Dynamical similarity and universality of drop size and velocity spectra in sprays

2018 ◽  
Vol 860 ◽  
pp. 510-543 ◽  
Author(s):  
K. Dhivyaraja ◽  
D. Gaddes ◽  
E. Freeman ◽  
S. Tadigadapa ◽  
M. V. Panchagnula
Keyword(s):  
Nozzle Exit ◽  
High Speed ◽  
Drop Size ◽  
Residual Effect ◽  
Liquid Sheet ◽  
Mathematical Form ◽  
High Speed Imaging ◽  
Size Spectra ◽  
Wide Range ◽  
Velocity Spectra

Sprays are a class of multiphase flows which exhibit a wide range of drop size and velocity scales spanning several orders of magnitude. The objective of the current work is to experimentally investigate the prospect of dynamical similarity in these flows. We are also motivated to identify a choice of length and time scales which could lead towards a universal description of the drop size and velocity spectra. Towards this end, we have fabricated a cohort of geometrically similar pressure swirl atomizers using micro-electromechanical systems (MEMS) as well as additive manufacturing technology. We have characterized the dynamical characteristics of the sprays as well as the drop size and velocity spectra (in terms of probability density functions, p.d.f.s) over a wide range of Reynolds ($Re$) and Weber numbers ($We$) using high-speed imaging and phase Doppler interferometry, respectively. We show that the dimensionless Sauter mean diameter ($D_{32}$) scaled to the boundary layer thickness in the liquid sheet at the nozzle exit ($\unicode[STIX]{x1D6FF}_{o}$) exhibits self-similarity in the core region of the spray, but not in the outer zone. In addition, we show that global drop size spectra in the sprays show two distinct characteristics. The spectra from varying $Re$ and $We$ collapse onto a universal p.d.f. for drops of size $x$ where $x/\unicode[STIX]{x1D6FF}_{o}>1$. For $x/\unicode[STIX]{x1D6FF}_{o}<1$, a residual effect of $Re$ and $We$ persists in the size spectra. We explain this characteristic by the fact that the physical mechanisms that cause large drops is different from that which is responsible for the small drops. Similarly, with the liquid sheet velocity at the nozzle exit ($u_{s}$) as the choice of velocity scale, we show that drops moving with a velocity $u$ such that $u/u_{s}<1$ collapse onto a universal p.d.f., while drops with $u/u_{s}>1$ exhibit a residual effect of $Re$ and $We$. From these observations, we suggest that physically accurate models for drop size and velocity spectra should rely on piecewise descriptions of the p.d.f. rather than invoking a single mathematical form for the entire distribution. Finally, we show from a dynamical modal analysis that the conical liquid sheet flapping characteristics exhibit a sharp transition in Strouhal number ($St$) at a critical $Re$.

Drop Size Spectra from Nozzles in High-Speed Airstreams

1985 ◽  
Vol 28 (2) ◽  
pp. 405-410 ◽  
Author(s):  
Wesley E. Yates ◽  
Robert E. Cowden ◽  
Norman B. Akesson
Keyword(s):  
High Speed ◽  
Drop Size ◽  
Size Spectra

scholarly journals Experimental analysis of particle clustering in moderately dense gas–solid flow

2021 ◽  
Vol 933 ◽  
Author(s):  
Kee Onn Fong ◽  
Filippo Coletti
Keyword(s):  
High Speed ◽  
Volume Fraction ◽  
Solid Volume Fraction ◽  
Stokes Number ◽  
Spherical Particles ◽  
High Speed Imaging ◽  
Dense Particle ◽  
Laboratory Observation ◽  
Laboratory Setup ◽  
Wide Range

