Multicomponent Droplet Research Articles

An automated imaging and control system for the continuous determination of size and relative mass of single compositionally dynamic droplets

A novel droplet imaging system (DIS) has been developed which, when interfaced with an electrodynamic balance (EDB), offers unique capabilities for characterizing size, mass, density, and composition of single, compositionally dynamic droplets. The DIS employs a standard closed-circuit video camera as a droplet sensor. Real-time hardware image processing techniques are employed to extract cross-sectional area and position information from the video signals. The position information is used to generate a control signal which is proportional to the droplet mass. The DIS capabilities are demonstrated for evaporating water and coal-water slurry droplets. The results presented clearly demonstrate that this system can be applied to easily detect compositional variations from one droplet to another. Accurate estimates of droplet composition can be obtained with little knowledge of the starting composition. The DIS has a wide range of applications and is particularly valuable when applied to the study of multicomponent droplets where the physical and chemical behavior often times depends on the droplet composition.

Simulation of the influence of various additives to a multicomponent droplet on its stability

Simulation of the influence of various additives to a multicomponent droplet on its stability

Simulation of the behaviour of a multicomponent droplet in varying atmospheres

Simulation of the behaviour of a multicomponent droplet in varying atmospheres

A semi-empirical approach to thermal and composition transients inside vaporizing fuel droplets

A simple, semi-empirical model of thermal and composition transients inside droplets is compared with experimental data and with a more detailed model. The semi-empirical model, which lumps the effects of internal circulation into effective thermal and mass diffusivities, is viewed as a possible practical alternative to the prohibitively complex nature of most transient droplet models. A Hill's vortex mixing model due to Tong & Sirignano for vaporizing multicomponent droplets at high gas phase Reynolds numbers is found to be well represented for the cases studied by increasing the molecular diffusivities by a factor of 2.25, while a significantly larger factor is found to best fit available high Reynolds number experimental data. This suggests that more extensive mixing occurred in the experiments than the Tong & Sirignano model would otherwise predict.

An experimental study of high-pressure bubble growth within multicomponent liquid droplets levitated in a flowing stream of another immiscible liquid

Measurements of vaporization rates of highly superheated liquid droplets are reported at reduced pressures up to 0.4 and reduced temperatures up to 0.95. Binary and ternary mixtures of n -pentane, n -hexane, and n -decane, were studied. The experimental method involved levitating a test droplet in a flowing stream of another immiscible liquid (glycerine). Levitation was achieved by balancing the buoyancy and drag force on the droplet. Bubble formation within the droplet was induced by isothermally decompressing the field liquid to pressures approaching the homogeneous nucleation limit of the levitated droplet. It was observed that a droplet consisting of a binary n -pentane- n -hexane mixture vaporized faster than either an n -pentane or n -hexane droplet, and that the vaporization rate decreased with increasing press­ure. Replacing part of the non-volatile component of a n -pentane— n -hexane mixture by a yet more non-volatile liquid ( n -decane), thus forming a ternary pentane-hexane-decane mixture, was found to increase the vaporization rate above that of a droplet consisting of a binary mixture of the most volatile component with any one of the other two components. The results are discussed from the perspective of bubble growth within a liquid mixture of infinite extent.

Multicomponent droplet vaporization in a high temperature gas

The problem of transient vaporization of a multicomponent droplet in a hot convective environment is examined. A simplified model which accounts for liquid-phase internal circulation, transient droplet diffusion, and axisymmetric gas-phase convection is obtained. The results are in close agreement with the results of a more detailed model. The present simplified model is also compared with several existing gas-phase and liquid-phase models. The spherical diffusion limit model and the present simplified model are recommended for stagnant and convective situations, respectively. Attempts have been made to compare the results of the various models with the existing experimental data. Due to large scattering in the experimental measurements, no final conclusion can be drawn based on the experimental data.

Multicomponent Transient Droplet Vaporization with Internal Circulation: Integral Equation Formulation and Approximate Solution

Multicomponent Transient Droplet Vaporization with Internal Circulation: Integral Equation Formulation and Approximate Solution

Aerosols of smoke, respiratory physiology and deposition.

Tobacco smoke is discussed as a multi-component droplet aerosol system of finite airborne life time which changes rapidly right after formation near the combustion zone of the tobacco and continues to change gradually when aging and approaching a multiphase steady state between vaporized smoke constituents mixed into the air and the dispersed particulate phase which will eventually vanish because of physical mechanisms removing the smoke particles from the airborne state. Data on chemical composition and physical characteristics of mainstream and sidestream cigarette smoke are compared and the dynamic models of aerosol particle behavior and deposition in the respiratory tract under different physiological conditions are discussed. While differences in chemical composition between mainstream and sidestream smoke aerosol systems are reported in the literature at least for the gas phase, there seems to be no reliable body of evidence confirming that mainstream and sidestream cigarette smokes have sufficiently different physical characteristics which would cause substantially different deposition patterns and different relative deposition in the respiratory tract. However, there are scanty experimental data in the literature on cigarette smoke deposition in the lung which seem to indicate that mainstream smoke deposition may exceed theoretical expectations while sidestream smoke may not. Further experimental results are needed before firm conclusions can be drawn. An ongoing experimental effort is described where deposition of sidestream cigarette smoke will be measured on mouth breathing test panels.

