High-speed Gas Jet Research Articles

Experimental investigation of natural gas leakage and dispersion characteristics in utility tunnels under the effects of real facility layout and forced ventilation

Natural gas leakage occurring in underground utility tunnels usually poses a significant threat to public safety. In order to prevent unexpected fires and explosions, the evolution mechanism of natural gas leakage and dispersion in utility tunnels is urgently needed. In this study, an experimental apparatus was built to facilitate the understanding of leakage and dispersion dynamics in utility tunnels. Methane with a purity of 99.9% is used as a surrogate for natural gas. A schlieren imaging system with a Z-shaped optical path was designed to visualize the high-speed gas jet. The concentration distribution, alarm time, dilution efficiency, and gas jet image were analyzed under the effect of various facility layouts, ventilation, and leakage rates. The results show that the gas leakage and dispersion process can be divided into three zones: upwind zone, leak zone, and downwind zone, characterized by complicated airflow collision, high-speed get jets, and stable dilution dispersion respectively. Scenario analysis highlights the significance of key facilities, such as cable brackets, in experimental and numerical modeling due to their impact on dispersion trajectories. Higher ventilation rates prove beneficial in reducing peak concentration, hazardous areas, and enhancing purge efficiency. Conversely, higher leakage rates exacerbate the likelihood and severity of gas explosions. Alarm time exhibits a V-shaped distribution relative to the leak point, while purge time correlates positively with sensor positions. These findings are of practical importance in enhancing quantitative risk assessment and designing mitigation strategies for gas leakage accidents, which helps to improve the safety-risk-control capabilities of utility tunnels.

Morphology and dynamics of the liquid jet in high-speed gas-assisted atomization retrieved through synchrotron-based high-speed X-ray imaging

The breakup of a liquid jet by a surrounding high-speed gas jet with different liquid Reynolds numbers and gas Weber numbers was studied using high speed phase-contrast X-ray imaging technique. Focusing on the liquid core, the portion of liquid that is connected to the nozzle, four distinct morphologies were observed and can be associated with changes in the large-scale configurations of the two-phase flow are reported in a phase diagram. Gas-to-liquid kinetic energy balance arguments capture several transitions between the liquid core regimes. The temporal evolution of the center of mass of the liquid core is extracted to quantify its motion, whose statistics can be utilized as a signature to distinguish different regimes. At low to moderate gas Weber numbers, the dynamics are strongly influenced by flapping, while long-time dynamics develop at high Weber numbers, that give way to quasi-periodic motions when swirl is impeded to the gas jet.

Experimental study of jet and cavity coupling under vertical motion of underwater vehicle

After the underwater vehicle detaches from the launch tube and starts its engine, the interaction between the tail cavity and the high-speed jet significantly impacts its motion stability and engine efficiency. Therefore, it is crucial to study the evolution mechanism of cavities in vertical motion. However, in traditional water tunnel experiments, the evolution of the cavity differs from the actual situation of vertical launch due to the influence of buoyancy. This study investigates the dynamics and motion trajectories of tail cavities and high-speed gas jets in the vertical movement of vehicles under different Froude numbers and nozzle stagnation pressure ratios through experimentation. The experimental results show two modes of tail cavity evolution: intact cavity (IC) and foam-conical cavity (FC-CC). Increasing the Froude numbers and stagnation pressure ratios facilitates the transition of the cavity from the IC mode to the FC-CC mode, effectively suppressing the formation and intensity of re-entrant jets, thereby reducing its impact on the bottom of the vehicle and enhancing motion stability. By deriving the formula for the length of the jet core area, it was found that the jet length is linearly related to the nozzle stagnation pressure, further revealing the mechanism of cavity evolution. These findings offer a new perspective for a deeper understanding and prediction of the dynamic behavior of underwater vehicles.

