Jet Length Research Articles

The transition from sheet to cloud cavitation

Recent studies indicate that the transition from sheet to cloud cavitation depends on both cavitation number and Reynolds number. In the present paper this transition is investigated analytically and a physical model is introduced. In order to include the entire process, the model consists of two parts, a model for the growth of the sheet cavity and a viscous film flow model for the so-called re-entrant jet. The models allow the calculation of the length of the sheet cavity for given nucleation rates and initial nuclei radii and the spreading history of the viscous film. By definition, the transition occurs when the re-entrant jet reaches the point of origin of the sheet cavity, implying that the cavity length and the penetration length of the re-entrant jet are equal. Following this criterion, a stability map is derived showing that the transition depends on a critical Reynolds number which is a function of cavitation number and relative surface roughness. A good agreement was found between the model-based calculations and the experimental measurements. In conclusion, the presented research shows the evidence of nucleation and bubble collapse for the growth of the sheet cavity and underlines the role of wall friction for the evolution of the re-entrant jet.

Comparison of PIV Experiment and Numerical Simulation on the Velocity Distribution of Intermediate Pressure Jets with Different Nozzle Parameters

AbstractThe need for a reliable research method of the external flow field has increased with the more widespread use of intermediate pressure jets. The experimental method and numerical simulation method were investigated, using the particle image velocimetry (PIV) technique and the volume of fluid‐level set (VOF‐level set) method. Both methods performed well on nozzles with different geometric parameters at distances close to the nozzle and further away from it. With reference to the different measurement windows, the velocities in vertical profiles along the x‐axis, the change of maximum jet velocities and the break‐up length ranges of jet flows were obtained. The two methods authenticate each other, and the results indicate that the maximum velocities of jets decrease and the distribution widths of velocity field also increase with increase in distance away from the nozzles. The PIV and simulation gave similar results at different distances from the nozzle, and the experimental values were slightly higher than the simulation values due to the pressure fluctuation with an error of less than 8%. The break‐up length of jets increases with pressures in situations where the experimental values were lower than the simulation values due to the effect of air. The experimental data from PIV agreed well with the simulation results. Therefore, the accuracy and reliability of the PIV experiment and the CFD (Computational Fluid Dynamics) result simulation were both validated. Copyright © 2017 John Wiley & Sons, Ltd.

Association of Placental Jets and Mega-Jets With Reduced Villous Density

Spiral arteries (SAs) lie at the interface between the uterus and placenta, and supply nutrients to the placental surface. Maternal blood circulation is separated from the fetal circulation by structures called villous trees. SAs are transformed in early pregnancy from tightly coiled vessels to large high-capacity channels, which is believed to facilitate an increased maternal blood flow throughout pregnancy with minimal increase in velocity, preventing damage to delicate villous trees. Significant maternal blood flow velocities have been theorized in the space surrounding the villi (the intervillous space, IVS), particularly when SA conversion is inadequate, but have only recently been visualized reliably using pulsed wave Doppler ultrasonography. Here, we present a computational model of blood flow from SA openings, allowing prediction of IVS properties based on jet length. We show that jets of flow observed by ultrasound are likely correlated with increased IVS porosity near the SA mouth and propose that observed mega-jets (flow penetrating more than half the placental thickness) are only possible when SAs open to regions of the placenta with very sparse villous structures. We postulate that IVS tissue density must decrease at the SA mouth through gestation, supporting the hypothesis that blood flow from SAs influences villous tree development.

Exhaustion of a silane jet into a space

Issues of explosion and fire safety of silane during its transportation and storage are intimately related to its possible catastrophic leak off from tanks if they become cracked. It turned out that silane self-ignition is possible at some leak off velocities. The interest of the aerospace industry to problems of ignition and combustion of silane combined with other fuels should be also noted. Experimental investigations of such problems, which are rather expensive and laborconsuming, provide primary information for the development and verification of mathematical models of physical and chemical processes, capable of predicting the characteristics of silane mixing, ignition, and combustion induced by its exhaustion. Recent experiments of Prof. Chen and his colleagues from Taiwan allowed them to determine the critical length of the silane jet (distance from the tube exit where ignition of exhausting silane occurred) and the time of silane ignition. In the present work, an attempt is made to simulate these phenomena within the framework of the concept of the critical concentration of silane with studying the dynamics of silane leak off from a tube.

