Transmitted light microscopy for visualizing the turbulent primary breakup of a microscale liquid jet

Reddemann, Manuel Armin; Mathieu, Florian; Kneer, Reinhold

doi:10.1007/s00348-013-1607-2

Cited by 13 publications

(8 citation statements)

References 38 publications

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“…The formation and breakup of diesel jets are commonly studied at atmospheric conditions, especially as a first step towards the development of a new optical diagnostic [23], or when a particular measurement technique cannot be applied reliably at engine conditions. In any case, results of studies on diesel breakup at atmospheric conditions cannot be expected to be fully representative of atomisation under normal engine operating conditions.…”

Section: Resultsmentioning

confidence: 99%

Microscopic imaging of the initial stage of diesel spray formation

Crua

Heikal

Gold³

2015

Fuel

119

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Detailed measurements of near-nozzle spray formation are essential to better understand and predict the physical processes involved in diesel fuel atomisation. We used long-range microscopy to investigate the primary atomisation of diesel, biodiesel and kerosene fuels in the near-nozzle region, both at atmospheric and realistic engine conditions. High spatial and temporal resolutions allowed a detailed observation of the very emergence of fuel from the nozzle orifice. The fluid that first exited the nozzle resembled mushroom-like structures, as occasionally reported by other researchers for atmospheric conditions, with evidence of interfacial shearing instabilities and stagnation points. We captured the dynamics of this phenomenon using an ultra-fast framing camera with frame rates up to 5 million images per second, and identified these structures as residual fluid trapped in the orifice between injections. The residual fluid has an internal vortex ring motion which results in a slipstream effect that can propel a microscopic ligament of liquid fuel ahead. We showed that this mechanism is not limited to laboratory setups, and that it occurs for diesel fuels injected at engine-like conditions with production injectors. Our findings confirm that fuel can remain trapped in the injector holes after the end of injection. Although we could not measure the hydrocarbon content of the trapped vapourised fluid, we observed that its density was lower than that of the liquid fuel, but higher than that of the in-cylinder gas. We conclude that high-fidelity numerical models should not assume in their initial conditions that the sac and orifices of fuel injectors are filled with in-cylinder gas. Instead, our observations suggest that the nozzle holes should be considered partially filled with a dense fluid

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Section: Resultsmentioning

confidence: 99%

Microscopic imaging of the initial stage of diesel spray formation

Crua

Heikal

Gold³

2015

Fuel

119

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“…St cut−off = τ d f = 1 2π (16) can be determined using the derived frequency of the oscillation f , as fundamentally introduced in work of Burger et al [47] for studying the particle response in oscillating flows. This cut-off Stokes number may be used analogous to the cut-off frequency of a first order low pass filter in order to assign different regimes of droplet motion in a turbulent gas flow.…”

Section: The Cut-off Stokes Numbermentioning

confidence: 99%

“…This cut-off Stokes number may be used analogous to the cut-off frequency of a first order low pass filter in order to assign different regimes of droplet motion in a turbulent gas flow. In the case under consideration, the droplet diameter of 4.9 µm is determined using Equation (16), which is graphically illustrated in Figure 14.…”

Section: The Cut-off Stokes Numbermentioning

confidence: 99%

Microscopic Imaging Spray Diagnostics under High Temperature Conditions: Application to Urea–Water Sprays

2019

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The quantitative investigation of droplet laden turbulent flows at high temperature conditions is of great importance for numerous applications. In this study, an experiment was set up for investigation of evaporating urea-water sprays, which are relevant for the effective reduction of nitrogen oxide emissions of diesel engines using Selective Catalytic Reduction. A shadowgraphy setup is pushed to its limits in order to detect droplet diameters as small as 4 µm and droplet velocities up to 250 m s −1 . In addition, the operating conditions of the gaseous flow of up to 873 K and 0.6 MPa are an additional challenge. Due to the high temperature environment, image quality is prone to be compromised by Schlieren effects and astigmatism phenomena. A water-cooled window and an astigmatism correction device are installed in order to correct these problems. The results to be presented include characteristics of the turbulent gas flow as well as detailed spray characteristics at different positions downstream of the atomiser. It is demonstrated that the velocity of the gas can be approximated by the velocity of the smallest detectable droplets with sufficient accuracy. Furthermore, the statistical analysis of velocity fluctuations provides data for predicting the turbulent dispersion of the droplets.

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“…Mostly large connected liquid structures are visible across the entire image. These connected structures consist of large ligaments (diameters up to 100 µm), which are often connected by sheets (sheet thickness ≤ 3 µm ) 4 . On most of these sheet structures possible interference patterns are visible.…”

Section: Iii) Steady-state Of the Injectionmentioning

confidence: 99%

Zooming into primary breakup mechanisms of high-pressure automotive sprays

Kirsch

Reddemann

Palmer

et al. 2017

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems

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In-cylinder mixture formation and combustion are highly influenced by primary breakup of injected fuel. Experimental investigation of this phenomenon directly at the outlet of a diesel injector requires a specialized transmitted light microscopy technique combined with a constant-pressure flow microscopy vessel. The method allows verification of the existence of an intact jet core for various states of injection and different fuels. The jet core is dominated by axisymmetric surface waves during the initial injection phase. By quantification of the wavelengths and comparison with existing breakup theories, boundary layer instabilities are identified as origin of surface waves. Boundary layer wavelengths are found to be larger for a higher fuel viscosity. An occasionally appearing non-cylindrical helical jet shape is visible during the injector's opening and closing phase. The helical jet shape is directly resulting from the nozzle outlet flow. Inner nozzle effects are found to be responsible for generation of the helical structure. A fuel dependence of the helical structure formation and its breakup could not be proved. Results also prove that fuel is exiting the nozzle even after the injector needle is closed, while air is simultaneously moving into the nozzle orifice.

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Transmitted light microscopy for visualizing the turbulent primary breakup of a microscale liquid jet

Cited by 13 publications

References 38 publications

Microscopic imaging of the initial stage of diesel spray formation

Microscopic imaging of the initial stage of diesel spray formation

Microscopic Imaging Spray Diagnostics under High Temperature Conditions: Application to Urea–Water Sprays

Zooming into primary breakup mechanisms of high-pressure automotive sprays

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