Compressible Gas Jet Research Articles

Statistical analysis of modelling approaches for CFD simulations of high-pressure natural gas releases

The use of computational fluid dynamics (CFD) in process safety to estimate the risk of a given incidental scenario has become ever more present in common industry practice. The simulation of high-pressure, compressible natural gas jets is often performed by modelling its source with a simpler notional diameter approach, such that the highly computationally expensive nearfield zone need not to be simulated; this is particularly determining when simulating a gas release in complex scenario like liquid natural gas (LNG) regasification plants. In this study, we analysed the structure of compressible and incompressible jets, using Birch 1984 (B84) and Birch 1987 (B87) models. In this work, a study on the positioning of the notional diameter with respect to the real orifice of the released gas is performed, along with a statistical analysis to assess the limits of the simpler model approaches.It was found that no spacing is needed between the virtual and real sources, as the potential core generated by the simpler model is as large as the fully simulated nearfield zone by the compressible model. Additionally, an end-of-transition zone position correlation is reported. The incompressible models can be used instead of the fully compressible model for a wide range of release conditions, with both models providing accurate predictions of axisymmetrical mole fraction, temperature, and velocity profiles between 2.5 and 130 bar of storage pressure at a 1-inch orifice diameter. However, as the diameter increases, B84 is not a viable model for a “full bore” (10-inch diameter size) release at 65 bar. While B84 is reliable, B87 is the superior model for its ability to account for the compressible effects of the expansion. Therefore, B87 should be used when simulating cases where temperature is of particular interest to the user.

An overview of cold spray coating in additive manufacturing, component repairing and other engineering applications

AbstractCold spray process (CSP) is a thermal spray technology in which coating (10–40 µm) is formed in the solid state by the impingement of power particles with supersonic velocity (200–1,200 m/s2) on coupon employing compressed gas jet, below the melting point of coating powder. It is commonly referred as cold gas dynamic spray, high velocity powder deposition, kinetic spray and kinetic energy metallisation process. Using CSP, various engineering materials (metals, polymers and ceramics) and its composites can be deposited. It is unique and promising approach for obtaining surface coating and offers various technological benefits over thermal spray as kinetic energy is employed for deposition rather than thermal energy. This offers great benefits in additive manufacturing (AM) to develop a component denser, low oxide coating free of tensile residual stresses, and undesired chemical reactions compared to conventional AM and coating techniques. Cold spray additive manufacturing (CSAM) is the powerful and emerging technique in the field of AM to develop engineering components with improved performance covering broad range of functionalities of surface, subsurface and interfaces. There are few flaws in this technique; however, extensive research work is going in CSAM and repairing of components to meet the real-time applications. The main objective of this review article is to summarise the history, effect of process parameters on surface coating, research and development in CSP along with its implementation in AM, component repairing and biomedical, antimicrobial and electrical applications. A discussion on future trends in CSAM is also provided at the end part of this article.

Acoustic analysis of starting jets in an anechoic chamber: implications for volcano monitoring

Explosive volcanic eruptions are associated with a plethora of geophysical signals. Among them, acoustic signals provide ample information about eruptive dynamics and are widely used for monitoring purposes. However, a mechanistic correlation of monitoring signals, underlying source processes and reasons for short-term variations is incomplete. Scaled laboratory experiments can mimic a wide range of explosive volcanic eruption conditions. Here, starting (non-steady) compressible gas jets are created using a shock tube in an anechoic chamber and their acoustic signature is recorded with a microphone array. Noise sources are mapped in time and frequency using wavelet analysis and their dependence from pressure ratio, non-dimensional mass supply and exit-to-throat area ratio is deciphered. We observed that the pressure ratio controls the establishment of supersonic conditions and their duration, and influences the interaction between shock, shear layer, and vortex ring. The non-dimensional mass supply affects the duration of the discharge, the maximum velocity of the flow, and the existence of a trailing jet. Lower values of exit-to-throat area ratio induce a faster decay of the acoustic fingerprint of the jet flow. The simplistic experiments presented here, and their acoustic analysis will serve as an essential starting point to infer source conditions prior to and during impulsive volcanic eruptions.

