Combustion Diagnostics Research Articles

Simultaneous Measurement of Multiparameter of Diesel Engine Exhaust Based on Mid-Infrared Laser Absorption Spectroscopy

Exhaust parameters of an engine carry vital information that can help to understand its transient duty cycles. We developed a mid-infrared wavelength modulation spectroscopic sensor for simultaneous measurement of exhaust temperature, pressure, and NO and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations by using two quantum cascade lasers (QCLs) for the first time. The dual-channel signals were demodulated and normalized respectively with a specially designed digital lock-in amplifier, achieving calibration-free measurements to adapt to the harsh and variable working conditions of diesel engines. Two-line thermometry was performed using a newly selected spectral line pair of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (1629.85 and 1630.33 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ). Simultaneous measurement of the quadruple parameter of diesel exhaust, i.e. temperature, pressure, and NO and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations, was first achieved using only three spectral lines and a developed multi-parameter inversion algorithm. The feasibility of the sensor was demonstrated in field tests in an in-service vehicle. Exhaust temperature, pressure, and NO and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations were obtained continuously with 0.1 s time intervals, which clearly show these parameters in the transition and steady stages of the engine, respectively. The method and results can be used to evaluate the engine operation performance and combustion diagnostics.

Flame detection via active plasma probing

Combustion diagnostics of highly diluted air fuel mixtures are of great importance for reducing the carbon footprint of various types of combustion systems. Flame detection techniques, such as ion sensing and optical diagnostics, have been reported for diagnosing combustion status. In this paper, a flame front detection technique based on active plasma probing is introduced and analyzed. Unlike the conventional ion sensing used in internal combustion engines, a separate electrode gap is used to detect the flame front arrival. Further, the voltage potential across the electrodes of the spark plug probe is modulated actively to be slightly below the breakdown threshold prior to flame arrival. At the arrival of the flame front, the ions in the flame tend to decrease the breakdown voltage threshold and trigger a breakdown event. An optically accessible constant volume combustion vessel is employed to investigate the efficacy of such active plasma probing for the detection of the flame front under quiescent and flow conditions. Three types of fuels are used in the tests, including methane, propane, and DME, with an air fuel ratio sweep (from stoichiometric to extremely lean) for each fuel. Efforts are made to characterize the probing criteria of minimum, yet adequate voltage to succeed in the detection of the arrival and departure of the flame front most sensitively and reliably, for all three types of fuels under various mixture strengths. For comparison, conventional ion current measurements are conducted under identical background conditions to benchmark the efficacy of flame detection. Results show that the active plasma probing can promptly detect the arrival and departure of the flame front, robustly regardless of the fuel type and excess air ratios. The precise control of the detecting voltage is key for reliable flame detection with high sensitivity.

Temperature dependent Raman spectra of pure, gaseous formaldehyde for combustion diagnostics

The combustion of renewable fuels such as methanol or ethanol produces comparatively large concentrations of formaldehyde (CH2O) as a combustion intermediate. This intermediate needs to be quantitatively measured using non-intrusive laser diagnostics to provide a better understanding of the chemical processes in the reaction zone. Spontaneous Raman scattering is used in reactive flow diagnostics to measure spatially resolved species concentrations. For diagnostics of CH2O using Raman scattering, the temperature-dependent Raman spectra of gaseous CH2O are required, but not yet available. One reason for this is that gaseous CH2O polymerizes very rapidly, especially at higher temperatures, and can only be made available in pure form for spectroscopic investigations by specific preparation. For this purpose, a continuous flow reactor was developed in which trioxane is pyrolyzed to monomeric CH2O by means of thermal decomposition in a tube reactor. Using a CW-Raman spectrometer, the products of a thermal decomposition at isothermal conditions are analyzed downstream of the tube reactor and CH2O is detected as the only product of the pyrolysis process. Raman spectra of gaseous CH2O are characterized for the first time using the continuous flow system. The Raman scattering in the CH-bend and CH-stretch regions show characteristic bands, which are, for instance, different in the spectral position to the ones from ethanol, allowing for a spectral discrimination. Raman cross sections reveal that the harmonic-oscillator assumption substantially deviates for the tetratomic CH2O, which underlines the relevance of an experimental characterization at elevated temperatures. Finally, the flow systems developed for the generation of monomeric gaseous CH2O can potentially be employed to improve diagnostics, such as laser induced fluorescence for a quantitative measurement of CH2O.

