Flame Emission Measurements Research Articles

Laminar burning velocity, emissions, and flame structure of dimethyl ether-hydrogen air mixtures

The present work reports experimentation and numerical simulations of laminar burning velocity, emission analysis, and flame structure of dimethyl ether-hydrogen mixtures. The volumetric hydrogen addition is considered into the DME fuel for a range of mixture equivalence ratios (0.6<ϕ < 1.7). The initial mixture temperature is raised to 373 K at constant atmospheric pressure of 1.0 bar. The heat flux method is utilized to investigate the laminar burning velocity in laminar premixed planar adiabatic flames. The laminar burning velocity (LBV), along with the flame position and emission measurements, are extracted from these flames. The computations were performed with freely propagating flames (LBV, species, sensitivity, etc. data), and the burner stabilizes flames (flame position and emission analysis) for the same mixture and initial conditions using various detailed mechanisms. An emphasis is paid to a newly developed Shrestha mechanism, which includes a radiation model and NO sub-mechanism. Both experimental data and competition results were validated against the pure DME air mixture data available in the existing literature, and further analysis was carried out. It is observed that planar flames can be stabilized only up to a hydrogen addition of 40%. Adding hydrogen enhanced the flame temperature, heat release rates, and burning velocity while reducing the CO and NO emissions. A slight shift of peak burning velocity on the rich side of the equivalence ratio (from 1.05 to 1.2) is observed with hydrogen addition (0–40%). The mixture temperature also enhances the flame temperature and laminar burning velocity. It is interesting to note that hydrogen-added mixtures show a relatively lower dependency on initial mixture temperature. The variation of dominant species and heat release rates are also discussed in detail. The sensitivity analysis is carried out to get information on dominant reactions participating in the combustion of DME hydrogen mixtures.

Early particle formation and evolution in iron-doped flames

In flame synthesis of nanoparticles, the temperature history experienced by the nascent particle aerosol defines the morphology, composition, and crystallinity of the resulting nanomaterial. Commonly, flame-synthesis processes are modeled with an isothermal approximation assuming that the particle temperature replicates that of the surrounding gas phase, avoiding inclusion of an additional internal coordinate in the population balance model, and thus reducing the computational cost. This simplification neglects the influence of matter- and energy-exchange as well as thermochemistry between the particle and reactive gas phase, impacting the particle temperature. In this work, we investigate the temperature history of the particles in incipient formation stages and their evolution in iron-doped flames, prototypical for many other transition-metal (oxide) synthesis systems. The temperature and relative volume-fraction distributions of early particles forming in H2/O2/Ar flames doped with iron pentacarbonyl were determined for the first time, based on spectrally and spatially resolved flame emission measurements and pyrometric analysis of the continuum emission emanating from the nascent particle aerosol. The nascent particle temperature was found to be several hundred degrees above the gas-phase temperature for all physically reasonable assumptions concerning particle composition and emission efficiency. Early particles volume fraction rises sharply shortly after the decomposition of iron pentacarbonyl and decreases steeply in the flame front, in excellent agreement with previous particle-mass spectrometry/quartz-crystal microbalance measurements. By modeling the evaporation process of isothermal iron particles, we show that vanishing of particles in the flame front cannot be explained by evaporation of particles that are in thermal equilibrium with the gas phase. A single-particle Monte-Carlo simulation based on a simple model comprising Fe-monomer condensation, concurrent with oxidation, reduction, etching, and evaporation occurring at the particle surface, captures both the flame structure with respect to early particle formation and their excess temperature compared to the gas phase.

Experimental investigation and demonstration of pilot-scale combustion of oil-water emulsions and coal-water slurry with pronounced water contents at elevated temperatures with the use of pure oxygen

In this study, a high-temperature combustion chamber was fueled with oil-water-emulsions (OWE) and coal-water-slurry (CWS) from pyrolytic carbon black in order to experimentally examine the feasibility of co-processing low-calorific value fuels in industrial high-temperature applications by using pure oxygen as oxidizer. The present paper is the first report on the oxy-fuel combustion characteristics of liquid fuels with a pronounced water content including flame characteristics, temperature distribution, thermal output and emission measurements in a semi-industrial combustion chamber at a thermal input of 300 kW and ambient pressure. The obtained results demonstrated the beneficial character of oxy-fuel combustion and showed that co-processing of OWE and CWS from pCB with pronounced water contents is technically feasible. A stable oxy-fuel combustion of OWE was achieved with water contents of up to 75 wt%. In addition, a successful combustion of CWS with a water content of 65 wt% and oxygen concentrations above 50 vol.% was observed. The obtained results further demonstrated the beneficial character of oxy-fuel combustion of OWE and CWS especially with regard to reduced NOx emissions. The combustion of OWE with a water content of 75 wt% resulted in a significant reduction of the NOx concentration compared to pure heating oil, namely from 760 to 110 ppm, whereas the maximum temperature of the combustion chamber decreased only slightly, while in the case of CWS combustion, the concentration of CO emitted decreased significantly from 34 to 9 ppm by increasing the oxygen concentration in the oxidizer from 50 to 100 vol.%.

