Droplet Evaporation Research Articles

Evaluation of Non-ideal Fluid Modeling for Droplet Evaporation in Jet-Engine-Like Conditions

Evaluation of Non-ideal Fluid Modeling for Droplet Evaporation in Jet-Engine-Like Conditions

Single bischofite precursor droplet evaporation: Modeling and simulation covering the effects of decomposition reactions and environmental variables

Spray pyrolysis is an excellent method to realize high value utilizing industrial waste bischofite. Interacting droplet evaporation characteristics under the combined influence of ambient pressure, temperature, and decomposition reactions are a prerequisite for constructing spray pyrolysis devices. Therefore, this work established a comprehensive heat and mass transfer model during the multi-component droplet evaporation incorporating decomposition reaction. It also delved into the interplay between temperature and pressure, elucidating their combined influence on the decomposition process during evaporation. The results suggest that raising either temperature or pressure alone can speed up the decomposition process; raising both will more successfully increase the rate of decomposition conversion. In addition, a set of non-dimensionless parameters, mth/ml (mass transfer contribution rate) and Qth/Qd (heat transfer contribution rate), are proposed to characterize the contribution of the decomposition reaction to the evaporation process. Under the actual spray pyrolysis conditions, the mass transfer contribution rate, mth/ml, which is varied in a range of 2 %–70 %, indicating the important role of decomposition reaction during the mass transfer process. The maximum value of the heat transfer contribution rate Qth/Qd is less than 10 %, indicating the dominant role of evaporation in the heat transfer process. Based on the analysis, a theoretical direction for modeling may be provided.

Insights into surrogate respiratory droplet behaviour on inclined surfaces: Implications for disease transmission

Disease transmission via fluid ejections from infected individuals presents a significant public health challenge. The ejected droplet frequently lands on substrates with varying orientations, including horizontal, vertical, and inclined. However, the behaviour of disease-causing droplets on inclined substrates remains unexplored. Recent research has demonstrated that droplet evaporation, flow, and colloidal deposition are significantly altered when surfaces are inclined. Changes in contact angle impact evaporative flux and induce flow between the top and bottom edges of the droplet. This study examines the behaviour of simulated respiratory droplets containing NaCl, mucin, and micrometer-sized particles as pathogen surrogates, focusing on substrate inclinations of 0°, 45°, and 90° on glass surfaces. As respiratory droplets evaporate, their solute concentration rises, altering both surface tension and viscosity. The study explores the coupled effects of solute concentration variations and asymmetric evaporative flux at varying relative humidity conditions and droplet volumes. The presence of salt and mucin induces Marangoni flow, potentially altering evaporation, flow patterns, and deposition dynamics. Additionally, sedimentation flow influences crystal nucleation sites and precipitation patterns. Results reveal unprecedented crystallization dynamics on inclined substrates, with notable differences in nucleation and growth based on the angle of inclination. Optical profilometry and confocal microscopy further show that surrogate bacterial particles accumulate excessively at the bottom edge of inclined droplets, suggesting an elevated risk of pathogen survival on inclined fomites. These findings highlight the critical importance of considering substrate orientation in understanding the transmission of disease through surface-bound droplets, offering insights that could inform public health strategies.

Suppression of the coffee-ring effect by controlling the solid particle density

Previous studies show that the coffee-ring effect can be suppressed by altering the droplet's evaporation rate, surface tension, surface properties, and shape of particles. This experiment used five types of particles with different densities to analyze their behavior during the droplet evaporation process. The results showed that when the particle density is close to the fluid density, the particles move within the droplet and accumulate at the edges, forming a pronounced coffee-ring effect. Conversely, with the higher difference between the particle density and the fluid density increases, they tended to deposit uniformly at the bottom of the droplet and were less likely to be pushed to the edges by capillary effects, effectively suppressing the coffee-ring effect. We also observed that the movement speed of high-density particles relatively slowed down through particle image velocimetry tracking technology. By analyzing the Peclet number and the timescale between particle sinking speed and particle movement speed due to capillary flow, we explained how particle density influences the critical factors of particle sinking and suspension, thereby inhibiting the formation of the coffee-ring effect.

