Multicomponent Droplet Research Articles

Bicomponent droplets evaporation: Temperature measurements and modelling

Development of improved energy conversion systems, having higher efficiencies and lower emissions, is central to reducing the production of green-house gases and to meeting air quality standards for other emissions. In non-premixed combustion systems, vaporization of the droplets and mixing of the vapour with the surrounding oxidizer control the overall rate of energy release. Droplet vaporization is affected by the nature of the liquid petroleum that contains species having extremely different volatilities. A fine understanding of multi-component droplet vaporization is therefore an important issue to optimize the combustion systems. This paper presents the application of a recently developed technique to measure the temperature of bi-component droplets. Based on the three-color laser-induced fluorescence (LIF) technique, droplet temperatures can be measured regardless of the composition. The method requires adding a small amount of a fluorescent organic dye to the fuel which is composed of ethyl-alcohol and acetone. The accuracy of the measurement is about ±1.3 °C. In this study, the emphasis is placed on the evaporation of the binary mixture in a hot air plume, the temperature of which (around 650 °C) largely exceeds the boiling point of the liquid components. An extensive study of the influence of the initial composition and droplet diameter is carried out. Finally, the experimental results are compared to an evaporation model based on the discrete components approach.

Validation of Activity Coefficient Models Using Resonances in Light Scattering from Evaporating Multicomponent Droplets

We present a technique for determining parameters of any activity coefficient model from the times at which resonances appear in light scattering spectra from an evaporating multicomponent droplet of known initial composition. Using individual component droplet evaporation data along with assumed values for the parameters of a chosen activity coefficient model, the size and the composition of the droplet as functions of time are theoretically predicted, and the times at which resonances appear are calculated from Mie theory. By minimizing the errors between the calculated and observed resonance appearance times, the optimum parameter values of the activity coefficient model are established. The technique has been applied to examine binary and ternary systems consisting of diethyl phthalate, dipropyl phthalate, and dimethyl phthalate. In addition, the technique has been validated by comparing the prediction from the optimum parameter values with independently determined activity coefficient data for binary mixtures of dioctyl phthalate and dimethyl phthalate.

Effects of temporally varying liquid-phase mass diffusivity in multicomponent droplet gasification

The relative roles of liquid-phase diffusional resistance and volatility differential in multicomponent droplet gasification are revisited, recognizing that liquid-phase mass diffusivities can be substantially increased as the droplet is progressively heated upon initiation of gasification, leading to a corresponding substantial weakening of the diffusional resistance. Calculations performed using realistic and temperature-dependent thermal and mass diffusivities indeed substantiate this influence. In particular, the calculated results agree with the literature experimental data, indicating that the gasification mechanism of multicomponent fuels is intermediate between diffusion and distillation limits. Investigation was also performed on gasification at elevated pressures, recognizing that the liquid boiling point and hence the attainable droplet temperature would increase with increasing pressure, causing further weakening of the liquid-phase diffusional resistance. This possibility was again verified through calculated results, suggesting further departure from diffusion limit toward distillation limit behavior for gasification at high pressures. The study also found that diffusional resistance is stronger for the lighter, gasoline-like fuels as compared to the heavier, diesel-like fuels because the former have overall lower boiling points, lower attainable droplet temperatures, and hence lower mass diffusivities in spite of their lower molecular weights.

Vaporization and pyrolysis modelling of a single droplet of heavy fuel oil using continuous thermodynamics

