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

Characterization of ethane jet from sub-critical to super-critical conditions through visible light and X-ray imaging

International audienceThe injection of fuel in a high-pressure gaseous environment, for automotive, aeronautical or rocket applications leads to thermodynamic conditions where pressure exceeds the critical pressure of working fluids and thus, the supercritical state of matter is reached. Providing reliable experimental results under these particular conditions is still nowadays a challenge, but it is of great importance for the validation of numerical codes. Indeed, at such a high pressure, the distinction between gaseous and liquid phases becomes blurred as surface tension decreases and the interface disappears completely. For such special conditions, experimental data are scarce and need to be consolidated. As an example, the modification of the local refractive index induced by density gradient makes the visible-light imaging technique to be used with care. The REFINE testbench (Real-gas Effect on Fluid Injection: a Numerical and Experimental study) has been designed at CORIA Lab to study the non-reactive injection of Ethane and Propane under sub-and supercritical conditions. The ambient gas pressure can be raised up to 6 MPa and warmed up to 573 K to scan sub-and trans-critical injection conditions. The chamber is equipped with two perpendicular accesses allowing different simultaneous diagnostics to be applied to the jet. Experimental data are collected from shadowgraph, diffused backlight illumination techniques and X-Ray. Quantitative measurements of jet spreading angle, breakup length and density maps are compared to literature results. Introduction The study of high pressure injection is a major topic of research in the transport industry (cars, aircrafts, rockets) because it conditions the combustion performance and therefore the formation of pollutants. Under certain conditions of pressure and temperature the liquid injected into the chamber does not behave as usual and the atom-ization process is replaced by a diffusion one [1, 2]. The experimental study of this transition is a challenge because it requires the development of a robust, precise and well-controlled experiment as well as adapted diagnostics. The number of experiments dealing with supercritical injection is therefore small and the amount of experimental data that can be used for modeling validation [3, 4, 5] is all the more scarce [6]. In particular, the high ambient pressure locally affects the refractive index gradient making the use of laser-based diagnostics questionable [7, 6] to provide quantitative local experimental data. Nevertheless, classical diagnostics such as shadowgraphy or schlieren techniques have been used up to now to observe the transition from a system composed of distinct liquid and gaseous phases [6, 8, 9] to a system where dense and light fluids mix together because the surface tension and the heat of vaporization diminish [10, 11]. Under supercritical conditions, a dense and dark core is visible at the injector exit on shadowgraph [12] and the analysis of the spreading angle shows that supercritical jet has a behavior closer to a gaseous jet than to a liquid jet [13, 14]. In this study, the first results coming from the research program REFINE (Real-gas Effect on Fluid Injection: a Numerical and Experimental study) are provided. It consists in an injection of Ethane into an environment of Nitrogen or Helium at high pressure and moderate temperature. The objective is to deliver a set of quantitative data for the dark core length and the spreading angle of a non-assisted jet, from subcritical to supercritical pressure with respect to the liquid injected, and for various levels of ambiant temperature. A tentative to deliver the value of density on the jet axis through the radiography of the liquid jet is also presented. Radiography is an X-ray imaging technique that has been used for the study of diesel jets [15, 16] or cryogenic injections under supercritical considerations [17]. The physical principle of radiography is the absorption of X-rays by the dense fluid. The transmission of X-rays thus depends on the mixture composition, i.e. the injected fluid and its surrounding. An advantage of X-ray absorption technique is that X-rays are not subject to deflection due to optical index gradients, contrary to laser-based techniques, and the attenuation of the transmitted light is proportional to the mass of fluid crossed by the X-ray beam

Entropy-Based Cavitation and Primary Atomization Analysis with a 2D Transparent Injector

International audienceA transparent scale-up injector with asymmetric incoming flow direction was designed to promote cavitation on mainly one-side of the orifice. This orifice has a rectangular cross-section that provides straightforward optical access to the internal flow. This geometry was designed to study the role of the internal flow, and particularly of the development of cavitation, on the modification of the primary atomization process occurring as soon as the liquid emanates from the injector. The internal flow is classified into four regimes based on the extent of the cavitation zone; 1-no-cavitation, 2-developing cavitation, 3-super cavitation and 4-semi-hydraulic flip cavitation. Image series of 500 images was recorded for different flow rates belonging to regimes 2-4. Image segmentation is applied to each individual image to identify liquid and vapour phase regions. Based on these segmented images, the mean and rms values of the cavitation extent are determined for each cavitating regime. Furthermore, a more detailed statistical analysis of the cavitation is obtained with the computation of an entropy image, bringing indication on interface between vapour and liquid and on the shed cavitation bubbles. The liquid jet fragmentation is qualified from the entropy analysis also. The primary atomization is associated with the region in the image where the liquid core is fragmented in detached ligaments and drops. This region is easily identified from the entropy analysis and the primary atomization is quantified through the computation of the area of this region. In the presence of cavitation, the primary atomization process is altered. The way liquid fragmentation is modified by cavitation is shown to be correlated to the extent of the cavitation zone in the orifice. In this paper we give a quantification of these modifications, clearly visible to the naked eye on the images

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

Behaviour of free falling viscoelastic liquid jets

[EN] In a recent work, a protocol to measure the relaxation time of dilute polymer solutions, known to be challenging, has been established [1]. This protocol is based on a 2D multi-scale description of free-falling low velocity viscoelastic liquid jets. Although the relaxation time reached an asymptotic value for high jet velocities, a significant dependence with the jet velocity is observed for low velocities. The present work reconsiders these previous experimental data using a 3D multi-scale analysis in order to identify the origin of the dependence between the relaxation time and the jet velocity. The 3D analysis demonstrates the importance of a velocity–dependent coalescence mechanism in the jet behaviour. Thanks to a simple model of jet deformation it is demonstrated that this coalescence mechanism prevents the elasto-capillary contraction of the smallest scales from occurring when the jet velocity is reduced.The authors acknowledge the financial support from the Frend National Research Agency (ANR) through the program Investissement d’Avenir (ANR-10 LABX-09-01), LABEX EMC3Tirel, C.; Renoult, M.; Dumouchel, C.; Blaisot, J. (2017). Behaviour of free falling viscoelastic liquid jets. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 241-248. https://doi.org/10.4995/ILASS2017.2017.4700OCS24124

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