Equivalence Ratio Measurements Research Articles

Machine Learning-Based Spectral Reconstruction for Equivalence Ratio Measurement in Premixed Air-Methane Flames Using RGB Imaging

ABSTRACT This paper introduced a machine learning-based method for reconstruction spectral information from RGB images for measurements of the equivalence ratio (Φ) of premixed air-methane flames. Digital color cameras capture color and spatial information of hydrocarbon premixed flames in the visible band. The color of the flame is the result of spectral integration of chemiluminescence in the visible wavelength band, and direct use for flame equivalence ratio measurements would result in large errors due to low spectral resolution. The mapping function was used to reconstruct the spectral characteristics of the premixed methane flame for the RGB image, which were built by three types of machine learning methods: support vector regression (SVR), random forest (RF) regression, and Levenberg-Marquardt backpropagation neural network (LM-BPNN), respectively. Finally, LM-BPNN-based reconstructed spectral features are used for the development of Φ measurement model, which measurement error below 0.01, accurately measures the equivalence ratio of premixed methane flames within the range of 0.73 to 1.47. Further analyze the two-dimensional spatial distribution of flame equivalence ratio.

Effects of gas temperature on equivalence ratio measurement in premixed methane–Air flame using laser-induced breakdown spectroscopy

Effects of gas temperature on equivalence ratio measurement in premixed methane–Air flame using laser-induced breakdown spectroscopy

Enhancing Accuracy of Flame Equivalence Ratio Measurements: An Attention-Based Convolutional Neural Network Approach for Overcoming Limitations in Traditional Color Modeling.

This paper addresses the inherent limitations in traditional color modeling techniques for measuring the flame equivalence ratio (Φ), particularly focusing on the subjectivity involved in threshold settings and the challenges posed by uneven 2D color distribution. To overcome these issues, this study introduces an attention-based convolutional neural network (ACN) model, a novel approach that transcends the conventional reliance on B/G color features (Tf). The ACN model leverages adaptive feature extraction, augmented by a spatial attention mechanism, to more effectively analyze flame images. By amplifying key features, autonomously minimizing background noise, and standardizing variations in color distribution, the ACN model in this experiment achieved a prediction accuracy of 99%, with a 76% reduction in error rate compared to the original model, significantly improving the accuracy and objectivity of flame Φ measurement. This method marks a substantial development in the precision and reliability of flame analysis.

Temporally modulating laser pulses to stabilize LIBS measurement locations under large gas temperature gradients

In combustion research, laser-induced breakdown spectroscopy (LIBS) has been widely employed in local equivalence ratio measurement. However, the potential temperature gradients in the probe volume can significantly affect the shape of induced plasmas, resulting in unstable measurement locations. In this work, we improved the stability of measurement locations by modulating the laser pulse duration. In a hot-cold gas flow interface with large temperature gradients, when using the original laser pulse with a full width at half maximum (FWHM) of 4 ns, the locations of initial plasma core were insensitive to gradient variations; however, the plasma expansion behaviors differed significantly after 3 ns. The hot spots of plasmas diverged bi-directionally under high temperature, resulting in two-lobe structures and unstable measurement locations. After the laser pulse was modulated to a shorter duration using a pressure chamber, the plasma expansion was suppressed which constrained the plasma volume. Specifically, using a modulated pulse with a FWHM of 1.9 ns, the two-lobe structure was eliminated across the interface, and the standard deviation of measurement locations was reduced to 0.27 mm. The measured equivalence ratios across the interface showed favorable agreement with the simulation.

