Research on the mechanics of underwater supersonic gas jets

An experimental research was carried out to study the fluid mechanics of underwater supersonic gas jets. High pressure air was injected into a water tank through converging-diverging nozzles (Laval nozzles). The jets were operated at different conditions of over-, full- and under-expansions. The jet sequences were visualized using a CCD camera. It was found that the injection of supersonic air jets into water is always accompanied by strong flow oscillation, which is related to the phenomenon of shock waves feedback in the gas phase. The shock wave feedback is different from the acoustic feedback when a supersonic gas jet discharges into open air, which causes screech tone. It is a process that the shock waves enclosed in the gas pocket induce a periodic pressure with large amplitude variation in the gas jet. Consequently, the periodic pressure causes the jet oscillation including the large amplitude expansion. Detailed pressure measurements were also conducted to verify the shock wave feedback phenomenon. Three kinds of measuring methods were used, i.e., pressure probe submerged in water, pressure measurements from the side and front walls of the nozzle devices respectively. The results measured by these methods are in a good agreement. They show that every oscillation of the jets causes a sudden increase of pressure and the average frequency of the shock wave feedback is about 5–10 Hz.     

Scaling of the flow field in a combustion chamber with agas——gas injector

The scaling of the flowfield in a gas--gas combustion chamber is investigated theoretically, numerically and experimentally. To obtain the scaling criterion of the gas--gas combustion flowfield, formulation analysis of the three-dimensional (3D) Navier--Stokes equations for a gaseous multi-component mixing reaction flow is conducted and dimensional analysis on the gas--gas combustion phenomena is also carried out. The criterion implies that the size and the pressure of the gas--gas combustion chamber can be changed. Based on the criterion, multi-element injector chambers with different geometric sizes and at different chamber pressures ranging from 3~MPa to 20~MPa are numerically simulated. A multi-element injector chamber is designed and hot-fire tested at five chamber pressures from 1.64~MPa to 3.68~MPa. Wall temperature measurements are used to understand the similarity of combustion flowfields in the tests. The results have verified the similarities between combustion flowfields under different chamber pressures and geometries, with the criterion applied.     

An Experimental Study of In-Situ Phase Fraction in Jet Pump Using Electrical Resistance Tomography Technique

We perform the experiments to investigate in-situ phase fraction in a jet pump using the electrical resistance tomography （ERT） technique. A new jet pump with ERT sensors is designed to measure in-situ phase fraction and flow regime. The study is based on laboratory experiments that are carried out on a 50-mm vertical flow rig for various gas and liquid phase superficial velocities. The different flow patterns of gas liquid in the jet pump and vertical pipe are studied using the ERT technique. The results suggest that the ERT system can be used to successfully produce images of gas-liquid flow patterns with frames rates of 58 fps and the in-situ phase fraction with frame rates of 5 fps can be obtained. The visualizations of a rapid mixing process in the throat of a jet pump obtained in this work provide a reliable basis for theoretical study and optimal design of jet pumps.     

Effect of staggered array structure on the flow field of micro gas chromatographic column

A new design of staggered array semi-packed micro gas chromatographic column was presented based on the micro electromechanical system(MEMS)technology.It was a sensor for gas sample analysis.The internal velocity fields of ten types of semi-packed micro gas chromatographic column were studied.The effects of array spacing and dislocation spacing on the flow field distribution were investigated.The results show that on the basis of ensuring the formation of virtual wall,with the increase of array spacing,the maximum velocity difference between the flow channels in the vertical direction decreases gradually,but the velocity difference in the flow channels a and b increases.When the inlet velocity was set to be 0.18 m/s,the maximum velocity difference in the channel of the staggered semi-packed micro gas chromatography column 3(CSAC3)was 0.05610 m/s.The maximum velocity difference in the channel a was 0.09160 m/s.The maximum velocity difference in the channel b was 0.02401 m/s.CSAC3 had a more uniform velocity field distribution,which can effectively suppress the laminar flow effect during chromatographic separation,and had a smaller pressure distribution,which puts forward lower requirements for carrier gas system.The staggered array semi-packed micro gas chromatography column proposed in this paper can effectively improve the velocity field distribution and pressure distribution in the channel,and provide a theoretical basis for the design of the new micro gas chromatography column structure.     

