Planar Laser-induced Fluorescence Research Articles

Spatiotemporal distribution characteristics analysis of local flameout in jet flames based on 5 kHz planar laser-induced fluorescence

Spatiotemporal distribution characteristics analysis of local flameout in jet flames based on 5 kHz planar laser-induced fluorescence

Topology characteristics of liquid ammonia swirl spray flame

The utilization of liquid ammonia in gas turbines can reduce energy loss and start-up time. However, the flash boiling phenomenon and the high latent heat of liquid ammonia make the spray flame difficult to stabilize. Increasing the preheated air temperature or adding a small amount of hydrogen as a piloted fuel are considered as effective methods to enhance the stability. To understand the flame topological structure, simultaneous Mie scattering and planar laser-induced fluorescence of OH (OH-PLIF) techniques were used to visualize the liquid ammonia spray structure and flame region information. Results show that the liquid ammonia swirl spray flame exhibits the flame topological structure of distinct zoning characteristics, including the droplet zone, the mixing zone, and the flame zone. Increasing the preheated air temperature accelerates the evaporation of liquid ammonia, leading to an increase in the local equivalence ratio and radial flame splitting. At lower air temperature conditions, increasing the hydrogen blending ratio has minimal impact on the flame topological structure. However, at higher temperature conditions, hydrogen blending significantly promotes reaction intensity upstream and reduces the flame lift-off height, which makes the mixing zone smaller. In general, to achieve a better flame stability effect, the two factors need to be reasonably matched, which has important reference value for the development of liquid ammonia fueled gas turbine combustors.

Structure and nitric oxide formation in laminar diffusion flames of ammonia–hydrogen and air

In this work, we present an investigation of the structure and formation of nitrogen oxides (NOx) in an axisymmetric laminar diffusion flame in which an ammonia–hydrogen fuel stream is surrounded by co-flowing air. Green ammonia, produced using renewable energy, is a promising carbon-free fuel for replacing hydrocarbon fuels over a diverse range of combustion applications. A key challenge for ammonia combustion is to understand the kinetics of nitrogen oxide formation for designing low-NOx combustion systems. In this experimental study, ammonia fuel is blended with hydrogen to increase fuel reactivity, with a fuel composition range of 15%–100% hydrogen. NOx emissions and fuel slip are measured by downstream sampling of NO, NO2, N2O and NH3 with a Fourier Transform Infrared (FTIR) gas analyzer. Flame structure is visualized by planar laser-induced fluorescence (LIF) for NO, NH, and OH radicals, as well as OH* by filtered chemiluminescence. Species concentration profiles are compared to predictions from 2-dimensional axisymmetric numerical models of the laminar flame using recent kinetic mechanisms developed for ammonia combustion. No measurable ammonia slip was recorded for any fuel composition, in agreement with model predictions. Good agreement between predictions and measurements was obtained for exhaust nitrogen oxide concentrations and for in-flame radical profiles, although larger variation and deviations are observed for predictions of NO2 and N2O than for NO. Numerical predictions for spatial distribution of flame radicals and flame liftoff height also showed good agreement with LIF and chemiluminescence measurements. For high fuel hydrogen content, the measured NH and NO profiles are shifted inward toward the fuel outlet, away from the peak temperature contour. Reaction path analysis is used to illustrate the important contributions to NO creation and destruction, including the differing role of key reaction pathways with varying fuel hydrogen content.

