Jet Droplet Research Articles

Effects of impacted dynamic hollows and extending dense drops on the concave surface with practical obstacles

In this study, the center of a concave surface was analytically studied using the volume of fluid approach to simulate hollow and dense droplets on a variety of solid obstacles. OpenFoam software was used to carry out the numerical simulations. The hollow droplet’s fluid phase, Glycerine, has an outer diameter of 5.25 mm and its gas phase, air, has a diameter of 4 mm. We looked at the laminar flow of an incompressible Newtonian fluid phase. Jet characteristics and droplet collision hydrodynamic behavior were investigated. Due to the interaction between droplets and shells on the obstacle’s surface and the concave surface, which causes a pressure difference and improves fluid movement, the largest jet size is consequently produced in rectangular obstacles. The sharp obstacle, on the other hand, molds the jet’s shortest length and height.

Design and performance analysis of a Swirl Pintle injector for a 1 MWth pressurized oxy-coal combustor

Pressurized Oxy-coal combustion has the potential to achieve high thermal efficiency by recovering the latent heat of steam from flue gases and providing options for up to 95% carbon capture. This study proposes a novel burner technology for pressurized oxy-coal combustion and investigates its operability. Three burner versions are designed and tested; no swirl, swirl numbers 0.9 and 1.2 to identify optimum design parameters and performance. A commercially available Dantec Dynamics software is used to conduct the shadowgraph experiments to observe burner performance. Coal water slurry is used as fuel with three different slurry ratios where the coal percentage by weights were 30, 40 and 50. Droplet size, distribution, spray angle and jet breakup lengths are measured and analyzed to quantify the performance. The study found that increasing the swirl number enhanced atomization. Implementing swirlers allowed the jet to spread through faster primary jet separation, thus improving atomization. In addition, higher slurry ratios improved the generation of smaller particles. However, it was also found that excessive increases in oxidizer momentums negatively impact the atomization process by jet shrinkage. The study concluded that operating a pintle burner at 1.2 swirl conditions with a 40/60 slurry ratio produces optimum performance.

Droplet Impact on a Micro-structured Hydrophilic Surface: Maximum Spreading, Jetting, and Partial Rebound

• Droplet impact on a hydrophilic microstructured substrate was studied. • Various impact outcomes were observed in our experiments. • The effects of impact velocity and fluid viscosity on impact phenomena were studied. • The scaling law for the maximum spreading diameter was reported. • The satellite droplets produced in jetting and partial rebound were analyzed. In this paper, we perform an experimental study of droplet impact on a partially wetting hydrophilic substrate composed of cylindrical micro-pillars. Water and glycerol are mixed at different ratios to primarily change liquid viscosity and keep surface tension approximately constant. We show that our microstructured hydrophilic surface can exhibit many of the same impact outcomes as hydrophobic surfaces including spreading, recoiling, jetting, and partial rebound. A regime map is constructed to convey the overall effects of the impact velocity and viscosity on the impact outcomes. Our data indicate that the maximum spreading factor β max generally follows the power law with the Weber number We as β max ∼ We 0.25 . However, the scaling relation of β max ∼ We 0.2 Re 0.04 provides a better correlation for β max because the viscous dissipative effect due to flow through the micro-pillars on the substrate becomes increasingly important for more viscous fluids. The rapid jet caused by the collapse of the air cavity in the recoil phase grows in a self-similar pattern. The relationship between the size of the top jet droplet and jet velocity is found to obey the same scaling law originally proposed for the bubble bursting jet. The partial rebound occurs only for low viscosity fluids with relatively high impact velocity. The size of the rebounding droplet emitted by the breakup of inertially stretched thick liquid thread in the partial rebound is found to be nearly independent of the impact velocity. The elapsed time between droplet impingement and partial rebound event scales with the capillary time.

