ILASS-Americas 29th Annual Conference on Liquid Atomization and Spray Systems, Atlanta, GA, May 2017

Ultra-short Pulse Off-axis Digital Holography for Imaging the Core Structure of Tran-sient Sprays.

M. Minniti*1, A. Ziaee1, J. Trolinger2, D. Dunn-Rankin1

1Department of Mechanical and Aerospace Engineering University of California, Irvine

Irvine, CA 92697 USA 2Metrolaser Inc,

Laguna Hills, CA 92653 USA

Abstract A single-shot Ultra-short Pulse Off-axis Digital Holography (UPODH) system is used to successfully image

microscopic details of fuel injection phenomena that are hidden from view by a dense cloud of droplets surrounding the near nozzle region. The experiment approximates the optically dense conditions typical of fuel injection in modern diesel engines. Under these conditions an outer layer of small droplets can hide a core of larger droplets or liquid ligaments; this core is inaccessible to most imaging techniques due to multiple-scattering in the outer layer. These conditions are mimicked by intentionally surrounding a core spray with a fine mist. The mist has a SMD of 4.28 microns. The core spray comes from driving water, with pressure ranging from 10 to 200 PSI, through single orifices of 0.1 and 0.3 mm diameter. The system shows nearly opaque transmissivities as low as 6x10-6. Transient phenomena, such as sheets of liquid becoming ligaments and their further break up into small particles are easily visible even when surrounded by the opaque mist with an optical density (OD) of 12. Holographic reconstruction allows these phenomena to be clearly observable in 3D, and a planar resolution of 30 microns is achieved. The 3D capability allows UPODH to bring into focus small particles and ligaments at different depth planes, even several millimeters apart.

*Corresponding author: mminniti@uci.edu

ILASS-Americas 29th Annual Conference on Liquid Atomization and Spray Systems, Atlanta, GA, May 2017

Introduction This research addresses an existing challenge in

current spray diagnostic techniques that have shown limited success so far in imaging the core of dense sprays; this is especially true for high-pressure combus-tion. Photography, holography and x-ray have all failed in providing ideal images [1] due to noise from multiple scattering from the particles surrounding the spray core. Institutions that conduct fundamental combustion re-search interested in understanding fuel sprays and drop-let breakup, fuel injector designers, and those who evaluate existing combustors can all benefit from a tool capable of imaging the core of dense sprays.

This research is a stepping stone towards the goal of successfully imaging the core of a high-pressure fuel spray: a spray setup that recreates the optically dense conditions of a diesel engine has been developed and high quality holographic images of fuel injection phe-nomena that are normally hidden were recorded over a wide range of optically dense conditions.

Technique description

We combined femtosecond off-axis digital holog-raphy with pseudo ballistic imaging to produce a sys-tem that measures the diameter of aerosol particles, distinguishes shapes, and locates particles in 3D. The method also provides a multiple framing capability for recording the dynamics of high frequency phenomena, eliminates most optical noise arising from multiple scattering, and is potentially deployable in combustion environments.

Holography uses coherence filtering to filter multi-ple scattering noise. The technique is based on record-ing the interference pattern (the hologram) generated when the wave diffracted by the object interferes with a reference wave of known properties that has travelled the same pathlength but without hitting the object [2]. If a reference wave identical to the one used to record the hologram is used to illuminate the recorded interference pattern the initial object wave is reconstructed, thus creating a 3-D image of the original object; this step is performed numerically in digital holography [2], [3].

In our system, an ultra-short laser pulse is used to remove multiple scattering noise. When using a light source with short coherence length only the portion of the object beam that is coherent with the reference is recorded in the hologram. Therefore, the portion of the pulse that underwent multiple scattering and traveled a longer path will not be coherent with the reference beam pulse by the time it reaches the sensor and it will be filtered out [4], [5]. This is defined as coherence filtering and we have found it to be more effective in filtering multiple scattering noise than other gating techniques, including Kerr-effect gating [4]–[6].

A beneficial peculiarity of holography is that any object placed in a wide range of positions between the

light source and the camera sensor can be recorded on a single hologram. Lateral resolution degrades as we re-construct images further away from the camera sensor. This is because in off-axis digital holography the object wave is modulated by being transmitted through the object; as opposed to reflectance based holography techniques where the depth of field of the recorded hol-ogram is limited to the coherence length of the light source. The current depth of field of the system is sev-eral centimeters with a sub millimeter scanning resolu-tion.