Soot and PAH formation in laminar diffusion flames of RP-3 jet kerosene and its surrogates at preheat temperature

For investigating the effects of preheat temperature on the soot/PAH formation of RP-3 kerosene surrogates, planar laser-induced incandescence (PLII), planar laser-induced fluorescence of PAH (PAH-PLIF) and chemical kinetic simulation are conducted in several laminar diffusion flames. The goal is to provide a surrogate that matches well with the soot formation characteristics of RP-3 kerosene and to investigate the mechanism behind the temperature sensitivity of different surrogates. Firstly, surrogate S1 (30.3% n-hexadecane, 39.8% iso-cetane, 29.8% o-xylene, in mole fraction) is formulated to obtain a surrogate with good reproducibility of soot formation characteristics corresponding to the RP-3 jet kerosene. Compared with another reported surrogate S2, S1 presents a higher soot yield due to its higher PAH formation and a stronger soot formation propensity. With elevated preheat temperature, soot yield of all the studied flames gets higher, and the temperature sensitivities for soot formation of S1 and S2 are shown with little difference. By further analyzing the PAH results experimentally and numerically, the temperature sensitivity for soot yield of S2 is more closely related to the temperature sensitivity of PAH precursors, while the subsequent processes after soot inception may be more important to the temperature dependence of S1.

Investigation of the effect of hydrogen addition on soot and PAH formation in ethylene inverse diffusion flames by combined LII and PAH LIF

It is shown that signals from both laser-induced incandescence (LII) of soot and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAHs) decrease with hydrogen addition (in volume fractions of 3%, 6% and 9% with respect to the fuel mixture) to ethylene–air inverse diffusion flames (IDFs). The structure of the IDF suppresses soot oxidation and the effect of hydrogen addition under these conditions has been studied. Experiments were performed using a frequency-doubled, pulsed dye laser to perform planar LIF of PAH, and a pulsed fibre laser to perform LII. In relative terms, the LII signal decreases more sharply than the PAH LIF signal. This would be consistent with the dependence of soot inception on PAH concentration as well as the suppression of soot growth via the reduced concentration of PAH and perhaps other precursors such as acetylene. Similar trends in relative signal decrease are observed at a range of heights above the burner, despite the measurement locations encompassing a wide range of absolute signal levels. As a comparison, the influence of adding methane to the IDF in the same volume fractions was also studied and found to suppress PAH LIF and LII signals but to a far lesser extent than in the case of hydrogen.

Investigation of the influence of the bluff-body temperature on a lean premixed DME/air flame approaching blowoff

In flames stabilized by a two-dimensional (2D) bluff body, the lean blowoff (LBO) limit can be extended by increasing the bluff-body temperature, which is verified experimentally in premixed dimethyl ether (DME)/air flames in this work. Two bluff-body temperatures of 573 K and 773 K are tested, respectively. High-speed OH* chemiluminescence (CL) imaging is applied to record the spatial distribution of the heat release. Particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) are performed simultaneously to explore the flow field and the combustion characteristics. Based on the results of OH*-CL, the heat release of the area near the bluff body is strengthened in the case of higher bluff-body temperature, and the enhanced chemical reactivity will further influence the downstream area. The turbulent flame speed obtained by OH-PLIF is increased in the entire area of interest. Due to the higher velocity gradient adjacent to the bluff body in the upstream region (y < 35 mm), the flame surface density (FSD), brush thickness, and flame wrinkling ratio are not sensitive to the bluff-body temperature. However, in the downstream region (y > 35 mm), a noticeable change can be found in these three parameters due to a higher turbulent flame speed and a much lower velocity gradient. Based on the PIV results, a limited effect on the velocity field by increasing the bluff-body temperature by 200 K. Furthermore, the probability density functions (PDF) of the vorticity and strain rate on the flame front are computed. The results indicate that the enhanced flame stability leads to a more pronounced overlapping between the flame front and the high-vorticity region, further increasing the values of these two parameters.

Impact of pilot flame and hydrogen enrichment on turbulent methane/hydrogen/air swirling premixed flames in a model gas turbine combustor

This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures.

