Bimodal Drop Size Distributions Research Articles

Impact of Rain on Retrieved Warm Cloud Properties Using Visible and Near-Infrared Reflectances Using Markov Chain Monte Carlo Techniques

Estimates of cloud droplet effective radius ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_e</i> ) and optical thickness (τ) can be derived using reflected sunlight in a visible non-absorbing channel combined with reflectances from a near IR channel that is absorbing (e.g., The bi-spectral method or BSM). Discrepancies between BSM-estimated <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_e</i> and collocated in situ measurements are commonly attributed to a violation of the assumptions used in the BSM algorithm such as plane parallel geometry, and a single mode droplet size distribution. This research uses Markov Chain Monte Carlo experiments to examine the impact of precipitation on BSM-retrieved <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_e</i> near optical cloud top by comparing the retrievals and associated uncertainties obtained from two types of experiments assuming a unimodal or bimodal drop size distribution. Where rain is present, BSM-retrieved <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_e</i> overestimates the true cloud mode <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_e</i> . Moreover, there is no longer a unique measure of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_e</i> within the precipitating liquid-phased clouds, resulting in a substantial increase in retrieval uncertainties. This leads to a corresponding loss of information on total number concentration and liquid water content near cloud top. It is found that <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_e</i> biases are not strongly correlated with properties exclusively pertaining to rain, such as rain water content or precipitation rates, but tend to be a function of the ratio between rain and cloud water content and the cloud total number concentration. These results highlight the need for additional independent information such as from an active or passive microwave sensor that can identify the presence of precipitation and constrain additional aspects of bimodal droplet distributions.

Raindrop size distribution and microphysical characteristics of a great rainstorm in 2016 in Beijing, China

This paper analyzes the characteristics of rain drop size distributions (DSDs) and vertical profiles of hydrometeors for a severe rainstorm event that occurred in Beijing, northern China, from 19 to 21 July 2016 using in situ measurements from two disdrometers (OTT-Parsivel2 and 2D-Video-Distrometer) and remote sensing data collected by a second-generation Micro Rain Radar (MRR2). This rainstorm was characterized by stratiform precipitation with embedded strong convection on 19 July and widespread, intense mixed convective-stratified rainfall resulting in persistent rainfall between 20 and 21 July. The temporal and spatial variations in rainfall rate R, liquid water content W, reflectivity Z, DSDs and the gamma DSD parameters in different precipitation episodes of this event are derived and compared between two meteorological stations in Beijing. Rainfall throughout the entire event was characterized by large concentrations of small- to medium-sized raindrops (diameters between 2 and 4 mm) and few larger particles (D > 4 mm), resulting in small values of reflectivity Z (<40 dBZ) and liquid water content W (<1.0 g m−3). In addition, the log10Nw-Dm scatter pairs recorded during the convective phase of this rainstorm indicate maritime-like convection. The time evolution of DSDs shows that most single peak DSDs occurred during stratiform rainfall, and bimodal DSDs were observed when the convective rainfall cells passed by and collisional breakups occurred. The gamma DSD model with a single peak in the stratiform phase of the storm and the bimodal DSD in the convective episode of the storm are quantitatively evaluated. In addition, in each precipitation episode, the differences in exponent b and coefficient A of the Z-R relationship between the two stations are smaller for stratiform rainfall than for convective rainfall due to the more evident variability of drop size and number concentration in convective rain. The physical nature of this relationship and the possible mechanisms leading to diverse characteristics in different phases of this heavy rainstorm are discussed.

DROP DYNAMICS IN HETEROGENEOUS SPRAY FLAMES FOR NANOPARTICLE SYNTHESIS

In flame spray pyrolysis (FSP), the understanding of the atomization and spray formation of the coaxial atomizer is a key step to control the mixing of reactants, and precursor release, as well as nanoparticle formation and growth mechanisms. The drop size and drop dynamics of fuel/precursor mixtures are relevant parameters to control the particle synthesis via gas-to-particle conversion for tailoring the nanoparticle. The investigation of the multiphase flow structures, especially in the dense spray region using sophisticated nonintrusive laser diagnostics, such as phase Doppler anemometry (PDA), enables a characterization of the burner with multiscale spatial and temporal resolution. In the present work, the evolution of drop size and velocity under varying iron nitrate (FNT) concentration of the precursor is investigated allowing for an estimation of precursor release and evaporation via detailed PDA measurements. The drop size distributions of pure ethanol, and ethanol + 2-ethylhexanoic acid, as well as for FNT-based spray flames, were studied in order to reveal the influence of metal precursors on the combustion. Drop size/velocity correlations were determined in order to resolve the size class averaged evolution of drop velocities. The evolution of the drop size distribution along the axial centerline revealed a variation of the evaporation rate in space and increased evaporation rates for FNT containing spray flames. Furthermore, bimodal drop size distributions were observed for heights above the burner larger than 40 mm in FNT precursor spray flames, indicating the occurrence of possible μ-explosions of drops, that have been reported in the literature for similar systems but only in single-droplet experiments so far.

