ILASS Americas, 28h Annual Conference on Liquid Atomization and Spray Systems, Dearborn, MI, May, 2016

Characterization of Liquid Jets in Subsonic Crossflows Using X-Ray Radiography

Kuo-Cheng Lin* Taitech, Inc.

Beavercreek, Ohio 45430

Campbell Carter and Scott Peltier Air Force Research Laboratory, Aerospace Systems Directorate

Wright-Patterson AFB, Ohio 45433

Alan Kastengren Argonne National Laboratory

Argonne, Illinois 60439

Ming-Chia Lai Wayne State University Detroit, Michigan 48202

Abstract

Spray structures of liquid jets in subsonic crossflows were characterized using X-ray radiography at the 7-BM beam-line 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 pure- or aerated-liquid jet at the desired injection condi-tions. 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, de-formation 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 shadowgraph-based 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 drop-let 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, air-

breathing propulsion system depends to a significant extent on liquid atomization performance, which de-termines the mixing behavior and the combustion effi-ciency. Furthermore, improved technology for the dis-tribution of specific quantities of atomized fuel to the desired locations will be required for the reduction of pollutant emissions from propulsion systems. There-fore, the fundamental physics of liquid-jet breakup pro-cesses must be understood, and better fuel injection schemes must be sought.

Among the candidate injection schemes for liquid-fueled 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 well-dispersed 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 tech-nique 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 tech-niques, including X-ray phase contrast imaging (PCI) and X-ray radiography.7-9 The studies with X-ray radi-ography in particular offered quantitative characteriza-tion 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 da-tasets. 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 pure- and 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 syn-chrotron 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 beam-line of the Advanced Photon Source at Argonne Na-tional 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 compo-nents 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 ef-fects 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 aerodynam-ic blockage created by the injected spray in the test sec-tion. 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 aerated-liquid 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 de-signed 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, respec-tively. 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 nearly-collimated, 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 sec-ond 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 pho-ton energy was set to 8 keV. This provides a good com-promise 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, locat-ed approximately 400 mm from the center of the hori-zontal 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 in-cident beam intensity is measured with a diamond pho-todiode 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 meas-urement 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 view of the spray. PIN diode signals were collected at a 2.5-MHz sam-pling rate for 0.5 s at each point (yielding over 1 million samples per probe location, allowing analysis of both mean and statistical spray behavior) with wide band-width.

According to Beer-Lambert Law, the relationship between the absorption and the properties of the materi-al through which the radiation is travelling can be de-scribed as: βMe

II −=0

(1)

where I and I0 are the intensity of the transmitted and incident radiation, respectively, measured by the PIN diode, β is the mass attenuation coefficient, and M is the projected density of the medium in the X-ray beam. For the present study, the medium of interest is a multi-phase flow consisting of air and a dispersed phase of water in the form of fine droplets and ligaments. The

absorption of the air can be neglected, allowing the projected density to be converted into the pathlength of water in the beam, x:

( )βρα

)ln(ln 00 IIIIEPLx −=

−=≡ (2)

where α is the pathlength absorption coefficient, ρ is the density (equaling 1.0 g/cm3 at room temperature), and β is the mass attenuation coefficient (equaling 10.4 cm2/g at 8.0 keV, from the NIST database13, in the ab-sence of coherent scattering). The equivalent path length (EPL) is the thickness of pure water required for the transmitting X-ray to generate the same amount of extinction as that generated from the dispersed spray at the same X-ray energy level. The value of EPL can, therefore, be related to the local density of the liquid/air mixture or local liquid mass fraction. In the present study, only the two-dimensional liquid line-of-sight EPL will be presented to depict the spray structure. Characterization of the gas mass distribution within the aerated-liquid jets was not attempted. PDPA Measurements

