Gas-Assisted Droplet Impact on a Solid Surface

2016 ◽  
Vol 138 (8) ◽  
Author(s):  
Andres J. Diaz ◽  
Alfonso Ortega
Keyword(s):  
Solid Surface ◽  
High Speed ◽  
Liquid Droplet ◽  
High Speed Photography ◽  
Droplet Deformation ◽  
Droplet Spreading ◽  
Carrier Gas ◽  
Air Jet ◽  
Vof Model ◽  
Theoretical Predictions

An experimental, numerical, and theoretical investigation of the behavior of a gas-assisted liquid droplet impacting on a solid surface is presented with the aim of determining the effects of a carrier gas on the droplet deformation dynamics. Experimentally, droplets were generated within a circular air jet for gas Reynolds numbers Reg = 0–2547. High-speed photography was used to capture the droplet deformation process, whereas the numerical analysis was conducted using the volume of fluid (VOF) model. The numerical and theoretical predictions showed that the contribution of a carrier gas to the droplet spreading becomes significant only at high Weo and when the work done by pressure forces is greater than 10% of the kinetic energy. Theoretical predictions of the maximum spreading diameter agree reasonably well with the experimental and numerical observations.

Experiments and Modeling of a Liquid Droplet Transported by a Gas Stream Impinging on a Heated Surface: Evaporative Regime

2009 ◽  
Author(s):  
Ryan P. Anderson ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Single Phase ◽  
Heat Dissipation ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Thin Liquid Film ◽  
Spray Cooling ◽  
High Speed Photography ◽  
Transfer Coefficients

Understanding the transport mechanisms involved in a single droplet impinging on a heated surface is imperative to the complete understanding of droplet and spray cooling. Evidence in the literature suggests that gas assisted sprays and mist flows are more efficient than sprays consisting only of liquid droplets. There has been few if any fundamental studies on gas-assisted droplets or spray cooling, in which a carrier gas or vapor stream propels the droplet to the target surface. The current work extends previous studies of a droplet impinging on a heated surface conducted by the same group from the single phase regime into the evaporative regime. For both regimes, understanding the transport physics due to the heat transfer from the heated surface to the droplet and then by convection and evaporation to the airflow is of fundamental importance. High-speed photography was used to capture the spreading process and yielded results that correlated well with previously published isothermal and single-phase results. The heat transfer was measured with a fitting approach by which the instantaneous temperature profile was matched to an analytic solution to determine the instantaneous value of the centerline heat transfer coefficient. A very large increase in the heat dissipation was observed when compared to previously published single-phase results. Heat transfer was optimized at Reynolds numbers that produced an optimally thin liquid film and high heat and mass transfer coefficients on the surface of the film.

scholarly journals The effects of surface tension on the spreading ratio during the impact of multiple droplets onto a hot solid surface

2018 ◽  
Vol 197 ◽  
pp. 08016
Author(s):  
Rafil Arizona ◽  
Teguh Wibowo ◽  
Indarto Indarto ◽  
Deendarlianto Deendarlianto
Keyword(s):  
Surface Tension ◽  
Ethylene Glycol ◽  
Solid Surface ◽  
High Speed ◽  
Frame Rate ◽  
Stainless Steel Surface ◽  
Droplet Spreading ◽  
Secondary Droplets ◽  
The Impact ◽  
Maximum Spreading

The impact between multiple droplets onto hot surface is an important process in a spray cooling. The present study was conducted to investigate the dynamics of multiple droplet impact under various surface tensions. Here, the ethylene glycol with compositions of 0%, 5%, and 15% was injected through a nozzle onto stainless steel surface as the multiple droplet. The solid surface was heated at the temperatures of 100 °C, 150 °C, and 200 °C. To observe the dynamics of multiple droplets, a high speed camera with the frame rate of 2000 fps was used. A technique of image processing was developed to determine the maximum droplet spreading ratio. As the result, the surface tension contributes significantly to maximum spreading ratio. As the droplet surface tension decreases, the maximum spreading ratio increases. The maximum spreading ratio appears when the percentage of the ethylene glycol is 15% at the temperature of 150°C. From the visual observation, it is shown that a slower emergence of secondary droplets (droplet splashing) is carried out under a lower surface tension. Hence, surface tension plays an important role on the behavior of emerging secondary droplets. Furthermore, results of the experiments are useful for the validation of available previous CFD models.

scholarly journals Gliding on a layer of air: impact of a large-viscosity drop on a liquid film

2019 ◽  
Vol 878 ◽  
Author(s):  
K. R. Langley ◽  
S. T. Thoroddsen
Keyword(s):  
Liquid Film ◽  
Impact Velocity ◽  
Solid Surface ◽  
High Speed ◽  
Solid Substrate ◽  
Compliant Surfaces ◽  
Theoretical Predictions ◽  
Viscous Drop ◽  
Water Drops ◽  
Air Film

