Increase In Droplet Diameter Research Articles

Characterization of the urea-water spray impingement in diesel selective catalytic reduction systems

Exhaust after-treatment selective catalytic reduction (SCR) systems based on urea-water solution are state-of-the-art technologies mitigating NOx emissions for diesel and lean combustion systems. Major challenges for implementing the systems are high NOx reduction performance, uniform mixture formation and solid deposits formation. This study presents a detailed analysis of the urea-water spray wall impingement and influences on reducing agent distribution and deposit formation, thus system performance. High speed images provide detailed information of the impingement process. Moreover, impinging spray mass flux distribution and droplet size distribution have been quantified under typical diesel exhaust flow conditions. The work has been performed with a commercial 3-Hole pressure-driven injector dosing into a flow channel.Under all tested conditions, the impingement is significant. At gas flow conditions of 300°C, 200kg/h, 35.6% of the injected fluid impinges on the opposed wall due to entrainment and evaporation. The entrainment level has been found to scale logarithmically with the gas flow momentum, related correlations are provided. Having an integrated analysis of a mechanical patternator and non-intrusive Phase Doppler Anemometry (PDA) results, it is concluded that droplets below 20μm are completely entrained or evaporated. The impingement rate is gradually increasing with increasing droplet diameter up to 90μm, while almost all larger droplets reach the opposed wall. High speed imaging shows in detail liquid film formation, film transport, liquid accumulation, nucleate boiling, urea crystallization and melting, as well as thin film evaporation prior to solid deposit formation. The spray impingement leads to liquid film formation. Relevant dimensions have been evaluated with digital image processing. Direct relationship of liquid accumulation to the solid deposit formation has been identified. The permanent solid deposit is consisting of cyanuric acid, biuret, ammelide and ammeline as identified by nuclear magnetic resonance spectroscopy.

Transmittance of semitransparent windows with absorbing cap-shaped droplets condensed on their backside

This study aims to investigate systematically light transfer through semitransparent windows with absorbing cap-shaped droplets condensed on their backside as encountered in greenhouses, solar desalination plants, photobioreactors and covered raceway ponds. The Monte Carlo ray-tracing method was used to predict the normal-hemispherical transmittance, reflectance, and normal absorptance accounting for reflection and refraction at the air/droplet, droplet/window, and window/air interfaces and absorption in both the droplets and the window. The droplets were monodisperse or polydisperse and arranged either in an ordered hexagonal pattern or randomly distributed on the backside with droplet contact angle θc ranging between 0 and 180° The normal-hemispherical transmittance was found to be independent of the spatial distribution of droplets. However, it decreased with increasing droplet diameter and polydispersity. The normal-hemispherical transmittance featured four distinct optical regimes for semitransparent window supporting nonabsorbing droplets. These optical regimes were defined based on contact angle and critical angle for internal reflection at the droplet/air interface. However, for strongly absorbing droplets, the normal-hemispherical transmittance (i) decreased monotonously with increasing contact angle for θc< 90° and (ii) remained constant and independent of droplet absorption index kd, droplet mean diameter dm, and contact angle θc for θc≥ 90° Analytical expressions for the normal-hemispherical transmittance were provided in the asymptotic cases when (1) the window was absorbing but the droplets were nonabsorbing with any contact angles θc, and (2) the droplets were strongly absorbing with contact angle θc> 90° Finally, the spectral normal-hemispherical transmittance of a 3mm-thick glass window supporting condensed water droplets for wavelength between 0.4 and 5 μm was predicted and discussed in light of the earlier parametric study and asymptotic behavior.

Comparison of spray collapses at elevated ambient pressure and flash boiling conditions using multi-hole gasoline direct injector

In this study, the spray characteristics of a five-hole gasoline direct injector were investigated in a constant volume vessel. The absolute ambient pressure ranged from 0.5 to 10.0bar, and the fuel temperatures were 20°C and 80°C. High-speed imaging and phase Doppler measurement technique were utilized to investigate the spray morphology and the droplet dynamics, respectively. Spray collapses were observed in the near field under the elevated ambient pressure (higher than 1.0bar) conditions and in the far field under the flash boiling conditions in both macroscopic and microscopic levels. In addition, under the elevated ambient pressure conditions, the droplets at the inner side of the target jet were larger than those at the outer side, due to the increased probability of droplet coalescence as a result of spray collapse. Whilst, slight increase in droplet diameter in the inner side of the target jet was also found under the flash boiling conditions. Furthermore, the near-field collapse at the elevated ambient pressure conditions was attributed to the jet-air interaction, termed as jet-induced spray collapse; and the far-field collapse under the flash boiling conditions was attributed to the dramatic temperature drop and the resultant vapor condensation, termed as condensation-induced spray collapse.