Effects of water vapor in high vacuum chamber on the properties of HfO_2 films

The influence of water vapor content in high vacuum chamber during the coating process on physical properties of HfO_2 films was investigated.Coatings were deposited on BK7 substrates by electron beam evaporation and photoelectric maximum control method.An in situ residual gas analyzer(RGA)was used to monitor the residual gas composition in the vacuum chamber.The optical properties,microstructure, absorption and laser-induced damage threshold(LIDT)of the samples were characterized by Lambda 900 spectrophotometer,X-ray diffraction(XRD),surface thermal lensing(STL)technique and 1064-nm Q- switched pulsed laser at a pulse duration of 12 ns respectively.It was found that a cold trap is an effective equipment to suppress water vapor in the vacuum chamber during the pumping process,and the coatings deposited in the vacuum atmosphere with relatively low water vapor composition show higher refractive index and smaller grain size.Meanwhile,the higher LIDT value is corresponding to lower absorbance.     

Mechanism from particle compaction to fluidization of liquid–solid two-phase flow

A new model of particle yield stress including cohesive strength is proposed, which considers the friction and cohesive strength between particles. A calculation method for the fluidization process of liquid–solid two-phase flow in compact packing state is given, and the simulation and experimental studies of fluidization process are carried out by taking the sand–water two-phase flow in the jet dredging system as an example, and the calculation method is verified.     

Statistical model for combustion of high-metal magnesium-based hydro-reactive fuel

<正>We investigate experimentally and analytically the combustion behavior of a high-metal magnesium-based hydroreactive fuel under high temperature gaseous atmosphere.The fuel studied in this paper contains 73%magnesium powders.An experimental system is designed and experiments are carried out in both argon and water vapor atmospheres. It is found that the burning surface temperature of the fuel is higher in water vapor than that in argon and both of them are higher than the melting point of magnesium,which indicates the molten state of magnesium particles in the burning surface of the fuel.Based on physical considerations and experimental results,a mathematical one-dimensional model is formulated to describe the combustion behavior of the high-metal magnesium-based hydro-reactive fuel.The model enables the evaluation of the burning surface temperature,the burning rate and the flame standoff distance each as a function of chamber pressure and water vapor concentration.The results predicted by the model show that the burning rate and the surface temperature increase when the chamber pressure and the water vapor concentration increase,which are in agreement with the observed experimental trends.     

Investigation on Rijke pipe＇s acoustic characteristics by numerical simulation： modeling the pulsing flow field coupled the inner of pipe with its outer space

Based on the results of fluid dynamics, heat transfer and acoustics, a Computational Fluid Dynamics （CFD） method was utilized to study the acoustic characteristics and self-excited pulsation mechanism inside a Rijke pipe. To avoid settling the irrational boundary conditions of the finite-amplitude standing wave in the Rijke thermo-acoustic system, the simulation modeling in the flow field, which coupled the inner of pipe with its outer space, was carried out to replace the traditional way in form of internal flow field numerical investigations. A hypothesis for heat source in energy equation including the relationship on unsteady heat of air around heat source, oscillation pressure and oscillation velocity was presented. To reflect the essence of Rijke pipe, simulation on self-excited oscillation was conducted by means of its own pulsation of pressure, velocity and temperature. This method can make the convergence process steady and effectively avoid divergence. The physical phenomenon of the self-excited Rijke pipe was analyzed. Moreover, the mechanisms on the Rijke pipe＇s self-excited oscillation were explained. Based on this method, comparative researches on the acoustic characteristic of the Rijke pipe with different size and different shape of nozzle were performed. The simulation results agreed with the experimental data satisfactorily. The results show that this numerical simulation can be used to study the sound pressure of nozzle for the engineering application of Rijke pipes.     

