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Characterization of Flow Structures Inside an Aerated-Liquid Jet Using X-Ray Diagnostics
| Content Provider | Semantic Scholar |
|---|---|
| Author | Peltier, Scott J. |
| Copyright Year | 2015 |
| Abstract | The present study examines the internal flowfield of aerated-liquid fuel injectors through X-ray radiography and fluorescence. An inside-out injector, consisting of a perforated aerating tube within an annular liquid stream, sprays into a quiescent environment at a fixed mass flow rate of water and nitrogen gas. The liquid and gas phases are doped with bromine and krypton, respectively, allowing discrimination between the phases through the respective X-ray fluorescence signals. For the present study, only the liquid mass distribution is examined. The injector housing is fabricated from beryllium (Be), which allows the internal flowfield to be examined (as Be has relatively low X-ray absorption). Time-averaged equivalent path length (EPL) and line-of-sight averaged density ρ(y) reveal the formation of the two-phase mixture, showing that the liquid phase is concentrated primarily below the aerating tube. As the mixture travels down the nozzle, the liquid mass distribution becomes increasingly co-annular. However, the spray region rapidly transitions to a Gaussian shape after a distance of 0.25 mm. The location and size of the aerating orifices largely affects only the formation of the two-phase mixture, and any differences due to the aerating tube are negligible in the injector nozzle and spray region. __________________________________________ * Corresponding author, Scott.Peltier.4.ctr@us.af.mil ILASS Americas 27th Annual Conference on Liquid Atomization and Spray Systems, Raleigh, NC, May 2015 INTRODUCTION The injection of liquid fuel into high-speed crossflow environments presents a considerable challenge to hypersonic air-breathing propulsion. In order to maximize combustion efficiency, the fuel jet must be atomized into small droplets. Liquid aeration has been shown to provide improved atomization of the fuel spray, while enhancing the dispersion of the plume [1]. The liquid fuel and aerating gas are combined internally in the injector body, forming a two-phase mixture. As the mixture enters the lower-pressure environment of the combustor, bulk dilatation of the gas bubbles shatters the liquid column and disperses the fuel droplets, thereby decreasing the time necessary for vaporization of the fuel spray [2]. Studies in subsonic crossflow environments have shown that liquid aeration improves both the penetration depth and droplet distribution of the fuel spray [3-4]. Dense sprays strongly scatter optical photons, making diagnosis (though optical techniques) challenging. Scattering of X-ray photons, on the other hand, is relatively small, due to an index of refraction very close to 1.0 (in both water and the surrounding gas). X-ray radiography and X-ray phase contrast imaging (PCI) have been successfully applied to the near-field spray regions of aerated sprays in quiescent environments, revealing both the structure of the fuel plume and the characteristics (droplet diameter, bubble diameter, and bubble film thickness) of the two-phase region [5-7]. Further studies using X-ray radiography have focused upon the liquid mass distribution of the spray, allowing for timeaveraged determination of the density, velocity, and momentum flux [8-9]. Additional studies have attempted to distinguish between the gas and liquid phases using X-ray radiography and X-ray fluorescence [10]. For this purpose, the aerating gas was argon (Ar), allowing the gas-phase to be identified by its X-ray fluorescence signal. It was shown that the gas and liquid phases diverge in the aerated spray, possibly due to the bounding influence of the barrel shock upon the gas streamlines. However, strong fluorescence signal trapping with Ar complicated analysis of the datasets. This issue has been overcome by i) using nitrogen (N2) doped with krypton (Kr) and water doped with bromine (Br) [11], as the respective gas and liquid, and by ii) increasing the incident X-ray photon energy to 14.7 keV. Quantitative measurements of the liquid and gas mass distributions supported the earlier observations of phase separation in the spray region. The examination of aerated-liquid jets has heretofore focused entirely upon the external spray region. In order to better understand the mechanisms by which internal geometry influences the external spray, measurements of the liquid and gas distributions inside the injector housing are highly desirable. An aerated-liquid injector has been manufactured from beryllium (Be) to allow examination of the internal flow. This material has much lower X-ray absorption than water, allowing interrogation of the internal flowpath. The objective of the current study is to examine the internal flowfield generated within an aerated-liquid injector using X-ray radiography and fluorescence. The paper will focus upon the application of this technique to two different aerating configurations, demonstrating the utility of this measurement for two-phase internal flows. Liquid