The laser source is a Coherent “Vitesse” with a “Legend Elite” mode-locked Ti:Sapphire oscillator. It generates 100 fs FWHM, 1 mJ pulses with a pulse repe-tition rate of 1 KHz and a central wavelength of 800 nm. The beam is converted to 400 nm using a doubling crystal; the 400 nm wavelength is preferred because it is easier to align and water absorption at this wave-length is negligible. The current 1kHz pulse repetition rate is not sufficient to image spray dynamics over mul-tiple frames so a single pulse is recorded into a holo-gram every 10 ms.

Off-axis holography is preferred to inline holog-raphy because it helps separate the hologram’s cross correlation term from the DC term and conjugate cross correlation term in the Fourier domain. This allows us to easily remove the multiple scattering noise that the latter two terms would add if they were included in the reconstruction [4]. Details on the reconstruction algo-rithm are not the focus of this paper and can be found in previous publications from the authors [4], [7].

Figure 1 Schematic of the off-axis digital holog-

raphy experimental setup at U.C. Irvine.

Experimental setup description

Figure 2 Schematic of the spray experimental set-

up.

The experimental setup shown in Figure 2 aims at recreating the optically dense conditions of diesel sprays where an outer spray of fine mist can potentially hide a core of larger droplets or liquid ligaments[6]. This is achieved by surrounding the target spray with a mist produced using an ultrasonic atomizer. Ultrasonic atomizers are used when a large quantity of small drop-lets is needed. In this device a piezoelectric transducer resonating at ultrasonic frequencies creates capillary waves on the liquid surface, and when these waves be-come too tall to support themselves small droplets fall off the tip of each wave resulting in atomization of the fluid. The mist has been sized using a Malvern Spray-tec droplet size measurement system. The Sauter Mean Diameter (SMD or D32) of the mist produced is 4.28 microns, the Dv50 is 7.8 µm and the Dv90 is 23.3 µm, meaning that 50% of the total spray volume is made by droplets with a diameter smaller or equal than 7.8 µm and 90% of the spray volume is made by droplets with a diameter which is less or equal than 23.3 µm.

These sizes compare well with the droplet sizing experimental data available in literature for the outer layer of diesel sprays [8]; therefore we believe that this setup is a good first approximation of the optical condi-tions that can be found in high-pressure diesel sprays

The current system uses a fast solenoid valve to control the core spray along with an exchangeable noz-zle (orifice diameter of 0.1 and 0.3 mm).

Water is pressurized to the desired value by a high-pressure Nitrogen tank and a pressure regulator. It is

then injected into an acrylic enclosure with 1 mm thick fused quartz windows. Two enclosure were built to create targets with a variable optical density, one has a window to window distance of 5 cm and the other of 10 cm.

The enclosure is filled with mist before the spray is activated, the press of a pushbutton initiates the spray-ing sequence: the Arduino microcontroller opens the spray’s solenoid valve and simultaneously sends a sig-nal via USB to the PC controlling the camera. Acquisi-tion of a sequence of images is started, each picture capturing a single 100 fs laser pulse, one image is ac-quired every 10 ms. After a user-defined amount of time the microcontroller stops the spray and camera acquisition.

Figure 3 Chamber designed to surround a fuel in-

jector with a dense droplet cloud. The pictures show that scattering is so severe that is impossible to follow the laser as it travels through the enclosure. Distance

between the fused quartz windows is 10 cm. A second chamber with a 5 cm distance between windows has

been built to experiment with lower OD values.

Results This section will showcase some of the reconstruc-

tion features of UPODH. The liquid used with all the different injectors presented in this study is water. All sprays are injected in a vessel at ambient pressure P=1 atm. Future research will include the use of fuels inject-ed in a pressurized environment.

Metric (µm) Value D32 4.28 DV(50) DV(90) D43 DV(10)

7.8 23.3 11.33 1.74

Table 1. Water mist properties measured by the Malvern Spraytec laser diffraction system.

