The Effect of Air-Assistance on Liquid Fuel Jet Disintegration in · PDF file · 2014-05-14The Effect of Air-Assistance on Liquid Fuel Jet Disintegration in a Cross-flow of Pre-Heated

ILASS Americas, 26th Annual Conference on Liquid Atomization and Spray Systems, Portland, OR, May 2014

The Effect of Air-Assistance on Liquid Fuel Jet Disintegration in a Cross-flow of Pre-Heated Air

Z. P. Tan*, E. Lubarsky*, O. Bibik, D. Shcherbik and B. T. Zinn

School of Aerospace Engineering Georgia Institute of Technology

Atlanta, Georgia 30332-0150 USA

Abstract This paper describes an experimental investigation of the effect of air-assistance on the outer/inner-edge penetrations and RMS fluctuations of the resulting spray when liquid Jet-A is injected into cross-flowing air at the temperature of 427oC and pressure of 350psia, where the injected fuel can develop into a multiphase liquid/vapor spray. The jet-in-cross-flow (JICF) spray was studied using high-speed visible-light (511nm) shadowgraph at 24,000Hz frame-rate. The liquid Jet-A injectant was injected from a plain-orifice nozzle with impinging air jets that were intended to assist the fuel jet’s disintegration in the cross-flow. Momentum-ratios (J) of the fuel jet to the cross-flow air were varied from 5 to 40. And, the amounts of air-assistance were controlled by changing the mass flow-rates of assisting-air, which were parameterized in terms of percentage static pressure-drop (dP) between the assisting-air supply-line and the cross-flow air. Air-assist dP of 0-2% were investigated, which corresponded to 0-17% of fuel mass flow-rate when J=5. It was observed that the assisting air-jets have the effect of increasing the effective J of the total fuel/assisting-air jet, thereby enhancing the spray’s outer-edge penetration. It was also observed that the air-assistance had pronounced effects on increasing the spray’s outer-edge unsteadiness, and on detaching the spray’s inner-edge from the test-channel’s wall. For future works, a novel UV/visible-light shadowgraph technique is being developed for multiphase spray characterization. The technique involves simultaneous 30Hz shadowgraph spray-imaging using 532nm and 266nm laser-lights. The 532nm light detects liquid Jet-A droplets through Mie-scattering, while the 266nm UV-light detects Jet-A in liquid and gaseous phase through Mie-scattering and Jet-A UV-absorption.

* Corresponding authors: [email protected], [email protected]


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Introduction In some modern jet engines’ combustors (e.g. [1],[2]), the liquid fuel is injected perpendicularly or at an angle into a high pressure (p) and temperature (T) cross-flow of air. The cross-flow then redirects the fuel jet into the flow direction and atomizes the fuel. In these engines, the cross-flow air can attain temperatures (T) up to 427oC and higher, possibly resulting in significant vaporization as atomization is taking place. In a simple jet-in-cross-flow (JICF) fuel injector/atomizer, the liquid fuel is typically injected from a plain orifice. However, a recent technique in enhancing the liquid fuel jet’s atomization process and shortening the required fuel-air mixing-length involved the use of air-jets to assist/blast the liquid fuel jet as it is being injected into the cross-flow [3].

The arrangement of the air-jets can take a variety of forms, from a perpendicular fuel-air impingement setup to a co-axial/concentric fuel-air jets setup. Literatures on air-assisted/blasted liquid JICF are few and originate mostly from the area of agricultural sprayers [3]. Hence, a good understanding is still lacking for this injection scheme when using injectants and operating conditions that are more relevant to gas-turbine engines. Thus, to support combustor development and CFD simulations of combustors employing air-assisted JICF designs, an understanding of the physics of liquid fuel (Jet-A) jet in a hot cross-flow and with air-assistance must be developed.

This paper describes the study performed on a JICF injector with a plain-orifice fuel nozzle

that was sunken into a shallow well. Liquid Jet-A at close to room temperature was injected through the plain-orifice. Meanwhile, air-jets at close to cross-flow T were injected from four slots surrounding the base of the well. The test operating conditions of the cross-flow air were: 350psia in pressure and 427oC in temperature. Liquid-to-cross-flow-air momentum-ratios were varied from J = 5 to 40. The degrees of air-assistance were controlled by changing the mass flow-rates of the assisting air, which were parameterized in terms of percentage static pressure-drop (dP) between the assisting-air line and the cross-flow air. dP between 0-2% were investigated, which corresponded to ~0-17% of fuel mass flow-rate when J=5.