De-agglomeration of nanoparticles in a jet impactor-assisted fluidized bed

Nanoparticles in agglomerated state lose their outstanding properties; hence, it is essential to break them up prior to use and prevent their re-agglomeration. Even though there are several dry techniques to disperse nanopowders, none of them have been able to produce truly nanoscale aerosols so far. Here, we study de-agglomeration of dry silica nanopowder via a jet impactor-assisted fluidized bed (JIAFB). The particle size distribution of fragmented powders was characterized by in-line scanning mobility particle spectrometry (SMPS) and offline transmission electron microscopy (TEM). In order to ascertain the jet length and that the kinetic energy of particles is sufficient for de-agglomeration, a CFD simulation was carried out. Both SMPS and TEM measurements imply that at a certain fluidization velocity, increasing the jet velocity shifts the particle size distribution towards smaller diameters, and at higher velocities the mode value reduced from 113 130 to 55–60nm. However, the geometric standard deviation or degree of polydispersity rises from 1.5 to 2.0 by increasing the jet velocity up to 197m/s, as it will increase the total superficial velocity and consequently entrainment of larger particles from the bed. In addition, the TEM results indicate that the range of individual particle sizes in the supplied nanopowder is wide; hence, increasing the geometric standard deviation can be an indicator of a higher level of agglomerate dispersion.

A Study of the Influence of the Surrounding Gas on the Plasma Jet and Coating Quality During Plasma Spraying

Coating quality is affected by arc and plume instabilities during plasma spraying. In closed chamber plasma spraying, gradual drift is one of the intermediate instabilities, which is mainly due to the electrode erosion. This work focuses on the source of the gradual drift of the plasma jet and the influence on coating quality. The ambient state inside the chamber was controlled by a ventilation system and a vacuum system. The variation in the plasma jet was observed by a particle flux image device based on a CCD camera. The optical spectrum of the plasma plume was measured and analyzed through an optical spectrometer. The results indicated that the addition of hydrogen to plasma gas induced the change in the plasma jet length and width with changing rates depending on the chamber state and the ventilation power. With poor ventilation, the intensity of Hα emission was found to become gradually stronger while Hβ and Hγ were found to become weaker. On closing the chamber and retaining enough ventilation power, it was observed that the ambient gas slowly turned red. Simultaneously, the coating weight and thickness were slightly decreased meanwhile the porosity ratio was obviously increased. The red ambient gas has been proved to be able to acidify the city water with pH value decreased from 7 to 1–3. Without hydrogen, the plasma jet was found to be stable without reddening and variation, but the plasma enthalpy was unfortunately low.

The objective design of triangular bucket for dam's spillway using Non-dominated Sorting Genetic Algorithm II: NSGA-II

Ski jump is one of the most e ective structures in energy dissipation over spillways. Spillways have long been of practical importance to safety of dams. The major criteria in hydraulic design are based on the analytical and empirical methods. In the current study, in order to increase chute spillway eciency, a multi-objective evolutionary algorithm known as the Non-dominated Sorting Genetic Algorithm II (NSGA-II) has been utilized to design the optimal triangular bucket angle and chute width. In design method, two separate objective functions have been used. In the rst objective function, equations of dynamic pressure of the bucket, the jet length after bucket, and the scour depth have been used. The second objective function is related to construction volume of chute spillway. For calibrating the rst objective function, characteristics of Karoon III dam have been used as a case study. The di erence between design parameters of Karoon III spillway and those from NSGA-II algorithm method is less than 12 percent. According to the results, if the jet length is considered as the most impressive parameter in the rst objective function, design of the spillway becomes frugal.

Effect of Atomization Parameters on Size and Morphology of Al-17Si Alloy Powder Produced by Free Fall Atomizer

Abstract: In the present work the effect of process parameters on the characteristics of Al-17Si alloy powder produced by gas atomization using a free fall nozzle of different apex angles. The axial gas velocity away from the nozzle exit is measured for different operating pressures. The alloy was melted to a superheat temperature of 100,150 and 200° C and atomized by varying the gas flow rate and melt flow rate. The powder produced during gas atomization process generally exhibits a wide size distribution. ASTM standard sieve with vibratory shaker is used for powder analysis. The characterization of powder particles was accomplished by the parameters such as mass median, average size of powder particles; standard deviation and morphology of powder were studied by scanning electron microscopy. It is observed that decrease of apex angle results in axial velocity decreases in entire applied gas pressures range. Increase in apex angle results in decrease of gas jet length to metal stream collision, the median particle diameter of the powder increases resulting in decrease in particle irregularity. The median particle diameter of the powder has been shown to decrease almost linearly with increase in gas to melt flow (G/M) ratio. The sauter mean diameter variation, however, decreases slowly with increase in G/M ratio. Median particle diameter is significantly reduced with increase in the melt superheat.