Combustion and microexplosion of freely falling multicomponent droplets

The combustion characteristics of isolated, low-Reynolds number, multicomponent droplets freely falling in a hot, oxidizing gas flow are studied. Experimental results on two-component fuels substantiate a three-staged combustion behavior, with diffusion being the dominant liquid-phase transport mechanism: that is, there exists an initial period during which the volatile component in the surface layer is preferentially gasified while the droplet temperature is relatively cold; a short, transition, period during which the droplet temperature increases rapidly, the burning rate is extremely low, the flame size shrinks, and the flame temperature diminishes; and a final, quasi-steady period during which the droplet interior concentration distribution remains constant, the surface region is more concentrated with the less volatile component, and the droplet temperature is relatively high. Experimental results on microexplosion show that its occurrence depends sensitively on the mixture concentration as well as the stability of the droplet generation mode, and that frequently nucleation appears to initiate in the vicinity of the droplet center.

Transport Phenomena with Single Aerosol Particles

This is a review of theory and experiments related to single aerosol particle transport processes. The theories of mass, heat, momentum, and charge transfer are outlined, with emphasis on mass transport in the continuum and noncontinuum regimes. Included in the discussion of mass transfer are single and multicomponent droplet evaporation, a comparison of the results of solutions of the Boltzman equations for Knudsen aerosol evaporation and growth and experimental methods for the study of single droplet evaporation or growth. Of particular concern here are the experimental apparati and techniques developed for single particle measurements. These range from the Millikan oil drop experiment through the electrostatic balance (the Millikan condenser with automatic stabilization of the particle) to the electrodynamic balance. The principles and applications of these instruments are reviewed.

On the evaporation rates of multicomponent oil droplets

An experimental method is presented for the measurement of the evaporation rates of supermicronsized multicomponent oil droplets. The fundamental principle of the method rests upon balancing the time-dependent gravitational force on the droplet against an adjustable electrostatic force. Applying continuum diffusion theory to the evaporation process, the total droplet vapor pressure is evaluated both as a function of time and percentage mass evaporated. The evaporation rates of 100 pale oil droplets at 40°C and no. 2 diesel oil droplets at 25 and 40°C are measured. Results indicate vapor pressures measured for 100 pale oil compare favorably with those values extrapolated from distillation curves. In addition for the multicomponent liquids studied here, the vapor pressure is well modeled as an exponential function of percentage mass evaporated.

The relative importance of atmospheric sulfates and nitrates in visibility reduction

Atmospheric visibility is largely determined by the concentration, size distribution and chemical composition of the ambient aerosol. In the present paper, total extinction coefficients were calculated for mixed NH 4NO 3-(NH 4) 2SO 4 aerosols of two different compositions under various humidity conditions. Pertinent thermodynamic and optical properties were measured at 25° C and incorporated into a computer model for droplet growth and Mie calculations. Log-normal particle size distributions with mass median diameters characteristic of the Los Angeles aerosol were assumed throughout the study. The results show that the extent of light extinction by aerosol particles depends on whether the aerosol is present as a multicomponent solution droplets of identical composition (internal mixture) or as a mixture of single-salt aerosols (external mixture|. The extinction coefficients calculated for externally mixed sulfate — nitrate aerosols compare remarkably well with published statistical correlations derived from the Los Angeles visibility data. In addition, it is shown that, for externally mixed sulfate-nitrate aerosols, the sulfate indeed contributes overwhelmingly to visibility reduction, a finding in agreement with general observations for atmospheric aerosols.

Theory of transient multicomponent droplet vaporization in a convective field

The evaporation of a multicomponent droplet in a convective field is analyzed at sufficently high Reynolds number so that internal circulation is fast with respect to diffusion. The theory is an extension of a previous single component analysis 1,2,3 to the case of any number of miscible components. The solution of the equations are shown for several two-component cases. It is found that the transient behavior prevails for both heat and mass transfer under the conditions analyzed. Even though circulation time is fast, gradients of both temperature and mass are significant, in discrepancy with the usual assumptions in this so-called “rapid mixing” limit. The more volatile component evaporates first from the surface, yet remains inside the core; the temperature history is then controlled by the heavy component, exceeding after some time the boiling point of the light component.

