Thermal durability in aero-engines is a key challenge with the advent of downsized and high-power density combustion chambers. Liner effusion cooling maintains appropriate heat fluxes, thereby improving flame-cooling air interaction (FCAI) processes. Beyond the thermal effectiveness of the wall, FCAI requires a better understanding compared to conventional flame-wall interactions, to assess the modifications of the flame topology. This study intends to examine how the thermal wall cooling performance and the flame topology are impacted when the momentum of a cooling air film (i.e. blowing ratio) is varied. Experiments are performed in a lab-scale combustion chamber operating at atmospheric pressure. A V-shaped turbulent lean premixed CH4/air flame stabilized on a ceramic rod is used to interact with an air-cooled steel wall. The cooling air film is generated by a splash cooling system located at the bottom of the wall. The flame topology, the 2D aerodynamic flow field and the 2D wall temperature distribution are measured simultaneously by Planar Laser-induced Fluorescence on hydroxyl radicals (OH-PLIF), Particle Image Velocimetry (PIV) and surface Phosphor Thermometry (PT). Results reveal various FCAI processes, depending on the blowing ratio induced by the cooling air film. For blowing ratios below unity, the cooling air film provides a limited impact on the thermal protection of the wall, while the flame topology exhibits a flame-wall interaction pattern with positive flame curvatures. For blowing ratios larger than unity, the cooling air film gives an additional insulation layer, reinforcing the wall thermal protection. The strong shear flow layer then governs the level of flame wrinkling. Located in the outer region of the shear layer, the flame is subjected to the effects of negative flow strain, while heat loss/dilution processes remain negligible. Novelty and significance statement Wall thermal management in combustion systems is critical in terms of safety and durability, but also essential to achieve high-efficient combustion systems. Routinely used in combustor liners, cooling air films create a thermal protection of the wall. However, the intrusive cooling air can influence the behavior of a flame, which still remains unclear. Differing from aerothermal studies and classical flame-wall interaction, this study considers the configuration of a flame cooling-air interaction, and systematically elucidates the role of a parietal cooling air film on the dynamics of a turbulent premixed flame. Taking advantage of laser-based planar diagnostics, this study highlights for the first time different regimes of flame-cooling air interaction, being driven by the cooling air momentum. Interestingly, the higher cooling air film efficiency is found to be unfavorable for the flame dynamics, with important flame straining and reduced flame activity.

The Rich burn-Quick mix-Lean burn (RQL) concept is a staged combustion technology that guarantees flame stability at all operating conditions while significantly reducing the concentration of pollutants at the outlet of a combustion chamber. A lab-scale RQL combustion module equipped with a new-generation double-circuit fuel injection system and large optical accesses is designed to be integrated into the visualization module of a highpressure combustion facility fed with aeronautic multi-component liquid fuels. This new optical RQL module will be devoted to perform a simultaneous study into the three RQL sections of the physico-chemical processes involved in soot production and oxidation and NOx formation under realistic high-pressure / high-temperature operating conditions encountered in helicopter combustors by means of advanced coupled laser-based diagnostics. To this end, the architecture of the RQL module has been designed to satisfy various constraints such as a reactive zone height comparable to that of a helicopter combustor sector, an overall pressure drop of similar to 3 %, realistic fuel / air mass flowrates, an efficient heat transfer between the walls and the flame and the ability to introduce laser sheets through the RQL areas for optical measurements. The optimized geometry of the RQL module was achieved by performing iterative Large-Eddy Simulations (LES) of the reactive flowfield of a kerosene vapor / air mixture with the AVBP numerical solver at a nominal operating condition (13 bar), then by LES simulations performed with a liquid kerosene / air mixture. Numerical LES results obtained from both singlephase and two-phase reactive flows are discussed. They highlight the ability to provide distinct RQL areas, a Vshaped jet opening at the injector outlet, a swirling flame topology, a predominant premixed / partially-premixed combustion regime in the primary rich area, as well as a high combustion efficiency of similar to 100 % at the outlet of the combustion chamber. Experimental results of kerosene / air flame emission at 8 bar and 11 bar confirmed the potential to produce well-distinct RQL combustion areas accessible with laser-based diagnostics while ensuring the combustor's thermal resistance. Finally, the effectiveness of the iterative LES methodology adopted to design the lab-scale optical RQL combustion chamber was also highlighted.