The relative efficiencies of the entrainment of mass, momentum and kinetic energy from a turbulent background

We derive expressions relating the entrainment fluxes of momentum and kinetic energy, relative to the mass flux entrained into a turbulent wake exposed to a turbulent background. These expressions contain correlations between the entrainment velocity and the turbulent fluctuations within the background. We perform high-resolution, simultaneous particle image velocimetry and planar laser-induced fluorescence experiments, and observe these correlations to be negligible in the far wake, such that momentum and kinetic energy are entrained into the wake with the same relative efficiency to mass as from an idealised, non-turbulent background. This is a useful result in the context of modelling, since the entrainment hypothesis (Turner, J. Fluid Mech., vol. 173, 1986, pp. 431–471) can still be used to model the entrainment of momentum and kinetic energy. Nevertheless, the entrainment rate of mass is shown to vary spatially, and with the specific nature of the background turbulence, so this in turn drives a spatial/background-turbulence-specific entrainment rate of momentum/kinetic energy. Contrastingly, in the near wake, whilst momentum is entrained from a turbulent background with the same relative efficiency to mass as for an idealised non-turbulent background, this is not the case for kinetic energy. Owing to the sum of multiple positive, small-valued correlations between the fluctuations in the background and the entrainment velocity, kinetic energy is entrained more efficiently than in the idealised case. This includes entrainment from a non-turbulent background, where small correlations are observed between the irrotational background fluctuations and the entrainment velocity. Evidence is also presented that the entrainment velocity scales with the Kolmogorov velocity scale when the background is turbulent.

Characterization of a Novel Additive Manufacturing Micromix Nozzle Burning Methane to Hydrogen

Abstract As the energy landscape transitions to low/zero-carbon fuels, gas turbine manufacturers are targeting fuel flexible operation with natural gas, syngas, and hydrogen-enriched mixtures. Having a single geometry that can support the different fuel blends requested by clients can accelerate the transition to cleaner energy generation and mitigate the environmental impact of gas turbines. Toward this goal, micromix combustion technology has received significant interest, and when coupled with additive manufacturing, novel injector geometries with unique configurations may be capable of stabilizing premixed, partially-premixed, and diffusion flames using fuel mixtures ranging from pure methane to pure hydrogen. In this work, a preliminary investigation of this micromix concept is performed in the Atmospheric Combustion Rig at the National Research Council (NRC) Canada. Flame stability maps are obtained for fuel lean mixtures of H2/CH4 ranging from 0/100, 70/30, 90/10, to 100/0%, by volume. Multiple flame shapes are observed depending on the fuel mixture and combustion mode selected. Particle image velocimetry (PIV), OH, and acetone planar laser-induced fluorescence (PLIF), and acoustic measurements provide additional insights into the combustion process of these novel burners to better understand the stability mechanisms. The quality of the fuel–air mixing from the premixed and micromix injectors is assessed using acetone as a tracer for the fuel, while simultaneous OH-PLIF measurements provide an indication of the postflame regions in the flow. Acoustic measurements complete the current dataset and provide combustion dynamics maps measuring the normalized pressure amplitudes and identifying the dominant frequencies. The preliminary characterization of this additive manufacturing (AM) micromix nozzle shows promising fuel flexibility with wide stability margins and low combustion dynamics for this single nozzle burner.

Measurement of Soot Volume Fraction in Counterflow Diffusion Flames Up to 10 Atm Using Optical Diagnostics

ABSTRACTCombustion pressure has an influence on formation and destruction of soot. The balance of these processes determines the net soot emissions. In this work, the soot volume fraction (fv) in ethylene air counterflow flames has been investigated at elevated pressures using laser-induced incandescence (LII) and two-dimensional line of sight attenuation (2D LOSA) measurements. Pressure has been varied from 3 to 7 atm for fv measurements using 2D LOSA whereas fv has been reported from 3 to 10 atm using LII. Above 7 atm, counterflow flames were not perfectly axisymmetric, and therefore LII has been used instead of 2D LOSA above 7 atm. The fuel mole fraction varied between 0.2 and 0.3. Attenuation of LII signal has been corrected using LOSA measurements. Planar laser-induced fluorescence was used to study the location of polycyclic aromatic hydrocarbons (PAHs) at 3 and 7 atm. Locations of various sizes of PAHs have been reported by detecting the fluorescence signal at ranges of wavelengths centered at 350, 400, 450 and 510 nm. fv measurements by 2D LOSA and LII showed a good agreement. The LII signal showed significant attenuation due to the optical density of the counterflow flames, which highlights the importance of correcting the LII signal before inferring the values of the soot volume fraction. An increase in pressure resulted in an increase of the soot volume fraction and the overlap of the PAH zones and soot zone increased at elevated pressures.