The objective of the overall study was to investigate the effects of the air-assistance upon the liquid spray’s penetration, trajectory fluctuations, atomization/plume-spread, and rate of vaporization, at different cross-flow temperatures, pressures, and liquid-to-air momentum-ratios (J). The study was aimed at leading to the construction of spray trajectory correlation functions which take air-assistance into account. This paper details the initial investigation and observations of the air-assistance’s effects at a high-T/p operating condition, including identification of any deviation and pattern of changes of the air-assisted JICF when compared to non-assisted JICF. As part of the suggested future works, the paper also presents the preliminary application of a novel UV/visible-light shadowgraph technique capable of detecting both liquid and gaseous Jet-A in the spray pattern.

Nomenclature 𝜌 density 𝜎 surface tension CD coefficient of drag dinj injector diameter dP air-assist pressure-drop J liquid-jet-to-cross-flow momentum-ratio Jeff effective momentum-ratio JICF jet-in-cross-flow l length along liquid column p pressure po reference pressure t time T temperature

u velocity in x w velocity in z U total velocity Weaero aerodynamic Weber number x injected jet direction z cross-flow direction Subscripts ∞ freestream airblast air-blast injectant assist air-assistance injectant cross-flow or cf cross-flow liquid or liq liquid injectant

Background

Two particular physical aspects of JICF are of primary concern to the researchers: 1) the breakup process of the injected fuel jet, and 2) its penetration/trajectory. Though not mutually independent, the jet breakup process can be largely characterized by the JICF’s aero Weber number (Weaero), defined as 𝑊𝑒 = 𝜌 𝑈 𝑑 /𝜎 . Meanwhile, the spray penetration can be largely

related to the liquid-to-cross-flow momentum ratio

J, defined as 𝐽 = . The background on jet

breakup and penetration are treated in separate sub-sections in below. Jet Atomization and Breakup Modes In a JICF with liquid fuel injectant and cross-flowing air, the injected fuel exists as an


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intact liquid column/jet near the injection orifice. Depending on the Weaero and J, the column may experience different types of disintegration and atomization [4], [5], [6], [7]. At low Weaero and J, surface waves form on the liquid column. The waves are then enhanced by aerodynamic forces from the cross-flow, and liquid ligaments strip off from the column waves. The ligaments then experience further disintegration into droplets, followed by droplets secondary breakups into increasingly smaller droplets until a critical Weaero is reached [8], [9], [10], [11], [12], [13]. At Weaero above 200, the dominant mode of breakup shifts from the column-to-ligament breakup process described above to a shear break-up process [14]. In the shear breakup regime, droplets are stripped directly from the liquid jet column without significant formations of ligaments.

Aside from the different breakup regimes defined by Weaero, J also has an effect on the jet breakup. Kush and Schetz [15] conducted JICF studies at supersonic cross-flows, and found that at J=1.5-6, the disturbance waves influenced the entire liquid column shape and column fracture location, resulting in irregular ligament sizes; while, at J>6, the waves grow on the surface regularly, spreading the liquid jet gradually and detaching regularly sized clumps of ligaments and droplets. Schetz [16] separately treated the windward and leeward waves, and attributed the large amplitude waves responsible for breakup to the acceleration process of the jet as it curves downstream under aerodynamic forces. Ingebo [17] studied the surface waves formation due to capillary and acceleration/aerodynamic forces, and found ligaments to form at the crest of the waves.

Nejad and Shetz [18], [19] found that lower surface tension (effectively higher Weaero) results in lower mean droplet diameters, because less surface tension forces are available to retain droplet integrity against aerodynamic forces. Meanwhile, Wu [6], and Reichenbach and Horn [20] found that the smaller droplets penetrate less than larger droplets, because they have less cross-stream momentum per subjected drag force.

Stenzler [21] employed Mie-scattering imaging of sprays to study the effects of J, Weaero and liquid injectant viscosity on the spray, and found the 3 parameters that constitute Weaero: liquid injectant surface tension, cross-flow velocity and cross-flow density, to be capable of independently affecting droplet sizes and spray penetrations.

