Breakup of a droplet at high velocity impacting a solid surface

We have studied the collision between a droplet of different liquids with high impact energy and a solid plate with varied surface roughness, which is characterized by a dimensionless Weber number (We, defined as the impact inertia of the droplet normalized by its surface force) extending up to 12,000 for water. To make such collision, a technique was developed to generate a single droplet with speed up to 42 m/s, which was initially driven by upstream air flow through a nozzle and accelerated to nearly the same velocity of the high-speed flow downstream. Via a high-speed photographing system, the various splashing mechanisms were investigated and a specific prompt splash on a smooth plate was found at sufficiently high We, which was different somehow from the conventionally defined one that was generally believed to occur only on a rough surface. The radius when multiple secondary droplets were shed out of the rim of the expanding lamella was found to scale almost invariantly with We at large values, whereas the coupled effect of liquid viscosity might affect the ultimate value.     

气相中双液滴正碰过程数值模拟研究

液滴碰撞现象普遍存在于动力装置燃烧室喷嘴的下游区域,影响燃料的雾化性能。为了揭示相同直径的双液滴中心碰撞机理,求解了轴对称坐标系下的N-S方程,采用VOF(Volume of Fluid)方法捕捉液滴碰撞过程中气液自由表面的演化规律。利用Qian等提供的实验结果对计算模型进行数值校验,验证了模型的准确性。在此基础上,研究了环境压强对液滴碰撞反弹后不同结果(分离和融合)的影响,分析了环境压强和Weber数对液滴碰撞分离的影响。结果表明,液滴在碰撞反弹后的状态(分离或融合)是由液滴间气膜压强与环境气动阻力共同作用的结果,环境压强对液滴碰撞分离过程基本没有影响;Weber数越大,碰撞过程中变形的幅度越大。     

Effects of fuel properties on the burning characteristics of collision-merged alkane/water droplets

The combustion characteristics of freely falling droplets, individually generated by the merging of colliding alkane and water droplets, were experimentally investigated. The outcome of the collision droplets was firstly studied and then the subsequent burning processes such as the flame appearance, ignition and burning behaviors were recorded, through either visual observation or microphotography with the aid of stroboscopic lightening. If the merged droplets were exhibited in an insertive manner, while the water droplet inserted into the alkane droplet, these yield the burning behaviors prior to the end of flame were very much similar to that of pure alkane. The burning was ended with droplet extinction for lower-C alkane, and with either droplet “flash vaporization” or extinction for hexadecane. And if the merged droplets were in adhesive manner, for hexadecane with large water content, they either could not be ignited for the large merged droplets, or be ignited with a much prolonged ignition delay, followed by a soot-reducing flame and an ending of droplet extinction for the small merged droplets. “Homogeneous” explosion was not observed in any of the tests, and “heterogeneous” explosion, induced by trapped air bubbles, occasionally occurred for merged droplets with C-atom in alkane is higher than dodecane. And the sudden disappearance of droplet definitely decreased the burning time and thus enhanced the burning intensity. Besides, the fuel mass consumption rates were increased, even in the cases that having droplet extinction, because of the enlargement of the surface area due to the stuffing of water droplet.     

An experimental study of the evaporation characteristics of emulsified liquid droplets

This paper presents the results of an experimental investigation, into the effect of water in diesel and kerosene emulsions, on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum). Experiments are performed at atmospheric pressure, and initial water volume concentrations of 10, 20, 30, and 40%. The wall temperatures ranging from 100–460 °C, to cover the entire spectrum of heat transfer characteristics from evaporation to film boiling. Results show that, qualitatively, the shapes of emulsion evaporation curves are very similar to that of pure liquids. Quantitavely, there are significant differences. The total evaporation time, for the emulsion droplets is lower than that for diesel and kerosene fuels, and decreased as water initial concentration increases, up to surface temperatures less than the critical temperature. The value of the critical surface temperature (maximum heat transfer rate), decreases as initial concentration of water increases. In the film-boiling region, the evaporation time for the emulsion droplets is higher than for diesel and kerosene droplets, at identical conditions.List of Symbols h_fg latent heat of vaporization, KJ/kg - m mass of the droplet, gm - T_b boiling temperature, °C - T_c critical temperature, °C - T_L Leidenfrost temperature, °C - T_s initial surface temperature of the hot surface, °C     

