Collinear, two-color optical Kerr effect shutter for ultrafast time-resolved imaging

Imaging with ultrashort exposure times is generally achieved with a crossed-beam geometry. In the usual arrangement, an off-axis gating pulse induces birefringence in a medium exhibiting a strong Kerr response (commonly carbon disulfide) which is followed by a polarizer aligned to fully attenuate the on-axis imaging beam. By properly timing the gate pulse, imaging light experiences a polarization change allowing time-dependent transmission through the polarizer to form an ultrashort image. The crossed-beam system is effective in generating short gate times, however, signal transmission through the system is complicated by the crossing angle of the gate and imaging beams. This work presents a robust ultrafast time-gated imaging scheme based on a combination of type-I frequency doubling and a collinear optical arrangement in carbon disulfide. We discuss spatial effects arising from crossed-beam Kerr gating, and examine the imaging spatial resolution and transmission timing affected by collinear activation of the Kerr medium, which eliminates crossing angle spatial effects and produces gate times on the order of 1 ps. In addition, the collinear, two-color system is applied to image structure in an optical fiber and a gasoline fuel spray, in order to demonstrate image formation utilizing ballistic or refracted light, selected on the basis of its transmission time.Comment: 13 pages, 10 figure

Ballistic Imaging of High-Pressure Fuel Sprays using Incoherent, Ultra- short Pulsed Illumination with an Ultrafast OKE-based Time Gating

We present an optical Kerr effect based time-gate with the collinear incidence of the pump and probe beams at the Kerr medium, liquid carbon disulfide, for ballistic imaging of the high-pressure fuel sprays. The probe pulse used to illuminate the object under study is extracted from the supercontinuum generated by tightly focusing intense femtosecond laser pulses inside water, thereby destroying their coherence. The optical imaging spatial resolution and gate timings are investigated and compared with a similar setup without supercontinuum generation, where the probe is still coherent. And finally, a few ballistic images of the fuel sprays using coherent and incoherent illumination with the proposed time-gate are presented and compared qualitatively.Comment: 7 pages, 7 figures, Presented at the 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics held at Lisbon, Portugal from 7th to 10th of July, 201

Liquid sheet thickness measurements using multi-pass, time-gated femtosecond imaging

International audienceThe present work focuses on the development and application of a non-invasive technique for measuring the thickness of a flat liquid sheet. The technique consists in separating a 100 femtosecond (fs) laser pulse into an imaging pulse which passes through the liquid sheet and a gating pulse, that travels only in air and whose path length can be adjusted using a delay line. The time delay ∆τ between the imaging and gating pulse is directly proportional to the liquid sheet thickness h and can be measured using Second Harmonic Generation (SHG) based time gate (here a Beta Barium Borate crystal is used for SHG). In order to enhance the thickness measurement resolution, an original multi-pass configuration was designed where the imaging pulse is passing twice (or more if needed) through the medium which increases the time delay between imaging and gating pulse. As a first step, we have checked the reliability of this technique by conducting measurements for a glass plate of known thickness (h g =120 µm). The measured thickness value (117 µm) is in close agreement with the expected value. Then, attention has been paid on flat liquid sheets produced by single-hole fan spray nozzles with various water-glycerol solutions. The streamwise evolution of the measured thickness of the sheet exhibits good agreement with the semi-analytical model of Dombrowski et al. [1]

Effect of cavitation on velocity in the near-field of a diesel nozzle

The entire process of atomization of the fuel in an internal combustion engine plays a very important role in determining the overall efficiency of these engines. A good atomization process could help the fuel to mix with the air properly leading to its efficient combustion, thereby reducing the emitted pollutants as well. The recent trend followed by the engineers focused on designing fuel injectors for more efficient atomization is to increase the atomization pressure while decreasing the nozzle orifice diameter. A consequence of this is the development of cavitation (formation of vapor cavities or bubbles in the liquid) inside the injector close to the nozzle. The main reason behind this is the sudden changes in the pressure inside the injector and these cavities or bubbles are usually formed where the pressure is relatively low.This work mainly focuses on studying the formation of cavitation and its effect on the velocity of the spray in the near nozzle region using asymmetrical transparent nozzle equipped with a needle lift sensor with nozzle diameter of 0.35 mm at 300 bar of injection pressure. The experiment consists in recording of several image-pairs, which are separated by about 300 ns, capturing the dynamics of the spray, a few millimeters from the nozzle in the direction of the flow. These image-pairs are then used to compute the velocity from the displacement of the liquid structures and ligaments by correlating the first image with the second. About 200 of such velocity graphs are then averaged to obtain a velocity map and is compared with the similar average velocity maps obtained at different times from the start of the injection. The angular spread of the spray from each of these images is calculated as well. The images showing cavitation inside the injector are also recorded at these same instants of time so as to understand the effects of cavitation on the velocity and angular spread of the spray close to the nozzle.Comment: 13th International Conference on Liquid Atomization and Spray Systems, Aug 2015, Tainan, Taiwan. 2015, https://iclass2015.tw

Quantitative comparison of fuel spray images obtained using ultrafast coherent and incoherent double-pulsed illumination