This article presents modelling of vaporization and pyrolysis of a single droplet of heavy fuel oil in a high ambient temperature field using the principle of continuous thermodynamics. Continuous thermodynamics reduces the computational simulation load compared with discrete thermodynamics, without compromising the quality of prediction of the complex combustion behaviours of such multicomponent complex fuels. Heavy fuel oil is represented by four fuel fractions and each of these fractions are assigned a separate distribution function. The prediction shows that in the modelling of heavy fuel oil, both heating rate and composition are very important parameters in coke formation. Coke formation can lead to engine degradation and increased exhaust smoke. The developed model shows good agreement with experimental results obtained by other researchers. References Goldsworthy, L., CFD Modelling of residual fuel oil combustion in the context of marine diesel engines. International Journal of Engine Research , 2005, 7: pp. 181--199. Baert, R. S. G., A mathematical model for heavy fuel oil droplet vaporization and pyrolysis in a heavy temperature inert gas. Combustion Science and Technology , 1993, 90: pp. 125--147. Hallett, W. L. H and C. Grimwood. A simple continuous mixture droplet evaporation model with multiple distribution functions. in Combustion Institute, 2001. Canadian Section. Spring Meeting. Hallett, W. L. H., A simple model for the vaporization of droplets with large numbers of components. Combustion and Flame , 2000, 121(1-2): pp. 334--344. doi:10.1016/S0010-2180(99)00144-3 Chen, C. S. and M. M.EI-Wakil, Experimental and theoretical studies of burning drops of hydrocarbon mixtures. Proceedings for Instrumentation and Mechanical Engineers, 1969, 184 (Pt. 3J): pp. 95--108. Ikegami, M., G. Xu, K. Ikeda, S. Honma, H. Nagaishi, D. L. Dietrich, and Y. Takeshita, Distinctive combustion stages of single heavy oil droplet under microgravity. Fuel , 2003, 82(3): pp. 293--304. doi:10.1016/S0016-2361(02)00257-0 Shyu, R. R., C. S. Chen, G. O. Goudie, and M. M. El-Wakil, Multi-component heavy fuel drop histories in a high-temperature flow field. Fuel , 1972, 51(2): pp. 135--145. Tamim, J. and W. L. H. Hallett, A continuous thermodynamics model for multicomponent droplet vaporization. Chemical Engineering Science, 1995, 50(18): pp. 2933--2942. doi:10.1016/0009-2509(95)00131-N Lippert, A. M. and R. D. Reitz. Modelling of multicomponent fuels using continuous distributions with application to droplet evaporation and sprays. in International Fall Fuels and Lubricants Meeting and Explosion. 1997. Tulsa, Oklahoma, 972882. Lippert, A. M., D. W. Stanton, C. J. Rutland, W. L. H. Hallett, and R. D. Reitz, Multidimensional simulation of diesel engine cold start with advanced physical submodels. International Journal of Engine Research , 2000, 1(1): pp. 1--27. Hallett, W. L. H. and N. A. Clark, A model for the evaporation of biomass pyrolysis oil droplets. Fuel , 2006, 85(4): pp. 532--544. doi:10.1016/j.fuel.2005.08.006

Evaporation of acoustically levitated multi-component liquid droplets

A theoretical model for the evaporation of multi-component liquid droplets based on the model by Abramzon and Sirignano is presented and applied to the evaporation of acoustically levitated droplets. The liquid phase is treated as a thermodynamically real fluid, using the UNIFAC method for calculating the component activities, and the gas phase as ideal. Computational results, which consist in the droplet surface and volume, temperature and composition as functions of time, are verified by experiments carried out with single droplets evaporating in an acoustic levitator. The results are in excellent agreement, suggesting that the model correctly captures the physico-chemical phenomena in multi-component liquid droplet evaporation.

Rainbow refractrometry on particles with radial refractive index gradients

The rainbow refractrometry, under its different configurations (classical and global), is an attractive technique to extract information from droplets in evaporation such as diameter and temperature. Recently a new processing strategy has been developed which increases dramatically the size and refractive index measurements accuracy for homogeneous droplets. Nevertheless, for mono component as well as for multicomponent droplets, the presence of temperature and/or of concentration gradients induce the presence of a gradient of refractive index which affects the interpretation of the recorded signals. In this publication, the effect of radial gradient on rainbow measurements with a high accuracy never reached previously is quantified.