Coherent anti-Stokes Raman Scattering (CARS) is a laser-based diagnostic dedicated to the measurement of temperature and major species concentrations in reactive flows. In the femtosecond excitation regime (∼100 fs), CARS enables instantaneous measurements at high repetition rates (i.e., 1–10 kHz), which makes it suitable for studying the temporal evolution of turbulent phenomena inherent to fast fuel mixing characteristics and subsequent dynamic processes inside modern propulsion systems. Among femtosecond CARS strategies, Chirped Probe Pulse CARS (CPP fs-CARS) process with a genetic algorithm has proved to provide reliable measurements of temperature, but suffers from long computation time. A new post-processing approach based on a Convolutional Neural Network (CNN) is proposed to perform temperature measurements from CARS spectra in quasi-real time, while maintaining good measurement accuracy. This method significantly reduces the processing time and paves the way for fast and efficient analysis of a large set of experimental spectra recorded at high repetition rates, thereby improving the practical usefulness of CPP fs-CARS in turbulent reactive conditions.Novelty and significance statement: This study presents a novel data processing strategy for the Chirped Probe Pulse Femtosecond anti-Stokes Raman Spectroscopy (CPP fs-CARS) diagnostic applied to reactive flows, which, to the best of your knowledge, has never been reported in this configuration. Although CPP fs-CARS enables temperature measurements in a single acquisition at high repetition rate, its widespread use has been limited by the substantial computational demands of usual spectral fitting with genetic algorithms. The proposed alternative, based on a Convolutional Neural Network (CNN), overcomes this limitation by enabling near real-time temperature estimation while maintaining a good accuracy required for combustion diagnostics. This advance significantly increases the practical value of CPP fs-CARS for time-domain studies of turbulent and unstable combustion.

The Rich burn-Quick mix-Lean burn (RQL) concept is a promising staged combustion technology that ensures flame stability at all operating conditions and significantly reduces the pollutants concentration at the outlet of a combustion chamber. The current paper aims to detail the design of an optimized RQL lab-scale combustion module equipped with large optical accesses and a new-generation Injection System from the French engine manufacturer Safran Helicopter Engines. It is intended to study soot oxidation and NOx reduction processes under realistic high-pressure conditions up to 14 bar by simultaneously investigating the Rich, Quick-mix and Lean regions by laser-based diagnostics. The design of this module is first ensured by performing Large-Eddy Simulations of the reactive flow produced by a kerosene-vapor/air mixture with the AVBP solver. Various geometric configurations of this module were tested at the nominal regime to gradually improve its performance while seeking a suitable solution optimizing flow, mechanical and thermal constraints. The main versions are first presented to show the progress made to well separate the three regions of the RQL combustor while preserving an ability to perform a detailed optical investigation. A reactive two-phase flow LES with liquid fuel kerosene is finally performed to validate the adopted design of the RQL module.

This study focuses on the development of strut fuel injectors for H-2/air combustion in aircraft engines. A reference architecture was first studied under non-reactive conditions. Optical acetone-PLIF measurements were used to determine the progress of fuel/air mixing close to the injector outlet and the relationship to the momentum flux ratio. The injector was then tested in an academic burner at atmospheric pressure to investigate its combustion performance. A combination of OH* chemiluminescence and NOx measurements using an exhaust sampling probe demonstrated good performance in terms of flame stability and low NOx emissions over a wide range of equivalence ratios. However, the flame length was not satisfactory. To overcome this limitation, strut injectors with novel architectures were developed. OH-PLIF measurements have revealed major improvements in the fuel/air mixture, leading to a significant enhancement in the flame length and flame stabilization, as well as a near-total mitigation of NOx emissions.