We present a quantitative comparison between the high-pressure fuel spray images obtained experimentally using classical imaging with coherent and incoherent ultrafast illuminations recorded using a compatible CMOS camera. The ultrafast, incoherent illumination source was extracted from the supercontinuum generated by tightly focusing the femtosecond laser pulses in water. The average velocity maps computed using time-correlated image-pairs and spray edge complexity computed using the average curvature scale space maps are compared for the spray images obtained with the two illumination techniques and also for the numerically simulated spray using the coupled volume of fluid and level set method for interface tracking (direct numerical simulation or DNS). The spray images obtained with supercontinuum-derived, incoherent, ultrafast illumination are clearer, since the artifacts arising due to laser speckles and multiple diffraction effects are largely reduced and show a better correlation with the DNS results.Comment: 8 pages, 9 figures, Presented at the ILASS-Europe 2014, 26th Annual Conference on Liquid Atomization and Spray Systems held at Bremen, Germany from 8th to 10th September 201

DRASC 1kHz mono-coup à sonde à dérive de fréquence : étude dynamique d'une flamme de diffusion à structures cohérentes

International audienceDepuis les années 2000, la présence sur le marché de sources femtoseconde commerciales haute puissance et haute cadence a permis de développer le diagnostic DRASC (Diffusion Raman anti-Stokes Cohérente) de mesure de température dans ce régime d’interaction. L’énergie par impulsion atteignant plusieurs mJ ainsi que la puissance crête élevée du laser femtoseconde permet de réaliser des mesures instantanées à une cadence de répétition élevée pouvant atteindre la dizaine de kHz. Dans la présente étude, le développement d’une méthodologie numérique et expérimentale a permis de réaliser mesurer la température de flamme avec une précision de l’ordre du pourcent. Combiné à cette précision, le dispositif de mesure DRASC permet d’analyser des processus dynamiques de flamme comme le développement de structures tourbillonnaires périodiques où des variations élevées de température sont présentes ainsi que de faire la démonstration de l’aptitude du système à capturer la dynamique d’une flamme présentant des structures tourbillonnaires périodiques sur une amplitude importante de valeur de température

Simultaneous high-speed internal and external flow measurements for a high-pressure diesel nozzle

We present an extensive experimental study focused on understanding the impact of cavitation in a high-pressure diesel nozzle on the macroscopic properties of fuel spray. Several high-speed videos of the liquid flow through a transparent, asymmetric cylindrical nozzle with a single orifice (phi = 0.35 mm) are recorded along with the videos of the resulting spray in the near-nozzle region, issued with an injection pressure of 300 bar at a frame-rate of 75 kHz. The high-repetition images of the internal flow are then used to estimate the onset of cavitation inside the transparent nozzle and the probability of development of cavitation in different regions of the nozzle with an average estimate of the amount of cavitation with time. On the other hand, recorded spray images are used to study spray penetration, cone-angles and velocity from the start of fuel injection. A novel approach is proposed for the measurement of perturbations that occur in form of big liquid structures along the spray boundary.Comment: in 27th European Conference on Liquid Atomization and Spray Systems, Sep 2016, Brighton, United Kingdom. 201

Imagerie femtoseconde d'un spray Diesel haute pression en proche sortie d'injecteur

La compréhension du processus d atomisation d'un jet liquide de carburant en proche sortie de l'orifice d'un injecteur Diesel est fondamentale si on veut optimiser la combustion dans les moteurs. Cependant, dans cette zone, les conditions difficiles (forte densité de liquide, vitesse élevée et petit champ d observation) rendent inopérantes les techniques classiques. Une technique d imagerie résolue en temps (imagerie balistique) est développée et son potentiel pour l étude des jets rapides est étudié. Le montage consiste à utiliser une source laser femtoseconde amplifiée (100 fs) et une porte optique permettant de sélectionner la lumière ayant interagi avec le spray en fonction du temps. Pour cela, deux configurations de porte optique sont proposées, toutes deux utilisant un montage pompe-sonde. L une est basée sur l effet Kerr optique généré dans du CS2 liquide et permet d avoir une résolution temporelle de l ordre de 1 ps. La résolution spatiale est grandement améliorée par l utilisation de deux longueurs d onde différentes pour le faisceau pompe et le faisceau imageur (montage à deux couleurs). L autre est basée sur une génération de second harmonique dans un cristal BBO et permet d obtenir une résolution temporelle sub-picoseconde. Par ailleurs, une mesure de vitesse des structures liquides du jet liquide est réalisée à l aide d un laser femtoseconde double impulsion dans une configuration d imagerie classique. On montre que l utilisation d une porte optique basée sur un cristal BBO permet l évaluation de la vitesse dans la zone la plus dense du jet.To understand the atomization process of a liquid jet in the near field of a Diesel injector is a key issue for the optimization of combustion engines. Unfortunately, the difficult conditions presented by Diesel sprays (high liquid density, high speed and small observation field) render classical techniques impractical. In this thesis a time resolved imagery technique (ballistic imaging) is developed and its potential for high speed liquid jet is studied. The experimental set up used in this work consists of an amplified femtosecond laser source (100 fs) and a time-dependent optical gate allowing the selection of light which interacts with the spray. Two configurations are examined, and both systems are arranged in a pump-probe configuration. The first system is based on an optical Kerr gate generated in CS2 liquid, resulting in a temporal resolution of about 1 ps. The system resolution is greatly improved by using two different wavelengths for the pump pulse and the imaging pulse (two-color configuration). The second system is based on the second harmonic generation in a BBO crystal, resulting in sub-picosecond temporal resolution. In addition, a velocity measurement of the structures of a liquid jet has been performed using a double pulse femtosecond laser in a classical imagery configuration. It has been shown that using an optical gate based on the BBO crystal allows the estimation of velocity in the densest part of the jet.ROUEN-BU Sciences Madrillet (765752101) / SudocSudocFranceF