Residence Time Of Droplets Research Articles

Study of a fogging system using a computational fluid dynamics simulation

Fogging is one of the most effective methods for lowering the air temperature in rooms and greenhouses. It also has many industrial applications, especially in gas turbines where this method presents great advantages over others in terms of achieving better turbine performance in hot weather conditions.With this in mind, a numerical study was performed in STAR-CCM+ to investigate mist dynamics at a turbine's inlet duct, specifically measuring: (i) residence time of water droplets; (ii) mass transfer between water and air; (iii) coalescence and agglomeration of the water droplets; and (iv) changes in air density and temperature inside the duct. The results were compared against the same variables taken from experimental wind tunnel data and found to be similar with respect to the behavior of temperature and relative humidity.Therefore, it was possible to conclude that the results obtained in the simulation were close to those reported experimentally with differences from 3 to 6%. Based on the profiles and contour graphs obtained, it was also found that the best mass and energy transfer occurs when an atomizing diameter of 20 µm is used with the lowest relative humidity possible. Working with this humidity and droplet diameter, it is possible to avoid water runoff on duct walls.

The design and performance evaluation of axial ventilator with honeycomb in the turbofan engine lubrication system

Numerical simulations have been carried out to investigate the performance of the axial ventilator equipped with honeycomb structure. The oil /air two-way coupling model based on the Realizable k–ɛ turbulence model and the droplet impact model are proposed. Based on verifying the rationality of numerical model, characteristics of flow resistance and oil–gas separation efficiency are calculated and analyzed. The results show that the axial ventilator with honeycomb has favorable separation efficiency, which is estimated as 99.6% for the oil droplet diameter of 5 µm. The honeycomb structure has little effect on flow resistance, but plays a major role in the oil–gas separation of axial ventilator, where the contribution to the oil separation accounts for 80% at least. Besides, the increase of rotation Reynolds number enhances the centrifugal force, resulting in the increase of separation efficiency, while the increase of nondimensional mass flow rate and environmental temperature reduce the residence time of oil droplets in the axial ventilator mainly, resulting in the decreasing of separation efficiency.

Spray drying with a two-fluid nozzle to produce fine particles: Atomization, scale-up, and modeling

ABSTRACTThis article presents experimental and modeling work to complete previously reported work on spray drying. Back-calculated droplet sizes have been verified by measurements with a laser imaging rig. Flow patterns in a cylindrical spray chamber have been simulated by computational fluid dynamics and demonstrated that droplet residence times are much shorter than expected. A droplet tracking population balance model has been implemented in gSOLIDS and shows how drying times vary with droplet diameter. Particle collection by cyclone and bag filter have also been compared experimentally.

Dynamic Modeling of LD Converter Steelmaking: Reaction Modeling Using Gibbs’ Free Energy Minimization

Slag–metal emulsion plays an important role in the oxidation kinetics of metalloids in oxygen steelmaking. The importance of droplet generation rate, droplet size, and its residence time in the slag–metal emulsion on the overall reaction kinetics has become evident in recent times. Residence times of the droplets are strongly dependent on the decarburization rate, the CO bubbles giving a buoyant force to the droplets. The present work aims at developing a mathematical model for predicting the composition evolutions of the slag and the metal phases as the blow proceeds in an LD converter. The process dynamics are modeled by dividing the LD convertor into three separate continuous stirred tank reactors. Oxidation reactions are assumed to be primarily taking place at the interface between the slag and the metal phases in the emulsion. Among the different mass transfer and reaction steps controlling the kinetics, the mass transfer of FeO in the slag phase and that of the metalloids within the metal droplet are assumed to be rate-controlling. For a Fe-C-X (X = Mn, Si etc.) droplet, simultaneous removal of elements have been modeled by Gibbs’ free energy minimization at the slag–metal interface. Effects of droplet size, mass transfer coefficient, and initial carbon content on the mean residence time of metal droplets in the slag–metal emulsion have also been identified. Mixing in the metal phase is simulated in terms of metal exchange rate and the reactor weight ratio between the upper and the lower parts of the bath.

Influence of the wall on the droplet evaporation

Evaporative influence of the wall material and its thickness has been investigated in the present study. The wall influence for heat exchangers is particularly important in the boiling transition regime and in the event of the Leidenfrost temperature. The experimental points significantly diverge in the transition area of the boiling crisis. This fact can be explained by a different residence time of droplet on the wall surface. The discrepancy between the experimental data also takes place at the Leidenfrost temperature. The lower the thermal diffusivity of the wall material (high thermal inertia), the more the wall is cooled under a droplet.

