Jet Diffusion Research Articles

Цилиндрические стримеры, формируемые в воздухе и азоте при низких давлениях

The results of experimental studies of analogs of columnar red sprites are presented. As is known, the development of sprites is due to the formation of cylindrical streamers at an altitude of 50-100 km from sea level. The velocities of a plasma diffuse jet (PDJ) obtained in a laboratory, which is an analogue of a sprite, were measured, and its emission spectra were recorded. Estimates are made of the magnitude of the reduced electric field strength in the PDJ at various distances from the electrodes initiating the discharge. It has been established that the shape and color of PDJ, which are also cylindrical streamers, at air pressures of 0.2-1.5 Torr corresponds to the color and shape of columnar red sprites. It is shown that the change in the color of diffuse plasma jets is associated with an increase in the reduced electric field.

The dynamics of an axisymmetric turbulent jet in ambient turbulence interpreted from the passive scalar field statistics

The effect of approximately homogeneous isotropic turbulence on the dynamics of an axisymmetric turbulent jet (Re = 10 600 and 5800) in an ambient with a negligible mean flow is interpreted from the statistics of the passive scalar field. The ambient turbulence is generated by a random jet array and scalar concentrations are measured in orthogonal cross sections of the jet using planar laser-induced fluorescence. Statistics of the scalar field of the jet in a turbulent ambient are compared to those in a quiescent ambient, using classical Eulerian averages and those conditioned on the jet centroid. A two-region model for the jet structure in ambient turbulence is proposed based on the centroidal statistics. Following the developing region of the jet, the ambient turbulence disrupts the jet structure, due to modulation of the jet interface, meandering of the jet by large eddies, and entrainment of the turbulent ambient fluid, resulting in a faster concentration decay and reduced entrainment compared to the quiescent ambient. Further downstream, once the ambient turbulence has destroyed the jet, only molecular and turbulent diffusion modify the scalar concentrations. The regions' relative lengths depend primarily on the relative turbulence intensity (ξ) between the ambient and the jet, as assessed using centroidal analysis, which removes the effect of the relative length scale (l) on the jet behavior in the turbulent ambient. The centroidal scalar statistics reveal self-similarity and self-preservation in mean scalar properties before jet breakup, which occurs abruptly once ξ> 0.5. The smaller scales of ambient turbulence modulate the jet boundary and, when entrained, lead to a wider range of centerline concentrations and root mean square concentrations, which are hypothesized to increase local concentration gradients within the jet and reduce jet mixing.

変形するノズルから流出する噴流

The purpose of this study is to control the jet diffusion by using the deforming nozzle. The shape of polypropylene nozzle can change from square shape into the cruciform shape or the octagonal shape smoothly. The velocity measurement of the jet flow was performed using X-type hot wire sensors and the constant temperature anemometer. The mean flow properties of the jet flow, such as the velocity distribution and the half value width were obtained. Moreover, to catch the flow feature of the jet near the nozzle the flow visualization was conducted by using the Laser Light Sheet and the high speed video camera. As a result, it has been found that the diffusion of the jet issuing from the deforming nozzle is promoted, comparing with the case of the square nozzle jet

Flow Visualization of Jet Flow Issuing from Pentagonal Duct

The purpose of this study is to control the jet diffusion by using the deforming nozzle. The shape of polypropylene nozzle can change from square shape into the cruciform shape or the octagonal shape smoothly. The velocity measurement of the jet flow was performed using X-type hot wire sensors and the constant temperature anemometer. The basic characteristics of the jet flow, such as the velocity distribution and the half value width were obtained. Moreover, to catch the flow feature of the jet near the nozzle the flow visualization was conducted by using the Laser Light Sheet and the high speed video camera. As a result, it has been found that the diffusion of the jet issuing from the deforming nozzle is promoted, comparing with the case of the square nozzle jet.

A Numerical Approach of the Influence of Water Temperature in the Extinction of Fire Developed in Closed Spaces

The aim of this work is to make a visual investigation of the fire behavior during the extinguish process with water mist at different temperatures, different diameters of the nozzle and different angles of diffusion jet, too. It was used a numerical simulation approach to analyze 24 scenarios regarding the influence of mist water in fire extinguish process. The main test was focused on a given enclosure with dimensions of 1.0 × 1.0 × 3.0 m and the fire source placed inside, in the middle. The enclosure is considered opened on the upper part. From the numerical simulation values, we concluded the best scenario in fire extinguish process using water mist.

