Flame Chemiluminescence Research Articles

Three-dimensional reconstruction method for flame chemiluminescence distribution with complicated structure.

In this study, reconstruction of flame chemiluminescence distribution with complicated structure was numerically investigated and experimentally validated. The ill-conditioned equations were constructed using the quasi-Monte Carlo method and solved by an algebraic reconstruction technique, where the convergence criterion was the Euclidean norm of the dimensionless displacement vector. Results of a phantom study revealed that the number of camera angles is the main restriction on reconstruction accuracy, and increase of the flame's nonhomogeneity improves the sensibility of reconstruction accuracy to image resolution. Results of experimental reconstruction showed the CH* distribution in a Meker burner flame. This work provides a better understanding in how to establish experimental systems for complicated flame reconstruction.

Flame front analysis of ethanol, butanol, iso-octane and gasoline in a spark-ignition engine using laser tomography and integral length scale measurements

Direct-injection spark-ignition engines have become popular due to their flexibility in injection strategies and higher efficiency; however, the high-pressure in-cylinder injection process can alter the airflow field by momentum exchange, with different effects for fuels of diverse properties. The current paper presents results from optical studies of stoichiometric combustion of ethanol, butanol, iso-octane and gasoline in a direct-injection spark-ignition engine run at 1500 RPM with 0.5 bar intake plenum pressure and early intake stroke fuel injection for homogeneous mixture preparation. The analysis initially involved particle image velocimetry measurements of the flow field at ignition timing with and without fuelling for comparison. Flame chemiluminescence imaging was used to characterise the global flame behaviour and double-pulsed Laser-sheet flame tomography by Mie scattering to quantify the local topology of the flame front. The flow measurements with fuel injection showed integral length scales of the same order to those of air only on the tumble plane, but larger regions with scales up to 9 mm on the horizontal plane. Averaged length scales over both measurement planes were between 4 and 6 mm, with ethanol exhibiting the largest and butanol the smallest. In non-dimensional form, the integral length scales were up to 20% of the clearance height and 5–12% of the cylinder bore. Flame tomography showed that at radii between 8 and 12 mm, ethanol was burning the fastest, followed by butanol, iso-octane and gasoline. The associated turbulent burning velocities were 4.6–6.5 times greater than the laminar burning velocities and about 13–20% lower than those obtained by flame chemiluminescence imaging. Flame roundness was 10–15% on the tomography plane, with largest values for ethanol, followed by butanol, gasoline and iso-octane; chemiluminescence imaging showed larger roundness (18–25%), albeit with the same order amongst fuels. The standard deviation of the displacement of the instantaneous flame contour from one filtered by its equivalent radius was obtained as a measure of flame brush thickness and correlated strongly with the equivalent flame radius; when normalised by the radius, it was 4–6% for all fuels. The number of crossing points between instantaneous and filtered flame contour showed a strong negative correlation with flame radius, independent of fuel type. The crossing point frequency was 0.5–1.6 mm−1. The flame brush thickness was about 1/10th of the integral length scale. A positive correlation was found between integral length scale and flame brush thickness and a negative correlation with crossing frequency.

Multi-directional 3D flame chemiluminescence tomography based on lens imaging.

Flame chemiluminescence tomography (FCT) has been widely used in flame diagnostics for three-dimensional (3D), spatially resolved measurements of instantaneous flame geometry and, to some extent, of species concentrations. However, in most studies, tomographic reconstructions are based on a traditional parallel projection model. Due to the light collection characteristics of a lens, a parallel projection model is not appropriate for the practical optical setups that are used for emission imaging, particularly at small F-numbers. Taking the light collection effect of the lens into account, this Letter establishes a complete and novel tomographic theory for a multi-directional tomography system consisting of a lens and CCD cameras. A modified camera calibration method is presented first. It determines the exact spatial locations and intrinsic parameters of the cameras. A 3D projection model based on the lens imaging theory is then proposed and integrated into the multiplicative algebraic reconstruction technique (MART). The new approach is demonstrated with a 12-camera system that is used to reconstruct the emission field of a propane flame, thereby resolving space and time.