Velocity And Concentration Measurements Of Supersonic Underexpanded Jets

The release of high-speed gas jets due to a tank leakage can pose a substantial safety hazard, as it can give rise to flammable, explosive, or toxic mixtures. Hence, understanding jet dispersion in real-world conditions is crucial for designing effective safety measures. Existing targeted or traceable studies provide velocity field data, but limitations in source-driven-pressures, nozzle geometries, or cross-flow conditions restrict their representativeness under real conditions. Indeed, an exhaustive experimental dataset for supersonic underexpanded gas jet dispersion under the influence of an Atmospheric Boundary Layer (ABL) is currently lacking. As part of a collaboration with the CEA, the von Karman Institute is continuing research efforts towards the development of high quality nonintrusive, optical measurement techniques for the characterization of the velocity and the concentration of a supersonic underexpanded jet submitted to different cross-flow conditions. The research explicitly targets measurements far from the release point within the self-similar region. Such measurements have already been tried on a supersonic jet without cross-flow and with a uniform velocity low-speed cross-flow [2], but never under the influence of an ABL. By filling this gap, the objective of the present work is to further improve the dispersion characterization by combining different techniques, such as Laser Mie Scattering and Light Extinction Spectroscopy (LES). To evaluate the dispersion within the self-similar region up to x/D ≈ 100, a significant FOV 1 is required. Therefore, Large-Scale Particle Image Velocimetry (LS-PIV) has been adapted and successfully applied to measure the velocity field. Prominent results have been obtained and comparable to standard PIV measurements. The pronounced variation in velocity magnitude observed while transitioning from the potential core to the self-similar region requires collecting multiple datasets to cover the broad spectrum of velocities comprehensively. A Mie scattering theory-based technique has been used to retrieve a relative concentration field. It is a non-intrusive technique that operates on the same experimental images produced by the LS-PIV campaign, assuming that light scattered by particles within a specific volume is proportional to the number of particles within this volume. The algorithm for concentration retrieval was adapted from and consequently improved for the present case. Discrepancies in the scaled concentration evolution among different nozzle diameters highlighted the necessity for refining the technique. This emphasizes accurately determining the reference concentration in a region free from intricate phenomena such as multiple scattering. The absolute and independent concentration measurement is done via Light Extinction Spectroscopy (LES). The LES technique measures the transmittance spectrum T of a collimated light beam passing through the particle-laden flow. This method has successfully been applied to several types of flows with similar seeding particles and concentration quantities (e.g. PSD 2 ). The extrapolation of absolute concentration parameters relies on the regularized solution of an ill-conditioned inverse problem. This solution is derived using the transmittance recorded during the test as the primary input parameter. The system to solve exhibits high sensitivity to small perturbations, making it challenging to employ numerical techniques confidently. Due to the substantial impact of minor perturbations in initial conditions on the system solution, a meticulous adaptation of the technique to the demands of compressible flows is necessary and still ongoing. The setup consists of a Deuterium-Tungsten Halogen UV-NIR light source emitting a divergent beam of light towards a 90◦ off-axis parabolic mirror, which collimates and deviates it into a beam to the jet. After passing through the jet, the resulting attenuated light beam goes into a collector mirror, redirecting it into a composite grating spectrometer UV-NIR with a 200 − 1100 nm wavelength range with 0.5 nm optical resolution. The results will examine the dispersion pattern and feature in terms of velocity and concentration of a supersonic underexpanded jet produced by an upstream tank with total pressure ranging from 5 to 9 bar without cross-flows. The latter will provide a foundation for successive studies on supersonic underexpanded jets subjected to an ABL.

A Numerical Simulation of the Underwater Supersonic Gas Jet Evolution and Its Induced Noise

To explore the complex flow field and noise characteristics of underwater high-speed gas jets, the mixture multiphase model, large eddy simulation method, and Ffowcs Williams–Hawking (FW–H) acoustic model were used for simulations, and the numerical methods were validated by the gas jet noise experimental results. The results revealed that during the initial stages, the jet collided with the water surface and created low-pressure high-temperature gas bubbles, accompanied by much high-frequency noise. When the jet reached its maximum length, its impact weakened, the bubble broke, the jet transformed into a conical shape, and the jet noise changed from high- to low-frequency. The pressure fluctuation peaked near the position at which the Mach number reached 1, indicating that the jet was the most unstable at the sonic point. Additionally, at low frequencies, the sound pressure levels between jets with different nozzle pressure ratios were similar, whereas above 400 Hz, under-expanded jets had higher sound pressure levels. This paper provides theoretical guidance for the study of underwater jet noise.