Influence of convection during droplet evaporation on corrosion product distribution in CFRP-AA2024 galvanic couple: Modeling and experimental approaches

This work aims to clarify the impact of convection, generated within an electrolyte droplet during evaporation, on the distribution of Al(OH)3 in the CFRP-AA2024 system. Both Finite Element and experimental approaches were employed to determine whether convection is significant or negligible. By coupling the Navier–Stokes and Nernst–Planck equations with appropriate geometric and boundary conditions, the transient distribution of corrosion products was numerically tracked. The results showed that Al(OH)3 preferentially deposits on the CFRP surface, a pattern absent when convection is excluded. Experiments confirmed that internal convective currents can transport the Al(OH)3 compound, aligning with the numerical predictions.

Wetting dynamics and heat transfer of droplet evaporation around boiling point: Facile manipulation by surface roughness

The evaporation of water–ethanol droplets on solid surfaces has vast potential for applications in thermal management, microfluidics, and biomedical sciences, while the approach of manipulating wetting dynamics and heat transfer of droplet evaporation around the boiling point remains open for investigations. The present study proposes a facile approach: fabricating solid surfaces with diverse roughness by sandpaper milling with different grit densities, which can alter the wetting and heat transfer characteristics of droplet evaporation. We discover that when the temperature of the heated surface exceeds the droplet boiling point, the existence of bubbles affects the solid–liquid wetting state; surface roughness alters the ability of bubble departure, thus determining the solid–liquid contact and the contact line retraction time. Consequently, in this case, the solid–liquid contact area and the heat absorbed from heated surfaces for droplets changes with surface roughness, altering the droplet evaporation time and the average heat flux at the solid–liquid interface. Bridged by droplet wetting dynamics, the heat transfer of droplet evaporation above the boiling point can be manipulated by adjusting surface roughness, offering a facile approach to tuning the process of droplet evaporation in the related industrial applications.

Evaporation and combustion of low asphaltene heavy oil droplets under conditions representative of practical applications

The evaporation and burning characteristics of isolated heavy fuel oil (HFO) droplets are experimentally studied at conditions representative of practical applications, producing novel, detailed results on the effect of oxygen concentration. Also, and for the first time for HFO, the temporal evolution of the flame stand-off ratio (based on CH* emission) and the cenosphere burning size are presented along with the droplet size histories. The effect of ambient oxygen concentration is first analyzed qualitatively, identifying different sub-stages. Later, a quantitative analysis is presented in terms of ignition delay time, micro-explosion regimes, shell swelling ratio, cenosphere size and several time metrics. The results show that, as ambient oxygen changes, the liquid and solid stages show marked differences not only in terms of time scales but also in the transitions of burning regimes. Cenospheres generated in oxygen-free conditions are found to be ∼1.4 times larger than those in droplet combustion. Although similar in size, the solid-to-liquid consumption time ratio in 5% O2 is found to be significantly longer, about twice than for the 10% O2 ambient. The characterization of these differences is thought to be relevant not only for oil flames but also for the promising future of heavy oil gasification applications.

Effect of flame temperature on structure and CO oxidation properties of Pt/CeO2 catalyst by flame-assisted spray pyrolysis

Flame synthesis offers the potential for the synthesis of structure-controlled catalysts. In this study, Pt/CeO2 nanoparticles were synthesized via flame-assisted spray pyrolysis (FASP) and used as CO oxidation catalysts. The catalysts were synthesized using a burner diffusion flame at three different flame temperatures (maximum flame temperatures, Tf = 1556, 1785, and 2026 K), and their particle structure and CO oxidation activity were evaluated. The synthesized Pt/CeO2 catalysts had a bimodal structure containing 100 nm-scale CeO2 loaded with 10 nm-scale Pt and fine CeO2 less than 10 nm loaded with highly dispersed Pt (less than 1 nm). As the flame temperature increases from 1556 to 2026 K, the formation of fine CeO2 particles dominates, resulting in an increase in BET specific surface area from 7.97 to 112 m2/g and Pt dispersion from 4.67 to 20.6%. Insight into the particle formation routes that determine the catalyst structure is provided by numerical simulation of droplet evaporation in a burner flame. CO oxidation experiments showed that the temperature at which CO conversion reached 100% (T100) decreased from 513 to 378 K with increasing flame temperature in FASP. In addition, the thermal stability test showed that the Pt dispersion after thermal degradation was higher for Pt/CeO2 catalyst made by FASP at Tf = 2026 K than that prepared by the impregnation method, and the T100 for CO oxidation was lower by 20 K.