Internal boiling and superheating in vaporizing multicomponent droplets

AbstractThe vaporization and combustion of a miscible multicomponent droplet in a quiescent atmosphere is analyzed by assuming transient‐diffusive transport within the droplet, quasi steady diffusive‐convective transport in the gas phase, and ideal solution behavior for the mixture. Results on the vaporization of a binary droplet show that owing to the significant liquid phase diffusional resistance, the vaporization process approximately consists of an initial transient regime, an intermediate, diffusion limited, almost quasi steady regime, and a final volatility limited regime. It is further demonstrated that the entrapment of the volatile components within the rapidly heated droplet interior may lead to the initiation of either homogeneous or heterogeneous nucleation, which can result in the fragmentation of the parent droplet with the internal pressure buildup.

Theory of convective, transient, multicomponent droplet vaporization

The vaporization of a multicomponent droplet in a strong convective gas stream is analyzed, with particular emphasis on the generation of internal circulation through the surface shear and its effects on the internal heat and mass transport processes. It is demonstrated that boundary layers exist in both the gas and liquid phases near the droplet surface over most of its lifetime, and that convection is an important transport mode within the droplet. An approximate analysis for the shoulder region then shows that: the more volatile compounds are always preferentially vaporized; in the liquid the more/less volatile compounds diffuse outward/inward respectively; and the relative rates with which the droplet heats up and conforms to the gas motion are expected to strongly influence the droplet behavior. A more general model, consisting of an exterior potential flow, an interior Hill's Vortex, and boundary layers near the interface, is outlined to study the transient behavior of the droplet.

Evaporation of volatile solvent from saline multi-component droplets carried in a stream of air

We present a theoretical and experimental study on the evaporation of drops of radii 1–8 μm carried through a wind tunnel in a turbulent stream of air at 5 m/sec. Saline solutions are made of mixtures of two electrolytes, NaCl and CaCl 2, in different concentration ratios. It has been demonstrated previously (El Golli et al., 1974) that the lifetime of saline droplets depends on the solvent activity in the solution. Measurements of static vapour pressure of solutions of electrolyte mixtures have been made up to salt saturation and the results of these measurements have been used to determine the lifetime of droplets in different conditions of humidity and temperature. The theory described previously (valid for one electrolyte only) has been extented to solutions of mixed electrolytes. The experimental results have confirmed the validity of the proposed theoretical model.

Multicomponent droplet combustion with rapid internal mixing

Two models are proposed to describe the gas-phase diffusion-controlled, unsteady combustion of a multicomponent droplet in a stagnant, unbounded atmosphere. The first, termed the Ideal-Mixture Model, assumes that the mixture behaves as an ideal mixture in its phase change characteristics, and that the composition and temperature within the droplet are spatially uniform but temporally varying. Expressions are obtained for the droplet vaporization rate and other quantities of interest. Sample solutions indicate that the components vaporize approximately sequentially in the order of their relative volatilities, and that the vaporization rate is insensitive to the mixture composition during combustion as well as during pure vaporization in hot environments. Available experimental evidence supports the theoretical model. The second model, termed the Shell Model, assumes a shelled distribution of the components such that quasi-steady, single-component vaporization prevails for each shell. Simplified solutions are derived and are shown to closely approximate the bulk vaporization behavior described by the more detailed Ideal-Mixture Model, particularly for the prediction of the total vaporization time.

An analysis of chemical reactor stability and control-XV The multicomponent spray reactor

The objective of this paper is to present a continuous stirred-tank model for processes in which a liquid is sprayed into a gas. In this model a uniform spray of droplets enters the reactor along with one or more gaseous reactants. Both the droplets and the gas are taken to be well-mixed in the reactor. The droplets evaporate and the resulting vapors combine homogeneously with the gaseous reactants. In the multicomponent case the droplets contain a mixture of two or more miscible components. Steady state and transient equations are derived, and the numerical methods of solution are discussed. Steady states are calculated for two-component systems of heptane-octane and hydrazine-water and for a three-component system of heptane-octane-nonane, with oxygen as the gaseous reactant in each case. In addition, a study is made of several parameters of the heptane-octane system. Stability is determined by calculating transients for selected perturbations from the steady states. Incorporated in the reactor model is an analytic model for the fluxes of vapors from a multicomponent droplet which exhibits the augmentation of transfer of one vapor due to the net flux of the remaining vapors. Also presented is an approximate model for the multicomponent spray reactor which gives good results for small droplets and requires only a fraction of the effort needed to solve the complete set of equations.

On models for evaporation of a multicomponent droplet

On models for evaporation of a multicomponent droplet

A model for evaporation of a multicomponent droplet

AbstractThe object of this study was to derive a model which would accurately describe the evaporation of a multicomponent droplet, especially near its boiling point. The model is derived and is shown to have the following properties: the transport of one component can be augmented by the bulk (Stefan) flow of the other components, and the droplet can never exceed its boiling point, a property not realized by previous models. The ordinary differential equations which constitute the model were integrated numerically, and results are given for both two‐ and three‐component droplets.