NOx emissions characteristics of the partially premixed combustion of H2/CO/CH4 syngas using artificial neural networks

Recently, global interest in energy depletion and the issue of rapid climate change have emerged. To address these issues, research and development related to clean coal technology are currently active. In this study, a combustion experiment is performed to investigate the combustion characteristics of H2/CO/CH4 syngas in a partially premixed model gas turbine combustor. Chemiluminescence measurements are undertaken to study the flame structure and characteristics of syngas combustion at an equivalence ratio range of 0.7–1.3. The Abel inversion method is applied to obtain 2-D chemiluminescence flame images from 3-D accumulated chemiluminescence images. EINOx is measured to investigate the relationship between the flame structure and the exhaust gas. In addition, a process using an artificial neural network (ANN) is employed to establish the EINOx prediction model. As a result, the EINOx characteristics of a partially premixed flame differ from those of a diffusion jet flame. Moreover, they are proportional to the flame temperature and correlated with the flame length at varying equivalence ratios ranging from 0.7 to 1.3 at the same heat input level. Overlapped OH*, CH*, and C2* chemiluminescence images are more accurate for estimating EINOx for various H2/CO/CH4 syngas compositions. This is confirmed from the ANN estimation results. The flame temperature, length, and input air flow rate are used in ANN to predict EINOx. The coefficients of correlation used for the predictions are 0.78 and 0.62 when using overlapped OH*, CH*, and C2* chemiluminescence images and only the OH* chemiluminescence image as input parameters, respectively. The weight partition method of this ANN process also confirms that EINOx formation is affected more by the flame temperature and length than by the air mass flow rate.

Visualization of Multiple Flame Interactions: Appearance Structure and Combustion of LPG-Air Premixed Laminar Flames

Interaction of multiple premixed laminar flames was studied by visualizations of the modified flame structures. Typical line-of-sight flame chemiluminescence image and spatial resolved OH distribution by means of planar laser-induced fluorescence (OH-PLIF) technique were applied for these observations. The premixed flames of liquefied petroleum gas (LPG)/air mixture with three inline nozzles were established to simulate the practical flame characteristics produced by the practical cooking burners. The obtained results are compared as a function of primary equivalence ratio (  ) and inter-distance (S/Do) between nozzles. Effect of S/Do and  on the multiple premixed flame structures is well understood via the clarifying reaction zone distribution, which is in-situ visualized by OH PLIF technique. S/Do has a strong effect on interactions between flames, resulting in either flame enhancement or flame inhibition, depending on  . CO and NOx emissions were measured in order to confirm the combustion performance of the multiple flames. It is recommended that the in-situ visualization is the focus for a study of the flame. This will help to avoid misunderstanding of the real combustion as it can be occurred by using the line-of-sight observation technique.

Investigation of Hydrogen Enriched Natural Gas Flames in a SGT-700/800 Burner Using OH PLIF and Chemiluminescence Imaging

The effect of hydrogen enrichment to natural gas flames was experimentally investigated at atmospheric pressure conditions using flame chemiluminescence imaging, planar laser-induced fluorescence of hydroxyl radicals (OH PLIF), and dynamic pressure monitoring. The experiments were performed using a third generation dry low emission (DLE) burner used in both SGT-700 and SGT-800 industrial gas turbines from Siemens. The burner was mounted in an atmospheric combustion test rig at Siemens with optical access in the flame region. Four different hydrogen enriched natural gas flames were investigated; 0 vol. %, 30 vol. %, 60 vol. %, and 80 vol. % of hydrogen. The results from flame chemiluminescence imaging and OH PLIF show that the size and shape of the flame was clearly affected by hydrogen addition. The flame becomes shorter and narrower when the amount of hydrogen is increased. For the 60 vol. % and 80 vol. % hydrogen flames the flame has moved upstream and the central recirculation zone that anchors the flame has moved upstream the burner exit. Furthermore, the position of the flame front fluctuated more for the full premixed flame with only natural gas as fuel than for the hydrogen enriched flames. Measurements of pressure drop over the burner show an increase with increased hydrogen in the natural gas despite same air flow thus confirming the observation that the flame front moves upstream toward the burner exit and thereby increasing the blockage of the exit. Dynamic pressure measurements in the combustion chamber wall confirms that small amounts of hydrogen in natural gas changes the amplitude of the dynamic pressure fluctuations and initially dampens the axial mode but at higher levels of hydrogen an enhancement of a transversal mode in the combustion chamber at higher frequencies could occur.