In this paper we contrast the early impact stage of a highly viscous drop onto a liquid versus a solid substrate. Water drops impacting at low velocities can rebound from a solid surface without contact. This dynamic is mediated through lubrication of a thin air layer between the liquid and solid. Drops can also rebound from a liquid surface, but only for low Weber numbers. Impacts at higher velocities in both cases lead to circular contacts which entrap an air disc under the centre of the drop. Increasing the drop viscosity produces extended air films for impacts on a smooth solid surface even for much larger velocities. These air films eventually break through random wetting contacts with the solid. Herein we use high-speed interferometry to study the extent and thickness profile of the air film for a large-viscosity drop impacting onto a viscous film of the same liquid. We demonstrate a unified scaling of the centreline height of the air film for impacts on both solid and liquid, when using the effective impact velocity. On the other hand, we show that the large-viscosity liquid film promotes air films of larger extent. Furthermore, the rupture behaviour becomes fundamentally different, with the air film between the two compliant surfaces being more stable, lacking the random wetting patches seen on the solid. We map the parameter range where these air films occur and explore the transition from gliding to ring contact at the edge of the drop dimple. After the air film ruptures, the initial contraction occurs very rapidly and for viscosities greater than 100 cSt the retraction velocity of the air film is ${\sim}0.3~\text{m}~\text{s}^{-1}$, independent of the liquid viscosity and impact velocity, in sharp contrast with theoretical predictions.

The mechanism of a splash on a dry solid surface

2011 ◽  
Vol 690 ◽  
pp. 148-172 ◽  
Author(s):  
Shreyas Mandre ◽  
Michael P. Brenner
Keyword(s):  
Solid Surface ◽  
High Speed ◽  
Liquid Droplet ◽  
Contact Point ◽  
Quantitative Agreement ◽  
Gas Pressure ◽  
Liquid Viscosity ◽  
Ink Jet ◽  
Jet Printing ◽  
The Impact

AbstractFrom rain storms to ink jet printing, it is ubiquitous that a high-speed liquid droplet creates a splash when it impacts on a dry solid surface. Yet, the fluid mechanical mechanism causing this splash is unknown. About fifty years ago it was discovered that corona splashes are preceded by the ejection of a thin fluid sheet very near the vicinity of the contact point. Here we present a first-principles description of the mechanism for sheet formation, the initial stages of which occur before the droplet physically contacts the surface. We predict precisely when sheet formation occurs on a smooth surface as a function of experimental parameters, along with conditions on the roughness and other parameters for the validity of the predictions. The process of sheet formation provides a semi-quantitative framework for studying the subsequent events and the influence of liquid viscosity, gas pressure and surface roughness. The conclusions derived from this framework are in quantitative agreement with previous measurements of the splash threshold as a function of impact parameters (the size and velocity of the droplet) and in qualitative agreement with the dependence on physical properties (liquid viscosity, surface tension, ambient gas pressure, etc.) Our analysis predicts an as yet unobserved series of events within micrometres of the impact point and microseconds of the splash.

A Numerical and Experimental Investigation of a Gas Propelled Liquid Droplet Impinging Onto a Heated Surface

2011 ◽  
Author(s):  
Andres Diaz ◽  
Alfonso Ortega
Keyword(s):  
Kinetic Energy ◽  
Contact Area ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Spray Cooling ◽  
High Heat Flux ◽  
Solid Contact ◽  
Droplet Spreading ◽  
Air Jet ◽  
The Impact

Due to the higher rates of heat transfer and the spatial homogeneity of heat removal that can be achieved with spray cooling, these systems have been widely proposed for cooling high heat flux electronics. In particular, gas-assisted spray cooling systems, in which a vapor phase jet propels the liquid phase droplets to a target surface, have been shown to be even more efficient in removing heat than sprays consisting of droplets alone. However, in all the studies found in the literature, in which the basic problem has been approached as a single-droplet event, only the behavior of a free falling droplet has been studied. To date, there is no fundamental investigation of the physics of gas or vapor-assisted spray cooling. To study this problem an experimental and numerical investigation of the deformation process of a liquid droplet transported by a gas stream impinging on a heated surface was performed. A preliminary study [1] has shown that increasing air jet velocities leads to an augmentation in liquid-solid contact area. Nevertheless, for low We*, the increase in droplet spreading diameter is only a consequence of the increase in droplet kinetic energy before the impact rather than the pressure and shear stress imposed by the gas during the spreading. An order of magnitude analysis showed that shear effects are negligible compared to the normal pressure of the jet. A first order analytical model of the droplet spreading behavior indicated that the jet stagnation pressure acting on the droplet surface becomes important at relatively low Weo and higher We* by contributing to the reduction in liquid film thickness and to the augmentation in liquid-solid contact area. It was shown that the work done by the gas stream in deforming the liquid droplet must be at least 10% of the initial kinetic energy of the droplet to start having a significant effect on the droplet deformation during the early stage of impact.

