Excitation Regime Two-line Atomic Fluorescence Research Articles

Effects of H2 enrichment on combustion characteristics in inverse swirl diffusion flames of H2-doped jet fuel using OH, CH2O, fuel and temperature imaging

Hydrogen (H2) and jet fuel co-firing is regarded as a promising transition strategy of decarbonization in aviation industry. Considering the simplicity and safety during highly hydrogen enrichment conditions, the inverse swirl diffusion (ISD) combustion holds research value. Given the current lack of understanding for the flame properties of H2-enriched pre-evaporative jet fuel ISD co-combustion, in this work, the simultaneous planar laser-induced fluorescence (PLIF) of hydroxyl (OH), formaldehyde (CH2O), and fuel, as well as the non-linear excitation regime two-line atomic fluorescence (NTLAF) thermography imaging, are used to study the effects of hydrogen enrichment on ISD flame intricate structure and temperature field. The results show the positive effects of H2 enrichment. The ISD flames display a blue hue and the increase in doping hydrogen helps the flames stabilize under global-lean conditions with almost no CO emission and low CO2 emission at the chamber exit. And the H2 enrichment contributes to inhibiting the CH2O formation upstream, reducing the generation of isolated reaction islands and promoting the strength of the flame wings, favoring the suppression of flame instability. In contrast, when at low H2 enrichment rate, the shift of the heat release zone and high-temperature zone, and the enlargement of the cold zone, are found in flame upstream. The analysis of the probability distribution functions (PDF) of temperatures confirms the effective effects of hydrogen on inhibiting the local flame fluctuations. Although the increase in the overall temperature level due to the hydrogen addition would lead to a promotion in NOx generation, the discussion on the structural analysis and flame propagation stability of ISD flames demonstrates the significance of H2 enrichment for the achievement of leaner low-NOx combustion of ultra-H2-enriched jet fuel in the future.

Temperature and flame structure imaging in kerosene swirl-stabilized spray flames at low air flow using TLAF and OH-PLIF

Air flowrate is an important parameter for airbrast nozzles and low air inlet flow rate in airbrast type combustors will lead to worse atomization, combustion instability, and increased pollutant emissions. Using non-linear excitation regime two-line atomic fluorescence (NTLAF) thermography imaging and planar laser-induced fluorescence of hydroxyl radical (OH-PLIF), temperature and flame structure in the near-nozzle area are investigated at different low air flow rate to study the combustion deterioration of kerosene swirl-stabilized spray flames. In this concerned area, combustion occurs mainly at the near-wall region but a few OH signals are present at the centerline region except for the kerosene LIF particles. With the air supply getting low, the near-wall OH signals get low, corresponding to the change of indium Stokes and anti-Stokes fluorescence. The temperature in the centerline region is presented to be reduced and the region for the temperature threshold filter expands, indicating the reduction of the downstream hot combustion products brought back by weak swirl, which is closely related to the extremely low air flowrate. Under extremely low air flow rate, the highly fluctuating temperature and weak gas-liquid mixed combustion at the centerline area may tend to cause combustion instability or ignition failure.

Water vapour effects on temperature and soot loading in ethylene flames in hot and vitiated coflows

Flames in hot, low oxygen environments exist in a variety of practical applications. These conditions result in significant mixing between fuel and combustion products, such as water vapour, or diluents included for emissions control. The chemical and physical effects of water vapour as a diluent are investigated in a series of ethylene flames in a jet in hot coflow burner to determine the effects on temperature and soot fields. The combined analyses of photographs, non-linear excitation regime two-line atomic fluorescence (NTLAF) of indium, planar laser-induced incandescence (LII) and one-dimensional opposed-flow flame simulations demonstrate the dominance of the chemistry, driven by the hot and vitiated oxidant, in soot reduction. Although photographs appear to suggest that both highly vitiated coflows, and highly diluted jet flames have global effect on the flames, detailed measurements reveal significantly different trends in their soot and temperature fields. The chemical contribution of water vapour as a reactant, as a third-body in ethylene decomposition and a source of H and OH in the rich mixture is further described, and trends subsequently identified, in the context of formation of polycyclic aromatic hydrocarbons and soot reduction.

Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

There is a wealth of existing experimental data of flames collected using laser diagnostics. The primary objective of this review is to provide context and guidance in interpreting these laser diagnostic data. This educational piece is intended to benefit those new to laser diagnostics or with specialisation in other facets of combustion science, such as computational modelling. This review focuses on laser-diagnostics in the context of the commonly used canonical jet-in-hot-coflow (JHC) burner, although the content is applicable to a wide variety of configurations including, but not restricted to, simple jet, bluff body, swirling and stratified flames. The JHC burner configuration has been used for fundamental studies of moderate or intense low oxygen dilution (MILD) combustion, autoignition and flame stabilisation in hot environments. These environments emulate sequential combustion or exhaust gas recirculation. The JHC configuration has been applied in several burners for parametric studies of MILD combustion, flame reaction zone structure, behaviour of fuels covering a significant range of chemical complexity, and the collection of data for numerical model validation. Studies of unconfined JHC burners using gaseous fuels have employed point-based Rayleigh-Raman or two-dimensional Rayleigh scattering measurements for the temperature field. While the former also provides simultaneous measurements of major species concentrations, the latter has often been used in conjunction with planar laser-induced fluorescence (PLIF) to simultaneously provide quantitative or qualitative measurements of radical and intermediary species. These established scattering-based thermography techniques are not, however, effective in droplet or particle laden flows, or in confined burners with significant background scattering. Techniques including coherent anti-Stokes Raman scattering (CARS) and non-linear excitation regime two-line atomic fluorescence (NTLAF) have, however, been successfully demonstrated in both sooting and spray flames. This review gives an overview of diagnostics techniques undertaken in canonical burners, with the intention of providing an introduction to laser-based measurements in combustion. The efficacy, applicability and accuracy of the experimental techniques are also discussed, with examples from studies of flames in JHC burners. Finally, current and future directions for studies of flames using the JHC configuration including spray flames and studies and elevated pressures are summarised.

Downstream evolution of n-heptane/toluene flames in hot and vitiated coflows

The scenario of fuel injected into hot surrounds is found in a range of practical combustion applications. These flame conditions have been emulated using a jet-in-hot-coflow-burner using prevaporised n-heptane and mixtures of n-heptane and toluene, relevant to gasoline and diesel fuel surrogates. This paper reports measurements of six lifted, turbulent flames, with a constant jet flow of a prevaporised fuel/N2 mixture at 380 K into various hot and vitiated coflow conditions. Five of these flames issued into coflows generated by the combustion of different mixtures of ethylene/air and one had a coflow from a natural gas/air flame. Two n-heptane/toluene fuel blends were also measured to study the effect of soot propensity. Gas sampling, non-linear excitation regime two-line atomic fluorescence (NTLAF) and laser-induced incandescence (LII) were used to characterise the flames, investigate the mixing between the hot coflow and the surrounding air, and measure the flame temperature for the different coflow configurations. A comparison of results of the flames issuing into hot coflows is presented, indicating that the hottest flame is not associated with the coflow containing the highest concentration of O2, but with the minimum soot loading and, consequently, the minimum radiative heat loss. Subsequent numerical simulations of canonical opposed-flow flames demonstrate that the soot loading in the downstream region of the flames is strongly dependent on PAH formation in the hot coflow region and further analyses reveal the chemical pathways which are most impacted by small variations in hot coflow composition.

Simple calibrated nonlinear excitation regime two-line atomic fluorescence thermometry.

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a promising two-dimensional (2D) thermometry technique for turbulent sooty flames. However, the complexity of calibrating three system parameters and expensive instruments restricts the application of the current NTLAF technique. Here we propose a simple and cheap NTLAF measurement approach based on a one-parameter model and tunable diode laser absorption spectroscopy (TDLAS) calibration. Using this methodology, only one system parameter, instead of three as in traditional NTLAF, is to be calibrated by path-averaged temperature acquired by the TDLAS technique. As a demonstration, instantaneous 2D thermometry data of a homemade burner were acquired using this approach, with measurement uncertainty of ∼4.5% and deviation from both reference TDLAS results and Raleigh scattering measurement results less than 50K, typically within 20K. This approach offers a novel simplified NTLAF solution for noncontact, in-suit, high-resolution 2D temperature measurement and is expected to greatly improve the compatibility of the NTLAF technique in scientific research and engineering applications.