Regimes of the length of a laminar liquid jet fragmented by a gas co-flow

Atomization occurs when a high-speed gas jet breaks a low-speed liquid jet coaxial to it, forming a spray via the production of ligaments, sheets, and drops. This mechanism has received a lot of attention and different fragmentation regimes have been identified qualitatively using imaging in the vicinity of the nozzle’s exit plane, as a proxy for the change of underlying break-up mechanisms. As the Weber number increases, i.e. as the gas aerodynamic stresses become larger with respect to the liquid surface tension, the liquid–gas interface suffers from instabilities that are governed by either capillarity, shear, or acceleration (or a combination of these). Several fragmentation regimes, in the low Weber number range, are typically deemed undesirable for applications such as propulsion or additive manufacturing, but no quantitative regime map is readily available. Using back-lit high-speed imaging in a multiscale approach, coaxial two-fluid fragmentation is studied in a broad range of gas Weber numbers that spans five fragmentation regimes. A fixed laminar liquid injection rate is maintained while the gas jet exit velocity is varied widely. The liquid core length, i.e. the extent of the liquid jet that is still fully connected to the nozzle, is an important metric of fragmentation, but most research has been focused on its average value in the bag break-up and fiber-type atomization regimes. The scaling laws of the first three statistical moments, the minimum value, and the correlation time of the liquid core length are reported across fragmentation regimes. The changes in scaling laws appear to be good indicators for several of the transitions explored.

A Numerical Study of the Flow Field Driven by a Submerged, High-Speed, Gaseous Jet

Abstract The analysis of the numerical study of underwater high-speed gas jets is presented in this study. This work aims to understand the development of the flow structure of the gas jets submerged in water and assess the performance of the jet in terms of the thrust under varying operating conditions. The behavior of the submerged gas jet is studied under two operating parameters, namely, the pressure ratio (ratio of the pressure of the gas jet at the nozzle exit to ambient pressure) and the depth of water at which the propulsion takes place. The effort utilizes computational fluid dynamics using the finite volume method to solve the unsteady Reynolds-averaged Navier–Stokes equations in a two-dimensional axisymmetric domain combined with the mixture model for the multiphase flow. The unsteady behavior of different flow variables under varying operating parameters is discussed in detail. Further, the flow physics of a submerged supersonic gas jet is compared with a supersonic gas jet expanded in the air under a similar set of operating parameters. The effects of density difference between the gas and water have been studied from the comparative analysis.

Observation of the jet transition at a single vertical nozzle under pool scrubbing conditions

Submerged gas injection and the subsequent hydrodynamic behavior are phenomena that significantly affect the pool scrubbing efficiency at the injection zone. The Weber number jet transition criteria which have been normally used in the pool scrubbing research could not cover the wide range of the flow rate conditions including high-speed gas jet. In this study, the submerged air injection experiment was conducted at a 5 mm vertical nozzle. Air injection behavior was visually observed with a high-speed camera and analyzed using the shadowgraphy method. The local void fraction along the centerline of the pool was measured by using an optical fiber probe. From the measurement results, a jet transition criteria was suggested based on the nominal Mach number conditions. The effect of flow regime transition on the pool scrubbing efficiency has been confirmed through decontamination factor (DF) experiments. The suggested transition criteria matched with the DF experiment results.

Оцінка параметрів занурення частинок реагенту з газом у розплав при інжекційній позапічній десульфурації чавуну

The aim of the work is to develop a mathematical model for estimating the immersion of reagent particles with gas in molten iron during desulfurization. The technologies of out-of-furnace injection ladle desulfurization of cast iron are based on the introduction of reagent particles into liquid cast iron with the help of high-speed gas jets. The patterns of behavior and interactions of two-phase jets (carrying gas+reagent) with cast iron melt are complex, as the individual dynamic characteristics of individual particles are lost. Also important is the question of the ratio of the number of particles that went deep into the metal and the particles that ended up in the floating bubble. In the work, a number of experimental studies were performed to evaluate the factors and parameters of the immersion of the reagent particle during ladle desulfurization of cast iron. Experiments have shown that the depth of immersion of a reagent particle with a radius of 0.5 mm in the presence of a cavern depends on the velocity of the particle. For an initial speed of 20 m/s, the length of the cavern is about 6 gauges, and for a speed of 140 m/s - about 10 gauges. However, as it follows from the calculations, the velocities of the reactant particles drop rapidly, which should lead to a change in the flow structure and the emergence of caverns. In this case, capillary forces will play a significant role in the penetration of the reagent into the melt. In the developed model, they are taken into account as an additional factor. It is shown that these effects have a rather weak character compared to the repulsive force of Archimedes.