Numerical Analysis and Verification of the Gas Jet from Aircraft Engines Impacting a Jet Blast Deflector

The process of the gas jet from aircraft engines impacting a jet blast deflector is not only a complex fluid–solid coupling problem that is not easy to compute, but also a safety issue that seriously interferes with flight deck environment. The computational fluid dynamics (CFD) method is used to simulate numerically the impact effect of gas jet from aircraft engines on a jet blast deflector by using the Reynolds-averaged Navier-Stokes (RANS) equations and turbulence models. First of all, during the pre-processing of numerical computation, a sub-domains hybrid meshing scheme is adopted to reduce mesh number and improve mesh quality. Then, four different turbulence models including shear-stress transport (SST) k-w, standard k-w, standard k-ε and Reynolds stress model (RSM) are used to compare and verify the correctness of numerical methods for gas jet from a single aircraft engine. The predicted values are in good agreement with the experimental data, and the distribution and regularity of shock wave, velocity, pressure and temperature of a single aircraft engine are got. The results show that SST k-w turbulence model is more suitable for the numerical simulation of compressible viscous gas jet with high prediction accuracy. Finally, the impact effect of gas jet from two aircraft engines on a jet blast deflector is analyzed based on the above numerical method, not only the flow parameters of gas jet and the interaction regularity between gas jet and the jet blast deflector are got, but also the thermal shock properties and dynamic impact characteristics of gas jet impacting the jet blast deflector are got. So the dangerous activity area of crew and equipments on the flight deck can be predicted qualitatively and quantitatively. The proposed research explores out a correct numerical method for the fluid–solid interaction during the impact process of supersonic gas jet, which provides an effective technical support for design, thermal ablation and structural damage analysis of a new jet blast deflector.

Multi-plume sprays interacting with subsonic compressible gas jets

Injection of multi-plume sprays into subsonic compressible cross-flow jets is commonly encountered in practical applications and it is investigated experimentally and computationally in this study for the Mach numbers of 0.35 and 0.58. It is shown that comparing with the volumetric shadowgraphy images, the instantaneous topology of the sprays issued into the cross-flowing jets is well resolved by the present large eddy simulation (LES) with the Eulerian-Lagrangian multiphase formulation. The droplet velocities obtained from the simulations compare well with the particle imaging velocimetry (PIV) measurements. Compared to the multi-plume spray evolution in a stagnant ambient, the presence of the cross-flow jet is found to accelerate the break-up and produce smaller droplets. The individual spray plumes are found to penetrate symmetrically with respect to the central plane of the cross-flow jet. The penetration of the multi-plume sprays into the core of the cross-flows is shown to alter the velocity and pressure field of the compressible jet.

Vortex break-down during the impact of a starting subsonic compressible gas jet on a multi-plume spray

The impact process and the consequent two-phase interaction for a compressible subsonic starting gas jet colliding on a multi-plume spray are investigated using large eddy simulation with Eulerian/Lagrangian multiphase approach, and the λ 2 criterion is used to visualize the temporal and spatial evolution of the vortical structures in the gas field. It is shown that before the impact a leading tip vortex ring is followed by smaller vortex rings in the quasi-steady region of the starting jet while the vortical structures inside the spray plumes known as spray-induced air jets are formed. After the impact the leading tip vortex ring and the following rings as well as spray-induced air jet vortices start to deform and eventually break down into smaller elongated vortex filaments. Unlike the injection of multi-plume sprays into the core of a steady cross flow gas jet, spray droplets are dispersed in a larger volume in all directions when impacted by the starting gas jet, beneficial for two-phase mixing enhancement. A pair of vortex rings is also observed merging into a new ring before reaching the impact zone.

Experimental investigation on shock waves generated by pressurized gas release through a tube

An experimental investigation on the flow structures and the strength of shock waves generated by high-pressure gas release through a tube into air was conducted. The results demonstrated that a leading shock wave was generated in front of the compressed gas jet and the shock wave speed increased firstly, then decreased and finally kept constant with an increase of the propagation distance in the tube. The experimentally measured Mach numbers of shock waves were close to those calculated from the theory of ideal shock tube flow. After spouting out of the tube, the normal shock quickly developed into a hemispherical shape. The Mach disk was observed in the under-expanded jet. For high-pressure combustible gas release, the concept of theoretical critical pressure of ignition was introduced and several theoretical critical pressures of common gaseous fuels were obtained.

Characteristics of compressed natural gas jet and jet-wall impingement using the Schlieren imaging technique

An experimental study was performed to investigate the compressed natural gas jet characteristics and jet-wall impingement using the Schlieren imaging technique and image processing. An injector driver was used to drive the natural gas injector and synchronized with camera triggering. A constant-volume optical chamber was designed to facilitate maximum optical access for the study of the jet macroscopic characteristics and jet-wall impingement at different injection pressures and injectors-wall distances. Measurement of the jet tip penetration and cone angle at different conditions are presented in this paper together with temporal presentation of the jet radial travel along the wall.