Modeling droplet breakup processes under micro-explosion conditions

This paper presents a numerical model of micro-explosion for multicomponent droplets. The first part of the model addresses the mass and temperature distribution inside the droplet and the bubble growth within the droplet. The bubble generation is described by a homogeneous nucleation theory, and the subsequent bubble growth leads to the final explosion (i.e., breakup). The second part of the model determines when and how the breakup process proceeds. Unlike ad hoc/empirical approaches reported in the literature, the size and velocity of sibling droplets were determined by a linear instability analysis. After validated against available experimental data for bubble growth and homogeneous nucleation, the developed model was first used to study the effects of various parameters on the onset of micro-explosion. It was found that, optimum composition and high ambient pressure favor micro-explosion; however, extremely high pressures suppress micro-explosion because the volatility differential decreases. The vaporization behavior of an oxygenate diesel blend was analyzed at the end. It was found that micro-explosion is possible under typical diesel engine environments for this type of fuel. Occurrence of micro-explosion shortens the droplet lifetime, and this effect is stronger for droplets with larger sizes or a near 50/50 composition.

Quantification of multicomponent droplet evaporation through optical resonances

The evaporation dynamics of a binary droplet is established by aligning resonances observed in light scattering with those calculated from an evaporation model. For an ideal solution droplet the size and composition as functions of time are determined with high precision from the observed resonances, and the underlying evaporation model is unquestionably validated by accurately recovering the properties of the individual components of the droplet. The results suggest that experimental resonances, each containing a unique relation between the size parameter and the refractive index, can be used to accurately quantify multicomponent droplet evaporation through complex models.

EXPERIMENTAL STUDY ON BINARY DROPLET EVAPORATION AT ELEVATED PRESSURES AND TEMPERATURES

The evaporation characteristics of single and multicomponent droplets hanging at the tip of a quartz fiber are studied experimentally at the different environmental conditions under normal gravity. Heptane and Hexadecane are selected as two fuels with different evaporation rates and boiling temperatures. At the first step, the evaporation of single component droplet of both fuels has been examined separately. At the next step the evaporation of several blends of these two fuels, as a binary component droplet, has been studied. The temperature and pressure range is selected between 400 and 700°C, and 0.1 and 2.5 MPa, respectively. A high-temperature environment was provided by a falling electrical furnace. The initial diameter of droplet was in the range of 1.1 and 1.3 mm. The evaporation process was recorded by a high-speed CCD camera. The results of binary droplet evaporation show the three-stage evaporation. In the first stage the more volatile component evaporates. The droplet temperature rises after an almost non-evaporative period and, in the third stage, a quasi-linear evaporation takes place. The evaporation of the binary droplet at low pressure is accompanied with bubble formation and droplet fragmentation and leads to incomplete microexplosion. The component concentration affects the evaporation behavior of the first two stages. The bubble formation and droplet distortion does not appear at high environment pressure.

Temperature measurements of binary droplets using three-color laser-induced fluorescence

Evaporation of multicomponent droplets is a critical problem in many engineering applications, for example spray combustion. Knowledge of droplet temperature is a key issue in understanding the highly complex heat and mass-transfer phenomena related to multicomponent droplet evaporation and combustion. In this work, optical diagnosis based on three color-laser-induced fluorescence was developed: the objective was to measure the temperature of binary droplets (ethanol and acetone mixtures), even when the composition varies with time. Demonstration on an overheated droplet stream of acetone–ethanol mixtures is described and the experimental data are compared with results from a numerical simulation based on the discrete-components model.