On the accuracy of Eulerian-Lagrangian CFD simulations for spray evaporation in turbulent flow

CFD simulation of droplet evaporation in turbulent flows is challenging as the accuracy and reliability of the results strongly depend on the available sub-models and their modeling parameters. This study presents a systematic sensitivity analysis focused on the impact of the most widely used discrete random walk (DRW) model, the constant time scale coefficient (CL), the turbulence model, and the drag coefficient model. CFD simulations with the Eulerian-Lagrangian approach are employed. The analysis is based on grid-sensitivity analysis and validation with measurements of spray evaporation in a heated turbulent airflow. The results show that using the DRW model leads to a good agreement between the CFD results and the experimental data of droplet size and droplet mean velocity, attributed to the turbulent fluctuations inducing droplet dispersion. The best performance is observed for the standard k-ε turbulence model with CL = 0.30 and 0.45. This is mainly attributed to the reasonable interaction time between droplets and turbulent eddies at these CL values. The three drag coefficient models (i.e., Spherical, Ischii-Zuber, and Grace) lead to similar results due to the low droplet Reynolds number.

Evaporation, spreading, and possible uptake of droplets on sorghum (Sorghum bicolor) and cowpea (Vigna unguiculata) leaves using an imaging-based technology.

Sprayed agrochemical droplets have a dynamic evolution on the leaf surface, undergoing changes in shape and volume due to spreading, evaporation, and adsorption. To better understand these processes, an accessible imaging-based experimental methodology is presented to precisely measure droplet spreading, evaporation, and potential uptake by a leaf within a controlled relative humidity environment. Laboratory experiments were conducted to determine the effect of hydrocarbon surfactants, accelerators (light mineral oil), and humectants (high fructose corn syrup) on droplet spread, evaporation, and potential uptake when applied to sorghum (Sorghum bicolor 'Tricker') and cowpea (Vigna unguiculata) leaves. Experiments on cowpea leaves showed uniform spreading and no change in evaporation compared to the predicted rate. In contrast, on sorghum leaves, results suggest that the volume loss rate exceeds the predicted evaporation rate (up to 23%), indicating a potential uptake by the leaves. Some accelerated dynamics on sorghum can be attributed to the lateral spreading observed on hairy leaves along the veins, increasing the contact area by an average of 65%. However, samples containing light mineral oil, typically considered an accelerant to aid in uptake, demonstrated the highest rate but exhibited minimal spreading. The study demonstrates how droplet composition affects droplet dynamics on waxy and hairy leaves by using an imaging-based methodology to measure evaporation rates, volume loss, contact angle, wetted area, and spreading behavior. The findings highlight some of the complex coupling between the crop protection product composition and droplet life cycle on a leaf. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Analysis of group evaporation characteristics of droplet clusters in an evaporating spray

Droplet clustering in sprays refers to the dynamic evolution of highly concentrated regions due to the preferential accumulation of the polydisperse droplets in the turbulent airflow entrained by the spray. In the current study, we aim to experimentally investigate the collective vaporization of the droplets in droplet clusters in an air-assisted acetone spray characterized by the Group number, $G$ . The magnitude of $G$ depends on the cluster length scale and interdroplet spacing, and it indicates the vaporization mode that may vary from the isolated mode ( $G \ll 1$ ) to external group mode ( $G \gg 1$ ). The droplet measurements were obtained under atmospheric conditions at different axial and radial locations within the spray. Application of the Voronoi analysis to particle image velocimetry images of the spray droplets facilitated the identification and characterization of the droplet clusters, which allowed the measurement of $G$ for each cluster. The results highlighted that multiscale clustering of the evaporating droplets leads to multimode group evaporation of the clusters (characterized by a wide range of $G$ : 0.001–10). The trend of interdroplet spacing versus cluster area allowed the classification of the droplet clusters into small-scale clusters (which are of the order of the Kolmogorov length scale) and large-scale clusters (that scale with the large-scale turbulent eddies), that are found to exhibit distinct group evaporation behaviour. A theoretical model is invoked to correlate $G$ with the droplet evaporation rate for individual clusters, and some interesting observations are identified, which are explained in the paper.

