Mixture Of Droplets Research Articles

Evidence of Oil Exchange between Oil-in-Water Emulsion Droplets Stabilized by Milk Proteins

Differential scanning calorimetry has been used to monitor the crystallization of hydrocarbons in oil-in-water emulsions containing a mixture of pure n-hexadecane droplets (10 wt%) and pure octadecane droplets (10 wt%) stabilized by either whey protein isolate or casein (0.5 wt%). The phase behavior of the emulsions changed with time in a manner consistent with oil exchange between the n-hexadecane and octadecane droplets. The rate of exchange was faster for the emulsion containing whey protein isolate than the one containing casein, and increased for both emulsions when additional protein (2.5 wt%) was added to the aqueous phase. The addition of NaCl (2.5 wt%) to the aqueous phase slightly decreased the rate of oil exchange. Measurements of the droplet size distribution of the emulsions during the course of the experiments indicated that the oil exchange was not caused by droplet aggregation or Ostwald ripening. We propose that the mechanism for oil exchange is the reversible binding of hydrocarbons to protein molecules (either at a specific binding site or by incorporation into micelles), and their subsequent transport through the aqueous phase separating the droplets.

Boundary layer and heat transfer in a two-phase gas-evaporating droplet medium

An asymptotic model of the flow in the laminar boundary layer of a gas-evaporating droplet mixture is constructed within the framework of the two-continuum approximation. The case of evaporation of the droplets into an atmosphere of their own vapor is examined in detail with reference to the example of longitudinal flow over a hot flat plate. Numerical and asymptotic solutions of the boundary layer equations constructed are found for a number of limiting situations (low droplet concentration, no droplet deposition, significant droplet deposition). The development of the flow with respect to the longitudinal coordinate is studied and it is shown that in the absence of droplet deposition a region of pure vapor may be formed near the surface. Similarity criteria are established and the mechanism of surface heat transfer enhancement is studied for a low evaporating droplet concentration in the boundary layer. In the inertial deposition regime the results of calculating the integral heat transfer coefficient are found to correspond with the experimental data [1].

Shock wave structure in a mixture of gas, vapour and droplets

The structure of the relaxation zone behind a shock wave of moderate strength in a mixture of gas, vapour and droplets is analysed. A model is presented for shock induced evaporation, which is based on wet-bulb equilibrium and on the absence of relative motion between droplets and gas. Experimental and numerical data on heterogeneous condensation induced by an unsteady rarefaction wave and on re-evaporation due to shock wave passage are reported for a mixture of water vapour, nitrogen gas and condensation nuclei. Pressure, temperature, saturation ratio and droplet size are experimentally obtained and are very well predicted by a numerical simulation based on the non-linear quasisteady wet-bulb model for phase transition, as well for the expansion wave as for the shock wave. During expansion, droplet number density decays much faster than predicted, which is not yet satisfactorily explained. Shock induced droplet evaporation is studied for post-shock saturation ratios ranging from 5×10−3 to 0.2, corresponding to shock Mach numbers of 1.2 to 1.9. The evaporation times are well predicted by the theoretical model. No evidence is found for droplet break-up for Weber numbers up to 13, and droplet radii of the order of 1μm.

Oil exchange between oil-in-water emulsion droplets stabilised with a non-ionic surfactant

Differential scanning calorimetry has been used to monitor crystallization and melting of hydrocarbons in oil-in-water emulsions containing a mixture of pure n-hexadecane droplets (10 wt%) and pure octadecane droplets (10 wt%) stabilized by a non-ionic surfactant, (2 wt% polyoxyethylene sorbiton monolaurate). The crystallization and melting behavior of the emulsions changed with time in a manner consistent with exchange of oil between the n-hexadecane and octadecane droplets. Complete mixing of the oils in the droplets took over 7 days. The rate of exchange increased when additional surfactant was added to the aqueous phase of the emulsion. The droplet size of the emulsions did not change during the course of the experiment. The probable mechanism for oil exchange between the droplets involved the solubilisation of the oil in the aqueous phase by surfactant micelles.

