“Swimming Jellyfish“: Visualizing Jet-Like Internal Flow in Coalescing Droplets

2019 ◽  
Vol 141 (10) ◽  
Author(s):  
Xiao Yan ◽  
Soumyadip Sett ◽  
Lezhou Feng ◽  
Leicheng Zhang ◽  
Zhiyong Huang ◽  
...  
Keyword(s):  
Flow Characteristics ◽  
Size Control ◽  
Internal Flow ◽  
Bulk Liquid ◽  
Working Fluid ◽  
Vortex Rings ◽  
Water Mixture ◽  
Droplet Coalescence ◽  
Visualization System ◽  
Significant Attention

Abstract Surface-tension-driven droplet coalescence has received significant attention due to its significant role in microfluidics, coalescence-induced droplet jumping, fluid mixing, and microscale heat and mass transfer. However, the study of internal flow characteristics of merging droplets remains a challenge due to visualization difficulty, limited droplet size control, poor droplet manipulation, and insufficient droplet front tracking. Here, in order to study droplet coalescence dynamics, a droplet dispensing and visualization system was developed. To control the size of droplets, monodispersed droplets with diameters of ≈ 40 μm were dispensed onto a superhydrophobic surface, enabling the target droplets to accumulate in volume and grow in radii. To track the internal flow front, an ethanol (20 wt %)-water mixture was used as the working fluid. Due to the unequal evaporation rate of water and ethanol, a density gradient was introduced at the liquid-gas interface of the droplets, resulting in a lower refractive index compared to that in the bulk liquid. The coalescence process of droplets with diameters of ≈ 232 μm and ≈ 1400 μm was imaged. We observed jet-like internal flow and vortex rings (“swimming jellyfish”) inside the merging droplet. It was shown that the jet-like flow underwent a significant deceleration while the vortex-ring experienced slow radial expansion. Our work not only presents a powerful platform capable of visualizing droplet coalescence hydrodynamics, but also provides insights into the internal flow dynamics during droplet coalescence.

scholarly journals A Comparative Study on Centrifugal Pump Designs and Two-Phase Flow Characteristic under Inlet Gas Entrainment Conditions

Energies ◽  
2019 ◽  
Vol 13 (1) ◽  
pp. 65 ◽  
Author(s):  
Qiaorui Si ◽  
Gérard Bois ◽  
Minquan Liao ◽  
Haoyang Zhang ◽  
Qianglei Cui ◽  
...  
Keyword(s):  
Pressure Pulsation ◽  
Two Phase Flow ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Phase Flow ◽  
Water Mixture ◽  
Centrifugal Pumps ◽  
Two Phase ◽  
Void Fractions

Capability for handling entrained gas is an important design consideration for centrifugal pumps used in petroleum, chemistry, nuclear applications. An experimental evaluation on their two phase performance is presented for two centrifugal pumps working under air-water mixture fluid conditions. The geometries of the two pumps are designed for the same flow rate and shut off head coefficient with the same impeller rotational speed. Overal pump performance and unsteady pressure pulsation information are obtained at different rotational speeds combined with various inlet air void fractions (α0) up to pump stop condition. As seen from the test results, pump 2 is able to deliver up to 10% two-phase mixtures before pump shut-off, whereas pump 1 is limited to 8%. In order to understand the physics of this flow phenomenon, a full three-dimensional unsteady Reynolds Average Navier-Stokes (3D-URANS) calculation using the Euler–Euler inhomogeneous method are carried out to study the two phase flow characteristics of the model pump after corresponding experimental verification. The internal flow characteristics inside the impeller and volute are physically described using the obtained air distribution, velocity streamline, vortex pattern and pressure pulsation results under different flow rates and inlet void fractions. Pump performances would deteriorate during pumping two-phase mixture fluid compared with single flow conditions due to the phase separating effect. Some physical explanation about performance improvements on handing maximum acceptable inlet two phase void fractions capability of centrifugal pumps are given.

Flow in Atomizers: Influence of Different Parameters on the Performance Characteristics of Plain Orifice Atomizer and Pressure Swirl Atomizer of a Fuel Injection System of Gas Turbine Combustor

2005 ◽  
Author(s):  
Digvijay B. Kulshreshtha ◽  
S. A. Channiwala
Keyword(s):  
Surface Tension ◽  
Drop Size ◽  
Cone Angle ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Performance Characteristics ◽  
Bulk Liquid ◽  
Atomization Process ◽  
Spray Cone ◽  
Pressure Swirl