The Effect of Freezing Drizzle, Sleet and Snow on Microphysical Characteristics of Supercooled Fog during the Icing Process in a Mountainous Area

Both the similar and different effects of freezing drizzle, sleet and snow on microphysical properties of supercooled fog were analyzed for fourteen events during a comprehensive wire icing, fog, and precipitation observation experiment conducted at Enshi radar station (30°17′N, 109°16′E; 1722 m a.s.l.) on a hilltop in Shibanling, Hubei, China. Liquid precipitation is in a relatively stable form in mountainous areas. Short-term precipitation (1–3 h) is dominant with temperature below 0 °C. The wet scavenging effect of freezing drizzle on small fog droplets with a size range less than 6–12 μm is weak but is stronger for fog droplets with a larger size, which is opposite to the effects of solid precipitation, broadening the fog droplet spectra significantly. As the fog droplet diameter increases, the distributions of droplet spectra change from leptokurtosis to platykurtosis and from positive skewness to negative skewness. Occurrence of freezing drizzle would improve the positive correlation of N-r in dissipation and oscillation periods, resulting in the N-r relationship having a weak negative correlation in the maturity period, and resulting in the transition of the N-L and N-r relationships into positive correlations in the development period. Meanwhile, the emergence of solid precipitation particles would result in negative values for the correlation coefficients of N-L and N-r. The change in relationships among the microphysical properties was caused by the occurrence of different phase precipitation, showing the influence on the main microphysical mechanisms of supercooled fog.

Detailed analysis of single drop coalescence—Influence of ions on film drainage and coalescence time

Coalescence in liquid–liquid systems is determined by multiple factors, including droplet size ratios and the concentration and type of any continuous phase ions present. When using the standardized EFCE (European Federation of Chemical Engineering) toluene/water standard test system, these factors were systematically investigated based on a high statistical relevant database. The influence on the film drainage time was captured with a high speed imaging system. Shorter coalescence times were found to correlate with higher coalescence probabilities and an observed increase in the coalescence time with increasing droplet diameter agrees well with simple drainage modeling. Coalescence inhibition was also observed with the addition of continuous phase electrolyte. An empirical correlation was developed to account for the influence of equivalent droplet diameter and ion concentration on the coalescence time.

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Three-dimensional Direct Numerical Simulations (DNS) in canonical configuration have been employed to study the combustion of mono-disperse droplet-mist under turbulent flow conditions. A parametric study has been performed for a range of values of droplet equivalence ratio ϕd, droplet diameter ad and root-mean-square value of turbulent velocity u′. The fuel is supplied entirely in liquid phase such that the evaporation of the droplets gives rise to gaseous fuel which then facilitates flame propagation into the droplet-mist. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for ϕd>1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion: the premixed mode ocurring mainly under fuel-lean conditions and the non-premixed mode under stoichiometric or fuel-rich conditions. The prevalence of premixed combustion was seen to decrease with increasing droplet size. Furthermore, droplet-fuelled turbulent flames have been found to be thicker than the corresponding turbulent stoichiometric premixed flames and this thickening increases with increasing droplet diameter. The flame thickening in droplet cases has been explained in terms of normal strain rate induced by fluid motion and due to flame normal propagation arising from different components of displacement speed. The statistical behaviours of the effective normal strain rate and flame stretching have been analysed in detail and detailed physical explanations have been provided for the observed behaviour. It has been found that the droplet cases show higher probability of finding positive effective normal strain rate (i.e. combined contribution of fluid motion and flame propagation), and negative values of stretch rate than in the stoichiometric premixed flame under similar flow conditions, which are responsible for higher flame thickness and smaller flame area generation in droplet cases.