Effects of Nozzle Pressure Figure 4 shows reconstructed images in the near

nozzle region of a 0.1 mm single orifice injector. At 150 PSI the injected fluid looks like a cylindrical col-umn; downstream from the nozzle ligaments start to form and eventually break down into droplets. As the water injection pressure is raised to 170 and 200 PSI the breakup mechanism changes, as the fluid appears to form sheets that downstream separate into ligaments and droplets. Holographic recording allows to focus individually on droplets or ligaments that are in differ-ent depth planes.

Figure 4. Reconstructed images of a water spray

from a 0.1 mm single orifice injector with increasing injection pressure. Reconstruction depth is 142 mm

from the sensor. Left image injection pressure 150 PSI, middle image 170 PSI, right image 200 PSI.

3D focusing

Figure 5 showcases digital holography’s 3D focus-ing capability. The right image is reconstructed 2 mm away from the left one. Looking at the left image, the ligaments and droplets in the large circle are in focus while the ones in the smaller one are out of focus. The opposite is true when looking at the right image, this tells us that these features reside in two depth planes 2 mm away from each other.

The short coherence length of the laser pulse does not limit the depth of reconstruction achieved by the system. Reconstructing images from the same femto-second hologram we routinely focused on droplets in the spray and on drops that were on windows centime-ters away from the spray.

Femtosecond holography records forward scattered light; these photons match the pathlength of the refer-ence wave over a large distance; unlimited by coher-ence length.

Figure 6 shows a detail from the spray produced by a Ford fuel injector. Water is injected at 150 PSI and the image is reconstructed 120 mm away from the cam-era sensor. Distinct droplets can be seen within the re-gion of interested (ROI) surrounded by the red rectan-gle. This region of interest is reconstructed in two dif-ferent depth planes in Figure 7. As we move 8 mm

closer to the sensor (left picture) the same particles that were visible in the 120 mm depth plane are still visible, but more appear on the right. When looking at the re-construction on the right, which is 32 mm away from the one on the left, all those particles disappear and a different droplet appears in focus at the center of the ROI.

Figure 5. Microscopic focusing in 3D. 0.1 mm

single orifice nozzle at 30 PSI.

Figure 6. Spray from a Ford automotive fuel injec-

tor. Injection pressure 150 PSI. OD 6.

Figure 7. Left picture shows the region of interest high-lighted in Figure 6 but reconstructed at a depth of 112

mm from the sensor. Right picture shows the same ROI reconstructed 144 mm away from the sensor.

Figure 8 shows how two fluid ligaments that re-side in depth planes 4 mm away from each other can be focused by changing the reconstruction depth. The red arrow indicates the ligament in focus.

Figure 8. 0.1 mm single orifice nozzle. Injection

pressure 30 PSI. Depth of reconstruction 140 mm.

Droplet microscopy in turbid media This section will demonstrate the system perfor-

mance when imaging a spray in extremely optically dense environments.

The scattering media surrounding the spray is a water droplet mist with SMD of 4.28 microns, this droplet size was chosen since the literature shows that it is a good approximation of the droplet size surrounding the core of a diesel spray [8]. Two enclosures were built, with a window-to-windows distance of 5 and 10 cm; so that by changing enclosure different optical den-sities can be obtained.

Optical density (OD) is defined as:

!" = − ln '(')

(1)

Where Io is the irradiance of the light before it en-ters the spray medium and Io is the irradiance after the

light has travelled through the higly scattering target enclosure.

In the larger enclosure transmissivities as low as 6x10-6 were reached, which corresponds to OD 13. These are considered to be extreme scattering condi-tions where techniques such as Ballistic Imaging (which does not allow for 3D reconstruction) achieved spatial resolution of 30-40 µm[1], [9].

Figure 9. Reconstructed mages of a spray from a

1996 automotive Ford fuel injector. Below each image the reconstruction distance from the camera sensor is

shown.

Figure 9 shows images reconstructed at different

depths of the spray produced by a Ford automotive fuel injector. The colored circles show once again how it is possible to resolve and focus different features residing at different depths within the spray.

Figure 10. Spray from a 1996 Ford automotive

fuel injector. The images show how the technique al-lows for scanning in depth as well as along the spray

direction.

Figure 10 shows images from the Ford fuel injec-tor operated at 100 PSI and immersed in a dense mist. The measured transmissivity of the whole target field is 1x10-3, which corresponds to almost OD 7. As we re-construct images at different depths, ligaments and droplets come into focus, and the breakup mechanisms are observable in 3D. The bottom row of images is tak-en 2.5 mm downstream from the location of the first row. The area imaged is approximately 9 mm2 and it is cropped from the whole hologram that in this configu-ration covers an area of 64 mm2.

