Hydrodynamics of a submerging drop: Lined structures on the crown surface

Macrophotography and high-speed videofilming are used to investigate the material transfer in a falling drop upon collision with the surface of a fluid at rest. In the experiments the drops of colored water, milk, mineral oil, and seed oil fell in pure or colored water. Emphasis was placed on recording the pattern of the drop material spreading over the surface of the receiving fluid. On the continuous surface of the primary cavity and the crown the drop material is concentrated in the form of thin fibers which form a regular streaky or netlike pattern in which triangular, quadrangular, and pentagonal cells are expressed. The cell rows are ordered in the form of layers on the lateral walls and the bottom of the cavity. The fiber dimensions and the degree of their expressiveness vary in the process of flow evolution. The upper row of structures on the crown surface is formed by vertical fibers.     

穿透现象和液滴形状的关系

拍摄了下落水滴周期性形变的照片，穿透现象（指水滴和水面碰撞后变成环状，穿入水中，直到池底）是在水滴为扁椭球形状或接近扁椭球形状时和水面碰撞发生的．表面张力是改变水滴形状的手段     

A comparison between spray cooling and film flow cooling during the rewetting of a hot surface

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics to investigate the rewetting of a hot surface. The rewetting of the hot surface by spray cooling has been analyzed in previous works. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., drops having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, drop diameter and velocity, and impact point of the spray. The working feature of the spray is based on the varicose rupture of the liquid jet: imposing a periodic (symmetrical) perturbation with appropriate amplitude and frequency on the jet surface, the flow is “constrained” to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper, previous results with spray cooling are compared with experimental runs in which the spray injection is replaced with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and using a digital image system based on the video image registered during the runs.     

Drop impact on a hot surface: effect of a polymer additive

The impact of a drop on a hot surface is studied for Weber numbers between 20 and 220, and wall temperatures between 120 and 180°C. Drops of pure water are compared with drops of a dilute polyethylene oxide water solution (0.02% M). The additive is shown to inhibit drop splashing, the ejection of secondary droplets and mist formation. As previously observed, the polymer can also prevent drops from bouncing off a cold wall. This is no longer true if the wall is above the dynamic Leidenfrost temperature, which is lower for the polymer solution.     

Evaluation of FFT-based cross-correlation algorithms for PIV in a periodic grooved channel

The design of a pneumatic droplet generator to produce small (~0.2 mm diameter) water droplets on demand is described. It consists of a cylindrical, liquid-filled chamber with a small nozzle set into its bottom surface, connected to a gas cylinder through a solenoid valve. Rapidly opening and closing the valve sends a pressure pulse to the liquid, ejecting a single droplet through the nozzle. Gas in the chamber escapes through a vent hole so that the pressure drops rapidly and more droplets do not emerge. We photographed droplets as they emerged from the nozzle, and recorded pressure fluctuations in the chamber. We determined the duration of the pressure pulse required to generate a single drop; longer pulses produced satellite drops. The length of the water jet when its tip detached and the diameter of the droplet that formed could be predicted using results from linear stability analysis. The peak pressure in the cavity could be increased by raising the supply pressure, increasing the width of the pressure pulse, or by reducing the size of the pressure relief vent.     

Droplet production from disintegrating bubbles at water surfaces. Single vs. multiple bubbles

We experimentally determine the droplet production rate at a water surface where either single or multiple bubbles (bubbly flow) with similar mean diameters disintegrate and produce film and jet droplets. A detailed assessment of film drop production from bubbly flow is important, since most presently used correlations are based on single-bubble measurements. Moreover, jet drops––even though they contain a much larger fluid volume––are de-entrained into the water surface in most technical and geophysical applications. Detailed phase Doppler anemometry (PDA) measurements are performed in the vicinity of the water surface with long sampling times. For a considered mean diameter of approximately 3 mm, the size distribution of the non spherical bubbles is determined from photographic images. From single-bubble measurements we find, consistent with literature data, a narrow size distribution of the jet drops with a mean diameter of 477 μm. For bubbly flow, the maximum is shifted to somewhat smaller jet drop diameters (425 μm) and the production of film droplets increases significantly. We relate this increase to the coalescence of bubbles prior to their disintegration at the surface. Our results therefore show that for a fixed bubble size and gas flow rate the number of film drops entrained from a bubbly flow is underestimated, if the estimate is based on single-bubble data.     

Lagrangian analysis of consecutive images: Quantification of mixing processes in drops moving in a microchannel

A tracking method and statistical analysis is introduced to quantify the mixing of moving droplets in the Lagrangian reference frame. Aqueous microrheology samples are produced as droplets in immiscible oil using a microfluidic T-junction. Samples from initially unmixed streams of the same viscosity-fluids (water/water) or different viscosity-fluids (water/glycerin solution) are dyed with different colors to visualize their internal motions and to quantify the extent of their mixing as a function of the age in the channel. The homogeneity of the material distribution in the drop is quantified by computing skewness of pixel intensity profiles or Shannon entropy index. Such analysis is important to ensure that samples are uniformly mixed for high-throughput rheological measurements using microrheology. Samples with a high viscosity ratio mix more rapidly than those with the same viscosities and the mixing length in traversing drops in the microchannel decays exponentially with traveling displacement until the drop reaches a diffusion limit.     