The atomization process is essentially one in which bulk liquid is converted into small drops. Basically, it can be considered as a disruption of the consolidating influence of surface tension by the action of internal and external forces. In the absence of such disruptive forces, surface tension tends to pull the liquid into the form of a sphere, since this has the minimum surface energy. Liquid viscosity exerts a stabilizing influence by opposing any change in system geometry. On the other end, aerodynamic forces acting on the liquid surface may promote the disruption process by applying an external distortion force to the bulk liquid. Breakup occurs when the magnitude of the disruptive force just exceeds the consolidating surface tension force. In twin fluid atomizers of the air-blast type and air assist type, atomization and spray dispersion tend to be dominated by air momentum forces, with hydrodynamic processes playing only a secondary role. With pressure swirl nozzles, the internal flow characteristics are of primary importance, because they govern the thickness and uniformity of the annular liquid film formed in the final discharge orifice as well as the relative magnitude of the axial and tangential components of velocity of this film. It is therefore of great practical interest to examine the interrelationships that exist between internal flow characteristics, nozzle design variables, and important spray features such as cone angle and mean drop size. The various equations that have been derived for nozzle discharge coefficient are discussed because this coefficient not only affects the flow rate of any given nozzle but also can be used to calculate its velocity coefficients and spray cone angle. Consideration is also given to the complex flow situations that arise on the surface of a rotating cup or disk. These flow characteristics are of basic importance to the successful operation of atomizers, because they exercise a controlling influence on the nature of the atomization process, the quality of atomization, and distribution of drop sizes in the spray. For plain orifice atomizers, the key geometrical variables are the orifice length and diameter. Final orifice diameter is of prime importance for pressure swirl atomizers. The absence of any theoretical treatment of the atomization process has led to the evolution of empirical equations to express the relationship between the mean drop size in a spray and the variable liquid properties. This paper includes the study of different parameters that affect the flow in plain orifice and pressure swirl atomizers. The paper also includes the performance characteristics of plain orifice and pressure swirl atomizers.

“Dancing Droplets”: Partial Coalescence on Superhydrophobic Surfaces

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
Xiao Yan ◽  
Lezhou Feng ◽  
Leicheng Zhang ◽  
Soumyadip Sett ◽  
Longnan Li ◽  
...  
Keyword(s):  
Capture Rate ◽  
Size Control ◽  
Superhydrophobic Surfaces ◽  
Droplet Coalescence ◽  
Satellite Droplet ◽  
Visualization System ◽  
Partial Coalescence ◽  
Primary Droplet ◽  
Moving Direction ◽  
Air Cushion

Abstract Droplet coalescence has received significant attention due to its significant role in fluid mixing, microfluidics, coalescence-induced droplet jumping, and heat and mass transfer applications. Coalescence of droplets has been extensively investigated from the perspectives of hydrodynamics and energy transfer. However, the study of coalescence characteristics of size-mismatched droplets on superhydrophobic surfaces remains a challenge due to visualization difficulty, limited droplet size control, and poor droplet manipulation. Here, in order to study coalescence dynamics of droplets with arbitrary initial sizes, a droplet dispensing and visualization system was developed. To control the size of droplets, monodispersed droplets with radii of ≈20 μm were dispensed using a frequency-controlled piezoelectric pulse injector onto a superhydrophobic surface, enabling the target droplets to accumulate in volume and grow in radii. The coalescence process of droplets having radii of ≈270 and ≈780 μm was imaged at a magnification of ≈25X and capture rate of 13000 fps. Surprisingly instead of completely merging together, the size-mismatched droplets underwent partial coalescence with the development of an additional satellite droplet. Specifically, the smaller droplet gave 'birth' to a secondary satellite droplet upon coalescence with the larger primary droplet due to liquid-bridge pinch-off dynamics, after which the satellite droplet bounced off upon collision with the primary droplet due to the presence of an air cushion that blocked contact between the two droplets. Meanwhile, the primary droplet continued to oscillate while the bouncing satellite droplet returned to the surface and eventually bounced off (moving direction is identified with arrows). Our work not only presents a powerful platform capable of both controlling and visualizing droplet coalescence hydrodynamics, but also provides insights into the flow hydrodynamics of droplets undergoing partial coalescence.

scholarly journals Numerical Investigation of the Effect of Air Leaking into the Working Fluid on the Performance of a Steam Ejector

2021 ◽  
Vol 11 (13) ◽  
pp. 6111
Author(s):  
He Li ◽  
Xiaodong Wang ◽  
Jiuxin Ning ◽  
Pengfei Zhang ◽  
Hailong Huang
Keyword(s):  
Flow Structure ◽  
Mass Fraction ◽  
Internal Flow ◽  
Working Fluid ◽  
Physical Parameters ◽  
Entrainment Ratio ◽  
Air Mass ◽  
Steam Ejector ◽  
Primary Fluid ◽  
Mass Fractions

This paper investigated the effect of air leaking into the working fluid on the performance of a steam ejector. A simulation of the mixing of air into the primary and secondary fluids was performed using CFD. The effects of air with a 0, 0.1, 0.3 and 0.5 mass fraction on the entrainment ratio and internal flow structure of the steam ejector were studied, and the coefficient distortion rates for the entrainment ratios under these air mass fractions were calculated. The results demonstrated that the air modified the physical parameters of the working fluid, which is the main reason for changes in the entrainment ratio and internal flow structure. The calculation of the coefficient distortion rate of the entrainment ratio illustrated that the air in the primary fluid has a more significant impact on the change in the entrainment ratio than that in the secondary fluid under the same air mass fraction. Therefore, the air mass fraction in the working fluid must be minimized to acquire a precise entrainment ratio. Furthermore, this paper provided a method of inspecting air leakage in the experimental steam ejector refrigeration system.

