Mach Disk Research Articles

Effects of ignition location and vent area on the external explosion in vented hydrogen explosions

In the current study, effects of vent area and ignition location on vented stoichiometric hydrogen-air mixture explosion were experimentally and numerically investigated. Internal and external pressure-time curves and flame behavior were focused. For low vent coefficient Kv = 4, the maximum internal pressure results from the vent rupture for front ignition and the external explosion for center ignition and rear ignition. The maximum internal pressure for IL4 is larger than those for other ignition locations. The maximum internal pressure is induced by the internal chemical reaction and increases monotonically with Kv for different ignition locations, and the maximum internal pressure occurs at center ignition as Kv ＞ 4. Mach disk in external flame begins to appear when Kv increases to 12.75 for front ignition and 8.65 for other ignition locations. External pressure histories show two pressure peaks P1 and P2 for center and rear ignitions. Although the pressure peak P1 results from the leading pressure wave, the pressure peak P2 is induced by the venting flame and external explosion for center ignition and rear ignition, respectively. Only one pressure peak was recorded for center ignition. For front ignition and IL2, the maximum external pressure increases with Kv while it firstly increases and then decreases for center ignition, IL4 and rear ignition, the critical value is Kv = 17.36 for center ignition and IL4 while it is Kv = 8.65 for rear ignition.

Numerical study of the transient behaviors of vapour-liquid equilibrium state CO2 decompression

The vapor–liquid equilibrium (VLE) state of CO2 commonly appears in pipelines for its transport, refrigeration systems, and large-scale trans-critical cycle systems. However, the behavior of sudden leaks in such systems remains unclear, which poses a challenge in assessing the risk of leakage. This work is focused on the decompression behavior of VLE state CO2 with different volume fractions of vapor phases in high-pressure pipeline leakage, specifically with the development of a computational fluid dynamics (CFD) model with a nonequilibrium phase transition and the real gas model. The results suggest that the Peng-Robinson Equation of State (PR EoS) is more conservative in predicting the degree of superheat than that of Span-Wagner (S-W) EoS and GERG-2008 EoS. Moreover, it is found that the initial volume fraction of the vapor phase in VLE state CO2 plays a crucial role in determining the characteristics of sudden leakage, such as pressure, temperature, and decompression wave speed both inside and outside the pipe. However, the position of the Mach disk remains unaffected by the initial volume fraction of the vapor phase. The initial vapor volume fraction of 0.2 has the most significant impact on the transient behaviors of the leakage, that is, the speed of the initial decompression wave is approximately 1.46 times slower than that of pure liquid CO2, the pressure and temperature start to decrease approximately 0.46 ms after those of pure liquid CO2, which is about 5.1 times later. Additionally, the peak pressure of the near-field jet is 6.3% higher and the maximum velocity is 11.4% higher than that of pure liquid CO2. It hopes that this work will contribute to the improvement of research models that assess the consequences of potential high-pressure pipeline rupture scenarios.

Effect of tab shape, length, and placement on the over-expanded free jet at Mach 2.0

Abstract Experiments have been carried out to investigate the over expanded free jet at Mach number 2.0 without and with tabs of different cross-sectional shapes, tab length penetration into the jet core defined as blockage ratio, and symmetrical and asymmetrical placement of tabs at the jet exit. The cross-sectional shapes such as triangular, square, and circular have been studied with the penetration depth of 3 %, 7 %, and 11 % blockage. The Pitot tube and Schlieren flow visualization were carried out in experiments. The basic features of the jet with Mach disk, shock cell, and its distortion with the adoption of tabs at various nozzle pressure ratios indicate a definite influence that helps in the reduction of the core jet length. This characteristic is also influenced by changes in tab cross-section and length of penetration. Since the basic jet has three-dimensional structures, the asymmetric placement of tabs is beneficial compared to the symmetric orientation of tabs. The jet width increases with adoption of these tabs and different shapes. Whereas, a maximum core jet length reduction of the order of 60 % could be achieved using the present techniques.

