Stream Of Drops Research Articles

Mass transfer to a drop stream from a field liquid

AbstractMass transfer from a stream of drops falling freely in a stagnant liquid was investigated. Drop streams were produced by a dripping method and by a jet breakup method. Water and isobutanol, mutually saturated, were used as the dispersed and the continuous phases. Sodium hydroxide was transferred from isobutanol to water drops which were initially free of solute. The mass transfer resistance is on the continuous phase side. The mass transfer coefficient and terminal velocity of drop streams were measured experimentally. The experimental results show that the mass transfer coefficient in the drop stream is affected by the shielding effect of the previous drops. The experimental data have been correlated as Kt/Ut0.5 versus interdrop distance l, a relationship describing the effect of the interdrop distance on the mass transfer coefficient in the continuous phase.

Investigation of the stochastic collisions of drops produced by Rayleigh breakup of two laminar liquid jets

The stochastic collisions of drops of two intersecting streams were investigated experimentally. The drop streams were produced by Rayleigh breakup of two laminar jets of propanol-2 and were arranged spatially so that they lie in one plane and intersect at an angle which was varied in the experiments. The collisional interactions of the drops were visualized using video equipment. In the zone between the drop streams downstream of the intersection point new drops occur which are formed by the collisions. The visualization showed that these new drops may be produced either by the merging of two colliding drops or by the breakup of liquid bridges formed between drops after off-center collisions. Measurements of velocity and size of the drops in the flow field were carried out using a phase-Doppler anemometer (PDA). These data and the frequency of drop arrival in the measurement control volume of the PDA give insight into the drop formation processes caused by the collisions and enable the computation of the collision frequency.

Effects of Relative Humidity on the Coalescence of Small Precipitation Drops in Free Fall

Abstract Observations of the effects of relative humidity on coalescence are limited to studies using supported drops or streams of drops, and the results are contradictory. In this paper, findings are presented on the effect of high and low relative humidity on collisions between freely falling drops. Comparisons between the collision outcomes (coalescence, bounce, and temporary coalescence with and without satellite drops) for high-humidity (RH > 95%) and low-humidity (RH ≈ 30%) experiments were made for small precipitation drops at terminal velocity and with minimal electric charge. Coalescence begins after the air-film between colliding drops is drained sufficiently to allow the drops to make contact. For temporary coalescence, the union of the two drops is not permanent because the rotational energy caused by a non-head-on collision is sufficient to pull the coalesing drops apart. One or more satellite drops form during a temporary coalescence when water filament between the separating drops breaks i...

Collisions between Small Precipitation Drops. Part I: Laboratory Measurements of Bounce, Coalescence, and Temporary Coalescence

Self-collection efficiencies were measured for isolated drop pairs failing at terminal velocity using orthogonal cameras to obtain the horizontal offset of the drops before collision and the collision outcome. Data were obtained on four different drop-size pairs over a range of impact Weber number (1–10) and size ratio (0.45–0.73). Collision offsets and outcomes were recorded during 45 experiment runs as a function of drop charge. The collision results from all 4200 events were tabulated by offset and charge, and the coalescence efficiency was determined for each run as a function of charge. Collision results revealed a coalescence region for small offset and a bounce region at intermediate-to-large offset and low-to-intermediate charge. The critical offset that separated the regions of coalescence and bounce was independent of charge. At higher values of charge, increasing charge was found to induce permanent and/or temporary coalescence from smaller and larger offsets until bounce was completely eliminated. In the offset range for temporary coalescence, the filament connecting the separating drops often collapsed into one and, occasionally, two satellite drops. Mean satellite sizes of 58–81-µm radius were generally consistent with previous measurements using colliding drop streams. The production of satellite drops by colliding precipitation drops should provide precipitation embryos that would accelerate the accretion of cloud water in warm-base convective clouds. Coalescence efficiencies of 15%–55% at minimal charge were significantly lower than previously reported for smaller drops; therefore, the results indicate a further reduction in the growth rate of precipitation drops. The efficiencies did not vary in a simple way with either Weber number or size ratio. For a constant size ratio (p ≈ 0.7) the coalescence efficiency decreased with increasing Weber number, whereas for a constant Weber number (We ≈ 4.2) the coalescence efficiency decreased with increasing size ratio. An excellent fit to the laboratory coalescence efficiencies, using the theoretical scaling for inelastic collisions, is presented in a companion paper. The resulting formulas for precipitation drops will allow application of these findings to self collection, a process that controls the spreading of raindrops to larger sizes and the growth of radar reflectivity.