Experimental study of vegetable oil droplets vaporization under low temperature conditions such as those found in diesel engine cold parts

This paper presents an analysis of three vegetable oil droplets vaporization process in the range temperature from 473 to 723 K corresponding to low temperature conditions found in diesel engine cold parts. This process is analyzed for a droplet evaporating in a hot environment at atmospheric pressure using the fiber-suspended droplet technique well used in the literature, and analyzing the droplet normalized square diameter and temperature evolution surrounding the droplet when vaporizing. The main difference between those already studied in literature is the range temperature which varies between 473 and 723 K in which vegetable oils vaporizing problems leading to deposits formation in the cold regions of diesel engines especially in direct injection. This presents a scientific challenge for resolving deposit formation. The findings reveal that vegetable oil droplets experience expansion and heating for temperatures below 623 K: no other changes are observed. Between temperatures of 623 and 683 K, an increase in temperature, expansion, and an inconsistent vaporization is observed. This is followed by a phase of low and constant vaporization. From 683 K, vegetable oils experience an initial stage of heating and expansion, which is accompanied by a phenomenon of puffing and bursting. Finally, in the last phase, residue formation occurs. Puffing and bursting phenomena manifest once the temperature reaches 683 K, indicating the emergence of substantial quantities of light compounds including carboxylic acids, aromatics, acrolein, ketene, and fatty acids. These compounds are formed through the thermal degradation and polymerization of vegetable oils. This process causes deposits to form in the colder areas of diesel engines.

Theoretical analysis of water-droplet condensation and evaporation in prescribed environment with radiation

Abstract A quasi-explicit algebraic solution of the problem of quasi-steady water droplet heat- and mass-transfer with condensation or evaporation in a prescribed environment including radiation has been obtained. The solution is based on the mass-fraction form of Fick’s law of species diffusive mass transport, which allows accounting for variation in gas density through the diffusion layer surrounding the droplet. A new term, the average vapor supersaturation through the diffusion layer, has been included that allows the model to be extended to environmental conditions significantly outside those for which it is theoretically derived (dilute or near-saturation), even to conditions of relatively high temperature, low relative humidity, and low pressure. The latter two have atmospheric significance because evaporative cooling can induce significant decrease in droplet temperature and possibly induce freezing. Basic modeling assumptions are confirmed by comparison with time-dependent temperature and evaporation-rate measurements of a laboratory laser-heated droplet. Accuracy of the new model is assessed by comparison with exact equations. For temperatures between −30 and 0°C and negligible radiation, relative-error magnitudes in droplet temperature and evaporation rate are less than 5%, even for relative humidity down to 10% and pressure down to 60 kPa. This represents an improvement in accuracy over the conventional model, in which property variations of density and supersaturation through the diffusion layer are not taken into account. Differences in predicted evaporation rates between the mass-fraction and the more common partial-density Fick’s laws, which can be significant (>10% above 70% relative humidity) in coupled, implicit equations, are shown to be made negligibly small (<0.5%) by linearizing about the saturation state.

Realistic Simulation of Dissolution Process on Rock Surface

Hydraulic dissolution, driven by carbon dioxide-rich precipitation and runoff, leads to the gradual breakdown and removal of soluble rock materials, creating unique surface and subsurface features. Dissolution is a complex process that is related to numerous factors, and the complete simulation of its process is a challenging problem. On the basis of deep investigation of the theories of geology and rock geomorphology, this paper puts forward a method for simulating the dissolution phenomenon on a rock surface. Around the movement of water, this method carries out dissolution calculations, including processes such as droplet dissolution, water flow, dissolution, deposition, and evaporation. It also considers the lateral dissolution effect of centrifugal force when water flows through bends, achieving a comprehensive simulation of the dissolution process. This method can realistically simulate various typical karst landforms such as karst pits, karst ditches, and stone forests, with interactive simulation efficiency.