Safran Helicopter Engines has recently patented the spinning combustion technology in which the burnt gases from one injector travel tangentially along the combustor annulus towards the neighboring injectors. Compared to conventional designs, the new kerosene injection systems are dedicated to improve air/fuel mixture ignition but also to further reduce NOx and soot particle emissions. Experimental studies are performed on these fuel injectors in a high-pressure/high-temperature combustion facility designed by the CORIA research laboratory. This test bench is able to reproduce the same operating conditions encountered in a helicopter combustor over the entire range of nominal operating conditions and has large optical accesses for the implementation of laser-based diagnostics. In the current paper, we present results concerning flame structure and NO formation in the primary zone under pressure conditions of up to 14 bar, using simultaneous OH-PLIF, NO-PLIF and kerosene-PLIF laser diagnostics. These experimental studies were supplemented by high-speed PIV measurements. A good spatial correlation between the distribution of liquid and vapour kerosene and the location of the flame front was observed. Depending on the operating conditions in terms of fuel/air ratio, mass flow rates and pressure, different flame structures resulting from the modification of the interaction between fuel injection and aerodynamics are observed. Furthermore, it was found that the Zeldovich pathway mainly controls the formation of NO in the vicinity of the flame front. In addition, the effects of FAR and pressure also have a significant impact on NO production. All these results are now intended to serve as a comprehensive validation database for the development and testing of high-fidelity LES tools dedicated to the simulation of reactive flows in aero-engine combustion chambers.

The present work proposes a methodology to include accurate kinetics for soot modeling taking into account real fuel complexity in Large Eddy Simulation (LES) of aeronautical engines at a reasonable computational cost. The methodology is based on the construction of an analytically reduced kinetic mechanism describing both combustion and gaseous soot precursors growth with sufficient accuracy on selected target properties. This is achieved in several steps, starting from the selection of the detailed kinetic model for combustion and soot precursors growth, followed by the determination of a fuel surrogate model describing the complex real fuel blend. Finally the selected kinetic model is analytically reduced with the code ARCANE while controlling the error on flame properties and soot prediction for the considered fuel surrogate. To perform all evaluation and reduction tests on canonical sooting flames, a Discrete Sectional Model (DSM) for soot has been implemented in Cantera. The resulting code (Cantera-soot) is now available for the fast calculation of soot production in laminar flames for any fuel. The obtained reduced kinetic scheme is finally validated in a Rich-Quench-Lean (RQL) burner of the literature in terms of soot prediction capabilities by comparison of LES coupled to the Lagrangian Soot Tracking model (LST) with measurements. Results show a significant improvement of the soot level prediction when using the reduced more realistic kinetics, which also allows a more detailed analysis of the soot emission mechanisms. This demonstrates the gain in accuracy obtained with improved reduced kinetics, and validates the methodology to build such schemes.

Reducing the anthropogenic pollutant emissions is a major concern in the aeronautical community. Implementing high-power density core engines made of lighter materials would undoubtedly increase the thermal efficiency and reduce CO 2 emissions. Nevertheless, near-wall combustion processes will become significant, raising concerns regarding thermal management. As a result, walls in aero-engine combustors are commonly cooled, but it also introduces more complex physical phenomena associated to pollutant formation during flame-cooling air interaction (FCAI). This experimental study aims at elucidating the role of the cooling air film on CO emissions during FCAI. The experiments are conducted in an atmospheric pressure and optically-accessible lab-scale combustion test rig dedicated to near-wall combustion. It generates a lean premixed turbulent methane/air V-shaped flame, with one branch of the flame that interacts with an oil-cooled stainless-steel wall. A splash-cooling plate system enables to generate a momentum-controlled parietal cooling air film. The novelty of this study is to develop and implement planar laser-induced fluorescence of the CO molecule (CO-PLIF), complemented by planar laser-induced fluorescence of the OH radical (OH-PLIF) as well as global CO emissions in the exhaust gases. Results show that the excitation of the Hopfield-Birge system enables to detect the AN ngström bands and the third positive system. However, significant interferences with C 2 and CN emissions are highlighted, being more pronounced in rich combustion conditions, and originating from the incomplete fuel oxidation as well as the high energy density of the two-photon excitation process. A broadband collection strategy of the AN ngström bands is selected, since these interferences are limited in lean combustion regimes. The analysis of the flame dynamics indicates that the increase of the cooling air film momentum shifts the reactive flow away from the wall, but also controls the flame wrinkling through modified aerodynamics. As a result, the flame is not influenced anymore by thermal quenching processes, and can effectively burn further away from the wall. Global emission measurements indicate an increase in CO when the cooling air film momentum increases. While thermal quenching favors a faster oxidation downstream of the flame, the establishment of the cooling air film leads to a longer flame with more production areas. CO-PLIF imaging concurs this phenomenon, with a CO distribution all along the wall when the cooling air film is well established.