The current study was performed with Weaero in the range of 1500-2500. This means that if the investigated JICF spray was injected from a plain nozzle without air-assistance, it would be well within the shear regime. However, in the presence of air-assist jets which surrounded the liquid column, the air jets may alter the effective cross-flow momentum (i.e., Weaero) experienced by the liquid jet in a few ways: 1) the assisting air may

have the effect of enhancing disintegration (higher effective Weaero) due to added disturbance on the liquid column, as experienced by Leong et al. [3], or 2) the air-jets may reduce the direct impact of the cross-flow on the liquid column; i.e., the air-jets would have to be sheared off by the cross-flow before the cross-flow can shear-strip droplets from the liquid column. Jet Penetration and Trajectory Correlation Aside from different modes of breakup, researchers are generally interested in the cross-stream penetration of the spray. Overall penetration, often evaluated as the average outer-edge trajectory of the JICF spray pattern, is commonly correlated to some power of liquid-to-air momentum ratio J [22], [23], with higher J resulting in higher penetration everywhere along the spray. On the other hand, the penetration versus downstream distance from the injection point can be constructed upon either a power law function [24], a logarithmic function [25], or a 3 parameter exponential function that accounts for 3 regimes of the spray (liquid column, ligaments, and droplets) [22].

Wu et al. [23] performed a simple drag force and jet momentum analysis to find the appropriate variables and proportionalities for correlating spray trajectories. The analysis assumed that the injected liquid column can be modeled as a cylinder having a constant diameter identical to the nozzle diameter dinj, while having a length l, which stretches from the orifice to the point of column fracture. In the model, mass loss by evaporation and droplets removal from the column were neglected. Body forces were also neglected, as they are small compared to aerodynamic forces. The effect of deformation and flattening of the column in the cross-flow was accounted through an effective drag coefficient 𝐶 . Additionally, the cross-stream velocity of the jet was assumed constant for the length of the analyzed column. As such, with the origin located the injection orifice, and z, w being the distance and liquid jet velocity along the cross-flow, while x, u the penetration and jet velocity perpendicular to the cross-flow, the stream-wise momentum equation is:

= 0.5𝐶 𝜌 𝑤 −

𝑤 𝑤 − 𝑤 + 𝑢 − 𝑢/𝑙𝑑

(1) In the model, 𝐶 was constant along the

column; 𝑢 − 𝑢 was less than 25% of 𝑤 − 𝑤 , and to be neglected; 𝑤 was less

than 16% of 𝑤 . Local 𝜌 in a low Mach flow was constant and equal to 𝜌 . As such, (1) can be simplified and 𝑤 found through integration with respect to time:


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= (2)

𝑤 = 𝑡 (3]

Hence, using the expression (3) with the assumption of constant cross-stream jet velocity, the penetration distance can be expressed as:

= 𝐽 (4)

From this simple analysis, the trajectory followed a parabolic profile, and the penetration has a 𝐽 . dependency. Many existing empirical correlations of spray trajectories contain exactly or nearly a square-root dependence of penetration upon J, including the correlations from Yates[25]:

= 1.1 𝐽 ln 1 + 10 (5)

from Geery and Margetts [26] and Hojnacki [24]:

= 2.1 𝐽.

(6)

from Chen et al.’s 3-parameter function [22]:

= 9.91𝐽 . 1 − 𝑒 . × 1 +

1.67𝑒 . 1 + 1.06𝑒 . (7)

and Wotel et al.[27]:

= 1.19𝐽 ..

(8)

Notice that the profiles of the trajectory in the correlations can take the form of exponentials, natural logs, or powers of far from the 0.5

which was derived in Wu et al.’s analysis. The correlations above were provided for

injection without air-assistance. Leong et al. [3], who studied air-blasted JICF injections calculated that for their air-blast injector, the momentum flux of the blasting-air can be orders of magnitude higher than the liquid jet, due to the large air-blast-nozzle area and high air-blast velocities. Consequently, significantly higher spray penetrations were observed in the presence of air-blasting. Therefore, a new effective momentum-flux formulation for the total air/liquid-fuel jet was proposed:

𝐽 = /( ) (9)

where: 𝐴 = 𝐴 + 𝐴 . With this new momentum-ratio, the correlation function can take on the simple form:

= 𝑐 × 𝐽 × (10)

where c's are correlation constants, which in the absence of air-blast: 𝑐 = 0.5 and 𝑐 = 0.5 , in agreement with Wu et al.’s analysis.

Despite the common dependence of correlations exclusively upon J, a study by Ingebo [28] also suggested that Weaero should be included in spray trajectory correlations- though as Stenzler [21] proposed, the Weaero term should perhaps not be independent of J.

