Detector Assessment for 1D Single-Shot Spontaneous Raman Scattering for Temperature and Multi-Species Measurements in Flames

International audienceThe critical aspect of 1D single-shot Spontaneous Raman Scattering (SRS) experiments in flames is the requirement of high efficiency of the detection system associated with a fast temporal gating. Single-shot SRS measurements in flames are performed either with ICCD or with back-illuminated CCDs associated with a fast shutter. Two types of back-illuminated CDD detectors are used: a back-illuminated CCD (BI-CCD) and electron multiplying CCD (BI-EMCCD). The purpose of the present paper is to compare the three detectors: the ICCD with its intensifier gating and the back-illuminated CCDs with a Pockels cell shutter developed in a previous work (Ajrouche et al, 2015). The accuracy and uncertainty of 1D single-shot SRS measurements of temperature and density are quantified in near-adiabatic CH4/air flames. This is performed for N 2 number density (high signal levels), and CO number density (low signal levels) corresponding to signal close to the detectability limit. The temperature is determined by modeling the vibration-rotation spectra of N 2 offering advantage of not requiring reference temperature and the modeled spectra are used as smoothed spectra to determine the instantaneous number densities. On one hand, the BI-CCD with the Pockels cell shutter is the most efficient detection systems in extreme low light situations for single-shot temperature measurements, and on the other hand the BI-EMCCD is the most powerful tool for best detectability of low density species

Simultaneous measurements of PIV, anisole-PLIF and OH-PLIF for investigating back-supported stratified flame propagation in lean and nonflammable mixtures

International audienceIn an effort to reduce pollutant emissions and increase energy efficiency, partially premixed combustion has been integrated into many new combustion technologies. The present study investigated lean back-supported flames in a stratified combustion regime. This strategy leads to hybrid combustion regimes, ranging between fully premixed and fully non-premixed reactants, with a large panel of flame structures and properties requiring to be characterized. Outwardly propagating flames were observed following ignition under laminar stratification conditions generated in a constant volume vessel. The quantitative analysis of the flame properties relied on simultaneous PIV measurements to obtain local flame burning velocities and stretch rates and used anisole-PLIF measurements to calculate the equivalence ratio. Simultaneous OH-PLIF measurements were used to differentiate between the burned gas boundaries and the active flame front. This differentiation was necessary to investigate the nonflammable mixture. The OH-gradient measurement proved to be suitable for distinguishing burned gas interfaces from active flame fronts. Simultaneous OH-and anisole-PLIF measurements were used to estimate the thermal flame thickness. Two flame families were investigated: in family A the flame was ignited in a lean mixture (φ=0.6) with a rich stratification; in family B the mixture in the chamber was nonflammable. In rich mixtures ignition compensated for the non-equidiffusive effects of the lean propane flame and reinforced the flame's stretch resistance. Both a flammable and a nonflammable mixture were investigated to determine the time scales of the back-supported propagation for the given stratification. The enhanced combustion regime allowed the flame to propagate with an active flame front, even in the nonflammable mixture. Combustion continued for a few milliseconds before the flame extinguished. The richer the stratification, the longer the combustion lasted in the nonflammable mixture

On the effect of separated oxygen and carbon dioxide injections on the stabilisation of diluted oxyfuel flames

