American Institute of Aeronautics and Astronautics

092407

1

Liquid Film Formation by an Impinging Jet in a High-Velocity Air Stream

Timothy A. Shedd1

University of Wisconsin, Madison, WI, 53706

and

May L. Corn2, Jeffrey M. Cohen3, Marco Arienti4, Marios C. Soteriou5

United Technologies Research Center, East Hartford, CT, 06108

Abstract

The impingement of a liquid jet on a solid surface, and the development of a shear-driven liquid film is characterized in a planar experiment. Weber number and momentum-flux ratios were chosen to be representative of gas turbine fuel injector operations. High-speed digital imaging was used to visualize the formation of the liquid film from impinging droplets and the development of a continuous, wavy film. Film thickness measurements indicated growth of the film along the length of the impinging surface (streamwise direction), and reduction in the film thickness in the cross-stream direction. In general, film thicknesses increased with increasing momentum flux ratio, as more liquid drops reached the filmer surface. Three different mechanisms of atomization from the liquid film were identified.namely droplet splashing, film surface atomization via aerodynamic instability and film breakup at channel exit.

Nomenclature ρl = liquid density ρa = air density μl = liquid dynamic viscosity σ = liquid surface tension d = orifice diameter Vl = liquid velocity Va = air velocity q = momentum-flux ratio of the liquid jet to the normal air flow, ρlVl

2 /ρaVa2

Rel = liquid Reynolds number, ρlVld /μλ

We = aerodynamic Weber number, ρaVa2d /σ

I. Introduction The injection and atomization of liquid fuels is important to many transportation combustion devices, including

gas turbines and internal combustion engines. Fuel droplet size, fuel spray spatial distribution and fuel/air mixing are all critical performance metrics for the injection system that directly affect the combustion performance of the device.

One common method of fuel atomization involves creating a thin film of fuel along a solid surface, and then subjecting that film to shear from high-velocity air flows. This process occurs in various types of internal combustion engines in which fuel can impinge on the walls of the combustion chamber. Because of stringent 1 Associate Professor, Department of Mechanical Engineering. 2 Staff Research Engineer, Thermal and Fluid Sciences Dept., AIAA Member. 3 Fellow, Thermal and Fluid Sciences Dept., AIAA Associate Fellow. 4 Staff Research Engineer, Thermal and Fluid Sciences Dept., AIAA Member. 5 Fellow, Thermal and Fluid Sciences Dept., AIAA Senior Member.

47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition5 - 8 January 2009, Orlando, Florida

AIAA 2009-998

Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

emissions regulations, it is important to avoid the formation of unburned hydrocarbons particularly in cold start conditions. For this reason, various researchers have studied film and droplet formation from the impingement of a spray onto a film in the absence of a crossflow of air (Mathews, 2003) as well as in the presence of a crossflow of air (Panao, 2005, Kiura, 2008). In gas turbine engines, the film formation and break-up process can be found in “air-blast” atomizers. One implementation of this method of atomization in a fuel injector consists of an array of plain orifices from which liquid jets impinge on an opposing filmer wall while being subjected to a cross-flowing air stream (Cohen, 1993, Becker, 2000). The concept of the plain-jet filming process in the presence of a high velocity crossflow or air is depicted in Figure 1 to illustrate the various phenomena that are assumed to occur in the atomization process. The creation of a thin, uniform film is critical, as this determines both the spatial distribution of the fuel (Cohen, 1993) and the primary droplet size (Lefebvre, 1989). As a result, the processes governing fuel film formation and spreading are of considerable interest to fuel injector designers. Current computational and modeling techniques have limited capability to resolve these processes for engineering-scale devices. There is, then, a strong need for both diagnostic and model-validation experimental data in this area.

Liquid jet injection

Air

Liquid jet-in-crossflow primary atomization

Film formation at the wall

Film transport under shear Film break-up

Drop splashing Film surface atomization

Figure 1. Plain jet film formation and break-up in the presence of a high velocity cross-flowing air stream.

The work presented in this paper represents our initial efforts in this area. The current experiment is a planar

representation of more realistic fuel injector geometries. This configuration offers a more canonical environment in which to study this complex problem, retaining in a simplified geometry the main relevant physics, while at the same time has the advantage of allowing easier diagnostic access for optical and visual techniques. The focus of this initial effort was to identify the physical behaviors governing the liquid film formation and droplet/film interactions when a canonical jet in crossflow is bounded by a solid surface.

