Droplet–turbulence interaction in a confined polydispersed spray: effect of turbulence on droplet dispersion

Sahu, Srikrishna; Hardalupas, Y.; Taylor, A. M. K. P.

doi:10.1017/jfm.2016.169

Cited by 27 publications

(30 citation statements)

References 61 publications

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“…In the present work, measurement of RDF was achieved by ILIDS measurements of droplets. We have verified in our previous work (Sahu et al 2016) that calculation of L c by an RDF based on ILIDS agrees well with the 'droplet counting in a cell' approach (Fessler et al 1994) based on focused spray droplet images. The expression for G in (3.16) is derived for uniform size of droplets in a cloud.…”

Section: Radial Distribution Function (Rdf)supporting

confidence: 77%

“…Also, the fact that the droplet-gas slip velocity decreases significantly for all droplet sizes far downstream of the injector exit (see Gounder, Kourmatzis & Masri 2012;Sahu et al 2016) further supports the above assumption. However, in our earlier work (Sahu et al 2014a) we have shown that the integral length scale of the droplet flow is smaller than that of the gas flow, and also, the gas velocity fluctuations are smaller than the droplet velocity fluctuations, though the differences are smaller for droplets of low Stokes number.…”

Section: Droplet Velocitysupporting

confidence: 61%

“…The correlation coefficient between fluctuations of droplet number density of different droplet size classes, denoted as R n i * n j , is useful for understanding droplet dispersion and clustering in isothermal sprays (Sahu et al 2016) and can provide an insight into the dispersion of evaporating droplets. The correlation coefficient of the fluctuations of droplet number density of two droplet size classes D i and D j can be expressed as…”

Section: Correlation Between Droplet Number Density Of Different Sizementioning

confidence: 99%

“…As mentioned earlier, since L c is of the order of L, the large scales of the turbulent flow are expected to influence the cluster size. In addition, the large eddies are responsible for the transport of these clusters, and thus the frequency of passage of droplet clusters and inter-cluster spacing at any location in the spray are expected to be governed by the large-scale eddies of the flow (Fessler et al 1994;Sahu et al 2016). …”

Section: Radial Distribution Function (Rdf)mentioning

confidence: 99%

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Interaction of droplet dispersion and evaporation in a polydispersed spray

2018

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The interaction between droplet dispersion and evaporation in an acetone spray evaporating under ambient conditions is experimentally studied with an aim to understand the physics behind the spatial correlation between the local vapour mass fraction and droplets. The influence of gas-phase turbulence and droplet-gas slip velocity of such correlations is examined, while the focus is on the consequence of droplet clustering on collective evaporation of droplet clouds. Simultaneous and planar measurements of droplet size, velocity and number density, and vapour mass fraction around the droplets, were obtained by combining the interferometric laser imaging for droplet sizing and planar laser induced fluorescence techniques (Sahu et al., Exp. Fluids, vol. 55, 1673, 2014b. Comparison with droplet measurements in a non-evaporating water spray under the same flow conditions showed that droplet evaporation leads to higher fluctuations of droplet number density and velocity relative to the respective mean values. While the mean droplet-gas slip velocity was found to be negligibly small, the vaporization Damköhler number (Da v ) was approximately 'one', which means the droplet evaporation time and the characteristic time scale of large eddies are of the same order. Thus, the influence of the convective effect on droplet evaporation is not expected to be significant in comparison to the instantaneous fluctuations of slip velocity, which refers to the direct effect of turbulence. An overall linearly increasing trend was observed in the scatter plot of the instantaneous values of droplet number density (N) and vapour mass fraction (Y F ). Accordingly, the correlation coefficient of fluctuations of vapour mass fraction and droplet number density (R n * y ) was relatively high (≈0.5) implying moderately high correlation. However, considerable spread of the N versus Y F scatter plot along both coordinates demonstrated the influence on droplet evaporation due to turbulent droplet dispersion, which leads to droplet clustering. The presence of droplet clustering was confirmed by the measurement of spatial correlation coefficient of the fluctuations of droplet number density for different size classes (R n * n ) and the radial distribution function (RDF) of the droplets. Also, the tendency of the droplets to form clusters was higher for the acetone spray than the water spray, indicating that droplet evaporation promoted droplet grouping in the spray. The instantaneous group evaporation number (G) was evaluated from the measured length scale of droplet clusters (by the RDF) and the average droplet size and spacing in instantaneous clusters. The mean value of † Email address for correspondence: ssahu@iitm.ac.in G suggests an internal group evaporation mode of the droplet clouds near the spray centre, while single droplet evaporation prevails near the spray boundary. However, the large fluctuations in the magnitude of instantaneous values of G at all measurement locations implied temporal variations in the mode of droplet cloud evapor...