Arbitrary position 3D tomography for practical application in combustion diagnostics

This work aims to make three-dimensional (3D) tomographic techniques more flexible and accessible to in-situ measurements in practical apparatus by allowing arbitrary camera placements that benefit applications with more restrictive optical access. A highly customizable, in-house developed tomographic method is presented, applying smoothness priors through Laplacian matrices and hull constraints based on 3D space carving. The goal of this paper is to showcase a reconstruction method with full user control that can be adopted to various 3D field reconstructions. Simulations and experimental measurements of unsteady premixed CH4/air and ethanol (C2H5OH) diffusion pool flames were evaluated, comparing arbitrarily placed cameras around the probed domain to the more commonly used in-plane-half-circle camera arrangement. Reconstructions reproduced expected topological field features for both flame types. Results showed slight decrease in reconstruction quality for arbitrarily placed cameras compared to in-plane-half-circle arrangement. However, at lower numbers of camera views (N q ⩽ 6) arbitrary placement showed better results. The introduced methodology will be useful for optically limited setups in terms of handling a priori information, camera placement and 3D field evaluation.

Optical diagnostics of methanol active-thermal atmosphere combustion in compression ignition engine

Methanol has become one of the research hotspots of internal combustion engine in recent years. One method of methanol compression ignition is that create in-cylinder active-thermal atmosphere to ignite direct injection (DI) of methanol. However, the combustion essences and related mechanisms in methanol active-thermal atmosphere combustion (ATAC) have not been comprehended. In current study, the characteristics of methanol ATAC were investigated on a light-duty engine using different optical diagnostics. Polyoxymethylene dimethyl ethers (PODE2) and n-heptane were chosen as port injection (PI) fuel respectively, and methanol was used as DI fuel. The experiment results indicate that methanol ATAC with PI of PODE2 is divided into two combustion processes. One process is the premixed combustion of PODE2 and the partially premixed combustion (PPC) of methanol, which appears that the premixed blue flames of PODE2 and the partially premixed yellow flames of methanol. The other is PODE2 premixed combustion and methanol diffusion combustion, which presents that the premixed blue flames of PODE2 and the yellow spray diffusion flames of methanol. Both the heat release rate and the flame structure of methanol ATAC can be adjusted by changing DI timing. The increase of PI energy proportion advances the timing of premixed blue flames of PODE2. The DI of methanol is driven by spray diffusion flames unless the methanol injection timing is earlier than high temperature heat release of PODE2. The capability of PODE2 to make methanol auto-ignited is higher than that of n-heptane. The methanol energy proportion can achieve 70% in PODE2 case, while achieve merely 30% in n-heptane case. Kinetic analysis reveals that the ignition delay times of PODE2 are lower than that of n-heptane even at lower equivalence ratio. The KL factor of methanol diffusion flame ranges from 0.02 to 0.04, which is remarkable lower than diesel spray flames. The methanol ATAC achieves higher substitution rate of methanol and the flexible combustion process can be obtained using PODE2 as PI fuel.

Supersonic combustion diagnostics with dual comb spectroscopy

Supersonic engine development requires accurate and detailed measurements of fluidic and thermodynamic parameters to optimize engine designs and benchmark computational fluid dynamic (CFD) simulations. Here, we demonstrate that dual frequency comb spectroscopy (DCS) with mode-locked frequency combs can provide simultaneous absolute measurements of several flow parameters with low uncertainty across a range of conditions owing to the broadband and ultrastable optical frequency output of the lasers. We perform DCS measurements across a 6800–7200 cm−1 bandwidth covering hundreds of H2O absorption features resolved with a spectral point spacing of 0.0067 cm−1 and point spacing precision of 1.68 × 10−10 cm−1. We demonstrate 2D profiles of velocity, temperature, pressure, water mole fraction, and air mass flux in a ground-test dual-mode ramjet at Wright-Patterson Air Force Base. The narrow angles of the measurement beams offer sufficient spatial resolution to resolve properties across an oblique shock train in the isolator and the thermal throat of the combustor. We determine that the total measurement uncertainties for the various parameters range from 1% for temperature to 9% for water vapor mole fraction, with the absorption database/model that is used to interpret the data typically contributing the most uncertainty (leaving the door open for even lower uncertainty in the future). CFD at the various measurement locations show good agreement, largely falling within the DCS measurement uncertainty for most profiles and parameters.