Primary atomization of liquid-fuel jets in confined turbulent pipe-flows of air at elevated temperatures

The primary atomization of evaporating laminar liquid jets of either pure n-pentane or n-heptane injected continuously from a circular nozzle concentrically and axisymmetrically into high-temperature turbulent coflows of air in confined a pipe has been studied experimentally. Fuel-jet bulk velocities up to 2.7 m/s, air coflow bulk velocities up to 37 m/s and temperatures up to 1150 K were investigated within a 25 mm round pipe. In the near-field region of the injector nozzle, liquid fuel jets were formed whose length increased with the fuel injection velocity and decreased with the air velocity. Smaller droplets were formed at both higher fuel and air velocities. Droplet diameters within the range from 200 to 900 µm were measured, indicating the polydispersed nature of the fuel spray. The mean droplet diameter increased non-monotonically with increasing axial distance from the nozzle due to a complex competition between droplet coalescence and evaporation. No significant effect of the air inlet temperature was observed within the investigated range of conditions on the jet length and droplet size. However, smaller droplets were obtained in flows with lower turbulence intensities and longer longitudinal integral lengthscales. Analysis of the data reveals that the mean droplet diameters can be predicted by the empirical expression (dmean/djet) = 34 × 103 We∆1.29Re∆-3, independent of the two perforated plates selected and used in the present work. The resulting data can contribute towards a better understanding of interfacial dynamics, and the development and validation of advanced multiphase-flow and reacting-flow models.

Membrane resonant based droplet ejector for micro-droplet jetting

A novel membrane resonant droplet ejector (MRDE) for viscous industrial liquids is proposed in the paper. In MRDE, a vibrating nozzle is built to generate micro-droplets and the nozzle clogging could be reduced. The droplet jetting mechanism is studied by a three-dimensional (3D) lattice Boltzmann (LB) model. The effects of voltage amplitude and driving frequency on droplet volume and velocity are explored. Experimental Results show that low-viscosity water and viscous glycol could be jetted stably in parameters of 30 V/16 kHz and 60 V/20 kHz, respectively. Moreover, the droplet jetting of MRDE and in-situ observation is set up to analysis the droplet jetting characteristics. The critical thresholds for MRDE are9.340≤Re≤17.65, 3.653≤We≤8.846 and 0.014≤oh≤0.411, which has a wider printable range than traditional piezoelectric printheads. With the compact structure, MRDE is easy of integration for mass industrial applications compared with the pneumatic and needle-based ejectors.

Effect of the Shape of Pulses on Heat Transfer at the Stagnation Point of a Nonstationary Axisymmetric Impact Gas–Droplet Jet

Effect of the Shape of Pulses on Heat Transfer at the Stagnation Point of a Nonstationary Axisymmetric Impact Gas–Droplet Jet

CFD SIMULATION OF A COUGHING DROPLET DIFFUSION IN THE ACCELERATING-MOTION METRO CABIN

Abstract. The metro cabin provides an effective way for the spread of the pneumonia virus when provides convenience to human beings. The cabin undergoes a series of motions of acceleration speed, uniform speed, and deceleration speed during the one-station travelling process. The induced flow of the accelerated moving cabin is the determinant of the air drag force on the cough droplets, and effect on the time-frequency characteristics and motion trajectories of droplets of different sizes. In this study, we established a momentum equation on droplets, affected by the inertial force correction term in the non-inertial frame. Then added the drag force from the velocity difference between the cough droplets and the background airflow, and simulated configurations as: 1) droplet spreading aerodynamic process from different face orientations of infected dummy, 2) six groups of size particles, and 3) variable speed motion phases. The numerical simulation provides a physical analysis idea for studying the relationship between the two-phase flow under the action of inertial force. This design of a ventilation environment in public transportation helps to reach a more profound understanding of the inertial precipitation mechanism of droplet jets.

Phase field simulation of electrohydrodynamic jet droplets and printing microstructures on insulating substrates

The phase field technique is a straightforward and effective mathematical model for resolving interfacial problems in E-Jet printing. The paper presents an exhaustive analysis for the Drop-on-demand electrohydrodynamic jet (DoD E-Jet) printing. The study introduces a phase field method to generate a stable cone jet morphology that can allow the production of micron/nano structures on different insulating substrates. Further, the impact of steel needle and quartz capillary were studied on the cone jet morphology for different insulating substrate. In trials, the optimal settings of flow rate and dc positive pulse voltage were adjusted to print drops for numerical simulation verification. However, a distinctive analysis of droplet formation on PET and glass substrates was demonstrated in order to lower droplet size and polarization of residual charges. The model is based on the Taylor–Melcher leaky dielectric model and employs the 2-phase field method to track the interface. The nozzle is capable of producing a magnificent micro-dripping jet and detaching stable drops from the substrate surface. As a result, the results are primarily concerned with droplet diameter and relative permittivity of ink. According to the simulation findings, microdroplets with a diameter of less than 100 μm are created on PET substrate with quick decay of residual charges, whereas drops with a diameter of less than 50 μm are formed on glass substrate. Furthermore, it is demonstrated in studies employing a nozzle with an internal diameter of 360 μm that is fixed to a quartz capillary with a diameter of 50 μm that it takes a significant amount of time, cost and effort. The approach can significantly guide printhead and parameter design for printing on insulating surfaces. It is a more convenient and efficient method for predicting pattern sizes with a small number of trials for MEMS device applications.