A two-component phase Doppler particle analyzer (PDPA), along with a TSI FSA4000 burst correlator and two 500-mW solid state lasers (operating at wave-lengths of 488- and 514.5-nm) were used to determine the properties of droplets and spray plumes. The clear aperture and focal length of the transmitter were 50 and 500 mm, respectively. The initial beam diameter was 1.77 mm. Beam separation was 20 mm. A beam ex-pander with an expansion ratio of 2 was used to reduce the size of the probe volume. The waist diameters of the probe volume for 488- and 514.5-nm wavelengths were 91.4 and 92.5 µm, respectively. The receiver has a clear aperture of 106 mm in diameter and a collecting lens with a focal length of 370 mm. The collected light was passed through a 50-µm slit to further reduce the probe volume. Light was collected at 30 degrees from the transmitter (refractive scattering). Droplet size, axial velocity, and volume flux were measured in the trans-verse and spanwise directions across the spray plume. Measurements were performed at several downstream locations, x/d0=10 to 100. A spatial increment of 1.27 mm was used for measurement in the cross stream (z) direction and an increment of 1.27 or 2.54 mm was employed for measurement in the jet injection (y) direc-tion, depending on the spray structure. Regions with unbroken liquid core, irregular ligaments, and non-spherical drops, where PDPA measurements cannot be carried out reliably, were avoided in the determination of detailed structures of the entire cross-sectional area. For spray plume areas with high number density, drop-let properties were averaged over more than 15,000

droplets at each location to reduce the experimental uncertainties. Otherwise, a constant duration of 10 sec-onds was used for data acquisition. Measurements were stopped when the measured liquid volume flux was below 0.05 cc/s/cm2. RESULTS AND DISCUSSION Near Field Structures

An EPL contour plot of water in the near field of a pure-liquid jet injected from a 1.0-mm nozzle into a Mach-0.3 crossflow is shown in Fig. 5(a). In this re-gion, X-ray radiography was conducted with radial scans (in the x direction) at various distances from the wind tunnel floor (in the y direction). The gradual bend-ing and flattening of the liquid column by the freestream are clearly depicted. The lateral movement of the liquid on the windward side of the column is also evident, as water EPL greater than 1.0 mm is found. After y = 4 mm, the plume density rapidly decreases, indicating a rapid breakup of the spray. (The discon-nected high-density region around y = 4.5 mm is an interpolation artifact.)

Figure 5(b) shows the standard deviation of the EPL contour. The region with a high value of standard deviation represents large temporal variation in liquid mass distribution. This is also the region where surface waves along the liquid column grow large enough to promote liquid shedding from the column surfaces tan-gential to the freestream flow. The highest standard deviation values are coincident with the most rapid de-crease in plume EPL, which suggests that this is indeed a point of liquid column breakup.

Time-average and standard deviation water EPL contours of the near fields of aerated-liquid jets at two aeration levels are shown in Fig. 6. The 0.5-mm nozzle was used for water injection into a Mach-0.3 crossflow. The aeration level is indicated by gas-to-liquid mass ratio (GLR). The effects of liquid aeration on spray at-omization can be clearly observed in the significant reduction in water EPL in the jet column. For a pure liquid jet injected from the same nozzle, the maximum water EPL should be 0.5 mm at the axis of the injector exit. Adding aerating gas creates voids inside the injec-tor; with aeration the measured water EPL is less than 0.1 mm and the contours exhibit a co-annular structure at both injection conditions (Figs. 6a and 6c). The re-duction in water EPL should come with an increase in local liquid flow velocity, as the total liquid flow rate is maintained. As the aeration level increases from GLR = 2% to GLR = 4%, the peak water EPL drops further, indicating a thinner annular liquid film moving at a faster speed into the crossflow. The time-average water EPL quickly decreases to less than 25 µm at y > 7 mm for both injection conditions. The standard deviation EPL contours show that the fluctuation in water EPL mainly takes place along the windward side of the jet

column, probably due to the interaction between freestream air and the relatively “porous” jet column.

Far Field Structures of Pure-Liquid Jet

Time-average and standard deviation of water EPL contours in the far field of a pure-liquid jet injected from a 0.5-mm nozzle into a Mach-0.3 crossflow are shown in Fig. 7. X-ray radiography was conducted with scans perpendicular to the tunnel floor (in the y direc-tion) at various freestream locations (in the x direction). Note that a nonlinear contour level was used to better differentiate the relatively large difference in gradient of liquid mass distribution between the near- and far-field regions in this figure. The high gradient in liquid mass distribution near the liquid column, x < 2.5 mm, indicates slow and limited liquid dispersion. The aver-age water EPL contour in Fig. 7(a) shows that after about x = 2.5 the injected liquid is quickly dispersed, with the peak water EPL decaying to around 50 µm at x = 40 mm (x/d0 = 80). The standard deviation water EPL contour in Fig. 7(b) indicates that the injected water can occasionally but infrequently appear outside the range captured by the average water EPL contour in the transverse (y) direction for a given x/d0 location. The corresponding droplet size and liquid volume flux are further characterized using the companion PDPA meas-urements (to be discussed later). Highly dynamic liquid breakup/shedding and unsteadiness in freestream and liquid flows are the likely contributing factors to the broadened liquid mass distributions in the spray plume depicted in Fig. 7(b).

