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. 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 and OH-gradient measurements were used to differentiate between the burned gas boundaries and the active flame front. This differentiation was necessary to investigate the nonflammable mixture. 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. Both a flammable and a nonflammable mixture were investigated to evaluate 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 for a few milliseconds before the flame extinguished

Accuracy and speed assessment of 3D cross-correlation algorithms for two-frame and multi-frame PIV

International audienceThe need to extract higher order volumetric experimental data from PIV such as acceleration and pressure fields benefits from fast and accurate cross-correlation methods. This paper reexamines the application of two-dimensional (2D) cross-correlation methods to three-dimensional datasets by Bilsky et al. [3] and the binning techniques of Discetti et al. [7]. A robust version of the 2D methods, which reconstructs the three-dimensional (3D) signal from the 2D cross correlation maps is proposed. This method is then extended to the the fluid trajectory evaluation from ensemble-averaged cross-correlation (FTEE) method. Performance tests based on computational time and accuracy for both two-frame and multi-frame PIV were carried out on synthetically generated data, where the errors and correlation signal can be investigated. The cases presented herein include uniaxial uniform linear displacements and shear, and a final comparison of the FTEE method on synthetic homogeneous isotropic turbulence (HIT) data. For the cross-correlation computation, the proposed algorithm is in the order of 10 times faster than a standard 3D FFT. The FTEE method reduces the bias and random errors in the HIT data, and we find that the proposed algorithms yields almost identical results with significant speed-up

TURBULENT BLUFF-BODY FLAMES CLOSE TO STABILITY LIMITS REVEALED BY COUPLING OF HIGH SPEED OPTICAL DIAGNOSTICS

International audienceThe understanding of flame stabilization is a requirement for the development of innovative combustion regimes associated to low pollutant emissions and high efficiency. Bluff-body burners are well adapted for the stabilization of turbulent non-premixed flames near extinction limit. In the present work, the stabilization mechanisms involved in turbulent bluff-body flames are investigated by means of time-resolved optical diagnostics. High-Rate Particle Image Velocimetry (HR-PIV) is coupled simultaneously to High-Rate flame imaging in order to point out the correlation between the aerodynamic fields and the flame structures from images time series. A periodic ejection of burning pockets is observed from the recirculation zone established at the burner exit to the base of the main lifted flame. The characteristic frequencies are determined by extracting local velocities and flame emission with time. This periodic behaviour is correlated to the intermittency of the aerodynamic structures. These results provide a better understanding of the physical phenomena controlling the stabilization of this turbulent flame

Enregistrement multi-angulaire sur une caméra pour les mesures 3D dans les écoulements

International audienceLes approches de mesures 3D sont utilisées de plus en plus fréquemment en mécanique des fluides, que ce soit pour mesurer la vitesse, la position d’une surface (vibration, interface gaz liquide) ou un champ scalaire (émission de flamme, LIF 3D, densité...). En pratique elles consistent à observer la zone d’intérêt sous différents angles avant de réaliser une reconstruction volumique, le nombre d’angles d’observation devant souvent être supérieur à trois. Cela conduit à complexifier notablement le montage optique et induit des couts importants (caméras, objectifs...). Pour la reconstruction de surface, il est dans certain cas possible de considérer des approches où plusieurs angles de vue sont enregistrés sur le même détecteur. Dans ce travail nous proposons ce type de dispositif en plaçant devant l’objectif un système optique à huit miroirs projetant quatre angles d’observation sur des zones distinctes d’un même capteur. Pour la localisation de surface, en utilisant la technique des "mouchetés", nous validons cette approche d’enregistrement sur quatre exemples différents nous servant à évaluer la précision de reconstruction de la sur-ace et montrer ses potentiels pour l’étude des interactions fluide/structure

Recent developments in two-phases fluorescence PIV: Application to the dynamic of high pressure gasoline sprays

