Combustion Gas Jet Research Articles

Evolution mechanism and interaction of the muzzle combustion-gas jets field of underwater dual-barrel synchronized launch

Despite a certain amount of research that has been done on the muzzle combustion–gas jet field of underwater launching in recent years, there are still few reports on the evolution mechanism and interaction of underwater dual-barrel synchronised launch flow fields. To fill this gap, this study aimed to numerically analyse the effects of different barrel intervals on the transient jet field evolution characteristics of the dual-barrel launch. The three-dimensional unsteady multi-phase mathematical model was created and compared with an underwater sealed emission experiment of data for verification. The volume of fluid multi-phase flow model is used to describe the gas–liquid two-phase flow process, and the k–ε turbulence model simulates the turbulent mixing phenomenon in the jet field. On this basis, the user-defined function coupling internal ballistics process was used to determine the transient jet field of a double-barrelled, 30 mm (d = 30) underwater artillery numerically at the barrel intervals of 3d, 4d and 5d. The calculation results indicate that the closer the interval is, the more chaotic the pressure field is, and the Mach disc structure forms later. When the barrel interval is 3d, the Mach disc formation time is ∼ 0.2 ms, while it forms at ∼ 0.15 ms when the barrel interval is 5d. The movement law of projectiles in water is consistent with the change of Mach disc displacement with time, and both satisfy the exponential growth law. The velocity field core is elliptical, and the vortices on both sides of the muzzle continue to grow within 0.4 ms when the launch interval is 3d. As the barrel interval increases, the core of the velocity field is bottle-shaped, the vortex on both sides of the muzzle increases first and then decreases within 0.4 ms, and the maximum temperature of the flow field increases by 15 %.

Unsteady flow of igniter combustion-gas heating base bleed propellant by convection

The performance and launching accuracy of the vehicle with the base-bleed unit (BBU) depend strongly on the complex flow field characteristics and reignition delay time of the base bleed propellant in the combustion chamber of the BBU because the rapid depressurization process will occur in the BBU with a significantly altered flow field structure and induced extinguishment of the base bleed propellant located on both sides of the BBU when the vehicle is out of the muzzle at an initial time. However, the base bleed propellant will be reignited under the continuous convective heating effect of the igniter combustion-gas jet in the BBU at a later time. In this study, numerical simulations are carried out to investigate the coupling flow field characteristics and heating mechanism of the igniter of an actual BBU under a rapid depressurization process. Indeed, it is found that when the base bleed vehicle is out of the muzzle, the BBU experiences a rapid depressurization process due to the pressure difference between the combustion chamber of the BBU and the atmosphere, resulting in a strong unsteady flow field structure under the Kelvin–Helmholtz instability effect from the velocity difference between the igniter combustion-gas and gun propellant combustion-gas. Meanwhile, the high-temperature combustion-gas from the igniter is pressed on the end face of the igniter, and then, the axial expansion becomes quicker than the radial expansion with the development of the depressurization process. Afterward, the continuous convective heating effect of the igniter combustion-gas jet on the base bleed propellant occurs due to the direct contact of the high-temperature combustion-gas jet from the igniter with the surface of the base bleed propellant.

Numerical Studies on Heat Transfer of Supersonic Combustion-Gas Jet Impingement on Perpendicular Flat Plate

This paper numerically investigates the heat transfer characteristics of supersonic high-temperature combustion-gas jet impinging to a perpendicular flat plate. First, the numerical method is validated using an experimental case and meanwhile different turbulence models are evaluated for heat transfer prediction. Then, the effects of multi-species, nozzle-to-plate distance, and nozzle pressure ratio on wall heat transfer are studied through a series of numerical experiments. Comparison of different turbulence models indicates that the variations of turbulent viscosity across the detached normal shock wave differ among the studied three models. Furthermore, if the real multi-species of combustion-gas are mixed as a single species with the same thermal-physical properties, the pressure and heat flux distributions are identical with the results of combustion-gas. Finally, when there is no stagnation bubble in the jet flow field, the heat flux on the flat plate produces a local minimum at the stagnation point for short nozzle-to-plate distance, while a local maximum appears at the stagnation point for large nozzle-to-plate distance. When the stagnation bubble exists, the other maximum of heat flux is produced at the edge of bubble.