Figure 8 shows the outer boundary or penetration height of the liquid plume measured from the average water EPL contours. Three EPL threshold values of maximum, 0.01 mm, and 0.005 mm are selected to de-fine the outer boundary at each x/d0 locations. These measured penetration heights are compared with predic-tions from penetration height correlation. These two correlations, one based on shadowgraph images and one based on PDPA measurements, were developed by Lin et al.,14 using the same experimental setup. They con-cluded that the predicted penetration height from the correlation using PDPA measurements is always higher than that predicted from the correlation using shadow-graph images.

The comparison in Fig. 8 shows that the measure-ments based on the water EPL threshold value of 0.005 mm give the deepest penetration, as expected. The measurements based on the water EPL threshold value of 0.01 mm agrees reasonable well with the predictions from the correlation using shadowgraph images. The penetration heights calculated from the correlation us-ing PDPA measurements significantly over-predict the present X-ray radiography measurements based on all three EPL threshold values. They are, however, more closely indicative of liquid presence based on the stand-

ard deviation water EPL contours seen in Fig. 7(b). Visual observation of the liquid plume showed that large droplets were present in low number density out-side the region depicted by the average water EPL con-tours. Liquid mass contribution from these large drop-lets will be discussed below, using the PDPA measure-ments. New correlations for spray penetration heights should be developed based on the threshold values seen here. Comparison between X-Ray Radiography and PDPA

Figure 9 shows the contour plots of liquid volume flux in both x and y directions, droplet velocities (up and vp), droplet size (in SMD), and droplet number density, from companion PDPA measurements at the x/d0=50 freestream location. Also shown in Fig. 9 are plume penetration heights by the present X-ray radiography measurements and from the correlation predictions of Lin et al.14 The penetration height predicted by the PDPA-based correlation (HPDPA) agrees reasonably well with the upper boundary of the volume flux contour. (In the present study, PDPA measurement was terminated when the measured liquid volume flux in the x direction was less than 0.05 cc/s/cm2.) The plume region beyond the upper boundaries specified by the present X-ray radiography measurements, including HEPL=0.005mm and HEPL=0.01mm, or by the prediction from the shadowgraph-based correlation (Hshadowgraph) contains large droplets (> 50 µm). These large droplets possess a low freestream velocity (up) and a high transverse velocity (vp), due to their relatively large mass. The large mass, of course, causes slow momentum exchange with the freestream air and preserves the original transverse momentum from initial injection. These large droplets, however, are low in number density, as seen in the visual obser-vation described above.

The measured peak liquid volume flux in Fig. 9 is located above the penetration heights measured by the X-ray radiography or predicted by the shadowgraph-based correlation. In order to better understand the liq-uid distribution from a different perspective, Fig.10 shows the distribution profiles of three integrated pa-rameters: the total liquid volume within a given volume by X-ray radiography (VL), the total droplet volume within a given volume by PDPA (Vd), and the liquid flowrate through a given volume by PDPA (QL) at three x/d0 locations. For easy comparison, the volume of the X-ray beam, which has a height (H) of 5 µm, a width (W) of 6 µm, and a length corresponding to the liquid plume width in the spanwise (z) direction, was used for the calculation of these parameters. This reference vol-ume was selected, in order to match the line-of-sight integration feature of the EPL measurement. Parameters obtained from the point PDPA measurements, such as the droplet number density (N), droplet size (dp), and

liquid volume fluxes (Fx and Fy in in the x and y direc-tion, respectively) can be integrated over the same X-ray volume to give the projected liquid properties in the X-ray line-of-sight direction. Derivations of these inte-grated parameters are shown as follows:

The total liquid volume within the volume of the X-ray beam, VL, can be directly obtained by multiplying the measured EPL value by the cross-sectional area of the X-ray beam:

( ) ( ) ( )WHyEPLyVL ××= (3)

The total droplet volume within the volume of the

X-ray beam, Vd, can be obtained by counting the total droplet volume:

( ) ( )∑

×∆×××=

iipiid dzHVNyV 3,6

π (4)

where ∆z is the PDPA probing increment in the spanwise direction and dp is the droplet diameter in SMD in the present study. Despite the differences in measurement principles between X-ray radiography and PDPA, both VL and Vd indicate the projected liquid vol-ume in the X-ray line-of-sight direction (or spanwise direction in the present wind tunnel setup) and should be theoretically identical. The measurement uncertain-ties associated with the PDPA technique, such the pres-ence of multiple droplets or none-spherical drop-lets/ligaments within the laser probe volume, may de-crease the accuracy in deriving Vd. Nonetheless, both volume-based parameters can give a direct comparison between both X-ray radiography and PDPA measure-ments.

The third integrated parameter is the liquid flowrate through the volume of the X-ray beam, QL:

( ) ( )( ) ( )( )( )∑ ∆×+∆×=i

iiyiixL zWFzHFyQ 2,

2,

(5)

The measured liquid volume fluxes by PDPA in both x and y directions are integrated over the corresponding surface areas of the X-ray beam to give the net liquid flowrate passing through the volume of the X-ray beam. Please note that QL is a flowrate-based parameter and cannot be directly compared with the volume-based parameters of VL and Vd. In the present study, QL pro-files are still included in Fig. 10, in order to qualitative-ly compare the distribution profiles and the peak value locations for all three integrated parameters.

Figure 10(a) illustrates the liquid distribution pro-files at the x/d0 = 25 location. At this location, the liquid jet is not well atomized. The presence of non-spherical droplets/ligaments and a high droplet number density

prevent the PDPA technique from giving accurate measurements. The X-ray radiography technique is free of the constraints relating to droplet shape and number density and should give an accurate quantitative value. The integrated droplet volume Vd by PDPA, however, exhibits a higher value than the integrated liquid vol-ume VL by X-ray radiography. Several factors may lead to this discrepancy. First, the use of SMD for dp in Eq. (4) can lead to a higher droplet volume. Instead, the actual droplet size distribution for each PDPA meas-urement point should be utilized to derive the local droplet volume for integration. Second, the present PDPA measurements give an integrated liquid flowrate across the y-z cross plane greater than the metered liq-uid flowrate through the injector exit orifice. Uncertain-ties in liquid flow metering and PDPA setup, which determines the laser probe volume for the derivation of droplet number density, can also lead the observed dis-crepancy. Efforts to resolve the discrepancy will be carried out in the future. Nonetheless, all three distribu-tion profiles of VL, Vd, and QL exhibit the similar loca-tion for the corresponding peak values.

At x/d0 = 50 (Fig. 10(b)), the similar discrepancy between VL and Vd is observed. The range of liquid dis-tribution in the transverse direction up to y/d0 ~ 48 and the location of peak values around y/d0 ~ 30 are almost identical for all three profiles. Both peak Vd and QL values are expected to be higher near the injector exit, where the plume exhibits limited dispersion with a higher peak liquid mass concentration. Both values at x/d0 = 50, however, are higher than those at x/d0 = 25, clearly indicating the technical challenges for the PDPA technique to give accurate measurements in the near field of a spray; here, the high probability for the pres-ence of multiple droplets and/or non-spherical droplets in the PDPA probe volume reduces the droplet valida-tion rate and, thus, droplet number density and liquid volume flux. Once again, the peak VL value by the X-ray radiography gives a reasonable trend in the decay of peak liquid mass distribution along the freestream di-rection.

At x/d0 = 100 (Fig. 10(c)), all three profile are similar. Note that the VL profile at x/d0 = 79 is shown in this figure. X-ray radiography was not performed at the x/d0 = 100 location for this injection condition. The slightly higher transverse locations for spray penetra-tion and peak values for both Vd and QL profiles by PDPA are simply due to the difference in x/d0.