Upconversion-enabled array spectrometer for the mid-infrared, featuring kilohertz spectra acquisition rates.

Mid-infrared spectroscopy is an essential analytical method in science and industry. Unlike in the near-infrared range, grating spectrometers for the mid-infrared are rarely employed, mostly due to the limited availability and performance of suitable line array detectors. In this work, continuous-wave nonlinear-optical upconversion is used to enable mid-infrared spectroscopy. A broad spectral window between 3.7 and 4.7 μm is upconverted to 825 - 867 nm for detection on a silicon-camera-based near-infrared grating spectrometer with a high sensitivity down to sub-picowatt of input power. A theoretical model is presented that accurately describes the upconversion process and the total system behavior. Spectroscopic flame emission measurements demonstrate the applicability towards the analysis of highly dynamic processes.

KHz-rate particle-image velocimetry of induced instability in premixed propane/air flame by millisecond pulsed current–voltage

Particle-image velocimetry (PIV) measurements were performed at 6kHz repetition rate in a premixed propane/air flame to examine the effects caused by applied millisecond-wide pulsed voltage–current below self-sustained breakdown. We have demonstrated significant structural changes to a burner-stabilized downward-propagating atmospheric pressure propane/air flame with overall flow speeds near 2m/s with +3kV pulsed applied voltages over 30mm gaps. Phase-locked, 2kHz broadband emission measurements of flame structure were also collected to support the PIV velocity data. The combined high-speed PIV and flame emission measurements were both capable of capturing changes from a single applied voltage pulse rather than using a phase matching approach requiring a highly repeatable disturbance as done previously [1]. The measured reductions in flame height, increases in local flow speeds, generation of large velocity gradients, and rapid oscillations in flame front are suggestive of an induced turbulence in an otherwise laminar flame. Taylor microscale lengths were calculated from the kHz PIV data and structures comparable to the reaction zone thickness were shown to increase during the applied voltage pulse. The timescale under which the flame flow changes combined with the accompanying flame emission measurements suggest that flame fluidics are modified by ion drift current induced net body force in or near the cathode fall at the base of the flame. The reduction in overall flame height and increase in speed near the base of the flame is suggestive of a ‘virtual’ bluff-body present in the flow. These fluidic changes force the flame to transition from a laminar to a highly unstable, transitioning to turbulence regime.

Spectroscopic measurements of premixed combustion in diesel engine

Digital imaging and spectroscopic techniques, with high temporal and spatial resolution, were applied in order to study the low temperature combustion process. Injection and combustion phases were analysed by digital imaging. Mixing process, autoignition and pollutants formation were investigated by broadband ultraviolet–visible extinction spectroscopy and flame emission measurements. Moreover, fuel distribution and oxidation were studied as well. Liquid fuel and vapour phase, injected around the top dead centre, were analysed. The liquid diesel fuel was observed by extinction measurements when the liquid jet reached the bowl rim and aromatic compounds due to fuel decomposition were identified. On the other side, the vapour fuel was detected about 2° after the injection start and liquid fuel disappeared. Then, radicals and species were detected in the combustion chamber. They are interesting in order to study the chemical kinetics of low temperature combustion process. The chemiluminescence spectra of HCCI combustion appeared as well as several distinct peaks corresponding to the emission from HCO, HCHO, CH, and OH. In particular, this latter was clearly evident during the whole premixed combustion and dominated the process also after the end of the premixed phase of the heat release. Advancing the combustion, bright spots due to not homogeneous charge were detected. They were the source of the very little soot amount detected at the exhaust pipe. Finally, the injection pressure effect on the development of low temperature combustion was analysed.