Linking continuous and recycle emulsification kinetics for in-line mixers

In-line high-shear mixers can be used for continuous or batch dispersion operations depending on how the pipework is arranged. In our previous work (Carrillo De Hert and Rodgers, 2017a) we performed a transient mass balance to establish the link in-between these two arrangements; however this model was limited to the estimation of the mode of dispersed phases yielding simple monomodal drop size distributions. In this investigation we expanded the previous model to account for the shape of the whole drop size distribution. The new model was tested by performing experiments under different processing conditions and using two highly viscous dispersed phases which yield bimodal drop size distributions. The results for the continuous arrangement experiments were fit using two log-normal functions and the results for the recycle arrangement by implementing the log-normal function in the previously published mass balance. The new model was capable of predicting the d¯32 for different emulsification times with a mean absolute error of 12.32%. The model presented here was developed for liquid blends, however the same approach could be used for milling or de-agglomeration operations.

Study of Bimodal Drop Size Distributions of Emulsion

In order to increase the present understanding of bimodal emulsion drop size distributions, systematic series of experiments have been carried out to investigate the effects of formulation variables on bimodal drop size distributions, and probability distribution functions were proposed to analyze the distribution. The results show that, the span of the drop size and Sauter mean diameter become larger when the dispersed phase volume fraction becomes higher and rotor speed becomes lower; the Frechet function represents the experimental data satisfactorily. The prediction model of Sauter mean diameter established by combining the prediction theory of the maximum stable drop diameter and experimental analysis results can fit the experimental data well.

Single-Pass Emulsification Processes in Two Different Inline High Shear Mixers

Single-pass continuous emulsifications were studied in kerosene– and silicone oil–aqueous systems with two inline high shear mixers (HSMs) adopting two main commercial configurations, including the dual-row ultrafine-toothed and the single-row blade–screen units. The effects of the processing parameters on the measured drop sizes and power consumptions were investigated. Bimodal drop size distributions (DSDs) were observed within both inline HSMs due to the inhomogeneity of the turbulence and shear levels accompanied with the recirculation and re-entrainment flow patterns. The drop sizes increase with increases in the dispersed-phase volume fraction and the continuous-phase flow rate and decrease with increases in the rotor speed and the continuous-phase viscosity. Correlations for the Sauter mean diameters were also obtained. The results obtained here are fundamental for the assessment of emulsification capability as well as the design and selection of inline HSMs.

Measurement and analysis of bimodal drop size distribution in a rotor–stator homogenizer

Rotor–stator homogenizers are widely used to manufacture liquid–liquid emulsions, but little emphasis has been placed on understanding the processing principles of these devices. In this study, a systematic series of experiments have been performed to investigate and quantify the effects of process parameters and formulation variables on the drop size distribution (DSD) and relative sizes of oil-in-water emulsions manufactured in a Rotor–stator homogenizer. The effects of rotor speed, the dispersed phase viscosity and the dispersed phase volume fraction were studied. The DSDs by volume were predominantly bimodal, and the smaller drop size peak appeared at 1μm for most of the oil-in-water emulsions in our experiment. Furthermore, the relationship between the Sauter mean diameter and the maximum stable drop diameter was found to be linear in emulsions with a bimodal volume distribution, and the proportionality constant somewhat decreases as the viscosity of the dispersed phase increases. Compared with the normal and log-normal function, the Frechet function, which is usually proposed to describe the extreme value problem, can describe the monomodal number DSD very well. The experiments also show that the low-viscosity droplets are stabilized by a combination of interfacial tension and viscous forces, while viscous forces primarily prevent breakage in high viscosity droplets. A modified version of Calabrese's model has been developed for combining the effect of the dispersed phase viscosity and volume fraction on drop size, which represents the experimental data well.