Automatic LIBS baseline correction by physical-constrained airPLS method: a case of equivalence ratio measurement in high temperature after-burn gas

Automatic LIBS baseline correction by physical-constrained airPLS method: a case of equivalence ratio measurement in high temperature after-burn gas

Development of Equivalence Ratio Measurement Model of Premixed Methane Flames Using Hyperspectral Imaging of C2* and CH* Chemiluminescence and Random Forest Algorithm

ABSTRACT The C2*/CH* model is often used to measure the equivalence ratio of combustion. But only the band of C2*(0,0) at 516.5 nm of the C2* swan band system was employed as the input of the C2*/CH* model for equivalence ratio measurement. The relations of other components of the C2 swan band system with flame equivalence ratios and their impact on this equivalence ratio measurement are unreported. In this study, the visible emission spectrum of premixed methane flame, ranging from 425 to 700 nm, was measured/obtained by a hyperspectral imaging system under equivalence ratios from 0.72 to 1.52. The influence of the C2* swan bands on the C2*/CH* model was analyzed, and the intrinsic characteristics of the C2* swan bands were found. After that, a more sensitive soft measurement model for equivalence ratio measurement based on the ratio of C2*(0,0)/C2*(2,0) was proposed. The monotonicity and stability of the proposed model were discussed. At last, the proposed model was further validated by the random forest (RF) algorithm.

Feasibility on equivalence ratio measurement via OH*, CH*, and C2* chemiluminescence and study of soot emissions in co-flow non-premixed DME/C1–C2 hydrocarbon flames

The effects of dimethyl ether (DME) addition to methane and ethylene fuels on the combustion characteristics of heat release, soot emissions, and flame temperature were investigated experimentally and numerically in a non-premixed laminar flame configuration. The flame-heat release soot-volume fraction was measured experimentally using CH*, OH*, and C2* chemiluminescence and planar two-color soot pyrometry, respectively. The CH*, OH*, and C2* were used to locate flame-heat release regions as well as to investigate the soot signal’s effect on their measurements. The ratios of the chemiluminescence pairs (OH*/CH* and OH*/C2*) were studied for the feasibility of map local equivalence ratios. Numerical calculations across a full range of DME mixing ratios were performed through 1D laminar flame simulations implemented with a detailed mechanism to provide an indication of the flame structures and profiles of key species including OH*, OH, CH*, CH, CH3, C3H3, C2H2, heat release rate (HRR), and flame temperature. An existing developed soot model was used in a 2D computational study to investigate its validity for modeling soot for DME (oxygenated fuel)/C2H4/N2 flames. Parametric studies have been carried out on some key parameters in the soot model to find optimum values that can be used in future studies. Although soot radiation intensities increased at a small amount (25%vol) of DME addition in the DME/methane flames, the soot pyrometry results showed a reduced soot volume fraction with an increased DME mixture ratio in both DME/methane and DME/ethylene flames studied, agreeing with the key conclusion of 1D numerical results. The flame HRR decreases with the increasing addition of DME to methane and ethylene flames and correlates with the trend of OH* and CH* profiles. The 1D simulation showed a non-monotonic correlation between OH*/CH* ratios and equivalence ratios, implying a limited use of OH*/CH* for the equivalence ratio measurement in non-premixed flames with DME additions.

Measurements of the local equivalence ratio and its impact on the thermochemical state in laminar partially premixed boundary layer flames

Local fuel–air equivalence ratios, gas phase temperature and CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CO}_2$$\\end{document} mole fractions were measured by a combination of laser-induced fluorescence of nitric oxide used as a tracer and dual-pump coherent anti-Stokes Raman spectroscopy in a vertically oriented partially premixed boundary layer flame under laminar flow conditions. By embedding a secondary effusive fuel inlet into the temperature-controlled wall of a sidewall-quenching configuration, different pyrolysis rates of a wall-embedded polymer are mimicked with reduced chemical complexity and well-controlled boundary conditions. The resulting boundary layer flames were investigated experimentally and numerically. The simulation results and measurements show a very good agreement on the complex interplay between local mixing and heat losses, even though inhomogeneities of the wall inlet complicate the comparison of data. Local equivalence ratios upstream of the reaction zone reach values of up to Φ=2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varPhi = 2$$\\end{document}. Under these conditions, a clear quenching location could not be identified based on the experimental data. A significant trend toward lower temperatures and CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CO}_2$$\\end{document} mole fractions with increasing amount of secondary fuel was found in the thermochemical state close to the temperature-controlled wall, downstream of the effusive inlet.Graphical abstract