Influence of the quality of fuel atomization on the emission of exhaust gases toxic components of combustion engines

Abstract The paper contains analysis of the impact on the quality of the fuel spray of droplets on the number and the value of the surface area of the droplets that have a direct impact on the efficiency of the combustion process. It presents a change in the level of toxic exhaust emissions with an increase in the so-called. Sauter Mean Diameter (SMD) of fuel droplets. The presented figures show that the increase in the average droplet diameter SMD in particular causes an increase of toxic exhaust gases emissions, i.e. mainly nitrogen oxides NO, carbon monoxide CO, hydrocarbons HC and Particulate Matter (PM). This tendency is not constant, because it is changing according to changes of the coefficient of equivalence Φ. For example, emissions of nitrogen oxides for the lean mixtures (Φ &lt;1), with an increase in average droplet diameter increases, while now for the rich mixtures (Φ&gt; 1), this trend is significantly changed, i.e. the level of this emission decreases.

Preparation of non-aqueous Pickering emulsions using anisotropic block copolymer nanoparticles

In this work, we show that amphiphilic diblock copolymer worms prepared via alcoholic RAFT dispersion polymerization can be used to stabilize non-aqueous Pickering emulsions. A previously reported synthesis protocol based on polymerization-induced self-assembly (PISA) was modified to enable the preparation of poly(2-(dimethylamino)ethyl methacrylate)-poly(benzyl methacrylate) (PDMA-PBzMA) worm-like particles directly in methanol at relatively high solids. A dilute dispersion of these highly anisotropic nanoparticles was then homogenized with sunflower oil to produce sunflower oil-in-methanol emulsions. The mean droplet diameter ranged from 9 to 104 μm, depending on the nanoparticle concentration and the stirring rate used for homogenization. The sunflower oil content was increased systematically, with stable emulsions being obtained up to a volume fraction of 0.60. In all cases, the sunflower oil droplets gradually increase in size on ageing for up to 4 days. However, stable emulsions were obtained after this time period, with no further change in the mean droplet diameter for at least 2 months on standing at ambient temperature. Turbidimetry studies of the continuous phase after sedimentation of the relatively dense emulsion droplets indicated that the initial adsorption efficiency of the PDMA-PBzMA worms is very high, but this is reduced significantly as the droplet diameter gradually increases during ageing. There is a concomitant increase in fractional surface coverage over the same time period, suggesting that the increase in droplet diameter is the result of limited coalescence, rather than an Ostwald ripening mechanism.

Statistical Analysis of Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Study

Turbulent combustion of mono-disperse droplet-mist has been analysed based on three-dimensional Direct Numerical Simulations (DNS) in canonical configuration under decaying turbulence for a range of different values of droplet equivalence ratio (ϕd), droplet diameter (a d ) and root-mean-square value of turbulent velocity (u ′). The fuel is supplied in liquid phase and the evaporation of droplets gives rise to gaseous fuel for the flame propagation into the droplet-mist. It has been found that initial droplet diameter, turbulence intensity and droplet equivalence ratio can have significant influences on the volume-integrated burning rate, flame surface area and burning rate per unit area. The droplets are found to evaporate predominantly in the preheat zone, but some droplets penetrate the flame front, reaching the burned gas side where they evaporate and some of the resulting fuel vapour diffuses back towards the flame front. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for ϕd > 1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets and this predominantly fuel-lean mode of combustion exhibits the attributes of low Damkohler number combustion and gives rise to thickening of flame with increasing droplet diameter. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion and the relative contribution of non-premixed combustion to overall heat release increases with increasing droplet size. The statistical behaviours of the flame propagation and mode of combustion have been analysed in detail and detailed physical explanations have been provided for the observed behaviour.