II. Background Several experiments of this type have been conducted by other researchers. Samenfink (1999) measured film

thicknesses and droplet size distributions, and used these measurements to develop correlations for the fraction of deposited droplets and other parameters of interest. Their experiments were particularly focused on the splashing behavior observed when a single droplet impinged on an already-existing liquid film. Mathews et al. (2003) investigated the impingement of a pulsed fuel jet on a flat plate in the absence of a gaseous crossflow and measured the temporal evolution of the resulting film. Panao and Moreira (2005) studied the effect of an impinging spray in the presence of a crossflow of air and found that the crossflow increased the formation of small droplets at the film surface because of the interaction between impinging droplets and the crowns on the film surface. Maroteaux et al. (2002) performed CFD simulations as well as experiments, investigating the process of atomization at the film separation point. Becker and Hassa (2000) conducted tests on the liquid jet filming in the presence of a crossflow under elevated pressure conditions and at both cold and hot air temperatures. Film formation and splashing occurred at both conditions. In addition, Becker and Hassa found that the spray formed at the hot test condition did not vary

American Institute of Aeronautics and Astronautics

092407

2

significantly in droplet size or cause a large increase in spray evaporation when compared with the the spray that was formed at the cold test condition. They attributed this observation to the interaction of the spray with the high temperature fuel film on the plate.

Limited data that characterize the detailed film and spray formation exist for the operating regimes of interest to gas turbine applications. The objective of this experiment was to acquire such measurements within an optically-accessible test section to investigate, under high magnification, the phenomena associated with film development and droplet formation at the film surface.

III. Experiment

A. Facility A sketch of the test section and liquid injection nozzle is shown in Figure 2. The test section and containment

vessel were fabricated out of 6.35 mm thick high clarity polycarbonate. Air was delivered from a large, centralized compressor and passed through both oil removal and particulate filters. A rotameter with needle valve was used to set the air flow rate; the rotameter readings were calibrated to the desired mean velocities in the test section. After the rotameter, the air passed through a 50.8 mm I.D. flexible hose, a series of screens, and finally a PVC-molded nozzle that transitioned from the hose to the rectangular test section.

Details of the actual test section are also shown in Figure 2. A primary flow channel, 10 mm tall by 44.5 mm wide was formed by polycarbonate plates, with a thin plate at the base of this channel separating a second small air channel from the primary one. The secondary channel was not used (i.e., no secondary air flow was present) in this initial study. The nozzle was fabricated from an aluminum rod to the dimensions and geometries shown in the figure (orifice diameter of 0.5mm, L/D of 2 with a total inlet orifice chamfer of 90 deg.) and installed flush with the upper wall of the primary flow channel.

Mineral spirits were chosen as a model fluid representative of the fuels that may be used in practical applications. The liquid was assumed to have a density, ρl = 780 kg/m3, dynamic viscosity, μl = 0.00085 kg/m-s and surface tension, σ = 0.024 N/m (Wypych, 2000). A variable-speed gear pump is used to draw mineral spirits from a reservoir and pump the liquid through a Coriolis flow meter and on through the liquid jet orifice. The mixture of air and mineral spirits exited the containment box into a large gravity/centrifugal separator that captures approximately 80% of the mineral spirits injected. The air/vapor mixture was exhausted via a laboratory exhaust system.

0.5 mm

1.0 mm

U1

U2

Gap Height: 1.0 mm Plate thickness:

1.0 mm

Gap Height: 10 mm

30mm

Channel spanwise width: 44.5 mm

Figure 2. Injector and test section geometries used in the experiment.

American Institute of Aeronautics and Astronautics

092407

3

B. Diagnostics The primary diagnostic tool used in this study was high speed digital imaging using a Phantom V7.0 camera and

a Mitotuyo long working distance 3x magnification telecentric microscope objective. A schematic of the imaging setup is shown in Figure 3a. The combination of camera resolution and microscope optics gave an image resolution of 12 microns per pixel. High speed imaging of microscopic behavior requires a great deal of light; this was provided primarily by an ARRI Junior 650 plus 600 W theatrical spotlight that combined a parabolic mirror and a Fresnel focusing lens to produce intense light over a limited area. Some of the imaging was obtained with an additional spotlight placed below the test section to provide light to highlight interfacial features at the surface of the liquid film.