Size and Velocity Measurements of Large Drops in Air and in a liquid‐liquid two‐phase flow by the phase‐Doppler technique

AbstractParticles comparable in size to or larger than the measurement volume need extra consideration when measured by a phase‐Doppler system. The phase of the Doppler burst received when such particles traverse the measurement volume depends not only on the size of the particle but also on its trajectory, since the particle is not uniformly illuminated. This paper presents a strategy for securing correct measurements even under such conditions, taking advantage of the three‐detector receiving optics of the Dantec Particle Dynamics Analyzer. The effectiveness of the approach is demonstrated for sizing drops in liquid‐gas and liquid‐liquid two‐phase flows: water drops in air, water drops in FC72 and FC72 drops in water. The combination of water and FC72 is also of interest because the relative refractive index is close to unity. Measurements of drops size were made on a monodisperse stream of drops about 2 mm in diameter, i.e. substantially larger than the measurement volume, and polydisperse distributions of drops ranging in diameter from below 0.2 mm to about 1 mm.

MECHANISMS OF AIR-ASSISTED LIQUID ATOMIZATION

A well-controlled experimental apparatus was used to investigate the air-assisted liquid drop atomization process. High-magnification, high-speed photography as well as conventional spray field photography was used to study the breakup of a monodisperse stream of drops injected into a transverse high-velocity air stream. The gas velocity profile, liquid drop velocity, and size were determined by laser Doppler velocimetry (LDV) and a phase Doppler particle analyzer (PDPA). The experiments gave information about the microscopic structure of the liquid breakup process, drop breakup regimes, drag coefficients of atomizing drops, breakup drop size distributions, and drop trajectories. At low gas-liquid relative velocities, the microscopic photographs confirmed the existence of bag and stripping breakup regimes. The photographs also revealed the nature of high-speed, "catastrophic," liquid atomization, which appears to be due to the development of instability waves on the liquid surface. A model based on aerodynamic liquid breakup theory was used to study the influences of the drop drag coefficient and the breakup time on the drop trajectory during the drop atomization process. Previously proposed expressions for the drop drag coefficient and the breakup time constant were tested by comparing the calculated trajectories with the measurements.

An apparatus for measuring angular dispersion of liquid drop streams

A new method to measure angular dispersion of liquid drop streams with several optical fiber probes has been developed. Measurements for the number of drops at a variety of direction angles were done at a consecutive period.

Variation of drag coefficients in an interacting drop stream

Variation of the drag coefficient of closely spaced drops in a stream injected into a turbulent flow is studied experimentally. Three different regions are identified. In the first region, close to the injector, drops flow in the wake of each other. The drag coefficient in this region is much smaller (by a factor of 4 to 5) than the standard drag coefficient, and its magnitude is dependent on the drop initial spacing. Shortly downstream of the injection point, the transition region starts, where the drag coefficient increases rapidly approaching the drag coefficient of a single isolated drop. And further down-stream when drops are dispersed significantly, the drag coefficient will behave the same way as a single isolated drop.

Changes in particle morphology during drying of drops of carbohydrate solutions and food liquids. 1. Effect of composition and drying conditions

Morphological changes of drops of sucrose and maltodextrin solutions, coffee extract, and skim milk have been monitored during drying. One method involved videotaping drops suspended in a stream of hot air. A second method was to sample a stream of drops of uniform initial size falling through a column with a controlled temperature profile. Suspended drops show a first period of near-spherical shrinkage, closely following predictions for a voidless sphere. This is followed by a second period of rapid inflate-deflate cycling (boiling) and a third and final period during which the drop grows and/or shrinks to reach a solidified morphology. The effects of composition, air temperature, initial solute concentration, and presence or absence of dissolved gases were determined. Particles of coffee extract from the falling-drop dryer evidence surface blowholes, replicating what is observed in commercial spray-dried coffee. This phenomenon is rationalized in terms of viscous resistance to sealing flows