A better understanding of the formation of soot particles is essential to improve combustion-related processes. For this purpose, this work reports for the first time the measurement of the size of soot particles under formation in a turbulent atmospheric flame in a planar configuration. This is ensured by the detection of the elastic scattering of a laser sheet thanks to two cameras positioned at different scattering angles (45 degrees and 135 degrees). The size information is then used to interpret the scattering signal collected by an additional camera positioned at 90 degrees in order to evaluate the soot number concentration. This last step relies on a calibration ensured by soot volume fraction and primary particle size previously determined by autocompensating laser-induced incandescence in the same flame. Moreover, the use of a 1 kHz repetition rate nanosecond laser and three high speed CMOS cameras enables the access to these physical parameters with a high temporal resolution. The 2D and time-resolved soot characterization is of interest because it enables the observation of the temporal and spatial modifications of soot structures when propagating in such turbulent flames. In particular, in this flame, the transport and mixing of soot pockets are clearly observed and the impact of these processes on the size and number concentration is quantified. (C) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The development of ultra-compact combustors raises challenges regarding wall thermal management. This study aims to understand the behavior of a reacting flow interacting with a cooling air film generated by an effusion wall. Experiments are carried out in an optically-accessible test rig operating at atmospheric pressure, and generating a premixed methane/air turbulent flame. Particle image velocimetry (PIV) and OH planar-laser induced fluorescence (OH-PLIF) are implemented to assess the cold-flow topology and the flame dynamics. Results indicate that an important turbulent flow activity is present in the developing shear layer, being independent of the hole diameter. The air film creates an aspiration of the mainstream. The blowing ratio modifies the flame wrinkling, amplified by the aspiration of the air film. Changing the main equivalence ratio induces a stronger flame-cooling air film interaction. Eventually, a larger hole diameter with a constant wall porosity increases the hole-to-hole distance, enabling the reactive flow to be engulfed at the wall.

In the context of air pollution, Safran Helicopter Engines patented an innovative design for helicopter combustors based on Spinning Combustion Technology. The development focuses on novel concepts of kerosene fuel injectors aiming to further reduce NOx and soot particle emissions. Experimental studies are performed on the fuel injectors in a high-pressure/high-temperature combustion facility designed by the CORIA laboratory. This test bench is able to reproduce the same conditions encountered in a helicopter combustor over the entire range of nominal operating conditions and has large optical accesses for the implementation of optical diagnostics. NOx and soot particles are assessed during three experimental studies, two of which focus on each pollutant individually and a third one specially dedicated for high-speed velocity measurements by PIV. Soot particles distribution, flame structure and fuel distribution were obtained by coupling the PLIF-OH, PLIF-kerosene and PLII diagnostics. On the other hand, the PLIF-NO combined with PLIF-OH and PLIF-kerosene allows to study the formation of NO with the combustion process and the fuel distribution.

Swirl-stabilized, turbulent, non-premixed kerosene-air flames were studied experimentally in an RQL (Rich burn/Quick-quench/Lean-burn) model combustor equipped with large optical accesses. The goal of these studies was to characterize the flame topology as well as soot and NO formation processes in the rich primary zone of the RQL combustor, and to establish a large database for future validation of numerical simulations. The experiments were performed under relevant operating conditions up to 4.5 bar. The aerodynamic flow field was measured by Particle Image Velocimetry, the flame structure, fuel and NO distributions by Planar Laser-induced Fluorescence and soot volume fractions by Planar Laser-induced Incandescence. Additional measurements were performed by a scanning mobility particle sizer technique to record the number of soot particles per unit volume as well as the particles size. Two equivalence ratio conditions were used to study the impact of relevant scalar parameters on NO and soot production. For each condition, instantaneous and average distributions of the measured parameters are presented and discussed. The coupling of the optical and intrusive measurement techniques has finally enabled to highlight the impact of the liquid and gas phases fuel distribution on the flame structure, but also on NO and soot formation.