The Role of Small Soluble Aerosols in the Microphysics of Deep Maritime Clouds

AbstractSome observational evidence—such as bimodal drop size distributions, comparatively high concentrations of supercooled drops at upper levels, high concentrations of small ice crystals in cloud anvils leading to high optical depth, and lightning in the eyewalls of hurricanes—indicates that the traditional view of the microphysics of deep tropical maritime clouds requires, possibly, some revisions. In the present study it is shown that the observed phenomena listed above can be attributed to the presence of small cloud condensation nuclei (CCN) with diameters less than about 0.05 μm. An increase in vertical velocity above cloud base can lead to an increase in supersaturation and to activation of the smallest CCN, resulting in production of new droplets several kilometers above the cloud base. A significant increase in supersaturation can be also caused by a decrease in droplet concentration during intense warm rain formation accompanied by an intense vertical velocity. This increase in supersaturation also can trigger in-cloud nucleation and formation of small droplets. Another reason for an increase in supersaturation and in-cloud nucleation can be riming, resulting in a decrease in droplet concentration. It has been shown that successive growth of new nucleated droplets increases supercooled water content and leads to significant ice crystal concentrations aloft. The analysis of the synergetic effect of the smallest CCN and giant CCN on production of supercooled water and ice crystals in cloud anvils allows reconsideration of the role of giant CCN. Significant effects of small aerosols on precipitation and cloud updrafts have been found. The possible role of these small aerosols as well as small aerosols with combination of giant CCN in creating conditions favorable for lightning in deep maritime clouds is discussed.

Cloud microphysical properties over Indian monsoon regions during CAIPEEX-2009

Cloud microphysical data collected from an instrumented aircraft in the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX-2009) during May–September 2009 have been used to examine the nature of cloud drop size distributions (DSD), cloud drop effective radius (RE) and their height variations at different locations during tropical Indian monsoons. Single mode drop size distributions were observed over Pathankot, Hyderabad and Bengaluru regions and bimodal DSD were recorded often over Guwahati and Bareilly regions. DSD spectral width showed height variation, being narrow at lower heights and broadening with increasing height. DSD spectra were narrow even at higher levels over Pathankot during pre-monsoon season and were very broad at Bareilly and Guwahati during the active phase of monsoon. The total concentrations of cloud droplets and percentage contribution of cloud droplet of radii≤10μm (small) and >10μm (large) showed interesting height variations and were different over different regions. The RE showed nearly linear increases with height over all the regions. However, the droplet growth rate is observed to be different over different regions, being less over north (Bareilly and Pathankot: 1.3–1.46μm/km), intermediate over central (Hyderabad: 1.74μm/km) and highest over northeast (Guwahati: 1.92μm/km) and south (Bengaluru: 1.99μm/km) India.For the first time an attempt has been made to collect and explore cloud microphysical characteristics using in-situ aircraft observations during Indian monsoon conditions.

Dynamics of Transitional Phase Inversion Emulsification: Effect of Addition Time on the Type of Inversion and Drop Size

Emulsification processes are very sensitive to the time scale during which the dispersed phase is introduced into the continuous phase. A series of experiments were conducted across a transitional phase inversion path, which often leads to formation of nanoemulsions, with the speed of incorporation of the second phase (water) into the first phase (oil) being altered. The optimum condition could only be achieved if the optimum composition was maintained for a critical time. A slow addition (addition time > 60 s) of water to the oil allows the transitional phase inversion to become operative, leading to formation of sub-micrometer droplets. A very fast addition of the water phase (<5.0 s) caused the catastrophic phase inversion mechanism to become dominant, leading to formation of rather large drops. In the intermediate range of addition time, 20−40 s, both inversion mechanisms have contributed to drop formation, at least locally, and as a result skewed or bimodal drop size distributions were formed. The results indicate that while spontaneous emulsification is fast, it is not instantaneous. At a low surfactant concentration, the droplet size was only slightly affected by the rate of addition.

The Drop Size in Membrane Emulsification Determined from the Balance of Capillary and Hydrodynamic Forces

Here, we investigate experimentally and theoretically the factors that determine the size of the emulsion droplets produced by membrane emulsification in "batch regime" (without applied crossflow). Hydrophilic glass membranes of pore diameters between 1 and 10 mum have been used to obtain oil-in-water emulsions. The working surfactant concentrations are high enough to prevent drop coalescence. Under such conditions, the size of the formed drops does not depend on the surfactant type and concentration, on the interfacial tension, or on the increase of viscosity of the inner (oil) phase. The drops are monodisperse when the working transmembrane pressure is slightly above the critical pressure for drop breakup. At higher pressures, the size distribution becomes bimodal: a superposition of a "normal" peak of monodisperse drops and an "anomalous" peak of polydisperse drops is observed. The theoretical model assumes that, at the moment of breakup, the hydrodynamic ejection force acting on the drop is equal to the critical capillary force that corresponds to the stability-instability transition in the drop shape. The derived equations are applied to predict the mean size of the obtained drops in regimes of constant flow rate and constant transmembrane pressure. Agreement between theory and experiment is established for the latter regime, which corresponds to our experimental conditions. The transition from unimodal to bimodal drop size distribution upon increase of the transmembrane pressure can be interpreted in terms of the transition from "dripping" to "jetting" mechanisms of drop detachment.