The planar imaging of laser induced fluorescence of fuel and hydroxyl for a wall-interacting jet in a single-cylinder, automotive-size, optically accessible diesel engine

The fuel penetration and reacting diesel jet development have been studied in a small-bore optical engine to better understand a swirl-influenced, wall-interacting diesel flame. The fuel injection was executed for long duration, creating negative ignition dwell conditions in which the start of combustion occurs before the end of injection. Planar laser-induced fluorescence imaging of hydroxyl (OH-PLIF), fuel-PLIF, and line-of-sight integrated chemiluminescence imaging were performed for various combustion stages identified by the in-cylinder pressure traces and apparent heat release rates. These include stages of vaporising fuel penetration, low-temperature reaction, and high-temperature reaction. The fuel-PLIF images show that the fuel penetration is strongly influenced by a swirl flow with the wall–jet penetration on the up-swirl side being shorter than that on the down-swirl jet. During the low-temperature reaction, cool flame chemiluminescence appears near the wall–jet head region. Interestingly, this region is where the initial turbulent ring-vortex is formed due to jet–wall interactions, suggesting that locally enhanced mixing induced the first-stage ignition. The OH-PLIF images show that the second-stage, high-temperature reaction starts to occur in the same initial head vortex region, which is largely different to the flame base found in the free jet region in heavy-duty, large-bore engines. Since the reaction occurs in the wall–jet region, the swirl flow impacts the high-temperature reaction significantly, as evidenced by more intense OH signals in the down-swirl jet. This is due to the influence of the swirl flow on the mixing process and the strain rate of the mixture, leading to earlier, faster and stronger high-temperature reaction on the down-swirl side. Upon the end of high temperature reaction, the heat release rate declines and the OH-PLIF signals slowly diminish.

Performance Evaluation of a Model Swirl Burner under Premixed or Stratified Inlet Mixture Conditions

AbstractThis work presents a comparative study of the performance between fully premixed and stratified propane flames stabilized in a disk burner configuration operating with a swirl coflow over a range of stoichiometric to ultra-lean conditions. A selection of radial equivalence ratio gradients up to the fully premixed case are regulated by staged premixing of propane and air within a double cavity formed along three concentric disks. Flame stabilization is established at the recirculation region of the afterbody disk. Measurements of temperatures, flame chemiluminescence images of OH* and CH*, and gas analysis provided information for the interpretation of the relative variations in flame structure and burner performance. The study elucidates some aspects regarding the influence of the variable inlet fuel–air profile and its interaction with the swirling coflow on the stabilization and disposition of the toroidal flame front. The results help to delineate important differences in the emission performan...

Influence of electric fields on premixed laminar flames: Visualization of perturbations and potential for suppression of thermoacoustic oscillations

In this experimental study the effects of weak DC electric fields on laminar flames are studied. In the first part transient DC electric fields were applied with different frequencies (1–200Hz) to a laminar Bunsen type flame for the investigation of the changes occurring in the flow field and the flame structure using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV). For various low frequencies similar response behavior of the flame was observed. After the activation of the electric field the flow field changes inducing a disturbance at the flame root which travels as a wave-like oscillation towards the flame tip until stationary conditions are reached. For higher frequencies different flame behavior was revealed since the process cannot establish a steady state anymore. For 100Hz the detected flame response time and the pulse duration of the electric field are similar leading to the largest flame oscillation amplitude. For 200Hz the time between activation and deactivation of the electric field is too short as compared to the flame response time. Therefore, the influence of the electric field is very weak. From this changed response behavior a limiting frequency of the ionic wind for technical application in the range of 100Hz was derived.In the second part the potential of electric fields to suppress acoustic flame instabilities was studied on a flat flame using the high-speed detection of the flame chemiluminescence. Acoustic oscillations were generated by an injector with defined injection frequencies resulting in a periodically lifted flame. Depending on the electric field strength the duration of flame oscillation could be reduced by up to 54%, so the flame returns quicker to its initial condition.

