Analysis of backlight images for spray measurement: how to accurately identify the liquid-gas interface in images

International audienceWe present in this paper the key points for using backlight images in spray measurement. The main features of a spray accessible from backlight images are the interface contours of the liquid elements. It is commonly considered that using this kind of images is straightforward, notably because these images are directly understandable by a human being. However, making image-based measurement needs a rigorous approach. The first point is the setting up of the optical system. Although no special expertise is required to make images of sprays, a particular attention must be put on the imaging system and light source arrangements when relevant measurement are expected. Also, different steps must be considered to correctly estimate liquid-gas interface localization on backlight images. Fist, attention must be put on the pretreatment of the images to correct light source defects such as spatial or temporal in-homogeneity. For the segmentation step that follows, local intensity variations in the images are analyzed to determine the localization of the liquid-gas interface contour in the images. It is shown that modeling the image of liquid elements is mandatory to correctly estimate this features. Based on the results of this modeling, several local parameters are proposed. It is shown how the local shape of the interface acts on the variation of these parameters. Finally, partial overlapping of images of liquid elements is considered and solutions for discriminating potential overlapping liquid elements in an image are proposed. Introduction Image analysis is among the most promising measurement methods for achieving improved characterization of sprays. Images result from the projection of 3D objects onto 2D image plane. As such, information relative to the third direction are lost or at least hidden. However, by appropriately analyzing images, some of these information can be recovered. Spray measurements are no longer confined to the spray drop size distribution but comprise now the objective to quantify more deeply the liquid-gas interface properties. First attempts were oriented to the determination of shape parameters but the objective is now to quantify the surface area of the interface of every liquid elements in a spray, whatever their shape. To do that, an accurate determination of the interface location is needed. Backlight imaging techniques are particularly well suited for this purpose. Indeed, at least the shape of the liquid elements directly appears on the images. However, all necessary precautions are not always taken in experiments, and this is mainly due to preconceived ideas about images. First, an imaging setup may seem easier to arrange as it does not require a rigorous optical alignment to obtain the image. The problem arise when starting to use this image to make the measurement. Second, the image is directly interpretable by a human being. This easy perception gives rise to the feeling that image will be measurable as soon as image can be "understood" to the naked eye. This is far from it as a measure needs signal processing operations that may fail in case of bad images. One important drawback in using images comes from the depth-of-field (DOF) of the optical system. A spray is three dimensional by nature so images of liquid elements are focused or not, depending of the location of these elements relative to the focus plane. In addition, when spray drop size distribution is considered, the drops must be counted in a given control volume, sorting the droplets by accounting their distance from the focus plane. This must be done through the use of objective criteria determined from signal modeling, as done for all other measurement techniques. A review of the key points of the optical setup is first addressed. Important steps in image analysis operations are then presented. Finally, some issues relative to the determination of the liquid-gas interface location in images are tackled

Drop Size and Drop Size Distribution Measurements by Image Analysis

International audienceOne important issue in drop sizing by image analysis is the determination of the contour of the drops. The grey level corresponding to this contour strongly depends on the degree of focusing of a drop. This grey level is determined from an imaging model based on Fourier optics formalism. The presented method allows estimating the correct contour of a focused or of an out-of-focus droplet, for a large range of sizes. A size-independent criterion for the selection of drops accordingly to their position relative to the focus plane is also presented. This criterion is based on the estimation of the PSF width. The criterion is used to select drops within a controlled range of out-of-focus positions, which is necessary for the determination of drop size distributions in spray applications. The method is applied to calibrated objects to control the method accuracy. Sources of sizing errors are evaluated and techniques to enhance the sizing procedure and to deal with overlapped images are presented

Response of coaxial air-assisted liquid jets in an acoustic field: atomization and droplets clustering