International audienceThe purpose of the present work is to develop and to demonstrate the ability of the two-phase PIV by fluorescence technique for the study of two-phase flows dynamics. In particular, the dynamic of a high-pressure spray and its interaction with surrounding air is investigated in this study. For the single-phase flows, the PIV is a well-established technique to measure the velocity of the continuous phase but not easily applicable on two-phase flows without adaptation and new developments. Ten years ago, fluorescence PIV were developed to investigate dynamic of spray injection [1][2]. For phase discrimination, each phase is labelled by two different dyes allowing the acquisition of two separate pairs of images which are then analysed separately and provide the instantaneous velocity fields of the two phases. In the present paper, an extension of our previous work is proposed [2] by associating two PIV lasers at 532 nm and 355 nm in order to make separation of fluorescence signals of the two phases more efficient and then to access to velocity measurements in the denser part of the spray. This arrangement permits also an optimisation of the two PIV delays in order to account for the high velocity difference between the phases in high pressure injections. For the imaging system, the two fluorescence signals are collected simultaneously on two 12 bits Hamamatsu cameras (C9300-4M pixels). Two selected passband filters are placed in front of the cameras, for the discrimination of the fluorescence in each phase. To ensure a perfect overlap of the velocity measurements, image deformation technique based on 5 th order polynomial functions are applied on particle images to correct any differences of magnifications, orientation and optical deformation of images. In the first part of the paper, the selection of a couple of fluorescent dyes adapted to our experimental conditions is addressed. In the second part, a novel PIV measurement based on pattern correlation is proposed to measure the velocity of the liquid phase in the dense part of the liquid jet near the injector nozzle. In the final part, our fluorescence PIV technique is validated on two high pressure gasoline injectors placed in a closed pressurized chamber (single and multi-holes injectors up to 100). bar). The major criteria of dyes selection are: maximum of absorption spectrum close to the laser wavelengths; fluorescence signal in a spectral range close to maximum quantum efficiency of the PIV cameras; high fluorescence efficiency, especially for the dye labelling the seeding particles; distinct fluorescence emission to permit the optical filtering of the two phases; the solubility of the dyes in the liquid phase and the seeding particles. Considering all these criteria and numerous tests, our dye selection has been stopped on Stilbene 420 for the seeding particles and Pyrromethene 597 for the liquid phase. Figure 1 presents instantaneous images in the case of single hole injector without any processing of the fluorescence signal of each dye. These images show that there is no signal from the seeding particles of the gas remaining on the spray image (Figure 1-A). The gas image (Figure 1-B) shows that the spray signal is correctly filtered in spite of the high intensity of droplets Mie scattering and fluorescence. This is confirmed in Figure 1-C by the superimposition of both phases at the location of the red rectangle plotted in Figure 1-B. Indeed, the spray droplets (in red) are clearly distinguished from the seeding particles of the gas (in black)

Determination of soot volume fraction and particle size distribution in turbulent non- premixed butane and ethylene jet flames through LII, PPS and SMPS measurements

International audienceThe present work reports soot volume fraction determined in turbulent sooting non-premixed jet flames using a Pegasor particle sensor (PPS) and also using the non-intrusive laser-induced incandescence (LII) technique. Additionally, soot particle size distribution is determined using Scanning Mobility Particle Sizer (SMPS). Turbulent flames with different fuels, namely: butane (Re = 26,000), 70% butane + 30% ethylene by volumes (Re = 20,000), and ethylene (Re = 10,000) are investigated. The axial distribution of PPS derived soot volume fraction is qualitatively in excellent agreement with that obtained using LII. The possible sources of quantitative discrepancy between soot concentration obtained through LII and that of the PPS are discussed. Axial evolution of the soot size along with the volume fraction is presented. These results form part of the proposed comprehensive database to validate advanced soot models

Digital in-line holography with a divergent beam for 3D velocity field measurement in small volumes

International audienceDigital holography is a well established technique for the study of dynamic volume of particle fields. In this work, an original digital in-line holography system is developed and used to determine 3D displacements of particles tracer in an air-flow. This system allows us to investigate a sample volume of few mm 3 and reconstruct a moving particle with velocities ranging up to 1m.s-1. The sample volume is reconstructed by computing the wavelet transformation (WT) for different value of the scale parameter. This parameter is related to the axial distance between a reconstruction plane and the CCD sensor. A fiber coupled laser diode is used as the recording light sources. The curvature radius of the reference wave emerging from this fiber forms geometrically magnified diffraction patterns of particles and thus acts as an optical magnification. We show that this enables to enhance the spatial resolution of the system. Here, a transverse magnification ratio of 4 allows us to reconstruct images of droplets with a diameter smaller than 5µm. Note that this configuration needs an accurate knowledge of curvature radius of the recording wave. By using an original calibration method, presented here, we show that the droplets can be localized from the reconstructed holograms with an accuracy of 1µm for lateral position and 20µm in depth. The potential of this system is shown in the case of a laminar air flow seeded with oil micro-droplets

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

Couplage de la PIV et de la PLIF de OH et d'anisole pour l'étude d'une flamme se propageant dans une stratification de mélange laminaire

International audienceLa combustion partiellement prémélangée est introduite dans de nombreuses technologies récentes de combustion pour réduire les émissions de polluants et augmenter l'efficacité énergétique. Ce papier s'intéresse à la combustion stratifiée prémélangée. Des mesures simultanées de PIV, PLIF-OH et PLIF-anisole ont été utilisées pour étudier la propagation de flamme allumée par étincelle dans une stratification de mélange. Deux familles de flammes propane/air ont été considérées en fonction que la flamme se propage vers un milieu pauvre inflammable ou ininflammable. Dans les deux cas nous avons pu montrer une amélioration de la combustion dans les zones pauvres, avec temporairement avant extinction, des propagations de flamme dans des mélanges ininflammables

Tomographic PIV study of lifted flames in turbulent Axisymmetric jets of methane

International audienceIn the present work, the capability of the tomo-PIV technique to measure 3D flow structures in reactive flow is evaluated in a lifted flame configuration. In combustion, two main problems can be encountered: the flame radiation and the imaging of the particle field throughout non-uniform distribution of the refractive index. In order to assess these two points, turbulent lifted flames of methane has been investigated. In that condition, the flame is detached from the burner and is in lifted-flame regime far below the blow-off condition. In this simple configuration, some parts of the methane jet is surrounded by the reaction zones and burned gases, both inducing large variations of the refractive index, which are time dependent. The main objective of this experiment is to compare the tomo-PIV results performed in reactive conditions to those obtained in the same optical arrangement without flame (free jet)