Effects of Mg/PTFE pyrotechnic compositions on reignition characteristics of base bleed propellants and heating mechanism

The effects of magnesium/polytetrafluoroethylene (Mg/PTFE) pyrotechnic compositions on the coupled flow field and reignition mechanism are important aspects governing the performance and range of base bleed projectiles (BBPs). Owing to a decrease in pressure and temperature when the BBP leaves the muzzle, rapid depressurization occurs, which extinguishes the base bleed propellant. The Mg/PTFE pyrotechnic composition pressed in the igniter of the base bleed unit (BBU) provides additional energy to the BBU via a chemical reaction. Thus, the extinguished base bleed propellant is reignited under the effect of high-temperature combustion gas jets from the igniter. In this study, a numerical analysis is conducted to evaluate the effects of PTFE and Mg granularity as well as Mg/PTFE pyrotechnic compositions. Owing to the rapid depressurization, the temperature and pressure was found to decrease for different Mg/PTFE pyrotechnic compositions. However, the depressurization time increased as the PTFE granularity increased, the Mg granularity decreased, and the Mg content increased. When the pressure in the combustion chamber of the BBU decreased to the atmospheric pressure, the combustion gas jets from the igniter expand upstream (rather than downstream). However, these combustion gas jets exhibit different axial and radial expansion characteristics depending on the pyrotechnic compositions used. The results show that the reignition delay time, td, of the base bleed propellant was 377.608, 94.27, 387.243, 523.966, and 221.094 ms for cases A–E, respectively. Therefore, it was concluded that the Mg/PTFE pyrotechnic composition of case B was the most beneficial for the reignition of the base bleed propellant, with the earliest addition of energy and mass to the BBP.

Study on draining off water mechanism and interaction characteristic of high-temperature and high-pressure combustion-gas jets with the water

A new method of underwater launching is proposed to overcome the water resistance in the barrel and realize continuous launching by forming gas curtain without any complex device. Namely, part of gunpowder combustion-gases behind the projectile is guided to the space in front of underwater projectile by opening pores around the circumference of the warhead, to form a dry launching path. Based on this, the expansion process of combustion-gas jets in the confining tube filled with water, especially complex interaction of high-pressure and high-temperature combustion-gas jets and water is studied by experiments and numerical calculations. The results show that, when the high-velocity combustion-gases inject into motionless liquid environment, multiple Taylor cavities are formed and typical structure of Taylor cavity presents two parts, including bubble head and combustion-gas path. The distribution of the recirculation zone is an important factor for the flow field structure and the effect of draining off water. The recirculation zone of the central jet expands downstream with time going on and is decreasing until it disappears. While, the recirculation zone of lateral jet is strengthening with developing downstream. In the whole expansion process, lateral combustion-gas jet plays a significant role on radial expansion process of Taylor cavities.

The gas jet behavior in submerged Laval nozzle flow

The behavior of the combustion gas jet in a Laval nozzle flow is studied by numerical simulations. The Laval nozzle is installed in an engine and the combustion gas comes out of the engine through the nozzle and then injects into the surrounding environment. First, the jet injection into the air is simulated and the results are verified by the theoretical solutions of the 1-D isentropic flow. Then the behavior of the gas jet in a submerged Laval nozzle flow is simulated for various water depths. The stability of the jet and the jet evolution with a series of expansion waves and compression waves are analyzed, as well as the mechanism of the jet in a deep water depth. Finally, the numerical results are compared with existing experimental data and it is shown that the characteristics of the water blockage and the average values of the engine thrust are in good agreement and the unfixed engine in the experiment is the cause of the differences of the frequency and the amplitude of the oscillation.

The influence of nozzle diameters on the interaction characteristic of combustion-gas jets and liquid

Abstract To investigate the controlling method of interior ballistic stability of bulk-loaded propellant guns, the combustion-gas generator and cylindrical stepped-wall chamber are designed aiming at the injection processes of combustion-gases in liquid. The expansion courses of Taylor cavities and the turbulent mixing characteristic of gas–liquid are recorded by means of high speed photographic system. Based on the experiment, three-dimensional unsteady model on the interaction of gas and liquid is established to simulate expansion characteristics of twin combustion-gas jets in liquid under different nozzle diameters. The distribution regularities of characteristic parameter in jet field are obtained and analyzed. The results show the pressure, velocity and temperature distributions under different nozzle diameters are basically the same at the initial time. As time goes on, these characteristic parameters under different nozzle diameters have large differences.