Effect of tab shape, length, and placement on the over-expanded free jet at Mach 2.0

Abstract Experiments have been carried out to investigate the over expanded free jet at Mach number 2.0 without and with tabs of different cross-sectional shapes, tab length penetration into the jet core defined as blockage ratio, and symmetrical and asymmetrical placement of tabs at the jet exit. The cross-sectional shapes such as triangular, square, and circular have been studied with the penetration depth of 3 %, 7 %, and 11 % blockage. The Pitot tube and Schlieren flow visualization were carried out in experiments. The basic features of the jet with Mach disk, shock cell, and its distortion with the adoption of tabs at various nozzle pressure ratios indicate a definite influence that helps in the reduction of the core jet length. This characteristic is also influenced by changes in tab cross-section and length of penetration. Since the basic jet has three-dimensional structures, the asymmetric placement of tabs is beneficial compared to the symmetric orientation of tabs. The jet width increases with adoption of these tabs and different shapes. Whereas, a maximum core jet length reduction of the order of 60 % could be achieved using the present techniques.

Experimental investigations on the jet flow field propagation characteristics and ignition mechanisms of stoichiometric CH4/O2/N2 premixed turbulent jet flames

The jet flow field (JFF) and flame propagation of stoichiometric CH4/O2/N2 mixtures under variable thermodynamic conditions were investigated experimentally using Schlieren and natural flame luminosity imaging techniques. A two-color method by wavelength integration was used to characterize the temperature field. The turbulent flame velocity (ST), Damköhler number (Da), Reynolds number (Re), and jet flame regime were explored by a combination of experimental data and theoretical calculations. The flame morphology shows that the propagation process can be subdivided into three stages: first, the JFF enters the main combustion chamber, then the flame propagates in the JFF, and finally, the JFF disappears after the flame front and the JFF front coincide. The vortex motion changes the thin reaction zone and the local high temperature region distribution. The normal jet shifts to the Mach disk jet at higher oxygen content. The ignition location changes from the jet tail to the shear layer. The top secondary vortex is more likely to generate at higher ambient temperatures, which enhances the ignition probability on the flame top. In addition, the degree of orifice blockage determines the jet flame type. Since the initial jet flame is more affected by orifice blockage, the variation in ST is strongly influenced by the change in the length-scale ratio. As the jet flame gradually moves away from the orifice, ST is mainly governed by turbulence intensity. Finally, a strong correlation between the Re number and Da number of the jet flame was found.

The influence of Mach number and overpressure on the structure of supersonic gas jets

ABSTRACT The Mach number is the primary parameter in determining the intrinsic driving properties, and effect on the environment of a supersonic jet. In the most basic form, a jet is released when a wall of a large high-pressure gas reservoir is punctured. The resulting high-pressure jet contains a configuration of shocks that continue to disturb the environment after the initial bow shock has passed. Here, we perform numerical simulations to determine the properties attributable to pure adiabatic hydrodynamic effects, taking a uniform stream out of a circular nozzle. We take a range of Mach numbers that, along with the jet overpressure, determine the flow pattern and shock locations. We distinguish conditions that generate Mach shock discs rather than a diamond pattern of oblique regular reflections. Potential observational diagnostics explored include the disc size, the distance from the nozzle, and oscillations in shock positions. Rapid oscillations occur in the divergent–convergent pattern through a feedback/hysteresis effect promoted by the ambient medium. The underlying flow patterns are independent of relative jet density, but heavy jets display both lower amplitude and lower frequency oscillations. We also study the energy transferred into the environment. Overpressured jets may contribute to noise and sound wave generation through screeching. However, these oscillations in the near-field are not sufficiently significant to regulate star and galaxy formation. We expect that upcoming high dynamic range and resolution observations will increasingly detect the shock patterns as jet gas transits from protostellar and galactic cores.