In collisional gas–solid flows, dense particle clusters are often observed that greatly affect the transport properties of the mixture. The characterisation and prediction of this phenomenon are challenging due to limited optical access, the wide range of scales involved and the interplay of different mechanisms. Here, we consider a laboratory setup in which particles fall against upward-moving air in a square vertical duct: a classic configuration in riser reactors. The use of non-cohesive, monodispersed, spherical particles and the ability to independently vary the solid volume fraction ( $\varPhi _V = 0.1\,\% - 0.8\,\%$ ) and the bulk airflow Reynolds number ( $Re_{bulk} = 300 - 1200$ ) allows us to isolate key elements of the multiphase dynamics, providing the first laboratory observation of cluster-induced turbulence. Above a threshold $\varPhi _V$ , the system exhibits intense fluctuations of concentration and velocity, as measured by high-speed imaging via a backlighting technique which returns optically depth-averaged fields. The space–time autocorrelations reveal dense and persistent mesoscale structures falling faster than the surrounding particles and trailing long wakes. These are shown to be the statistical footprints of visually observed clusters, mostly found in the vicinity of the walls. They are identified via a percolation analysis, tracked in time, and characterised in terms of size, shape, location and velocity. Larger clusters are denser, longer-lived and have greater descent velocity. At the present particle Stokes number, the threshold $\varPhi _V \sim 0.5$ % (largely independent from $Re_{bulk}$ ) is consistent with the view that clusters appear when the typical interval between successive collisions is shorter than the particle response time.

scholarly journals High-speed imaging of ice nucleation in water proves the existence of active sites

2019 ◽  
Vol 5 (2) ◽  
pp. eaav4316 ◽  
Author(s):  
Mark A. Holden ◽  
Thomas F. Whale ◽  
Mark D. Tarn ◽  
Daniel O’Sullivan ◽  
Richard D. Walshaw ◽  
...  
Keyword(s):  
High Speed ◽  
Active Sites ◽  
Complex Problem ◽  
Ice Nucleation ◽  
Crystal Formation ◽  
High Speed Imaging ◽  
Wide Range ◽  
Nanoscale Topography ◽  
Salient Factor ◽  
And Control

Understanding how surfaces direct nucleation is a complex problem that limits our ability to predict and control crystal formation. We here address this challenge using high-speed imaging to identify and quantify the sites at which ice nucleates in water droplets on the two natural cleavage faces of macroscopic feldspar substrates. Our data show that ice nucleation only occurs at a few locations, all of which are associated with micron-size surface pits. Similar behavior is observed on α-quartz substrates that lack cleavage planes. These results demonstrate that substrate heterogeneities are the salient factor in promoting nucleation and therefore prove the existence of active sites. We also provide strong evidence that the activity of these sites derives from a combination of surface chemistry and nanoscale topography. Our results have implications for the nucleation of many materials and suggest new strategies for promoting or inhibiting nucleation across a wide range of applications.

scholarly journals Internal flow characteristics in scaled pressure-swirl atomizer

2018 ◽  
Vol 180 ◽  
pp. 02059 ◽  
Author(s):  
Milan Malý ◽  
Marcel Sapík ◽  
Jan Jedelský ◽  
Lada Janáčková ◽  
Miroslav Jícha ◽  
...  
Keyword(s):  
High Speed ◽  
Laser Doppler Anemometry ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Liquid Sheet ◽  
Wide Range ◽  
Swirl Atomizer ◽  
Pressure Swirl ◽  
Pressure Swirl Atomizer ◽  
Working Liquid

Pressure-swirl atomizers are used in a wide range of industrial applications, e.g.: combustion, cooling, painting, food processing etc. Their spray characteristics are closely linked to the internal flow which predetermines the parameters of the liquid sheet formed at the discharge orifice. To achieve a better understanding of the spray formation process, the internal flow was characterised using Laser Doppler Anemometry (LDA) and high-speed imaging in a transparent model made of cast PMMA (Poly(methyl methacrylate)). The design of the transparent atomizer was derived from a pressure-swirl atomizer as used in a small gas turbine. Due to the small dimensions, it was manufactured in a scale of 10:1. It has modular concept and consists of three parts which were ground, polished and bolted together. The original kerosene-type jet A-1 fuel had to be replaced due to the necessity of a refractive index match. The new working liquid should also be colourless, non-aggressive to the PMMA and have the appropriate viscosity to achieve the same Reynolds number as in the original atomizer. Several liquids were chosen and tested to satisfy these requirements. P-Cymene was chosen as the suitable working liquid. The internal flow characteristics were consequently examined by LDA and high-speed camera using p-Cymene and Kerosene-type jet A-1 in comparative manner.