A Comparative Study of the Mass and Heat Transfer Dynamics of Evaporating Ethanol/Water, Methanol/Water, and 1-Propanol/Water Aerosol Droplets

The mass and heat transfer dynamics of evaporating multicomponent alcohol/water droplets have been probed experimentally by examining changes in the near surface droplet composition and average droplet temperature using cavity-enhanced Raman scattering (CERS) and laser-induced fluorescence (LIF). The CERS technique provides a sensitive measure of the concentration of the volatile alcohol component in the outer shell of the droplet, due to the exponential relationship between CERS intensity and species concentration. Such volatile droplets, which are probed on a millisecond time scale, evaporate nonisothermally, resulting in both temperature and concentration gradients, as confirmed by comparisons between experimental measurements and quasi-steady state model calculations. An excellent agreement between the experimental evaporation trends and quasi-steady state model predictions is observed. An unexpectedly slow evaporation rate is observed for the evaporation of 1-propanol from a multicomponent droplet when compared to the model; possible explanations for this observation are discussed. In addition, the propagation depth of the CERS signal, and, therefore, the region of the droplet from which compositional measurements are made, can be estimated. Such measurements, when considered in conjunction with quasi-steady state theory, can allow droplet temperature gradients to be measured and vapor pressures and activity coefficients of components within the droplet to be determined.

A quantitative demonstration of the enhancement of cavity enhanced Raman scattering by broad band external laser seeding

We demonstrate in this letter an ability to manipulate threshold concentrations for the observation of cavity enhanced Raman scattering (CERS) in microdroplets, and to enhance the intensity of scatter from a selected component in binary component (nitrate/water) droplets. Measurements from droplets of varying composition have shown that the concentration of the minor component can be quantitatively determined from the optically seeded signal. Various strategies are described by which this method may be more generally applied to enhance stimulated Raman scattering intensities of minor components in multicomponent droplets.

Measurements of multicomponent microdroplet evaporation by using Rainbow Refractometer and PDA

The evaporation process of a multi-component droplet has been studied utilizing optical diagnostic techniques. Investigation focused on the measurements of the different volatility mixtures and the variation of the mass fraction within a droplet of different components. The method for determining the temperature of a multicomponent droplet containing nonvolatile and volatile compositions has been proposed. For a droplet with two volatile compositions, such as water and ethanol mixture, the model of evaporation for droplet temperature measurements was also suggested based on the experimental data. Droplets with different mass fractions of each component under different temperature conditions were measured utilizing a primary rainbow refractometer and a recently improved phase-Doppler analyzer. It was also found that the droplet temperature could hardly reach its saturation temperature within the droplet unlike explained by previous researchers. The effects of temperature gradient within an evaporating microdroplet and the temperature discontinuity on the evaporating surface have been discussed.

A model for the evaporation of biomass pyrolysis oil droplets

This paper presents a numerical model for the evaporation and pyrolysis of a single droplet of pyrolysis oil derived from biomass. Continuous thermodynamics theory for multi-component droplet evaporation is used, with the fuel being represented by four fractions: organic acids, aldehydes/ketones, water, and pyrolytic lignin, each of which is described by a separate distribution function. Pyrolysis of the lignin fraction is included, and detailed properties for all fractions are presented. The model is compared with the results of suspended droplet experiments, and is shown to give good predictions of the times of the major events in the lifetime of a droplet.

A Film-Theory-Based Model for a Multicomponent Droplet Evaporation at Both Low- and High-Pressure Environments

Modeling of a multicomponent droplet evaporation is investigated based on the film theory in both low- and high-pressure environments. Unlike the classical film theory, effects of blowing due to evaporation on gas-side heat and mass transfer are included through the film thicknesses. The corresponding gas-side heat and mass transfer equations are derived in terms of film thicknesses. In a high-pressure situation, the real gas behavior is considered. Based on the derived equations, a new model for multicomponent droplet evaporation is formulated and validated against the published data.

Droplet and Particle Dynamics during Flame Spray Synthesis of Nanoparticles

Particle formation by flame spray pyrolysis is investigated theoretically and compared with particle size distribution data of ZrO2 production at 100−300 g/h. Spray droplet size distributions as well as droplet and gas velocities are measured by phase Doppler anemometry along the axis and radius of the solid-cone-type spray. Multicomponent droplet combustion is simulated, accounting for droplet polydispersity, transport, and evaporation. The dynamics of the product particle size distributions are investigated, accounting for coagulation and sintering by population balance equations. The effect of the initial droplet size and liquid feed rate on the evolution of the product particle size and polydispersity is analyzed.