Innovative low-carbon aero-engines rely on ultra-compact and high energy density combustors, increasing the thermal loads of the walls. This study aims at experimentally enhancing the understanding of the interrelated physical processes between a flame, a wall and a cooling technology. The test rig operates at atmospheric pressure and generates a methane/air turbulent flame interacting with a wall that is air-cooled by means of a splash cooling plate device. Optical diagnostics are simultaneously implemented, including reactive particle image velocimetry (PIV) to assess the flow field, planar laser induced fluorescence on the OH radical (OH-PLIF) to study the flame structure, and laser induced phosphorescence to measure the wall temperature in contact with the reactive flow. Results show that the mean flame location is shifted away from the wall as the blowing ratio is increased. The wall temperature reveals that to enhance cooling efficiency, blowing ratio and flame location must not be considered without each other. Eventually, a classical flame-wall interaction process is retrieved when the blowing ratio is small, whereas flame strain, originating from the aerodynamic shear layer, pilots the flame dynamics for large blowing ratios.

This paper illustrates the use of chirped-probe-pulse fs-CARS thermometry in combination with a high-pressure optical test rig for lean-burn combustor development. Temperature measurements were performed at 1 kHz in kerosene/air combustion at 0.75 MPa.

Surface phosphor thermometry is an attractive remote temperature diagnostic to study heat and mass transfer processes. The spectral intensity ratio method enables to obtain two-dimensional temperature measurements with good spatial resolution, but suffers from sources of uncertainties that limit its use in terms of temperature uncertainties. This study aims to evaluate the improvement of the temperature uncertainty of the Mg 4 FGeO 6 :Mn 4+ ratio-based phosphor thermometry when selecting different optical filters for the detection. This selection is done with the use of a numerical optimization procedure and multiple objective functions. Assuming a shot-noise limited regime for the phosphorescence signals, a series of experimental phosphorescence spectra recorded at various temperatures are combined with different virtual optical filters in order to minimize the relative temperature uncertainty as well as its standard deviation on the 300–750 K temperature range. The best combination of optical filters is composed of a filter centered on the 660 nm emission band, the other one being shifted towards the red wing. A further analysis indicates that the relative sensitivity is favored while the signal-to-ratios are fairly constant. A parametric study on the impact of the multi-objective function as well as the extent of the explored temperature range highlights that the mean temperature uncertainty criterion mainly drives the optimization procedure, notably with a weighting given to high-temperature spectral properties. Finally, a realistic temperature imaging arrangement composed of an ICCD camera, an image doubler and commercial bandpass filters, validates this new strategy. The relative temperature uncertainty is ≈ 2.9 % between 300 and 750 K offering enhanced performances (up to three times) when compared to usual optical filters reported in the literature and centered on the two main emission bands.

The current experimental study is focused on the analysis of the interactions between a V-shaped lean turbulent premixed methane-air flame and a parietal cooling air film generated by means of a splash-cooling system. The flame structure, the reactive aerodynamics and the wall temperature are simultaneously studied by implementing planar laser-induced fluorescence of the OH radical (OH-PLIF), reactive particle image velocimetry (PIV) and surface intensity-ratio phosphor thermometry (PT). Results show that the mean flow field and the mean flame topology significantly differs according to the level of blowing ratio. Further joint correlations of experimental measurements highlight various FCAI regimes. For blowing ratios below unity, a common flame-wall interaction process occurs and the air film has a limited influence on the flame dynamics. Increasing the blowing ratio enables to establish a cooling air film at the wall. Though it effectively increases the level of thermal protection, it does not have much influence on the flame topology. Eventually, for larger blowing ratios, the flame dynamics is piloted by the shear layer dynamics, associated with strong flow strain. The flame is pushed away from the wall and is located in the outer region of the shear layer. The cooling effectiveness is not improved anymore and shows no more dependencies with the blowing ratio.

