Air evolution during drop impact on liquid pool

CFD-DPTM-VOF numerical simulation of particle motion and entrainment under the action of single and double bubbles

Recently, an aqueous discharge reactor was developed to facilitate reformation of liquid fuels by in-liquid plasma. To gain a microscopic understanding of the physical elements behind this aqueous reactor, we investigate nanosecond discharges in liquid n-heptane with single and double gaseous bubbles in the gap between electrodes. We introduce discharge probability (DP) to characterize the stochastic nature of the discharges, and we investigate the dependence of DP on the gap distance, applied voltage, gaseous bubble composition, and the water content in n-heptane/distilled-water emulsified mixtures. Propagation of a streamer through the bubbles indicates no discharges in the liquids. DP is controlled by the properties of the gaseous bubble rather than by the composition of the liquid mixture in the gap with a single bubble; meanwhile, DP is determined by the dielectric permittivity of the liquid mixture in the gap with double bubbles, results that are supported by static electric field simulations. We found that a physical mechanism of increasing DP is caused by an interaction between bubbles and an importance of the dielectric permittivity of a liquid mixture on the local enhancement of field intensity. We also discuss detailed physical characteristics, such as plasma lifetime and electron density within the discharge channel, by estimating from measured emissions with a gated-intensified charge-coupled device and by using spectroscopic images, respectively.

In this chapter drop impacts onto a liquid layer of the same liquid as in the drop are considered. The chapter begins with consideration of such weak drop impacts on a liquid layer that they result only in capillary waves propagating over the surface. An interesting feature of these waves is that they are self-similar (Section 6.1). In the following Section 6.2 crown formation in strong (high-velocity) impacts onto thin liquid films is considered. Normal and oblique impacts of a single drop onto a wet wall are studied, as well as crown–crown interaction in sprays impacting the wall. Also, the evolution of the free rim on top of the crown is described. Then, in Section 6.3 drop impacts onto a thick liquid layer are considered and the dynamics of the crater formation is explained. Drop impacts onto a wet wall leave a residual liquid film on the wall which is addressed in Section 6.4. Drop impacts onto deep liquid pools produce a plethora of interesting morphological structures considered in Section 6.5. In the following Section 6.6 bending instability of a free rim is considered and the splashing mechanism is discussed. Splashing resulting from impacts of drop trains one-by-one is discussed in Section 6.7, where its physical mechanism and the link to splashing of a single drop impacting onto a liquid layer are elucidated. Several other regimes of drop impact are also mentioned. Drop Impact onto Thin Liquid Layer on a Wall: Weak Impacts and Self-similar Capillary Waves Consider patterns of capillary waves propagating over the free surface of a thin liquid film from the point where it was impacted normally by a tiny droplet or a stick (Fig. 6.1), as an example of a relatively weak (low-velocity) impact. For scales of the order of several millimeters the gravity effect on these waves is negligibly small, and for time scales of the order of several milliseconds viscosity effects can also be neglected.

We present experiments showing vertical jetting from the apex of a viscous drop which impacts onto a pool of lower viscosity liquid. This jet is produced by the ejecta sheet which emerges from the free surface of the pool, and moves up and wraps around the surface of the drop. When this sheet of liquid converges and collides at the top apex of the drop it produces a thin upward jet at velocities of more than 10 times the drop impact velocity. This jetting occurs for a limited range of impact conditions, where the ejecta speed is sufficient for the sheet to travel around the entire drop periphery, but not so fast that it separates from the drop surface. The lower bound for the jetting region is thereby set by a minimal Reynolds number, but the upper bounds are subject to a maximum-Weber-number criterion. The strongest observed jets appear for viscous drops impacting onto liquid pools with the lowest viscosity as well as lowest surface tension, such as acetone and methanol. Jetting has also been observed for drops which are immiscible with the pool liquid, under a different range of impact conditions. However, jetting is never observed for pools of water, as the surface tension is then significantly larger than that of the drop. We believe that Marangoni stresses act in this case to promote separation of the sheet to prevent the jetting. A movie is available with the online version of the paper.

The penetration of drop-formed vortex rings into pools of liquid

The splashing phenomenon associated with the impact of a liquid drop on a liquid pool is investigated in this study using the volume of fluid method. The different outcomes of this phenomena largely depend on the height (/depth) of a liquid pool and the impinging drop velocity. The impingement angle, drop shape, fluid properties, and other non-isothermal effects also play a role, but we have eliminated those dependencies by considering no variation in these parameters. The different phenomena that are observed when a drop impacts a liquid pool are controlled by (i) crater depth and wave-swell (rim of the crater) expansion, (ii) wave-swell retraction followed by crater side retraction, and (iii) crater base retraction. During splashing, a deep crater is produced in the receiving liquid after the drop impact. At its rim, a crown-like cylindrical liquid film is ejected out of the crater. Small droplets are normally shed from this rim. It is seen that the depth of the pool has dramatic effects on the dynamics of the crown formed during splashing. When observed even more comprehensively, the physical attributes of the crown, such as crown height and crown radius, are found to strongly relate to the velocity of the falling drop. Finally, we try to demarcate the regions of splashing with and without the formation of secondary droplets on the regime map of Weber number–dimensionless pool depth.