Figure 11. Reconstructed images of the 0.1 mm

single orifice nozzle – top row shows water injected at 200 PSI, bottom row shows water injected at 50 PSI. Transmissivity varies from 10-5 to 10-4 (OD ~9-11).

Figure 11 shows the spray from a 0.1 mm single orifice nozzle where water is injected at 200 PSI (top row) and 50 PSI (bottom row). The overall target transmissivity is 2x10-5, or OD 11. In this extreme scat-tering condition the breakup process is still visible even though the image quality is deteriorated.

Figure 12. Reconstructed image with no surround-ing mist (left) and with a mist of OD 12 (right).

Figure 12 shows a comparison where the same

spray is imaged with and without mist. The OD is ap-proximately 12, meaning that only 2 photons out of a million traverse the target field without encountering a scattering event. The target is essentially opaque.

Figure 13. Reconstructed images from a 0.3 mm

single orifice nozzle. Injection pressure 30 PSI. Depth of reconstruction 142 mm.

Figure 13 Shows the spray created from a 0.3 mm single orifice nozzle as it develops over time. Images are taken 100 ms one after the other. A higher frame rate would be desirable to image spray dynamics but the system is limited by the 1 KHz repetition rate of the Ti-Sapphire laser.

Figure 14. Spray from a 0.3 mm single orifice

nozzle injected at 30 PSI as it develops over time. Top row shows the spray with no mist, bottom row shows the spray surrounded by a mist with transmissivity as

low as 10-5 (OD 12).

Figure 14 shows the development over time of the spray generated from a 0.3 mm single orifice injector, with and without mist. Water is injected at 30 PSI. The top row is imaged without any mist; the jet starts as a liquid column, and it then develops instabilities that form ligaments and droplets. Eventually liquid sheets form which break up into ligaments and then droplets. It is possible to scan through the hologram in depth reconstructing hundreds of differently focused image planes; a resolution of 25 micrometers over 10 centime-ters of depth is achieved.

The bottom row shows the same spray imaged in OD 12 conditions. Even though the mist clearly deterio-rates the image quality it is still possible to resolve 25 micron particles.

Comparing this value to the theoretical lateral reso-lution:

∆+ = ,-./01

(2)

Where ! is the laser central wavelength, 400 nm, z

is the depth of reconstruction which in our setup is in the order of 100 mm, and LC is the laser pulse coher-ence length which is 30 microns. These values give us a theoretical lateral resolution of 40 microns; which com-pares well with what we could resolve experimentally.

Conclusions

This paper described the results obtained using an ultrashort-pulse off-axis digital holography system to image targets within highly scattering turbid media.

We found that using high-energy ultra-short pulses in digital holography maximizes the efficacy of coher-ence gating in suppressing multiple scattering noise, furthermore it can be beneficial when imaging fast tran-sient phenomena, such as atomizing sprays.

Droplets as small as 25 microns were identified in optically dense conditions with OD up to 12. The depth scanning capability of the technique was demonstrated by reconstructing and resolving fluid ligaments and droplets residing in depth planes centimeters away from each other.

Future work

The 1 kHz repetition rate of the laser source limits the hologram acquisition rate at one hologram every 10 ms; this rate is too slow to image many dynamic spray conditions. The feasibility of holographic interferome-try will be examined in future work where one frame is interfered with another to determine phase changes which can be used for flow visualization and refractive index variation views. We also plan on applying this technique in a pressurized spray enclosure to finally establish its efficacy in high-pressure combustion appli-cations.

Funding National Science Foundation (NSF); Grant Opportuni-ties for Academic Liaison with Industry (GOALI); CBET 1233728. Additional funds were provided by the NASA GRC SBIR project; Contract No. NNX16CACC90P; Dr. Yolanda Hicks, project officer. Ongoing research is sponsored by the Army Research Office under Grant Number W911NF-17-1-0061. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office or the U.S. Gov-ernment. The U.S. Government is authorized to repro-duce and distribute reprints for Government purposes

notwithstanding any copyright notation herein.

Acknowledgements We thank Dr. Andrei Dioumaev for his assistance

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