Modeling of wavy water film by application of artificial neural network - a state of art study

Nowadays, artificial intelligence (AI) tools have attained a state of maturity and have advanced at a quick rate of achievement in various fields of study. Thus, the predictive and classification of artificial intelligence AI capabilities in manufacturing, commercial, and scientific research applications have resulted in remarkable success. It is widely known that two-phase flow is such a complex problem, and it has a vital state of study due to its importance and wide range of applications. Such as water film phenomena are critical for various industries, including nuclear protection, energy generation, absorption processes, internal combustion engines, chemical reactors, and spray cooling. The present study presented a realistic model for a particular two-phase flow problem of water film thickness. Also, we used neurotic networks for prediction and classification, such as the conventional neural network (CNN) and Long Short-Term Memory (LSTM). The proposed model was evaluated using various raw data, including the input liquid Reynolds number, the Reynolds gas number, the angle of inclination, and the thickness of the water film. The water film thickness was determined using a planar laser-induced fluorescence (PLIF) technique on the various inclination platforms. The current experimental work's inclination angle varied between 0 and 50 degrees. However, when the Reynolds number of the input water changed from 175 to 435, the thickness of the water film was determined; Reynolds gas numbers range from 6950 to 34800.The current study findings indicate an effective alternative method for detecting and forecasting water films on a sloping plate. Nash-Sutcliffe Efficiency (NSE) affected 99 %, 97 %, and 94 % of forecasts, respectively. The findings show tremendous improvement when deep learning uses both LSTM and CNN. The gravitational pressure and air share tension push the water film.

Effect of filament length on the flow field of a rotationally oscillating cylinder with an attached filament

Experimental studies are conducted to examine the effect of filament length on the flow field and forces in the flow past a rotationally oscillating cylinder with an attached filament. The diagnostics are flow visualization through planar laser-induced fluorescence technique and characterization of the velocity and vorticity field using planar particle image velocimetry measurements. The streamwise force is estimated through control volume analysis, using an improved expression, described in Sunil et al., J. Fluid Mech. 930, A3 (2022). With an increase in the filament length, the wake structure transitions from a Kármán wake to a reverse Bénard–Kármán wake and subsequently to an asymmetric wake. This happens at a Strouhal number of StA = 0.25 and 0.6, respectively. The transition from a Kármán wake to a reverse Kármán wake precedes the drag-to-thrust transition. Thrust generation is observed to be a function of both the cylinder forcing parameters and the filament length. Shorter filaments (L/D = 0.5 and 1) produce drag, whereas longer filaments (L/D = 2, 3, and 4) produce thrust at FR≥ 2.5. Normalized values of circulation scale well with the filament length, L, and the filament tip velocity, VTE, in the case of longer filaments. The effect of filament length on vortex dynamics and thrust generation is examined through a time evolution of the instantaneous vorticity, velocity, and kinetic energy data. Maximum momentum and kinetic energy content is observed in the flow field near the instants when the vortices are shed in the wake.

