Investigation of pure- and aerated-liquid jets using ultra-fast X-ray phase contrast imaging

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Abstract

Pure- and aerated-liquid jets were observed using the ultra-fast X-ray phase contrast imaging technique. Highly convoluted wrinkle structures were seen on the column surface of a turbulent pure-liquid jet, gas bubbles were discovered inside droplets and ligaments of aerated-liquid sprays, and apparently homogenous two-phase mixtures were observed inside the aerated-liquid injector. The major limitation of this X-ray technique lies in its line-of-sight nature, which can create overlapped objects/interfaces on the X-ray images.

Introduction

Liquid jet atomization plays an important role in establishing stable and efficient combustion inside the combustor of a liquid-fueled air-breathing propulsion system. For applications requiring both deep fuel penetration into high-speed crossflows and small droplets in the liquid spray, a superior liquid injection scheme must be sought. Among the possible candidates, the aerated-liquid (or effervescent, or barbotage) jet has exhibited promising characteristics in both reacting and nonreacting environments [1], [2], [3]. An aerated-liquid injector features an internal mixing region for liquid to mix with a small amount of aerating gas prior to discharging to the applications [1], [2]. While macroscopic and far-field features of the aerated-liquid jets have been extensively examined, detailed near-field spray structures cannot easily be explored, due to the difficulties of diagnosing dense spray regions [4], which may, for instance, appear as opaque regions in shadowgraphs.

This paper presents recent efforts in investigating the structures of pure- and aerated-liquid jets both inside and outside (near-field region) an aerated-liquid injector, using the ultra-fast X-ray phase contrast imaging (PCI) technique available at the XOR 32-ID beamline of the Advanced Photon Source (APS) at the Argonne National Laboratory [5], [6]. Advantages and limitations of utilizing the X-ray PCI on liquid sprays will be discussed.

Section snippets

Experimental methods

The experiment was conducted at the XOR 32-ID beamline of the Advanced Photon Source (APS) at Argonne National Laboratory. The undulator source provides the high X-ray brilliance necessary for the white-beam ultra-fast imaging technique. With an optimized undulator gap, most of the intensity was located within the first harmonic at 13.3 keV, with a peak irradiance of 1014 ph/s/mm2/0.1%bw and a natural bandwidth of 0.3 keV FWHM. A fast scintillator crystal (LYSO:Ce with a 40 ns decay time) converted

Results and discussion

Composite X-ray images of pure-liquid jets injected from the axisymmetric aerated-liquid injector at various liquid flow rates are shown in Fig. 1. The composite image shown in this paper consists of randomly-selected individual X-ray images taken at the corresponding spatial locations. A sequence of 10 images was obtained at each imaging location. The respective water flow rate and Reynolds number (Re) range from 9.1 to 36.3 g/s and from Re=1.14×104 to 4.58×104. Note that with Re>4×103, the

Conclusion

Unique features of pure- and aerated-liquid jets were observed with the ultra-fast X-ray PCI technique. Highly convoluted wrinkle structures on the column surface of a turbulent pure-liquid jet were observed. Entrained gas voids inside droplets of aerated-liquid sprays were also observed in the X-ray images. These gas bubbles, usually with large diameters, can be mistreated as large droplets by conventional diagnostics. In addition, droplet/bubble size can be measured and breakup mechanisms can

Acknowledgements

This work was sponsored by the AFRL/Propulsion Directorate under contract number FA8560-08-D-2844 (Robert Behdadnia, monitor) and supported by the U.S. Air Force Office of Scientific Research (Julian Tishkoff, monitor). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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