Combustion dynamics for energetically enhanced flames using direct microwave energy coupling

An atmospheric high-Q re-entrant cavity applicator is used to couple microwave (2.45 GHz) electromagnetic energy directly into the reaction zone of a premixed laminar methane–oxygen flame for flame enhancement. As microwave energy increases, a transition from electric field enhancement to microwave plasma discharge is observed. At low microwave powers (1–5 W), the flame is influenced by an electromagnetic field only. When power is increased, ionization and eventually breakdown of gas molecules result in a plasma plume with significant increase in the flammability limit. 2-D laser induced fluorescence imaging of hydroxyl radicals (OH) and carbon monoxide (CO) are conducted in the reaction zone over this transition, as well as spectrally resolved flame emission measurements to monitor excited state species and derive rotational temperatures using OH chemiluminescence for a range of equivalence ratios ( ϕ = 0.9–1.1) and total flow rates. In the electromagnetic field only phase (1–5 W), flame stability, excited state species, and temperature slightly increased with power while no significant change in OH number density was detected. With the onset of a plasma plume, a significant rise in both excited state species, CO and OH number density was observed. The importance of in-situ fuel reforming in plasma coupled flames is shown through the concentration of CO, which increases ∼18% with 30 W microwave power.

Determination of particle temperatures in a silica-generating counterflow flame via flame emission measurements

We propose a simple technique to measure particle temperatures in a particle generating counterflow flame. The silica particle temperature was derived from flame light emission measurements. This technique allows the non-intrusive measurement of particle temperatures over 2000 K. In addition, the OH concentration distribution in the hydrogen–oxygen flame was estimated from flame emission spectra in the ultraviolet region. A numerical model of the combustion processes, which included the reactions of SiCl 4 leading to the formation of silica particles, verified that the measured particle temperatures and OH concentration were close to the theoretical values.

Laminar hydrocarbon flame structure

From the very first experimental studies, based essentially on flame propagation velocity and flame emission measurements, successive improvements in analytical and numerical techniques have contributed to make the analysis of laminar flame structure a powerful tool for extending the knowledge on combustion chemistry, thermodynamics, and transport properties. This better knowledge is very beneficial to design efficient combustion devices with reduced pollutant emission. Overall net species production rates are derived from the experimental determination of the evolution of the gas stream velocity, temperature, and species concentrations in the direction normal to the flame front. For species involved in a limited number of reactions, rate constants can be calculated at the next step. The development, in the early 1980s, of numerical codes for simulating the structure of one-dimensional laminar premixed flames the flame structure data to be directly used for validating detailed reaction mechanisms. Species analyses are still performed with techniques based on local gas sampling by probes, despite flame perturbations, but flame structure analyses have been markedly enriched by the use of non-intrusive spectroscopic techniques. The former allow the analysis of a large variety of species, and they have proven to be very well adapted to the large number of intermediate species formed in rich flames or in flames fed by heavy fuel molecules. The molecular beam mass spectrometry technique has been recently improved by the use of new photoionization sources that allow identification of isomers and extend the knowledge on intermediate species involved in formation of benzene, polycyclic aromatic hydrocarbons, and soot in flames. Amongst various spectroscopic techniques applied to flame structure analyses, laser-induced fluorescence has been largely used to perform accurate quantitative measurements of intermediate radicals that play a key role in the prompt-NO mechanism. In this study, the contribution of flame structure studies to a better knowledge of formation mechanisms of benzene and NOx is briefly reviewed.

Computational and experimental study of a forced, time-dependent, methane–air coflow diffusion flame

Forced, time-varying flames are laminar systems that help bridge the gap between laminar and turbulent combustion. In this study, we investigate computationally and experimentally the structure of a periodically forced, axisymmetric laminar methane–air diffusion flame in which a cylindrical fuel jet is surrounded by a coflowing oxidizer jet. The flame is forced by imposing a sinusoidal modulation on the steady fuel flow rate. Rayleigh and spontaneous Raman scattering are used to generate the temperature and major species profiles. Particle image velocimetry is used to determine the magnitude of the velocity at the exit of the burner and the phase of the forcing modulation. CH ∗ flame emission measurements are used to provide an indication of the overall flame shape. Computationally, we solve the transient equations for the conservation of total mass, momentum, energy, and species mass with detailed transport and finite rate chemistry submodels. The governing equations are written using a modified vorticity–velocity formulation and are solved on an adaptively refined grid using implicit time stepping and Newton’s method nested with a Bi-CGSTAB iterative linear system solver. Results of the study include an investigation of the start-up features of the time-dependent flames and the time it takes for initial transients to dissipate. We include a detailed description of the fluid dynamic-thermochemical structure of the flame at a 20 Hz forcing frequency for both 30% and 50% sinusoidal velocity perturbations. Comparisons of experimentally determined and calculated temperature, CO and H 2O mole fraction profiles provide verification of the accuracy of the model.