Intensity-ratio based phosphor thermometry is a well-established technique to obtain surface temperature measurements, which however suffers from important pixel-to-pixel noise, mainly due to the signal-dependent photon shot noise. In order to enhance signal-to-noise ratio levels, spatial filtering is recognized as a common post-processing routine, with the implication of strongly alleviating the spatial resolution (SR) of measurements. In this study, a dataset of phosphorescence images using Mg 4 FGeO 6 :Mn 4+ thermographic particles and an ICCD camera is constituted. Various spatial filtering strategies are applied to these images, such as software pixel binning, moving average and Gaussian filters, in order to estimate their relative performances in terms of SR and temperature uncertainty. In addition, a Fourier space low-pass Butterworth filter is benchmarked against these common filters. Results show that the pixel binning strategy provides a limited improvement in the temperature uncertainty when compared to the loss in SR. Conversely, the Gaussian and moving average filters are found to effectively enhance the temperature uncertainty, though the 5th-order Butterworth filter is more selective, by providing an excellent mitigation of high-frequency noise with a minor attenuation of low-frequency information. Eventually, a joint spatial-spectral filtering strategy is investigated, which however does not present significant advantages compared to a sole filtering strategy.

Temperature uncertainty improvements of surface phosphor thermometry imaging are reported, with the intensity-ratio method. Analysis of optical filters/post-processing spatial filters with Mg4FGeO6:Mn4+ thermographic phosphors results in a ≈1% temperature uncertainty between 300 and 825 K.

The development of lighter low-carbon technologies in the aviation sector raises a technological challenge regarding material durability and wall thermal management. This study aims to introduce a new optically accessible test rig to investigate the interaction between a premixed CH4/air turbulent flame and a parietal cooling air film for various blowing ratios. Laser Doppler velocimetry (LDV) is implemented to measure the cooling air and the mainstream inflow conditions. The impact of the blowing ratio on the mean flame topology is conducted with OH* chemiluminescence. Results show that the flame-wall distance increases with the blowing ratio and eventually reach a plateau, while the flame length constantly decreases. OH Planar-laser induced fluorescence (OH-PLIF) and phosphor thermometry (PT) are performed to assess the near-wall flame structure and the cooling effectiveness. The flame seems to be aerodynamically controlled while the cooled-air momentum has a beneficial impact on the wall temperature.

Soot particles are one of the main causes of today's pollution because of their negative contribution to global warming and human health. Aviation is one of the domains dealing with soot reduction as the new engine concepts are developed to reduce fuel consumption and global emissions. Despite the fact that most combustion devices used for air transportation operate at high pressure (e.g., aircraft gas turbines up to 40 bar), our understanding of soot formation and oxidation in such conditions is not yet at an appropriate level, as there is still a fundamental lack of experimental data and corresponding predictive models in the literature. Thus, the objective of the current study is to evaluate soot formation and oxidation processes in stratified, swirled, premixed ethylene/air flames examined with a variety of laser diagnostics designed to simultaneously measure soot particle and soot precursor 2D-distributions, as well as the flame structure and the aerodynamic field. For that, the SIRIUS burner was selected because of its ability to produce flames with topologies similar to those encountered in aircraft combustors. Soot particle distributions are measured by Planar Laser-induced incandescence (PLII) diagnostic. The flame structure is obtained by detecting the hydroxyl radicals (OH) with Planar laser-induced fluorescence (PLIF). A second PLIF diagnostic is also used to investigate the production of polycyclic aromatic hydrocarbons (PAH) with the detection of two benzene rings molecules which are recognized as good precursors of soot nucleation and growth. Finally, the particle Image Velocimetry (PIV) diagnostic is used for measuring the velocity distributions. These laser diagnostics are coupled together in order to obtain cross-correlations between several scalar parameters playing a determining role in the soot formation/consumption processes. The experimental results collected at atmospheric pressure are reviewed and critically assessed. A scenario describing the link between the soot inception, growth, aggregation and oxidation processes is proposed by analyzing velocity, OH, PAHs and soot distributions. In particular, the data reveal the presence of distinct regions for these processes. Incipient soot production zone is strongly function of specific local conditions of velocity, PAH concentration, and strain rate encountered at the interface of the internal recirculation zone and the fuel/air jet. The central part of the inner recirculation zone in which large structures move at low velocities provides suitable conditions for the aggregation of nascent soot particles while an oxidation region located in the upper zone of the internal recirculation zone favors the consumption of soot.