Additionally, higher cross-flow velocities [6], [29] and densities (at elevated pressure) [30], [3] can both also independently lead to lower droplets SMD and less penetration, through increasing We and/or decreasing J. Leong et al. [3] in their air-blast JICF investigation observed that at higher pressures the jet disintegrated into a wider spray pattern that could result in a spray inner-edge that impinge/attach along the lower wall that contained the injection orifice. When air-blast was applied, the resulting enhanced spray-penetration detached the inner-edge from the wall. In accordance with the observation, a correlation function that took pressure into account independently of J was proposed:

= 𝑐 × 𝐽 × × (11)

where 𝑐 was negative. The lower wall impingement phenomenon was also independently observed by the authors of this paper in a previous non-air-assisted JICF investigation [31].

The current study of air-assisted JICF was performed at conditions more typical of gas-turbine engines’ compressor output air: temperatures of 427oC and pressures of 350psia (24atm). The degrees of air-assistance was tested from dP=0% to 2% of cross-flow pressure, and momentum-ratio J ranged from 5 to 40.

Experimental Setup High-Temperature/Pressure JICF Facility

The current JICF study was carried out in a high-temperature/pressure facility capable of providing continuous flow cross-flow air at up to 350psia and 427oC. During experiments, the hot, high-pressure air flowed from a supply line into an 8in diameter plenum (see Fig. 1), then into a 2.45𝑖𝑛 × 1.70𝑖𝑛 test-channel via a bell-shaped in-take. The flow was then further accelerated through a converging section into the 1.25𝑖𝑛 × 1.0𝑖𝑛 test-section. The air-assisted injector was mounted on one side of the test-section, while the other three

walls were quartz windows for optical access. The windows were thin and designed to hold the hot air at a differential pressure of ~2psi. A thicker outer chamber with thick quartz windows (not illustrated) held high-pressure cooling-air that surrounded the test-section.

A fuel line delivered liquid Jet-A at near room temperature to the injection orifice, while another thermally insulated line delivered the assisting-air at close to 𝑇 . When Jet-A was not being injected, high-pressure nitrogen was used to purge and cool the fuel-injection line to prevent fuel-coking. The test-section with optical


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access from three sides allowed a variety of optical diagnostic techniques (including shadowgraph using visible and UV lights) to be applied to characterize the JICF spray.

Upon exiting the test-section, the fuel-air mixture was combusted in an afterburner before being released through the exhaust duct. Injector A schematic of the air-assist injector is shown in Fig. 2. The injector featured a central plain-orifice sunken into a spray-well for liquid Jet-A injection. The diameter of the orifice was ~0.6mm, while the spray-well was 2.5mm in diameter and 2mm deep. The assisting-air was first delivered into a cylindrical plenum within the injector hardware, before being injected through slots around the bottom of the spray-well. The angled injection slots were ~0.5mm wide, and encircled nearly the entire spray-well circumference, broken only by four struts that connected the central fuel-injection component to the rest of the injector.

The mass flow-rates of fuel and assisting-air were controlled by valves upstream on supply-lines. The fuel flow-rate was measured by a turbine flow-meter, while the assisting-air flow-rate was monitored through a differential pressure sensor that measured static pressure difference between the air-assist line and the test-channel interior.

Spray Optical-Diagnostic Systems Two different optical diagnostic techniques were used to investigate the air-assisted JICF: 1) 24,000Hz high-speed (HS) shadowgraph based on 511nm light from a copper-vapor pulsed laser with 30ns pulse-width, and 2) 30Hz UV/visible-shadowgraph based on 266nm and 532nm (UV and visible) lights from a Nd:YAG pulsed laser with 10ns pulse-width. The HS-shadowgraph setup is shown in Fig. 3. The copper-vapor pulsed laser generated 511nm laser pulses, which were guided by an optical fiber to the JICF test rig. The pulsed laser-lights were directed onto a light-diffusing plate mounted on the side of the test-channel window. The illuminated plate formed the back-light for the shadowgraph-imaging of the test-section and spray. A Memrecam GX-1 high-speed camera which was synchronized to the laser pulses aimed through the windows of the test-section at the illuminated plate. When liquid fuel droplets from the spray are present in the line-of-sight of the camera, they were detected as dark shadows against the back-light. More details about the setup can be found on the previous investigations by the authors [4]. The UV/visible-shadowgraph (setup shown in Fig. 4) is a novel technique currently under development by the authors, and is further described in the “Work in Progress and Future Works” section below.