Experimental investigation of soot evolution in a turbulent non-premixed prevaporized toluene flame

The formation, growth, and oxidation of soot in turbulent prevaporized toluene diffusion flames stabilized on a jet-in-hot-coflow (JHC) burner are investigated in this study. Flame structure, local gas temperature as well as local soot volume fraction and primary soot particle diameter, are simultaneously detected by means of OH planar laser-induced fluorescence (PLIF), non-linear excitation regime two-line atomic fluorescence (nTLAF) of indium, and time-resolved (TiRe) laser-induced incandescence (LII), respectively. The collected data sets were used to generate joint statistics of soot properties and flame characteristics and provided new insights into the interaction of the OH layer and soot in turbulent flames. The interaction of OH and soot as a driving mechanism for soot oxidation is of particular interest as it has been proven to be challenging to model. Statistics of soot volume fraction and primary particle size in the OH layer are employed to gain deeper insights into the soot oxidation process. Mean soot volume fraction and primary soot particle size conditioned on temperature and OH signal intensity indicate that, due to differential diffusion of soot with respect to the chemical species, high soot volume fraction and primary soot particle diameter of up to 50 nm are present at low temperatures and low OH concentration. In the soot oxidation region, statistical analysis of the soot parameters disclose that clusters of high soot volume fraction mostly consist of large primary particles. Observations from instantaneous images and the presence of large primary particles inside the OH layer suggest that the oxidation is not sufficiently fast to burn the soot completely.

Temperature and reaction zone imaging in turbulent swirling dual-fuel flames

Gaseous and liquid dual-fuel flames present both a practical approach to emissions reduction and a challenge to current state-of-the-art combustion modelling. This paper uses simultaneously imaged temperature and normalised OH signal fields to investigate flame structure and provide experimental data for model validation across a range of swirl-stabilised n-heptane spray flames. These data are obtained by non-linear excitation regime two-line atomic fluorescence (NTLAF) of indium, and planar laser-induced fluorescence (OH-PLIF), respectively. Swirling gas streams are varied by flowrate (63–88% of blow-off), premixed equivalence ratio (including air-only), and by type of gaseous fuel (natural gas and hydrogen). Results are used to describe how hot combustion products interact with the fuel spray: heating and diluting the region above the apex of the spray cone at low air flowrates but drawing fuel into outer branches of the flame with increasing air flowrates. Adding natural gas to the swirling air stream, at a concentration below the lean flammability limit, gives rise to a temperature increase in the outer branches with little effect on the hot region above the apex of the spray, along the burner centreline. The size of this region is significantly reduced; however, using hydrogen. As the concentration of gaseous fuel increases towards the lean flammability limit, peak temperatures shift towards the outer branch of the flame. Exceeding the lean flammability limit, an additional reaction zone begins to form in the premixed swirling stream, adjacent to the outer branch of the swirl flame. Stable outer branches of the swirl flame, however, become less prevalent and the peak temperatures of the spray flame return to burner centreline. This study provides insight into the complex behaviour of dual-fuel flames, a complementary dataset to related, PLIF-only studies and validation data for the development of numerical modelling tools.

Influence of nozzle diameter on soot evolution in acoustically forced laminar non-premixed flames

The current study investigated the soot evolution in a series of acoustically forced laminar flames affected by the variations in the nozzle diameter, in-flame residence time and buoyancy. Measurements were performed for three laminar co-flowing non-premixed jet flames in which sinusoidal pressure fluctuations were imposed to the fuel stream. Non-linear excitation regime Two-Line Atomic Fluorescence (NTLAF), Laser-Induced Incandescence (LII), Time-Resolved Laser-Induced Incandescence (TiRe-LII) and Planar Laser-Induced Fluorescence of hydroxyl radicals (OH-PLIF) were performed simultaneously to acquire the phase-resolved measurements of temperature, soot concentration, primary particle size and the location of reaction zones. Additionally, Particle Imaging Velocimetry (PIV) was employed independently to characterise the velocity field. The peak soot concentrations in all the forced flames are double those measured in their steady counterparts, consistent with previous measurements, whereas the maximum particle size for the forced flames is only 10% larger than that for the steady flames and is independent of the nozzle diameter. Nevertheless, a systematic variation of the fuel tube diameter shows that the toroidal vortex scale affects the flame structure which leads to an impact on the spatial distribution and the total volume of soot. In addition, residence time analysis shows that the enhancement of the largest particle size, as well as the peak soot volume fraction, scales with the in-flame residence time.