International audienceOxyfuel combustion with exhaust gas recycle coupled with CO 2 capture and storage (CCS) is a promising way to meet low CO 2 emission standards in industrial facilities with limited economical impact. In such systems, the flame stability is very sensitive to the dilution by injection of exhaust gases, particularly in configurations where they are not premixed with the oxygen, making the design of the burners more complex but offering a larger operation flexibility and a better control of flame heat transfer. In order to study the strategies of injection of the oxygen and exhaust gases, this paper presents an experimental study of the aerodynamic mechanisms influencing the stabilization of CO 2-diluted oxy-fuel flames, for four different configurations of 23 kW quadri-coaxial burners with separated injections for oxygen and carbon dioxide. The four burners have same axisymmetric geometry consisting in injections of methane in the center surrounded successively by a first oxygen (O 2i) inner annular jet, the CO 2 co-flow and a second oxygen (O 2e) outer annular jet. Dimensions of burners are chosen to keep constant CH 4 and O 2i injections and to be able to change independently CO 2 and O 2e velocities for constant thermal power, total equivalence ratio, oxygen repartition and dilution ratio. The interaction between combustion and the aerodynamic features is investigated by CH* chemiluminescence imaging and Particle Image Velocimetry (PIV). Mean tomographic images of the flame structure are obtained by Abel's inversion of averaged chemiluminescence images. PIV measurements are performed for two fields of view in order to obtain the global aerodynamic features of the turbulent oxyfuel flames and a more precise characterization in the vicinity of the burner exit. For the latter, the spatial resolution of the measurements is optimized by the development of a specific multi-step PIV processing. Low flow-velocity and high flow-velocity configurations are tested at maximum CO 2 dilution allowed by the burners. Different structures of flames are obtained with a long continuous annular shape or with local extinction for some operating conditions. The intensity of the mixing processes and the resulting stability of the flame depend largely on the shear constraints between CO 2 and O 2e jets. A better stabilization is found for low CO 2 velocity, which favors its mixing with outer oxygen annular jet prior to direct dilution of the flame. An increase of O 2e velocity further improves centrifugal entrainment of CO 2 and then reduces the radial stratification around the flame. These results obtained in a reference configuration are useful guides for the design of flexible and efficient oxy-fuel industrial burners for CCS units

Simultaneous gas and spray PIV measurements in an optical engine

International audienceEngine internal aerodynamic reveals complex flow involving multi-scale turbulence, flow structure compression, two-phase interactions and are responsible for the mixing process and the engine performance and efficiency. Recent efforts have been made to describe in details in-cylinder flows either with temporal (HR-PIV) or spatial (Tomo-PIV) resolution. However, for gasoline direct injection engine (GDI), the dynamic coupling between gas and spray droplets should also be undertaken for a correct evaluation of in-cylinder aerodynamics. Simultaneous measurements of instantaneous gas and droplets velocities during intake and compression strokes are proposed by means of two-phase PIV based on fluorescence [1, 2]. The technique is adapted to the constraints of optical engine and associated to specific algorithms development for the liquid phase. The engine test bench consists in a mono-cylinder GDI engine (AVL) which operates up to 3000 rpm in optical configuration with a displacement volume of 450 cm 3 and a compression ratio of 8.5. The optical accesses to the combustion chamber is enabled by a quartz-glass liner. The injection system is composed of a solenoid multi-hole injector Bosch fed up by a pressurized volume to ensure a stable injection pressure up to 100 bar. An injector power control module (EFS IPOD) is used to drive the injector and control injection timings in the engine cycle. The two-phase PIV technique is based on the use of two different dyes dissolved in the seeding particle and gasoline, producing fluorescent emissions on separated spectral bands for each phase [1, 2]. The phase separation is enabled by a detection system consisting of a dichroic window distributing the collection signal on two synchronized PIV cameras (Hamamatsu 12 bits 2018×2048 pixels) equipped with Nikkor lenses (50mm f/#2) and adapted pass-band filters. Angular controls are mounted at the base of the dichroic sheet and both cameras in order to adjust with precision the common camera field of view. A refined adjustment based on a polynomial approach of 5 th degree is then numerically performed from calibration grid images to ensure a perfect images overlap and to correct image distortion induced by the glass liner. Dyes excitation is performed with two lasers at different wavelengths (532 nm for the gas and 355 nm for the spray) in order to independently adjust the PIV acquisition delays to the high velocity shift between phases in the early stage of injection. The use of two wavelength also improve the spectral separation of fluorescence signal and then the phase discrimination. An original synchronisation of the laser and camera with the engine cycle is ensured by means of a programmable time board to get rid of engine speed fluctuations and guarantee a fixed working frequency for the lasers while limiting injection and fouling to the acquisition triggering. Velocities of the gas and the liquid phases can thus be acquired simultaneously for engine conditions where the two phases are present, typically early after the start of injection [2, 3]. Prior to the correlation step, a pre-processing of the fluorescence images is performed to enhance the correlation level. A masking technique, with adaptation of the masking surface at each angular position is also used. The PIV post-processing is then adapted to the present configuration with two different algorithms for each phase. The velocity fields of the gaseous phase are obtained by a multi-pass subpixel shift correlation algorithm based on the correlation of the seeding patterns [4]. Interrogation window size of 32×32 pixels (1.85×1.85 mm 2) with an overlap of 50% has been used with a vectors filtering based on a minimum value of the Signal to Noise Rate (SNR) and on a median filter which are adapted to each experimental condition. This enables to reject most of non-valid vectors. Gas phase velocity calculation for internal engine flow is validated in our configuration by means of simultaneous Mie-based PIV and fluorescence based PIV. Comparison of mean and instantaneous velocities show less than 5 % differences. The density of the liquid phase is heterogeneous with a very dense part near the injector nozzle and a dispersed part after the breaking of the liquid sheet. In the spray dispersed part, velocities are processed with a particle approach, whereas in the dense part of the spray, a specific algorithm based on pattern correlation is developed. Preferential direction and topology of the spray are taken into account through the window shape and size