Effects of Damkhöler number of evaporation on the morphology of active layer and the performance of organic heterojunction solar cells fabricated by electrospray method

The electrospray (ES) process has emerged as a scalable and efficient fabrication method for organic photovoltaic cells (OPVs). However, the ES process could often involve numerous parameters, which impose a major challenge on uncovering the interplay among process, morphology of active layers, and device performance. This work attempts to reduce the parameter space and capture the essence of the ES process using the Damkhöler (Da) number of evaporation, which is the ratio of the droplet residence time over evaporation time. We first derived an explicit equation for Da that links nine different parameters affecting the process. Experimental results indicate that Da number shows strong effect on morphology and crystallinity of the active layer composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Most remarkably, the power conversion efficiency exhibits monotonically dependence on Da values spanning more than one order of magnitude, e.g., from 0.13 to 1.52. This finding suggests that Da number analysis can reduce the large parameter space in electrospray deposition, thus provides a simple way to control and predict the morphology of active layer and the performance of the solar cells.

Droplet characteristics and local equivalence ratio of reacting mixture in spray counterflow flames

Spray combustion was studied by injecting monodisperse 125 or 200μm ethanol droplets in a premixed natural gas fuel flame flowing against an opposed heated air jet. Phase Doppler anemometry and chemiluminescence measurements allowed to characterise both droplet parameters and the local reacting mixture. Both types of droplets crossed the flow stagnation plane and entered the opposite jet. However, 125μm droplets reversed their motion and oscillated around the stagnation plane, leading to increased droplet residence time in hot regions. The fuel vapour released by droplets close to the stagnation plane, mixed with the surrounding air and led to the ignition of a second fuel vapour flame below the natural gas flame. For 125μm droplets, mean local equivalence ratio of the fuel vapour flame was about 0.8, suggesting lean-premixed combustion; whereas 200μm droplets led to stoichiometric combustion. “Group Combustion” number was estimated from measurements and suggested that 125μm and 200μm droplets burned in different “Group Combustion” regimes.

Experimentally studying TV3-117 gas-turbine unit characteristics at superheated water injection into a compressor

The results from experimentally studying TV3-117 gas-turbine unit (GTU) characteristics at injection of cold and superheated (metastable) water to the inlet of the GTU compressor are presented. In the latter case, the finer water atomization is obtained. The water injection makes it possible to considerably increase the unit power. At a constant temperature of the working fluid downstream of the turbine combustion chamber, water injection in an amount of 1% of the air flow rate provides an increase in the turbine power by approximately 12% and expands GTU controlling potentialities. The use of the metastable superheated water atomization enables one to more reliably implement the technology of water injection into a compressor, especially into intermediate compressor stages. However, it requires accounting for operational conditions of particular installation. Due to small water droplet residence time in the compressor flow path, even with fine water atomization, in aircraft engine derivative power turbines, about 15–20% of moisture injected have no time to completely evaporate within the compressor. When injecting cold water, this figure is from 5 to 10% larger.

Development of an axisymmetric population balance model for spray drying and validation against experimental data and CFD simulations

An incremental model for spray drying, including a full droplet size distribution, has been implemented in a flowsheeting package incorporating tracking of distributed particle properties. Results were compared with expected trends based on standard theory and with results from a laboratory-scale spray dryer with a two-fluid nozzle for atomization. Predicted trends were as expected, with larger droplets giving substantially longer drying times and higher final moisture content. Predicted final moisture content was lower than measured values, as the very short residence times for fine particles were inadequately represented by first-order falling-rate drying kinetics. Dryer gas flow patterns were simulated by computational fluid dynamics. Calculated droplet residence times were much lower than for a plug-flow or fully mixed gas flow, because a high-velocity gas flow zone from the two-fluid atomizer persists down a substantial part of the dryer.

