Impingement of high-speed cylindrical droplets embedded with an air/vapour cavity on a rigid wall: numerical analysis

2019 ◽  
Vol 864 ◽  
pp. 1058-1087 ◽  
Author(s):  
Wangxia Wu ◽  
Bing Wang ◽  
Gaoming Xiang
Keyword(s):  
Shock Wave ◽  
High Speed ◽  
Spray Coating ◽  
Rigid Wall ◽  
Liquid Jets ◽  
Two Phase ◽  
Transmitted Waves ◽  
Mean Pressure ◽  
Local Pressures ◽  
Air Cavities

The high-speed impingement of hollow droplets embedded with a cavity has fundamental applications in various scenarios, such as in spray coating and biomedical engineering. The impingement dynamics is modulated by the wrapping medium, different from that of denser solid droplets. With air and vapour cavities, the impingement of two kinds of hollow cylindrical droplets is simulated in the present study to investigate the morphology and physical mechanisms regarding droplet and cavity dynamics. The compressible two-phase Eulerian model is used to couple with the phase transition procedure. The results detail the evolution of droplets and collapsing dynamics of the two kinds of cavities. Processes are captured in which the impinging water-hammer shock wave interacts with the cavity, and vertical liquid jets are induced to impact the embedded cavity. For the case of the air cavity, a transmitted shock wave is formed and propagates inside the cavity. The air cavities are compressively deformed and broken into a series of small cavities. Subsequently, a range of intermittent collapsing compression wavelets are generated due to the interface collapse driven by local jets. As for the vapour cavity in the saturated state, initially, once it is impacted by the impinging shock wave, it gradually shrinks accompanied by local condensation but without generation of transmitted waves. Following the first interaction between the lower and upper surfaces of the cavity, the vapour cavity undergoes continuous condensation and collapse with repeated interface fusion. The vapour cavity finally turns into liquid water blended into the surroundings, and the strong collapsing shock waves are expanded inside the droplet. The radius ratios and initial impinging speeds are chosen to analyse the variation of the collapsing time, maximum collapsing pressure and mean pressure on the rigid wall. The pressure withstood by the wall due to the collapsing cavity increases with the initial size of the cavity and initial impinging speed. The maximum local pressures in the entire fluids and the mean pressure on the wall during the collapsing of the vapour cavities are higher than those for the air cavities.

Tandem cavity collapse in a high-speed droplet impinging on a constrained wall

2021 ◽  
Vol 932 ◽  
Author(s):  
Wangxia Wu ◽  
Bing Wang ◽  
Qingquan Liu
Keyword(s):  
Shock Wave ◽  
High Speed ◽  
High Energy ◽  
Tandem Array ◽  
Small Scale ◽  
Two Phase ◽  
Collapse Pressure ◽  
Cavity Collapse ◽  
Pressure Peak ◽  
Energy Convergence

A focusing shock wave can be generated during the high-speed impact of a droplet on a $180^\circ$ constrained wall, which can be used to realise energy convergence on a small scale. In this study, to realise high energy convergence and peak pressure amplification, a configuration of droplets embedded with cavities is proposed for high-speed impingement on a $180^\circ$ constrained wall. A multicomponent two-phase compressible flow model considering the phase transition is used to simulate the high-speed droplet impingement process. The properties of the embedded cavities can influence the collapse pressure peak. The collapse of an embedded single air cavity or vapour cavity, as well as the cavities in a tandem array, is simulated in this study. The physical evolution mechanisms of the impinging droplet and the embedded cavities are investigated qualitatively and quantitatively by characterising the focusing shock wave generated inside the droplet and its interaction with different cavity configurations. The interaction dynamics between the cavities is analysed and a theoretical prediction model for the intensity of each cavity collapse in the tandem array is established. With the help of this theoretical model, the influencing factors for the collapse intensities of the tandem cavities are identified. The results reveal that the properties of the initial shock wave and the interval between the cavities are two predominant factors for the amplification of the collapse intensity. This study enhances the understanding of the physical process of shock-induced tandem-cavity collapse.