Modeling and Analysis of Droplet Evaporation at the Interface of a Coupled Free-Flow–Porous Medium System

AbstractEvaporation of droplets formed at the interface of a coupled free-flow–porous medium system enormously affects the exchange of mass, momentum, and energy between the two domains. In this work, we develop a model to describe multiple droplets’ evaporation at the interface, in which new sets of coupling conditions including the evaporating droplets are developed to describe the interactions between the free flow and the porous medium. Employing pore-network modeling to describe the porous medium, we take the exchanges occurring on the droplet–pore and droplet–free-flow interfaces into account. In this model, we describe the droplet evaporation as a diffusion-driven process, where vapor from the droplet surface diffuses into the surrounding free flow due to the concentration gradient. To validate the model, we compare the simulation results for the evaporation of a single droplet in a channel with experimental data, demonstrating that our model accurately describes the evaporation process. Then, we examine the impact of free-flow and porous medium properties on droplet evaporation. The results show that, among other factors, velocity and relative humidity in the free-flow domain, as well as pore temperature in the porous medium, play key roles in the droplet evaporation process.

Flow characteristics and performance optimization of temperature and pressure reducing valve with droplets evaporation

The influence of water spray arrangement on temperature reduction performance is significant in temperature and pressure reducing valves (TPRV). A verified vapor-liquid two-phase flow in TPRV was established by Euler-Lagrange flow and droplets evaporation model. The heat and mass transfer processes in droplet evaporation are presented. The influence of nozzles number, spray position, injection direction on internal flow characteristics and temperature reduction performance of TPRV was analyzed. An optimized spray water arrangement was determined via nonlinear fitting model, the great temperature reduction performance was obtained. The results show that the mass flow rate of TPRV inlet and outlet has a non-linear variation with the valve opening. The significant pressure gradient occurs in the valve core and chamber. The upward oblique jet at throat makes the temperature near the spray area low and the temperature at throat downstream high. The range of high Mach numbers increases with the increasing of valve opening. The temperature variation of parallel injection and reverse injection is different with the increasing of the nozzles number. The vortex intensity and range have a stronger influence on temperature reduction performance ability. The temperature reduction performance of optimized TPRV is improved by 0.6 %- 3.98 %. The optimization of water spray arrangement on temperature reduction performance is applicable to various loads.

Phase-change ultrasound contrast agents for proton range verification: towards an in vivo application

Objective.In proton therapy, range uncertainties prevent optimal benefit from the superior depth-dose characteristics of proton beams over conventional photon-based radiotherapy. To reduce these uncertainties we recently proposed the use of phase-change ultrasound contrast agents as an affordable and effective range verification tool. In particular, superheated nanodroplets can convert into echogenic microbubbles upon proton irradiation, whereby the resulting ultrasound contrast relates to the proton range with high reproducibility. Here, we provide a firstin vivoproof-of-concept of this technology.Approach.First, thein vitrobiocompatibility of radiation-sensitive poly(vinyl alcohol) perfluorobutane nanodroplets was investigated using several colorimetric assays. Then,in vivoultrasound contrast was characterized using acoustic droplet vaporization (ADV) and later using proton beam irradiations at varying energies (49.7 MeV and 62 MeV) in healthy Sprague Dawley rats. A preliminary evaluation of thein vivobiocompatibility was performed using ADV and a combination of physiology monitoring and histology.Main results.Nanodroplets were non-toxic over a wide concentration range (<1 mM). In healthy rats, intravenously injected nanodroplets primarily accumulated in the organs of the reticuloendothelial system, where the lifetime of the generated ultrasound contrast (<30 min) was compatible with a typical radiotherapy fraction (<5 min). Spontaneous droplet vaporization did not result in significant background signals. Online ultrasound imaging of the liver of droplet-injected rats demonstrated an energy-dependent proton response, which can be tuned by varying the nanodroplet concentration. However, caution is warranted when deciding on the exact nanodroplet dose regimen as a mild physiological response (drop in cardiac rate, granuloma formation) was observed after ADV.Significance.These findings underline the potential of phase-change ultrasound contrast agents forin vivoproton range verification and provide the next step towards eventual clinical applications.