International audienceHigh-frequency combustion instabilities have been proven to be extremely harmful to liquid rocket engine operation , even leading to the destruction of the combustion chamber. The coupling between acoustic field and combustion heat release rate in the combustion chamber is considered as the driving phenomenon. Experiments have shown that intense acoustic field can deeply affect atomization process thereby causing a non-uniform heat release distribution which can couple with the resonant mode shapes of the combustion chamber and consequently trigger or sustain combustion instability. The effects of acoustic acting on atomization of coaxial air-assisted liquid jets have been investigated experimentally and results are presented in this paper. The experimental setup is composed of three coaxial injectors installed on the roof of a semi-open resonant cavity provided with 4 compression drivers. An acoustic field corresponding to the 2 nd transverse mode of the cavity is forced into that at a frequency of 1 kHz. Acoustic levels up to 174 dB are produced. High speed visualizations are performed in order to observe the response of the jet to the acoustic perturbations. In the case of low Weber numbers (We < 30) the jet can be considered as cylindrical and depending on the position of the injector with respect to the acoustic axis different responses can be observed. If the injector is placed in correspondence of the velocity antinode the jet is flattened into a liquid sheet perpendicular to the acoustic axis, if the injector is located in correspondence of an intensity antinode the jet is deviated toward the velocity antinode. Combined response can be observed at intermediate positions. For higher Weber numbers the jet is no more cylindrical and a spray is formed, characterized by with a certain spray angle. Such a spray is can still be affected by the acoustics but it is not always possible to get evidence of this from observation of raw images. To quantify these effects, image analyses have been carried-out to determine how spatial distributions of droplets are affected by acoustics. Results are presented for Weber numbers ranging from 30 to 1500, with and without acoustic. Clustering of droplets is shown as well as improvement of atomization process

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

Acoustic response of a feeding system to high-frequency transverse acoustic field

International audienceThe acoustic coupling between the injection system and the acoustic fluctuations in liquid rocket engine combustion chambers is an important issue in the understanding of the thermo-acoustic instability phenomenon. This paper presents results of a large parametric investigation of a two-phase injection system acoustic response, to the excitation produced by a high-amplitude transverse acoustic field forced into a main resonant cavity. Two domes, one for the gas and one for the liquid, were expressly designed to feed three identical coaxial injectors. Characterization of domes internal mode shapes were performed by measuring pressure signals at different locations in the domes. Experimental mode shapes showed good agreement with those predicted by numerical simulations. Acoustic pressure amplitudes up to 17% of the the one induced in the main cavity can be found in both gas and liquid dome. The maximum acoustic response is observed in a configuration in which acoustic boundary conditions does not correspond to the maximum injection system solicitation conditions

Acoustic response of a feeding system to high-frequency transverse acoustic field

International audienceThe acoustic coupling between the injection system and the acoustic fluctuations in liquid rocket engine combustion chambers is an important issue in the understanding of the thermo-acoustic instability phenomenon. This paper presents results of a large parametric investigation of a two-phase injection system acoustic response, to the excitation produced by a high-amplitude transverse acoustic field forced into a main resonant cavity. Two domes, one for the gas and one for the liquid, were expressly designed to feed three identical coaxial injectors. Characterization of domes internal mode shapes were performed by measuring pressure signals at different locations in the domes. Experimental mode shapes showed good agreement with those predicted by numerical simulations. Acoustic pressure amplitudes up to 17% of the the one induced in the main cavity can be found in both gas and liquid dome. The maximum acoustic response is observed in a configuration in which acoustic boundary conditions does not correspond to the maximum injection system solicitation conditions

Acoustic response of an injection system to high-frequency transverse acoustic fields

International audienceThe acoustic coupling between the injection system and the acoustic fluctuations in liquid rocket engine combustion chambers is an important issue in the understanding of the thermo-acoustic instability phenomenon. This paper presents the results of a wide-ranging parametric investigation of the acoustic response of a two-phase injection system submitted to a forced high-amplitude transverse acoustic field. Two domes, one for the gas and one for the liquid, were expressly designed to feed three identical coaxial injectors. The internal mode shapes of the domes were characterized by measuring pressure signals at different locations in the domes. Experimental mode shapes showed good agreement with those predicted by numerical simulations. Acoustic pressure amplitudes up to 23% of those induced in the main cavity can be found in both the gas and liquid domes. The response efficiency in a dome depends on the position of the injectors' exit in the acoustic field