A multicomponent multiphase lattice Boltzmann model with large liquid–gas density ratios for simulations of wetting phenomena

A multicomponent multiphase(MCMP) pseudopotential lattice Boltzmann(LB) model with large liquid–gas density ratios is proposed for simulating the wetting phenomena. In the proposed model, two layers of neighboring nodes are adopted to calculate the fluid–fluid cohesion force with higher isotropy order. In addition, the different-time-step method is employed to calculate the processes of particle propagation and collision for the two fluid components with a large pseudoparticle mass contrast. It is found that the spurious current is remarkably reduced by employing the higher isotropy order calculation of the fluid–fluid cohesion force. The maximum spurious current appearing at the phase interfaces is evidently influenced by the magnitudes of fluid–fluid and fluid–solid interaction strengths, but weakly affected by the time step ratio.The density ratio analyses show that the liquid–gas density ratio is dependent on both the fluid–fluid interaction strength and the time step ratio. For the liquid–gas flow simulations without solid phase, the maximum liquid–gas density ratio achieved by the present model is higher than 1000:1. However, the obtainable maximum liquid–gas density ratio in the solid–liquid–gas system is lower. Wetting phenomena of droplets contacting smooth/rough solid surfaces and the dynamic process of liquid movement in a capillary tube are simulated to validate the proposed model in different solid–liquid–gas coexisting systems. It is shown that the simulated intrinsic contact angles of droplets on smooth surfaces are in good agreement with those predicted by the constructed LB formula that is related to Young's equation. The apparent contact angles of droplets on rough surfaces compare reasonably well with the predictions of Cassie's law. For the simulation of liquid movement in a capillary tube, the linear relation between the liquid–gas interface position and simulation time is observed, which is identical to the analytical prediction. The simulation results regarding the wetting phenomena of droplets on smooth/rough surfaces and the dynamic process of liquid movement in the capillary tube demonstrate the quantitative capability of the proposed model.     

Influence of Inert Gas Pressure on Growing Rate of Nanocrystalline Silicon Film Prepared by Pulsed Laser Deposition

Nanocrystalline silicon film (nc-Si) was prepared by pulsed laser deposition in different inert gas atmospheres such as He, Ne and Ar. The influence of inert gas pressure on growing rate of the film was investigated. The results show that with increasing gas pressure, growing rate first increases and reaches its maximum and then decreases; the gas pressure at the maximum of growing rate is proportional to the reciprocal of atomic mass of gas. The rate maximum is 0.315 A/pulse when He gas pressure is 8.3 Pa. The dynamic process is analysed theoretically by means of resputtering from the film surface and scattering of ablated particles. Ehrthermore, our results are compared with those in the case of Ag target.     

Characteristics of a Novel Water Plasma Torch

Relying on heat generated by plasma arc heating liquid water into steam as a swirl gas, a water plasma torch has the distinctive steam generation structure, which has various applications such as in the treatment of organic waste and hydrogen production for fuel cells in future vehicles. The operational features of the water plasma torch and water phase change process in the discharge chamber are investigated based on the temporal evolution of the voltage and current. The optical emission spectrum measurement shows that the water molecule in the plasma is decomposed into 14, OH and O radicals. As the electrodes do not require water-cooling, the thermal efficiency of the torch is very high, which is confirmed by analytical calculation and experimental measurement.     

Suppression effect of jet flow on pulsating pressure of cavity using scale-adaptive simulation model

The suppression of the aerodynamic noise in the cavity has a great significance to solve relevant puzzles of weapon bays.Acoustic field of the standard cavity model is simulated by using the computational fluid dynamics technology based on scale-adaptive simulation(SAS)model.The results obtained by the proposed method in this paper show reasonable agreement with experiments.On the basis of this,effect of different jet flow rates on the time-averaged variables,turbulent kinetic energy,root mean square(RMS)of sound pressure,sound sources distribution and the pulsating pressure distribution in the cavity is studied.The analysis shows that the jet flow has great influence on the cavity flow field and the distribution of pulsating pressure RMS by changing the morphology of the shear layer.The most obvious of these measures is spout4 configuration,the influence mainly in the form of reducing the pulsating pressure of the whole cavity and changing the sound pressure level in the far field.The results show that different jet flow rates have different control effects on pulsating pressure in the cavity and sound pressure level in the far field.Furthermore,the jet flow rates and the suppression effect on the pulsating pressure have no linear relation.     