Slurry flow characteristics numerically analysed using sand water mixture

2021 ◽  
Author(s):  
Varinder Singh ◽  
Satish Kumar ◽  
Dwarikanath Ratha
Keyword(s):  
Flow Characteristics ◽  
Water Mixture ◽  
Slurry Flow

scholarly journals Effect of Air Injection on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

Processes ◽  
2021 ◽  
Vol 9 (7) ◽  
pp. 1182
Author(s):  
Seung-Jun Kim ◽  
Yong Cho ◽  
Jin-Hyuk Kim
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Air Injection ◽  
Low Flow ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Vortex Rope

Under low flow-rate conditions, a Francis turbine exhibits precession of a vortex rope with pressure fluctuations in the draft tube. These undesirable flow phenomena can lead to deterioration of the turbine performance as manifested by torque and power output fluctuations. In order to suppress the rope with precession and a swirl component in the tube, the use of anti-swirl fins was investigated in a previous study. However, vortex rope generation still occurred near the cone of the tube. In this study, unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted with a scale-adaptive simulation shear stress transport turbulence model. This model was used to observe the effects of the injection in the draft tube on the unsteady internal flow and pressure phenomena considering both active and passive suppression methods. The air injection affected the generation and suppression of the vortex rope and swirl component depending on the flow rate of the air. In addition, an injection level of 0.5%Q led to a reduction in the maximum unsteady pressure characteristics.

Magnetophoresis Effects on the Flow Characteristics of Oil-Based Ferrofluids in Rectangular Enclosures

2015 ◽  
Vol 15 (10) ◽  
pp. 7451-7456
Author(s):  
Hyeon-Seok Seo ◽  
Jin-Hyo Boo ◽  
Youn-Jea Kim
Keyword(s):  
Magnetic Fields ◽  
Permanent Magnet ◽  
Flow Characteristics ◽  
Flow Fields ◽  
Working Fluid ◽  
Magnetic Flux Density ◽  
Magnetic Field Direction ◽  
Analysis Model ◽  
Rectangular Enclosure ◽  
Flow Phenomena

This study numerically investigated the flow characteristics in a rectangular enclosure filled with oil-based ferrofluid (EFH-1, Ferrotec.) under the influence of external magnetic fields. The rectangular enclosure contained obstacles with different shapes, such as a rectangle and a triangle mounted on the top and bottom wall surfaces. In order to generate external magnetic fields, a permanent magnet was located in the lower part of the rectangular enclosure, and its direction was selected to be either horizontal or vertical. Our results showed that the ferrofluid flow fields were affected by the applied external magnetic field direction and eddy flow phenomena in the working fluid were generated in the vicinity of high magnetic flux density distributions, such as at the edge of the permanent magnet. It was also confirmed that the magnetophoretic force distributions in the analysis model played a significant role in the development of the ferrofluid flow fields.

Internal Flow Characteristics of a Plastic Kaolin Suspension During Extrusion

2011 ◽  
Vol 95 (2) ◽  
pp. 494-501 ◽  
Author(s):  
Brooks D. Rabideau ◽  
Pascal Moucheront ◽  
François Bertrand ◽  
Stéphane Rodts ◽  
Yannick Mélinge ◽  
...  
Keyword(s):  
Flow Characteristics ◽  
Internal Flow ◽  
Kaolin Suspension

A particle image velocimetry study on the pulsatile flow characteristics in straight tubes with an asymmetric bulge

2000 ◽  
Vol 214 (5) ◽  
pp. 655-671 ◽  
Author(s):  
S C M Yu ◽  
J B Zhao
Keyword(s):  
Particle Image Velocimetry ◽  
Steady Flow ◽  
Pulsatile Flow ◽  
Flow Condition ◽  
Particle Image ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Working Fluid ◽  
Flow Conditions ◽  
Image Velocimetry

Flow characteristics in straight tubes with an asymmetric bulge have been investigated using particle image velocimetry (PIV) over a range of Reynolds numbers from 600 to 1200 and at a Womersley number of 22. A mixture of glycerine and water (approximately 40:60 by volume) was used as the working fluid. The study was carried out because of their relevance in some aspects of physiological flows, such as arterial flow through a sidewall aneurysm. Results for both steady and pulsatile flow conditions were obtained. It was found that at a steady flow condition, a weak recirculating vortex formed inside the bulge. The recirculation became stronger at higher Reynolds numbers but weaker at larger bulge sizes. The centre of the vortex was located close to the distal neck. At pulsatile flow conditions, the vortex appeared and disappeared at different phases of the cycle, and the sequence was only punctuated by strong forward flow behaviour (near the peak flow condition). In particular, strong flow interactions between the parent tube and the bulge were observed during the deceleration phase. Stents and springs were used to dampen the flow movement inside the bulge. It was found that the recirculation vortex could be eliminated completely in steady flow conditions using both devices. However, under pulsatile flow conditions, flow velocities inside the bulge could not be suppressed completely by both devices, but could be reduced by more than 80 per cent.

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