Initial film thickness measurements have been performed with an optical method described in principle by Shedd and Newell (1998), with applications to impinging sprays described by Mathews (2003). The concept is shown in Figure 3b. Light, in this case from a small solid state laser, strikes a diffusing coating on the bottom of the filming plate shown in Figure 2. For this application, the diffusing coating is formed by a thin adhesive tape with a very uniformly distributed white pigment (Academix Brand). The diffuse light enters the transparent plate and a percentage is reflected back to the diffusing coating on the underside of the plate, creating a “ring” of light surrounding the central, bright spot. This is demonstrated on the right of Figure 3b. When a liquid layer exists on the plate, the light enters the liquid and reflects from the liquid/air interface, causing the diameter of the reflected light ring to increase. The difference in the diameters of these two light rings is directly proportional to the liquid film thickness. This measurement system has been independently validated for shear driven thin films and has been used in previous impinging spray studies (Rodriguez, 2004; Kiura., 2008; Mathews, 2003).

Phantom V 7.0High Speed Digital Camera

Mitutoyo 3xLong working distance Telecentric objectiveW.D. 72.5 mm, nom.DOF 0.06 mm, nom.Image resolution of 12 microns/pixel

ARRI Junior 650 Plus600 W spotlight

Ground Glass Plate

(a) (b)

Figure 3. (a) High speed imaging setup (b) Conceptual description of film thickness measurement

C. Operating Conditions Various researchers classify the break-up of the liquid jet-in-crossflow by regime maps. One of the more

commonly-used regime maps incorporates the jet-to-crossflow momentum-flux ratio (q) and the aerodynamic Weber number (We) (Wu, 1997). The momentum-flux ratio is a first order indicator of liquid jet penetration in the cross-flowing stream. The aerodynamic Weber number has a strong influence on the mechanism of breakup which shifts from a liquid column breakup dominated mode to a shear stripping mode as this parameter is increased.. The liquid Reynolds number should also be considered in break-up regime maps because the surface wave instabilities introduced by turbulence for Re > 5000 contribute to the primary break-up process. (Madabhushi, 2006). However, this study will focus first on the effect of the q and We on the film formation.

The full range of operating conditions that were studied are presented in Table 1 and are plotted with respect to the q-We atomization regime map of Wu (1997) in Figure 4. Most of the conditions fall in the shear break-up regime where droplet stripping from the liquid column occurs because of the shear generated by the crossflow, while a few conditions lie in the surface break-up regime where droplet stripping results from instabilities generated by the liquid momentum. These regimes were selected to simulate the break-up regimes encountered in the air-blast type injectors. The highlighted conditions in Table 1 will be discussed in more detail in this paper.

American Institute of Aeronautics and Astronautics

092407

4

0

1

10

100

1000

1 10 100 1000 10000

Weber Number (We_g)

Jet m

omen

tum

Flu

x R

atio

Surface break-up

Column break-up

Bag break-up Shear break-upMulti-mode

Figure 4. Operating conditions shown relative to the atomization regime map of Wu (1997), with highlighted conditions denoted by red circles.

Table 1. List of operating conditions with specific cases highlighted for further discussion.

Case Liquid Jet

Velocity (m/s) Crossflow Velocity

(m/s)

Liquid Rel Aerodynamic We

q Impingement Type

1 4.2 72 1935 155 1.9 Spray 2 8.5 72 3870 155 7.6 Spray 3 12.7 72 5800 155 17.1 Spray 4 17.0 72 7740 155 30.4 Jet 5 21.2 72 9670 155 47.4 Jet

6 4.2 81 1935 195 1.5 Spray 7 8.5 81 3870 195 6.0 Spray 8 12.7 81 5800 195 13.5 Spray 9 17.0 81 7740 195 24.0 Jet

10 21.2 81 9670 195 37.5 Jet

11 4.2 99 1935 290 1.0 Spray 12 8.5 99 3870 290 4.0 Spray 13 12.7 99 5800 290 9.0 Spray 14 17.0 99 7740 290 16.1 Spray 15 21.2 99 9670 290 25.1 Jet