Observations of soot during droplet combustion at low gravity: heptane and heptane/monochloroalkane mixtures

Experimental observations of the combustion of sooting fuel droplets, performed in a drop tower to create a low gravity environment, are reported. Free n-heptane droplets and suspended droplets of heptane, monochloroalkanes, and mixtures of monochloro-octane and heptane were studied. Initial droplet diameters ranged from 0.4 to 1.1 mm. The results suggest that soot may influence droplet vaporization rates. Spherical symmetry of the flame allowed for extended observation of soot agglomerates inside the droplet flame. Effects of slight convective flows were also observed, both through variations of natural convection around the suspended droplets and through variations in the drift velocities of the unsupported droplets. Slight convective flows around the suspended droplets reduced flame luminosity as well as soot accumulation inside the flame and increased droplet vaporization rates. Mixing monochloro-octane with n-heptane demonstrated the effectiveness of n-heptane in reducing soot emissions from the flames of the chlorinated fuels. Finally, trends of initial droplet diameter with burning rate were observed and may be linked to the effect of droplet size upon soot formation inside the flame.

Calculations of the auto-ignition of liquid hydrocarbon mixtures as single droplets

A mathematical model for the auto-ignition of single droplets of multicomponent liquid hydrocarbon mixtures is described, and applied to predict ignition delay times as a function of composition for mixtures approximating commercial diesel fuels. The mixtures devised for the calculations allowed independent variation of the distillation properties and chemical make-up. The results show that the distillation curve of the fuel is in itself no indicator of ignition behaviour; rather, ignition is governed primarily by the chemical nature and boiling points of the most volatile constituents of the fuel.

The Combustion Mechanism and Combustion Promotion of Coal-Water Mixture Fuel.

The combustion behavior of coal-water mixture (CWM) droplets was observed in the electric furnace, where the measurement of droplet temperature, video recording and high-speed photography were carried out at sequential steps of the combustion process. The effects of furnace temperature, slurry concentration and oxygen concentration were examined in order to elucidate the combustion mechanism of CWM droplets. The results indicate that they are influenced considerably at each step of the combustion process such as vaporization, ignition, visible envelope flame combustion and char combustion. The effects of the addition of C-heavy oil, A-heavy oil, methanol and kerosene were examined in order to elucidate the feasibility and the mechanism of microexplosion of CWM droplets. The results show that the explosion makes fairly short both ignition delay time and total combustion time in the fuel of C-heavy oil in addition to the CWM.

Aerial Spray Deposition in Cotton

The effect of aerial application parameters on the deposition of water-based sprays on mid-season cotton canopies was evaluated in three field studies. Fluorescent dye tracer spray deposits were measured on cotton leaves and three different types of artificial samplers at the top and midway down in the plant canopy, and on ground-cover sheets between the cotton rows. Over 400 samples collected over three years provided sufficient data to determine the relationship of spray mixture rate, droplet size, droplet density, temperature, relative humidity, and wind speed to active spray ingredient deposition on the crop canopy. These studies show that droplet spectra and spray rate can be modified or adjusted by the applicator to improve spray deposition and distribution in broadleaf row crop canopies. Environmental factors shown to adversely influence deposition may be avoided by spray applicators to maintain deposition efficiency.

Soot formation during combustion of unsupported methanol/toluene mixture droplets in microgravity

An experimental study is reported concerning the influence of liquid composition on soot formation and burning rate of a droplet composed of a binary miscible mixture of liquids. The mixture components represented a highly sooting fuel, toluene, and a non-sooting fuel, methanol. The experimental observations were made in a microgravity environment to create near spherically symmetric burning of the droplets. Mixtures of 5%, 25% and 50% by volume toluene in methanol were burned in room temperature air. Initial droplet diameters ranged from 0.47 mm to 0.60 mm, and the available experimental time was sufficient to record the complete droplet burning history. Toluene concentration in methanol was shown to dramatically influence flame luminosity and soot production. However, neither burning rates nor propensity for flame extinction appeared to be significantly affected by toluene mixture fractions. 5% toluene mixture droplets behaved like pure methanol droplets in terms of burning rate, lack of flame luminosity, and extinction. Increasing the toluene concentration in the droplets to 25% increased flame luminosity, yet no visible soot agglomerates were observed. The 50% mixture droplets, however, burned with highly luminous flames and large amounts of soot agglomerates collecting inside the flame. None the less, all the mixture droplets showed similar burning rates to those of pure methanol and likewise exhibited flame extinction before complete droplet vaporization.