Study of sol–gel processing for fabrication of hollow silica–aerogel spheres

Preliminary results were previously reported indicating that uniform, hollow silica–aerogel spheres of controlled size and thickness can be fabricated by controlled hydrolysis and condensation of tetraethylorthosilicate (TEOS). The method consists of first producing a stream of hollow drops of a mixture of TEOS, H2O, ethanol, and a catalyst using a dual‐nozzle system and then introducing the drops into a gelation chamber where the drops solidify into a rigid form while being levitated by a gelation gas mixture. A detailed further study designed to understand and control the kinetics of the sol–gel processing that is responsible for the hollow silica–aerogel sphere formation is described. Specifically, the optimal rheology and stoichiometry of the reactant solution, the make‐up of the gelation/levitation gas mixture, and the characteristics of the resulting silica–aerogel spheres, such as the size, thickness, porosity, pore size, and density, are investigated.

A compact, laser diode based phase Doppler system

Recent advances in laser diode technology provide the opportunity to reduce the size and cost of laser Doppler and phase Doppler anemometry systems. The application of laser diodes to phase Doppler systems is discussed, and the construction of a compact, laser diode based phase Doppler system is described. Test measurements in a monosized drop stream, a water spray and a fluidized bed are reported, demonstrating the functionality of such a system.

Collision of water drops with a plane water surface

The dependence of the outcome of the collision of uncharged water drops with a plane water surface on the impact angle α, the velocity v1 and the radius r1 of the drops has been investigated experimentally. The impact parameters were varied over the intervals: v1=0.40–1.05 m/sec, r1=75–150μm, and α=16-85°. The method employed made it possible to avoid having to monitor the individual high-speed impact process. A stream of drops, produced in a “vibrating reed” type monodisperse droplet generator, was directed at the target. The impact parameters were measured by means of pulsed illumination. The results are expressed in the form of the dependence of the rebound probability and the coalescence coefficient ES on the impact parameters. The existence of alternating conditional rebound-coalescence-rebound zones for different impact angles is established, together with a decrease in ES with increase in r1 and v1. The data obtained generalize the results of previous experiments.

Size distribution of detached drops

The law governing the size distribution of detached gas-liquid streams of drops has been determined analytically, and a comparison is carried out against experimental data existing in the literature. The derived theoretical relationships offer an excellent description of existing experimental results.

Solidification by radiative cooling of a cylindrical region filled with drops

Transient cooling by radiation is analyzed for a cylindrical region filled with axially flowing streams of drops that are becoming solidified. This is of interest for the dissipation of waste heat from orbiting power system in space. The drops absorb, emit, and scatter radiation, and the surroundings are at a lower uniform temperature. The radiative properties are assumed gray, and the scattering is isotropic. The radiating region is a two-phase mixture that remains at the melting temperature of the drops. Its temperature uniformity maintains a high emissive power as energy is lost. This is an advantage over a sensible heat radiator in which the temperature decreases, thereby reducing the emissive power. The results provide the axial length that remains two-phase and the fraction of energy dissipated within this length in which the emissive power has not decreased because of sensible cooling. It is also shown how the radial distribution of the axial velocity of the drops can be modified to increase this energy fraction.

Drop-on-demand operation of continuous jets using EHD techniques

In some applications of continuous jet drop generators, it is helpful to produce a single isolated drop instead of the stream of drops normally encountered. A voltage pulse applied to a nearby electrode can produce an isolated drop, but only if the size and shape of the pulse are carefully chosen. The proper choice is determined by decomposing the input voltage into Fourier components, following each one along the jet, and then recombining them to give a snapshot of the jet at the breakoff point. A computer program has been developed that produces a graphical profile of the jet response that demonstrates how the jet would appear in actual operation. Visual inspection of the calculated jet profile while adjusting the pulse parameters leads to several suitable voltage pulses for drop-on-demand operation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Charging up raindrops

It has long been recognised that charge can affect the results of collisions between water drops and, therefore, may significantly alter the evolution of precipitation in clouds and thunderstorms. In the past, experimenters have obtained data on the coalescence of charged drops from streams of drops, drops hitting a flat water surface and drops fired at suspended drops.