Characterization of Spray Field for Water-Emulsified Diesel Using a Pressure Swirl Atomizer Under a Nonreacting Environment

Abstract Employing a mixture or an emulsion of water and diesel fuel is considered a way to reduce gas emissions such as NOx and soot in a gas turbine. This study presents detailed experimental results on the spray characteristics of a water–diesel emulsion injected by a pressure swirl atomizer with a 90-degree spray angle and a flow number of 0.58 under a nonreacting environment at high pressure and temperature conditions. Acquiring these data is a key step when configuring a combustor that will employ emulsified fuels. In addition, this study seeks to confirm that the emulsion stays intact when it gets sprayed into the combustor. Furthermore, this study attempts to understand if a water–diesel emulsion prepared by a sonicator improves fuel atomization as compared to a water–diesel mixture prepared by a static mixer, i.e., not a proper emulsion. Tests are conducted in a high pressure and temperature testing facility at two ambient pressures and three ambient temperatures and the water to diesel ratio (W/D) is varied from 11% to 100% by mass. Phase Doppler Particle Anemometry (PDPA) is employed to measure the spray characteristics. Through a backlit high-speed photography, overall spray patterns over different test conditions are visualized. Mie-scattering and planar laser-induced fluorescence imaging are utilized to visualize the mixture field. In general, the results indicate that emulsion stays intact as it gets sprayed into the combustor; and emulsion is a better solution to reduce emissions than a statically mixed mixture.

Simultaneous measurements of kinetic and scalar energy spectrum time evolution in the Richtmyer–Meshkov instability upon reshock

The Richtmyer–Meshkov instability (Richtmyer, Commun. Pure Appl. Maths, vol. 13, issue 2, 1960, pp. 297–319; Meshkov, Fluid Dyn., vol. 4, issue 5, 1972, pp. 101–104) of a twice-shocked gas interface is studied using both high spatial resolution single-shot (SS) and lower spatial resolution, time-resolved, high-speed (HS) simultaneous planar laser-induced fluorescence and particle image velocimetry in the Wisconsin Shock Tube Laboratory's vertical shock tube. The initial condition (IC) is a shear layer with broadband diffuse perturbations at the interface between a helium–acetone mixture and argon. This IC is accelerated by a shock of nominal strength Mach number $M = 1.75$ , and then accelerated again by the transmitted shock that reflects off the end wall of the tube. An ensemble of experiments is analysed after reshock while the interface mixing width grows linearly with time. The kinetic and scalar energy spectra and the terms of their evolution equation are calculated and compared between SS and HS experiments. The inertial range scaling of the scalar power spectrum is found to follow Gibson's relation (Gibson, Phys. Fluids, vol. 11, issue 11, 1968, pp. 2316–2327) as a function of Schmidt number when the effective turbulent Schmidt number is used in place of the material Schmidt number that controls equilibrium scaling. Further, the spatially integrated scalar flux follows similar behaviour observed for the kinetic energy in large eddy simulation studies by Zeng et al. (Phys. Fluids, vol. 30, issue 6, 2018, 064106) while the spatially varying scalar flux exhibits back scatter along the centre of the mixing layer and forward energy transfer in the spike and bubble regions.

Experimental investigation of NO reduction by H2 on Pd using planar laser-induced fluorescence

This study investigates the NO reduction by H2 on a Pd/Al2O3 catalyst in a temperature range of 100–300 °C and NO/H2 ratios from 0.5–2, aiming to gain a deeper understanding of the reaction kinetics and its interaction with mass transfer. Planar laser-induced fluorescence (PLIF) is used to visualize the NO distributions over the catalyst, supplemented by end-of-pipe gas analysis of other components. The reduced Pd-based catalyst undergoes a slow deactivation after exposure to the reactive flow, leading to reduced overall NO conversion and decreased selectivity towards N2. The NO-PLIF measurements are only conducted on the reduced catalyst without considering the temporal evolutions. Despite the overall NO conversion varying only around 50%–65% across all the investigated conditions, the spatially resolved NO distributions reveal three distinct regimes that limit the overall NO conversion: the regime governed by intrinsic reaction rates, the regime constrained by H2 availability, and the regime restricted by NO diffusion. These findings, demonstrating the interaction between reaction kinetics and mass transfer over a heterogeneous catalyst, highlight the significance of analyzing spatially resolved concentration distributions obtained through PLIF measurements. This approach complements the conventional end-of-pipe analysis, offering a more comprehensive understanding of the underlying processes.