Droplet entrainment by high-speed gas jet into a liquid pool

For the optimal design and safety evaluation of sodium-cooled fast reactors (SFRs), the risk estimation of tube failure in the steam generator (SG) via numerical analysis is very important. This requires the understand of the droplet entrainment behavior induced by high-speed steam injection from the heat transfer tube into the liquid sodium in the shell; however, there is a lack of detailed information such as droplet diameter and velocity due to measurement difficulties. Therefore, this study aims to understand the droplet entrainment caused by high-speed gas injection into a liquid pool and to obtain experimental data for developing proper analysis models. Air was injected into a water pool at various air superficial velocities and the droplets were visualized via the frame-straddling method, obtaining clear images of the droplet generation from the gas–liquid interface. The droplet entrainment was caused by the liquid phase stretching and tearing due to the gas jet shear stress. Based on the visualization results, the droplet diameter and velocity were simultaneously measured at various locations through image processing; the number of fast droplets decreased as the diameter increased. The droplet diameter increased along with the distance from the nozzle. At the center of the jet, the droplet velocity was higher near the nozzle and lower at larger distances. The droplets tended to be larger and slower near the jet interface than near the center of the jet. When the jet interface fluctuation became larger, the droplet diameter increased, while the velocity decreased. Therefore, to develop droplet entrainment analysis models for the risk evaluation of tube failures in SFR SGs, the effect of the jet interface fluctuation on the droplet behavior must be evaluated.

Working Mechanism and Behavior of Collison Nebulizer

The Collison nebulizer (as well as its variations) has been widely used for generating fine aerosol droplets of a few microns from liquids of viscosity up to 1000 centipoise. It was originally developed for producing medical aerosols in inhalation therapy, and now has become an important component as pneumatic atomizer in the Aerosol Jet(R) direct-write system for additive manufacturing. Qualitative descriptions of its working mechanism were given in the literature as an expanding high-speed gas jet creates a negative pressure to syphon liquid into the jet stream, where the liquid is subsequently blown into sheets, filaments, and eventually droplets. But quantitative analysis and in-depth understanding have been lacking until rather recently. In this work, we present a logical description of the working mechanism of Collison nebulizer based on OpenFOAM(R) CFD analysis of compressible jet flow in the jet expansion channel. The positive-feedback liquid aspiration mechanism becomes clear by examining the CFD results as the jet expansion channel geometry is varied. As a consequence, the output mist density can be rather insensitive to the liquid viscosity, which is illustrated by a set of experiments with the Collison-type pneumatic atomizer in an Aerosol Jet(R) direct-write system. Thus, an intrinsic self-regulation mechanism is elegantly incorporated in the Collison nebulizer design. As intuitively expected, experimental data also supports the notion of the existence of an upper limit for liquid holdup in the limited space of jet expansion channel; therefore, the output mist density cannot increase indefinitely by increasing the atomization gas flow rate.

High-speed turbulent gas jets: an LES investigation of Mach and Reynolds number effects on the velocity decay and spreading rate

The aim of this work is the investigation of Mach and Reynolds numbers effects on the behaviour of turbulent gas jets in order to gain new insights into the fluid dynamic process of turbulent jet mixing and spreading. An in-house solver (Flow-Large Eddy and Direct Simulation, FLEDS) of the Favre-filtered Navier Stokes equations has been used. Compressibility has been analyzed by considering gas jets with Mach number equal to 0.8, 1.4, 2.0 and 2.6, and Re equal to 10,000. As concerns the influence of Re on gas jets, four cases have been investigated, i.e. mathrm{Re} = 2500, 5000, 10,000 and 20,000, with Mach number equal to 1.4. The results show that, in accordance with previous experimental and numerical studies, the potential core length increases with Mach number. As regards the velocity decay and the spreading rate downstream of the potential core, compressibility effects are not relevant except for the jet with Mach number of 2.6. The normalized turbulent kinetic energy along the centerline as a function of the normalized streamwise distance shows a similar peak at the end of the potential core for all jets, except for the case with Mach number of 2.6. By increasing Re, the length of the potential core decreases up to the same value for all Re higher than 10,000. In the region downstream of the potential core, the velocity decay decreases as Re number increases from 10,000 to 20,000, whereas, for lower values of Re, the influence is almost negligible.