Effect of flow rate and subcooling on spray heat transfer on microporous copper surfaces

In this work, we experimentally investigated spray boiling heat transfer performance with degassed HFE-7100 as the coolant on a conductive microporous copper surface, and observed enhanced heat transfer performance compared to that on a plain surface. Spray heat transfer data were measured using two full-cone spray nozzles spanning a range of volumetric flow rate from 1.1cm3/s to 15.8cm3/s. We also investigated the effect of different liquid subcooling levels ranging from 30°C to 0°C on the heat transfer data. Spray impingement on the microporous surface showed an enhancement of 300–600% in the heat transfer coefficient at a given wall superheat compared to spray impingement on a plain surface. The critical heat flux also increased by up to 80% for the case of spray impingement on a microporous coated surface as compared to impingement on a plain surface, depending on the flow rates and the subcooling levels. Contrary to the results in the literature, for a given nozzle we observed that the liquid spray at near-saturated temperature (0°C subcooling) had higher heat transfer performance and critical heat flux than the subcooled spray on both plain and microporous surfaces except at the lowest flow rates. This likely results from the limited residence time of the liquid droplets in contact with the heater surface and the much higher efficiency of phase change heat transfer. The near-saturated spray undergoes phase change much faster than the subcooled liquid, removing heat more efficiently than the subcooled liquid. A modified correlation, based on the Estes–Mudawar correlation (1995) [22], utilizing the experimental data from the present study and literature is proposed for the critical heat flux for spray impingement on both plain and microporous surfaces.

Characterization method of dielectric properties of free falling drops in a microwave processing cavity and its application in microwave internal gelation

Microwave internal gelation (MIG) is a chemical process proposed for the production of nuclear particle fuel. The internal gelation reaction is triggered by a temperature increase of aqueous droplets falling by gravity by means of non-contact microwave heating. Due to the short residence time of a solution droplet in a microwave heating cavity, a detailed knowledge of the interaction between microwaves and chemical solution (shaped in small drops) is required. This paper describes a procedure that enables the measurement of the dielectric properties of aqueous droplets that freely fall through a microwave cavity. These measurements provide the information to determine the optimal values of the parameters (such as frequency and power) that dictate the heating of such a material under microwaves.

Near-Field Electrospray Microprinting of Polymer-Derived Ceramics

Ceramic microelectromechanical systems (MEMS) sensors are potentially game-changing devices in many applications in high-temperature and corrosive environments, where the use of conventional MEMS materials such as silicon is prohibited. However, the microfabrication of ceramic MEMS devices remains a major technical challenge. Here, we report a method to directly print micro ceramic patterns using near-field electrospray (ES) of polymer-derived ceramics (PDCs). We demonstrated that the viscous ceramic precursor liquids can be printed reliably without any clogging problem. The spray self-expansion due to Coulombic repulsion force among charged droplets can be suppressed by decreasing the droplet residence time in space. A spray expansion model is used to predict the line width, and the results are in decent agreement with the experiments. We demonstrated a 1-D printed polymer feature as narrow as 35 μm and a micro pentagram pattern. Moreover, after the pyrolysis of PDC at 1100 °C in nitrogen, amorphous alloys of silicon, carbon, and nitrogen (SiCN) are obtained. The samples show good integrity and adhesion to the substrate. The near-field ES PDC printing can become a useful addition to the toolbox of high-temperature MEMS.

Droplet impact on superheated micro-structured surfaces

When a droplet impacts upon a surface heated above the liquid's boiling point, the droplet either comes into contact with the surface and boils immediately (contact boiling), or is supported by a developing vapor layer and bounces back (film boiling, or Leidenfrost state). We study the transition between these characteristic behaviors and how it is affected by parameters such as impact velocity, surface temperature, and controlled roughness (i.e., micro-structures fabricated on silicon surfaces). In the film boiling regime, we show that the residence time of droplets impacting upon the surface strongly depends on the drop size. We also show that the maximum spreading factor Γ of droplets in this regime displays a universal scaling behavior Γ [similar] We3/10, which can be explained by taking into account the drag force of the vapor flow under the drop. This argument also leads to predictions for the scaling of film thickness and velocity of the vapor shooting out of the gap between the drop and the surface. In the contact boiling regime, we show that the structured surfaces induce the formation of vertical liquid jets during the spreading stage of impacting droplets

Radiative shielding by water mist : comparisons between downward, upward and impacting injection of droplets

Radiative shielding with water curtain has been studied numerically, investigating three different possibilities of droplet injection : downward, upward and impacting on a wall to be protected. The efficiency has been evaluated based on radiation attenuation predicted considering a given incident flux attenuated when crossing the area where water is injected. For upward and downward injection, a simple water curtain is considered. For the impacting spray case, a water film streaming on the wall is considered in addition to the spray (an idealized film with constant and fixed thickness for the moment). The dynamics has been imported from an Eulerian-Lagrangian simulation and radiative transfer has been addressed with a Monte Carlo method. Results show that the upward injection performs better than the downward injection due to a favoring dynamics that increases the residence time of droplets. The impacting spray could be even more efficient owing to the possible high attenuation efficiency of films, but present results still make use of simplifications on the water film falling on the wall and present promising observations require further verification.