Water in oil emulsion droplet size characterization using a pulsed field gradient with diffusion editing (PFG-DE) NMR technique

This paper describes a proton nuclear magnetic resonance (NMR) technique, pulsed field gradient with diffusion editing (PFG-DE), to quantify drop size distributions of brine/crude oil emulsions. The drop size distributions obtained from this technique were compared to results from the traditional pulsed field gradient (PFG) technique. The PFG-DE technique provides both transverse relaxation (T2) and drop size distributions simultaneously. In addition, the PFG-DE technique does not assume a form of the drop size distribution. An algorithm for the selection of the optimal parameters to use in a PFG-DE measurement is described in this paper. The PFG-DE technique is shown to have the ability to resolve drop size distributions when the T2 distribution of the emulsified brine overlaps either the crude oil or the bulk brine T2 distribution. Finally, the PFG-DE technique is shown to have the ability to resolve a bimodal drop size distribution.

Bimodal drop size distributions during the early stages of shear induced coalescence

Drop sizes and drop size distributions were determined by means of an optical shear cell in combination with an optical microscope for the systems polyisobutylene/poly(dimethylsiloxane) [I] and poly(dimethyl- co-methylphenylsiloxane)/poly(dimethylsiloxane) [II] at low concentrations of the suspended phases and at different constant shear rates ranging from 10 to 0.5 s −1. After pre-shearing the two-phase mixtures [I: 50 s −1; II: 100 s −1] for the purpose of producing small drop radii, the shear rate was abruptly reduced to the preselected value and coalescence was studied as a function of time. In all cases one approaches dead end drop radii, i.e. breakup is absent. The drop size distributions are for sufficiently long shearing always unimodal, but within the early stages of coalescence they are in some cases bimodal; the shape of the different peaks is invariably Gaussian. The results are discussed by means of Elmendorp diagrams and interpreted in terms of collision frequencies and collision efficiencies.

A population balance model for disperse systems: Drop size distribution in emulsion

A mechanistic model for the description and prediction of the drop size distribution in a turbulently agitated emulsion dispersion is developed. Based on this model, breakage rate and relative distribution functions are determined and used to solve the general population balance equation for a continuous flow vessel. Parameters of the models are evaluated from experimental data by means of Marquardt’s method. The model is found to be capable of predicting bimodal drop size distributions of emulsions.

Dynamic simulation of bimodal drop size distributions in low-coalescence batch dispersion systems

Abstract Transient drop size distributions in a batch stirred tank are a manifestation of the drop breakage and coalescence mechanisms. A recently developed experimental configuration using a laser diffraction technique suitable for on-line monitoring of drop size distributions, showed increased sensitivity in measuring the small diameter drops. For all experimental conditions tested it was observed that the batch low-coalescing dispersion system assumed characteristic bimodal distributions within very short agitation time. The laser diffraction technique revealed the fine changes of the small size peak of the distribution with sufficiently high resolution. The objective of the present study was to establish a breakage mechanism in order to predict the dynamic behaviour of the bimodal drop size distributions by using the population balance formalism. The proposed model considers breakage of the drops into one daughter and several satellite drops. The volume ratio between the generated drops as well as the number of satellite drops are considered to be proportional to the parent drop volume. The obtained agreement between the experimental and theoretical bimodal drop size distributions is quite satisfactory for a range of experimental conditions.

The Life Cycle of Valley Fog. Part II: Fog Microphysics

Extensive measurements were made of the microphysics of valley fog in the Chemung River Valley near Elmira, New York. This paper discusses data on drop size distributions, drop concentrations, liquid water contents, and haze and cloud nucleus concentrations obtained on eight fog nights. The behavior patterns of the microphysical variables were found to be extremely consistent. Shallow ground fog usually occurs prior to the formation of deep valley fog. The data show that ground fog is characterized by droplet concentrations of 100 to 200 per cubic centimeter in the 1 to 10 μm radius range with mean radii of 2 to 4 μm. As deep fog forms aloft, droplet concentration near the surface decreases to less than 2 cm−3 and the mean radius increases from 6 to 12 μm. Droplets of radii <3 μm disappear. Thereafter, droplet concentration and liquid water content increase gradually until the first visibility minimum at the surface when typical values range from 12 to 25 cm−3 and 50 to 150 mg cm−3, respectively. The small droplets reappear at first visibility minimum. Subsequently, bimodal drop size distributions occur in approximately half of the fogs with one mode at 2–3 μm radius and a second mode between 6 and 12 μm. Aloft, drop size distributions become narrower and the mean radius decreases with both increasing altitude and increasing age of the fog. The cloud nucleus concentration active at S = 3.0% is usually between 800 and 1000 cm−3 near the surface and decreases to 500–800 cm−3 at 300 m. It is argued from the data that supersaturation in the thin ground fog exceeds that in deep fog. The initial surface obscuration in deep fog appears to be due to droplets that form aloft and are transported downward into unsaturated air by turbulent diffusion. New droplets are apparently not generated near the surface until after the first visibility minimum.