Windowed Fourier transform and cross-correlation algorithms for molecular tagging velocimetry

Simulated and experimental molecular tagging velocimetry (MTV) images have been analyzed with a technique commonly used to process grid images on surfaces, the windowed Fourier transform with local spectrum analysis (WFT-LSA). A systematic synthetic image study of the modulation transfer function (MTF) and error tendencies of the WFT-LSA was performed and compared with a PIV-style cross-correlation algorithm to see if advanced strategies such as iterative image deformation can improve analysis of gridded images with high noise levels. Testing of single-pass algorithms showed that in typical MTV images, the WFT-LSA yields significantly lower bias errors than cross-correlation (CC) at displacements greater than 1 pixel but slightly higher random error at all displacements and image conditions. Analysis of the MTF shows that CC provided better resolution of spatial fluctuations than the WFT-LSA in many combinations of grid size and interrogation window. Tests of image deformation algorithms showed that the gap in performance between CC and WFT-LSA is maintained even as both methods improve. Additionally, WFT-LSA and CC methods are applied to real MTV experiments in high-speed gas jet flows. A preliminary analysis of phosphorescence lifetime provided by acetone vapor excited at 266 nm is used for assessing the required gas speeds for making MTV application feasible. The application of WFT-LSA to real MTV images demonstrates the ability of the algorithm to handle further real-world effects that could not be considered in the synthetic image analysis, like reduced signal-to-noise ratio and non-uniform intensity of the tagging grid across the image introduced by the actual laser beam energy distribution. With experimental images, CC is more accurate with shear flows but less robust to high noise levels than WFT-LSA, as predicted by the synthetic image analysis.

Investigation of characteristics of diesel fuel atomization with a high-speed gas jet

Using the modern contactless interferometric method for determining the droplet diameter (IPI), a gas-drop flow was experimentally investigated when diesel fuel is atomized with a high-speed gas jet. The size distribution of fuel drops was obtained in different operating parameters. The dependences of drop sizes on regime parameters are revealed and analyzed.

Investigation of the structure of the gas flow from the nozzle of a spray-type burner

The flow structure in a high-speed gas jet released from the nozzle of a promising liquid-fuel spray-type burner is studied experimentally and theoretically. The velocity distribution in a single-phase gas flow is measured by means of particle image velocimetry (PIV) at various operating parameters. Using the one-dimensional isentropic flow approximation, the gas-dynamic parameters are estimated for the characteristic operating regimes of the burner with supply of superheated water steam, as well as with air supply. With the use of the ANSYS Fluent CFD package, two-dimensional numerical simulation of aerodynamic structure of air stream from the nozzle is carried out. The results are compared with the measured velocity field and the flow pattern, typical of a supersonic underexpanded jet, visualized by the direct shadow photography method near the nozzle orifice.

Liquid atomization in a high-speed coaxial gas jet

The paper studies the process of liquid atomization in high-speed gas jets with application to a subject of high-rate fuel nozzles. Experiments were carried out for gas-liquid jet with the central-axis feeding of liquid to the outlet of a confuser-type nozzle with pumping of air in subsonic and supersonic flow regimes. The energy balance approach was developed for describing a gas-liquid jet. This provided us the needed data for comprehensive description of the gasliquid jet: gas velocity field without liquid, shadow visualization of geometry and wavy structure of a jet with liquid and with pure gas, velocity profiles for liquid phase, spray droplet size, spray concentration and spatial distribution. The gas-liquid flow was characterized by Weber number from the time of liquid jet breakup till the final spray.

