ILASS Americas, 22nd Annual Conference on Liquid Atomization and Spray Systems, Cincinnati, OH, May 2010

Ultrafast X-ray Phase-Contrast Imaging of High-Speed Fuel Sprays from a Two-Hole Diesel Nozzle

S. Moon, Z. Liu, J. Gao, E. Dufresne, K. Fezzaa and J. Wang*

X-Ray Science Division

Argonne National Laboratory 9700 South Cass Ave, Argonne, IL 60439 USA

X. Xie and M. -C. Lai

Department of Mechanical Engineering Wayne State University

5050 Anthony Wayne Dr., Detroit, MI 48202 USA

Abstract Near-nozzle fuel jet morphology during high-pressure diesel injection by axially symmetric single-hole nozzles has been unveiled from recent studies. Such a study had not extended to more realistic and practical multi-hole injectors. This study focuses on near-nozzle jet morphology of high-speed fuel sprays from a two-hole nozzle under varied injection conditions. The two-hole nozzle is a simplified version of multi-hole nozzle, where two orifices are sym-metrically aligned around the nozzle axis. The jet morphology of the two-hole nozzle was studied with the high-intensity and high-brilliance x-ray beam available at the Advanced Photon Source (APS) in Argonne National Labo-ratory with a sub-ns (150 ps) temporal duration and 1 µm spatial resolution. It was observed that turbulent flow in swirl-like morphology originated from inter-nozzle vortex flow was formed inside fuel sprays from the two-hole nozzle. This turbulent vortex flow of the two-hole nozzle led to unstable spray development, wider cone angle and faster break-up of the spray compared to the single-hole nozzle spray. Unique surface and internal morphology of the fuel sprays can be identified at both lower (<70 MPa) and higher injection pressures (>70 MPa). The stability of the spray jets is also investigated.

*Corresponding author: wangj@aps.anl.gov

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1. Introduction Near-nozzle fuel jet development dominated by in-

ternal nozzle flow and related fluid-dynamic instability governs the primary break-up process of the injected fuel. Initial break-up process of the spray initiates the mixing of the injected fuel with surrounding oxidant gas, which is crucial to achieve highly-efficient and clean combustion. Visible-light imaging techniques have been dominantly used to visualize the internal- and near-nozzle fuel flow and led to remarkable pro-gress in analysis of cavitation inside the nozzle and near-nozzle spray geometry [1-4]. However, these tech-niques have revealed their limitation in the visualization of near-nozzle jet morphology especially for dense and high-speed fuel sprays which are optically dense. To overcome this, single-shot ultrafast x-ray phase-contrast imaging technique has been introduced recently for dense high-speed fuel sprays [5]. X-ray phase-contrast imaging technique records phase variations of the emerging radiation from liquid/gas interfaces rather than absorption-induced intensity variations when the x-ray beam passes through an object [6].

Recent trends in fuel injection nozzles for high-pressure diesel injection are reducing the size of orifices and increasing the number of orifices (multi-hole noz-zle concept) to achieve better fuel atomization and im-proved fuel/air mixing process. To place the multi-orifices in an injector nozzle, the orifices are located off-axis to the nozzle axis and symmetrically aligned around the nozzle axis. This off-axis location of the orifices makes the near-nozzle spray to be strongly af-fected by the vortex flow inside the nozzle sac other than axial laminar-like flow. The structure of the vortex flow and cavitation inside the multi-hole diesel nozzles has been unveiled from some simulative studies [1, 2, 7, 8]. The results showed that counter-vortex flow is formed inside the nozzle orifice and it strongly affects the near-nozzle spray development [1, 2, 8].

A previous study focused on the near-nozzle jet morphology of high-pressure diesel spray injected by a single-hole nozzle which center of hole is located on the nozzle axis [5]. Their results revealed that the lami-nar-like jet flow is formed very near the nozzle exit and this jet flow becomes unstable due to its fluid-dynamic instabilities and turbulence generated inside the nozzle. In the case of smooth orifice inlet, mono-dispersed pla-nar wave is formed on the jet surface, while totally tur-bulent jet morphology rather than wavy structure was observed for sharp orifice inlet. However, such a study had not extended to more realistic and practical multi-hole nozzles which jets are strongly affected by the vortex flow inside the nozzle.

The aim of this study is to investigate the near-nozzle jet morphology of the high-speed fuel spray in-jected by a two-hole nozzle under varied injection con-ditions. The two-hole nozzle is a simplified version of

multi-hole nozzle. The near-nozzle jet morphology of the two-hole nozzle was obtained by the single-shot ultrafast x-ray phase-contrast imaging technique, im-plemented by the high-intensity and high-brilliance x-ray beam available at the Advanced Photon Source (APS) in Argonne National Laboratory. The near-nozzle jet morphology of the two-hole nozzle was com-pared to that of the single-hole nozzle during the quasi-steady injection process.