Modeling and simulation of a turbulent jet diffusion flame of a biodiesel surrogate composed of MD, n-Hept, MC and EtOH

The more properties of biodiesel one wishes to represent, the more complex the surrogate mixture becomes. Often the aim is to represent the H/C and O/C ratios and molecular weight using a small number of pure components. In this article, a turbulent jet diffusion flame of a biodiesel surrogate, composed of 50% Methyl Decanoate, 40% n-heptane, 9% methyl crotonate and 1% ethanol is modeled and simulated. The strategy used is to apply the Directed Relation Graph technique for a first reduction, and Layerless Neural Network to define the main chain and obtain a skeletal mechanism. The results obtained for temperature and mass fractions of CO2, CO and H2O agree reasonably with literature data for a mechanism of 147 reactions and 45 species. The small number of species facilitates the simulation of the coupling between turbulence and chemical kinetics, a desirable feature of reduced mechanisms.

Improving the mixing of a turbulent jet diffusion flame of methane via linear quadratic regulator control

The purpose of this article is to show the improvement of the mixture of a turbulent jet diffusion flame of methane by means of linear quadratic regulator control. The model is based on low Mach and high Damköhler numbers. Large eddy simulation is used to simulate a turbulent flow with the Smagorinsky submodel for turbulent viscosity. The control employs a pair of actuators that act transversally to the flow direction on the walls of the burner. They consist of rectangular openings added along the depth of the duct. Numerical tests were performed by changing the position of one of these actuators. The results show that the controllers alter the jet diffusion flame, generating vortices of different sizes and dispersion patterns, which quickly leads to highly complex flow patterns.

Effect of Application of Electric Force to Diesel Fuel Droplets Charged Through Millikan Oil-Drop Experiment on Physical and Thermal Characteristics of Flame

Abstract This paper proposes a new feasible method to allow continuous change in the primary injection spray cone angle of liquid fuel droplets, which are injected from nozzles in liquid fuel combustion systems, to control the flame shape and thermal characteristics of the flame. The method is based on electric force applied to fuel droplets charged through frictional effects between the internal surface of the nozzle and the fuel flow as the liquid fuel is sprayed (based on the Millikan oil-drop experiment). A sprint computational fluid dynamics code was developed to investigate the effect of application of electric force to charged diesel fuel droplets, which were injected from a pressure swirl atomizer, on physical and thermal characteristics of a two-dimensional axisymmetric turbulent jet diffusion flame. The results show that an electric field applied to charged fuel droplets (electric force) changes the spatial distribution of the liquid fuel droplets in the flame reaction zone. An applied electric force in (−y) direction diverts the fuel droplets towards the axis centerline of the furnace and, consequently, decreases the primary injection cone angle and increases the concentration of the evaporated droplets around the axis centerline, which enhances the fuel-oxidant mixing rate and raises the flame temperature. Unlike an applied electric force in (−y) direction, an applied electric force in (+y) direction decreases the flame temperature. However, as the primary injection cone angle is decreased, an applied electric force in (+y) direction increases the flame temperature.

Environmental Impact of Discharge Patterns and Ambient Wind on the Jet Discharge from a Mechanical Upper Vent of a Tunnel

Flow characteristics and pollutant dispersion characteristics of a mechanical upper vent of a tunnel under various discharge patterns and ambient wind were studied by using the computational fluid dynamics (CFD) numerical method and an experimental model. On this basis, variations in the environmental impact radius rcriti changing with the ambient wind velocity U for pollutants from vertical and horizontal outlets were also analyzed with the set discharge capacity. According to our findings, the pollutant emission of the upper vent under a vertical discharge pattern can be considered as free buoyant jet diffusion under the impact of the momentum of ambient wind, while the emission under the horizontal discharge pattern can be regarded as semi-confined buoyant jet diffusion under the impact of the momentum of ambient wind. Moreover, the rcriti of the vertical outlet increased with the increase of U when U ≤ 3 m/s and was minimally affected by U for U > 3 m/s. Additionally, the rcriti of the horizontal outlet decreased with the increase of U. When the discharge capacity was constant, the rcriti of the upper vent under both vertical and horizontal discharge patterns declined with the increase of the discharge velocity V0. Note that the rcriti of the vertical outlet was smaller than that of the horizontal one for the range of parameters considered. These results can provide a theoretical reference for optimizing the design of a vent on the top of an urban tunnel.