Normal shock-wave structure in two-phase vapour-droplet flows

A study of the structure of stationary, fully and partly dispersed, normal shock waves in steady vapour–droplet, two-phase flow is presented. Pure substances only are considered, but, unlike most previous work, the droplet population is allowed to be polydispersed. It is shown how the effects of thermal relaxation for such a mixture can be elegantly incorporated into the analysis.Three types of fully dispersed wave are identified. Type I waves are dominated by thermal relaxation and an approximate analytical solution is presented which gives results in close agreement with accurate numerical solutions of the governing equations. The analysis predicts some unexpected behaviour of the thermodynamic variables and demonstrates the correct scaling parameters for such flows. An approximate analysis is also presented for Type II waves, dominated by both velocity and thermal relaxation. Type III waves, where all three relaxation processes are important, are of little practical significance and are only briefly discussed. Partly dispersed waves are also considered and the results of a numerical simulation of the relaxation zone are presented. A linearized solution of this problem is possible but, unlike other relaxing gas flows, does not give good agreement with the more exact numerical calculations.The apparent discontinuity in the speed of sound in a vapour–droplet mixture as the wetness fraction tends to zero has been responsible for some confusion in the literature. This problem is reconsidered and it is shown that the transition from the two-phase equilibrium to the single-phase frozen shock wave speed is continuous.

Should bulk cloudwater or fogwater samples obey Henry's law?

In this work we prove that mixing of droplets with different pH that are individually in Henry's law equilibrium with the surrounding atmosphere always results in a bulk mixture that is supersaturated with weak acids like S(IV) and HCOOH and bases like NH3 with respect to the original atmosphere. The degree of supersaturation of the bulk liquid water sample for a particular species depends on its dissociation constant, on the initial pH of the bulk droplet mixture, and on the distribution of the pH and of the liquid water over the droplet spectrum. High supersaturations result only when the pH of the bulk droplet mixture exceeds the pKa of the species, in which pH range large pH differences among droplets of different sizes lead to large deviations from Henry's law for the bulk mixture. The deviation is shown to depend on the ratio of the arithmetic mean to the harmonic mean of the hydrogen ion concentrations of the droplets with the liquid water content used as weighting factor in the calculation of the means. The theory developed can explain observed discrepancies from Henry's law in atmospheric samples and also other observed phenomena like the reported increase of pH values of bulk aqueous samples during storage.

HEAT TRANSFER IN TWO PHASE FLOW OVER A FLAT PLATE

The theoretical model of heat and mass transfer during laminar two phase (air-water) flow along a flat plate is carried out. The governing continuity and energy equations with its boundary conditions are written for the free stream of two phase flow , the air water droplets mixture boundary layer and the water film over the plate. The equations are given for determining;the mass flux of water evaporation from film surface, the convective heat flux and the mass flux of droplets depositing on the flat plate. The analysis shows high flux resulting from the superposition of the film evaporation process from the plate surface and the heat transfer by convection. The convective heat flux appears higher than during dry air flow due to the enhancement effect of the droplets. Experimental work is carried out. The comparison between the results obtained in the theoretical model and those obtained from the experimental results is presented and it was found an agreement.

Combustion of unsupported methanol/dodecanol mixture droplets at low gravity

Experiments are reported on the combustion of free or unsupported methanol/dodecanol mixture droplets in a reduced gravity environment. The experiments were conducted in a drop tower. The Grashof number (based on drop diameter) in the moving frame of reference was on the order of 10 −5 for the period of droplet burning. Combustion of the mixture droplets was characterized by a multi-stage process indicative of diffusion controlled burning. The flame was not visible during the first stage because of the lighting used, but was visible (and spherical) in the final stage. The results suggested that the mixture droplets did not extinguish. By contrast, methanol droplets consistently extinguished. Microexplosions were not observed for mixtures with initial concentrations of equal volumes of methanol and dodecanol. However, most of the droplets containing initially 75% (by volume) of methanol exploded. The initiation of microexplosion, as defined by the occurrence of homogeneous nucleation within the droplet, is explained by comparing the calculated nonlinear variation of superheat limit with composition for a methanol/dodecanol mixture with the boiling point of dodecanol.