LIF diagnostics of hydroxyl radical in a methanol containing atmospheric-pressure plasma jet

In this paper, a pulsed-dc CH_3OH/Ar plasma jet generated at atmospheric pressure is studied by laser-induced fluorescence(LIF) and optical emission spectroscopy(OES). A gas–liquid bubbler system is proposed to introduce the methanol vapor into the argon gas, and the CH3OH/Ar volume ratio is kept constant at about 0.1%. Discharge occurs in a 6-mm needle-to-ring gap in an atmospheric-pressure CH_3OH/Ar mixture. The space-resolved distributions of OH LIF inside and outside the nozzle exhibit distinctly different behaviors. And, different production mechanisms of OH radicals in the needle-to-ring discharge gap and afterglow of plasma jet are discussed. Besides, the optical emission lines of carbonaceous species, such as CH, CN, and C_2 radicals, are identified in the CH_3OH/Ar plasma jet. Finally, the influences of operating parameters(applied voltage magnitude, pulse frequency, pulsewidth) on the OH radical density are also presented and analyzed.     

The hydrothermal wave of large-Prandtl-number fluid in a shallow cavity

The hydrothermal wave was investigated numerically for large-Prandtl-number fluid (Pr = 105.6) in a shallow cavity with different heated sidewalls. The traveling wave appears and propagates in the direction opposite to the surface flow (upstream) in the case of zero gravity when the applied temperature difference grows and over the critical value. The phase relationships of the disturbed velocity, temperature and pressure demonstrate that the traveling wave is driven by the disturbed temperature, which is named hydrothermal wave. The hydrothermal wave is so weak that the oscillatory flow field and temperature distribution can hardly be observed in the liquid layer. The exciting mechanism of hydrothermal wave is analyzed and discussed in the present paper.     

Laser-Induced Particle Jet and Its Ignition Application in Premixed Combustible Gases

A hot particle jet is induced as a laser pulse from a free oscillated Nd：YAG laser focused on a coal target. The particle jet successfully initiates combustion in a premixed combustible gas consisting of hydrogen, oxygen, and air. The experiment reveals that the ionization of the particle jet is enhanced during the laser pulse. This characteristic is attributed to the electron cascade process and the ionization of the particles or molecules of the target. The initial free electrons, which are ablated from the coal target, are accelerated by the laser pulse through the inverse Bremsstrahfung process and then collide with the neutrals in the jet, causing the latter to be ionized.     

Numerical and experimental study on shear coaxial injectors with hot hydrogen-rich gas/oxygen-rich gas and GH2 /GO2

The influences of the shear coaxial injector parameters on the combustion performance and the heat load of a combustor are studied numerically and experimentally. The injector parameters, including the ratio of the oxidizer pressure drop to the combustor pressure (DP ), the velocity ratio of fuel to oxidizer (R V ), the thickness (WO ), and the recess (HO ) of the oxidizer injector post tip, the temperature of the hydrogen-rich gas (TH ) and the oxygen-rich gas (TO ), are integrated by the orthogonal experimental design method to investigate the performance of the shear coaxial injector. The gaseous hydrogen/oxygen at ambient temperature (GH2 /GO2 ), and the hot hydrogen-rich gas/oxygen-rich gas are used here. The length of the combustion (LC ), the average temperatures of the combustor wall (TW ), and the faceplate (TF ) are selected as the indicators. The tendencies of the influences of injector parameters on the combustion performance and the heat load of the combustor for the GH2 /GO2 case are similar to those in the hot propellants case. However, the combustion performance in the hot propellant case is better than that in the GH2/GO2 case, and the heat load of the combustor is also larger than that in the latter case.     