Characterization of a new ultra-high pressure shock tube facility for combustion and propulsion studies.

A new shock tube facility has been designed, constructed, and characterized at the University of Central Florida. This facility is capable of withstanding pressures of up to 1000 atm, allowing for combustion diagnostics of extreme conditions, such as in rocket combustion chambers or in novel power conversion cycles. For studies with toxic gas impurities, the high initial pressures required the development of a gas delivery system to ensure the longevity of the facility and the safety of the personnel. Data acquisition and experimental propagation were implemented with remote access to ensure safety, paired with a LabVIEW- and Python-based user interface. Thus far, test pressures of 270 atm, blast pressures of 730 atm, and temperatures approaching 10 000K have been achieved. The extreme limitations of this facility allow for emission spectroscopy to be performed during the oxidation of fuel mixtures, e.g., alkanes diluted in argon and carbon dioxide. Ignition delay times were determined and compared to simulations using chemical kinetic mechanisms. The design, experimental procedures, processes of analysis, and uncertainty determination are outlined, and typical pressure profiles are compared with a new gas dynamics solver and empirical correlations developed across multiple shock tube facilities. Preliminary reactive mixture analyses are included with further investigation of the mixtures outlined.

Review of Measurement Techniques of Hydrocarbon Flame Equivalence Ratio and Applications of Machine Learning

Abstract 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.

A Survey for 3D Flame Chemiluminescence Tomography: Theory, Algorithms, and Applications

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 full-field 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.

Artificial neural networks modeling of combustion parameters for a diesel engine fueled with biodiesel fuel

In the present study, numerous artificial neural networks were employed to predict the combustion characteristics of a four-stroke, single-cylinder, naturally aspirated diesel engine, including multilayer perceptron (MLP), adaptive neuro-fuzzy interference system (ANFIS) and radial basis function network (RBFN). The actual data derived from measurements and calculations were applied in model training, cross-validation, and testing. Biodiesel fuel ratio, engine load, air consumption, and fuel flow rate data were considered as model-input parameters, which are related to main engine operating variables and also affect the combustion characteristics. These kinds of model-input data were especially preferred due to being commonly direct measurable with the main engine sensors or found in engine maps/look-up tables managed by the electronic control unit (ECU). The main parameters obtained from the direct analysis of the measured in-cylinder pressure data and the heat release analysis results were also determined as model-output parameters. Equally, to ensure a more accurate, straightforward and practical approach to prediction, in every category of neural networks, three training algorithms were adopted, including Levenberg-Marquardt (LM), back propagation (BP) and conjugate gradient (CG). Moreover, a sensitivity analysis was accomplished by assessing the strengths of the links between input and output parameters for every topology. As such, the researcher revealed that all proposed neural networks could predict maximum heat release rate (HRRmax), ignition delay (ID), maximum cylinder pressure (Pmax), maximum cylinder pressure location (θPmax), maximum in-cylinder pressure rise rate (dPmax), indicated mean effective pressure (IMEP), crank angle of center heat release rate (CA50), and combustion duration (CD), with a high accuracy rate. Results of model-output parameters are also important parameters in the combustion diagnostics which are influential on engine thermal efficiency and pollutant formation process. In addition, it is necessary to note that MLP architectures that incorporated the LM algorithm presented superior results. With regards to the optimal ANN model, the linear coefficient values: 0.999848, 0.999847, 0.999955, 0.999780, 0.999378, 0.999929, 0.999766, and 0.999216, were found for ID, HRRmax, Pmax, θPmax, dPmax, IMEP, CA50 and CD, respectively.