Global network design for robust operation of microfluidic droplet generators with pressure-driven flow

This study investigates the influence of both local generator design and global network architecture in improving the stability and operational performance of microfluidic droplet generators. We identify naturally occurring short-term and long-term oscillations that are related to changes in the flow of the two phases. Short-term oscillations are related to the creation of each droplet and are quantified by tracking droplet speed in the network. Long-term oscillations are caused by dynamic feedback associated with the periodic change in the hydrodynamic resistance of the network as droplets enter and exit the system. Our analysis identifies that these long-term oscillations are best quantified by measuring changes in droplet spacing rather than the conventional method of using droplet size. Furthermore, we find that these long-term oscillations have a periodicity that matches the residence time of droplets within the network. In combination with experiments, a simple compact model is developed to study these oscillations and guide the network design of droplet generators. As part of this analysis a set of design rules is developed to help improve overall generator performance using pressure-driven flow.

Microfluidic Elucidation of the Effects of Interfacial Rheology on Droplet Deformation

Proteins and peptides, adsorbed at a fluid–fluid interface, can form a mechanically strong cohesive interfacial network, which significantly affects the behavior of foams and emulsions. Droplet deformation in the presence of interfacial protein and peptide networks was systematically investigated in defined hydrodynamic extensional flows using a specially designed microfluidic chip. A correlation between dynamic droplet deformation and the Capillary number was developed for pure systems (oil in water and the buffer solution) and for the low molecular weight surfactant sodium dodecyl sulfate (SDS), which can be used to estimate interfacial tension following observation of deformation. Surprisingly, we found that droplet deformation in the presence of proteins (β-lactoglobulin, lysozyme and β-casein) was described by this correlation, which indicated that proteins did not significantly affect the droplet deformation behavior in the systems we examined. However, the peptide AFD4, which forms rapidly a mechanically strong cohesive film, reduced droplet deformation. Further investigation revealed that the kinetics of protein network formation was slower than the kinetics associated with interfacial tension reduction; the short residence time of droplets in the microfluidic channel (10 s) did not provide sufficient time for protein network formation. Conversely, peptide AFD4 formed the interfacial cohesive network rapidly after the formation of droplets because of its rapid interfacial adsorption due to its small size (2345 Da). The correlation obtained based on interfacial tension was found to be also valid for droplet deformation in the presence of a strong interfacial cohesive network, provided interfacial elasticity replaces interfacial tension in the Capillary number. Combining different techniques including Profile Analysis Tensiometer (PAT), CIT, and microfluidics, this study provides direct insight into the behavior of proteins and a peptide surfactant (AFD4) at the fluid–fluid interface in confined flows and reveals a similarity in effects for interfacial elasticity and tension in controlling droplet deformation.

An In Vitro Assessment of Aerosol Delivery Through Patient Breathing Circuits Used with Medical Air or a Helium–Oxygen Mixture

The bench experiments presented herein were conducted in order to investigate the influence of carrier gas, either medical air or a helium-oxygen mixture (78% He, 22% O2), on the droplet size distribution and aerosol mass delivered from a vibrating mesh nebulizer through a patient breathing circuit. Droplet size distributions at the exit of the nebulizer T-piece and at the patient end of the breathing circuit were determined by laser diffraction. Additional experiments were performed to determine the effects on measured size distributions of gas humidity and of the droplet residence time during transport from the nebulizer to the laser diffraction measurement volume. Aerosol deposition in the nebulizer, breathing circuit, and on expiratory and patient filters was determined by photometry following nebulization of sodium fluoride solutions into the breathing circuit during simulated patient breathing. With no humidification of the carrier gas, droplet volume median diameter (VMD) at the exit of the nebulizer T-piece was 5.5±0.1 μm for medical air, and 4.3±0.1 μm for helium-oxygen. Varying the aerosol residence time between the nebulizer and the measurement volume did not affect the measured size distributions; however, humidification of the carrier gases reduced differences in VMD at the nebulizer exit between medical air and helium-oxygen. At the patient end of the breathing circuit, droplet VMDs were 1.8±0.1 μm for medical air and 2.2±0.1 μm for helium-oxygen. The percentages of sodium fluoride recovered from the nebulizer, breathing circuit, patient filter, and expiratory filter were, respectively, 29.9±8.3, 40.4±5.6, 8.3±1.5, and 21.5±2.1% for air, and 32.6±2.2, 36.3±0.7, 12.0±1.4, and 19.1±1.1% for helium-oxygen. Ventilation with helium-oxygen in place of air-oxygen mixtures can influence both the droplet size distribution and mass of nebulized aerosol delivered through patient breathing circuits. Assessment of these effects on aerosol delivery is important when incorporating helium-oxygen into patient ventilation strategies.