mass distributions are examined throughout the internal geometry, comparing the effects of aerating orifice size and location on the two-phase mixture. EXPERIMENTAL METHODS X-ray radiography and fluorescence measurements were conducted at the 7-BM beamline of the Advanced Photon Source (APS) at Argonne National Laboratory. The axisymmetric injector was mounted on a 3-axis traversing mechanism and translated in the x-y plane, where the xand y-directions denote the axial (positive down) and transverse directions, respectively. Data were collected by raster scanning, traversing the injector laterally through the focused X-ray beam at specified axial positions. The aerated-liquid injector was oriented vertically and supplied with a fixed mass flow rate of nitrogen gas and water (see Fig. 1). A catchment system was placed below the injector to collect the liquid spray and prevent the attachment of liquid droplets to the exterior of the injector body, which would interfere with measurements of the internal flowfield. Injector Geometry The final section of the injector geometry has been manufactured from Be (> 99.4% purity) due to its low X-ray absorption. Figure 2 shows the injector (with aerating tube installed), denoting the four flow regions: aerating, mixing, nozzle, and spray. In the aerating region, the aerating gas was injected into the annular liquid flow through a perforated tube (described further below). The internal diameter of this region is dm = 4.8 mm (0.188”), and the gap between the wall and the aerating tube is 0.8 mm (0.0315”). The mixing region consists of a volume downstream of the mixing tube but upstream of the nozzle contraction, where the liquid and aeration gas can mix. A gap of H = 3.2 mm (0.125”) was maintained between the end of the aerating tube and the nozzle entrance. In the nozzle region, the inner diameter narrows to d0 = 1.02 mm (0.04”), with a length L/d0 = 10. For all tests, the mass flow rates of nitrogen and water were held constant. Water was injected at a rate of mL = 18.2 g/s (0.04 lbm/s) into the annular region at the top of the injector (see Fig. 1). A constant gas-toliquid mass ratio (GLR) of 4% was maintained throughout each data set. Aerating Tubes The aerated-liquid injector produces a two-phase mixture by injecting the aerating gas into an annular liquid stream. This inside-out configuration has been studied previously by [10], but only in the spray region. A total of six aerating tube designs were utilized in the experiment, varying the number, size, spacing, and angular offset of the aerating orifices. In the current study, only two cases are examined in detail, labelled as Case #2 and Case #5. Both tubes are manufactured from stainless steel, with an outside diameter dt,o = 3.2 mm (0.125”) and internal diameter dt,i = 2.2 mm (0.085”). The downstream end of each tube is sealed with a solid plug, and the tubes are centered in the injector at a distance H = 3.2 mm (0.125”) from the nozzle entrance. Due to the use of stainless steel, the flow within the aerating tube was inaccessible to the X-ray diagnostics. The designs of the tubes differ only in the placement and size of the aerating orifices. A schematic of the aerating tubes is shown in Fig. 3, and the relevant parameters are given in Table 1. The orifices in Case #2 have a diameter of da = 0.76 mm (0.030”) and are arranged in four rows with two holes each. Each row of holes is offset from the preceding pair by 90° and separated by a distance of 1.3 mm (0.05”). The aerating orifices of Case #5 are similarly spaced, but each hole measures only da = 0.51 mm (0.020”), arranged in ten rows of two holes each. The orifice area Aa of each geometry is approximately equal to the tube area At. The selection of Cases #2 and #5 illustrate the effect of mixing length on the internal and external flow, as the aerating gas of Case #5 is injected over a distance 2.5 times that of Case #2. X-Ray Diagnostics The 7-BM beamline is configured for spray diagnostics using X-ray radiography and fluorescence [12]; a schematic of the setup for this experiment is shown in Fig. 4. Nearly collimated, broadband X-ray emission is generated by a synchrotron bending magnet. The size of the X-ray beam is trimmed by a pair of slits, and the broadband emission is converted to a monochromatic beam by a double multilayer monochromator (1.40% ∆E/E). Focusing was performed with a pair of 300 mm Kirkpatrick-Baez focusing mirrors, and the focus was placed 400 mm from the center of the horizontal focusing mirror. The X-ray beam focused size is 56 μm FWHM (VH), but at the spray region, the beam size is about 20 μm. . The X-ray measurement process follows the procedures used in ref. [11]. Pointwise X-ray radiography was performed by raster scanning across the injector geometry and spray region. X-ray transmission was measured by an unbiased silicon PIN diode, 300 μm thick. Due to the 1-second averaging time at each point, only time-averaged behavior is examined in this study. The attenuation of the X-ray beam |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.ilass.org/2/conferencepapers/9_2015.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