Effect of ferrocene as a combustion catalyst on the premixed combustion flame characteristics of Jatropha biodiesel

Fuel additives are widely used to optimize the combustion process, reduce the harmful emissions, and improve the performance of engines. Among the metal-based additives, ferrocene is soluble in most fuels, including biodiesel. Furthermore, it is easily available and has high stability, low toxicity, and high lipophilicity. It can evenly disperse in biodiesel without causing any change in the physicochemical properties. Therefore, it can serve as an effective combustion catalyst for biodiesel. In this study, the effect of ferrocene as a combustion catalyst on the pyrolysis and combustion performance of Jatropha biodiesel is investigated. The coupled technique of thermogravimetry-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) is used to analyze the evolution characteristics of multicomponent gaseous products generated during the pyrolysis and high-temperature oxidation of Jatropha biodiesel (JB100) and Jatropha biodiesel + ferrocene (JBF). The laminar premixed combustion flame and pollutant emissions of JB100 and JBF are examined using a variety of techniques, including OH planar laser-induced fluorescence (OH-PLIF), flue gas analysis, spectral analysis, transmission electron microscopy, etc. The results reveal that after adding ferrocene, the activation energy for the pyrolysis reaction of JB100 decreases by 4.66 kJ/mol and the complete weight loss temperature decreases by 10 ℃. Further, the addition of ferrocene reduces the endothermic heat by 0.38 mW/mg during the initial stage of the high-temperature oxidation reaction of JB100, and the amounts of final products CO2 and H2O are increased. Compared with JB100, JBF shows an enhanced OH group intensity in the combustion flame and a lower soot spectral peak. Furthermore, compared with JB100, under JBF combustion, the pollutant PM2.5 emissions decrease by 13.3 %, PM10 emissions decrease by 13.2 %, PM> 10 particulate matter emissions decrease by 19.3 %, CO emissions decrease by 6.8 %, NOx emissions increase by 21.2 %, and the size of soot particles decreases by 20.8 %.

Dynamics of multilayer Rayleigh–Taylor instability at moderately high Atwood numbers

This paper investigates the multilayer Rayleigh–Taylor instability (RTI) using statistically stationary experiments conducted in a gas tunnel. Employing diagnostics such as particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF), we make simultaneous velocity–density measurements to study how dynamics and mixing are linked in this variable density flow. Experiments are conducted in a newly built, blow-down three-layer gas tunnel facility. Mixing between three gas streams is studied, where the top and bottom streams are comprised of air, and the middle stream is an air–helium mixture. Shear is minimized between these streams by matching their inlet velocities. The four experimental conditions investigated here consist of two different density ratios (Atwood numbers 0.3 and 0.6), each investigated at two instability development times (or equivalently, two streamwise locations), and all experiments are with the same middle stream thickness of 3 cm. The growth of the middle layer is measured using laser-based planar Mie scattering visualization. The mixing width is found to grow linearly with time at late times. Various quantitative measures of molecular mixing indicate a very high degree of molecular mixing at late times in the multilayer RTI flow. The vertical turbulent mass flux$a_y$is calculated. In addition to mostly negative values of$a_y$, typical of buoyancy-dominated flows due to negative correlation between velocity and density fluctuations, positive regions are also observed in profiles of$a_y$due to entrainment and erosion at the lower edge of the mixing region. Global energy budgets are calculated for the multilayer RTI flow at late times and it is found that the majority of potential energy released has been dissipated due to viscous effects, and a large value of mixing efficiency ($\sim$60 %) is observed.