A concise algorithm for calculating absorption height in spray tower for wet limestone–gypsum flue gas desulfurization

In this paper, a concise model for wet limestone–gypsum flue gas desulfurization system with spray tower has been presented, aiming at the prediction of the absorption height in a spray tower. The equations of droplet motion and mass balance were incorporated and developed to simulate the droplet movement, distribution and mass transfer of SO2 in the absorption section. The calculation results were in good agreement with the plant values of the absorption height. The influences of operating parameters on the absorption height were analyzed. The results show that absorption height declines with increasing liquid–gas ratio and pH value of the slurry, and increases with increasing droplet diameter, gas flow rate, gas temperature, inlet SO2 concentration and absorption efficiency, respectively. As the most sensitive parameter, the droplet diameter is crucial for the good prediction of absorption height. This model can provide engineering guidance to design a spray tower absorber.

Numerical Analysis on Droplet Deformation and Breakup in High Gravity Field

Research slope stability under rainfall condition in geotechnical centrifuge is an ideal test method, however, the influence of high centrifugal force field produced by running geotechnical centrifuge cannot be neglected. Droplet deformation and breakup under different gravity and of different diameters were studied with VOF method, the results shows that the process of droplet deformation and breakup is similar under condition of different g-value and diameters, droplet breakup in a very short time in high gravity field, and with the increase of g-value, the breakup time of droplet became shorter, with the increase of droplet diameter, the breakup time of droplet became longer under same gravity acceleration. Studies in this paper have important significance in developing geotechnical centrifuge artificial rainfall equipment.

Effects of the ambient temperature and initial diameter in droplet combustion

Results are presented from transient numerical simulations of combustion of an isolated heptane (n-C7H16) droplet in nearly quiescent ambient air. The focus is on the effects of environment temperature and droplet size on the combustion characteristics. Droplet sizes range from 10μm to 1mm, environment temperatures range from 800K to 1600K, and pressure is 1atm.The results show that ignition delay times have a very moderate dependence on droplet diameter. With an increase in droplet diameter, the ignition delay time first decreases near the ignitable diameter and then increases far from the ignitable diameter. The ignition stand-off ratio always decreases with an increase in droplet diameter. A strong but short-lived convection, which is generated following the ignition, is demonstrated for the first time. The convection is caused by gas expansion which ‘pushes’ the droplet toward the incoming flow. At lower temperatures, the transition from the kinetically controlled regime to the diffusion controlled regime is shifted to larger droplet diameters, and a larger range of droplet sizes is affected by the transition. Flame stand-off ratios are higher, and flame temperatures are lower, for small droplets. The transient evolution of heat release rate in droplet flames is presented for the first time. The heat release rate first exhibits thermal runaway (ignition), then decreases once the diffusion flame is established, and finally increases precipitously towards the end of the droplet lifetime due to an increase in strain rate. The heat release rates exhibit a strong dependence on droplet diameter.Finally, a comparison of the combustion characteristics of methanol and heptane droplets is presented, which establishes the differences in the phenomena reported in this paper and those reported for methanol. The increase in flame stand-off ratios and fuel vapor accumulation effects as droplet size is reduced, are qualitatively the same as those reported for methanol. The gasification rates and droplet surface temperatures for heptane exhibit less variation. Heptane flame locations lie in the fuel-rich zone whereas methanol flames are located at near stoichiometric mixture fractions.

Determination of required hydrophilic–lipophilic balance of citronella oil and development of stable cream formulation

Context: Citronella oil is reported to have excellent mosquito-repellent activity. To develop a stable cream formulation (emulsion), its hydrophilic–lipophilic balance (HLB) value is important.Objective: To determine required hydrophilic–lipophilic balance (rHLB) value of citronella oil and to develop stable cream formulation.Materials and Methods: Emulsions of citronella oil were prepared by phase inversion temperature technique using water, Tween 80 and Span 80. A first series of 11 emulsions with HLB values ranging from 5.0 to 15.0 and a second series of eight emulsions with smaller interval in HLB values from 11.0 to 13.8 were prepared. Emulsions were evaluated for creaming index, droplet size and turbidity to determine rHLB. Utilizing determined rHLB, citronella oil cream was formulated and evaluated for different texture parameters. rHLB of light liquid paraffin was also determined for validation of methodology.Results: rHLB of light liquid paraffin and citronella oil was determined to be 11.80 and 12.60, respectively. Stable citronella oil cream was developed with 10% emulsifier blend. Texture parameters were found to be consistent over the entire storage period.Discussion: Creaming index, droplet diameter, percent increase in droplet diameter and turbidity are the established parameters to determine rHLB and to develop stable emulsion. Emulsions with optimum emulsifier concentration resulted in less percentage creaming index, smallest droplet, less percentage increase in droplet diameter and highest turbidity. Texture properties evaluation ensures the stability of the developed cream.Conclusion: rHLB value of citronella oil was found 12.6 and a stable cream was formulated utilizing determined rHLB.