ВИБРОМОНИТОРИНГ ДВИГАТЕЛЯ ВНУТРЕННЕГО СГОРАНИЯ

The work is devoted to the problem of diagnostics of automotive internal combustion engines.The problem of monitoring the state of internal combustion engine is now most relevant dueto the increase in the number of cars and the tightening of environmental requirements. In thework the consequences of operation of faulty internal combustion engine are considered. The purposeof the work is to justify the choice from existing diagnostic methods of such a method, whichcan help to detect the fault most accurately and quickly. For this purpose, the work details moderndiagnostic tools, highlights the principles of work, advantages and disadvantages. With the adventof modern technologies, the long-known method of estimating the state of internal combustionengine by sound can become the most advanced, as the human factor is excluded, for signal processingthe computational technique of analysis of the audio spectrum in which is carried out withthe help of artificial neural networks is used. The use of artificial neural networks for sound spectrumanalysis has found application in speech recognition and for diagnosis of respiratory systemdiseases. The article considers mechanisms that are capable of generating sound signals duringinternal combustion engine operation, some of them are phased, i.e. they are tied to operatingcycles, some are not phased. The proposed diagnostic technique allows to distinguish "useful"sounds from the total number of internal combustion engine noises, after comparative analysis topoint to the node the sound of which differs from the reference, serviceable one. Scientific noveltyconsists in the fact that the diagnostic process becomes automated, all sounds captured by sensorsare processed in a computer or a special scanner, the display shows information about the condition of certain nodes, unlike traditional methods where the diagnosis is carried out visually or byear. This increases diagnostic accuracy and reduces overall labor intensity by avoiding partial orcomplete engine disassembly

Chemiluminescence-based characterization of heat release rate dynamic in a micro gas turbine combustion chamber

Chemiluminescence is a major spontaneous emission inflame and can be used in combustion diagnostics to evaluate various flame characteristics. OH∗ chemiluminescence is a good indicator for heat release rate analysis in alkane flames. In this paper, a high-resolution ultra-violet imaging system was used to capture the OH∗ chemiluminescence images. The experiments were conducted in a pilot-scale system with a 300 kW combustion chamber. The variety of temperature distribution is evaluated using the outlet temperature distribution factor. The heat release rate dynamic characteristics were discussed by criteria of distribution uniformity on the spatial and temporal dimensions and the characteristic frequency results obtained by three different methods. The result shows that operating conditions with a uniform outlet temperature distribution always have more uniform spatial distribution and smaller temporal fluctuation amplitude of heat release rate. Frequency results obtained by different methods match very well when the heat load ratio is over 70%. The flame frequency characteristics correlate well with the turbulence property compared with the unmixedness of fuel and air. But the oscillation amplitude of heat release is mainly affected by the unmixedness.

Single-camera stereoscopic 3D multiplexed structured image capture for quantitative fuel-to-air ratio mapping

A single-camera three-dimensional Multiplexed Structured Image Capture (3D-MUSIC) based stereoscopic imaging system is demonstrated to obtain a three-dimensional fuel-to-air ratio (FAR) map of a methane-air Bunsen flame at standard atmosphere conditions. Images of the C2* (500nm) and CH* (430nm) emissions are spatially modulated and captured through two optical paths and combined into a single image on the camera. The C2* / CH* ratio is computationally recovered by low-pass filtering in the spatial frequency domain and exhibits a linear trend to a FAR map of the flame. Each channel's unique FAR map is fused to generate a line-of-sight three-dimensional FAR map by triangulating points from each channel's perspective. As an imaging technique, the 3D MUSIC is capable of obtaining multi-spectral multi-view 3D images using spatial modulation and recovery in one camera without any special illumination. It may be extended for applications in combustion, biomedical, and other key diagnostics.

Synchronic measurements of temperatures and concentrations of OH, NH, and NO in flames based on broadband ultraviolet absorption spectroscopy