One major difference among VL, Vd, and QL pro-files can be observed in the region close to the tunnel floor (y/d0 < 10) for all three freestream locations in Fig. 10. In this region, VL from the X-ray radiography technique detects the presence of a significant amount of water. Vd and QL from the PDPA technique, howev-er, do not capture the noticeable presence of water, as further indicated by the cross-sectional contours of

droplet and plume properties at the x/d0 =50 location in Fig. 9. Clipping of the PDPA laser beams by the tunnel floor can be significant in the region with y/d0 < 3, es-pecially for the measurement of the transverse (y) com-ponent properties in the present setup. Still, the inte-grated VL value by X-ray radiography at the tunnel floor is close to half of its peak value at x/d0 ≥ 50. The inte-grated Vd value by PDPA, however, does not exhibit the similar features. This discrepancy should be explored in the future.

The comparison between the integrated liquid dis-tribution profiles in Fig. 10(b) and the cross-sectional volume flux contours by PDPA in Fig. 9 at the x/d0 = 50 location shows that the peak volume flux is located close to the top of the plume cross section and is almost coincident with the upper boundary of the spray plume defined by EPL = 0.005 mm (HEPL=0.005mm in Fig. 9). The peak value of the integrated VL, Vd, or QL is actual-ly located at a lower position, close to the upper bound-ary of the spray plume defined by EPL = 0.01 mm (HEPL=0.01mm) or predicted by the shadowgraph-based correlation (Hshadowgraph) in Fig. 9.

Once again, existing correlations or the definition of spray penetration height should be used with care. Interpretation or use of the measurement from a specific diagnostic technique should be exercised with the cor-responding working principles and technical constraints in mind. It is prudent to combine data sets from various spray diagnostic techniques for an improved under-standing of the spray structures. With X-ray radiog-raphy, the line-of-sight path-integration feature inhibits a detailed picture of the liquid distribution over a plume cross section. The combination of X-ray fluores-cence15,16 and focusing polycapillary optics17 can poten-tially give point measurements in the spanwise direc-tion to overcome this limitation. This technical ap-proach should be explored in the future. Furthermore, the X-ray radiography results in the present study are time averaged over a million samples at a fairly high sampling rate of 2.5 MHz. Even though the standard deviation of the collected data is presented to illustrate the statistical variation in the features of the sprays, time-resolved spray dynamics should be explored in the future.

Far Field Structures of Aerated-Liquid Jet

Water EPL contours depicting the far-field struc-tures of aerated-liquid jets are shown in Fig. 11. Once again, the nonlinear contour level was used to differen-tiate the gradient in liquid mass distribution in both near and far fields. With the creation of semi-coannular two-phase flow prior to injection, as depicted in Fig. 6, wa-ter is quickly dispersed in the plume. The measured water EPL is fairly low in the far-field region. The low liquid flow rate of 2.3 g/s for the injection conditions also contribute to the low water EPL. As in Fig. 7, the

standard deviation water EPL contours exhibit a wider range of liquid distribution in the transverse direction than do the time-average images.

Figure 12 shows a comparison of measured pene-tration heights based on various threshold water EPL values for the injection conditions depicted in Fig. 11. The increase in aeration level from GLR = 2% to GLR = 4% produces similar penetration heights. With the in-jection condition with GLR = 4% in Fig. 12(b), the in-jected water is so dispersed that the maximum water EPL decays below 10 µm at the x/d0 = 8 location. Also shown in Fig. 12 are penetration heights predicted from the existing correlations for aerated-liquid jets by Lin et al.14 The comparisons between measurements and pre-dictions in Fig. 12 are similar to those in Fig. 8. The penetration heights based on the water EPL threshold value of 0.01 mm show reasonable agreement with the predictions from the shadowgraph-based correlation. The penetration heights calculated from the PDPA-based correlation are more representative of liquid presence as depicted in the standard deviation water EPL contours.

Contour plots for plume and droplet properties from PDPA measurements at the x/d0 = 50 location for the injection condition with GLR = 2% are shown in Fig. 13. Once again, the penetration height predicted from the PDPA-based correlation (HPDPA) agrees rea-sonably well with the upper boundary of the volume flux contours. The other penetration heights, HEPL=0.005mm, HEPL=0.01mm, and Hshadowgraph, coincide with the transverse (y) region with a high liquid volume flux in the x direction.