Etude des variabilités cycliques dans le cylindre d'un moteur essence à injection directe par la PIV haute cadence

International audienceLes résultats obtenus à partir des mesures expérimentales par PIV haute cadence dans la chambre de combustion d'un moteur essence à injection directe ont été analysés à l'aide de la décomposition propre orthogonale (POD) dans le but d'isoler les structures cohérentes au cours d'un cycle moteur et identifier leurs variabilités cycliques. Des investigations basées sur l'analyse de la position du centre du tumble en fonction de la phase moteur, i.e. à un degré vilebrequin déterminé, montrent que les plus fortes variations apparaissent lors de la compression. Il a été démontré que les dispersions aérodynamiques dans la chambre de combustion se comportent différemment en fonction de l'ouverture de la tubulure d'admission

Étude de l'interaction d'une décharge électrique avec un écoulement laminaire par diagnostics optiques

International audienceLes technologies récentes mises au point en moteur à allumage commandé et injection directe essence sont tournées vers le downsizing et la combustion diluée par EGR. Ces modes de combustion sont prometteurs pour la réduction des émissions CO2 mais restreignent fortement la stabilité de l'allumage, particulièrement pour les mélanges pauvres. L'allumage par décharge électrique présente un fort potentiel pour répondre à ces contraintes, à condition de maîtriser les couplages existants entre la décharge plasma et les mécanismes d'initiation de la combustion. Une caractérisation des propriétés physico-chimiques de la décharge ainsi que l'étude de la topologie de l'arc et de l'écoulement environnant sont réalisées par diagnostics optiques. La décharge électrique est générée par une bobine crayon (Marque Audi) entre deux électrodes placées en configuration pointes-pointes et écartées de 3 mm dans l'air à pression atmosphérique. Les évolutions temporelles du courant et de la tension délivrés aux électrodes sont mesurées pour évaluer l'énergie maximum qui peut être déposée dans le gaz. Les résultats obtenus apportent des éléments de compréhension des mécanismes de couplage entre une décharge électrique et l'écoulement mais constituent également une base de données pour la validation de simulation numérique

Planar Laser Induced Fluorescence Measurement of [ OH] in the Stabilization Stage of a Spray Jet Flame

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PLIF Imaging Measurements of a Co-Axial Rocket Injector Spray at Elevated Pressures.

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