16 4.2 108 1935 350 0.8 Spray 17 8.5 108 3870 350 3.4 Spray 18 12.7 108 5800 350 7.5 Spray 19 17.0 108 7740 350 13.4 Spray 20 21.2 108 9670 350 21.0 Jet

IV. Results and Discussion

A. General Liquid Jet and Droplet Behavior The behavior of the liquid jets as a function of the aerodynamic Weber number (We) and momentum-flux ratio

(q) is shown in Figure 5. The first, second, and third rows of images correspond to Weber numbers of 155, 195, and

American Institute of Aeronautics and Astronautics

092407

5

290, respectively. Within each row of constant We, the images are arranged in order of decreasing q. Each image of Figure 5 represents an average of 500 separate images, or a time averaged image of 0.02 seconds. As can be seen, the increase in liquid jet/droplet transverse penetration into the gaseous crossflow was primarily caused by an increase in q while the shear break-up of the jet is a function of increasing We. These trends are consistent with the findings of other researchers (Wu, 1997, Madabhushi, 2006). All of the sprays presented in Figure 5 exhibited surface and shear break-up at the column which confirms that the selected q and We operating conditions were consistent with the break-up regime map in Figure 4.

From the images one can also observe whether the liquid impinged directly on the filmer while still in the form of a nearly intact liquid column (i.e., before primary break-up of the column) or as a spray of droplets. The liquid jet impinged directly on the filmer for momentum-flux ratios larger than 20, as seen in the left column of images in Figure 5and as observed with the rest of the conditions noted in Table 1. For sprays with momentum-flux ratios greater than 20, there did not appear to be a dense distribution of droplets within the test section, which implies that most of the spray was filming the wall. As the momentum-flux ratio decreased, the spray penetration decreased and resulted in an increasing of the spray density in the test section. Sprays that appeared to exit without impinging on the wall, as seen in the rightmost column of images in Figure 5 for momentum-flux ratios less than 8, showed a higher spray density in the cross-flow.

Figure 6 presents images of the same sprays shown in Figure 5 but from a different viewpoint. In this set of images, the camera viewed the spray from the bottom of the test section, through the transparent filmer plate. The image plane was focused on the impingement side of the filmer plate. Note that the dark semi-circle at the left is the nozzle assembly, which blocked the initial jet structure from view. Each image is marked with dashed lines that denote the boundary of the film formed on the wall that was observed from the movies. The film on the wall can be differentiated from the spray in the freestream because the film is in focus and moves slower along the plate relative to the spray in the freestream. As can be seen in Figure 6, a liquid film was formed due to droplet impingement in every flow condition shown in Figure 5, even at the lowest momentum-flux ratio. At these low momentum-flux conditions, the film was likely formed by the largest droplets in the flow, which tended to penetrate farther into the gas flow..

We = 155q = 30.4

We = 195q = 24.0

We = 290q = 16.1

We = 155q = 17.1

We = 195q = 13.5

We = 290q = 9.0

We = 155q = 7.6

We = 195q = 6.0

We = 290q = 4.0

30 mm

10 m

m

Figure 5. Spray trajectory and penetration as a function of Weber number (We) and momentum-flux ratio (q).

American Institute of Aeronautics and Astronautics

092407

6

We = 155 q = 7.6

We = 195 q = 6.0

We = 290 q = 4.0

We = 155 q = 17.1

We = 195 q = 13.5

We = 290 q = 9.0

We = 155 q = 30.4

We = 195 q = 24.0

We = 290 q = 16.1

30 mm

20 mm

Figure 6. Film formation as a function of We and q.

B. Filming and Atomization The impact of the filming surface on the atomization process is clear from Figure 7. Even at the lowest

momentum-flux ratios, a significant stream of fine droplets was generated from the filming surface. The volumetric flow of these droplets emerging from the film increased with the amount of liquid directly impinging on the surface. As the momentum-flux ratio increased to values of 20 and higher, a second distribution of droplets appeared from the filming surface due to the liquid film flowing off the surface and breaking up. Thus, two modes of droplet generation from the liquid film existed: a fine mist formed by film surface atomization before the liquid reached the edge of the plate, and the break-up of the film sheet at the filmer lip. The actual generation of the fine mist from the surface of the film formed by an impinging jet in a crossflow has not been well-characterized in previous studies and will be the focus of this study.