Figure 14 shows the liquid distribution profiles in terms of VL, Vd, and QL at two x/d0 locations. Even with the similar location for the peak values, the magnitude of the peak value between VL by X-ray radiography and Vd by PDPA exhibits a relatively large discrepancy at the x/d0 = 25 location. The VL profile shows that a sig-nificant amount of water is distributed close to the tun-nel floor. Note that X-ray radiography was not per-formed at the x/d0 = 50 location, due to high uncertain-ties in EPL measurements. The spray is highly dis-persed at this freestream location. The spanwise-integrated liquid volume VL is relatively uniform in the transverse region 10 < y/d0 < 40 with a significant amount of water staying close to the tunnel floor. The spanwise-integrated droplet volume Vd by PDPA, how-ever, still exhibits a typical bell-shaped distribution profile with a peak value at y/d0 ∼ 40 and a relatively small amount of water close to the floor.

The liquid distribution profiles in terms of VL, Vd, and QL for the injection condition with GLR = 4% are shown in Fig. 15 at two freestream locations. Compar-ing with Fig. 14, the VL profile shows that the injected liquid is further dispersed over a slightly larger cross-sectional area with a reduced peak. The maximum VL at

x/d0 = 39 is actually located on the tunnel floor, possi-bly indicating the presence of a thin liquid film. This phenomenon was confirmed by visual observation but is absent in the companion PDPA measurements. SUMMARY

For the first time, to our knowledge, spray struc-tures of liquid jets in subsonic crossflows were charac-terized using the X-ray radiography technique. 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. An axisymmetric aerated-liquid injector fitted with an exchangeable adaptor was used to gener-ate a pure- or aerated-liquid jet at the desired injection conditions. Line-of-sight radiography measurements, recorded at a 2.5 MHz sampling rate (for a 0.5-s period) were processed to give quantitative liquid mass distri-butions for the spray at various injection conditions. The present results were also used to derive spray pene-tration heights for comparison with predictions from previously formulated correlations. Companion PDPA measurements were performed to compare with the X-ray radiography measurements and to expand the un-derstanding of the plume structures. Major findings of the present study are as follows:

1. X-ray radiography can quantify liquid mass distri-

butions within both the near field, such as the jet column, and the far field of liquid jets in subsonic crossflows.

2. In the near field, deformation of the liquid column of pure-liquid jets and the co-annular-like column structure of the aerated-liquid jets can be quantita-tively depicted by the present X-ray technique.

3. The present efforts to compare the measured pene-

tration heights in the far field, based on various threshold values, with predictions from the existing penetration height correlations give a new perspec-tive to characterize spray penetrations in cross-flows.

4. In general, the penetration heights predicted from

shadowgraph-based correlations are in agreement with the time-average water mass contours. These penetration heights, however, whether measured or predicted, may neglect a significant amount of the injected liquid mass in the far field.

5. Direct comparison between liquid mass distribution

from X-ray radiography and PDPA measurements is afforded by spanwise integration of the PDPA-measured properties. In spite of the difference in the integrated quantities, peak heights for the inte-grated quantities match reasonably well.

6. A discrepancy was observed in X-ray mass distri-

bution measurement and the spanwise-integrated PDPA measurement near the tunnel floor, especial-ly for highly-dispersed sprays. Contributing factors should be explored in the future.

The combination of the X-ray fluorescence tech-

nique and focusing polycapillary optics may be capable of depicting the liquid mass distribution in the spanwise (y-z) cross section and should be carried out in the fu-ture. The obtained results should be compared with the cross-sectional contour plots obtained in the present PDPA measurements.

ACKNOWLEDGEMENTS This work was sponsored by the AFRL/Aerospace Sys-tems Directorate under contract number FA8560-14-D-2316 (Contract monitor: Steven Smith) and by the Air Force Office of Scientific Research (AFOSR). Use of the Advanced Photon Source at Argonne National La-boratory was supported by the U. S. Department of En-ergy, Office of Science, Office of Basic Energy Scienc-es, under Contract No. DE-AC02-06CH11357. REFERENCES 1. Mathur, T., Lin, K.-C., Kennedy, P., Gruber, M.,

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5. Wu, P. K., Kirkendall, K. A., Fuller, R. P., and Nejad, A. S., “Spray Structures of Liquid Jets At-omized in Subsonic Crossflows,” Journal of Pro-pulsion and Power, Vol. 13, No. 2, 1998, pp. 173-182.