Turbulent Flame Speed and Flame Characteristics of Lean Premixed H2-CH4 Flames at Moderate Pressure Levels

Abstract Carbon dioxide emissions in gas turbine power generation can be reduced by adding an increasing amount of hydrogen to the existing natural gas-fueled combustion systems. To enable safe operation, more insight on how H2 addition affects turbulent flame speed and other important flame characteristics is needed. In this work, the investigation of hydrogen addition effects on certain flame properties has been carried out in a high-pressure axial-dump combustor at gas turbine relevant conditions. OH planar laser induced fluorescence (PLIF) was applied to retrieve flame front contours and turbulent flame speed. The results show that as the concentration of hydrogen in the fuel mixture increases, turbulent flame speed and flame characteristics change drastically. Two main regimes can be identified: From 0 to 50% vol. Hydrogen, the turbulent flame speed increases weakly in an almost linear fashion, while from 50% vol. to 100% vol. the trend sharply changes and the higher reactivity of hydrogen, combined with a lower Lewis number, cause thermal-diffusive instability and preferential diffusion effects to become increasingly strong, leading to very high burning rates. The presented results help to understand and to define the relevant modifications that are necessary to successfully operate gas turbine combustor systems with high H2 content fuels.

Effects of hydrogen on PAH and soot formation in laminar diffusion flames of RP-3 jet kerosene and its surrogate

PAH and soot are harmful substances that can be produced in any type of combustion equipment including aircraft engines. The co-combustion of hydrogen and jet fuel has been applied in aero-engine combustors and large-scale hydrogen addition is a promising solution for reducing the consumption of fossil fuels in the aviation industry. However, it remains unclear how H2 influences the soot and PAH formation characteristics of jet kerosene. In this study, to investigate the impact of H2 on soot and PAH formation, planar laser-induced incandescence (PLII), planar laser-induced fluorescence of PAH (PAH-PLIF) and chemical kinetic simulation were conducted for the laminar diffusion flames of RP-3 jet kerosene and its surrogate S1 with different H2 doping rates. It is found that the introduction of H2 leads to the increased soot formation. However, the promotion effect of H2 on the PAH formation weakens as the number of PAH rings increases, and the formation of A4 is significantly inhibited. But the rapidly increase of benzene and alkynes in the H2-doped kerosene flame may ultimately lead to the promotion of soot formation. Furthermore, the changes in direct synthesis reactions and PAH=>PAH- jointly affect the converse changes in A1 and A4 formation. These findings will contribute to the development of the soot model and soot/PAH-reduction strategy for the co-combustion of jet fuels and hydrogen.

Ambient pressure operando catalytic characterization by combining PM-IRRAS with planar laser-induced fluorescence and surface optical reflectance imaging

We present a combination of optical operando techniques that allow us to bridge the pressure gap in heterogeneous catalysis. By combining Polarization Modulated - InfraRed Reflection Absorption Spectroscopy (PM-IRRAS) with two dimensional-Surface Optical Reflectance (2D-SOR) and Planar Laser Induced Fluorescence (PLIF), we can simultaneously measure the adsorbed species on the catalyst surface, monitor the surface oxide formation across the catalyst surface and image the gas phase right above the catalyst surface, respectively. In a single measurement, we are able to follow heterogeneous catalytic reactions temporally- and spatially resolved with all three optical techniques, which are additionally supported by Mass Spectrometry (MS). To validate the experimental setup, we perform two experiments studying CO oxidation on Pd(100) at 150 mbar and 910 mbar by ramping the sample temperature. PM-IRRAS and 2D-SOR reveal that the formation of well-defined ultrathin surface oxide coincides with the disappearance of CO adsorption on the surface. At the same time, PLIF and MS confirm the simultaneous transition into a mass-transfer-limited (MTL) regime. A difference between 150 and 910 mbar can be seen in the light-off temperature caused by different partial pressures of CO and in the spatial distribution of the gas cloud across the surface in space caused by gas diffusion. This emphasizes the need for spatially-resolved gas phase diagnostics in heterogeneous catalysis. The combination of all techniques aids our understanding of the gas-surface interaction.