Diesel spray velocity and break-up characterization with dense spray imaging

International audienceThis work presents analysis methods for categorizing breakup morphology in a diesel spray produced by a single-hole, plain orifice diesel injector issuing into ambient atmospheric conditions. Velocity data and images which include the near-nozzle region of a diesel spray were obtained using both time-gated ballistic imaging (BI) and high-resolution ultrafast shadow imaging (USI) measurements. The USI results provide high-resolution visualization of the spray edges and resolved droplets within the depth-of-field of the collection optics, while the BI results provide a view of the spray at a modified dynamic range which mitigates interferences from refracted light and multiple-scattering noise, revealing additional spatial information. Time-correlated image-pairs obtained by both techniques were filtered and cross-correlated on a variety of scales to produce velocity profile data and identifiable structures which can be exploited to differentiate the breakup modes observed in the diesel spray. In addition, a multi-scale analysis was applied to the image data, demonstrating an approach whereby physical parameters can be derived from the image data to quantify the degree of atomization exhibited by a diesel spray

Spatio-temporal analysis of a liquid jet using a sub-picosecond optical gate

International audienceA Diesel jet visualization based on an ultra-short light pulse and a time-resolved detection is proposed. An optical gate, consisting of a BBO (Beta Barium Borate) crystal, allows to separate ballistic, refractive and scattered light. An imaging pulse passes through the studied medium while a gating pulse is used to open the gate. With this configuration, a spatial resolution of 10 µm and a temporal resolution of 270 fs are obtained. The gate duration is compatible with the typical dimensions of a liquid jet generated by a commercial automotive fuel injector. Preliminary results show that spatio-temporal diagrams of transmitted light through a spray may be acquired, showing clearly the separation between ballistic, refracted and scattered light

VELOCITY, INTERFACE COMPLEXITY AND DROPLETS PRODUCTION IN THE NEAR NOZZLE REGION OF A DIESEL SPRAY: COMPARISON BETWEEN EXPERIMENTAL ANALYSIS AND DIRECT NUMERICAL SIMULATION

International audienceA transillumination imaging arrangement using a double-pulsed femtosecond laser system coupled to a double frame camera was used to record high-resolution time-correlated image-pairs in the near-nozzle region of a high pressure Diesel jet. On the other hand, direct numerical simulations (DNS) using coupled volume of fluid/level set (VOF/LS) method for interface tracking have been carried out for this kind of injection. For the sake of precision of these numerical simulations, the flow inside the injector was also computed by using a commercial CFD code (Fluent 6.3). A quantitative comparison between numerical simulations and experimental images has been performed by applying the same tools. From the numerical point of view, some jet characteristics are well reproduced and this study allowed detecting the features that need improvement

Single shot 1 kHz CPP fs CARS : Dynamic investigation of a buoyant flame

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Spatio-temporal analysis of a liquid jet using a sub-picosecond optical gate

International audienceA Diesel jet visualization based on an ultra-short light pulse and a time-resolved detection is proposed. An optical gate, consisting of a BBO (Beta Barium Borate) crystal, allows to separate ballistic, refractive and scattered light. An imaging pulse passes through the studied medium while a gating pulse is used to open the gate. With this configuration, a spatial resolution of 10 µm and a temporal resolution of 270 fs are obtained. The gate duration is compatible with the typical dimensions of a liquid jet generated by a commercial automotive fuel injector. Preliminary results show that spatio-temporal diagrams of transmitted light through a spray may be acquired, showing clearly the separation between ballistic, refracted and scattered light

Diesel spray velocity and break-up characterization with dense spray imaging

International audienceThis work presents analysis methods for categorizing breakup morphology in a diesel spray produced by a single-hole, plain orifice diesel injector issuing into ambient atmospheric conditions. Velocity data and images which include the near-nozzle region of a diesel spray were obtained using both time-gated ballistic imaging (BI) and high-resolution ultrafast shadow imaging (USI) measurements. The USI results provide high-resolution visualization of the spray edges and resolved droplets within the depth-of-field of the collection optics, while the BI results provide a view of the spray at a modified dynamic range which mitigates interferences from refracted light and multiple-scattering noise, revealing additional spatial information. Time-correlated image-pairs obtained by both techniques were filtered and cross-correlated on a variety of scales to produce velocity profile data and identifiable structures which can be exploited to differentiate the breakup modes observed in the diesel spray. In addition, a multi-scale analysis was applied to the image data, demonstrating an approach whereby physical parameters can be derived from the image data to quantify the degree of atomization exhibited by a diesel spray

Experimental study of the effect of droplets on opposed stretched premixed flames

International audienceThe effect of fuel droplets on the burning velocity of strained laminar premixed flames was investigated experimentally using optical diagnostics. The twin counterflow burner configuration was used to create premixed acetone vapour flames with the addition of fuel droplets. Particle image velocimetry (PIV) was used to measure 2D velocities for nominal equivalence ratios in the range (0.8-1.21) as a function of strain rate (250-550 s 1). Measurements of reference flame speeds upstream of the flames were made for both reference methane/air flames and acetone vapour/droplet flames, and compared to simulated values of the purely gaseous flames. Spontaneous Raman spectroscopy was used to measure temperature and major species molar fractions across reference methane flames. The results were shown to be in good agreement with the simulations for most of the species, except CO and CO 2 , which had too low a signal-to-noise ratio in the product zone. Preliminary measurements of acetone spray/vapour flames showed the capability of the setup to measure Raman signals in the presence of droplets at lowered laser energy. High laser energies led to prompt ignition of the droplets. Potential means of resolving the problem are suggested

Time gate, optical layout, and wavelength effects on ballistic imaging

Idlahcen, Said Mees, Loic Roze, Claude Girasole, Thierry Blaisot, Jean-BernardA method to distinguish a hidden object from a perturbing environment is to use an ultrashort femtosecond pulse of light and a time-resolved detection. To separate ballistic light containing information on a hidden object from multiscattered light coming from the surrounding environment that scrambles the signal, an optical Kerr gate can be used. It consists of a carbon disulfide (CS(2)) cell in which birefringence is optically induced. An imaging beam passes through the studied medium while a pump pulse is used to open the gate. The time-delayed scattered light is excluded from measurements by the gate, and the multiple-scattering scrambling effect is reduced. In previous works, the two beams had the same wavelength. We propose a new two-color experimental setup for ballistic imaging in which a second harmonic is generated and used for the image, while the fundamental is used for gate switching. This setup allows one to obtain better resolution by using a spectral filtering to eliminate noise from the pump pulse, instead of a spatial filtering. This new setup is suitable for use in ballistic imaging of dense sprays, multidiffusive, and large enough to show scattered light time delays greater than the gate duration (tau=1.3 ps). (C) 2009 Optical Society of Americ