Temperature is an important parameter influencing the combustion reaction path and rate and determining the combustion and energy exchange efficiency. The OH, NH, NO and other species are involved in the key elementary reactions of combustion and determine the generation of NO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; pollutants. Therefore, temperature and concentration measurements of OH, NH, and NO are of great significance for combustion diagnostics and research on reaction or emission mechanisms. In this work, a measurement system with high spatial resolution based on broadband ultraviolet absorption spectroscopy is established to realize simultaneous measurements of the temperature and concentrations of OH, NH, and NO in flames. Low detection limits of these three species are achieved by using the established measurement method. The 1&lt;i&gt;σ&lt;/i&gt; detection limit of NH is 1.8 ppb·m (1560 K), which is realized for the first time in atmospheric-pressure flames using absorption spectroscopy. The 1&lt;i&gt;σ&lt;/i&gt; detection limits of OH and NO are 60 ppb·m (1590 K) and 1 ppm·m (1380 K), respectively, which are obviously better than the existing results obtained by using infrared laser absorption spectroscopy. Then, the distributions of temperatures and concentrations of OH, NO and NH are acquired at various heights in an atmospheric-pressure NH&lt;sub&gt;3&lt;/sub&gt;/CH&lt;sub&gt;4&lt;/sub&gt;/air premixed flat flame with a high spatial resolution of nearly 0.1 mm. The broadband absorption spectra of OH and NH are acquired simultaneously inside the flame front, and the spectra of OH and NO are acquired simultaneously above the flame front. Inside or near the flame front, the temperatures deduced from the spectra of OH, NH, and NO are consistent, verifying the ability of these three species to be used to measure temperature. In addition, OH, NH, and NO are found to be suitable for different regions in combustion. The OH absorption is suitable for the post-combustion region with temperatures higher than 1000 K, the NH absorption can be used to acquire the temperature inside the flame front in complex combustion, and the NO absorption was able to provide the temperature in the region before or outside combustion at lower temperatures. Additionally, the experimental temperature and concentration profiles are in good agreement with the computational fluid dynamics predictions based on the mechanism, exhibiting the accuracy of the simultaneous temperature and concentration measurements by using broadband ultraviolet absorption spectra. Moreover, the differences in temperature and OH concentration between experiments and simulations indicate that the carbon sub-mechanism in the mechanism given by Okafor et al. [Okafor E C, Naito Y, Colson S, Ichikawa A, Kudo T, Hayakawa A, Kobayashi H &lt;ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1016/j.combustflame.2017.09.002"&gt;2018 &lt;i&gt;Combust. Flame&lt;/i&gt; &lt;b&gt;187&lt;/b&gt; 185&lt;/ext-link&gt;] should be further improved for more accurate predictions of NH&lt;sub&gt;3&lt;/sub&gt;/CH&lt;sub&gt;4&lt;/sub&gt; combustion.

Herriott cell enhanced SMF-coupled multi-scalar combustion diagnostics in a rapid compression expansion machine by supercontinuum laser absorption spectroscopy

A novel supercontinuum laser absorption spectroscopy (SCLAS) approach combining a planar external Herriott multi-pass cell (HMPC) with fully single-mode fiber (SMF) coupling is presented for broadband multi-scalar combustion measurements in a rapid compression expansion machine (RCEM). For the HMPC, 1-dimensional refractive index gradients occurring in the RCEM combustion chamber are analyzed via ray-tracing with respect to axial and vertical beam steering. The impact of beam steering on the SCLAS signal is compared with the ray-tracing analysis and high-speed flame luminosity images. SCLAS based measurements are presented during compression, auto-ignition and combustion in the RCEM for n-heptane/methane mixtures at varied AFR (air-fuel ratio) and n-heptane/EGR (exhaust gas recirculation) mixtures at temperatures exceeding 1800 K and pressures up to 80 bar. Simultaneous temperature and mole fraction courses of H2O, CH4, and CO2 are inferred from NIR (near-infrared) broadband absorbance spectra detected by a Czerny Turner spectrometer (CTS) in a spectral range of 1374 nm to 1669 nm. The multi-pass approach with SMF coupling, which avoids multi-mode fiber induced noise, allows for high-speed multi-species SCLAS measurements with low standard deviations; for temperature this amounts to about 5 K and partially below at a temporal resolution of 25 µs.