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Section: Radial Distribution Function (Rdf)supporting

confidence: 77%

Section: Droplet Velocitysupporting

confidence: 61%

Section: Correlation Between Droplet Number Density Of Different Sizementioning

confidence: 99%

Section: Radial Distribution Function (Rdf)mentioning

confidence: 99%

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Interaction of droplet dispersion and evaporation in a polydispersed spray

2018

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“…The local concentration is measured along the wall-normal plane by counting the number of particle centroids detected. This approach was used for inertial particles in air (Yang & Shy 2005;Sahu et al 2014;Sahu et al 2016) and in water (Kiger & Pan 2002). Knowles & Kiger (2012) showed that, in water, laser-based measurements of particle concentration can be misestimated by as much as 30%; however, they considered volume fractions one order of magnitude higher than the present case.…”

Section: Measurement Methodsmentioning

confidence: 99%

Velocity and spatial distribution of inertial particles in a turbulent channel flow

2019

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We present experimental observations of the velocity and spatial distribution of inertial particles dispersed in the turbulent downward flow through a vertical channel at friction Reynolds numbers Re τ = 235 and 335. The working fluid is air laden with size-selected glass micro-spheres, having Stokes numbers St = O(10) and O(100) when based on the Kolmogorov and viscous time scales, respectively. Cases at solid volume fractions φ v = 3×10 −6 and 5×10 −5 are considered. In the more dilute regime, the particle concentration profile shows near-wall and centerline maxima compatible with a turbophoretic drift down the gradient of turbulence intensity; the particles travel at similar speed as the unladen flow except in the near-wall region; and their velocity fluctuations generally follow the unladen flow level over the channel core, exceeding it in the near-wall region. The denser regime presents substantial differences in all measured statistics: the near-wall concentration peak is much more pronounced, while the centerline maximum is absent; the mean particle velocity decreases over the logarithmic and buffer layers; and particle velocity fluctuations and deposition velocities are enhanced. An analysis of the spatial distributions of particle positions and velocities reveals different behaviors in the core and near-wall regions. In the channel core, dense clusters form which are somewhat elongated, tend to be preferentially aligned with the vertical/streamwise direction, and travel faster than the less concentrated particles. In the near-wall region, the particles arrange in highly elongated streaks associated to negative streamwise velocity fluctuations, several channel height in length and spaced by O(100) wall units, supporting the view that these are coupled to fluid low-speed streaks typical of wall turbulence. The particle velocity fields contain a significant component of random uncorrelated motion, more prominent for higher St and in the near-wall region.

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Two-Phase Characterization for Turbulent Dispersion of Sprays: A Review of Optical Techniques

Sahu

Manish

Hardalupas

2017

Energy, Environment, and Sustainability

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In liquid fueled combustion the interaction of spray droplets with surrounding turbulent air flow is crucial since it influences the evaporation rate of the fuel droplets and the process of air and fuel vapor mixture preparation. For detail understanding on the droplet-turbulence interaction mechanisms as well as to validate numerical simulations of sprays, simultaneous measurement of both dispersed and carrier phases of the spray is essential. This way the vortical interaction of droplets can be fully characterized such that important issues such as local spray unsteadiness and spatio-temporal inhomogeneity of droplet concentration due to droplet clustering, and group evaporation of droplets can be addressed. This review focuses on the advances in the optical diagnostics (especially the planar techniques) in the last few decades to meet these requirements. Due to broad size and velocity distributions of spray droplets, the application of the two-phase measurement techniques to sprays encounters challenges especially in phase discrimination. Additionally, for sprays, it is not sufficient to simultaneously measure two-phase velocities but the droplet size is also important since the dynamic drag on droplets is according to their size and velocity relative to the surrounding gas flow. The techniques with such capability are explored. The sources of uncertainty, and advantages and limitations of different two-phase measurement approaches are analyzed according to their application to dense and dilute sprays.

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Droplet–turbulence interaction in a confined polydispersed spray: effect of turbulence on droplet dispersion

Cited by 27 publications

References 61 publications

Interaction of droplet dispersion and evaporation in a polydispersed spray

Interaction of droplet dispersion and evaporation in a polydispersed spray

Velocity and spatial distribution of inertial particles in a turbulent channel flow

Two-Phase Characterization for Turbulent Dispersion of Sprays: A Review of Optical Techniques

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