Peculiarities of development and extinguishing fires at the objects where liquified petroleum gas is stored

The possibility of using sprayed water jets is established, primarily radial air jets, in vertical and horizontal planes in order to reduce the temperature of the burning Liquefied Petroleum Gas, starting from the cut-off of its expiration into free space. Various technical means of supplying water in a spray and spray nozzles for their creation, which are used in solving the problem of reducing the temperature of the burning Liquefied Petroleum Gas are viewed. It has been established that the active phase of the sprayed water jet is most effective (0,5-0,75 of the lengths), which, acting under the cut of the flame is crushed to a finely dispersed phase under the influence of a high-speed gas jet. As can be seen from the above, the heating rate of the finely dispersed water phase is increased up to the gaseous state, which leads to a decrease of the temperature of the flame in its middle combustion zone. Consequently, in the flame temperature will be much lower than the temperature of the flame cone, and, as a result, close to the extinction temperature. The authors recommend the use of modern jet-forming devices to realize this goal – nozzles NRT-5, NRT-10, NRT-20, NRS, as well as hand nozzles of the kind RSK-50, RS-А, RS-B, PROTEK. In order to cool, for example, one horizontal ground steel tank on both sides, it is necessary to use at least two hydraulic guns, which will act as maneuvering. In this case, the flow of water from the hydraulic gun with a diameter of the nozzle 25 mm will be from 16 to 18 liters per second. Therefore, the task is to find, calculate and experimentally prove the efficiency of the thermal screen, which is created in the form of a moving water surface that can effectively protect a particular physical object from the thermal impact of the heat zone. In order to reduce the flow of water to cool the tanks, the authors suggest using the NRS to create radial (flat) water jets. Application of such a nozzle allows to increase the area of simultaneous cooling of the maximum surface area per unit time due to the expansion of the angle of the spray jet spark. The water consumption does not exceed 13 liters per second.

Numerical and experimental study of oscillatory behavior of liquid surface agitated by high-speed gas jet

This study examines high-speed air jet impingement on water surface in the penetration mode using CFD simulation and experimental observation. An axisymmetric approach was taken along with the volume of fluid (VOF) method and the k−ω SST turbulence model for simulations. Moreover, a test rig was designed and fabricated to diagnose the phenomenon by tracking the interface motion using a high-speed camera in conjunction with an image processing procedure, and to evaluate the validity of the numerical simulations. Important aspects of the oscillatory behavior of the interface, cavity depth oscillation and rising ligaments and sheets, are addressed in detail. The simulation results offer a very close prediction of the peak frequencies of the cavity oscillation, and the number of ejected liquid sheets and ligaments despite some discrepancies in the range of the cavity motion and the height of ligament and sheets. Induced waves on the interface, and bubbles escaping from the cavity, which are the main source of instability of this two-phase system, are reproduced successfully.

Solid distribution in fluidized and fixed beds with horizontal high speed gas jets

High speed jet penetration in fluidized beds can be used for particle size control or comminution processes. Thus, particle breakage is determined by the kinetic energy of the solids and the frequency of particle-particle impacts inside the flow. Both aspects are influenced by the solid concentration inside the jets and at the boundary. For understanding the fundamental breakage mechanism, the determination of the solid distribution in those complex flows is an important contribution.In order to access the turbulent and delicate multiphase flow, a modern fast X-ray computed tomography unit was used in this work. To obtain quantitative solid concentrations, the measurement system was optimized by means of hardware filtering, exposure time and reconstruction algorithm. These methods allowed the investigation of influencing parameters like jet velocity, fluidization velocity or particle size on the solid distribution in a single jet. With the obtained experimental data, correlations for the axial and radial solid distribution were developed. In addition, the influence of multiple jet injection with a common focal point is presented.

Growth of diamond structures using high speed gas jet deposition activated in heated tungsten channels

A method for obtaining diamond films using gas jet deposition with activation on extended surfaces made of tungsten heated up to high temperatures is considered in the article. The original reactor had been designed for activation of carbonaceous gas mixtures. A distinguishing feature of this method is to provide an extended interaction of hydrogen, methane, and its fragments with a heated tungsten surface. It is proposed to implement the increase of catalytic thermal dissociation of hydrogen molecules by providing multiple collisions during passage through the heated tungsten channels. In the researches carried out the effect of thermo-gas-dynamic parameters on resulting carbon structures had been studied. During the film deposition on the molybdenum surface, a strong dependence of the film morphology on the temperature of the tungsten surface was observed in the range of 2000–2150°C. As a result of conducted research next objectives has obtained: the optimal substrate temperature, the value of the pressure in the reaction zone, ways of minimizing the carburization of the activation surfaces were found.