Test Results Mean Spray Outer Trajectories

The JICF sprays were imaged using the high-speed shadowgraph technique at the cross-flow conditions of 350psia, 800oF (426.7oC). The shadowgraph images (~3600 frames per test-point) were corrected for background and back-light non-uniformity. The images were then binarized by applying intensity thresholds. The instantaneous spray outer-edge trajectories were obtained from each binarized frames, then averaged to form a mean spray outer trajectory. The threshold values were selected based on values that provided the least changes in trajectories per change in thresholds.

Fig. 5 shows the mean spray outer trajectories for the cross-flow velocity Ucf~60m/s and liquid-to-cross-flow momentum ratios J~5 and 20, while air-assist dP~0-2%. Similarly, Fig. 6 shows the J~5 and 40 trajectories for Ucf~75m/s. It was observed that at up to dP~2%, the air-assistance’s influence on the spray trajectory was more significant at lower momentum-ratios (J~5), while barely noticeable at J>20.

In both Figs. 5 and 6’s test conditions, Weaero>1500 (i.e., the sprays were well within the shear-breakup regime), and thus changes in the spray penetrations may be primarily attributed to

changes in momentum-ratios J. Hence, the observed enhancement in penetrations in the presence of air-assistance at lower J’s may be attributed to the increase in total liquid/assisting-air jet momentum due to added momentum from the assisting-air. On the other hand, it was observed that air-assist did not enhance spray penetrations noticeably at higher J’s. This was because when dP is fixed and J is increased, the momentum flow-rate of the assisting-air remains nearly constant, while the liquid jet momentum flow-rate increases. Therefore, at higher J’s, the assisting-air contributes to a proportionally lesser increase in the total jet momentum.

Comparing Figs. 5 and 6 also shows that at a fixed dP and J, the air-assistance had less penetration-enhancement at higher cross-flow velocities. This was because to reach higher cross-flow velocities at a fixed cross-flow pressure, a control valve downstream of the test-channel was opened to process higher mass flow-rates. In turn, the liquid fuel control valve also increased mass flow-rates of the injected liquid Jet-A to maintain the same liquid-to-air J. On the other hand, the air-assistance orifices had fixed dimension, and thereby nearly constant mass flow-rates at a given dP and test-channel pressure. Therefore, as the cross-flow velocity is increased, the liquid fuel


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momentum flow-rate increases, while air-assistance momentum flow-rate remains constant. Consequently, the assisting-air contributes to proportionally lesser increases in the total jet momentum at higher cross-flow velocities, resulting in lesser enhancements on spray penetrations.

To compare the measured spray trajectories against the common J~0.5 dependency discussed in the background section, the trajectories at both test conditions for J~5-40 and dP~0-2% were normalized by the square-root of the liquid-to-cross-flow J’s and the resulting plots are shown in Fig. 7. In the absence of air-assistance, the normalized trajectories were all reasonably well-collapsed into a single trajectory, showing that the spray penetrations followed the J~0.5-dependency. It is also shown that within the tested range, cross-flow velocities had no influence on the spray trajectories when J is constant.

When air-assistance was present, the spray normalized trajectories at high J’s were relatively unaffected. However, the normalized trajectories at lower J’s were significantly higher. When dP~2%, the normalized trajectories failed to be collapsed into one.

In observance of Leong et al’s [3] air-blast JICF study, where the normalizing parameter of the spray trajectory was switched from the liquid-to-cross-flow J to an effective liquid-and-air-blast-to-cross-flow momentum ratio Jeff, the Jeff values were also calculated for the currently investigated air-assisted JICF. Similar to Leong et al., it was assumed in the calculation that all the momenta emanating from the air-assist jet slots and fuel orifice were conserved and formed the total momentum of the injected liquid/air jet. Unlike Leong et al.’s air-blast nozzle where the air-blast’s momentum flow-rate significantly exceeded the liquid injectant’s momentum flow-rate, it was discovered that for the present injector design, the momentum flow-rate of the air was significantly lower than the momentum flow-rate of the liquid fuel jet even at dP~2% (the highest tested assisting-air flow-rate) and J~5 (lowest fuel flow-rate). Consequently, the Jeff’s were at most 6% higher than the original liquid-to-cross-flow J, and normalization through Jeff did not improve the collapsing of the trajectories in Fig. 7 into one.