Formation of oil droplets in plasticized starch matrix in simple shear flow

This paper describes the effect of simple shear flow on the formation of triglyceride oil droplets in a plasticized starch matrix. An in-house developed shearing device was used that enabled the application of controlled shear flow and rheological characterization of the native maize starch–triglyceride blends at shear stresses of up to 37kPa. Due to the high viscosity of starch matrix, the viscosity ratio of the continuous starch phase and the dispersed triglyceride phase varied between 10−7 and 10−5. It was possible to create small droplets with a droplet diameter of 2.1μm using simple shear flow only. An increase in shear rate had no influence on droplet diameter. However, an increase in oil content led to a vast increase in droplet diameter indicating the occurrence of coalescence. The results further show that the maximum stable droplet size in plasticized starch is significantly smaller (up to 100 times) than the predicted values for a Newtonian matrix. The differences of plasticized starch to Newtonian matrices are discussed in detail.

D102 高温面に衝突する微小液滴の熱伝達 : 衝突速度と液滴径(OS1 マイクロエネルギー変換)

The present paper describes experimental results of the behavior of a small water droplet impinging onto a hot surface and the water spray cooling. The effects of impinging velocity and diameter of droplet on the solid-liquid contact time, maximum spreading ratio on a hot surface and cooling time until the surface temperature drops to 100℃ from 500℃ are examined. The surface used here is SUS304 steel cylinder with the value of roughness, Ra 0.04. The length of contact time decreases as the impinging velocity decreases and increases as the droplet diameter increases. The maximum spread ratio gets bigger with increase in impinging velocity and droplet diameter. The cooling rate increases with increase in impinging velocity. The cooling rate also increases with decrease in droplet diameter when the flow rate of water is constant.

New Mathematical Model for Interpreting pH-Stat Digestion Profiles: Impact of Lipid Droplet Characteristics on in Vitro Digestibility

The pH-stat method is commonly used to characterize the in vitro digestibility of lipids under simulated small intestine conditions. This method measures the fraction of free fatty acids (FFA) released from triacylglycerols over time. A new mathematical model has been developed to characterize the FFA versus time profiles generated by the pH-stat method, which can be used to quantify the influence of physicochemical parameters on the rate (k) and extent (phi(max)) of lipid digestion. In this model, k is the amount of FFA produced per unit time per unit surface area, whereas phi(max) is the maximum fraction of digestible FFAs released. This model is used to quantify the influence of lipid droplet characteristics (size, concentration, composition, and emulsifier type) on the digestion of emulsified lipids. The rate (k) of lipid digestion increased with decreasing lipid content (from 2.5 to 0.5 wt %), increasing droplet diameter (from d = 200-15000 nm), and decreasing fatty acid molecular weight (MCT versus corn oil). The extent (phi(max)) of lipid digestion was also considerably less for corn oil than for MCT. The rate and extent of lipid digestion did not depend strongly on initial emulsifier type: beta-lactoglobulin, Tween 20, lecithin, or lyso-lecithin. These results are interpreted in terms of differences in the concentrations of reactants, products, catalysts and cofactors at the lipid droplet surfaces during digestion, for example, triacylglycerols, emulsifiers, FFA, lipase, and bile salts. This model provides a useful means of quantifying the influence of specific parameters on lipid digestion using the pH-stat method.