Investigation of superheated liquid carbon dioxide jets for cutting applications

Water jet cutting is of increasing interest in manufacturing technology. By using carbon dioxide (CO2) instead of water, a dry processing becomes possible. To realize cutting results comparable to water, liquid and coherent CO2-jets with high specific energy have to be formed. However, CO2 does not exist as a liquid in thermodynamic equilibrium at atmospheric pressure. During the expansion through a nozzle from a liquid state to atmospheric pressure the saturation curve is crossed and a spray consisting of a gaseous and solid phase is generated. We show that with sufficiently low injection temperatures the phase change is delayed and liquid jets emanate from the nozzle even at atmospheric pressure. These jets disintegrate closer to the nozzle than water jets at same pressure and nozzle conditions. The shorter jet length results in lower kerf depths. But the kerf width is smaller with CO2 and a more precise cutting is possible.

Effect of non-Newtonian rheology on electrified jets of polymer nanofibers in electrospinning process based on bead–spring model

In the recent decades, electrospinning was mainly concerned with the formation of nanofibers from synthetic polymers and natural polymers targeting predominantly technical applications areas such as textiles and filters. In this paper, the effect of nonlinear matter flow on the dynamics of electrified jets in electrospinning process is examined by analyzing the bead–spring model. The ordinary differential equation is derived from momentum equation which is solved numerically using the fourth-order Runge–Kutta method. The simulations show good agreement with the experimental data and provide a qualitative understanding of the role of viscoelasticity in the early stage of the electrospinning experiment. The results show that a non-Newtonian rheology can slow down the elongation dynamics, thereby reducing the stable jet length.

Effect of jet length and ambient temperature on the performance of a two-phase jet impingement heat sink refrigeration system

The jet impingement heat sink integrated with a compact oil-free R-134a vapor compression refrigeration system introduced in a previous work (Oliveira and Barbosa, 2017) is now further evaluated in terms of the influence of the compressor piston stroke, applied thermal load, orifice-to-heater distance (jet length) and ambient (hot end) temperature. The proposed heat sink is a compact active thermal solution for concentrated heat loads because it integrates the evaporator and the expansion device into a single unit, making use of a single two-phase impinging jet as the cooling mechanism. The present analysis is based on the coefficient of performance and other steady-state heat transfer parameters associated with the impinging jet (heat transfer coefficient and heater surface temperature). A reduction of the jet length promoted a more vigorous splattering of the jet on the heated surface, enhancing the droplet breakup, which in turn reduced significantly the critical heat flux. An increase of the hot reservoir temperature increased the jet impingement heat transfer coefficient.

Production of long, laminar plasma jets at atmospheric pressure with multiple cathodes

Plasma jets from conventional non‐transferred arc plasma devices are usually operated in turbulent flows at atmospheric pressure. In this paper, a novel non‐transferred arc plasma device with multiple cathodes is introduced to produce long, laminar plasma jets at atmospheric pressure. A pure helium atmosphere is used to produce a laminar plasma jet with a maximum length of >60 cm. The influence of gas components, arc currents, anode nozzle diameter, and gas flow rate on the jet characteristics is experimentally studied. The results reveal that the length of the plasma jet increases with increasing helium content and arc current but decreases with increasing nozzle diameter. As the gas flow rate increases, the length of the plasma jet initially increases and then decreases. Accordingly, the plasma jet is transformed from a laminar state to a transitional state and finally to a turbulent state. Furthermore, the anode arc root behaviours corresponding to different plasma jet flows are studied. In conclusion, the multiple stationary arc roots that exist on the anode just inside the nozzle entrance are favourable for the generation of a laminar plasma jet in this device.

Attrition characteristics of iron ore by an air jet in gas-solid fluidized beds

The effects of superficial gas velocity (Ug=1.25–3.00m/s) and distributor hole size (8.0–12.4mm) on the attrition rate of iron ore in a gas-solid fluidized bed with 0.076m ID×3.7m height with or without solid circulation have been determined. The particle density and the Sauter mean diameter of fresh iron ore were 3705kg/m3 and 357μm, respectively. When the kinetic energy rate from the orifice was equal or >180J/s, we could determine an attrition rate trend by measuring the fractional mass of fine particle formation (under 500μm fraction) during 30min without solids circulation. In the experiments with solids circulation, the attrition rate was determined by measuring the fractional mass of fine particle formation (under 63μm fraction; variation of threshold size). The attrition rate increases with increasing kinetic energy rate from the orifice (180J/s<EK<608J/s). The kinetic energy rate from the orifice was calculated using the mass flow rate and orifice nozzle velocity. The correlation of the attrition rate with the kinetic energy rate from the orifice isΦ/N=0.1214EK−1.3587 above the jet length.