Compressibility Effects in the Gas Phase for Unsteady Annular Two-Phase Flow in a Microchannel

2006 ◽  
Author(s):  
Alexandru Herescu ◽  
Jeffrey S. Allen
Keyword(s):  
Shock Wave ◽  
Gas Phase ◽  
High Speed ◽  
Gas Flow ◽  
Two Phase Flow ◽  
Flow Behavior ◽  
Flow Section ◽  
Phase Flow ◽  
Two Phase ◽  
Gas Liquid Flow

High speed microscopy experiments investigating two-phase (gas-liquid) flow behavior in capillary-scale systems, that is, systems where capillary forces are important relative to gravitational forces, have revealed a unique unsteady annular flow with periodic destabilization of the gas-liquid interface. Standing waves develop on the liquid film and grow into annular lobes similar with those observed in low-speed two-phase flow. The leading face of the lobe will decelerate and suddenly become normal to the wall of the capillary, suggesting the possibility of a shock wave in the gas phase at a downstream location from the minimum gas flow section. Visualization of the naturally occurring convergent-divergent nozzle-like structures as well as a discussion on the possibility of shock wave formation are presented.

On high-speed impingement of cylindrical droplets upon solid wall considering cavitation effects

2018 ◽  
Vol 857 ◽  
pp. 851-877 ◽  
Author(s):  
Wangxia Wu ◽  
Gaoming Xiang ◽  
Bing Wang
Keyword(s):  
Phase Transition ◽  
Shock Wave ◽  
High Speed ◽  
Solid Wall ◽  
Fluid Model ◽  
Droplet Diameter ◽  
Droplet Surface ◽  
Rarefaction Waves ◽  
Two Phase ◽  
The Impact

The high-speed impingement of droplets on a wall occurs widely in nature and industry. However, there is limited research available on the physical mechanism of the complicated flow phenomena during impact. In this study, a simplified multi-component compressible two-phase fluid model, coupled with the phase-transition procedure, is employed to solve the two-phase hydrodynamics system for high-speed cylindrical droplet impaction on a solid wall. The threshold conditions of the thermodynamic parameters of the fluid are established to numerically model the initiation of phase transition. The inception of cavitation inside the high-speed cylindrical droplets impacting on the solid wall can thus be captured. The morphology and dynamic characteristics of the high-speed droplet impingement process are analysed qualitatively and quantitatively, after the mathematical models and numerical procedures are carefully verified and validated. It was found that a confined curved shock wave is generated when the high-speed cylindrical droplet impacts the wall and this shock wave is reflected by the curved droplet surface. A series of rarefaction waves focus at a position at a distance of one third of the droplet diameter away from the top pole due to the curved surface reflection. This focusing zone is identified as the cavity because the local liquid state satisfies the condition for the inception of cavitation. Moreover, the subsequent evolution of the cavitation zone is demonstrated and the effects of the impact speed, ranging from $50$ to $200~\text{m}~\text{s}^{-1}$ , on the deformation of the cylindrical droplet and the further evolution of the cavitation were studied. The focusing position, where the cavitation core is located, is independent of the initial impaction speed. However, the cavity zone is enlarged and the stronger collapsing wave is induced as the impaction speed increases.

Bubble Dynamics Observed in a Gas-Liquid Venturi Flow

2011 ◽  
Author(s):  
Wataru Nishi ◽  
Masanori Nogami ◽  
Hiroyuki Takahira
Keyword(s):  
High Speed ◽  
Bubble Dynamics ◽  
Propagation Speed ◽  
Video Camera ◽  
Bubble Surface ◽  
Liquid Jets ◽  
Pressure Waves ◽  
Two Phase ◽  
Pressure Transducers ◽  
Two Bubbles

The present study deals with the experiments for the gas-liquid two-phase flow inside an acrylic Venturi tube using a high-speed video camera. Some interesting phenomena on the bubble dynamics are observed in the tube. First, the volume and surface oscillations of two interacting bubbles are observed in converging section of the tube when one bubble enters the throat. The volume oscillation of the bubble that enters the throat is caused by the detachment of the tip of the downstream surface of the bubble. The pressure wave irradiated from the bubble that enters the throat induces the volume and surface oscillations of the bubble that remains at the converging section. The parametric excitation is the reason for the surface oscillations. Second, the bubble deformations at the throat in a Venturi or a converging tube are investigated. The experiments show that two kinds of liquid jets are formed on the bubble surface; one is a forward jet that develops from the upstream surface to the downstream surface and the other is a counter jet in which the direction of the jet is opposite to the forward jet. It is shown that the counter jet occurs only when the distance between two bubbles in the throat is sufficiently short. The interactions between two bubbles cause the counter jet. It is also shown that the velocity of the forward jet becomes faster when the bubble is pinched off more upstream in the converging section. Finally, the propagations of the pressure waves are measured with pressure transducers. The impulsive pressure associated with the collapse of cavitation bubble cloud is measured when a bubble enters the throat of the tube. Also, the propagation speed of pressure waves is evaluated with the cross-correlation function. The results show that the propagation speed and damping of the pressure waves are dependent on the number density of bubbles at the downstream part in the tube.