J = 52Weg = 157Crossflow Velocity: 74 m/s

J = 42Weg = 193Crossflow Velocity: 82 m/s

J = 34Weg = 238Crossflow Velocity: 91 m/s

J = 28Weg = 287Crossflow Velocity: 100 m/s

J = 24Weg = 335Crossflow Velocity: 108 m/s

h1800-250-2500fps.avi

l2000-250-2500fps.avi

p2200-250-2500fps.avi

t2400-250-2500fps.avi

J = 19Weg = 157Crossflow Velocity: 74 m/s

J = 15Weg = 193Crossflow Velocity: 82 m/s

J = 12Weg = 238Crossflow Velocity: 91 m/s

J = 10Weg = 287Crossflow Velocity: 100 m/s

J = 8.8Weg = 335Crossflow Velocity: 108 m/s

f1800-150-2500fps.avi

j2000-150-2500fps.avi

n2200-150-2500fps.avi

r2400-150-2500fps.avi

Figure 7. Qualitative visualization of variations in atomization due to changes in gas and liquid jet flow.

C. Experimental Film Thickness Experimental film thickness measurements were obtained for the 16 conditions highlighted in Table 1 and are

presented in Figure 8. These measurements were made on the filmer plate along the jet centerline 23 mm

American Institute of Aeronautics and Astronautics

092407

7

downstream of the jet nozzle. It is estimated that there is a bias uncertainty of ±5 microns in these results due to uncertainty in the zero calibration. An additional ±7 microns of precision uncertainty exists on each data point. Thus, two data points can be compared with one another assuming an uncertainty of ±7 microns in each measurement, but the accuracy of any given data point is limited to about ±9 microns (the bias and precision uncertainties combined in quadrature).

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120Air Velocity [m/s]

Film

Thi

ckne

ss [m

icro

ns]

0.00065 kg/s0.0013 kg/s0.002 kg/s0.0026 kg/s

0

20

40

60

80

100

120

140

0 0.0005 0.001 0.0015 0.002 0.0025 0.003Liquid Flow [kg/s]

Film

Thi

ckne

ss [m

icro

ns]

72 m/s81 m/s99 m/s108 m/s

Figure 8. Film thickness plotted as a function of air velocity (left) and total liquid jet flow (right).

Figure 8 demonstrates that, in general, the film thickness increased with jet liquid flow and decreased with increasing cross flow. It was expected that the liquid film thickness would be inversely related to the gas shear, but the relationship to the jet liquid flow is less clear, as liquid film thickness will depend on the volumetric flux of liquid impinging on the surface rather than the total jet flow. Figure 9 shows the strong influence of momentum-flux ratio on the degree of droplet impingement on the filmer plate. This is expected, given that the jet and spray penetration monotonically increased with the momentum-flux ratio, as seen in Figure 5 and Figure 7. Figure 9 shows that film thickness correlates to momentum-flux ratio more strongly than to either the gas or liquid flow rates directly.

In addition, liquid film thickness measurements were obtained for a constant flow condition at various locations beneath the atomizing jet. These results are summarized in Figure 10 for a q of 13.5 and a We of 195. The liquid film thickness can be seen to increase non-linearly with distance from the nozzle. In addition, the film thickness decayed fairly rapidly with distance from the centerline. These results agree qualitatively with the flow visualization experiments to be discussed later, suggesting that the liquid film thickness reaches a mean value at steady state flow conditions that is directly proportional to the local droplet flux impinging on the filmer plate.

American Institute of Aeronautics and Astronautics

092407

8

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35

Momentum Flux Ratio [-]

Film

Thi

ckne

ss [m

icro

ns]

Figure 9. Liquid film thickness as a function of momentum-flux ratio.

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25Distance from Jet Nozzle [mm]

Film

Thi

ckne

ss [m

icro

ns]

Film thickness along the centerline

Film thickness 4 mm away from the centerline

Air velocity = 81 m/sLiquid mass flow rate = 0.002 kg/sq = 13.5, We = 195

Figure 10. Liquid film thickness as a function of distance from the liquid jet nozzle.