scholarly journals Internal flow and air core dynamics in simplex and spill-return pressure-swirl atomizers

2017 ◽  
Author(s):  
Milan Maly ◽  
Lada Janáčková ◽  
Jan Jedelský ◽  
Jaroslav Sláma ◽  
Marcel Sapík ◽  
...  
Keyword(s):  
High Speed ◽  
Laser Doppler Anemometry ◽  
Temporal Stability ◽  
Internal Flow ◽  
High Speed Imaging ◽  
Dimensionless Numbers ◽  
Swirl Chamber ◽  
Air Core ◽  
Wide Range ◽  
Pressure Swirl

It is well known that the spray characteristics of pressure-swirl atomizers are strongly linked to the internal flow andthat an unstable air core may cause instabilities in the spray. In this paper, a 10:1 scale transparent Plexiglas (PMMA) model of a pressure-swirl atomizer as used in a small gas turbine is introduced. The internal flow is examined using high-speed imaging, laser-Doppler anemometry and computational fluid dynamics tools. The experimental and numerical results were analysed and compared in terms of the air core morphology and its temporal stability. Two different liquids were used, Kerosene-type Jet A-1 represented a commonly used fuel while p-Cymene (4-Isopropyltoluene) matched the refractive index of the Plexiglas atomizer body. The internal flow characteristics were set using dimensionless numbers i.e. the Reynolds number and Froude number. The flow test conditions were limited to inlet Reynolds numbers from 750 to 1750. Two atomizers were examined to represent a simplex and spill-return (SR) geometry. In a comparative manner, the SR atomizer features a central passage in the rear wall of the swirl chamber. The main advantage of this concept is that the fuel is always supplied to the swirl chamber at a high pressure therefore providing good atomization over a wide range of the injection flow rate. However, the presence of the spill orifice strongly affects the internal flow even if the spill-line is closed. The air core in the simplex atomizer was found fully developed and stable. The SR atomizer behaved differently, the air core didnot form at all, and the spray was therefore unstable.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4995

scholarly journals Improved evaluation of granular media flows using an X-ray scanning compatible cone-plate setup

2021 ◽  
Vol 249 ◽  
pp. 03020
Author(s):  
Zohreh Farmani ◽  
Jing Wang ◽  
Ralf Stannarius ◽  
Martina Bieberle ◽  
Frank Barthel ◽  
...  
Keyword(s):  
Granular Materials ◽  
High Speed ◽  
Granular Media ◽  
Flow Behavior ◽  
Three Dimensional ◽  
High Speed Imaging ◽  
Flow Measurements ◽  
Flow Rates ◽  
X Ray ◽  
Wide Range

To understand the typically heterogeneous flowing behavior of granular materials, it is important to combine flow tests with three-dimensional imaging. To probe the flow behavior of granular materials over a wide range of flow rates, it is imperative to be able to impose such flow rates in a well controlled manner while performing imaging tests that are compatible with all imposed flow rates. Achieving both flow control and bulk imaging capacity is challenging for a number of reasons. Here, we describe the design of a setup in which we are able to do imaging while imposing a constant overall shear rate on a granular material. We characterize the setup in which flow tests will be performed, which consists of a bottom-driven cone-plate or double-cone design. We show that the setup can be integrated in x-ray microtomography devices to aid particle tracking based flow measurements. The design is also compatible with typical rheometer setups. We also perform high speed imaging of a granular flow in an ultra-fast x-ray scanner, for which we provide proof-of-principle data in a simplified shear setup. The designed flow geometry is also compatible with said high speed imaging facility, where particle image velocimetry can be employed to extract quantitative flow field data.