Jet fires involving releases of gas and solid particle

The high-pressure leakage of underground pipeline could entrain the nearby sands and soils, and thus cause the jet fire of gas-solid mixtures. In addition, the thermal runaway of Lithium ion batteries could also cause a typical gas-solid jet diffusion flame. In comparison with the gaseous jet diffusion flame and the gas-solid-air jet premixed flame, the gas-solid jet diffusion flame lacks of exploration. Thus, a new facility consisting of a jet fire apparatus and a sand feeder, was designed to explore the gas-solid jet fire. The test repeatability was justified for the proposed facility. The dynamical differences of gas-solid and gaseous jet fires are clarified. It is found that the gas-solid jet fire holds a larger lift-off height, a less visible flame height and a less radiative fraction than the purely gaseous jet fire. In particular, for gas-solid jet fires at high exit velocities, the lift-off height increases, but the visible flame height and radiative fraction seem to decrease, as the particle size of sand decreases to increase the jet height of sand. (1) Inside the flame volume, the sand plays a role of heat sink to decrease the flame temperature and thus the flame burning velocity. Moreover, in the lift-off region, the sand dilutes the gas fuel and isolates the surrounding oxygen to inhibit the combustion. Both effects of sand cause a large lift-off height, especially for the sand of little particle size. (2) A large lift-off height means the increase of the air mass entrained from flame base or the portion of the premixed flame to the whole flame. In addition, the sand particle-laden flow would significantly increase the ambient air entrainment rate of fire plume. Both effects result in a less visible flame height. (3) The portion of the premixed flame increases in the bottom, which reduces the total soot amount inside the whole flame volume. The decrease of the flame temperature and the total soot amount reduces the radiative fraction. (4) The sand significantly affects the flame geometry and radiative fraction, which alters the radiant heat flux distribution and challenges the development of thermal radiation model for gas-solid jet fires. The gas-solid jet fire holds a less peak radiant heat flux but a higher positon of peak radiant heat flux than the gaseous jet fire. In addition, the line source radiation model available in literature is revised to well predict the radiant heat flux distribution of the gas-solid jet fires.

Numerical study of modeling methods and evaluation indexes for jet fans

With the rapid development of turbulent jet technology, jet fans have been widely used in building ventilation system, especially for underground buildings. Researchers often use CFD to design the impulse ventilation system, but the study of modeling methods for jet fans have not been studied comprehensively. Because of the characteristic of jet fan, different kind of inlet boundary condition types coupled with different turbulence models lead to different flow fields. In order to find the most suitable modeling method, this paper studied the effect of the combination of 8 turbulence models and 3 flow boundary conditions on modeling accuracy of jet fans. In addition to comparing velocity and turbulence intensity, the research adopted jet core area length and diffusion angle to evaluate the pros and cons of the simulation results. Results showed that the Fan Boundary Method (FBM) integrated with the k-w SST or RSM (Stress-BSL) turbulence model; the Velocity Boundary Method (VBM) integrated with the k-w SST turbulence model; the Momentum Source Method (MSM) integrated with the standard k-ε turbulence model can represent most jet fans' performance with free jet. When the main task is to verify jet fan nozzles performance or optimize nozzles design, the RSM (Stress-BSL) and k-w SST turbulence models are recommended. When the main focus is on the fan range or the performance of the impulse ventilation system, the standard k-ε turbulence model is recommended. The choice of the final model depends on the available data and the application scenario.

Numerical Study of the Influence of Secondary Air Uniformity on Jet Penetration and Gas-Solid Diffusion Characteristics in a Large-Scale CFB Boiler

The uniformity of secondary air (SA) in large-scale CFB boilers has an important influence on gas-solid flow and combustion, but was seldom considered in previous studies. Numerical simulation based on the Eulerian–Eulerian and RNG k-ε turbulence models was conducted to explore the influence of SA uniformity and load variation on jet penetration, diffusion characteristics and gas-solid mixing in the first 600 MW supercritical CFB boiler. The results showed that better SA uniformity was conductive to the uniformity of SA penetration and gas-solid mixing along the furnace height, although the penetration depth and diffusion distance showed an opposite trend. In addition, the penetration depth and diffusion distance got enhanced with higher boiler load. The inner and outer SA jets could not cover the furnace width, and the uneven SA uniformity led to a huge deviation of the solid concentration within 10 m of the air distributor. Eventually, a calculation model was successfully established for predicting the penetration depth of inclined thermal SA jets during boiler operation.