Flow structure of conical distributed multiple gas jets injected into a water chamber

Based on an underwater gun firing project, a mock bullet with several holes on the head was designed and experimented to observe the combustion gas injected into a cylindrical water chamber through this mock bullet. The combustion gas jets contain one vertical central jet and 4 to 8 slant lateral jets. A high speed camera system was used to record the expansion of gas jets in the experimental study. In numerical simulations, the Euler two-fluid model and volume of fluid method were adopted to describe the gas-liquid flow. The results show the backflow zone in lateral jet is the main factor influencing the gas-liquid turbulent mixing in downstream. On cross sections, the gas volume fraction increased with time but the growth rate decreased. With a change of nozzle structure, the gas fraction was more affected than the shock structure.

Study on the gas-curtain generation characteristics by the multiple gas jets in a liquid-filled tube

To further understand the gas-curtain generation characteristics during the launching process of the underwater guns, a simulated gas-injector with multiple nozzles is designed and the expansion process of these multiple combustion-gas jets is experimentally captured. Experiment results indicate that, with turbulent mixing between the gas jets and the liquid medium, the gas curtain is generated in the observation chamber. Based on the experiment, a three dimensional unsteady model is established to simulate the expansion process of the multiple jets. Numerical result shows, with the injection of the gas jets, pressure declines in the Taylor cavity along the axial direction at the beginning. During the expansion process, the multiple jets contact and interact with each other, and pressure rises in the mixing area. After merging, backflow vortexes interact and converge, and the vortexes move along the axial direction with the expansion of the combustion gas jets.

Study on interaction characteristics between multi gas jets and water during the underwater launching process

To further understand the mechanism of new submerged water piercing gun launcher, the simulation device of an imitated projectile is designed with using multi combustion gas jets to drain off the water in gun tube. The expansion characteristic of multi gas jets and generation mechanism of the gas-curtain in the liquid medium are experimental observed by a high speed camera. The experiment shows that, with the mixing process between the multi combustion gas jets and the liquid medium, the gas-curtain is generated, which provides a gas path for the projectile motion. On the base of experimental observation result, the influences of injection pressure and sprayer structure on projectile motion are discussed. Experiment result indicates: increasing injection pressure, both axial expansion velocities of gas-curtain and projectile moving speed increase; magnifying the center nozzle cannot enhance the drainage performance of the gas-curtain and will restrain projectile motion in the later stage; with enlarging the oblique plane nozzles, the axial velocity of gas-curtain tend to rise after converging and projectile kinematic performance is also improved.

The interaction between multiple high pressure combustion gas jets and water in a water-filled vessel

A static experimental system based on the background of the underwater gun firing process is designed to simulate the expansion of conical-distributed multiple gas jets in a water-filled vessel. The gas is generated from the gunpowder combustion and injected into water through five orifices on a mock bullet. A high-speed digital camera is taken in experiment to record such expansion and the Euler-Euler multiphase model is utilized in simulation to describe the gas-liquid flow. Results show that the contraction of Taylor cavity in liquid and the gas-liquid entrainment both affect the shock structure in gas. The expansion zone near the orifice shrinks and the gas speed downs when the central gas jet has a contraction, but lateral jets have no contractions due to the obstruction from the wall. The contraction has a greater impact on the shock structure than the gas-liquid entrainment and occurs earlier under a higher injection pressure.

Expansion characteristics of multiple wall jets in cylindrical observation chamber

The objective of this study is to investigate the drag reduction effect of a new launch technology by utilizing propellant gases to drain off water in the front of the projectile inside the gunbarrel for submerged launch process of underwater gun. A static analogue visualized device was designed and the high-speed digital photographic system was adopted to investigate expansion processes of multiple wall jets in cylindrical observation chamber. The evolution laws of Taylor cavities can be obtained by dealing with the expansion sequence maps. Based on the experiments, a three-dimensional unsteady mathematical model is established to simulate the interaction process between the multiple wall jets and liquid medium, which can achieve the detailed distribution characteristics of phase, pressure, temperature and velocity in flow field. Results show that, by increasing four wall jets to eight wall jets, the decrease of the inlet turbulent kinetic energy of each jet leads to a decline in the initial axial velocity. However, the Taylor cavities achieve to converge more quickly for the case with more numbers of combustion-gas jets due to less nozzle interval distance. In addition, the convergence of Taylor cavities can enhance the drainage effect of the multiple wall jets and achieve excellent drag reduction effect for the underwater launching process, wherein the drainage effect for the case of six wall jets is the most outstanding.