Experimental investigation of acoustic forcing on temperature, soot volume fraction and primary particle diameter in non-premixed laminar flames

The influence of the acoustic-driven oscillation frequency on the structure and the soot formation of a laminar non-premixed ethylene flame is presented. The flames were forced at either 20 and 40 Hz, corresponding to Strouhal numbers of 0.23 and 0.46. Two-dimensional phase-resolved measurements of gas temperature, soot volume fraction and the diameter of primary particles were measured simultaneously for one steady and two forced flames with non-linear excitation regime Two-Line Atomic Fluorescence, Laser-Induced Incandescence and Time-Resolved LII, respectively. Simultaneous measurements of gas temperature and soot volume fraction provide details of the difference between the flame structure and the soot distribution in the forced and unforced flames. The distinctive features of the forced flames are the occurrence of necking near to the fuel tube and the formation of a hollow soot “shell”. Despite the distinctive structure, the soot region is confined to a restricted range of temperatures, approximately 1700–1800 K. The strong spatial correlation between the presence of soot and a narrow range of temperature is well established for steady laminar flames. The present measurements also reveal the complex relationship between diameter of primary particles and soot volume fraction for these flames. Planar gas temperature images and OH* chemiluminescence profiles show that the reaction zone extends periodically upstream from the exit plane of the fuel tube for the flame with St=0.23. This phenomenon, which only occurs for some conditions, is an important consideration for the modelling of these flames because the boundaries of the computational domain typically starts at the fuel flow exit plane.

Simultaneous measurements of gas temperature, soot volume fraction and primary particle diameter in a sooting lifted turbulent ethylene/air non-premixed flame

Simultaneous, planar measurements of flame temperature (T), soot volume fraction (fv), primary particle diameter (dp) and the derived number density of primary particles (Np) are reported in a well characterized, lifted ethylene jet diffusion flame, both to increase confidence in measurement accuracy and to provide new joint statistics. Planar measurements of temperature were performed using non-linear excitation regime two-line atomic fluorescence (nTLAF) of indium with an improved optical arrangement over those reported previously, and were found to yield good agreement with previous measurements obtained with coherent anti-Stokes Raman spectroscopy (CARS). Planar measurements of soot volume fraction and primary particle diameter were performed using time-resolved laser-induced incandescence (TiRe-LII). On the flame centreline, both the measured values of dp and fv grow with axial distance to peak near to the mid height of the flame. The joint probability density functions (PDF) of the measured T, fv, dp and derived Np were obtained from the two-dimensional images and assessed at 15 locations in the flame (5radial×3axial locations). Strong correlations were found between fv, dp and Np, while they exhibit a moderate correlation with flame temperature. The changes in PDFs with radial and axial locations and the most probable values are also reported.

Improvement of precision and accuracy of temperature imaging in sooting flames using two-line atomic fluorescence (TLAF)

Severe interference to the fluorescence signal from seeded indium has previously limited the application of current non-linear excitation regime two-line atomic fluorescence (NTLAF) to low sooting flames. The present paper reports new details of these interferences, from spatially- and temporally-resolved spectral measurements in a laminar, non-premixed sooting C2H4 flame burning in air. Three types of interference to the indium LIF signal have been identified: laser-induced fluorescence (LIF) from polycyclic aromatic hydrocarbons (PAH), elastic laser scattering (ELS) and laser-induced incandescence (LII) from soot particles. This knowledge was used to propose new methods to suppress interferences and select customized narrow-band filters for the signal collection, with a narrow bandwidth of ∼1.2 nm (full-width at half-maximum, FWHM) and a high peak transmission of ∼95%. The efficacy of the narrow-band filters for interference suppression in planar NTLAF measurements was assessed by comparing these measurements with those obtained with a set of conventional band-pass filters (FWHM = 10 nm), in both a soot-free flame and a sooting flame. The application of the narrow-band filters was found to increase both the accuracy and the precision of the temperature measurement, so that measurement accuracy that is due to the presence of interferences has been improved from 198 K to 10 K at a location where the soot volume fraction (fv) is 1.2 ppm. An average reduction of 40% in the standard deviation of measured temperature in single-shot measurements has also been achieved with the use of high transmission filters. With the use of narrow-band filters, NTLAF can be reliably applied in laboratory target sooting flames with peak soot volume fraction of several ppm.