Experimental research on droplets releasing characteristics of the bubble bursting behavior at a free surface with an aerosol

Bubble burst phenomena exist widely in industrial and environmental processes and exert a subtle influence. Bubbles may be produced by gas injection or boiling and will burst after floating at the free surface of the pool for a while. The produced film droplets and jet droplets will entrain the liquid substance into the gas, especially in reactor accidents. When aerosols retain in the pool, radioactive aerosol-laden droplets may exert a long-lasting source term that needs to be addressed to evaluate this source more precisely. The bubble bursting and film droplets producing behavior at the free surface under different conditions are investigated by visualization. Image processing software is applied to measure the size, number, and speed of film droplets. By comparing the bubble cap film thickness and the film roll-up speed to the number of droplets under different liquid temperatures both in distilled water and aerosol suspension, we show that the increase of the cap film thickness (decrease of the bubble lifetime) attributes to the decline of the number and average speed of film droplets and the increase of average size. The normalized droplet speed obeys the multimodal distribution. The normalized droplet size distribution obeys the gamma distribution and the Rayleigh distribution. The goodness of fit for gamma distribution of distilled water can reach 88%, and aerosol suspension is 70%. The increase of the liquid temperature will slightly increase the number and average size of film droplets but decrease the average speed for distilled water and aerosol suspension. Under low temperature, adding TiO2 aerosol may increase the number of film droplets; under high temperature, the aerosol may reduce the number of droplets.

Effect of Arc Behaviour and Metal Transfer Process on Aluminium Welds during Ultrasonic Frequency Pulsed GMAW

In order to study the influence of a novel current waveform control method of ultrasonic-frequency pulse (UFP) on arc behaviour, metal transfer process and the welds formation, monitoring system and image processing algorithms were employed for extracting the welding characters. Mechanics, defect, microstructure and mechanical property analysis based on pulsed GMAW was carried out. The results showed that, compared with the conventional pulsed GMAW, the ultrasonic-frequency pulse not only increased axial arc plasma jet force, arc force and droplet falling acceleration, but also suppressed the welded porosity formation, decelerated the microstructure regional element segregations, refined the grain and increased the microhardness property of welded joint. The study helped to reveal the mechanism for improving welding quality of ultrasonic-frequency pulsed GMAW.

Deep-Learning-Based Microfluidic Droplet Classification for Multijet Monitoring.

Inkjet printing, the deposition of microfluidic droplets on a specified area, has gained increasing attention from both academia and industry for its versatility and scalability for mass production. Inkjet printing productivity depends on the number of nozzles used in a multijet process. However, droplet jetting conditions can vary for each nozzle due to multiple factors, such as the surface wetting condition of the nozzle, properties of the ink, and variances in the manufacturing of the nozzle head. For these reasons, droplet jetting conditions must be continuously monitored and evaluated by skillful engineers. The present study presents a deep-learning-based method to identify the droplet jetting status of a single-jet printing process. A convolutional neural network (CNN)-based on the MobileNetV2 model was employed with optimized hyperparameters to classify the inkjet frames containing images captured with a CCD camera. By accumulating the classified class data in order by frame time, the jetting conditions could be evaluated with high accuracy. The method was also successfully demonstrated with a multijet process, with a test time of less than a second per image.