Low-pressure flashing mechanisms in iso-octane liquid jets

2007 ◽  
Vol 572 ◽  
pp. 121-144 ◽  
Author(s):  
M. M. VIEIRA ◽  
J. R. SIMÕES-MOREIRA
Keyword(s):  
Shock Wave ◽  
Two Phase Flow ◽  
Liquid Core ◽  
Wave Structure ◽  
Low Pressure ◽  
Liquid Jets ◽  
Phase Flow ◽  
Shock Wave Structure ◽  
Two Phase ◽  
Very High

This paper examines a flashing liquid regime that takes place at very high ratios of injection to discharge pressures in flow restrictions. Typically, the flashing phenomenon has been observed in laboratory experiments where a liquid flows through a short nozzle into a low-pressure chamber at a pressure value considerably lower than the liquid saturation pressure at the injection temperature. By using two visualization techniques, the schlieren and the back-lighting methods, it was possible to identify some compressible phenomena associated with the liquid flashing process from the nozzle exit section. The schlieren method was used to capture the image of a shock-wave structure surrounding a liquid core from which the phase change takes place, and the optical technique allowed us to observe the central liquid core itself. The work corroborates previous physical descriptions of flashing liquid jets to explain an observed choking behaviour as well as the presence of shock waves. According to the present analysis, flashing takes place on the surface of the liquid core through an evaporation wave process, which results from a sudden liquid evaporation in a discontinuous process. Downstream of the evaporation discontinuity, the two-phase flow reaches very high velocities, up to the local sonic speed that typically occurs at high expansion conditions, as inferred from experiments and the physical model. That sonic state is also a point of maximum mass flow rate and it is known as the Chapman–Jouguet condition. The freshly sonic two-phase flow expands freely to increasing supersonic velocities and eventually terminates the expansion process through a shock-wave structure. This paper presents experimental results at several test conditions with iso-octane.

Setting up Experiments for Analyzing Disturbances of the Head Shock Wave Due to the Presence of Particles in the Supersonic Flow

2021 ◽  
pp. 34-46
Author(s):  
S.I. Gerasimov ◽  
V.I. Erofeev ◽  
E.G. Kosyak
Keyword(s):  
Shock Wave ◽  
High Speed ◽  
Dust Particles ◽  
Particle Collisions ◽  
Two Phase ◽  
Experimental Physics ◽  
High Enthalpy ◽  
Initial Stage ◽  
Ballistic Experiment ◽  
Phase Medium

The research in the field of devices and methods of experimental physics includes, in particular, the creation of installations for conducting experiments in the physics of multiphase nonequilibrium flows. Multiphase flow around bodies is of significant importance in various fields of technology, for example, in power engineering, contact visualization methods, aerosol technologies, in the application of various coatings, etc. In the high-speed two-phase flow around bodies, the physics of particle collisions with the surface and the interaction of flying particles with the head shock wave play an important role. Most of the experiments in this area, including the passage of the model through the zone of rain, snow, dust, cooled clouds, etc., are carried out in the reverse setting. In this case, the model is fixed, and the flow is made by one or another high-enthalpy aerodynamic installation. This approach does not correspond to the initial stage of the interaction formed before the entrance to the zone of the two-phase medium, between the head shock and the incident particle. Nevertheless, for some approaches, it is of interest to visually confirm the possibility of an oncoming particle being ejected at a supersonic speed by the front of the head shock wave and to see the deceleration of the model from the shadow pattern with a change in the Mach line. The paper considers a direct ballistic experiment which by shadow visualization tests the possibility of cavern formation near a body crossing the trajectory of the atmosphere saturated with dust particles

GENERATION AND SHOCK WAVE CHARACTERISTICS OF UNSTEADY PULSED SUPERSONIC LIQUID JETS

2003 ◽  
Vol 13 (5-6) ◽  
pp. 475-498 ◽  
Author(s):  
K. Pianthong ◽  
B. E. Milton ◽  
M. Behnia
Keyword(s):  
Shock Wave ◽  
Liquid Jets ◽  
Wave Characteristics
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