Gas mixture conditions conducive to backdraft phenomenon

This work examines the influence of gas mixture compositions on backdraft phenomenon. A series of experiments were conducted in a reduced-scale enclosure subjected to various fire configurations, including a 25.0 kW and a 37.5 kW methane fire and a 16.7 kW and a 25.0 kW propane fire. Three different metrics were used to evaluate backdraft phenomenon: (1) internal conditions within the enclosure before an anticipated backdraft; (2) ignition of a local gas mixture resulting in a backdraft; and (3) intensity of resulting backdraft. The gas mixture composition within the compartment before a backdraft was determined using measurements obtained from an enhanced phi meter and gas analyzer. Ignition resulting in a backdraft was evaluated from the equivalence ratio and gas species concentration measurements surrounding a triggered spark ignitor. Backdraft intensity was analyzed by quantifying the total heat release of the exiting fireball via carbon dioxide generation calorimetry. Real-time equivalence ratio measurements showed the progressive state of the gas mixture composition within the compartment before ignition. Backdraft was observed to occur when the gas mixture surrounding the ignition soure was greater than stoichiometric conditions. The total heat release of backdrafts was found to correspond to the initial mass fraction of fuel residing within the compartment for experiments with propane fires.

Experimental and Simulated Study of the Relationship Between Color Camera Imaging and Color-Modeled Equivalence Ratio Measurement

A simulation methodology was proposed to reconstruct color images under different camera spectral sensitivity functions ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CSSFs</i> ) by integrating the hyperspectral images. Based on this proposed simulation method, the effects of different <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CSSFs</i> , Bayer patterns, and demosaicing algorithms on color-modeled <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CH</i> * and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C₂</i> * measurement have been investigated. Firstly, initial full-color images were reconstructed under the CIE 1931 XYZ matching function ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CMF</i> ). Then, the Bayer pattern encoded images were obtained from the initial reconstructed images by removing the pixel values according to different Bayer patterns. Next, three different conventional demosaicing algorithms were employed to calculate the full interpolated color images, and last, the effects of demosaicing algorithms on color-modeled <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CH</i> * and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C₂</i> * measurement were analyzed. It was found that the effects of Bayer patterns and demosaicing algorithms on color-modeled global equivalence ratio (Φ) measurement were limited, whereas the most significant impact is <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CSSF</i> . Therefore, an improved color-modeled Φ measurement model was also proposed using a color correction algorithm under <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CMF</i> , which may provide a quick, efficient, and generalized method for color-modeled Φ measurement.

Numerical Investigation of the Fuel/Air Ratio Sensor Sensitivity in a Port-Fuel-Injected Spark-Ignition Engine Equipped With Three-Way Catalysts

Abstract The internal combustion engine will continue to be the primary source of power for transportation. Spark ignition (SI) engines are still widely used for mobility due to their wide range of operating conditions. The key operating variables of an engine are primarily controlled by an engine control unit that has been calibrated. However, a less accurate sensor can lead to large variations in engine performance and emissions. The purpose of this study was to investigate the importance of air–fuel ratio sensor precision during operation of various engines. In this study, a one-dimensional (1D) computational fluid dynamics (CFD) model was used to analyze the engine response due to the variation of the equivalence ratio sensor precision at different engine speeds and loads, to explore the main indicators influenced by the precision of equivalence ratio measurements, and to propose a discriminant criterion for evaluating the suitability of the proposed equivalence ratio precision in relation to the conversion rate of three-way catalyst and vehicle emissions. The results show that for engine performance, it varies slightly with small changes in the fuel-to-air ratio. At higher engine speeds, a slight change in the air–fuel ratio leads to a smaller change in emissions. At the same time, changes in fuel-to-air ratio have a significant effect on carbon monoxide (CO) and nitrogen oxides (NOx) emissions. Carbon monoxide is the most sensitive to the air–fuel ratio, followed by nitrogen oxides, while unburned hydrocarbons are not sensitive to it. And for the three measurement accuracies studied in this paper (0.5%, 1%, and 2%), the accuracies are acceptable, but combining the relative errors of the actual emissions of CO and in order to achieve accurate combustion control, it is recommended that the sensor accuracy should be at least higher than 1% for the port fuel injected engine investigated in this study.