CFD-DEM Analysis of Particle Attrition in a Jet in a Fluidised Bed

In fluidised bed processes, the solids are in vigorous motion and thus inevitably subjected to mechanical stresses due to inter-particle and particle-wall impacts. These stresses lead to a gradual degradation of the particles by surface wear, abrasion and body fragmentation commonly termed attrition. One significant contribution of attrition comes from the air jets of the fluidised bed distributor. Particles are entrained into the air jet, where they get accelerated and impacted onto the fluidised bed particles. The jet induced attrition only affects the part of the bed which is limited by the jet length, where the mode of attrition is largely collisional. The overall jet attrition rate is therefore the result of the combination of the single particle damage and the flux of particles entering into that region. The attrition behaviour of particles in the jet region is analysed by evaluating their propensity of breakage experimentally and by simulating an air-jet in a bed of particles by CFD-DEM. The frequency of collisions and impact velocities are estimated from which the attrition due to a single air-jet is predicted.

A low power miniaturized dielectric barrier discharge based atmospheric pressure plasma jet.

In this paper, a dielectric barrier discharge plasma based atmospheric pressure plasma jet has been generated in a floating helix and floating end ring electrode configuration using argon and helium gases. This configuration is subjected to a range of supply frequencies (10-25 kHz) and supply voltages (2-6 kV) at a fixed rate of gas flow rate (i.e., 1 l/min). The electrical characterization of the plasma jet has been carried out using a high voltage probe and current transformer. The current-voltage characteristics have been analyzed, and the consumed power has been estimated at different applied combinations for optimum power consumption at maximum jet length. The obtained optimum power and jet length for argon and helium gases are 12 mW and 32 mm, and 7.7 mW and 42 mm, respectively. It is inferred that besides the electrode configurations, the discharge gas is also playing a significant role in the low power operation of the cold plasma jet at maximum jet length. The obtained results are interpreted on the basis of penning processes.

Non-equilibrium steady-state kinetics of He-air atmospheric pressure plasmas

A non-equilibrium, steady-state collisional-radiative kinetics model is developed to study atmospheric pressure discharges produced in He mixed with dry air (79% N2 and 21% O2). The model is based on a self-consistent solution of the Boltzmann equation for the electron energy distribution function coupled to a system of non-linear equations for species that govern plasma chemistry (electrons, ions, radicals, atoms, and molecules in ground and excited states). The main plasma parameters, including the maintaining electric field and species densities, are obtained as a function of He-to-air ratio. The results indicate that the concentration of air strongly influences the plasma. Notably, the He metastables and ion densities collapse at air concentrations above 0.1%, while the power required to maintain the plasma sharply increases as the concentration of air exceeds 1%. The model is applied to study the plasma characteristics along the length of an atmospheric pressure plasma jet using He as a carrier gas.

Rheumatic Giant Left Atrium—An Overview

Aim: To present the aneurysmal dilatation of left atrium due to rheumatic mitral valve disease and its clinical consequences such as arrhythmic, thromboembolic and compressive manifestations. Introduction: Extreme enlargement of left atrium, usually referred to as giant, gigantic or aneurysmal dilatation is an uncommon finding with a reported incidence of 0.3% in rheumatic heart disease. It is an important clinical risk identifier to predict the outcome of cardiovascular disease. Case reports: Aneurysmal left atrium correlating with the length of pure mitral regurgitation jet in a 18-year-old girl, posterior mitral leaflet prolapse with regurgitation jet swirling around the entire interatrial septum in a 37-year-old male, Giant left atrium in mixed mitral valve disease in a 37-year-old female and a thrombosed giant left atrium resembling as “coconut” in a 50-year-old female were reported. Conclusion: Giant left atrium may be misinterpreted as right-sided pleural effusion, pericardial effusion and mediastinal tumor on X-ray chest and so echocardiographic evaluation is mandatory to exclude the aneurysmal left atrium in such conditions.