On the gasification of droplets of azeotropic mixtures: Theory and experiment

Recognizing the potential formation of azeotropic mixtures from blending hazardous chemical wastes for incineration, a fundamental experimental and theoretical investigation of the volatility reversal and gasification characteristics of droplets of azeotropic and zeotropic mixtures has been conducted. Experimental results on the temporal variation of the droplet composition of binary positive azeotropic mixtures show that while the low-boiling-point component indeed gasifies faster throughout the droplet lifetime for low initial concentrations, it becomes the slower gasifying component at high initial concentrations. Consequently there exists an intermediate, azeotropic point such that a droplet at this initial composition will remain so throughout its lifetime. This gasification azeotropic point is different from the thermodynamic azeotropic point and is determined by the specific droplet combustion parameters. Corresponding control experiments on zeotropic mixtures show that the more volatile component remains as the faster gasifying component for all initial concentrations. Theoretical calculations on binary mixture droplets completely substantiate the experimental observations. The behavior of the azeotrope with the addition of a third component of greater or lesser volatility has also been examined. Experiments on some ternary mixtures show that the azeotropic behavior can be broken, for additions of either volatility.

Laser ignition of liquid propellants

Following previous studies of electrical ignition of liquid propellants in plasma jets, initiation by focused laser beams is studied both by fast cine shadowgraphy and by recording permanent gas release as a measure of decomposition. Unlike military propellants, droplets of aqueous mixtures of nitrate salts of hydroxylamine with a 4-carbon aliphatic amine do not ignite when an approx 10 J laser pulse is focused within the liquid. Ignition is achieved by producing breakdown in air in the vicinity of propellant droplets, though little decomposition results, at atmospheric pressure. Precursory electrolysis of varying duration is attempted, in view of postulated chemical mechanisms which suggest that solution of NO in the propellant should enhance ignition. This indeed greatly promotes the vigour of the reaction and leads to an amount of decomposition which increases with increasing time of electrolysis. It appears that the propellant is permanently modified by this simple procedure, leading to obvious practical as well as fundamental conclusions.

Combustion of a droplet of coal-water mixture heated by laser beams

A heating system using a CW argon ion laser beam has been used simultaneously with a two color pyrometer and a high speed camara to study different steps occurring during the combustion of a coal water mixture (CWM) droplet, positioned around a microthermocouple junction. Experiments have been performed using by-products of coal processing (schlamms) from Freyming-France coal mixed with 38% water. Four distinct steps have been characterized including: water evaporation, devolatilization, volatile combustion and heterogeneous char combustion. A detailed parametric study has been made. The main results are as follows: the duration of heterogeneous combustion does not depend on the heating power, but surprisingly, this duration is increased when the particle size of droplet constitutive coal decreases. This is attributed either to ash effect or to porosity evolution. Ash does not influence the evaporation and total heterogeneous combustion durations. A computer model has been developed to describe the different steps. The water evaporation and fixed carbon combustion are assumed to be mass diffusion controlled, the combustion following a shrinking core model. Good agreement is obtained between computed and measured parameters.

Crystallization in hydrocarbon-in-water emulsions containing a mixture of solid and liquid droplets

Time-dependent measurements of the velocity of ultrasound at constant temperature in hydrocarbon-in-water emulsions containing a mixture of solid and liquid droplets show that crystallization is induced in liquid droplets when solid droplets are present. We suggest that this may be caused by the solid droplets colliding with liquid droplets and initiating crystallization within them.