Computational fluid dynamics investigation of pesticide spraying by agricultural drones

Precise control over pesticide spraying by agricultural drones requires fundamental research of multiphase flow, heat and mass transfer of liquid droplets in downwash flow. In this study, water was used as a carrier agent in pesticide formulations, in which the liquid droplets exhibit a Newtonian fluid behavior. A general computational fluid dynamics (CFD) model that characterizes propeller motion coupled with droplet evaporation and deposition was developed. The downwash flow generated by the propeller rotation was simulated using a multiple reference frame method along with the SST-k-ω turbulence model. Spraying of liquid droplets was simulated using a discrete phase model. The flow model validation was conducted by comparing the simulated velocities with experimental data, and the spray model validation was completed by a comparison of droplet depositions from CFD predictions and experiments. The results indicated that the droplet deposition fluctuates with an increase of propeller speed, and that both temperature and relative humidity affect the droplet deposition. In addition, the relation between the number of particles and the impact velocity was identified quantitatively, impact velocity with respect to particle diameter was predicted, and the effects of surface inclination, surface roughness, and crosswind speed on droplet deposition were analyzed. Moreover, an in-depth discussion about the intrinsic relation between downwash flow and droplet spraying was conducted.

Rapid and Accurate Identification of Dyes on Ancient Textiles by SERS With a Negligibly Invasive Approach

ABSTRACTNowadays, surface‐enhanced Raman spectroscopy (SERS) has become a powerful tool for rapidly detecting and analyzing textile cultural relics due to its ability to provide chemical information with single‐molecule sensitivity. However, preserving a high level of detection sensitivity while avoiding sample damage remains a persistent challenge. In this work, we developed a SERS approach with both microextraction and detection functions. The alcohol–water droplet with iodide‐modified Au nanoparticles (AuIMNPs) is directly dropped on the textile, where dyes strongly bound on textiles can be extracted by ethanol (EtOH). As a result, the sample can be well preserved from being damaged. In particular, the volatility of EtOH allows the molecules to be captured in the hot spots through the capillary effect during droplet evaporation, resulting in a dramatic increase in Raman signal intensity. This highly sensitive strategy can be used to measure dyes in plant extracts and mock‐up textiles. Furthermore, the capability of SERS to provide fingerprint information allows us to distinguish different dyes in overdyeing textiles. Eventually, this approach is successfully applied to identify dyes of authentic ancient Chinese textiles. This rapid, universal, and negligibly invasive approach provides a powerful way to study textile cultural relics.

Investigation on ignition enhancement characteristics by high-energy spark in a scramjet combustor under low flight Mach conditions

Ignition characteristics of the long electrode distance high-energy spark igniter (LHSI) in a cavity-stabilized scramjet combustor at low flight Mach has been numerically and experimentally investigated and compared with those of conventional lower energy spark igniter (SI). The effects of ignition and injection schemes on ignition performance were analyzed and the enhancement of greater spark energy were verified. The results show that the widest ignition boundary of SI is observed when the kerosene is injected in far field due to the sufficient evaporation of kerosene droplets and the disturbance caused by upstream cavity. Due to the greater energy deposition by LHSI, the ignition performance is markedly improved and the ignition boundary of combustor is extended from 0.131–0.148 to 0.091–0.224 under the scheme of I1+L2. The increased energy deposition of spark plasma ignites more combustible mixture and enhances the initial combustion, leading to a larger high-temperature zone. This is crucial for ensuring the survival of the flame kernel against dissipation of airflow in the scramjet combustor operating at low fight Mach numbers due to the short duration of spark discharge.