Flame Dynamics With Hydrogen Addition at Lean Blowout Limits

Lean premixed combustion is widely used in power generation due to low nitric oxide emissions. Recent interest in syngas requires a better understanding of the role of hydrogen addition on the combustion process. In the present study, the extinction process of hydrogen enriched premixed flames near lean blow out (LBO) in a swirl-stabilized combustor has been examined in both unconfined and confined configurations. High speed images of the flame chemiluminescence are recorded, and a proper orthogonal decomposition (POD) procedure is used to extract the dominant flame dynamics during the LBO process. By examining the POD modes, the spectral information and the statistical properties of POD coefficients, the effect of hydrogen addition on the LBO processes are analyzed and described in the paper. Results show that in unconfined flames, the shear layer mode along with flame rotation with local quenching and reignition is dominant in the methane-only case. For the open hydrogen enriched flames, the extinction times are longer and are linked to the lower minimum ignition energy for hydrogen that facilitates reignition events. In confined methane flames, a conical flame is observed and the POD mode representing the burning in the central recirculation zone appears to be dominant. For the 60% hydrogen enriched flame, a columnar burning pattern is observed and the fluctuation energies are evenly spread across several POD modes making this structure more prone to external disturbances and shorter extinction times.

Plasma Actuator Control of a Lifted Ethane Turbulent Jet Diffusion Flame

A dielectric barrier discharge (DBD) actuator is used to stabilize the base of a lifted ethane turbulent jet diffusion flame by modifying the coflow velocity field. The velocity field and flame base are measured by particle image velocimetry and unfiltered flame chemiluminescence imaging, respectively. An axisymmetric DBD actuator, integrated onto the nozzle body and driven by 8 kHz, 10-12-kV peak-to-peak sinusoidal voltage, generates the directional flow that opposes the coflow that surrounds the jet nozzle. This flow induces a separation bubble that retards and diverts the surrounding fluid further away from the fuel jet. The turbulent jet diffusion flame liftoff height is found to be significantly reduced by this plasma actuation.

Impact of the Flame-Holder Heat-Transfer Characteristics on the Onset of Combustion Instability

In this article, we investigate the impact of heat transfer between the flame and the flame-holder on the dynamic stability characteristics of a 50-kW backward-facing step combustor. We conducted a series of tests where two backward step blocks were used, made of ceramic and stainless steel, whose thermal conductivities are 1.06 and 12 W/m/K, respectively. Stability characteristics of the two flame-holder materials were examined using measurements of the dynamic pressure and flame chemiluminescence over a range of operating conditions. Results show that with the ceramic flameholder, the onset of instability is significantly delayed in time and, for certain operating conditions, disappears altogether, whereas with the higher conductivity material, the combustor becomes increasingly unstable over a range of operating conditions. We explain these trends using the heat flux through the flame-holder and the change in the burning velocity near the step wall. Results suggest a potential approach using low-thermal-conductivity material near the flame-holder as passive dynamics suppression methods.

Volume-resolved flame chemiluminescence and laser-induced fluorescence imaging

There is significant need for optical diagnostic techniques to measure instantaneous volumetric vector and scalar distributions in fluid flows and combustion processes. This is especially true for investigations where only limited optical access is available, such as in internal combustion engines, furnaces, flow reactors, etc. While techniques such as tomographic PIV for velocity measurement have emerged and reached a good level of maturity, instantaneous 3D measurements of scalar quantities are not available at the same level. Recently, developments in light field technology have progressed to a degree where implementation into scientific 3D imaging becomes feasible. Others have already demonstrated the utility of light field technology toward imaging high-contrast particles for PIV and for imaging flames when treated as single-surface objects. Here, the applicability and shortcomings of current commercially available light field technology toward volumetric imaging of translucent scalar distributions and flames are investigated. Results are presented from imaging canonical chemiluminescent and laser-induced fluorescent systems. While the current light field technology is able to qualitatively determine the position of surfaces by locating high-contrast features, the correlation-based reconstruction algorithm is unable to fully reconstruct the imaged objects for quantitative diagnostics. Current analysis algorithms are based on high-contrast correlation schemes, and new tools, possibly based on tomographic concepts, will have to be implemented to reconstruct the full 3D structure of translucent objects for quantitative analysis.