Assessment of weighted-sum-of-gray-gases models for gas-soot mixture in jet diffusion flames

The global radiative property models are widely used in combustion simulation due to its high computational efficiency. This study assesses the weighted sum of gray gases (WSGG) models for gas-soot mixture in the simulations of turbulent jet diffusion flame at atmospheric pressure with fully coupled combustion and radiation heat transfer. Non-adiabatic tabulated chemistry approach and the method of moments were employed for combustion modeling and soot particle dynamics. The radiative transfer equations are solved by the discrete ordinate model. Two kinds of WSGG model based on mixture modeling (MM) and superposition method (SM), and their combination with the gray and non-gray soot radiative property models are involved in the comparison. First, the radiative property models are evaluated on the one-dimensional parallel plate system using line-by-line model as the benchmark, and then the WSGG models are evaluated on the basis of the FSCK model in the combustion simulation. The results show that MM and SM provide similar prediction accuracy, while MM has much lower computational cost. The MM combined with the non-gray soot radiative property model (MMNS) provides the best accuracy for gas-soot mixture. Moreover, the relative error of MM combined with gray soot radiative property model (MMGS) is less than 10% for radiation source term, which is acceptable in engineering simulation. The gray implementation of WSGG model has low accuracy, which indicates that it is necessary to use the non-gray implementation of WSGG model in CFD simulations.

Analysis on the Shape and Impact Pressure of the High-Pressure Water Jet during the Hydraulic Flushing Cavity Technique

A large proportion of minable coal seams in China belong to low-permeability soft coal seams. Such coal seams suffer serious coal and gas outburst hazards and endure a high incidence of major disasters in coal mines. The adoption of the high-pressure water jet (HPWJ) hydraulic flushing cavity can effectively promote the gas drainage efficiency and volume and eliminate the hidden danger of gas disasters. Nevertheless, the shape and impact pressure of rotating HPWJ are rarely researched. In this study, on the basis of the numerical simulation, the axial and radial stress distributions of HPWJ and the energy-gathering effect of a conical-cylindrical combined nozzle were analyzed. It is concluded that the submerged condition will accelerate the attenuation of jet velocity and reduce the impact strength of the jet. The jet diffusion angle grows with the increases in the nozzle diameter and water pressure, and 24° is the optimal contraction angle. Finally, the influences of factors such as the rotation speed on the shape and impact pressure of HPWJ were explored, and the results show that the rotation speed should be controlled within 90 r/min. The research findings lay the foundation of the study on the mechanism of coal crushing by HPWJ and provide technical support for the research and development of drilling and flushing integrated equipment.

A study of the influence of coflow on flame dynamics in impinging jet diffusion flames

Non-premixed impinging jet flames with different coflow conditions are performed using PIV technology combined with numerical simulation to investigate flame instability in the vicinity of wall. Results indicate that the increase of coflow velocity results in a more chaotic flow field and higher fuel efficiency, and the increase of coflow temperature leads to ignition advance and the increase of NO concentration. These can be attributed to the coupling effect of Kelvin-Helmholtz instability, convective instability and Rayleigh-Taylor instability. High coflow velocity is more likely to induce Kelvin-Helmholtz instability and convective instability, and the increase of coflow temperature enhances Rayleigh-Taylor instability and convective instability. Due to the impact effect in the vicinity of wall, the flame instability is more likely to be induced at high coflow velocity. Meanwhile, the increase of coflow temperature can inhibit flame wrinkles. The flame dynamics is affected by turbulent mixing, head-on collision, shear and convective behaviors in non-premixed flames.

Atmospheric diffuse plasma jet formation from positive-pseudo-streamer and negative pulseless glow discharges

Atmospheric gas discharge is very likely to constrict into filaments and diffuse plasma formation is inefficient in most cases. Developing cost-efficient atmospheric diffuse plasma devices represents a significant challenge for high performance in biomedical decontamination and material processing. Here, we propose an alternative roadmap to produce a diffuse argon plasma jet by expanding and quenching the existing filamentary discharge at the initial or middle stage of streamer development. Possible mechanisms are summarized. With the gas flow velocity comparable to the ion drift one, enhancing ambipolar diffusion near the edge of the positive-streamer channel promotes the radial diffusion of newly-produced electrons, realizing the radial expansion of channel. Weakening electric field in front of the streamer head through head expansion and field offset, prevents the further development of streamer, leading to a positive-pseudo-streamer discharge. Reducing electric field in front of the negative-streamer head through ion compensation, impedes the initial growth of streamer, resulting in a negative pulseless glow discharge. The positive-pseudo-streamer and negative pulseless glow discharges function together to form the diffuse plasma jet.