2. Experimental Setup and Conditions 2.1 Experimental setup

Experiments are performed using a phase-contrast x-ray imaging setup at XOR 7ID beamline in the APS. The x-ray beam is generated from an insertion device (undulator) in the APS electron storage ring. The spe-cial beam pattern (hybrid singlet mode) shown in Fig. 1 was used in this experiment. This pattern contains a single electron bunch (150 ps duration and carrying 16 mA current) isolated from the remaining electron train bunch (472 ns long, 96 mA) by symmetrical 1.594 µs gaps. To reduce the heat power, the x-ray beam was gated by two mechanical shutters: the slow shutter op-erating at 1Hz frequency with 10 ms opening duration and the fast shutter operating at 2 kHz frequency with 9 µs opening duration [9]. Synchronized operation of these two shutters cuts-off more than 99% of the beam heat power.

After transmitted to the spray, the x-ray beam gen-erates the phase-contrasted image on the scintillator crystal (LYSO:Ce), which converts the transmitted x-ray beam into visible light (432 nm). This image is re-flected by a 45o mirror and then captured by a charge-coupled device (CCD) camera (Sensicam, 13761040 pixels, from Cooke). The camera was gated at the tim-ing when only the singlet electron bunch passes through the shutters. The remaining electron bunches were cut-off by closing the camera gate at 1.5 μs after the gate opening (exposure time of 1.5 s). The field of view of the camera was 1.751.32 mm2 when a 10 objective lens (NA=0.14) is used.

The fuel was injected into a spray chamber using a common-rail injection system, composed of fuel tank, motor, high-pressure pump, pressure valve and com-mon-rail. The pressure inside the common rail was con-trolled via feedback control of measured pressure inside the common-rail and bleeding fuel flow rate of the pressure valve. The spray chamber has two Kapton windows which allow the x-ray beam to pass through this without decrease in intensity.

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Figure 1. Experimental setup for phase-contrast x-ray imaging

Nozzle Single-hole nozzle (1-hole, D0=0.135mm)

Two-hole nozzle (2-holes, D0=0.130mm, =135o)

Injection Pressure, Pinj (MPa) 30, 40, 50, 70, 100

Injection Pulse Duration, Tq (ms) 3.0

Injection Duration, Tinj (ms) 4.0

Ambient Pressure and Temperature Room Conditions

Ambient gas Nitrogen (N2)

Density ρ (kg/m3) Viscosity μ (mm2/s) Fuel

Biodiesel 888.0 5.02

Table 1. Experimental Conditions

2.2 Experimental conditions Table 1 shows the experimental conditions. A two-

hole nozzle (THN) with the orifice diameter (Do) of 0.130mm and the angle between two-orifices of 135o was used as the test nozzle (see Fig. 2). A reference nozzle, single-hole nozzle (SHN) with the orifice di-ameter (Do) of 0.135mm, was also used in this study for the comparison with two-hole nozzle. Figure 2 shows the internal structure of SHN and THN visualized with higher energy x-ray beam and longer exposure time [10]. These two nozzles have same sac diameter and similar level of hydro-grinding, but has slightly differ-ent nozzle hole diameter. The difference in nozzle hole diameter (around 4%) seems resulted from imperfect

manufacturing. This subtle difference should not cause severe change in near-nozzle fuel flow. The applied injection pressures were ranged from 30MPa to 100MPa. Relatively longer injection pulse duration (3.0 ms) than that of real diesel engines was applied to in-vestigate the near-nozzle flow characteristics in the quasi-steady injection stage. The real injection duration, determined by the spray images, was around 4.0 ms in this injection pulse duration. Experiments were per-formed under room pressure and temperature conditions. Biodiesel was used as the test fuel in this study.

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Figure 2. Internal nozzle structure of two-hole (left) and single-hole (right) nozzles captured using phase-contrast imaging technique (detailed methodology can be found in [10])

3. Results and Discussion 3. 1 Near-Nozzle Jet Morphology of Two-Hole and Single-Hole Nozzles

Figure 3 shows the near-nozzle jet morphology of THN and SHN at the injection pressure of 30MPa. The images shown are captured during the quasi-steady in-jection process (around 2.0ms after the start of injec-tion).

In case of the THN spray, swirl-like wavy structure was clearly observed near the nozzle tip. The helical flow of the diesel spray was once reported from a pre-vious study [11], even though the wavy structures pre-sented in that study were not clear as the phase-contrasted images in this study. This wavy structure confirms the existence of rotational flow inside the two-hole nozzle [1, 2]. Similar wavy structure was also ob-served from the previous studies focused on the helical instability of a uniformly rotating viscous liquid jet formed by a rotating tube [12, 13], even though applied fuel and injection conditions are quite different with this study. The results of these researches [12, 13] showed that at downstream of the undisturbed liquid region, a very short region of planar instability is formed and this planar instability rapidly evolves into a helical instability with spiraling shape (named as helical

wave). This helical wave structure experiences secon-dary instability which initiates the break-up of the spi-raling structure [14]. Similar spray structure and break-up process are observed from the THN sprays, but there are some differences with that from the uniformly rotat-ing liquid jet.