Gas-Substrate Heat Exchange During Cold-Gas Dynamic Spraying

In this study, the temperature distribution of the surfaces of several substrates under an impinging gas jet from a cold spray nozzle was determined. A low-pressure cold-gas dynamic spraying unit was used to generate a jet of hot compressed nitrogen that impinged upon flat substrates. Computer codes based on a finite differences method were used to solve a simplified 2D temperature distribution equation for the substrate to produce nondimensional relationships between the surface temperature and the radius of the impinging fluid jet, the axial velocity of the cold spray nozzle, the substrate thickness, and the heating time. It was found that a single profile of the transient nondimensional maximum surface temperature could be used to estimate the dimensional maximum surface temperature, regardless of the value of the compressed gas temperature. It was found further that, as the thermal conductance of the substrate increased, the maximum surface temperature of the substrate beneath the gas jet decreased. Heat exchange between the substrate and the compressed gas jet during motion of the nozzle to produce heat conduction within the substrate was characterized by the nondimensional Peclet number. It was found that lower Peclet numbers produced higher temperatures within the substrate. The close agreement of the numerical results with the experimental results suggests that the nondimensionalized results may be applied to a wide range of conditions and materials.

Direct numerical simulation of a liquid sheet in a compressible gas stream in axisymmetric and planar configurations

A thin liquid sheet present in the shear layer of a compressible gas jet is investigated using an Eulerian approach with mixed-fluid treatment for the governing equations describing the gas–liquid two-phase flow system, where the gas is treated as fully compressible and the liquid as incompressible. The effects of different topological configurations, surface tension, gas pressure and liquid sheet thickness on the flow development of the gas–liquid two-phase flow system have been examined by direct solution of the compressible Navier–Stokes equations using highly accurate numerical schemes. The interface dynamics are captured using volume of fluid and continuum surface force models. The simulations show that the dispersion of the liquid sheet is dominated by vortical structures formed at the jet shear layer due to the Kelvin–Helmholtz instability. The axisymmetric case is less vortical than its planar counterpart that exhibits formation of larger vortical structures and larger liquid dispersion. It has been identified that the vorticity development and the liquid dispersion in a planar configuration are increased at the absence of surface tension, which when present, tends to oppose the development of the Kelvin–Helmholtz instability. An opposite trend was observed for an axisymmetric configuration where surface tension tends to promote the development of vorticity. An increase in vorticity development and liquid dispersion was observed for increased liquid sheet thickness, while a decreasing trend was observed for higher gas pressure. Therefore surface tension, liquid sheet thickness and gas pressure factors all affect the flow vorticity which consequently affects the dispersion of the liquid.

Oblique shock/bow shock interference in rarefied flow past a cylinder

Direct statistical simulation is employed to study the flow of a rarefied diatomic gas past a cylinder in the presence of an incident oblique shock. The distinctive features of the formation of a high-pressure compressed-gas jet in the case of interference between the oblique shock and the bow shock are studied for different Reynolds numbers. The variation of the pressure and the heat transfer to the surface with the shock position relative to the center of the cylinder, the Reynolds number, and the surface temperature is analyzed. The results obtained are compared with the experimental data and the results of the numerical solutions of the Euler and boundary layer equations. Free-molecular-to-continuum flow transition is demonstrated with reference to the example of interference-free flow past a cylinder.

Interference of a bow shock with an oblique shock and an isentropic compression wave

The features of the flow in the zone of interaction between a plane bow shock and an oblique shock or an isentropic compression wave are studied. The limiting interaction regimes are considered analytically, the similarity conditions are formulated, and the limiting values of the flow parameters are determined for the high-pressure compressed gas jet formed in the interference and for the body surface. On the basis of a numerical solution of the Euler equations the flow specifics in the neighborhood of the spreading line on the body are determined and ways of reducing the dynamic and thermal loadings on this line are proposed.