However, as an alternative method of using Jeff as a parameterizing variable, the J-normalized spray trajectories in the presence of air-assist can be compared to trajectories without air-assist; and, the observed %-changes in trajectories can be plotted against the degree of air-assist (expressed in the %-changes in Jeff relative to non-air-assisted Jeff). Fig. 8 shows such a plot, where the regions of comparison were taken from z/dinj=10-15, 30-35 and 50-55. New trends emerged in Fig 8. It was observed that the %-changes in J-normalized spray penetrations increased almost linearly with the

increase of %-change in Jeff. However, the trend was not very clear/consistent until the %-changes in Jeff > 3.5.

Fig. 9 shows the %-changes in J-normalized penetrations plotted against Jeff, and split into groups of low/high-dP. It is apparent that the air-assist’s effect on spray penetration enhancements was significant at dP~2%. While at dP~1%, the enhancements were overall about half of dP~2% (reflecting the linear trend in Fig. 8), except when the spray’s Jeff~5. At Jeff~5, the spray penetration responded strongly to dP~2% of assisting-air, but weakly to dP~1%. Fig. 9 also shows that spray penetration enhancements at dP~2% were highest for J~5 (the lowest tested J), and dropped off rapidly as J increased.

These observations suggest that the air-assist had an effect of increasing the total effective jet momentum and enhancing the spray penetration. However, unlike Leong et al.’s conclusion, it was found that Jeff was a better indicator of air-assist’s effect when expressed as changes in penetration versus changes in Jeff. However, when Jeff of the spray is low, the amount of penetration enhancement may have non-linear sensitivity to the amount of air-assist. Spray Outer Trajectories RMS

To investigate the effects of air-assist on the spray’s dynamics, the root-mean-square (RMS) fluctuations in spray outer-edge trajectories were calculated and shown in Figs. 10. The RMS profiles of J=20 and 40 cases were nearly identical for different dP’s, and therefore have been omitted from the plots for clarity. It is observed in Figs. 10 that a spray of higher J had higher RMS in general. RMS also increased downstream as the spray pattern widened and where increasing vaporization may take place.

In Figs. 10, the Ucf~75m/s, J~10 cases had RMS values that are very similar for all dP’s. However, the J~5 RMS’s were noticeably increased by increasing dP. The coinciding of higher RMS with higher air-assistance dP can also be observed for the Ucross-flow~60m/s cases.

It is worth noting that although the dP~2% air-assisted J~5 trajectories penetrated at most as high as the non-assisted J~10 trajectories, their RMS’s were all comparable to or much higher than the J~10 sprays’ RMS’s. Thus, these observations suggest that the air-assistance may have more influence on spray fluctuations than penetrations, which constitutes a non-J-related effect that is not captured by the Jeff parameter. The very high J~5 RMS’s when air-assistances were present may also coincide with the spray penetration-enhancement’s non-linear/inconsistent dependence on the degree of air-assistance at J~5.


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Variation-Based Processing for Spray Inner-Edge Characterization

In addition to characterizing the air-assistance’s effects on the spray outer-edge trajectories, the interior of the spray pattern (especially the spray’s inner-edge trajectory) was also investigated. A variation-based processing method that emphasizes spray edges was used for this investigation. In Fig. 9, the original averaged shadowgraph image is shown on the left. The maximum intensity-variation at each point in the left-image was calculated by taking the difference of the maximum and minimum intensity values in a 3×3 pixels box centered at the evaluated point. The resulting plot of image intensity variations is shown on the right of Fig. 9. The regions of high variation value within the variation-image mark the edges of the spray pattern. Figs. 10-11 show variation-images for the sprays that have been binarized through the application of variation value thresholds.

Figs. 10 and 11 show that when dP=0%, the spray’s inner-edge (the jet column wake region) appeared to be attached to the test-channel’s wall.

In Fig. 10, when dP was raised to ~1%, the inner-edge became “lifted” from the wall and penetrated deeper into the x direction, even though the spray’s outer-edge trajectory did not exhibit significant changes. At dP~2%, the inner and outer-edges both had increased penetration. These phenomena were in agreement with Leong et al.’s [3] study of an air-blast JICF at elevated pressure. At J~10 (Fig. 11), the trends were similar, where the inner-edges were lifted higher from the wall for higher dP’s. Thus, the air-assistance had the effect of reducing the amount of spray materials that impinged upon the test-channel/injector wall.

Figs. 10-11 also show that when dP~2%, the blue-colored high-variation regions were noticeably smaller. Smaller high-variation regions can be related to a smoother image, which may be an indication of more rapid mixing within the spray pattern. Thus, it is likely that the air-assistance led to an enhanced spray mixing process. The indication of enhanced mixing may also be related to the increased spray outer-edge RMS’s described earlier.