Recognition of jet modes in electrohydrodynamic direct-writing based on image segmentation

The jet modes are directly related to the stable and accurate deposition in electrohydrodynamic direct writing. In order to analyze the state of the jet through the image, it is essential to segment the image into needle area, droplet area and jet area. The small feature scale, unclear boundary and fast response speed bring troubles to the region segmentation. An algorithm of region segmentation is proposed to achieve the region segmentation. A scan method is used to extract the needle area automatically and the open-operation is used to separate the droplet area and jet area. The characters of droplet and jet are analyzed to judge the mode of the jet, which directly reflects the stability of the ejection state. This work is beneficial to the real-time controlling of the EDW and the stability and uniformity of micro/nanofibers.

Study on the dynamic characteristics of stable formation of single droplet in gas-liquid co-flow device

The dynamic characteristics of the stable formation of a single droplet in a gas-liquid co-flow device are studied experimentally. The scaling laws of the dimensionless necking radius R of a droplet with dimensionless remaining time τ for different parameters are analyzed, and the reasons for the transformation between these scaling laws are discussed. The results show that the change in the main pressure has little effect on the scaling law. In the earlier necking, R follows a 1/5 power law with respect to τ. In the later collapse, a 2/3 power law is matched. When the auxiliary pressure changes, the pneumatic shear force has a large impact on the earlier necking, causing the scaling law to change, but only a slight effect on the later collapse. Simultaneously, the droplet size and jet limit length decrease significantly with an increase in auxiliary pressure. The results are of great significance for revealing the mechanism of droplet formation and reducing droplet formation size in gas-liquid co-flow devices and provide a theoretical reference for further study of stable droplet formation.

Working principle and relevant physical properties of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) evaluation.

PurposeTo describe and evaluate relevant physical properties of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) for ocular surface sensitivity measurement.MethodsCharacteristics of Liquid Jet (LJ) droplets (consisting of isotonic saline solution) were analysed: vertical and horizontal displacement and speed of LJ droplets were recorded with the aid of the High Speed Photron FASTCAM NOVA S6 camera (stimulus duration: 40 ms). Stimulus mass was assessed for 20 sets of 10 LJs with aid of a microbalance (pressure range of 100–1500 mbar).ResultsBecause continuous flow LJ disintegrated into droplets in the lower pressure range (<700 mbar), pulsed stimuli were applied in order to obtain similar stimulus characteristics across the applied pressure range. For all measurements, very little variability was observed. Vertical and horizontal displacement did not exceed 0.13 mm in either direction. The mass per shot showed an unexpected cubic dependency on pressure. Up to approximately 700 mbar, LJ speed showed an almost linear relationship. For the pressure range of >700–1500 mbar, variability increased and speed decreased compared to the expected in a linear manner. However, this may be caused by the difficulty of identifying pattern changes of LJ droplets from one high speed image frame to the next with increasing stimulus speed, when determining LJ speed via pixel count.ConclusionsSwiss Liquid Jet Aesthesiometer for Corneal Sensitivity was shown to deliver fine droplets with a pulsed stimulus mode, in a repeatable manner with precise localisation to the ocular surface. Very little variability was observed in LJ speed and mass for the typical pressure range required for clinical sensitivity measurements.

Impact force characteristics of liquid droplets and compressor blades

To study the influencing factors of liquid droplets on the cleaning effects of the scale layer, a three-dimensional droplet-impacting-a-blade model is constructed. The finite volume method and the volume of fluid method are used to numerically study the dynamic characteristics of a droplet impacting the fouling layer of a blade. The results show that under certain conditions, increased velocity and diameter of the droplet lead to an increase in the impact force of the droplet, and the greater the curvature of the blade surface, the smaller the vertical impact force of the droplet; likewise, the greater the impact force on cutting, the larger the corresponding spreading coefficient. The impact of liquid on the surface of the blade reduces the peak value of the droplet impact force. Finally, the increase in droplet diameter and velocity will improve the cleaning effect of the jet droplet. The larger the blade surface curvature, the wider the droplet cleaning area will be.

Dynamic Behaviors after Droplet Impact onto Liquid Surface

In this paper, we present the dynamic behaviors of crown formation, central jet, and secondary droplets generated with droplet impact onto a liquid surface by using experimental and computational approaches. In our experiment, the dynamic behaviors after a droplet impact are recorded using a high-speed camera with appropriate resolution and exposure time. On the other hand, we simulate numerically the similar behaviors using the VOF (volume of fluid) solver in the OpenFOAM. As a fluid field, we consider the multiphase flows with free surfaces based on incompressible Navier-Stokes equations in the software codes. Some qualitative comparisons between the experimental and the computational results demonstrate the workability and validity of the present approaches.