Time-dependent measurements of length and area of the contact line in contact-boiling regime

2021 ◽  
Vol 926 ◽  
Author(s):  
Mohammad Khavari ◽  
Tuan Tran
Keyword(s):  
Contact Line ◽  
Contact Area ◽  
High Speed ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Bubble Nucleation ◽  
High Speed Imaging ◽  
Wide Range ◽  
Length And Area ◽  
The Impact

During the impact of a liquid droplet on a sufficiently heated surface, bubble nucleation reduces the contact area between the liquid and the solid surface. Using high-speed imaging combined with total internal reflection, we measure and report how the contact area decreases with time for a wide range of surface temperatures and impact velocities. We also reveal how formation of the observed fingering patterns contributes to a substantial increase in the total length of the contact line surrounding the contact area.

Determination of the Drop Size During Air-Blast Atomization

2019 ◽  
Vol 141 (12) ◽  
Author(s):  
T.-W. Lee ◽  
J. E. Park
Keyword(s):  
Kinetic Energy ◽  
Viscous Dissipation ◽  
High Speed ◽  
Drop Size ◽  
Integral Form ◽  
Initial Kinetic Energy ◽  
Air Blast ◽  
Dissipation Term ◽  
Wide Range ◽  
Drop Size Distributions

We have used the integral form of the conservation equations, to find a cubic formula for the drop size during in liquid sprays in coflow of air (air-blast atomization). Similar to our previous work, the energy balance dictates that the initial kinetic energy of the gas and injected liquid will be distributed into the final surface tension energy, kinetic energy of the gas and droplets, and viscous dissipation. Using this approach, the drop size can be determined based on the basic injection and fluid parameters for “air-blast” atomization, where the injected liquid is atomized by high-speed coflow of air. The viscous dissipation term is estimated using appropriate velocity and length scales of liquid–air coflow breakup. The mass and energy balances for the spray flows render to an expression that relates the drop size to all of the relevant parameters, including the gas- and liquid-phase velocities and fluid properties. The results agree well with experimental data and correlations for the drop size. The solution also provides for drop size–velocity cross-correlation, leading to computed drop size distributions based on the gas-phase velocity distribution. This approach can be used in the estimation of the drop size for practical sprays and also as a primary atomization module in computational simulations of air-blast atomization over a wide range of injection and fluid conditions, the only caveat being that a parameter to account for the viscous dissipation needs to be calibrated with a minimal set of observational data.

Experimental Investigation of Tip Cavitating Vortices in Axial-Flow Pump

2017 ◽  
Author(s):  
Desheng Zhang
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Axial Flow ◽  
Operating Conditions ◽  
High Speed Imaging ◽  
Cavitation Performance ◽  
Stable Operating ◽  
Wide Range ◽  
Axial Flow Pump ◽  
Flow Pump

The primary goal of this work focuses on the cavitating vortices in the tip region of an axial-flow pump with 3 and 4 blades mainly based on the high-speed imaging experiments, with special attention on the trajectory and dynamics of a large-scale cavitation structure. The hydraulic and cavitation performance between two impellers were compared, and it can be found that the model with 4 blades has a relative wide range of stable operating conditions as well as the better anti-cavitation ability. By the analysis of the cavitation curves, it confirms that the highly unsteady tip cavitation cloud near the blade trailing edge should be responsible for the severe degradation of the performance. According to the detailed study on the cavitation evolution in the two impellers, it is observed that the trajectory of tip cavitating vortices for different flow rates seems very similar determined by the operating conditions. However, the dynamics varies significantly, which is associated with the blade loading and flow passage width.

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