Wiener-Hopf Technique Solution to a Rewetting Model with Precursory Cooling

This paper reports on an exact solution using the Wiener-Hopf technique derived for a rewetting model. In this mixed-boundary model, a constant heat transfer coefficient is assumed in the wetted part of the solid slab, whereas an exponentially decaying heat flux is assumed in that part of the solid that is cooled by a mixture of vapor and liquid droplets (the precursory cooling). The solution, which is of a simple form, is found to be in excellent agreement with a solution by separation of variables, previously obtained for the same problem. Numerical results are presented for a wide range of model parameters.

Biomodulator-mediated susceptibility of endogenous lipid droplets from rat adipocytes to hormone-sensitive lipase

The amount of fatty acid release by a fat cell homogenate without pretreatment with epinephrine was found to be slightly more than that released from fat cells by epinephrine, suggesting that fat cells contain high lipolytic activity even in the absence of lipolytic agents. Fat cells contain high hormone-sensitive lipase activity (1383 mumole free fatty acids/g/hr) in the absence of epinephrine, and addition of epinephrine to the cells did not increase the activity, significantly. Like epinephrine, DBcAMP and/or theophylline also elicited marked release of glycerol from fat cells without activating the hormone-sensitive lipase activity. However, although fat cells contain a large amount of hormone-sensitive lipase, lipolysis was negligible in the absence of these lipolytic agents. These results suggest that lipolytic agents such as epinephrine, DBcAMP, and theophylline induce lipolysis in fat cells through some mechanism other than activation of hormone-sensitive lipase and that in the absence of lipolytic agents, some system in fat cells inhibits lipolysis of endogenous lipid droplets by hormone-sensitive lipase. The lipid droplets in fat cells consist mainly of triglyceride with phospholipids, cholesterol, carbohydrate, and protein as minor constituents. The phospholipid fraction was found to consist of 75% phosphatidylcholine and 25% phosphatidylethanolamine. Of the minor constituents of endogenous lipid droplets, only phosphatidylcholine strongly inhibited hormone-sensitive lipase activity in a [3H]triolein emulsion. These results suggest that phosphatidylcholine in endogenous lipid droplets may be responsible for inhibition of hormone-sensitive lipase. Then, a cell-free system was established in which epinephrine, DBcAMP, and theophylline stimulated lipolysis of endogenous lipid droplets from fat cells by lipase solution. In this system, these lipolytic agents did not induce lipolysis in the absence of added lipase. Lipolysis in the mixture of the endogenous lipid droplets and lipase solution was accelerated by phospholipase C with concomitant loss of epinephrine-induced lipolysis. After pretreatment of the endogenous lipid droplets with phospholipase C, these lipolytic agents no longer induced lipolysis. Pretreatment of the endogenous lipid droplets with phospholipase C reduced their phospholipid content with the formation of phosphorylcholine, but did not affect their triglyceride and cholesterol contents. Treatment of the endogenous lipid droplets with phospholipase D did not affect lipolysis in the cell-free system. These results suggest that phosphatidylcholine in the endogenous lipid droplets may inhibit their lipolysis by hormone-sensitive lipase in fat cells and also be involved in the mechanisms of the stimulatory effects of epinephrine, DBcAMP, and theophylline on lipolysis.

Analysis of vapor barrier experiments to evaluate their effectiveness as a means to mitigate HF cloud concentrations

Accidental release of Hydrogen Fluoride (HF) can result in initially dense, highly reactive and corrosive gas clouds. These clouds will typically contain a mixture of gases, aerosols and droplets which can be transported significant distances before lower hazard levels of HF concentration are reached. Previous related field and laboratory experiments have been analyzed to estimate the effectiveness of barrier devices. The experiments were examined to determine their relevance to Hydrogen Fluoride spill scenarios. Wind tunnel and field data were compared where possible to validate the laboratory experiments. Barrier influence on peak concentrations, cloud arrival time, peak concentration arrival time, and cloud departure time were determined. These data were used to develop entrainment models to incorporate into integral and depth averaged numerical models. The models were then run to examine barrier performance for a typical Hydrogen Fluoride spill for a wide range of vapor barrier heights, spill sizes, meteorological conditions and release configurations. Finally the results of the data analysis and numerical sensitivity study were interpreted and expressed in a form useful to evaluate the efficiency of vapor barrier mitigation devices.