One-dimensional equivalence ratio measurements by femtosecond laser filament-triggered discharge plasma spectroscopy

Equivalence ratio measurements are of great importance for combustion systems. In this paper, femtosecond laser filament-triggered discharge plasma spectroscopy is proposed for one-dimensional equivalence ratio measurements of combustion flow fields. The optical emission spectra of femtosecond laser filament-triggered discharge plasma in methane-air premixed laminar flames were measured. The spectral intensity ratios of different species show a linear correlation with the equivalence ratio. It can be demonstrated that femtosecond laser filament-triggered discharge plasma spectroscopy can be used for equivalence ratio measurements. We investigated the effect of the temperature of the methane-air mixture on the equivalence ratio measurements. We further analyzed the one-dimensional spatial intensity distributions of the plasma spectral lines. The results demonstrated the capability of femtosecond laser filament-triggered discharge plasma spectroscopy for one-dimensional equivalence ratio measurements. Finally, femtosecond laser filament-triggered discharge plasma spectroscopy was applied to a methane diffusion flame and measured the one-dimensional equivalence ratios of the diffusion flame at different heights. Femtosecond laser filament-triggered discharge plasma spectroscopy offers a new method for one-dimensional equivalence ratio measurements of combustion flow fields.

Four-Line C2*/CH* Optical Sensor for Chemiluminescence Based Imaging of Flame Stoichiometry

In the present work, an optical sensor was developed and calibrated for the purpose of non-intrusive equivalence ratio measurements in combustion systems. The sensor incorporates a unique four-line, single-sensor chemiluminescence imaging-based approach, which relies on the ratio of C2* and CH* radical-species intensities to obtain measurements of equivalence ratios. The advantage of the four-line sensor is the use of additional filtering to mitigate broadband luminescence signals, and its improvements over conventional two-line chemiluminescence diagnostics are discussed. The sensor was calibrated using a premixed bluff-body jet burner with a propane–air flame operating over a wide range of equivalence ratios. The results showed that the four-line processing technique improved the signal-to-noise ratio of the chemiluminescence images for all test cases. Calibrations of C2*/CH* intensity ratio to equivalence ratio were developed for both the four-line and two-line techniques. The calibrations were then used to create maps of local equivalence ratios in the flame-holding region. The maps revealed a non-uniform field of equivalence ratios due to the nature of the radical-species intensity profiles within the flame. Therefore, special consideration is required for calibration in order to accurately quantify equivalence ratios and apply these to diffusion flames.

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.

Laser-induced breakdown spectroscopy for local equivalence ratio measurement in opposed jet methane-air flames

Laser-induced breakdown spectroscopy (LIBS) has been applied to both non-reacting and reacting opposed jet flows to evaluate the measurement accuracy of instantaneous, local air-fuel ratio. However, the LIBS signal is much weaker in flames than in the corresponding non-reacting gases and more work is required to investigate the potential of this technique for instantaneous measurements in reacting flows and quantify measurement uncertainties. The present study reported local equivalence ratio measurements of methane/air flames in an opposed-jet burner, which allowed independent control of flow rate and air-fuel ratio. Images and emission spectra of laser-induced plasma were investigated simultaneously using both an Intensified CCD (ICCD) and a spectrometer with a high degree of spatial and temporal resolution. The influences of the camera delay time, exposure time and laser pulse energy on the LIBS measurements were investigated as well. The dependence of spectral intensity ratios of H/O and C2/CN on air-fuel ratio was quantified over a wide range of conditions extending from pure air to pure fuel. It was found that H/O and C2/CN intensity ratios depend monotonically on the mole fraction of methane in the ranges of 0.0–0.8 and 0.3–1.0 respectively. The presence of a flame within the laser beam led to significant measurement deterioration relative to the corresponding non-reacting flows. This could lead to increased measurement uncertainty, and was therefore corrected by increasing the laser pulse energy and applying a proposed data processing method; the proposed correction method was able to reduce the equivalence ratio measurement uncertainty to be within 10% for mixtures with methane mole fractions lower than 50% and within 15% for mixtures with higher methane mole fraction. The established correlations between the intensity ratios of H/O and C2/CN with local air-fuel ratio allowed measurement of the spatial gradient of air-fuel ratio across opposed jet diffusion flames. The LIBS measurements of air-fuel ratio in non-premixed flames were finally compared successfully with CHEMKIN simulations, which demonstrates the ability of LIBS to accurately measure the spatial gradient of the air-fuel ratio.