An experiment on the breakup of impinging droplets on a hot surface

Breakup characteristics of liquid droplets impinging on a hot surface are investigated experimentally with the wall temperatures in the Leidenfrost temperature range of 220–330°C for n-decane fuel. Factors influencing droplet breakup are wall temperature, impinging velocity, droplet diameter and impinging angle. The 50% breakup probability shows that the impinging velocity decreases linearly with the droplet diameter increase and there exists an optimum impinging angle near 80° having the minimum value in the impinging velocity for given wall temperature and droplet size. Near the wall temperature of 250°C corresponding to the Leidenfrost temperature, a peculiar nonlinear behavior in the breakup probability is observed.This work was supported by the Turbo and Power Machinery Research Center, Seoul National University.     

Investigation of droplet collisions for solutions with different solids content

The collision behaviour of droplets and the collision outcome are investigated for high viscous polymer solutions. For that purpose, two droplet chains produced by piezoelectric droplet generators are directed towards each other at a certain angle so that individual droplet pairs collide. For recording the collision event, one double-image and one high-speed CCD camera were used. One camera is positioned perpendicular to the collision plane recording the outcome of the collision, and the second camera is aligned parallel to the collision plane to assure that the droplet chains are exactly in one plane. A new approach for tracking droplets in combination with an extended particle tracking velocimetry algorithm has been developed. Time-resolved series of pictures were used to analyse the dynamics of droplet collisions. The three different water soluble substances were saccharose and 1-Ethenyl-2-pyrrolidone (PVP) with different molecular weights (K17, K30). The solvent was demineralised water. The solids contents ranged from 20 to 60 %, 5 to 25 % and 5 to 35 %, yielding dynamic viscosities in the range of 2–60 mPa s. Results were collected for different pairs of impact angles and Weber numbers in order to establish common collision maps for characterising the outcomes. Here, relative velocities between 0.5 and 4 m/s and impact parameters in the interval from 0 to 1 for equal-sized droplets (Δ = 1) have been investigated. Additionally, satellite formation will be discussed exemplarily for K30. A comparison with common models of different authors (Ashgriz and Poo in J Fluid Mech 221:183–204, 1990; Estrade et al. in Int J Heat Fluid Flow 20:486–491, 1999) mainly derived for low viscous droplets revealed that the upper limit of their validity is given by an Ohnesorge number of Oh = 0.115 and a capillary number of Ca = 0.577. For higher values of these non-dimensional parameters and hence higher dynamic viscosities, these models are unable to predict correctly the boundaries between collision scenarios. The model proposed by Jiang et al. (J Fluid Mech 234:171–190, 1992), which includes viscous dissipation, is able to predict the boundary between coalescence and stretching separation for higher viscosities (i.e. Oh > 0.115 and Ca > 0.577). However, the model constants are not identical for different solution properties. As a conclusion, an alteration of the collision appearance takes place because of the relative importance between surface tension and viscosity.     

Modeling of droplet collision-induced breakup process

The present article proposes a new droplet collision model considering droplet collision-induced breakup process with the formation of satellite droplets. The new model consists of several equations to investigate the post-collision characteristics of colliding droplets and satellite droplets. These equations are derived from the conservations of droplet mass, momentum, and energy between before and after collision, and make it possible to predict the number of satellite droplets, and the droplet size and velocity in the analytical way. To validate the new collision model, numerical calculations are performed and their results are compared with experimental data published earlier for binary collision of water droplets. It is found from the results that the new model shows good agreement with experimental data for the number of satellite droplets. It can be also shown that the predicted mean diameter by the new model decrease with increasing the Weber number because of the collision-induced breakup, whereas the O’Rourke model fails to predict the size reduction via the binary droplet collision.     

Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (∼10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (∼100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000–2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.     

Experimental investigation of the impaction of water droplets on cylindrical objects

The impaction of water droplets on isothermal cylindrical wires has been investigated experimentally in the present study. Mono-size droplets of 110, 350 and 680 μm in diameter were generated using piezoelectric droplet generators. The effects of droplet velocity and wire size were varied parametrically to reveal the impacting phenomena. Typical modes of the impaction outcome are disintegration and dripping. For droplets impacting on small wires, finer drops are disintegrated if the impacting droplet velocity is high, and larger dripping drops are observed if the velocity is low. For droplets impacting on large wires, bigger pendent drops are gradually formed which would eventually detach from the wires under the influence of gravity. In addition, droplets impacting on wires with waxy surface generate smaller dripping drops than that of the non-waxed wires. A non-dimensional regime map and new formulations in terms of the droplet Weber number, the wire Bond number and the size ratio of the wire diameter to incoming droplet diameter have been established to identify the regime for each mode of outcome and to predict the size of the dripping drops within the experimental limits.     

Collision dynamics of high-speed droplets upon layers of variable thickness

The collision dynamics between a droplet and a film has been studied with high-impact energy that can be grouped in a dimensionless Weber number, We, as normalized by surface energy. To accomplish this, we have developed a technique based on cutting of a high-speed jet, which can generate a single droplet with speed up to 23 m/s and We on the order of thousands. It was found that the boundaries indicating the occurrence of a central jet and that of a secondary droplet disintegrated from the jet decreased monotonically with increased dimensionless film thickness, H, and remained constant when the film thickness was larger than the crater depth. However, the transition designating multiple droplets that are originated from a central jet shows a non-monotonic trend with the variation of H, with a minimum We being at H ≈ 3, which is about the maximum crater depth, owing to a tuning behavior. The critical We for splashing that occurs at an early phase immediately after the impact is relatively sensitive to the film thickness only when H is between 1 and 2, which increases with reduced H. At large We (≳2,570 for high H), the ejected crown is closed to form a bubble and the transition boundary reveals a similar dependence on H as that for creation of a central jet.     

Numerical modeling and experimental measurements of water spray impact and transport over a cylinder

This study compares experimental measurements and numerical simulations of liquid droplets over heated (to a near surface temperature of 423 K) and unheated cylinders. The numerical model is based on an unsteady Reynolds-averaged Navier–Stokes (RANS) formulation using a stochastic separated flow (SSF) approach for the droplets that includes submodels for droplet dispersion, heat and mass transfer, and impact on a solid surface. The details of the droplet impact model are presented and the model is used to simulate water spray impingement on a cylinder. Computational results are compared with experimental measurements using phase Doppler interferometry (PDI). Overall, good agreement is observed between predictions and experimental measurements of droplet mean size and velocity downstream of the cylinder.     

Artificial boiling heat transfer in the free convection to carbonic acid solution

Free convection phenomenon has been experimentally investigated around a horizontal rod heater in carbonic acid solution. Because of the tendency of the solution to desorb carbon dioxide gas when temperature is increased, bubbles appear when cylinder surface is heated. The bubbles consists mainly carbon dioxide and also a negligible amount of water vapor. The results present that dissolved carbon dioxide in water significantly enhances the heat transfer coefficient in compare to pure free convection regime. This is mainly due to the microscale mixing on the heat transfer surface, which is induced by bubble formation. In this investigation, experiments are performed at different bulk temperatures between 288 and 333 K and heat fluxes up to 400 kW m⁻² at atmospheric pressure. Bubble departure diameter, nucleation site density and heat transfer coefficient have been experimentally measured. A model has been proposed to predict the heat transfer coefficient.     