Deflagration to Detonation Transition Processes by Turbulence-Generating Obstacles in Pulse Detonation Engines

The results from a series of detonation experiments conducted to characterize the deflagration-to-detonation transition (DDT) process for ethylene-air mixtures in a 44-mm-square, 1.65-m-long tube are described. Experiments were conducted for both single-shot detonations involving quiescent mixtures as well as multicycle detonations involving dynamic fill. For the experiments, high-frequency pressure and flame emission measurements were made to obtain the compression wave and flame speeds, respectively. In addition, schlieren and hydroxyl-radical/planar-laser-induced-fluorescence (OH-PLIF) imaging were applied to investigate the interactions between the shock-wave and combustion phenomena during both deflagration and detonation. For ethylene-air mixtures, strategically placed obstacles were necessary to achieve DDT. The effect of the presence of obstacles on flame acceleration was systematically investigated by changing the obstacle configuration. The parametric study of obstacle blockage ratio, spacing between obstacles, and length of the obstacle configuration indicated that for successful detonations the obstacle needs to accelerate the flame to a minimum flame speed of roughly half the Chapman-Jouguet detonation velocity. Differences in the flame and compression wave velocities demonstrated the development of a coupled feedback mechanism as the wave propagated along the tube. A series of simultaneous schlieren and OH-PLIF images showed that the obstacle plays a major role in generating small/large-scale turbulence that enhances flame acceleration. Localized explosions of pockets of unburned mixture further enhanced the shock-wave strength to continuously increase the flame speed. The results of this experimental study support the importance of obstacles as a means to enhance DDT and provide a potential solution for practical pulse-detonation-engine applications.

Local Chemiluminescence Spectra Measurement in Laminar Methane/Air and Propane/Air Premixed Flames.

Spatially resolved chemiluminescent spectra were obtained in the reaction zone in a laminar premixed methane/air and propane/air flame (φ = 0.9-1.5) using local flame emission measurements with a new Cassegrain optics and spectrometers. Highly spectrally resolved OH*, CH* and C*2 emission were obtained at the local flame front in both methane and propane. Detailed structure of methane spectra and propane spectra were corresponding each other very well. The linear correlations between the peak intensity rations of OH*/CH*, C*2/CH*, and C*2/OH* to the equivalence ratio were estimated using the power fitting of y = A×xB in experimentally. These functions could be applicable to the prediction of local stoicheometry in hydrocarbon flames. Futhermore, chemiluminescnecne spectra dependence on temperature were examined and it was found that spectral structure less depended on temperature.

An experimental estimation of mean reaction rate and flame structure during combustion instability in a lean premixed gas turbine combustor

The interactions between the local flame structure produced by a premixed swirl-stabilized injector with combustion instabilities were experimentally studied for a model gas turbine combustor operating at high pressure and temperature. The model gas turbine combustor studied utilizes a sudden-expansion dump combustor with a single swirler and bluff body for enhancing mixing rate and flame stabilization, respectively. Laser-based measurements were made for both stable and unstable operating conditions. The local flame front structure was visualized using planar laser-induced fluorescence (PLIF) from the OH⊙, and the global heat release zone was interpreted from flame emission measurements. For stable combustion conditions, the mean reaction rate estimated independently from both OH-PLIF and OH * chemiluminescence measurements showed good agreement, thereby indicating confidence in the use of OH-PLIF measurements for extracting the local mean reaction rate. For unstable combustion conditions, the flamefront characteristics, including flame surface density and mean reaction rate, were evaluated together with the information from the OH * chemiluminescence measurements to identify the boundary of the heat release region at discrete phases of the unstable flame. Analysis of the flame structures during combustion instability indicated significant variations during different phases of the instability. The heat release flow field, particularly in the recirculation regions appearing at the corner and inner face of the dump plane, varied substantially. Rayleigh index information indicated that the recirculation zones play an important part in driving the instability. In contrast, the high shear layer formed along the interface between reactants and hot products produced a region where the instability was damped due to a lowering of the heat release.

Spatially and Spectrally Resolved Measurement of Chemiluminescence in Laminar Methane/Air Premixed Flames. Correlation between Emission Intensity Ratio of OH*/CH* to the Equivalence Ratio.

Spatially resolved chemiluminescent spectra were obtained in the reaction zone in a laminar premixed methane/air flame (φ=0.9-1.5) using local flame emission measurements with a new Cassegrain mirror system and spectrometers. The distributions of the OH*, CH*, and C2* emission intensities in the flame front were obtained at atmospheric pressure using this method. C2* was found to have the thinnest reaction zone. Highly spectrally resolved OH*, CH* and C2* emission were obtained at the local flame front. The linear correlations between the peak intensity ratios of OH*/CH*, C2*/CH*, and C2*/OH* in the reaction zone to the equivalence ratio could be used to investigate the local flame stoichiometry. OH*/CH* could be used to determine the local flame stoichimetry in the reaction zone of premixed flames for a wider range of equivalence ratios.

