Direct comparison of shadowgraphy and x-ray imaging for void fraction determination

Dash, Amitosh; Jahangir, Saad; Poelma, Christian

doi:10.1088/1361-6501/aaea49

Cited by 9 publications

(4 citation statements)

References 28 publications

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“…Also, remains unaltered with an increase of threshold from 0.1 to 0.15. The observed deviation could be attributed to the difficulty in defining the vapour content (vapour fractions) and thus cavity in shadowgraphs (Dash, Jahangir & Poelma 2018). The vapour bubbles detached from the cavity may appear to be a part of it in shadowgraphs, overestimating cavity lengths.…”

Section: Resultsmentioning

confidence: 99%

The characteristics of bubbly shock waves in a cavitating axisymmetric venturi via time-resolved X-ray densitometry

Gawandalkar,

Poelma

2024

J. Fluid Mech.

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The bubbly shock-driven partial cavitation in an axisymmetric venturi is studied with time-resolved two-dimensional X-ray densitometry. The bubbly shock waves are characterised using the vapour fraction and pressure changes across it, propagation velocity, and Mach number. The sharp changes in vapour fraction measured with X-ray densitometry, combined with high-frequency dynamic pressure measurements, reveal that the interaction of the pressure wave with the vapour cavity dictates the shedding dynamics. At the lowest cavitation number ( $\sigma \sim 0.47$ ), the condensation shock front is the predominant shedding mechanism. However, as $\sigma$ increases ( $\sigma \sim 0.78$ ), we observe an upstream travelling pressure discontinuity that changes into a condensation shock as it approaches the venturi throat. This coincides with the increasing strength of the bubbly shock wave as it propagates upstream, manifested by the increasing velocity of the shock front and the pressure rise across it. Consequently, the Mach number of the shock front increases and surpasses the critical value 1, favouring condensation shocks. Further, at higher $\sigma$ ( ${\sim }0.84\unicode{x2013}0.9$ ), both the re-entrant jet and pressure wave can cause cavity detachment. However, at such $\sigma$ , the pressure wave likely remains subsonic. Hence cavity condensation is not favoured readily. This leads to the re-entrant jet causing the cavity detachment at higher $\sigma$ . The shock front is accelerated as it propagates upstream through the variable cross-section of the venturi. This enhances its strength, favouring cavity condensation and eventual shedding. These observations explain the existence of shock fronts in an axisymmetric venturi for a large range of $\sigma$ .

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Section: Resultsmentioning

confidence: 99%

The characteristics of bubbly shock waves in a cavitating axisymmetric venturi via time-resolved X-ray densitometry

Gawandalkar,

Poelma

2024

J. Fluid Mech.

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“…Historically, backlit or shadowgraph imaging has been a popular method for studying the near-field region of a spray (Castrejón-García et al, 2011;Castrejón-Pita et al, 2013;Settles and Hargather, 2017;Dash et al, 2018). However, white beam X-ray radiography has recently gained traction because the technique provides information-rich images and enables rapid data collection.…”

Section: Advantages Of X-ray Imagingmentioning

confidence: 99%

Fluid Flow Observations of the Spray Near-Field Using High-Speed X-Ray Imaging

Bothell

Morgan

Kastengren

et al. 2022

J Flow Vis Image Proc

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Sprays are important to many industrial processes, but the near-field region where spray formation takes place is difficult to visualize and characterize through visible light imaging because this region is typically optically dense. X-ray imaging is one technique that can penetrate this optically dense region and provide unique observations of the spray formation process. This study uses high-speed white beam X-ray imaging conducted at the Advanced Photon Source to capture spray formation dynamics from a canonical airblast atomizer. A range of momentum flux ratios are visualized by varying the gas flow rate while the liquid flow rate is held constant. The X-rays show overlapping flow structures including bags, ligaments, mushrooms, and webs. Air bubbles are captured within the liquid regions, and a very dynamic crown forms at the liquid needle exit at higher momentum flux ratios. Finally, at the highest momentum flux ratios, air bubbles and liquid droplets are observed migrating upstream in the center of the crown, and this motion is attributed to a toroidal recirculation cell at the nozzle exit.

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“…Frame rates up to 8,000 fps were achieved for each irradiated slice having a thickness of 1 mm, hence the dynamically-evolving cavitating flow could be quantified to some extent. Finally, Dash et al [41] used a medical-type CT setup to obtain the time-averaged, three-dimensional void fraction distribution of the transient twophase mixture arising at the throat of a axisymmetric converging-diverging nozzle.…”

Section: Introductionmentioning

confidence: 99%

X-ray phase contrast and absorption imaging for the quantification of transient cavitation in high-speed nozzle flows

et al. 2021

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High-flux synchrotron radiation has been employed in a time-resolved manner to characterise the distinct topology features and dynamics of different cavitation regimes arising in a throttle orifice with an abrupt flow-entry contraction. Radiographs obtained though both X-ray phasecontrast and absorption imaging have been captured at 67,890 frames per second. The flow lied in the turbulent regime (Re=35,500), while moderate (CN=2.0) to well-established (CN=6.0) cavitation conditions were examined encompassing the cloud and vortical cavitation regimes with pertinent transient features, such as cloud-cavity shedding. X-ray Phase-Contrast Imaging (XPCI), exploiting the shift in the X-ray wave phase during interactions with matter, offers sharp-refractive index gradients in the interface region. Hence, it is suitable for capturing fine morphological fluctuations of transient cavitation structures. Nevertheless, the technique cannot provide information on the quantity of vapour within the orifice. Such data have been obtained utilising absorption imaging, where beam attenuation is not associated with scattering and refraction events, and hence can be explicitly correlated with the projected vapour thickness in line-of-sight measurements. A combination of the two methods is proposed, as it has been found is capable of quantifying the vapour content arising in the complex nozzle flow, while also faithfully illustrating the dynamics of the highly-transient cavitation features.

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Direct comparison of shadowgraphy and x-ray imaging for void fraction determination

Cited by 9 publications

References 28 publications

The characteristics of bubbly shock waves in a cavitating axisymmetric venturi via time-resolved X-ray densitometry

The characteristics of bubbly shock waves in a cavitating axisymmetric venturi via time-resolved X-ray densitometry

Fluid Flow Observations of the Spray Near-Field Using High-Speed X-Ray Imaging

X-ray phase contrast and absorption imaging for the quantification of transient cavitation in high-speed nozzle flows

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