Modeling of binary droplet collisions for application to inter-impingement sprays

In this paper, we focused on modeling the collision phenomenon between two liquid droplets for application in spray simulations. It has been known that the existing O’Rourke collision model widely used in CFD codes is inaccurate in determining collision outcomes and droplet behavior. In addition, since the collision probability of the model follows a statistical approach involving computational cell geometry, the prediction results should be strongly dependent on the cell size. As a result, to more accurately calculate droplet collisions, the technique for predicting the droplet velocity and its direction after collision must be extended for use in spray modeling. Further, it is also necessary to consider all the possible collision outcomes, such as bouncing, stretching separation, reflexive separation and coalescence. Therefore, this paper describes the appropriateness of a composite concept for modeling collision outcomes and the implementation of deterministic collision algorithms into a multidimensional CFD code for the calculation of post-collisional droplet movements. Furthermore, the existing model does not consider the formation of satellite droplets. For this reason, our present modeling concept includes a fragmenting droplet collision model. Using the present model, we have validated the collision interactions between liquid droplets under high Weber number conditions by comparing our calculations with experimental results from a binary droplet collision. This paper also deals with the application of the model to inter-impingement sprays by analyzing the atomization characteristics, such as mean droplet size and velocity, spray tip penetrations and spray-shapes of the impinging spray using the suggested collision algorithms and then comparing the results with available experimental data.     

Oldroyd-B黏弹性液滴碰撞过程的数值模拟

复杂的流变特性使凝胶推进剂的雾化过程存在一定困难,这制约了它的发展.聚合物胶凝剂的加入使凝胶推进剂具有黏弹性,从而在雾化时会产生黏弹性液滴,因此为了进一步认识凝胶推进剂的雾化机理、提高凝胶推进剂的雾化性能,对黏弹性液滴的碰撞行为进行数值模拟研究.针对凝胶推进剂雾化过程中出现的液滴撞击现象,考虑流体具有的黏弹性效应,采用...     

Experiments on the dynamics of droplet collisions in a vacuum

Highly controlled experiments of binary droplet collisions in a vacuum environment are performed in order to study the collision dynamics devoid of aerodynamic effects that could otherwise obstruct the experimental observations by causing distortion or even disintegration of the coalesced mass. Pre-collision droplets are generated from capillary stream break-up at wavelengths much larger than those generated with the typical Rayleigh droplet formation in order to reduce the interactions among the collision products. Experimental results show that the range of droplet Weber number necessary to describe the boundaries between permanent coalescence and coalescence followed by separation is several orders of magnitude higher than has been reported in experiments conducted at standard atmospheric pressures with lower viscosity liquids (i.e. hydrocarbon fuels and water). Additionally, the time periods of both the oblate and prolate portions of the coalesced droplet oscillation have been measured and it is reported for the first time that the time period for the prolate portion of the oscillation grows exponentially with the Weber number. Finally, new pictorial results are presented for droplet collisions between non-spherical droplets. Received: 30 June 1998/Accepted: 15 October 1999     

A new predictive model for fragmenting and non-fragmenting binary droplet collisions

A new predictive model for collisional interactions between liquid droplets, which is valid for moderate to high Weber numbers (>40), has been developed and validated. Four possible collision outcomes, viz., bouncing, coalescence, reflexive separation and stretching separation, are considered. Fragmentations in stretching and reflexive separations are modeled by assuming that the interacting droplets form an elongating ligament that either breaks up by capillary wave instability, or retracts to form a single satellite droplet. The outcome of a collision, number of satellites formed from separation processes and the post-collision characteristics such as velocity and drop-size are compared with available experimental data. The comparisons include colliding mono- and poly-disperse streams of droplets of different fuels under atmospheric conditions, and the results agree reasonably well.     