Impact mechanism of gas temperature in metal powder production via gas atomization

This paper aims at studying the influence mechanism of gas temperatures(300 K, 400 K, 500 K, and 600 K) on gas atomization by simulating the integral atomization process of the close-coupled nozzle in vacuum induction gas atomization(VIGA). The primary atomization is simulated by the volume of fluid(VOF) approach, and the second atomization is studied by the discrete phase model(DPM) combined with the instability breakage model. The results show that, at an increased gas temperature, the influences of gas–liquid contact angle and gas temperature in the recirculation zone on the primary atomization are virtually negligible. However, increasing the gas temperature will increase the gas–liquid relative velocity near the recirculation zone and decrease the melt film thickness, which are the main reasons for the reduced mass median diameter(MMD, d50) of primary atomized droplets. During the secondary atomization, increasing the gas temperature from 300 K to 600 K results in an increase in the droplet dispersion angle, which is beneficial to the formation of spherical metal powder. In addition, increasing the gas temperature, the positive effect of gas–liquid relative velocity increase on droplets refinement overweighs the negative influence of the GMR decrease, resulting in the reduced MMD and diameter distribution interval. From the analysis of the atomization mechanism, the increase in atomization efficiency caused by increasing the temperature of the atomizing gas, including primary atomization and secondary atomization, is mainly due to the increase in the gas drag force difference between the inner and outer sides of the annular liquid film.     

Structural response of aluminum core–shell particles in detonation environment

Natural aluminum particles have the core–shell structure. The structure response refers to the mechanical behavior of the aluminum particle structure caused by external influences. The dynamic behavior of the structural response of aluminum core–shell particles before combustion is of great importance for the aluminum powder burning mechanism and its applications. In this paper, an aluminum particle combustion experiment in a detonation environment is conducted and analyzed; the breakage factors of aluminum particles shell in detonation environment are analyzed. The experiment results show that the aluminum particle burns in a gaseous state and condenses into a sub-micron particle cluster. The calculation and simulation demonstrate that the rupture of aluminum particle shell in the detonation environment is mainly caused by the impact of the detonation wave. The detonation wave impacts the aluminum particles, resulting in shell cracking, and due to the shrinkage-expansion of the aluminum core and stripping of the detonation product, the cracked shell is fractured and peeled with the aluminum reacting with the detonation product.     

Close-coupled nozzle atomization integral simulation and powder preparation using vacuum induction gas atomization technology

We simulate the gas-atomization process of a close-coupled annular nozzle for vacuum induction gas atomization at a three-dimensional scale.Moreover,the relationship between the simulated droplet type and experimentally metallic powder is established by comparing the morphology of droplets with powders.Herein,the primary atomization process is described by the volume-of-fluid(VOF)approach,whereas the prediction of powder diameter after secondary atomization is realized by the VOF-Lagrangian method.In addition,to completely reflect the breaking and deformation process of the metallic flow,we employ the VOF model to simulate the secondary atomization process of a single ellipsoidal droplet.The results show that the primary atomization process includes the formation of surface liquid film,appearance of serrated ligaments,and shredding of ligaments.Further,gas recirculation zone plays an important role in formation of the umbrella-shaped liquid film.The secondary atomization process is divided into droplet convergence and dispersion stages,and the predicted powder diameter is basically consistent with the experiment.In general,the four main powder shapes are formed by the interaction of five different typical droplets.     

Numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet

A numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium–air plasma jet is presented. The generation and loss mechanisms of the OH radicals in a positive half-cycle of the applied voltage are studied and discussed. It is found that the peak OH density increases with water content in air(varying from0% to 1%) and reaches 6.3×1018m-3when the water content is 1%. Besides, as the water content increases from 0.01%to 1%, the space-averaged reaction rate of three-body recombination increases dramatically and is comparable to those of main OH generation reactions.