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Characterization of Liquid Jets in Subsonic Crossflows Using X-Ray Radiography
| Content Provider | Semantic Scholar |
|---|---|
| Author | Beavercreek Carter, Campbell D. Peltier, Scott |
| Copyright Year | 2016 |
| Abstract | Spray structures of liquid jets in subsonic crossflows were characterized using X-ray radiography at the 7-BM beamline of the Advanced Photon Source at Argonne National Laboratory. A small-scale wind tunnel with a test section of 51 mm (H) × 51 mm (W) × 152 mm (L) provided a freestream flow up to Mach 0.3. The wide windows of the test section are fitted with a thin polyimide film for high X-ray transmittance. An axisymmetric aerated-liquid injector fitted with an exchangeable adaptor was used to generate a pureor aerated-liquid jet at the desired injection conditions. The transmitted X-ray intensities were processed to give quantitative liquid mass distributions within the spray at various injection conditions. The present results were also used to derive spray penetration heights for comparison with predictions from the existing correlations. Companion PDPA measurements were carried out to compare with the X-ray radiography measurements and to expand the understanding of the liquid jets in crossflows. The present study shows that the present techniques provide quantitative measurements of liquid mass distribution within both the near field, including the jet column, and the far field of liquid jets in subsonic crossflows. In the near field, deformation of the liquid column in the pure-liquid jets and the co-annular-like column structure in the aerated-liquid jets were also measured. In the far field, the present efforts to compare the measured penetration heights, based on various threshold values, with predictions from existing penetration height correlations offer new perspectives on characterizing spray penetration in crossflows. In general, the penetration heights predicted from shadowgraphbased correlations are in agreement with the time-averaged water mass contours and may ignore a significant amount of injected liquid mass in the far field. The penetration heights predicted from PDPA-based correlations are in agreement with the standard deviation water mass contours and are more indicative of the outer boundary of droplet presence. The approach to comparison of liquid mass distribution using spanwise-integrated liquid volumes from both X-ray radiography and PDPA measurements is relatively well illustrated in the present study. A discrepancy between X-radiography and PDPA measurements of liquid mass distribution near the tunnel floor was observed. The factors contributing to this discrepancy should be explored in the future. __________________________________________ * Corresponding author, Kuocheng.Lin.ctr@us.af.mil ILASS Americas, 28h Annual Conference on Liquid Atomization and Spray Systems, Dearborn, MI, May, 2016 INTRODUCTION The successful design of a liquid-fueled, airbreathing propulsion system depends to a significant extent on liquid atomization performance, which determines the mixing behavior and the combustion efficiency. Furthermore, improved technology for the distribution of specific quantities of atomized fuel to the desired locations will be required for the reduction of pollutant emissions from propulsion systems. Therefore, the fundamental physics of liquid-jet breakup processes must be understood, and better fuel injection schemes must be sought. Among the candidate injection schemes for liquidfueled high-speed air-breathing propulsion systems, liquid aeration is a plausible approach to enhance liquid atomization in a high-speed crossflow environment.1 A two-phase mixture created inside the injector with a small amount of gas mixed with liquid fuel produces an aerated-liquid jet that is capable of generating a welldispersed plume to mix efficiently with the ambient air for efficient combustion. This approach can reduce the time and space required for the breakup of the liquid column. It has been shown that the liquid aeration technique can generate a spray that penetrates well into the flow and produces a large fuel plume containing a large fraction of small droplets.2,3 Characterization of liquid jets in crossflows mainly rely on high-speed imaging or optical measurements, such as phase Doppler particle analysis (PDPA).3-6 Macroscopic spray structures, as well as plume and droplet properties, are typically presented. Spatial liquid mass distribution, however, is typically not measured. Recently, the near-field structures of liquid jets have been explored with various X-ray diagnostic techniques, including X-ray phase contrast imaging (PCI) and X-ray radiography.7-9 The studies with X-ray radiography in particular offered quantitative characterization of liquid mass distribution within the near fields of aerated-liquids.10,11 Liquid-based plume properties, such as averaged density, velocity, and momentum flux, were readily derived from the X-ray radiography datasets. In these studies, however, the aerated-liquid jets were injected into a quiescent environment, in contrast to that found inside high-speed air-breathing propulsion