scholarly journals Liquid Droplet Impact on a Sonically Excited Thin Membrane

2021 ◽  
Author(s):  
Abba Abubakar ◽  
Bekir Yilbas ◽  
Hussain Al-Qahtani ◽  
Ammar Alzaydi
Keyword(s):  
High Speed ◽  
Liquid Droplet ◽  
Excitation Frequency ◽  
Membrane Dynamics ◽  
Contact Period ◽  
Restitution Coefficient ◽  
Time Duration ◽  
Hydrophobic Membrane ◽  
Droplet Spreading ◽  
Low Frequencies

Abstract Impacting droplet characteristics on hydrophobic surfaces can be altered by introducing surface oscillations. Impacting water droplet contact duration, spreading, retraction, and rebounding behaviors are examined at various sonic excitation frequencies of the hydrophobic membrane. Membrane oscillation and droplet behavior are analyzed by utilizing a high-speed camera. The restitution coefficient and membrane dynamics are formulated and the findings are compared with those of the experiments. It is found that the mode of membrane oscillation changes as the sonic excitation frequency is changed. The droplet spreading and retraction rates reduce while rebound height and restitution coefficient increase at a sonic excitation frequency of 75 Hz. However, further increase of the excitation frequency results in reduced rebound height because of increased energy dissipation on the impacted surface. The droplet contact (transition time) duration reduces as the excitation frequency increases. Increasing droplet Weber number enhances the droplet contact period on the membrane, which becomes more apparent at low frequencies of sonic excitation.

Numerical Study on High-Speed Impact Between a Water Droplet and a Deformable Solid Surface

2012 ◽  
Author(s):  
Yongqiang Han ◽  
Yonghui Xie ◽  
Di Zhang
Keyword(s):  
Solid Surface ◽  
Water Droplet ◽  
High Speed ◽  
Numerical Study ◽  
Contact Radius ◽  
Deformable Solid ◽  
Theoretical Predictions ◽  
Steam Turbine Blade ◽  
Set Up ◽  
The Impact

In this study an axisymmetric model is set up to study the impact of a spherical water droplet with a planar deformable solid surface using the Lagrange-Euler coupling method which is based on a penalty formulation. The diameter and velocity of the droplet are 0.4 mm and 500 m/s respectively, while the solid is a kind of steam turbine blade material. The generated pressure distribution in the droplet and its variation with time, the formation of lateral jet, the deformation and stress distribution in the solid are obtained and investigated. It is shown that the compressibility of the droplet and the solid plays a significant role during the impact. The water-hammer pressure and the maximum contact edge pressure are calculated and in good agreement with the existing theoretical predictions. The calculated contact radius for shock departure is larger than that of the conventional theoretical prediction, which is analyzed and attributable to the radial motion of the liquid in the compressed region. The formation of the high-speed lateral jet is calculated and the time for the observable jetting is much later than that of the shock departure. This delay is discussed and the reason needs more research. The pressure of the contact edge region remains highest even after a considerable time of shock departure and lateral jetting. In the mean time, a saucer-shaped depression is generated in the center of the impact. The stress waves in solid move faster even before shock departure in the liquid. This causes disturbance of the solid surface before the high-speed lateral jetting and provides site for the scouring action of it, and subsequently may cause material damage and erosion.

scholarly journals Lateral Diffusion of a Free Air Jet in Slot-Die Melt Blowing for Microfiber Whipping

Polymers ◽  
2019 ◽  
Vol 11 (5) ◽  
pp. 788 ◽  
Author(s):  
Sheng Xie ◽  
Wanli Han ◽  
Xufan Xu ◽  
Guojun Jiang ◽  
Baoqing Shentu
Keyword(s):  
High Speed ◽  
Lateral Diffusion ◽  
Polymer Melt ◽  
Nonwoven Material ◽  
Fiber Formation ◽  
High Speed Photography ◽  
Melt Blowing ◽  
Lateral Velocity ◽  
Air Jet ◽  
Slot Die

In melt blowing, microfibrous nonwoven material is manufactured by using high-speed air to attenuate polymer melt. The melt-blown air jet determines the process of polymer attenuation and fiber formation. In this work, the importance of lateral velocity on the fiber was first theoretical verified. The lateral diffused characteristic of the air flow field in slot-die melt blowing was researched by measuring the velocity direction using a dual-wire probe hot-wire anemometer. Meanwhile, the fiber path was captured by high-speed photography. Results showed that there existed a critical boundary of the lateral diffusion, however, air jets in the x–z plane are a completely diffused field. This work indicates that the lateral velocity in the y–z plane is one of the crucial factors for initiating fiber whipping and fiber distribution.

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