A numerical study on the ability to predict the heat release rate using CH* chemiluminescence in non-sooting counterflow diffusion flames

Numerical studies on 1-D, non-sooting counterflow diffusion flames were performed to determine the precision with which the total heat release rate can be calculated using light emission, namely, chemiluminescence, from the reaction zone. A detailed reaction mechanism, incorporating sub-reaction models for excited state radicals (CH* and OH*, where * denotes the excited state), was employed in this study. A set of 1-D, steady state conservation equations was solved under standard atmospheric conditions over a counterflow configuration utilizing the CHEMKIN-PRO package. A variety of fuels (CH4 and C3H8), velocities (0.1m/s − near the extinction condition), diluents (N2, H2O, CO2, and Ar), detailed reaction mechanisms for C1–C3 hydrocarbons (GRI-Mech 3.0, Hai Wang’s-, and San Diego-mechanism), different sub-reaction models for excited radicals, and excited radical transport properties were examined for the current purpose. It was found that a one-to-one correlation between total chemiluminescence from CH* and the total heat release rate cannot be sustained when the flame experiences a relatively high stretch and dilution, even though the condition is still far away from extinction. This trend is consistent with the different types of fuels, and it is understood that the reduction of the ethynyl radical (C2H), a potential precursor of CH*, is the main cause of the one-to-one correlation not being sustained. To this end, it was concluded that the observable light emission can only be used to predict the total heat release rate when non-sooting diffusion flames exist under velocity conditions from 0.1m/s to 1.5m/s. In other words, the chemiluminescence intensity does not always correlate with the total heat release rate of highly stretched flames found in practical combustors.

Three-Dimensional Chemiluminescence Imaging of Unforced and Forced Swirl-Stabilized Flames in a Lean Premixed Multi-Nozzle Can Combustor

A tomographic image reconstruction technique has been developed to measure the 3D distribution of CH* chemiluminescence of unforced and forced turbulent premixed flames. Measurements are obtained in a lean premixed, swirl-stabilized multi-nozzle can combustor. Line-of-sight images are acquired at equally spaced angle increments using a single intensified charge-coupled device camera. 3D images of the flames are reconstructed by applying a filtered back projection algorithm to the acquired line-of-sight images. Methods of viewing 3D images to characterize the structure, dynamics, interaction and spatial differences of multi-nozzle flames are presented. Accuracy of the reconstruction technique is demonstrated by comparing reconstructed line-of-sight images to measured line-of-sight downstream-view images of unforced flames. The effect of the number of acquired projection images on the quality of the reconstruction is assessed. The reconstructed 3D images of the unforced multi-nozzle flames show the structure of individual flames as well as the interaction regions between flames. Forced flame images are obtained by phase-synchronizing the camera to the forcing cycle. The resulting 3D reconstructions of forced flames reveal the spatial and temporal response of the multi-nozzle flame structure to imposed velocity fluctuations, information which is essential to identifying the underlying mechanisms responsible for this behavior.

Spray flame structure of rapeseed biodiesel and Jet-A1 fuel

The spray combustion characteristics of rapeseed methyl esters (RME) were compared to Jet-A1 fuel using a gas turbine type combustor. The swirling spray flames for both fuels were established at a constant power output of 6kW. The main swirling air flow was preheated to 350°C prior to coaxially enveloping the airblast-atomized liquid fuel spray at atmospheric pressure. Investigation of the fundamental spray combustion was performed via measurements of the fuel droplet sizes and velocities, gas phase flow fields and flame reaction zones. The spray flame droplets and flow fields in the combustors were characterised using phase Doppler anemometry (PDA) and particle imaging velocimetry (PIV) respectively. Flame chemiluminescence imaging was employed to identify the flame reaction zones. The highest droplet concentration zone extends along a 30° angle from the symmetry axis, inside the flame zone. Only small droplets(<17μ) (<17μm)are found around the centreline region, while larger droplets are found at the edge of the spray outside the flame reaction zone. RME exhibits spray characteristics similar to Jet-A1 but with droplet concentration and volume fluxes four times higher, consistent with the expected longer droplet evaporation timescale. The flow field characteristics for both RME and Jet-A1 spray flames are very similar despite the significantly different visible characteristics of the flame reaction zones.