Temperature imaging of turbulent dilute spray flames using two-line atomic fluorescence

This paper reports the first application of nonlinear excitation regime two-line atomic fluorescence imaging (NTLAF) of indium to measure temperature in turbulent flames of dilute sprays. Indium chloride is dissolved in acetone fuel which is atomised with an ultrasonic nebuliser and supplied with carrier air into a standard piloted burner. It is found that the indium fluorescence signal is not affected by scattering from the droplets or fuel vapour and that no changes to the optical arrangement used with gaseous flames were required. Notwithstanding the lower temperature thresholds of 800 K imposed by the population of excitation species for the NTLAF method and of 1,200 K imposed by the mechanism of releasing gas-phase indium from its salt, the comparisons of conditional and pseudo-unconditional means with thermocouple measurements performed in a range of turbulent spray flames are quite favourable. The NTLAF signal quality deteriorates on the jet centreline at upstream locations and on the lean side of the flame, the former being largely due to insufficient conversion of indium chloride to indium atoms and the latter potentially due to indium oxidation. Nevertheless, the signal-to-noise ratios obtained in the reaction zone regions are good and the results reveal the expected temperature trends in the turbulent spray flames tested here. Further developments are necessary to resolve the mechanism of indium formation and to broaden the temperature range.

Simultaneous planar measurements of temperature and soot volume fraction in a turbulent non-premixed jet flame

New measurements are reported of the soot–temperature interaction in a turbulent non-premixed ethylene–hydrogen–nitrogen attached jet flame with an exit Reynolds number of 15,000. Spatially resolved, two-dimensional temperature and soot volume fractions were measured simultaneously using non-linear excitation regime two-line atomic fluorescence (NTLAF) and laser-induced incandescence (LII) techniques, respectively. The soot–temperature correlation is presented and analyzed through representative images of single-shot simultaneous temperature and soot volume fraction at various heights, as well as through joint probability density functions (PDFs) of soot volume fraction (SVF) and temperature. A strong influence of temperature on SVF is found, which is consistent with current understanding of their inter-dependence. Axial and radial plots of mean SVF categorized into temperature bands of 300K are also reported. These reveal that, while the mean SVF is a function of both temperature and axial distance, the joint PDF depends only weakly on radial distance. The study highlights the value of the simultaneous measurements for understanding soot behavior in a turbulent environment and for model development and validation.

Temperature measurements in turbulent non-premixed flames by two-line atomic fluorescence

The temperature measured by non-linear excitation regime two-line atomic fluorescence, NTLAF, is compared with previous data in a well characterised turbulent non-premixed flame, known as the TNF DLR-A flame. The comparison of the previous detailed single-point measurements with the NTLAF measurements is used to assess the accuracy, and limitations, of the NTLAF technique. The NTLAF measurements were obtained using two different seeding methods, both separately and together, namely by seeding the indium as a solution of indium chloride conveyed as a fine mist with the fuel and by directly seeding neutral indium atoms into the fuel stream by laser ablation of an indium rod. Both instantaneous images and radial profiles of the mean and RMS data are reported for the different techniques. The calculated inter-pixel uncertainty of the measurements is estimated to be ∼50K in the mean, and 8% uncertainty on an instantaneous basis. The comparison is performed on a conditional basis, given that the NTLAF measurements are limited to a lower temperature threshold and to the stoichiometric and rich regions of the flame. On this basis, the NTLAF method is found to generally agree with the TNF DLR-A data to within approximately 100K.

Assessment of interferences to nonlinear two-line atomic fluorescence (NTLAF) in sooty flames

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a laser-based thermometry technique that has application in turbulent flames with soot. However, no assessment of the various interferences from soot or its precursors in flames with high soot loadings on the technique is available. To examine these issues, both on- and off-wavelength NTLAF measurements are presented and compared for laminar nonpremixed ethylene-air flames. Laser-induced incandescence (LII) measurements were used to determine the corresponding soot concentration and location in the investigated flames. The measurements indicate that interferences, such as spurious scattering and laser-induced incandescence from soot, are not significant for the present set of flame conditions. However, interferences from soot precursors, predominantly condensed species (CS) and perhaps polycyclic aromatic hydrocarbons (PAH), can be significant. Potential detection schemes to correct or circumvent these interference issues are also presented.