Computational Models for Simulating Multicomponent Aerosol Evaporation in the Upper Respiratory Airways

An effective model for predicting multicomponent aerosol evaporation in the upper respiratory system that is capable of estimating the vaporization of individual components is needed for accurate dosimetry and toxicology analyses. In this study, the performance of evaporation models for multicomponent droplets over a range of volatilities is evaluated based on comparisons to available experimental results for conditions similar to aerosols in the upper respiratory tract. Models considered include a semiempirical correlation approach as well as resolved-volume computational simulations of single and multicomponent aerosol evaporations to test the effects of variable gas-phase properties, surface blowing velocity, and internal droplet temperature gradients. Of the parameters assessed, concentration-dependent gas-phase specific heat had the largest effect on evaporation and should be taken into consideration for respiratory aerosols that contain high volatility species, such as n-heptane, at significant concentrations. For heavier droplet components or conditions below body temperatures, semiempirical estimates were shown to be appropriate for respiratory aerosol conditions. In order to reduce the number of equations and properties required for complex mixtures, a resolved-volume evaporation model was used to identify a twelve-component surrogate representation of potentially toxic JP-8 fuel based on comparisons to experimentally reported droplet evaporation data. Due to the relatively slow evaporation rate of JP-8 aerosols, results indicate that a semiempirical evaporation model in conjunction with the identified surrogate mixture provide a computationally efficient method for computing droplet evaporation that can track individual toxic markers. However, semiempirical methodologies are in need of further development to effectively compute the evaporation of other higher volatility aerosols for which variable gas-phase specific heat does play a significant role.

Multi-component droplet growth. II. A theoretical model

A droplet growth model is formulated that describes the growth of homogeneous multi-component droplets suspended in a dilute gas–vapor mixture. The droplet is suspended in a real carrier gas, in which the vapors are diluted. The model is applicable for all Knudsen numbers. It is shown that the model can easily be extended to describe the growth of layered droplets. The model calculations are compared to the results of droplet growth experiments of supersaturated water and/or supersaturated n-nonane in methane. The model performs well for all the cases, except for the case when supersaturated n-nonane vapor is added to supersaturated water vapor in methane gas. In the latter case it is proposed that the discrepancies originate from the incomplete wetting of n-nonane on liquid water at the given conditions.

Multi-component droplet growth. I. Experiments with supersaturated n-nonane vapor and water vapor in methane

Droplet growth rates of droplets suspended in methane gas and supersaturated water and/or n-nonane vapor are experimentally determined. The experiments are performed by applying the nucleation pulse principle using a modified shock tube. The droplets are optically detected using a combination of constant angle Mie scattering and light extinction measurements. From the analysis of the droplet growth rates in the binary systems the diffusion coefficients of water in methane and n-nonane in methane have been determined at two different conditions, being 11 bar and 242 K, and 44 bar and 247 K. The droplet growth rates in the ternary system have been determined at the same two conditions. From the analysis of these experiments it is evident that supersaturated water vapor does not condense onto n-nonane droplets while supersaturated n-nonane vapor does condense onto water droplets. This can be related to the wetting properties of liquid water on liquid n-nonane and vice versa.

Determination of the size and composition of multicomponent ethanol/water droplets by cavity-enhanced Raman scattering

Cavity-enhanced Raman scattering is used to determine the size and composition of multicomponent ethanol/water droplets in the concentration range 7.5–19% ethanol by volume. Under the experimental conditions presented here, the integrated CERS signal from ethanol shows an exponential increase with increase in ethanol concentration when compared with the integrated intensity of the water band. The calibration is shown to be invariant with particle size over the droplet radius range 20–35 μm. In addition to providing a method for determining particle size and composition, initial studies show that the evaporation dynamics of these multicomponent droplets can be probed by CERS.