The mechanism of air bubble entrainment in self-aerated flow

The mechanism by which air bubbles are brought into a quick moving waterflow with free surface was studied. The processes taking place at the surface were visualized with a stroboscope. It shows, how the bubbles are generated by falling drops. The height of fall and the velocity with which a water drop must hit the surface in order to form an air bubble can be calculated by an energy balance. Measurements of diameter, height of fall and velocity of the drops as well as of the size of the bubbles coincide well with the obtained formulae. Results are valid for pure water and slightly dirty waste water. Such flows occur in steep smooth channels, spillways and strongly inclined partially filled conduits and pipelines.     

横向气流穿过垂直下落液滴场的二维流动数值计算

对Cl2/He混合气体横向穿过垂直下落的BHP(按重量25%的KOH,25%的H2O2及50%的H2O)液滴场的化学反应流动作了数值计算.模拟的流场是气体/液滴两相流场,在气相方程中,考虑了液滴与Cl2反应产生及释放O2(1Δ)的质量源项及表示液滴对气流阻碍作用的动量源项.由于气相动量小,液滴在下落过程横向偏移小,下落...     

Collision between immiscible drops with large surface tension difference: diesel oil and water

The collision outcomes of immiscible drops with large surface tension difference, namely, a water drop and a diesel oil drop, were observed experimentally. In a near head-on collision between immiscible drops with large surface tension difference, an “overlaying” action for the drop of the smaller surface tension, i.e., the diesel oil drop, to go around the surface of the drop of the larger surface tension, i.e., the water drop, occurs during the collision. This overlaying action reduces the reflex energy for head-on collisions, making reflex separation more difficult to occur. At the same time, due to the immiscibility, the liquid bridge during stretching separation becomes narrower, which makes stretching separation easier to happen. No coalescence could be observed for a collision of Weber number greater than 60. In addition, compound drops are produced frequently.     

Transformation of the bridge during drop separation

The geometry of flows during separation of pendant drops of liquids with significantly different physical properties (alcohol, water, glycerin, oil) has been studied by high-speed video recording. The dynamics of the processes involving the formation of bridges of two characteristic shapes—slightly nonuniform in thickness and with thinning of the upper and lower ends—has been investigated. It has been shown that the shape change of the separated bridge has a number of stages determined by the properties of the liquid. As a result, the bridge is transformed into a small drop—a satellite drop.     

The fate of soluble and insoluble surfactant monolayers subjected to drop impacts

Surfactant monolayers were formed on a water surface and subjected to water drops falling from a nozzle. Surface tension was measured during these experiments to determine the effect of the drop impacts on the surfactant monolayer. The purpose of this work was to determine whether monolayers can be altered by drop impacts without the formation of a splash. Accordingly, a small fall height was used to avoid drop splashes and concomitant surfactant loss by droplet ejection. The relevance of this work pertains to the fate of surfactant monolayers during rain events. Results are presented for a soluble and insoluble surfactant. The results show that the insoluble monolayer is virtually unaffected by the drops, indicating that the monolayer immediately reforms after the drop impact. The soluble monolayer shows significant changes in measured surface tension during droplet impact when the surfactant concentration is high.     

Experimental comparison of measurement techniques for drop size distributions in liquid/liquid dispersions

An online measurement technique for drop size distribution in stirred tank reactors is needed but has not yet been developed. Different approaches and different techniques have been published as the new standard during the last decade. Three of them (focus beam reflectance measurement, two-dimensional optical reflectance measurement techniques and a fiber optical FBR sensor) are tested, and their results are compared with trustful image analysis results from an in situ microscope. The measurement of drop sizes in liquid/liquid distribution is a major challenge for all tested measurement probes, and none provides exact results for the tested system of pure toluene/water compared to an endoscope. Not only the size analysis but also the change of the size over time gives unreasonable results. The influence of the power input on the drop size distribution was the only reasonable observation in this study. The FBR sensor was not applicable at all to the used system. While all three probes are based on laser back scattering, the general question of the usability of this principle for measuring evolving drop size distributions in liquid/liquid system is asked. The exterior smooth surface of droplets in such systems is leading to strong errors in the measurement of the size of the drops. That leads to widely divergent results. A different measurement principle should be used for online measurements of drop size distributions than laser back scattering.     