Inertia dominated flow and heat transfer in liquid drop spreading on a hot substrate

The present work deals with computational modeling of the fluid flow and heat transfer taking place in the process of impact of a cold liquid drop (T_d = 20-25 °C) onto a dry heated substrate characterized by different thermophysical properties. The computational model, based on the volume-of-fluid method for the free-surface capturing, is validated by simulating the configurations accounting for the conjugate heat transfer. The simulations were performed in a range of impact Reynolds numbers (Re = 2000-4500), Weber numbers (We = 27-110) and substrate temperatures (T_s = 100-120 °C). The considered temperature range of the drop-surface, i.e. liquid-solid system does not account for the phase change, that is boiling and evaporation. The model performances are assessed by contrasting the results to the reference database originating from the experimental and complementary numerical investigations by Pasandideh-Fard et al. [Pasandideh-Fard, M., Aziz, S., Chandra, S., Mostaghimi, J., 2001. Cooling effectiveness of a water drop impinging on a hot surface. International Journal of Heat and Fluid Flow, 22, 201-210] and Healy et al. [Healy, W., Hartley, J., Abdel-Khalik, S., 2001. On the validity of the adiabatic spreading assumption in droplet impact cooling. International Journal of Heat and Mass Transfer, 44, 3869-3881]. In addition, the thermal field obtained is analyzed along with the corresponding asymptotic analytical solution proposed by Roisman [Roisman, I.V., 2010. Fast forced liquid film spreading on a substrate: flow, heat transfer and phase transition. Journal of Fluid Mechanics, 656, 189-204]. Contrary to some previous numerical studies, the present computational model accounts for the air flow surrounding the liquid drop. This model feature enables a small air bubble to be resolved in the region of the impact point. The reported results agree reasonably well with experimental and theoretical findings with respect to the drop spreading pattern and associated heat flux and temperature distribution.     

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.     

Characterization of diabatic two-phase flows in microchannels: Flow parameter results for R-134a in a 0.5 mm channel

An optical measurement method for two-phase flow pattern characterization in microtubes has been utilized to determine the frequency of bubbles generated in a microevaporator, the coalescence rates of these bubbles and their length distribution as well as their mean velocity. The tests were run in a 0.5 mm glass channel using saturated R-134a at 30 °C (7.7 bar). The optical technique uses two laser diodes and photodiodes to measure these parameters and to also identify the flow regimes and their transitions. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions were detected and observed also by high speed video. It was also possible to characterize bubble coalescence rates, which were observed here to be an important phenomena controlling the flow pattern transition in microchannels. Two types of coalescence occurred depending on the presence of small bubbles or not. The two-phase flow pattern transitions observed did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. Time averaged cross-sectional void fractions were also calculated indirectly from the mean two-phase vapor velocities and compared reasonably well to homogeneous values.     

On the quenching of steel and zircaloy spheres in water-based nanofluids with alumina,silica and diamond nanoparticles

The quenching curves (temperature vs time) for small (∼1 cm) metallic spheres exposed to pure water and water-based nanofluids with alumina, silica and diamond nanoparticles at low concentrations (?0.1 vol%) were acquired experimentally. Both saturated (ΔT_sub = 0 °C) and highly subcooled (ΔT_sub = 70 °C) conditions were explored. The spheres were made of stainless steel and zircaloy, and were quenched from an initial temperature of ∼1000 °C. The results show that the quenching behavior in nanofluids is nearly identical to that in pure water. However, it was found that some nanoparticles accumulate on the sphere surface, which results in destabilization of the vapor film in subsequent tests with the same sphere, thus greatly accelerating the quenching process. The entire boiling curves were obtained from the quenching curves using the inverse heat transfer method, and revealed that alumina and silica nanoparticle deposition on the surface increases the critical heat flux and minimum heat flux temperature, while diamond nanoparticle deposition has a minimal effect on the boiling curve. The possible mechanisms by which the nanoparticles affect the quenching process were analyzed. It appears that surface roughness increase and wettability enhancement due to nanoparticle deposition may be responsible for the premature disruption of film boiling and the acceleration of quenching. The basic results were also confirmed by quench tests with rodlets.