The Reattachment Process of a Lifted Jet Diffusion Flame by Repetitive DC Pulse Discharges

On research of plasma assisted combustion, effects of electric and plasma discharges in DC, AC and pulse forms on reattachment of a lifted flame have attracted extensive attention. However, the detailed plasma assisted reattachment process and mechanism and roles of induced corona discharge and corona-induced ozone on the reattachment process are still unclear and undocumented. The forced reattachment process of a lifted diffusion jet flame by repetitive DC electric pulse discharges was experimentally investigated in this study using high-speed flame imaging, conditioned particle image velocimetry (PIV), and planar ozone concentration imaging. The forced reattachment process can be divided into three stages in sequence: ionic wind prior to corona initiation, corona initiation, and corona enhancement propagation. The conditioned PIV results showed that the instantaneous flame base propagation velocity is sufficiently enhanced beyond the laminar burning velocity for high pulse-repetition-frequency (PRF) cases at the instant of pulse discharge (on pulse) due to the enhanced oxidation of the corona induced ozone. By observing the dynamic flame-base behavior and evolution characteristics of the short-lived corona induced ozone for various PRFs, the novel forced reattachment process and mechanism of a lifted jet flame induced by repetitive DC electric pulse discharges is proposed.

Risk control analysis of safety accident in Hydrogen refueling station based on PHAST software

This paper uses PHAST software to simulate the hydrogen diffusion distance, explosion and jet fire radiation safety distance of hydrogen refueling station under certain conditions. Comparing the code data and the PHAST simulation results, in special circumstances such as high wind speeds and densely populated areas, it is necessary to increase the distance from the crowd gathering place on the basis of the existing code data, and explain the measures taken by the hydrogen refueling station to prevent leakage accident risks.

Analysis of Global and Local Hydrodynamic Instabilities on a High-Speed Jet Diffusion Flame via Time-Resolved 3D Measurements

Measurements of 3D flame edge dynamics were made on a high-speed jet diffusion flame to assess the global/local hydrodynamic instability. The flame was generated by issuing high-speed ethylene (Uj = 170 m/s) into a low-speed vitiated hot coflow (Uc = 1.5 m/s), resulting in a hydrodynamic shear layer instability at the interface between combustion products and ambient flow. The measurements used a high-speed camera combined with nine-headed fiber endoscopes to simultaneously collect both the soot radiation and chemiluminescence projections of the flame from nine views, based on which 3D flame edges were obtained via computed tomography at 15 kHz. The measurements clearly capture the time-varying, 3D instantaneous flame edge structures with fine-scale corrugations, enabling the observation of small-scale vortices' evolution. The flame edge deformations induced by those vortices were calculated globally and locally to infer the relationship between global and local flame edge oscillations. Results show that various local oscillation frequencies exist at different locations along the flow direction for such a highly sheared flame. They are dominated by the periodical formation and motion of various localized, small-scale vortices. The local oscillations are much severer than the global oscillation, indicating a self-suppression of the instabilities between these local oscillations. The suppression mechanism is attributed to the constructive and destructive interference behavior of the local disturbances. The global oscillation of the flame edge turns out to be the linear superposition of local oscillations at different locations.

Investigation on magnetic and flow field in magnetorheological jet machining

The instability and spreading out of jet flow outside the focusing tube involved in abrasive waterjet machining can be prevented by using magnetorheological (MR) fluid. The collimated and coherent MR jet in the presence of external axial magnetic field is potential to give a preferable erosion footprint and processing performance. In this research, a magnetic generator was developed and installed on an abrasive waterjet machining system to conduct MR jet erosion experiments on alumina. The induced magnetic field and jet flow field were numerically analyzed to evaluate their effects on material removal. The results indicated that the magnetic flux density increases with an increment of excitation current. The concentration of jet flow can be significantly enhanced by applying external magnetic field, and the velocity attenuation along the flowing direction due to the jet diffusion is restrained. The experimental results indicated that the range of crater is relatively smaller, and the erosion depth is larger when applying magnetic field, which can verify the validity of simulation results.