Enhancing the phi-meter by incorporating carbon dioxide measurement

This paper describes the development, calibration and experimental application of an enhanced equivalence ratio measurement apparatus, or ‘phi-meter’. The original phi-meter developed by Babrauskas et al. used only an oxygen analyser and chemically removed the CO2. In this study, an enhanced phi-meter was developed to measure the equivalence ratio from experimental enclosure fires by O2 and CO2 measurements. As with all combustion calorimetry, the accuracy of the results increases with the number of species being directly measured and removing the need for chemical removal of the CO2 is an advantage. This is of practical importance as scrubbing agent required to remove CO2 from the combustion products requires careful handling and increases test costs. Moreover, in severely under-ventilated conditions, the production of CO significantly increases, and its measurement by phi-meter provides real-time validation of the occurrence of complete combustion within the phi furnace. The original equations derived for the phi-meter incorporated the depletion of oxygen, whereas the phi-meter equations developed in this paper incorporate O2 and CO2 measurements. The equivalence ratios measured in compartment fires for heptane, LPG and timber crib fuels ranging from 0.2 for oxygen-rich to 3.0 for fuel-rich conditions are found to agree with theoretical values.

Optical Equivalence Ratio Measurement of a Dual Fuel Burner for Natural Gas and Kerosene

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.

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.

A Numerical Analysis of the Effects of Equivalence Ratio Measurement Accuracy on the Engine Efficiency and Emissions at Varied Compression Ratios

Increasingly stringent regulations to reduce vehicle emissions have made it important to study emission mitigation strategies. Highly accurate control of the air-fuel ratio is an effective way to reduce emissions. However, a less accurate sensor can lead to reduced engine stability and greater variability in engine efficiency and emissions. Additionally, internal combustion engines (ICE) are moving toward higher compression ratios to achieve higher thermal efficiency and alleviate the energy crisis. The objective of this investigation was to analyze the significance of the accuracy of air-fuel ratio measurements at different compression ratios. In this study, a calibrated 1D CFD model was used to analyze the performance and emissions at different compression ratios. The results showed that carbon monoxide (CO) and nitrogen oxides (NOx) were sensitive to the equivalence ratio regardless of the compression ratio. With a slight change in the equivalence ratio, a high compression ratio had little effect on the change in engine performance and emissions. Moreover, with the same air-fuel ratio, an excessively high compression ratio (CR = 12) might result in knocking phenomenon, which increases the fluctuation of the engine output parameters and reduces engine stability. Overall, for precise control of combustion and thermal efficiency improvement, it is recommended that the measurement accuracy of the equivalence ratio is higher than 1% and the recommended value of the compression ratio are roughly 11.

In situ determination of the equivalence ratio in a methane/air flow field by femtosecond filament excitation

The equivalence ratio is a key parameter in understanding pyrolysis, combustion and gasification of fuels for improving combustion efficiency and lowering net pollution emissions. In this study, we investigate the feasibility of measuring the local equivalence ratio in a gaseous flow field using femtosecond filament excitation. We demonstrate that quasi-collimated-beam-induced filamentation in a premixed methane/air flow can produce a long and uniform free-of-atomic-line emission channel, and that the intensity ratios of the molecular emissions in this channel, such as methylidyne radical (CH) (331 nm)/N2 (337 nm), CH (331 nm)/N2 (357 nm), and CH (331 nm)/CN (388 nm), are all linear correlations to the equivalence ratios of the methane/air flow. Our results provide a possibility for the in situ measurement of long-range and multi-dimensional equivalence ratio of flow fields with femtosecond filament excitation.