The simulative results from the previous studies revealed the existence of counter-vortex flow inside the nozzle orifice [1, 2, 8]. The line-of-sight phase-contrast images of the THN sprays show the superposed wavy structures from the counter-vortex. The different wave-length of each wavy structure indicates that each vortex has different azimuthal velocity components. However, only one or no wavy structures were observed in some other sample images. It means the counter-vortex flow inside the nozzle orifice is quite unstable even during the quasi-steady injection process.

The SHN sprays showed more laminar-like flow characteristics and more stable spray morphology and near-nozzle spray angle compared to the THN sprays. The near-nozzle spray angle was smaller than that of the THN spray. Due to relatively stable flow character-istics, the break-up of the SHN spray occurred at farther downstream of the nozzle exit compared to the THN spray.

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Figure 3. Near-nozzle jet morphology of two-hole nozzle and single-hole nozzle (Fuel: Biodiesel, Pinj: 30 MPa, Tinj: 4.0 ms, Images taken at 2.0ms after the start of injection (ASOI))

Figure 4. Near-nozzle jet morphology of two-hole nozzle under various injection pressures (Fuel: Biodiesel, Tinj: 4.0 ms, Images taken at 2.0ms ASOI)

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Figure 5. Definition and measurement of wavelength

(a) and wavelength results of THN spray under various injection pressures (b) (Fuel: Biodiesel, Tinj: 4.0 ms,

Images taken at 2.0ms ASOI) 3. 2 Injection Pressure Effect on Two-Hole Nozzle Spray Development

Figure 4 shows the near-nozzle jet morphology of THN under various injection pressures. Even though the swirl-like wavy structure is still observable until 70MPa injection pressure (image not included), it be-comes unclear and disturbed as the injection pressure increases. It seems relatively large-scale vortex flow breaks-up into fine-scale turbulences upon increase in injection pressure and it leads to unclear wavy structure of the two-hole nozzle jet. It supposedly results from the increase in Reynolds number at higher injection pressures. In case of the pipe flow, it is well known that the flow becomes more turbulent as the Reynolds num-

ber increases (Re=VD/µ; is fuel density, V is fuel velocity, D is pipe diameter and is fuel viscosity).

Figure 5 shows the results of wavelength and its shot-to-shot deviation under various injection pressures. The wavelength in the 100MPa injection case is not included here, since totally turbulent jet morphology other than wavy structure is observed in this injection pressure. The wavelengths were measured using power spectrum of the intensity profile along the wavy struc-ture (see Fig. 5a). As shown in Fig. 5b, the wavelength decreases as the injection pressure increases. Consider-ing higher injection pressure increases the rotational speed of vortex flow, this wavelength dependence on injection pressure follows the trend of a previous study [12] that shows increased level of helical instability at higher rotational speed of jets with shorter wavelength and higher wave amplitude of the helical waves. This shorter wavelength and higher wave amplitude cause faster break-up of the liquid jet at shorter distance from the nozzle exit [12]. The images in Fig. 4 confirm the shorter break-up length of the spray at higher injection pressures. Unfortunately, the wave amplitude was not measurable from the obtained phase-contrast images since the overlapped morphology of two-vortices made hard to distinguish the amplitude of each wavy struc-ture.

Interestingly, the shot-to-shot deviation in wave-length and near-nozzle spray angle was considerably decreased as the injection pressure increased. Probably, the flow transition from larger-scale unstable vortex flow to the finer-scale turbulences caused this rela-tively stable spray structure at high injection pressures.

4. Conclusions

In this study, near-nozzle jet morphology of a two-hole nozzle, a simplified version of multi-hole diesel nozzle, was investigated using ultrafast x-ray phase-contrast imaging technique during quasi-steady injec-tion process. The flow characteristics of the two-hole nozzle spray were then compared to that of an axially symmetric single-hole nozzle spray.

Sprays from the two-hole nozzle are dominated by vortex flow inside the nozzle. Swirl-like wavy structure of the emerging spray verifies the existence of rota-tional flow inside the nozzle. The overlapped multi-wavy structures inside the two-hole nozzle spray indi-cate that the counter-vortex formed inside the nozzle affects the two-hole nozzle spray development. Unsta-ble flow characteristic of the two-hole nozzle caused higher shot-to-shot variation in spray morphology, lar-ger spray angle and faster break-up of the spray com-pared to the single-hole nozzle spray which has lami-nar-like flow characteristics.

More turbulent jet morphology was observed in-side the two-hole nozzle spray upon increase in injec-tion pressure. Wavelength of the wavy structure was

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decreased at high injection pressure and it caused faster break-up of the emerging spray. The shot-to-shot de-viation in spray angle and wavelength was decreased upon increase in injection pressure supposedly due to break-up of the larger-scale unstable vortex flow into the finer-scale turbulences. Acknowledgement

This work at and use of the APS and CHESS are supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Science. The beamline support at the APS Sector 7ID is highly appreciated. This work is also partially supported by the U.S. De-partment of Energy, Office of Vehicle Technology. We also thank Dr. Richard Winsor of John Deere Product Engineering Center (PO BO Box 8000, Waterloo, IA 50704-8000) for providing the injection nozzle.

Nomenclature D diameter viscosity P pressure angle Re Reynolds number density T temperature V velocity Subscripts inj injection

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