Charge effects on the coalescence of water drops in free fall

It has long been recognized that charge can affect the results of collisions in streams of water drops1,2, between drops and a flat water surface3, between two drop streams4–6, and for drops fired at suspended drops7,8. Here we present the first observations of the effect of charge on the coalescence of colliding water drops that were initially falling freely at terminal velocity. Other experiments using uncharged drops show large discrepancies between drop streams or suspended drop results and free fall, terminal velocity results9–11. Therefore, it is probable that previous work on charge effects should not be directly applied to clouds. The present findings support this contention, because the charge required to induce complete coalescence differs from previous results for similar drop sizes and size ratios by an order of magnitude. The following new features of charged drop interactions were observed: a unique impact angle dividing the coalescence from the non-coalescence region that is independent of charge, a two order of magnitude range of relative charge between drop bounce and complete charge-induced coalescence, a wide range of charge where a satellite drop occurs with temporary coalescence at higher charge levels the beginning of charge-induced permanent coalescence at the most grazing collisions.

New technique for producing highly uniform droplet streams over an extended range of disturbance wavenumbers

A recent discovery about a well-studied phenomenon leads to the formation of ultracoherent streams of liquid drops. A remarkably straightforward, although previously unreported, method of drop generation from capillary streams, which involves the imposition of an amplitude modulated sinusoidal carrier disturbance, is discussed. The amplitude-modulated disturbance results in the production of highly stable droplet streams over a previously unattainable range of wavenumbers. A decrease in the speed variation of the drops within the stream, which is a result of the amplitude modulation, is demonstrated over a range of nondimensional wavenumbers from 0.4 to 0.04. The nondimensional wavenumber is the ratio of stream circumference to disturbance wavelength. It is suggested that amplitude modulating the capillary stream’s disturbance may also decrease the diameter variation of the drops.

Fluid dynamics and mass transfer of interacting drops -a mathematical modelling approach

An accurate and computationally efficient description of the transport mechanisms of accelerating, interacting drops in a gaseous environment is important for the basic understanding of this particular class of two-phase flows and for the improvement/design of trace gas absorption, dense spray, and fuel droplet combustion processes. Modelling of such a complex system requires: (i) a system conceptualization which renders the multiple drop analysis mathematically tractable, (ii) the development of governing equations detailed and flexible enough to represent differential as well as integral system parameters, and (iii) an accurate and efficient numerical scheme for a useful computer simulation model. We started out with a lumped parameter approach where trace gas absorption with chemical reaction in a jet-like stream of falling drops was considered. This analysis yielded the aerodynamics of the jet-induced entrainment corridor, the velocities of the accelerating drops, and the mass transfer rate of ambient trace gases into the stream of drops. In order to gain more insight into the dynamics of interacting drops, we developed a distributed parameter model for the coupled two-phase momentum/mass transfer of spherical drops in the Reynolds number range 50 <Re <400. After model verification, the influence of major system parameters, e.g. inter-drop distance, reaction rate, Peclet numbers etc., was analyzed in well-defined computer experiments. Presently, the local two-drop analysis is being integrated into the global stream-of-drops configuration.

FACTORS GOVERNING SURFACE MORPHOLOGY OF SPRAY-DRIED AMORPHOUS SUBSTANCES

ABSTRACT Changes in particle morphology (size, shape, and appearance) have been monitored during drying of drops of foods and food-related materials. The apparatus produces a single stream of drops of uniform size, using a vibrating-orifice device for drop production. The drop size and the time-temperature history of the drops as they fall can be varied and controlled. Qualitative observations are reported for drying of aqueous solutions of lactose, maltodextrin, skim milk, and coffee extract, with different feed concentrations. Particular emphasis is placed upon the tendency for development of folds upon the particle surface. A mechanistic model is developed, relating the tendency for folding to the extent of viscous flow of surface material in response to a surface-energy driving force. This model gives semi-quantitative agreement with observations for solutes of different molecular weight (and hence different viscosity) and for different feed concentrations.