Experimental study on gas-curtain generation characteristics by multicombustion-gas jets in the cylindrical liquid chamber

The simulator for multicombustion-gas jets injection is designed to study the gas-curtain generation characteristics by multicombustion-gas jets during the underwater gun launch process. The expansion process and interaction process of combustion-gas jets in the liquid are studied under the different injection pressures and nozzle diameters by means of a high-speed camera system, and then the turbulent mixing during the merging are numerically discussed. The experimental results indicate that the gas-curtain is formed by the turbulent mixing process and the turbulent mixing process is enhanced with the injection pressure increasing before gas-curtain forms. In the initial phase, the expansion velocity of the gas-curtain is affected by the center jet and the final expansion velocity is determined by the side jets.

Study on the influences of interaction behaviors between multiple combustion-gas jets on expansion characteristics of Taylor cavities

The purpose of this study is to investigate means of controlling the interior ballistic stability of a bulk-loaded propellant gun (BLPG). Experiments on the interaction of twin combustion gas jets and liquid medium in a cylindrical stepped-wall combustion chamber are conducted in detail to obtain time series processes of jet expansion, and a numerical simulation under the same working conditions is also conducted to verify the reliability of the numerical method by comparing numerical results and experimental results. From this, numerical simulations on mutual interference and expansion characteristics of multiple combustion gas jets (four, six, and eight jets) in liquid medium are carried out, and the distribution characteristic of pressure, velocity, temperature, and evolutionary processes of Taylor cavities and streamlines of jet flow field are obtained in detail. The results of numerical simulations show that when different numbers of combustion gas jets expand in liquid medium, there are two different types of vortices in the jet flow field, including corner vortices of liquid phase near the step and backflow vortices of gas phase within Taylor cavities. Because of these two types of vortices, the radial expansion characteristic of the jets is increased, while changing numbers of combustion gas jets can restrain Kelvin–Helmholtz instability to a certain degree in jet expansion processes, which can at last realize the goal of controlling the interior ballistic stability of a BLPG. The optimum method for both suppressing Kelvin–Helmholtz instability and promoting radial expansion of Taylor cavities can be determined by analyzing the change of characteristic parameters in a jet flow field.

Study on the effect of distance between the two nozzle holes on interaction of high pressure combustion-gas jets with liquid

The combustion-gas generator and cylindrical stepped-wall observation chambers with five stages are designed to study the expansion characteristic of twin combustion-gas jets in liquid working medium under high temperature and high pressure. The expansion processes of Taylor cavities formed by combustion-gas jets and the mixing characteristics of gas–liquid are studied by means of high-speed digital camera system. The effects of the distance between the two nozzle holes, injection pressure and nozzle diameter on jet expansion processes are discussed. The experimental results indicate that, the velocity differences exist on the gas–liquid interface during expansion processes of twin combustion-gas jets, and the effect of Taylor–Helmholtz instability is intense, so interfaces between gas and liquid show turbulent folds and randomness. The strong turbulent mixing of gas and liquid leads to release of combustion-gas energy with the temperature decreasing. Moreover, the mixing effectiveness is obviously enhanced on the corners of each step of the cylindrical stepped-wall structure, forming radial expansion phenomenon. The reasonable matching of multi-parameter can restrain the jet instability and make the combustion-gas energy orderly release. Based on the experiments, the three-dimensional unsteady mathematical model of interaction of twin combustion-gas jets and liquid working medium is established to obtain the density, pressure, velocity and temperature distribution characteristics in jet flow field. Discussions are focused on variation law of characteristic parameters of the flow field under different nozzle intervals. The results indicate that, as the ratio of diameter increment of each step to the nozzle interval ΔD/S≧0.6, twin combustion-gas jets begin converging earlier, and the jets can also reach the exit of observation chamber earlier. But as ΔD/S<0.6, the twin jets are closer to the chamber wall, and radial inductions from the expanding steps are stronger, so twin jets do not appear the collection effect on the space and time scale of the observation chamber and the pressure distributions of jet flow field are always more violent pulsation state. The calculated data of axial displacement of Taylor cavity heads are in good agreement with the experimental ones according to dealing with the density profiles and experimental sequence processes.