Experimental investigation of phase inversion in an oil–water flow through a horizontal pipe loop

An experimental study was made of phase inversion in an oil–water flow through a horizontal pipe loop. The experiments started with the flow of a single liquid through the pipe loop; thereafter the second liquid was gradually added (using different injectors and different injection flow rates) until inversion took place. It was found that in this way the point of inversion could be postponed to high values of the dispersed phase volume fraction (>0.8). Samples were taken from the flowing mixture and inspected with the aid of a microscope. Multiple drops consisting of oil droplets in water drops were observed, but multiple drops consisting of water droplets in oil drops were never found. The significance of these observations for the phase inversion mechanism is discussed.     

A Eulerian/Lagrangian model to calculate the evolution of a water droplet spray

We introduce a Eulerian/Lagrangian model to compute the evolution of a spray of water droplets inside a complex geometry. To take into account the complex geometry we define a rectangular mesh and we relate each mesh node to a node function which depends on the location of the node. The time-dependent incompressible and turbulent Navier-Stokes equations are solved using a projection method. The droplets are regarded as individual entities and we use a Lagrangian approach to compute the evolution of the spray. We establish the exchange laws related to mass and heat transfer for a droplet by introducing a mass transfer coefficient and a heat transfer coefficient. The numerical results from our model are compared with those from the literature in the case of a falling droplet in the atmosphere and from experimental investigation in a wind tunnel in the case of a polydisperse spray. The comparison is fairly good. We present the computation of a water droplet spray inside a complex and realistic geometry and determine the characteristics of the spray in the vicinity of obstacles.     

A pneumatic droplet-on-demand generator

The design of a pneumatic droplet generator to produce small (~0.2 mm diameter) water droplets on demand is described. It consists of a cylindrical, liquid-filled chamber with a small nozzle set into its bottom surface, connected to a gas cylinder through a solenoid valve. Rapidly opening and closing the valve sends a pressure pulse to the liquid, ejecting a single droplet through the nozzle. Gas in the chamber escapes through a vent hole so that the pressure drops rapidly and more droplets do not emerge. We photographed droplets as they emerged from the nozzle, and recorded pressure fluctuations in the chamber. We determined the duration of the pressure pulse required to generate a single drop; longer pulses produced satellite drops. The length of the water jet when its tip detached and the diameter of the droplet that formed could be predicted using results from linear stability analysis. The peak pressure in the cavity could be increased by raising the supply pressure, increasing the width of the pressure pulse, or by reducing the size of the pressure relief vent.     

Forces acting on water droplets falling in oil under the influence of an electric field: numerical predictions versus experimental observations

The combination of an electric field and a moderate turbulent flow is a promising technique for separating stable water–oil emulsions. Field-induced charges on the water droplets will cause adjacent droplets to align with the field and attract each other. The present work describes the forces that influence the kinematics of droplets falling in oil when exposed to an electric field. Mathematical models for these forces are presented and discussed with respect to a possible implementation in a multi-droplet Lagrangian framework. The droplet motion is mainly due to buoyancy, drag, film-drainage, and dipole–dipole forces. Attention is paid to internal circulations, non-ideal dipoles, and the effects of surface tension gradients.Experiments are performed to observe the behavior of a droplet falling onto a stationary one. The droplet is exposed to an electric field parallel to the direction of the droplet motion. The behavior of two falling water droplets exposed to an electric field perpendicular to the direction of their motion is also investigated until droplet coalescence. The droplet motion is recorded with a high-speed CMOS camera. The optical observations are compared with the results from numerical simulations where the governing equations for the droplet motion are solved by the RK45 (Runge Kutta) Fehlberg method with step-size control and low tolerances. Results, using different models, are compared and discussed in detail. A framework is otlined to describe the kinematics of both a falling rigid spherical particle and a fluid droplet under the influence of an electric field.     

Effect of impingement angle on the outcome of single water drop impact onto a plane water surface

Experiments of single water drop impact onto a plane water surface were carried out to investigate the effect of impingement angle on the total mass of secondary drops produced during the collision. When the impingement angle (the angle between the velocity vector of primary drop and the normal vector to water surface) was less than 50°, an increase in the impingement angle led to a remarkable increase in the total mass of secondary drops; this could be attributed to a significant increase in the secondary drop size. However, no secondary drop was observed within the experimental ranges tested when the impingement angle exceeded 70°.     

Fluctuating flow in a liquid layer and secondary spray created by an impacting spray

A spray impacting onto a wall produces a flow of secondary droplets. For relatively sparse spray these secondary droplets are produced by the splashing of the impacting drops and their interactions. For dense sprays, like Diesel injection sprays, these secondary droplets are created by the fluctuating liquid film created on the wall. In the present paper hydrodynamic models are presented for these two extreme cases. The velocities of the secondary droplets produced by the crown splash in a sparse spray are described theoretically. Next, the fluctuations in the motion of the liquid film created by a dense impacting spray are analyzed statistically. This motion yields the formation of finger-like jets, as observed in experiments of a Diesel spray impacting onto a rigid wall. The characteristic size and velocity of the film fluctuations are estimated. These two theoretical models are validated by comparison with the experimental data.