Study on the characteristics of the transient flow field under different underwater environments

The underwater muzzle transient flow field is an unsteady, multiphase complex flow field interacting with projectiles and containing various shock wave structures. The turbulent mixing of gunpowder gas and water has a significant impact on the development of the muzzle gas flow field. Moreover, the muzzle gas flow field disturbs the motion of the projectile, thereby affecting shooting accuracy. As part of this research, an unsteady multiphase flow model of the underwater muzzle transient flow field is established by combining the theories of multiphase flow and turbulent mixing. The volume of fluid model is employed to trace the two-phase interface, while the gas–liquid turbulent mixing is described by the standard k–ε turbulence model. Furthermore, the cavitation model is used to describe the cavitation phenomenon caused by the motion of the projectile. The established numerical model is validated by comparing underwater launching experimental results. Accordingly, the muzzle flow field of a 30 mm underwater gun under different water depth conditions is numerically calculated. The results demonstrate that, as the water depth increased, the gunpowder gas is exposed to relatively high water pressure during the expansion process, resulting in a continuous decrease in the core area of the gas, and the Mach disk is also increasingly closer to the muzzle. At different water depths, the diameter of the Mach disk conforms to the binomial law with time, while the displacement of the Mach disk from the muzzle increases exponentially with time.

Numerical Simulation of Underexpanded Sonic Jet

Underexpanded sonic jet is numerically simulated using commercial CFD software. Three dimensional Navier Stokes equations alongwith two equation turbulence models are solved. Computed centerline distribution and profiles of various flow variables at various axial stations agree quantitatively with experimental results. The comparison of the flow variables computed with four different two equation models reveals that k - e and RNG k - e models perform very well in predicting both pressure and temperature throughout the flow filed; whereas SST and k - w models fail to predict the flow behavior beyond the first Mach disc.

Muzzle flow field characteristics of underwater sealed launch system under different projectile shapes

The underwater sealed launch is one of effective launch technologies for enhancing the velocity of underwater vehicle and also for achieving better interior ballistic performance. However, the underwater muzzle flow field based on this sealed technology is a major consideration as it decides initial kinetic stability and subsequent shooting accuracy. Hence, this study establishes a comprehensive model involving in a modified sealed interior ballistic model, muzzle multiphase turbulent flow model and supercavitation model due to high-velocity moving vehicle, and mainly investigates the effect of the vehicle shapes on the underwater sealed muzzle field. An experiment for the muzzle field of underwater sealed launch is also carried out using a 12.7 mm-caliber launch device and water tank. Comparisons on flow patterns of muzzle field and expansion displacements of Taylor cavity on computational results are done with experimental studies, and a reasonable match has been obtained in these comparisons. The results show that the gas sealed in the launch barrel firstly injects into the water forming an initial Taylor cavity once the sealed device is broken. Then, the propellant combustion-gas just with high pressure and high temperature injects into the water with the moving vehicle. Consequently, a complicated interaction effect between these two types of gases and high-density water takes place, generating complicated shock wave phenomena in the muzzle field. Different vehicle shapes can affect the formation of the shock waves and expansion characteristics of Taylor cavity in the water. Eventually, the diameter of the Mach disk is always the largest for the cylindrical underwater vehicle and the axial pressure fluctuation for the vehicle with a cavitator are most significant with a lowest temperature field due to strong gas-liquid interaction.

Numerical study on coupled flow characteristics of a gas-solid two-phase jet in a supersonic crossflow

In this study, the Eulerian-Lagrangian scheme coupled with the gas-solid two-phase Reynolds-average Navier-Stokes (RANS) method was used to study the airflow and particles flow characteristics of particles of different diameters injected into a supersonic flow field by a sonic jet. The results show that introducing small particles has little effect on the flow field. When the particle diameter is 1e-6 m, most particles are mainly distributed in the shear layer, and some parts of the particles are sucked into the surface trailing counter-rotating vortex pairs (TCVP) and the wall boundary layers by the airflow. The introduction of large diameter particles hinders the jet's development, reduces the Mach disk's height, and makes the counter-rotating vortex pair (CVP) closer to the wall. This leads to a decrease in the streamwise velocity and an increase in the wall-normal velocity near the wall downstream of the jet. The streamwise velocity increases and the wall-normal velocity decreases at the far wall downstream of the jet. At the same time, it exacerbates the Kelvin-Helmholtz (K-H) instability in the shear layer on the windward side of the jet, forming a large-scale coherent structure between the shear layer and crossflow. The penetration depth and diffusion range of particles increase with particle diameter and density, and the velocity decreases with particle diameter increase by comparing the trajectories of particles with different diameters and densities.