Experimental and computational study of CH, CH *, and OH * in an axisymmetric laminar diffusion flame

In this study, we extend the results of previous combined numerical and experimental investigations of an axisymmetric laminar diffusion flame in which difference Raman spectroscopy, laser-induced fluorescence (LIF), and a multidimensional flame model were used to generate profiles of the temperature and major and minor species. A procedure is outlined by which the number densities of ground-state CH (X(sup 2)II) excited-state CH (A(sup 2)Delta, denoted CH*), and excited-state OH (A(sup 2)Sigma, denoted OH*) are measured and modeled. CH* and OH* number densities are deconvoluted from line-of-sight flame-emission measurements. Ground-state CH is measured using linear LIF. The computations are done with GRI Mech 2.11 as well as an alternate hydrocarbon mechanism. In both cases, additional reactions for the production and consumption of CH* and OH* are added from recent kinetic studies. Collisional quenching and spontaneous emission are responsible for the de-excitation of the excited-state radicals. As with our previous investigations, GRI Mech 2.11 continues to produce very good agreement with the overall flame length observed in the experiments, while significantly under predicting the flame lift-off height. The alternate kinetic scheme is much more accurate in predicting lift-off height but overpredicts the over-all flame length. Ground-state CH profiles predicted with GRI Mech 2.11 are in excellent agreement with the corresponding measurements, regarding both spatial distribution and absolute concentration (measured at 4 ppm) of the CH radical. Calculations of the excited-state species show reasonable agreement with the measurements as far as spatial distribution and overall characteristics are concerned. For OH*, the measured peak mole fraction, 1.3 x 10(exp -8), compared well with computed peaks, while the measured peak level for CH*, 2 x 10(exp -9), was severely underpredicted by both kinetic schemes, indicating that the formation and destruction kinetics associated with excited-state species in flames require further research.

Computational and experimental study of a forced, timevarying, axisymmetric, laminar diffusion flame

Forced, time-varying flames are laminar systems that help bridge the gap between laminar and turbulent combustion. In this study, we investigate computationally and experimentally the structure of an acoustically forced, axisymmetric laminar methane-air diffusion flame, in which a cylindrical fuel jet is surrounded by a coflowing oxidizer jet. The flame is forced by imposing a sinusoidal modulation on the steady fuel flow rate. Rayleigh scattering and spontaneous Raman scattering of the fuel are used to generate the temperature profile. Particle image velocimetry (PIV) is used to measure the fuel tube exit velocity over a cycle of the forcing modulation. CH flame emission measurements have been done to predict the excitedstate CH (CH * ) levels. Computationally, we solve the transient equations for the conservation of total mass, momentum, energy, and species mass with detailed transport and finite-rate C 2 chemistry submodels to predict the pressure, velocity, temperature, and species concentrations as a function of the two independent spatial coordinates and time. The governing equations are written in primitive variables. Implicit finite differences are used to discretize the governing equations and the boundary conditions on a nonstaggered, noniumiform grid. Modified damped Newton's method nested with a Bi-CGSTAB iteration is utilized to solve the resulting system of equations. Results of the study include a detailed description of the fluid dynamic-thermochemical structure of the flame at a 20-Hz frequency. A comparison of experimentally determined and calculated temperature profiles and CH * levels agree well. Calculated mole fractions of species indicative of soot production (C 2 H 2 , CO) are compared against those levels in the corresponding steady flame and are observed to increase in peak concentration values and spatial extent. Analysis of acetylene production rates reveals additional significant production in the downstream region of the flame at certain times during the flame's cyclic history.

Experimental and Theoretical Comparison of the Precision of Flame Atomic Absorption, Fluorescence, and Emission Measurements

Theoretical equations and experimental evaluation procedures for the determination of the precision of flame atomic absorption, emission, and fluorescence measurements are presented. These procedures and noise power spectra are used to evaluate the precision and noise characteristics of atomic copper measurements with all three techniques under the same experimental conditions in an H2-air flame. At the detection limit, emission and fluorescence measurements are limited by background emission shot and flicker noise whereas absorption measurements are limited by flame transmission lamp flicker noise. Analyte flicker noise limits precision at higher analyte concentrations for all three techniques. Fluctutations in self-absorption and the inner filter effect are shown to contribute to the noise in atomic emission and fluorescence measurements.