Effect of grouped-hole nozzle geometry on the improvement of biodiesel fuel atomization characteristics in a compression ignition engine

This paper describes the influence of grouped-hole nozzle geometry on the atomization characteristics of a biodiesel fuel. The effects of injection rate profile, spray evolution, droplet mean diameter, and mean velocity of the droplets are investigated in a high-pressure injection system. A grouped-hole nozzle with 16 holes driven by piezo stack was used in the experiments and the obtained results are compared with those obtained using a single-hole nozzle with the same overall nozzle area. The effect of using a split fuel injection strategy on the spray atomization was analysed using a grouped-hole nozzle with 12 holes. The time-based injection rate profiles for a biodiesel fuel, i.e. the mass flowrate of the fuel injector nozzle were obtained from the pressure variation generated when the fuel was injected into a tube filled with fuel. The spray and atomization performance for biodiesel fuel injected through a grouped-hole nozzle were visualized and measured by a visualization system and droplet analyser system, respectively. It was revealed that the biodiesel injection rate profile from a single-hole nozzle with the same overall nozzle area shows a similar injection rate profile to that of the grouped-hole nozzle. The injection rate profiles for a split injection strategy are lower than for a single injection and the peak value of the second injection is lower than for the first injection. The Sauter mean diameter distributions of biodiesel from the grouped-hole nozzle have a larger diameter because the small droplets injected through the grouped-hole nozzle collide and coalesce with each other, which leads to an increase in droplet diameter. From these results, it can be concluded that the use of a grouped-hole nozzle and split injection strategy does not improve on the fuel atomization performance for biodiesel fuel for a single-hole nozzle.

Effect of viscosities of dispersed and continuous phases in microchannel oil-in-water emulsification

Although many aspects of microchannel emulsification have been covered in literature, one major uncharted area is the effect of viscosity of both phases on droplet size in the stable droplet generation regime. It is expected that for droplet formation to take place, the inflow of the continuous phase should be sufficiently fast compared to the outflow of the liquid that is forming the droplet. The ratio of the viscosities was therefore varied by using a range of continuous and dispersed phases, both experimentally and computationally. At high viscosity ratio (ηd/ηc), the droplet size is constant; the inflow of the continuous phase is fast compared to the outflow of the dispersed phase. At lower ratios, the droplet diameter increases, until a viscosity ratio is reached at which droplet formation is no longer possible (the minimal ratio). This was confirmed and elucidated through CFD simulations. The limiting value is shown to be a function of the microchannel design, and this should be adapted to the viscosity of the two fluids that need to be emulsified.

High-pressure-homogenised cream liqueurs: Emulsification and stabilization efficiency

Recent progress in high-pressure technology has led to new opportunities for homogenisation processes. In this study, a high-pressure homogeniser and a standard radial diffuser homogeniser (Gaulin-type) were evaluated and compared in terms of their efficiency in model cream liqueur homogenisation and stabilization. Cream liqueurs with finely-dispersed droplets were produced on single-pass homogenisation using the high-pressure homogeniser. However, an increase in the homogenisation intensity above optimal conditions increased the mean fat droplet diameter. The standard radial diffuser homogeniser required multiple homogenisation passes to produce a stable cream liqueur. High-pressure homogenisation markedly increased cream liqueur temperature, due to pressure build-up and shear effects associated with conversion of kinetic energy into heat. Cream liqueurs with small mean fat droplet diameters, homogenised using the high-pressure homogeniser, displayed a slower rate of increase in droplet diameter and in viscosity during storage at 45 °C than those produced using a low pressure homogenisation process.

The formation of water droplets in an air-breathing PEMFC

Air-Breathing Proton Exchange Membrane Fuel Cells (AB-PEMFC) have the potential to supersede lithium-ion batteries in portable electronics. However, their water management issue has yet to be resolved to ensure optimum cell performance and safe system operation. In this paper, the formation of water droplets and their aggregation in the cathode flow channels of an operating AB-PEMFC is investigated by direct visualisation under various operating conditions. The developed optical set-up enables observation of droplet formation on the surface of the membrane from the top and side view of the channels simultaneously. The two orthogonal views reveal that during formation the receding and advancing droplet contact angles are almost identical with values that increase, in a similar trend to the droplet height, with increasing droplet diameter. Water films were able to develop and maintain direct contact with the side wall of the channels even under the effect of gravitational force. The aggregation of water droplets in the channels was strongly influenced by the change in the air and hydrogen stoichiometry conditions. However, these operating parameters appear to have no significant effect on the water extraction from the channels contrary to load and temperature, where temperature has proved to be the most effective water removal mechanism with minimum reduction in the current density of AB-PEMFC.