systems. The objective of the present study is to apply X-ray radiography to measure the liquid phase concentrations within both pureand aerated-liquid jets in subsonic crossflows. A portable wind tunnel was designed and fabricated to provide the crossflow environment. This wind tunnel is limited in size and flow capacity, due to the requirement to fit the tunnel assembly inside a synchrotron X-ray facility (with very limited available space). Nonetheless, this is the first measurement, to our knowledge, by synchrotron radiography of a jet in crossflow. Companion PDPA measurements were also accomplished in the present study, in order to provide a better understanding of the spray structures. EXPERIMENTAL METHODS Experimental Setup The experiment was conducted at the 7-BM beamline of the Advanced Photon Source at Argonne National Laboratory. A portable wind tunnel was set up inside the X-ray hutch to provide subsonic crossflows flowing perpendicular to the X-ray beam path. Figure 1 shows the portable wind tunnel with critical components identified. The test section has a dimension of 51 mm (H) × 51 mm (W) × 152 mm (L) and can be fitted with clear sidewalls for visual observation or optical measurements. For the present study, the sidewalls were replaced by a pair of 50-μm-thick polyimide films for high X-ray transmittance, as illustrated in Fig. 2. The polyimide films bow in slightly toward the tunnel flow during tunnel operation, due to a small differential pressure across the side wall. Flow distortion created by the deformed polyimide films has been determined to be less than 2% of the ideal tunnel mean flow. Its effects on spray properties are, therefore, ignored. This wind tunnel is capable of flow up to Mach 0.3 inside the test section. An automatic control loop is available to maintain a given flow Mach number at the entrance of the test section, in order to overcome the aerodynamic blockage created by the injected spray in the test section. The wind tunnel can be operated continuously for nonstop data acquisition. For the present experiments, the wind tunnel was rigidly mounted on a traversing table, which provided movement normal to the X-ray beam. An axisymmetric injector was flush mounted on the bottom floor of the wind tunnel to inject an aeratedliquid jet, with the regions of interest positioned in the path of the X-ray beam. Water and aerating gas were supplied into the aerated-liquid injector at desired flow rates to form a liquid jet. The injector body features a so-called outside-in aerating configuration with the aerating gas discharged through aerating orifices from an annular passage to create a two-phase mixture inside the mixing chamber. The injector hardware was designed to accommodate exchangeable adaptors, as shown in Fig. 3. The adaptors feature an exit orifice diameter, d0, of either 0.5 or 1.0 mm, with a length to diameter ratio, L/d0, of 10. Water and nitrogen were selected as the liquid injectant and aerating gas, respectively. X-Ray Measurements The 7-BM beamline is dedicated to ultrafast X-ray radiography and tomography experiments for fuel sprays and associated phenomena. The X-ray source is a synchrotron bending magnet, which produces nearlycollimated, broadband X-ray emission. The beamline consists of two radiation enclosures. The first enclosure (7BM-A) houses a pair of slits to limit the X-ray beam size and a double multilayer monochromator (4.2% ∆E/E). The monochromatic beam passes into the second radiation enclosure (7BM-B), which houses the experimental equipment. More information regarding the beamline performance can be found in the study of Kastengren et al.12 For the current experiments, the X-ray beam photon energy was set to 8 keV. This provides a good compromise between absorption of the beam by the spray and excessive absorption by the X-ray windows and ambient air. The beam was focused using a pair of 300 mm long Kirkpatrick-Baez focusing mirrors. The beam focus is approximately 5 × 6 μm FWHM V × H, located approximately 400 mm from the center of the horizontal focusing mirror. The effective size of the beam for the current sprays (which are several mm wide) is somewhat greater than this minimum focus size; the divergence of the focused x-ray beam is approximately 2 × 3 mrad V × H. The experimental setup at the 7-BM beamline with a typical injection configuration is shown in Fig. 4. For the present study, the injection stand is replaced by the portable wind tunnel. The radiography measurements are based on a measurement of X-ray absorption from water. The incident beam intensity is measured with a diamond photodiode 52 μm thick. The transmitted beam intensity is measured with a reverse-biased silicon PIN diode, 300 μm thick. The X-ray intensity when the beam passes outside the spray is compared with the X-ray intensity when the beam passes through the spray. This measurement gives a measure of spray density for one line of sight through the spray. Automated raster scanning is used to interrogate a wide field of |
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| Alternate Webpage(s) | http://www.ilass.org/2/conferencepapers/10_2016.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