Numerical simulation and analysis of the 3D transient muzzle flow field of underwater artillery

The in-depth study of the muzzle flow field contributes to the improvement of the gun and the design of the projectile. This study establishes an unsteady muzzle multiphase flow model of the underwater sealed launch with a three-dimensional scale. Among that, the VOF model is employed to trace the two-phase interface and the gas-liquid turbulent mixing is described by the standard k-ε turbulence model. Also, the established numerical model was validated by comparing underwater sealed launching experimental results. On this basis, numerical analysis on the muzzle flow field of the 30 mm artillery under different launching conditions were carried out by coupling with revised interior ballistic equations. The results show that the “necking” phenomenon occurs during gas-expansion process due to the interaction between water and projectile. Then, a “bowl-shaped” shock area is obviously formed at the muzzle. In addition, the Mach disk diameter and the corresponding growth rate are closely related to the projectile head cavitation diameter. The growth rate of Mach disk diameter increases with an increase of the cavitation diameter. When the muzzle velocities are different, the positions of the Mach disk formed are also different, presenting that the Mach disk is farther away from the muzzle with the increase of muzzle velocity.

Research on multiphase flow field characteristics of underwater gun double‐tube parallel firing

AbstractThe underwater muzzle flow field formed by the double‐tube launch will interfere with each other, resulting in large changes in the characteristic parameters of the flow field and shock wave morphology. Therefore, the numerical model of three‐dimensional unsteady multiphase flow for underwater gun‐sealed launching was established. Meanwhile, an experimental platform for the underwater sealed launch was built and the rationality of the model was verified. The interior ballistic process compiled by UDF was coupled with the dynamic mesh technology, and the VOF multiphase flow model integrated with the Schnerr‐Sauer cavitation model was chosen to numerically calculate the muzzle flow field of the 30 mm underwater gun double‐tube parallel launch, and the numerical results were compared to those of the single‐tube launch. The results show that the gas is expelled from the muzzle and rapidly expands to form a gas cavity, and the “necking” phenomenon of the gas cavity occurs at 0.2 ms, while the Mach disk structure has formed. Due to the mutual interference between the flow fields formed by each tube, the diameters of the Mach disk are slightly different, and the flow field structure has a certain asymmetry in the evolution process. The core area of the shock wave is bowl‐shaped of the single‐tube launch, while it is not completely filled of the double‐tube launch. Within 0.5 ms, the diameter of the Mach disk increases monotonously when the single‐tube is launched, while it first increases and then decays by a double‐tube.

Study on the influence of helicopter rotor flow on projectile motion

In this paper, the muzzle flow field generated during helicopter gun firing is numerically studied. Based on Navier-Stokes equation and Standard k − ε turbulence model, the projectile motion is realized by using overlapping mesh technology. The development of the muzzle flow field formed by complex wave systems such as Mach disk and shock wave under the influence of helicopter rotor flow at different speeds, as well as the force and motion state of the projectile in the flow field are analysed. The results show that the degree of asymmetry of the flow field, the force and offset of the projectile are positively correlated with the rotor flow velocity. The research will help to improve the understanding of the muzzle flow field of helicopter gun and provide a new idea for improving the firing accuracy of helicopter gun..

The influence of cryogenic temperature on the shock structure of impinging under-expanded flow over a convex surface

This study comprehensively investigates the effect of cryogenic nozzle inlet temperature on the flow structure and interactions of an under-expanded supersonic jet with a spherical solid surface. A combined experimental and numerical approach was employed to achieve this goal, utilizing high-speed Z-type schlieren visualization and Reynolds-averaged Navier–Stokes simulations with a Redlich–Kwong real gas equation of state. This study is significant as it addresses a relatively unexplored area of research on the flow structure of the cryogenic under-expanded supersonic jet. The study examines the shock pattern and interaction region through varying static inlet temperature (Tin = 178–290 K) and nozzle pressure ratio (NPR 5–14). Additionally, parameters including nozzle exit-to-throat area ratio (A/A* = 1.277), the distance between the sphere and the nozzle (1.5 cm), and the diameter of the sphere (d = 1.5 cm) were considered fixed. The results show that the supersonic jet exhibits a change in shock patterns in the first shock cell concerning the location and width of the Mach disk, accompanied by a shift in the location of the last shock crossing point and the shock plate. The simulation provides a more detailed insight into the flow, indicating a temperature drop to 105 K in the case of the cryogenic nozzle inlet. At such a low temperature, the compressibility factor exhibits a 5% reduction from unity, while in the case of the ambient nozzle inlet, the minimum temperature at the nozzle exit reached 170 K, leading to only a 1% drop in the compressibility factor, which is negligible. It triggers different flow structures concerning the nozzle inlet temperature. These findings can contribute to the complex flow structures of supersonic jets seen in different industrial and scientific fields.