Antimicrobial Cu-functionalized surfaces prepared by bipolar asymmetric DC-pulsed magnetron sputtering (DCP)

Cu-cotton fabrics were functionalized by bipolar asymmetric DC-pulse magnetron sputtering (DCP). The DCP of Cu-particles on cotton proceeds at a higher energy than DC-magnetron sputtering (DC). The different sputtering mode showed effects on the structure of the Cu-film on the textile. The Cu-layer thickness was observed to be a function of DCP time being the rate of atomic deposition of 2.5 × 10 15 atoms/cm 2 s at 300 mA. The fastest Escherichia coli inactivation was observed within 10 min when Cu was sputtered on cotton Cu for 60 s. This led to a film thickness of 30 nm (150 Cu-layers) with 1.7 × 10 17 atoms/cm 2. The Cu-textiles became darker at longer sputtering times as detected by diffuse reflectance spectroscopy (DRS). By transmission electron spectroscopy (TEM), Cu-particles 35–50 nm in size were found and became more compact on the cotton surface as a function of deposition time. X-ray photoelectron spectroscopy (XPS) was used to determine the surface atomic concentration of O, Cu C, and N along the states of oxidation of the Cu-ions during the redox process leading to E. coli inactivation. The oxidation of the E. coli on the Cu-cotton surface was a function of reaction time and was monitored by the oxidation index of the carbon species on the fabric according to the ratio: (C–OH)/(C–C, C C, C–H). The increase in hydrophobicity of the Cu-cotton was followed as a function of the contact angle and droplet residence time for different samples. The results obtained for the E. coli inactivation on the Cu-films are discussed suggesting a possible reaction mechanism.

Spray Combustion and Particulate Matter Emissions of a Wood Derived Fast Pyrolysis Liquid-Ethanol Blend in a Pilot Stabilized Swirl Burner

Biomass fast pyrolysis liquid (or bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better understand the combustion behavior and emissions of bio-oil, a 10 kW spray burner was designed and constructed. The effect of swirl, atomization quality, ignition source energy, air/fuel preheat, and equivalence ratio on the particulate matter emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests. Increasing the residence time of spray droplets in the hot combustion zone by increasing the swirl number promotes the burnout of solid residues. Decreasing the mean diameter of fuel droplets by increasing the atomizing air flow rate has a similar effect. Ignition source energy, air/liquid fuel preheat, and equivalence ratio have either a weak or ambiguous effect on the measured particulate emissions. The residual material collected from the exhaust is composed of both carbonaceous matter and fly ash. However, the majority of particulate matter consists of ash, even at relatively poor combustion conditions. These results suggest that it is possible to provide enough oxygen availability and residence time for droplets to undergo nearly complete burnout during combustion. Under such conditions, the total particulate matter emissions could be further mitigated by reducing the inherent ash content in the fuel.

Upward vs downward injection of droplets for the optimization of a radiative shield

A numerical and experimental study of a spray has been carried out at the laboratory scale, focusing on the comparison between downward and upward injection situations. The simulation has been carried out thanks to a dedicated numerical code addressing the two-phase flow with an Eulerian–Lagrangian approach, and treating the radiative transfer with a Monte Carlo method. The experimental setup involves a FTIR spectrometer and an IR camera, both characterizing simultaneously the spectral attenuation through the spray when irradiated by a blackbody. Numerical results and experimental data both show a better ability of the spray to attenuate radiation when injected upward. The residence time of droplets is highly increased when they are injected upward. This results in an increased attenuation ability of the spray. A gain by a factor 3 or more is possible regarding the attenuation of radiation if water is injected upward. This observation has to be considered with care however because of a possible poor stability of the spray. The influence of an air flow affecting the spray dynamics has also been investigated numerically, tests being done with a 2 m/s air flow. The droplet motion is strongly altered and the radiation attenuation is affected. In the upward injection case the deformation of the spray may be so strong in its upper part that the attenuation tends to zero because droplets are carried away.