Concluding Remarks

� 511nm visible-light HS-shadowgraph was used to capture images of an air-assisted liquid Jet-A jet-in-cross-flow under high-temperature/pressure conditions, for liquid-to-cross-flow momentum ratios of 5-40. The images were used to investigate the effects of air-assistance on the spray’s outer-edge trajectories, outer-edge trajectory RMS fluctuations, and the spray’s inner-edge/jet-wake regions.

� The spray outer-edge trajectories suggested that higher flow-rates of assisting-air lead to more enhanced spray penetrations. While, at a constant flow-rate of the assisting-air, the air-assist effects are generally more pronounced for lower liquid flow-rates. The change in Jeff due to air-assistance is a suitable indicator of the degree of penetration enhancement.

� The spray outer-edge’s RMS fluctuations data suggested that air-assistance increases the spray’s outer-edge unsteadiness, especially when the liquid jet’s flow-rate is lower. Additionally, the influence of air-assist on RMS is typically more pronounced than the influence of air-assist on spray trajectory, suggesting the presence of non-Jeff-related effects from the air-assistance.

� The spray image-variation plots suggested that at high-temperature/pressure, the spray’s inner-edge/jet-wake region is attached to the test-channel’s wall. Air-assistance has the ability to lift the inner-edge away from the wall, even when the outer-edge is not noticeably affected. High levels of air-assistance enhance both inner/outer-edge penetrations. The air-assistance may also enhance the rate of fuel-air mixing in the spray.

Work in Progress and Future Works Because the air-assisted JICF was investigated under high temperatures, significant vaporization of the initially liquid Jet-A fuel may occur within the test-section. Conventional visible-light shadowgraph does not provide gaseous-fuel detection capability. Therefore, a novel method of combined UV and visible-light (UV/vis) shadowgraph is being developed by the authors. As shown in Fig. 4, the setup was similar to a conventional shadowgraph setup, but relied on two

wavelengths (532nm visible light and 266nm UV) for back-lighting. Two separate cameras were also used, each detecting only one of the two wavelengths of light. An intensifier was installed in front of the 266nm-imaging camera to convert the UV signal into a wavelength suitable for the camera sensor. The UV/vis-shadowgraph technique shared similar principles with the more common 0.6328𝜇𝑚/3.39𝜇𝑚 visible-light-scattering/infrared-absorption (IRA) technique [32],[33],[34]. In IRA


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measurements of multiphase hydrocarbon sprays, a 0.6328 𝜇𝑚 back-light is scattered by the liquid droplets in its line of travel to the camera. However, the light is transmitted through regions of gaseous hydrocarbon. A second 3.39 𝜇𝑚 back-light, however, has a wavelength coinciding with the C-H stretching bond energy of hydrocarbons, and is thus absorbed by both gaseous and liquid hydrocarbon (in addition to being scattered by liquid). Thus, a shadowgraph image from the two wavelengths of light can be used to detect and distinguish regions of liquid and gaseous hydrocarbon in a spray.

The UV/vis-shadowgraph technique under development replaces the liquid-sensitive wavelength with 532nm visible light, while the liquid/gas-sensitive wavelength with 266nm UV-light. Past researches by Baranger et al. [35] and the authors [31] indicated that Jet-A/kerosene in both liquid/gaseous-phase absorbs 266nm UV-light strongly. Hence, a 266nm shadowgraph is capable of detecting both liquid and gaseous Jet-A. Replacing the 3.39 𝜇𝑚 IR light with 266nm UV has the potential benefit of reducing the Jet-A’s wavelength-dependent absorption cross-section’s sensitivity to temperatures, making the technique more suitable for charactering an entire JICF spray pattern that may contain tempertures from 25oC (injection T) to 427oC (cross-flow T). Fig. 12 shows a sample comparison of the averaged 532nm visible-light shadowgraph images

against the 266nm UV shadowgraph images at three conditions: 1) 350psia/27oC “Cold-flow”, 2) at 280psia/316oC “Low-TP”, and 3) at 350psia/426.7oC “High-TP”. At the Cold-flow conditions, vaporization of the liquid jet was minimized, and both visible and UV images should only show liquid droplets. However, the UV image has a different distribution of light-intensities from the visible-light image, suggesting that the two wavelengths were differently sensitive to liquid droplets. This was expected, as 532nm light was only scattered by the droplets, whereas 266nm was both scattered and absorbed.

