABSTRACT FROM AUTHOR]

Time-gated picosecond laser-induced breakdown spectroscopy (ps-LIBS) for the determination of local equivalence ratios in atmospheric-pressure adiabatic methane–air flames is demonstrated. Traditional LIBS for equivalence-ratio measurements employ nanosecond (ns)-laser pulses, which generate excessive amounts of continuum, reducing measurement accuracy and precision. Shorter pulse durations reduce the continuum emission by limiting avalanche ionization. Furthermore, by contrast the use of femtosecond lasers, plasma emission using picosecond-laser excitation has a high signal-to-noise ratio (S/N), allowing single-shot measurements suitable for equivalence-ratio determination in turbulent reacting flows. We carried out an analysis of the dependence of the plasma emission ratio Hα (656 nm)/NII (568 nm) on laser energy and time-delay for optimization of S/N and minimization of measurement uncertainties in the equivalence ratios. Our finding shows that higher laser energy and shorter time delay reduces measurement uncertainty while maintaining high S/N. In addition to atmospheric-pressure flame studies, we also examine the stability of the ps-LIBS signal in a high-pressure nitrogen cell. The results indicate that the plasma emission and spatial position could be stable, shot-to-shot, at elevated pressure (up to 40 bar) using a lower excitation energy. Our work shows the potential of using ps-duration pulses to improve LIBS-based equivalence-ratio measurements, both in atmospheric and high-pressure combustion environments. [ABSTRACT FROM AUTHOR]

This paper presents the results of real-time monitoring of air combustion of aluminum nanopowder and its mixtures with aluminum micropowder and iron oxide III. An optical system with a brightness amplifier was used along with visual monitoring to characterize the reflectivity of the sample surface. The reflectivity was analyzed after registering by a photodiode the intensity of radiation reflected from the surface and then amplified by the brightness amplifier at 510.6- and 578.2-nm wavelengths. Analysis of the images obtained with a high-speed camera and the photodiode output oscillograms showed that the intensity of the brightness amplifier output corresponds to the main stages of the combustion process, including the beginning of combustion, spreading of the heat wave and rise of the second combustion wave. The proposed technique is adequate for real-time monitoring of the combustion process with temperatures of 2200 °C–2400 °C, which is accompanied by intensive lighting. [ABSTRACT FROM AUTHOR]

A measurement technique for determination of the global and local equivalence ratios from the flame chemiluminescence for a swirl-stabilized lean premixed combustion of natural gas and kerosene is presented. First, we conducted spectrally resolved chemiluminescence studies using an imaging spectrometer to correlate the ratio of individual chemiluminescence signals to the equivalence ratio. Flame spectra were recorded for a multitude of different lean operating conditions for natural gas and kerosene combustion. The spectra show that, without background correction, the CH*/CO2* ratios for both natural gas and kerosene combustion exhibited a monotonic relationship to the equivalence ratio in the investigated range. Subsequently, bandpass-filtered images of CH* and CO2* chemiluminescence were acquired simultaneously on one camera chip using an image doubler to investigate the local relationship of the CH*/CO2* ratio with the equivalence ratio. The ratio images corroborate the monotonic relationship of the CH*/CO2* ratio to the equivalence ratio. Furthermore, the ratio was found to be influenced by the local reaction zone temperature. The presented technique allows high temporal resolution determination of the local equivalence ratio in lean premixed natural gas and kerosene flames and can thus be applied to quantify equivalence ratio oscillations during unstable combustion.

The measurement techniques for delineating fuel-air mixing and transport in gas turbine combustion, as well as examples of representative results, are provided in this overview. The summary is broken into applications for gaseous fuels and liquid fuels since many diagnostics which are specific to the phase of the fuel have been developed. Many possible methods for assessing the general mixing have been developed, but not all have been applied to practical systems either under scaled or under actual conditions. With respect to gaseous mixing processes, planar laser-induced fluorescence (PLIF) based on acetone is now starting to be successfully applied to actual systems and conditions. In spray-fired systems, the need to discriminate between phases leads to considerable complication in delineating fuel-air mixing. Methods that focus on the discrete phase have successfully provided details relative to the droplets. These include phase Doppler interferometry (PDI), which is becoming ubiquitous in application to practical devices and under practical conditions. PDI is typically being applied to quantify droplet sizes, although the volume flux, which is relevant to fuel-air mixing, in practical systems is also being reported. In addition, PLIF strategies that focus upon the behaviour of the droplets are now being developed. However, PLIF strategies that can discriminate between phases either in the fuel or with respect to the liquid fuel and combustion air are also being developed. In terms of characterizing the vector fields associated with the mixing process, laser anemometry (LA), although it is tedious to apply, has proven reliable even in the presence of droplets. Newer methods such as DPIV and FRS have seen only limited application in practical systems but appear promising. In terms of scalar fields, LIF and PLIF have also been applied successfully to these systems, and examples of the measurements of concentrations of various radical species such as OH are found throughout the literature.

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Studying the line‐broadening effects of combustion intermediates in flames is the key to derive quantitative information via spectroscopic techniques. In this communication, an extension to polarization spectroscopy via employing two probe beams, one of which is copropagating whereas the other one is counterpropagating to the pump beam, is proposed. This gives rise to dual‐probe polarization spectroscopy, which enables recording a Doppler‐broadened and a Doppler‐free signal simultaneously. [ABSTRACT FROM AUTHOR]

The development of laser and sensor technologies have provided unprecedented opportunities for the extended applications of volumetric tomography. The recent progresses in computed tomography of chemiluminescence (CTC) have facilitated the understanding of turbulent flows and combustion instability. However, the current demonstrations of CTC can only provide either an instantaneous measurement with a good number of projections to achieve a good spatial resolution or time-resolved measurements (kHz-rate) but with a reduced number of projections which may cause a failure in resolving small details of the flames. In this work, we aim to develop a time-resolved endoscopic CTC system with 17 projections to achieve both good spatial and temporal resolutions. A new method was proposed here to calibrate projections that cover a field of view larger than 180 degrees. The system was then applied to a non-premixed turbulent swirl flame to reconstruct its time-resolved 3D structures. The experimental studies have shown that when only nine projections were used, parts of the flame structures would be lost. To fully recover the flame structures, a minimum of 16 projections should be used. Proper orthogonal decomposition and dynamical mode decomposition were then applied to analyze the time serious of 3D structures of a turbulent swirl flame. [ABSTRACT FROM AUTHOR]

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]