Numerical Investigation on the Thermal Protection Characteristics of a New Active Jet Design Parameter for Hypersonic Flight Vehicle

This study investigated the thermal protection performance of an active jet thermal protection system (PRsAJ-TPS) based on a new jet design parameter PRs for hypersonic flight vehicles (HFVs). The new parameter PRs is defined as the relationship between the jet flow total pressure and the free flow total pressure behind the outer bow shock. A 20° tilted nozzle design is employed together with the PRs to form the PRsAJ-TPS. Theoretical and numerical analysis is performed to prove the advantages of using the PRs. A conventional in-house CFD solver with the k ‐ ω SST turbulence model is utilized to perform the calculation. The influence of different PRs (working in the short penetration mode) and flight angles of attack on the performance of the PRsAJ-TPS is also studied. The simulation results confirmed that with a constant PRs, the Mach disk location stays the same and the normalized heat flux reduction is similar at different flight conditions. Nearly linear relationships exist between the new design parameter PRs and thermal protection performance indicators. The PRsAJ-TPS also exhibits good protection when flight angle of attack (AoA) varies between 0° and 40°, with the best results achieved at AoA = 0 ° . This study provides valuable information for the engineering application of the jet-based TPS to future HFVs.

Numerical investigation on muzzle flow characteristics for small combustion chamber with embedded propelled body

Achieving a smaller size and lighter weight without affecting the launch performance is an important goal in the design of special launch equipment systems such as micro air vehicles and gun propulsion systems. An embedded structure is applied to small chamber systems to save launch space. This paper aims to illustrate the launch performance and muzzle flow characteristics of the embedded structure. The mathematical model of the launch process with a two-stage ignition is established. The muzzle flow model is used to simulate the launch process. The numerical method is verified by experiment. Numerical results indicate that a 21.1% space saving and a 4.2% muzzle velocity increase can be achieved by the embedded structure. The effect of the embedded structure on the muzzle flow is investigated. In the launch process, the Mach disk of the traditional structure travels farther, but that of the embedded structure catches up due to the greater velocity. The pressure oscillation of the embedded structure is larger, but the range of the oscillation is smaller. In the development process, compared with the traditional structure, the main parameters of the embedded structure, such as pressure and Mach number, are greater, which may lead to the launch risk. The findings will be applied to improve the design of small launchers.

Numerical simulation of ideal and non-ideal under-expanded supersonic jets with adaptive grids

This paper numerically investigates the effect of flow non-ideality on the Mach disk pattern occurring in axisymmetric under-expanded supersonic jets. The explored conditions are found in the non-ideal gas region close to the liquid–vapor saturation curve and the critical point, where involved thermodynamics cannot be accurately described by the ideal gas law. Numerical simulations of steady inviscid compressible flows are carried out by adopting the polytropic ideal gas model and improved Peng–Robinson thermodynamic model, via the open-source computational fluid dynamics code SU2. To properly resolve the Mach disk position and radius, a mesh adaptation procedure is used. An assessment of the most suitable mesh adaptation criteria for the considered ideal and non-ideal flow configurations is carried out, considering isotropic and anisotropic mesh adaptation criteria. The best mesh adaptation criterion for ideal and non-ideal under-expanded jets results to be the anisotropic criterion based on the Hessian of the density and Mach number field. Then, the effects of the flow non-ideality on the Mach disk are investigated by simulating different operating conditions of under-expanded nozzle jets of five different vapors. For nitrogen, oxygen, carbon dioxide, the Mach disk moves downstream and has a smaller radius with a higher level of non-ideality. Conversely, for siloxanes MM and MDM, the Mach disk moves upstream and has a larger radius as the non-ideality of the flows increases.