Expansion characteristics of twin combustion gas jets with high pressure in cylindrical filling liquid chamber

To deal with the problem of how to control the interior ballistic stability in the bulk-loaded liquid propellant gun, the expansion and mixing process of the twin combustion-gas jets with high temperature and pressure in a liquid medium is studied in the cylindrical filling liquid chamber. A series of the jet expansion shapes is obtained by using a high-speed photographic system. The influences of the jet pressure on the jet expansion shape are discussed. Based on the experiments, the three-dimensional mathematical model is established. The expansion processes of the twin gas jets in the liquid medium are simulated by means of fluent to get the pressure, density, temperature, velocity contours and evolutionary process of vortices. Results show that the jet external outline and tops are all irregular. The Kelvin-Helmholtz instability is shown in the whole expansion process. The numerical simulation results of the axial displacement of the twin gas jets in liquid agree well with the experiment.

Study on Expansion Characteristic of Twin Combustion Gas Jets in Five-Stage Cylindrical Stepped-wall Observation Chamber

The five-stage cylindrical stepped-wall observation chamber is designed to investigate the method of controlling the interior ballistic stability in bulk-loaded propellant guns. The expansion and mixing process of twin combustion gas jets in liquid is studied by means of high speed photographic system. The influence of multiple parameters on jet expansion shape is discussed. Based on the experiment, the three-dimensional mathematics model is established to simulate the expansion process of twin gas jets in liquid. The pressure, density, temperature, velocity contours and evolutionary process of vortices are obtained. Results show that vortices behind the corner of the steps are formed due to the inducing effect of steps. The jets can expand along the axial and radial direction simultaneously, weakening the Kelvin-Helmholtz instability. The numerical simulation results of axial expansion displacement are in good agreement with the experimental data.

Experimental Study on Expansion Process of High Pressure Twin Combustion-Gas Jets in Liquid

In order to investigate the multipoint ignition process and the combustion stability controlling mechanism of the bulk-loaded liquid propellant gun, the cylindrical stepped-wall observation chambers and cylindrical observation chambers are designed. The expansion process and interaction of high-speed twin combustion-gas jets in liquid are studied by high speed photographic system. The influence of chamber structure, nozzle diameter, dual-orifice interval, jet pressure on twin gas jets expansion process has been discussed.

Current Status and Future Prospects of Warm Spray Technology

A modification of high-velocity oxy-fuel (HVOF) thermal spray process named as warm spray (WS) has been developed. By injecting room temperature inert gas into the combustion gas jet of HVOF, the temperature of the propellant gas can be controlled in a range approximately from 2300 to 1000 K so that many powder materials can be deposited in thermally softened state at high impact velocity. In this review, the characteristics of WS process were analyzed by using gas dynamic simulation of the flow field and heating/acceleration of powder particles in comparison with HVOF, cold spray (CS), and high-velocity air-fuel (HVAF) spray. Transmission electron microscopy of WS and CS titanium splats revealed marked differences in the microstructures stemming from the different impact temperatures. Mechanical properties of several metallic coatings formed under different WS and CS conditions were compared. Characteristics of WC-Co coatings made by WS were demonstrated for wear resistant applications.

Effects of Particle Strength of Feedstock Powders on Properties of Warm-Sprayed WC-Co Coatings

Warm spray (WS) process, which can control the temperature of a combustion gas jet used to propel powder, has been successfully applied to deposit WC-Co coatings. Detrimental reactions resulting from dissolution of WC into Co binder and decarburization were suppressed effectively by keeping the WC-Co particles’ temperature below the m.p. of the binder phase. In this study, three nano-structured WC-12Co powders with different particle strengths were prepared by changing the sintering conditions of spray-dried powder and were deposited by WS. The deposition efficiency and porosity of the coatings decreased with increasing the particle strength. The coating deposited from the powder with very low particle strength showed significant phase changes, while those deposited from the higher particle strengths showed almost no change. Particle Image Velocimetry revealed significant disintegration of the weakest powder, which explains the changes observed. The hardness and wear properties of the former coating, therefore, were inferior to the other two.