On the phase between pressure and heat release fluctuations for propane/hydrogen flames and its role in mode transitions

This paper presents an experimental investigation into mode-transitions observed in a 50-kW, atmospheric pressure, backward-facing step combustor burning lean premixed C3H8/H2 fuel mixtures over a range of equivalence ratios, fuel compositions and preheat temperatures. The combustor exhibits distinct acoustic response and dynamic flame shape (collectively referred to as “dynamic modes”) depending on the operating conditions. We simultaneously measure the dynamic pressure and flame chemiluminescence to examine the phase between pressure (p′) and heat release fluctuations (q′) in the observed dynamic modes. Results show that the heat release is in phase with the pressure oscillations (θqp≈0) at the onset of a dynamic mode, while as the operating conditions change within the mode, the phase grows until it reaches a critical value θqp=θc, at which the combustor switches to another dynamic mode. According to the classical Rayleigh criterion, this critical phase (θc) should be π/2, whereas our data show that the transition occurs well below this value. A linear acoustic energy balance shows that this critical phase marks the point where acoustic losses across the system boundaries equal the energy addition from the combustion process to the acoustic field. Based on the extended Rayleigh criterion in which the acoustic energy fluxes through the system boundaries as well as the typical Rayleigh source term (p′q′) are included, we derive an extended Rayleigh index defined as Re=θqp/θc, which varies between 0 and 1. This index, plotted against a density-weighted strained consumption speed, indicates that the impact of the operating parameters on the dynamic mode selection of the combustor collapses onto a family of curves, which quantify the state of the combustor within a dynamic mode. At Re=0, the combustor enters a mode, and switches to another as Re approaches 1. The results provide a metric for quantifying the instability margins of fuel-flexible combustors operating at a wide range of conditions.

Visualizing CH* chemiluminescence in sooting flames

This investigation presents and validates a method to visualize CH* chemiluminescence in a sooting non-premixed flame. Excited-state CH radicals (A2Δ→X2Π, denoted CH*), upon relaxing, emit a photon in the visible portion of the electromagnetic spectrum, giving what is seen as a faint blue glow. Optical methods, especially those that rely on passive emission, provide a simple and non-intrusive means to characterize combustion; however, in the presence of hot solid particles – particularly soot – chemiluminescent emission can be obscured by blackbody radiation. This paper describes an image post-processing technique where flame emission is imaged through multiple filters near the CH* emission band in order to infer and filter the contribution of continuum radiation from soot in the CH* band. This information is then used to produce a soot-free image of CH* chemiluminescence.

Structures and performances of laminar impinging multiple premixed LPG–air flames

Three different imaging techniques were utilized to study the laminar, impinging, multiple, premixed liquefied petroleum gas (LPG)–air flames for simulating the premixed impinging flames of practical cooking burners. Two line-of-sight observation techniques are introduced, i.e. direct photography of a typical visible flame and the imaging of flame chemiluminescence, whereas the spatially resolved imaging is carried out by means of the OH-PLIF technique as the in situ visualization. Effects of the key parameters of heating height (H/D) and inter-port spacing (S/Do) on the flame structure and flame interaction are systematically studied and compared. The burner’s performances in terms of CO emissions and thermal efficiency are experimentally studied to correlate with the flame structure. Among the three techniques, the OH-PLIF measurement is the most reliable technique to uncover the combustion situation existing in the flames. H/D is a major parameter controlling the multiple flame structure and the corresponding burner’s performance, whereas S/Do has a less degree effect as compared with H/D. The optimum condition is found at H/D=2.1 and S/Do=1.5, where OH-PLIF profiles are well correlated with the corresponding burner’s performance. This study results in better understanding of insights into the structure of the multiple premixed impinging flames, which is valuable for further work of designing an efficient, practical cooking burner.

Spray Flame Study Using a Model Gas Turbine Swirl Burner

A model gas turbine burner was employed to investigate spray flames established under globally lean, continuous, swirling conditions. Two types of fuel were used to generate liquid spray flames: palm biodiesel and Jet-A1. The main swirling air flow was preheated to 350 °C prior to mixing with airblast-atomized fuel droplets at atmospheric pressure. The global flame structure of flame and flow field were investigated at the fixed power output of 6 kW. Flame chemiluminescence imaging technique was employed to investigate the flame reaction zones, while particle imaging velocimetry (PIV) was utilized to measure the flow field within the combustor. The flow fields of both flames are almost identical despite some differences in the flame reaction zones.