Shock waves, overpressure, and spontaneous ignition of pressurized hydrogen in T-shaped tubes

The accidental release of high-pressure hydrogen into a tube can induce self-ignition, and special tube configurations can complicate their characteristics. Hence, this paper studies the shock waves and self-ignition in tubes with T-shaped structure. Results show that the shock wave intensity decreases after passing through the T-shaped region, and its intensity is lower in the branch pipeline. The flow field is extremely complex after shock waves enter the T-shaped region, accompanied by different shockwave structures (e.g. barrel shock and Mach disk), multiple reflections, and shock-shock interaction. In addition, the location of the bifurcation point affects not only the downstream velocity evolution but also the time of upstream pressure to reach the peak pressure. When the bifurcation point is the closest to the rupture disc, spontaneous ignition is least likely to take place. The flame intensity decreases after passing via a T-shaped zone. Besides, if the T-shaped zone is far from rupture disc, even extinguishment can occur.

Experimental investigation on high-pressure methane jet characteristic single-hole injector

High-pressure gas direct injection (DI) technology benefits engines with high efficiency and clean emissions, and the gas jet process causes crucial effects especially inside an mm-size space. This study presents an investigation on the high-pressure methane jet characteristics from a single-hole injector by analysing jet performance parameters including jet impact force, gas jet impulse, and jet mass flow rate. The results show that the methane jet exhibited a two-zone behaviour along the jet direction in the spatial dimension induced by high-speed jet flow from the nozzle: zone 1 near the nozzle—the jet impact force and jet impulse increased consistently except for a fluctuation due to shock wave effects induced by the sonic jet and no entrainment occurs, and zone II farther away from the nozzle—the jet impact force and jet impulse became stable when the shock wave effects became weak and the jet impulse was conserved with a linear conservation boundary. The Mach disk height was exactly the turning point of two zones. Moreover, the methane jet parameters, such as the methane jet mass flow rate, jet initial jet impact force, jet impulse, and Reynolds number had a monotonous and linearly increasing correlation with injection pressure.

Explosion risk analysis of R290 leakage into a limited external space

R290 is considered as a promising alternative refrigerant to be used in household air conditioners. To promote the safety application of R290 in air conditioners, a numerical model of refrigerant leakage and diffusion in a limited space was developed, and the reliability of the model was verified by the experimental results on the concentration field of refrigerant leakage from a split air conditioner indoor unit. On this basis, the field strength characteristics of R290 leaking from holes with different diameters and shapes to the external space were investigated, and the explosion risk caused by R290 leakage was further analyzed. The results showed that when R290 leaked from circular leakage holes with different diameters or leakage holes with different shapes to the external space, respectively, Mach disks were formed in the flow area, and the location and size of the Mach disks had certain differences. For circular leakage holes, with the increase of the leakage diameter, the diameter of the Mach disk inside the flow area and the distance from the Mach disk to the leakage hole increased, and the range of the flammable area formed by the leaked R290 in the external space did not change significantly in the horizontal direction, but increased significantly in the vertical direction, which were 0.42 m, 0.84 m, 1.26 m, and 1.7 m, respectively. For leakage holes with the same area but different shapes, the diameter of the Mach disk formed by the gas leaking from the circular hole was larger, and the gas was distributed symmetrically in the horizontal direction with the jet axis as the center in the external space. The gas leaking from the triangular hole was approximately distributed in a triangular shape, and then it diffused along the three centerlines of the triangle. The gas leaking from the square hole diffused along the diagonal of the square in the horizontal direction, and finally the flow area gradually changed from a square to a circle. The gas leaking from the rectangular slit was elliptically distributed in the external space, and the gas mainly diffused along the long axis direction of the ellipse, and the diffusion of the short axis direction was not significant. The range of the flammable area formed by the gas leaking from holes of different shapes did not change significantly in the external space.