At the Low/High-TP points, where significant spray vaporization was expected, the visible-light images show more tapered profiles compared to Cold-flow (i.e., more rapid rates of reduction in droplets number densities due to vaporization). The UV images, however, are similar throughout the three test-conditions. This confirmed that Jet-A vapors from the evaporated droplets at the Low/High-TP points were invisible to the 532nm light, but detected by the 266nm shadowgraph through the light-absorption process.

At the point of writing, efforts are still in progress for characterizing the absorption coefficient of Jet-A in different phases, temperatures and pressures, and developing the UV/visible-light shadowgraph technique for quantitative multi-phase spray characterization.

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8

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Figure 1. A schematic of the jet-in-cross-flow spray characterization test-hardware.

Figure 2. Schematic of the air-assisted JICF injector.


10

Figure 3. A schematic of the 511nm high-speed shadowgraph optical diagnostic system.

Figure 4. A schematic of the UV/Visible-light shadowgraph optical diagnostic system.


11

Figure 5. Shadowgraph-derived outer trajectories of sprays at 350psia, (800oF) 426.7oC, Ucross-flow~60m/s.

Figure 6. Shadowgraph-derived outer trajectories of sprays at 350psia, (800oF) 426.7oC, Ucross-flow~75m/s.

Mean Outer Trajectories: Ucross-flow~75m/s

Mean Outer Trajectories: Ucross-flow~60m/s


12

Figure 7. Trajectories normalized by the square-root of liquid-fuel-to-cross-flow J, for dP~0-2% and Ucross-

flow~60-75m/s.

0

1

2

3

4

5

6

7

8

0 20 40 60

x/d

inj

Liquid-J Normalized Trajectoriesdp~0%

J5, Ugas=75 J5, Ugas=60




0

1

2

3

4

5

6

7

8

0 20 40 60

x/d

inj

dp~1%

0

1

2

3

4

5

6

7

8

0 20 40 60

x/d

inj

z/dinj

dp~2%

J~5, Ucf~75 J~10, Ucf~75 J~20, Ucf~75

J~40, Ucf~75

J~5, Ucf~60 J~10, Ucf~60 J~20, Ucf~60

J~40, Ucf~60


13

Figure 8. Changes in average penetration at 3 different z/dinj regions as a function of changes in the Jeff relative

to a non-assisted spray.

Figure 9. Changes in average penetrations from 3 different z/dinj regions as a function of air-assist dP and Jeff.

-10

0

10

20

30

40

50

60

0 2 4 6 8 10

%C

han

ge in

Pen

etra

tio

n

%Change in Effective J

%Change in J-Normalized Penetration Relative to Non-Assisted Spray

z/d=10-15

z/d=30-35

z/d=50-55

-10

0

10

20

30

40

50

60

0 10 20 30 40 50

%C

han

ge in

Pen

etra

tio

n

Effective J

%Change in J-Normalized Penetration Relative to Non-Assisted Spray

dP~1% dP~2%


14

Figure 10.RMS fluctuations of spray outer trajectories for High T/p points, J~5 & 10, dP~0-2%, Ucross-flow~60 &

70m/s.

Figure 9. Comparison of the raw averaged shadowgraph image (left) and the averaged intensity-variation

distribution plot (right).

0

0.5

1

1.5

2

2.5

3

3.5

4

0 20 40 60

Pene

trat

ion

Stde

v (d

inj)

z/dinj

HS-shadowgraph RMS: Ucross-flow~75m/s

0

0.5

1

1.5

2

2.5

3

3.5

4

0 20 40 60z/dinj

HS-shadowgraph RMS: Ucross-flow~60m/s

J~5, dp~0% J~11, dp~0%

J~5, dp~1% J~11, dp~1%

J~5, dp~2% J~11, dp~2%

J~5, dP~1% Averaged Shadowgraph Image J~5, dP~1% Variation Plot


15

Figure 10.Threshold-applied variation distributions of the sprays for J~5, Ucross-flow~60m/s and dP=0-2%.

Figure 11. Threshold-applied variation distributions of the sprays for J~10, Ucross-flow~60m/s and dP=0-2%.

High-TP:

High-TP:

High-TP:

High-TP:

High-TP:

High-TP:

J~5, dP~0% Variation w/ Threshold







16

Figure 12. Comparison of 532nm visible-light shadowgraph against 266nm UV shadowgraph at high-pressure,

high vs. low temperature conditions.

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