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(a) (b) Figure 1. (a) Photograph of the portable wind tunnel with main components identified, (b) Photo graph of the test section with clear sidewalls for visual observation and optical measurement (injector at the top for illustration).

Figure 2. Photograph of the test section fitted with thin Polyimide films for X-ray access (injector at the bottom for actual X-ray radiography and PDPA measurements).

Figure 3. Photograph of axisymmetric aerated-liquid injector with exchangeable nozzle adaptors

Figure 4. X-ray and injection stand setup at the 7-BM beamline at the Argonne National Laboratory. For the pre-sent study, the injection stand is replaced by the portable wind tunnel.

(a) (b) Figure 5. Liquid mass distribution in terms of EPL within the near field of a pure-liquid jet. (a) Time-average EPL, (b) Standard deviation EPL. d0 = 1.0 mm, M = 0.3, mL = 10.7 g/s, q = 13.8, GLR = 0. Freestream air from left to right.

(a) (b)

(c) (d) Figure 6. Liquid mass distributions in terms of EPL within the near field of aerated-liquid jets. (a) GLR = 2%, time-average EPL, (b) GLR = 2%, standard deviation EPL, (c) GLR = 4%, time-average EPL, (d) GLR = 4%, standard deviation EPL. d0 = 0.5 mm, M = 0.3, mL = 2.3 g/s. Freestream air from left to right.

(a) (b) Figure 7. Liquid mass distribution in terms of water EPL for a pure-liquid jet. (a) Time-average EPL, (b) Standard deviation EPL. d0 = 0.5 mm, M = 0.3, mL = 4.0 g/s, q = 30.3, GLR = 0. Freestream air from left to right.

Figure 8. Outer boundaries of a pure-liquid jet based on various threshold EPL values. d0 = 0.5 mm, M = 0.3, mL = 4.0 g/s, GLR = 0. Freestream air from left to right. Also shown are penetration heights predicted by the correlations of Lin et al.14

Figure 9. Contour plots for plume and droplet properties from PDPA measurements. Also shown are penetration heights by the X-ray radiography measurements and by correlation predictions from Lin et al.14 d0 = 0.5 mm, M = 0.3, mL = 4.0 g/s, GLR = 0, x/d0 = 50.

(a) (b) (c) Figure 10. Distribution profiles for spanwise-integrated liquid volume (VL by X-ray radiography and Vd by PDPA) and liquid flow rate (QL by PDPA). (a) x/d0 = 25, (b) x/d0 = 50, (c) x/d0 = 100. d0 = 0.5 mm, M = 0.3, mL = 4.0 g/s, GLR = 0.

(a) (b)

(c) (d) Figure 11. Liquid mass distributions in terms of EPL within the far field of aerated-liquid jets. (a) GLR = 2%, time-average EPL, (b) GLR = 2%, standard deviation EPL, (c) GLR = 4%, time-average EPL, (d) GLR = 4%, standard deviation EPL. d0 = 0.5 mm, M = 0.3, mL = 2.3 g/s. Freestream air from left to right.

(a) (b) Figure 12. Outer boundaries of aerated-liquid jets based on various threshold EPL values. (a) GLR = 2%, (b) GLR = 4%. d0 = 0.5 mm, M = 0.3, mL = 2.3 g/s. Freestream air from left to right. Also shown are penetration heights pre-dicted by the correlations of Lin et al.14

Figure 13. Contour plots for plume and droplet properties from PDPA measurements. Also shown are penetration heights by the X-ray radiography measurements and by correlation predictions from Lin et al.14 d0 = 0.5 mm, M = 0.3, mL = 2.3 g/s, GLR = 2%, x/d0 = 50.

(a) (b) Figure 14. Distribution profiles for spanwise-integrated liquid volume (VL by X-ray radiography and Vd by PDPA) and liquid flow rate (QL by PDPA). (a) x/d0 = 25, (b) x/d0 = 50. d0 = 0.5 mm, M = 0.3, mL = 2.3 g/s, GLR = 2%.

(a) (b) Figure 15. Distribution profiles for spanwise-integrated liquid volume (VL by X-ray radiography and Vd by PDPA) and liquid flow rate (QL by PDPA). (a) x/d0 = 25, (b) x/d0 = 50. d0 = 0.5 mm, M = 0.3, mL = 2.3 g/s, GLR = 4%.