PurposeThe purpose of this paper is to present a numerical approach for investigating different phenomena during multiple liquid drop impact on air‐water interface.Design/methodology/approachThe authors have used the coupled level‐set and volume‐of‐fluid (CLSVOF) method to explore the different phenomena during multi‐drop impact on liquid‐liquid interface. Complete numerical simulation is performed for two‐dimensional incompressible flow, which is described in axisymmetric coordinates.FindingsDuring drop pair impact at very low impact velocities, the process of partial coalescence is observed where the process of pinch off is different than single drop impact. At higher impact velocities, phenomena such as bubble entrapment are observed.Originality/valueIn this paper, a new approach has been developed to simulate consecutive drop impact on a liquid pool.

Role of chemical reaction and drag force during drop impact gelation process

Velocity characteristics of microjets generated by double bubbles near a rigid wall under ultrasound

Zeolitic imidazolate frameworks-based porous liquids with low viscosity for CO2 and toluene uptakes

The present annual report summarises the purpose of the project and the ongoing investigations performed at the Institut fur Industrielle Fertigung und Fabrikbetrieb Universitat Stuttgart (IFF) on the numerical study of paint drop impacting onto dry solid surfaces. Both Newtonian and yield-stress viscous droplets were applied. Detailed numerical observations of the drop impact dynamics with the focus of air entrapment were obtained. It has been found that at the early stage of the droplet spreading there is no contact line movement, but only direct contact of the droplet outline with the substrate, which results in the formation of an air disc under the impact point. The maximum air disc is reached, when the drop spreading is driven by the movement of the fully wetted contact line. Numerical results showed much more bubble entrapment at the interface between liquid and solid for Newtonian droplets. For shear thinning non-Newtonian fluids the created air disc and air bubbles during drop spreading are reduced tremendously because of the quite low liquid viscosity. The effects of the drop properties, impact velocity and static contact angles on the maximum air disc and on the air bubble release from the droplet film were analysed.

Thermo-hydrodynamic analysis of drop impact calcium alginate gelation process

Coalescence of non-spherical drops with a liquid surface

This paper is concerned with the interaction between a weak-plane shock wave and bubbles. The behavior of a liquid microjet produced by the collapse of single and double bubbles was observed using a water shock tube, Ima-Con high speed camera and a PVDF pressure sensor with a high frequency response. The behavior of single collapsing bubble agrees qualitatively with previous experimental results, and it is shown that the present experimental method is effective in investigating the bubble collapse and generation of impulsive pressure. The speed of a microjet within the bubble is estimated to be about 100 m/s, or about ten times as much as that of a microjet penetrating the other side of the bubble surface. In the case of double collapsing bubbles the direction of microjet formation is different from the case of single bubble collapse.

A large amount of paramount research on interface instability has been carried out in fluid mechanics field, particularly on Richtmyer-Meshkov instability (RM instability), which has strong background and research value in both academic and engineering applications. The scholarly community has given the issue of RM instability a great deal of attention ever since it was raised. In the event of a severe accident, the high-temperature molten fuel and coolant may come into contact leading to a potential steam explosion. Then, the high-temperature melt’s surface may be triggered, endangering the integrity of the reactor cavity outside the pressure vessel, as a result of the RM instability due to the interaction between the explosion shock wave and the vapor bubble/coolant interface initialized by the coolant’s evaporation. In this study, the RM instability of the vapor-liquid interface driven by hydrodynamic effect under single and double bubbles’ condition is numerically modeled in two-dimensional geometry. The vapor-liquid interface is tracked by using the coupled Volume of Fluid (VOF) and Level-Set methods. We utilize mesh adaptive techniques, refining the mesh appropriately based on changes in the gas-liquid interface during the calculation process. This enhances the resolution of the flow field adjacent to the vapor-liquid interface, facilitating a clearer observation of the occurrence of RM instability. Under varying shock wave strengths propagating in the same direction along the buoyancy force, the velocity field, pressure field, and vapor-liquid two-phase distribution in the calculation domain are obtained. The duration time for the bubble to departure from the spherical wall is discovered to have an inverse proportional relation with the triggering pressure, the larger the triggering pressure (0.5–2MPa), the shorter the bubble’s departure time from the spherical wall. The amplitude of the disturbance at the vapor-liquid interface starts to show at the same time that the trigger pressure increases to a threshold (1 MPa in this investigation). Entrainment of droplet in the bubble will happen as a result of RM’s instability, and the trigger pressure has a quadratic relationship with the entrained droplet’ quality. The entrainment of droplets may affect the surface heat transfer efficiency of high-temperature molten materials and worsen heat transfer. The bigger the entrainment mass of the droplet in the bubble, the earlier the interfacial disturbance emerges, and the more fully the interfacial perturbation develops with an increase in triggering pressure. Furthermore, in the double bubble condition, both two bubbles will still have RM instability and droplet entrainment, but the bubble shape at the end of the bubble detachment from the spherical wall will be quite different from that in the single bubble condition. This work can offer a phenomenological reference for the development of RM instability at the vapor-liquid interface, and provide technical assistance in clarifying the triggering stage of fuel-coolant interaction in severe accident.