Large eddy simulation of cough jet dynamics, droplet transport, and inhalability over a ten minute exposure.

High fidelity simulations of expiratory events such as coughing provide the opportunity to predict the fate of the droplets from the turbulent jet cloud produced from a cough. It is well established that droplets carrying infectious pathogens with diameters of remain suspended in the air for several hours and transported by the air currents over considerable distances (e.g., in meters). This study used a highly resolved mesh to capture the multiphase turbulent buoyant cloud with suspended droplets produced by a cough. The cough droplets' dispersion was subjected to thermal gradients and evaporation and allowed to disperse between two humans standing 2 m apart. A nasal cavity anatomy was included inside the second human to determine the inhaled droplets. Three diameter ranges characterized the droplet cloud, , which made up 93% of all droplets by number; 5 to 100 μm comprised 3%, and m comprising 4%. The results demonstrated the temporal evolution of the cough event, where a jet is first formed, followed by a thermally driven puff cloud with the latter primarily composed of droplets under 5 μm diameter, moving with a vortex string structure. After the initial cough, the data were interpolated onto a more coarse mesh to allow the simulation to cover ten minutes, equivalent to 150 breathing cycles. We observe that the critical diameter size susceptible to inhalation was , although most inhaled droplets after 10 min by the second human were approximately . These observations offer insight into the risk of airborne transmission and numerical metrics for modeling and risk assessment.

Physical and Mathematical Modeling of the Interaction of Water Droplets and High-Speed Gas Flow

Present work is devoted to physical and mathematical modeling of the secondary disintegration of a liquid jet and gas-dynamic breakup of droplets in high-speed air flows. In this work the analysis of the experiments of water droplet breakup in the supersonic flow with Mach numbers up to M = 3 was carried out. The influence of shock wave presence in the flow on the intensity of droplets gas-dynamic breakup is shown. A developed empirical model is presented. It allows to predict the distribution of droplet diameters and velocities depending on the gas flow conditions, as well as the physical properties of the liquid. The effect of the Weber and Reynolds numbers on the rate of droplets gas-dynamic breakup at various Mach numbers is shown. The obtained data can be useful in the development of mathematical models for the numerical simulation of two-phase flows in the combined Lagrange-Euler formulation.

Active control of jet breakup and droplet formation using temperature modulation

Different regimes of temperature modulated breakup were studied using a slender jet model. The instability is induced through capillary and Marangoni stresses and can be used to control droplet formation in terms of intact length and resultant drop size distribution, which is otherwise irregular due to the inevitable presence of background noise. When thermal modulation is weak, the surface tension gradient forces act only as a trigger and curvature-gradient forces soon take over and grow exponentially downstream of the jet due to inertio-capillary growth. When thermal modulation is strong, the surface tension gradient forces not only act as a trigger, but remain significant till breakup.

Droplet Ejection and Liquid Jetting by Visible Laser Irradiation in Pyro‐Photovoltaic Fe‐Doped LiNbO3 Platforms

AbstractControlling liquid dispensing and jetting from a reservoir drop or a liquid film require strong electric fields. One efficient method proposed some years ago is based on the high pyroelectric‐electrohydrodynamic (EHD) effect presented by lithium niobate when it undergoes a high temperature change. Additionally, first experiments generating droplets using the photovoltaic effect of Fe‐doped lithium niobate crystal (LiNbO3:Fe) have been recently reported. Here, it is shown how the excitation of the photovoltaic and pyroelectric effects of LiNbO3:Fe by visible light irradiation allows droplet dispensing and jetting. A basic characterization of the process, including the important role of the excitation light intensity, is reported. The experimental investigation demonstrates that efficient droplet ejection and liquid jetting can be easily achieved in different optical configurations and with various liquid solutions (water, alcohol, aqueous suspension of particles or polymers). This new explored method is analyzed by discussing some of its intrinsic and attractive advantages, namely the flexible and versatile control offered by light excitation and the activation of the photovoltaic and pyroelectric effects. The results let us to foresee that this strategy will have very good chances to become a viable inkjet printing method for addressing new challenges in directed liquid dispensing and patterning.