Numerical analysis of hollow droplet impact on a flat surface

Many researches of a dense droplet impacting on a flat surface have been reported in the literature. However, the mechanism of a hollow droplet impacting on a flat surface has not yet been well addressed. A mathematical model is developed in the present research to resolve this impacting process. The model couples level set and volume of fluid method, and considers heat transfer and contact resistance between the droplet and surface. The validation of the model is carried out by comparing simulation results with experiment data. Different impact behaviors are observed in the impacting processes of both the dense droplet and the hollow droplet on a flat surface, obtained from the simulation result. The hydrodynamics and heat transfer behaviors of the hollow droplet impacting on a flat surface and the formation of central jetting are also explored. The effects of impact velocity and surface wettability on the impacting behavior of the hollow droplet are also analyzed. The results show that in the impacting process, the hollow droplet presents a spread and central jetting pattern, accompanying liquid shell contraction and breakup, while only spread and liquid shell contraction are observed in the dense droplet impacting process. It is also observed that the central jetting of the hollow droplet peels off the surface in the final impacting stage. The dimensionless spread factor for the hollow droplet is less than that of the dense droplet with the same initial kinetic energy in spread stage. The pressure gradient inside the hollow droplet is the main factor resulting in the spread and central jetting. The temperature distribution in the liquid shell and the surface is more uniform than in the central jetting, which is caused by the secondary breakup of the liquid shell. The spread factor of the hollow droplet remains unchanged as the impact velocity increases but is closely related to the surface wettability. The spread factor of the hydrophilic surface is larger than that of the hydrophobic surface. The effects of the surface wettability on the spread factor gradually reduce with the increase of the impact velocity. The effects of the impact velocity on the dimensionless jet length and the average wall heat flux are significant, while the surface wettability plays a negligible role in them. Improving the impact velocity increases the dimensionless length of the central jetting and the average wall heat flux, but this influence diminishes under a high impact velocity condition. Neither the dimensionless time spans of reaching the maximum spread factor nor the maximum average wall heat flux for the hollow droplet is influenced by the impact velocity and surface wettability and the development of the spread falls behind the heat transfer. Furthermore, the maximum spread factor increases with Reynolds number, and when Reynolds number is higher than 500, the increase in the maximum spread factor is no longer significant.

Numerical investigation of multiphase flow in flue gas desulphurization system with rotary jet stirring

There exits multiphase flow of oxygenized air, lime slurry and limestone particles in power plant flue gas desulfurization (FGD) system. A rotary jet stirring device (RJSD) is installed in the slurry pond at the bottom center of absorption tank to get an appropriate mixing flow field so far as possible to get off sulfur dioxide. By employing the standard k-ε turbulence model and the Eulerian multiphase model, numerical investigations for liquid-solid and liquid-solid-gas flow field are presented to discuss the distribution of flow hydrodynamic parameters under a certain operational condition, respectively. In liquid-solid flow, limestone particle concentration shows a decreasing trend with the increase of the rotation angular velocity. The core jet length of the rotary mixer tend to be much longer with the increase of jet velocity under the surrounding pressure, but the dispersed slurry velocity tends to reduce after an initial increasing during the starting period due to the inadequate mixing process. This phenomenon shows that the stirring down-stream is more important than the advection stream in liquid-solid condition. Compared to the liquid-solid condition, the distribution of the air volume of fraction in gas-liquid-solid flow is getting uniform because of the participation of the solid phase, and the down-stream plays an important role in air lateral distribution. The concentration of the limestone particle in liquid-solid condition is higher than in the multiphase flow field. The reason is that the particle distributing velocity near the tank wall is lower and the vertical velocity is higher than in the liquid-solid condition when the jet down-stream velocity is equal. The numerical results could be a useful exploration for an attempt usage of rotary jet device in FGD system.

Cold Plasma Jet Construction to Use in Medical, Biology and Polymer Applications

Cold atmospheric pressure plasma jets were capable of generating cold plasma plumes that were not confined by electrodes, which make them very enticing for biological, medical and polymer applications. During this work, experimental study of a low frequency, atmospheric plasma-jet discharge in nitrogen has been presented. The experimental operation of this device was conducted with commercially neon power supply. The discharge process operated by using nitrogen as input gas with different flow rates. The experimental results showed that the maximum plasma jet length of 7 mm was detected at 3 Kv input voltage corresponding to 14 l/min as a nitrogen flow rate. The effect of distance from nozzle, nitrogen flow rate and input voltage on the plasma temperature was examined. The gas temperature decreased continuously as the flow rate of nitrogen increasing from 2 l/min to 14 l/min at 3 Kv as a fixed input voltage.