Three-Dimensional Temporal Instability of Compressible Gas Jets Injected in Liquids

A temporal linear stability analysis was conducted to study the growth rates of three-dimensional disturbances on a compressible inviscid gas jet injected into an incompressible viscous liquid coflow in the absence of gravitational effects. The primary parameters that governed the growth rates of the disturbances were the gas Weber number, Mach number, Ohnsorge number, density ratio, and velocity ratio. As the Mach number was increased, the range of unstable wave numbers and the growth rates of the unstable modes were increased. Also, the maximum growth rate was shifted to larger wave numbers (smaller wavelengths). The disturbance velocities were small, on the order of the coflowing liquid velocity. The growth rates and range of unstable wave numbers were reduced as the liquid coflow was increased. With an increase in liquid viscosity, the growth rates were reduced and the maximum growth rates were shifted to longer wavelengths. A reduction in the ratio of the liquid density to the mean gas density resulted in an increase in the growth rates of the unstable disturbances and the disturbance velocities

Three-dimensional temporal instability of compressible gas jets injected in liquids

Three-dimensional temporal instability of compressible gas jets injected in liquids

Instability of cylindrical compressible gas jets in viscous liquid streams

A linear stability study of cylindrical compressible gas jets in a moving incompressible viscous liquid medium subject to varicose disturbances is described. It was found that the gas jet is always unstable for a range of wavenumbers at any flow condition. When the gas and liquid velocity are not equal, temporal instability is enhanced by surface tension effects for small Weber numbers, and by aerodynamic interaction between the gas and liquid phase for high Weber numbers (where surface tension has a stabilizing influence). Increasing liquid viscosity always reduces the growth rate and the dominant wavenumber, whereas increasing gas density always increases gas jet instability. It was also found that the relative, rather than the absolute, velocity of the gas and liquid controls temporal instability. Increasing gas compressibility always increases the maximum growth rate and dominant wavenumber. On the other hand, for equal gas and liquid velocities, increasing surface tension always destabilizes, while increasing gas density always stabilizes, the gas jet. For absolute and spatial (or convective) instability, it was shown that the critical Weber number, separating the region of absolute from that of spatial instability, decreases monotonically as the liquid velocity is increased. For a stationary liquid medium, the gas jet is always absolutely unstable, and spatial instability does not exist, in contrast to liquid jets in a stationary gas medium. For sufficiently large liquid velocities, the gas jet is spatially unstable, whereas absolute instability disappears. Further, the absolute velocity of gas and liquid flow controls not only the growth of unstable disturbances, but also the characteristics of the instability. Increasing viscous effects tends to suppress absolute instability, while increasing both gas density and compressibility promotes absolute instability for small liquid velocities (however, their effect diminishes as liquid velocity is increased).

Instability analysis of the transition from bubbling to jetting in a gas injected into a liquid

Submerged gas injection into liquids is a widely applied processing technique. At low gas flow rates, a bubble plume forms in the liquid. With increasing gas flow rates, the gas bubbles in the plume coalesce and a gas jet is formed. Transition from bubbling to jetting occurs in the transonic region. To date, there is no sufficient theoretical explanation for this transition. In this paper, a basic theory is developed to explain the transition from bubbling to jetting. The instability of a circular compressible gas jet in a liquid was studied. For the axisymmetric mode, it was found that there is a peak growth rate for both the temporal and spatial instabilities when the Mach number approaches unity. The instability quickly reduces to vanishing values at supersonic gas velocities. However, in the supersonic region, it was shown that the helical instability mode may become important. Gas pressure perturbations have a destabilizing effect in the subsonic region but a stabilizing effect in the supersonic region. The problem of absolute instability was studied in order to explain the physical phenomenon of the transition from bubbling to jetting. Absolute instability was found in the subsonic region and a gas jet always breaks up into bubbles in the subsonic region. No absolute instability was found in the supersonic region, and the gas jet may remain stable. This transition from absolute to non-absolute instability occurs in the transonic region which was observed to be the transition from bubbling to jetting.

Hybrid processing for phase measurement in metrology and flow diagnostics

A hybrid optical/digital processing scheme for measuring phase distributions is described and demonstrated. It is intended to be an alternative to interferometric methods of measuring optical path length changes in flow diagnostics and can also be used as a flow visualization technique. The processing scheme enables one to make accurate measurements of phase at arbitrary points in the image plane. The system is based on a simple coherent optical Fourier processor but incorporates three separate measurements and postdetection digital processing to eliminate extraneous parts of the signal. The addition of a holographic filter to the system enables one to measure deformation or displacement of diffusely reflecting opaque objects. The technique is demonstrated by using it to visualize the flow of an expanding compressible gas jet and to measure the optical path length through a heated plume of air.