American Institute of Aeronautics and Astronautics

092407

9

D. Liquid Film Formation At all but the highest momentum-flux ratio conditions where direct jet splashing occurred, the liquid film formed

from droplets impacting the solid surface and flattening from a combination of their initial momentum and the shear force imposed by the gaseous crossflow. High-speed imaging indicated that droplets tended to distribute liquid streamwise from the point of impact. A continuous film formed only after multiple drop impacts merged. Imaging of the liquid film from an oblique angle above the surface of the filmer plate indicated that during initial film formation, virtually all of the liquid in each impinging droplet was incorporated into the liquid film (see Figure 11a). The liquid film does not appear to spread significantly in the direction transverse to the streamwise flow.

The sequence of images in Figure 11a shows a droplet near the beginning of the film formation opposite the atomizing jet at the filmer plate. The images were acquired from beneath the film with lighting directed to refract through the curved surfaces on the liquid film to emphasize its topology. The images were captured at 66,000 frames per second; each successive frame represents an elapsed time step of 15 microseconds. In this sequence, the droplet impacted the surface and spread to become a stable, relatively slow-moving ripple on the surface of the filming plate. Figure 11b complements this series of images with a view from above the film. Although these images were not obtained simultaneously with those in Figure 11a, the images provide another view of the droplet impacting the filming surface with the film absorbing the entire droplet.

Contrasting this is the series of images in Figure 11c from a position approximately 10 mm downstream of those in Figure 11b where the mean thickness of the base film is significantly thicker. Note that a partial crown now forms on the downstream side of the impact. This crown rapidly disintegrates into droplets that cannot be resolved by the optical system (with 12 microns/pixel of resolution). Figure 11d presents a set of images obtained in the same manner as in Figure 11a, showing similar behavior from beneath the film. A drop impact can be clearly seen with a crown forming downstream of the impact. This crown appears to disintegrate at its center and leave very little liquid behind. In fact, unlike the cases in Figure 11a and Figure 11b, there is very little evidence of the droplet impact after the event in Figure 11c and Figure 11d.

1.5 mm

1.5 mm

(a) (b) (c) (d)

Figure 11. Imaging of the formation of the liquid film and droplet generation at the film interface due to atomization of a liquid jet in crossflow: (a) and (b) illustrate droplet impact and absorption during the initial film formation via (a) a view looking upward from below the test section and (b) an oblique view above the filming surface; (c) and (d) show the oblique and bottom-upward views of drop generation 10 mm downstream of the location shown in (a) and (b).

American Institute of Aeronautics and Astronautics

092407

10

Imaging of both sides of the liquid film suggests that once the liquid had become sufficiently thick, significant

atomization occurred directly from the liquid film. Two different mechanisms were apparent. As noted above, impinging droplets created a semi-circular crown that appeared to disintegrate into fine droplets rather than re-incorporate into the liquid film. The second mechanism appeared to be caused by instabilities forming directly on small ripples. Figure 12 shows a time sequence in which a small ripple in the center of the ellipse experiences an instability. In image b, a small dark region obscures the ripple; this dark region grows through image c and d in the sequence, and dissipates in image e. At the end of this 75 microsecond sequence, image f shows that the ripple has disappeared. A hypothesis that explains this behavior is that the ripple experiences a Kelvin-Helmholtz instability which grows until the liquid is broken up into fine droplets. The Kelvin-Helmholtz instability theory gives a critical wavelength of about 21 microns for the conditions shown in Figure 12 (i.e., relative velocity of 80 m/s and fluid properties for air and mineral spirits) (Ghiaasiaan, 2008).

1.5 mm

a b c

d e f

Figure 12. An image sequence of liquid atomization directly caused by instability of the liquid film.

V. Summary and Conclusions A study of the liquid jet in crossflow in the presence of a bounding wall has been undertaken. Various

momentum-flux ratio and aerodynamic Weber numbers were tested to span a range of sprays that included high and low penetrating sprays with high to low surface stripping rates. From a macro scale, two types of liquid impingement were observed. Direct jet impingement occurred at momentum-flux ratios above a certain value (e.g., q=20 for this experiment) while impingement by the spray on the filmer surface occurred at momentum-flux ratios below this key value.

The film formation was characterized on a micro scale for the spray impingement cases. Two types of droplet/film impact were observed. At the initial stages of film formation, the droplets impacted and were entirely absorbed into the film. As the film developed and grew in thickness along the axial length of the filmer, the collision of the droplets with the film surface resulted in splashing, and the formation of a crown and satellite droplets. In addition to droplets formed by shear of the liquid jet, drops were formed from this splashing effect and from critical-scale surface ripples in the film. Droplets were also formed (intentionally) at the edge of the filmer plate due to aerodynamic shear of the free film. Additional film thickness measurements are planned to identify the film thickness at which the transition from droplet absorption to film surface atomization occurs.