Flame emission tomography based on finite element basis and adjustable mask

Flame emission tomography is a promising tool for combustion diagnostics. Up to now, this technique relies on the assumption that the distribution within a voxel is uniform. Such an assumption suffers from a large gradient between adjacent voxels and the loss of information within the voxel. Hence, this work aims to develop a method based on the finite element basis to address the above shortcomings. From the reconstruction side, an adjustable mask is developed to suppress the artifacts and speed up reconstruction. Both simulative and experimental studies have been designed and conducted. The simulative results show that the finite element basis can decrease the reconstruction errors (defined as normalized 2-norm difference) from 0.616 to 0.241 and 0.386 to 0.203 by the algebraic reconstruction technique for two representative flame phantoms. The adjustable mask can decrease the reconstruction error for both bases, especially under low voxel resolution. The experimental results show that the predicted projection error of the finite element basis with the adjustable mask is decreased by 57% compared with that of the uniform voxel basis with the fixed mask when the voxel resolution is 16 × 16 × 20. Furthermore, the finite element basis can generate better reconstructions with fewer voxels. Both simulative and experimental studies suggested the superiority of the finite element basis with the adjustable mask.

Development of an infrared laser absorption sensor for non-intrusive gas temperature measurements

Energetic materials have extremely high volumetric and specific energy densities, making them attractive and important in combustion systems. To improve their combustion performance, reliable temperature acquirement method is highly demanded. A laser sensor was developed for in situ and quantitative measurements of gas temperature. Scanned-wavelength direct absorption spectroscopy was used for line-of-sight temperature measurements. Multiple H2O absorption features in the near-infrared combination band (v1+v3) and mid-infrared fundamental band (v3) were selected to establish four absorption line pairs with good temperature sensitivity. Three infrared distributed feedback lasers (DFB) were used to cover the selected absorption lines. The accuracy and uncertainty of the sensor were first numerically evaluated in a wide temperature range of 1000–3000 ​K under different Gaussian white noise levels (5–20%). A free-space optical setup was established to experimentally evaluate the sensor performance by measuring benchmark laminar premixed flames, which were compared with additional thermocouple measurements, chemical kinetic modeling and computational fluid dynamics simulations. The good performance of the current sensor indicates the potential of being used in non-intrusive, in-situ and quantitative diagnostics of the energetic materials combustion.

Equivalence Ratio Measurements in CH4/Air Gases Based on the Spatial Distribution of the Emission Intensity of Femtosecond Laser-Induced Filament

Femtosecond lasers have been used in combustion diagnostics. Based on the characteristics of femtosecond laser filamentation, many diagnostic techniques have been developed. Here, we propose a method, based on femtosecond laser filamentation, for equivalence ratio measurements in CH4/air gases. By measuring the spatially resolved spectra of the femtosecond laser-induced filament, we found that the variation of the equivalence ratio in the flow field would affect the spatial distribution of the emission intensity of femtosecond laser-induced filament. On this basis, the equivalence ratio was calibrated by using the relative spatial positions of N2 (337 nm) and C2 (516.5 nm) signals in the filament. This method overcomes the interference of local air disturbance, having lower measurement uncertainty.

The gaseous hydrocarbon fuel combustion process diagnostics using laser-spark emission spectrometry

To solve the gaseous hydrocarbon fuel combustion process control and diagnostics problem, it is proposed to apply the laser-spark emission spectrometry methods. In propane-air mixture combustion, three modes are investigated: stoichiometric combustion, an enriched mixture, and a lean mixture. A laboratory stand has been developed to study combustion processes by laser-spark emission spectrometry. The plasma radiation spectral characteristics an experimental study results formed in a flame when exposed to laser radiation are presented.

Quantum cascade laser spectroscopy-based high-sensitive temperature measurement technology

The development of temperature sensors with ultra-fast response, high sensitivity, and wide range of temperature is of important academic value for the study of flame structure and combustion characteristics under extreme conditions. Recently, tunable diode laser absorption spectroscopy (TDLAS) is developed rapidly as an important means of combustion diagnostics due to its outstanding characteristics, such as non-invasiveness, rapid response, and high realizability. The absorption lines of TDLAS are extended to the mid-infrared region. Two spectral lines of H2O at center wavelength of 2136.14 and 1874.31 cm − 1 are used as candidate pair, which provides a wide measuring range (800 to 4000 K) and high-temperature sensitivity (4.55 at 1000 K) sensor for temperature measurement. We used two quantum cascade lasers with wavelength of 4.68 and 5.34 μm to scan the selected line pair of H2O rapidly in the combustion environment and obtained the temperature information of the combustion field. Our work provides a good solution for the temperature measurement of extreme combustion environment with ultra-fast response, high sensitivity, and wide temperature range.