The film development was assessed with film thickness measurements acquired at various operating conditions at one location and at multiple locations for one operating condition. The results showed that the film thickness increased with an increase in momentum-flux ratio, which probably caused an increase in spray penetration and droplet impact and absorption at the film. The liquid continued to add to the film thickness along the length of the filmer plate. The film thickness was also observed to decrease at locations that were away from the axial centerline of the film, indicating little transverse spreading of the impinging jet.

Future studies will utilize Phase Doppler Interferometry to measure the droplet sizes and velocities formed at the film surface. Another point of interest in future studies is the assessment of the percentage of spray originating from the film versus the spray formed by liquid jet atomization by the crossflow.

American Institute of Aeronautics and Astronautics

092407

11

American Institute of Aeronautics and Astronautics

092407

12

Acknowledgments The authors thank David Fautsch for his contribution in fabricating the test section and acquiring data for this

study and acknowledge the support of the United Technologies Research Center.

References Becker, J. and Hassa, C., “Plain Jet Kerosene Injection into High Temperature, High Pressure Crossflow with and without

Filmer Plate,” Eighth International Conference on Liquid Atomization and Spray Systems, Pasadena, CA, USA, July 2000. Cohen, J.M. and Rosfjord, T.J., “Influences on the Sprays Formed by High-Shear Fuel Nozzle/Swirler Assemblies,” Journal

of Propulsion and Power, Vol. 9, No. 1, 1993, pp. 16-27. Ghiaasian, S.M., Two-Phase Flow, Boiling, and Condensation, Cambridge University Press, 2008. Kiura, T., Shedd, T.A., and Blaser, B.C., “Investigation of Spray Evaporation and Numerical Model Applied for Fuel-

Injection Small Engines,” 2008-32-0064 (SAE) / 20084764 (JSAE). Lefebvre, A.H, Atomization and Sprays, Hemisphere Publishing Corp., 1989. Madabhushi, R.,K., Leong, M.Y., Arienti, M., Brown, C.T., and McDonell, V.G., “On the Breakup Regime Map of Liquid

Jet in Crossflow,” 19th ILASS-Americas, Toronto, Canada, May 2006. Maroteaux, F., Liory, D., Le Coz, J.-F., Habchi, C., “Liquid Film Atomization on Wall Edges—Separation Criterion and

Droplets Formation Model,” Journal of Fluids Engineering, Vol. 124, 2002, pp. 565-575. Mathews, W.S., Lee, C.F. and Peters, J.E., “Experimental Investigations of Spray/Wall Impingement,” Atomization And

Sprays, Vol. 13, 2003, pp. 223-242. Panao, M.R.O. and Moreira, A.L.N., “Experimental Characterization of an Intermittent Gasoline Spray Impinging Under

Cross-Flow Conditions,” Atomization and Sprays, Vol. 15, 2005, pp. 201-222. Rodríguez, D.J., “Characterization of Bubble Entrainment, Interfacial Roughness and the Sliding Bubble Mechanism in

Horizontal Annular Flow,” Ph.D. Thesis, University of Wisconsin-Madison, 2004. Samenfink, W., Elsaber, A., Dullenkopf, K., and Wittig, S., “Droplet Interaction with Shear-Driven Liquid Films: Analysis

of Deposition and Secondary Droplet Characteristics,” International Journal of Heat and Fluid Flow, Vol. 20, 1999, pp. 462-469. Shedd, T.A. and Newell, T.A., “Automated Optical Liquid Film Thickness Measurement Method,” Review of Scientific

Instruments, Vol. 69, No. 12, 1998, pp. 4205 – 4213.

Wu, P.K., Kirkendall, K.A., Fuller, R.F., and Nejad, A.S., “Breakup Processes of Liquid Jets in Subsonic Crossflows,” Journal of Propulsion and Power, Vol. 13, 1997, pp. 64-73.

Wypych, G. Knovel Solvents – A Properties Database. ChemTec Publishing, 2000. http://www.knovel.com/knovel2/Toc.jsp?BookID=635&VerticalID=0.