FLUENT软件在射流模拟中的应用

介绍了利用CFD软件对二维射流作用下的吸气流动的模拟,通过与实验研究得出的结果进行比较,结果吻合得很好。同时,与理论分析方法进行了比较,证明利用商业软件对射流作用的吸气运动进行研究是可行的。

一种新的模拟固体火箭发动机射流铝颗粒燃烧的方法

在总结以往火箭射流数值模拟的基础上,采用多组分有限速率化学反应模型模拟气体在喷管后的复燃,并引入拉格朗日颗粒轨道模型来模拟由Al和Al_2O_3组成的混合颗粒。通过快速欧拉模型得到Al_2O_3烟尘的速度场,然后通过质量、动量、能量源项将气体相、颗粒相和烟尘相三者联立耦合计算来模拟铝颗粒的燃烧和燃烧产生的烟尘颗粒对整个射流结构的影响,为以后更准确地计算射流红外辐射特性提供了理论工具。

一种新型串联战斗部前级装药结构的数值模拟研究

针对串联战斗部前级装药结构兼顾大的侵深与扩孔,设计了一种阶梯状药型罩。采用数值模拟的方法研究了阶梯状药型罩射流的形成机理以及对45号钢的侵彻能力,并与锥角药型罩形成的射流参数以及对45号钢的侵彻深度、扩孔大小进行对比。研究结果表明:相同装药条件下,阶梯状药型罩比锥角药型罩的侵彻深度提升约25%,对45号钢板的扩孔直径扩大约1.5倍;并且形成杵体较小,更加符合串联战斗部前级装药结构的特性。

大气压非平衡等离子体射流活性粒子产生机制

大气压非平衡等离子体中的高能电子和大量活性粒子如O、OH等能够同污染物发生反应,达到促进污染物降解的目的。因此通过数值模拟和实验的方法对大气压脉冲直流等离子体射流推进以及OH产生机理进行了研究。模拟结果表明:介质管内表面附近形成的鞘层将等离子体通道和介质隔离开来,介质边缘的强电场增强了气体混合区域的电离,因此射流头部呈现出环状结构。Penning电离在电离过程中起主导作用并提高了等离子体通道的电导率。电子分解水分子,电子和水离子的复合以及O(1D)分解水分子是产生OH最主要的3个反应,其中电子分解水分子在3者中作用最大。工作气体中掺入额外的N2、O2和水蒸气能够小幅度提高OH的密度是因为Penning电离增加了电子密度。由于空气的扩散,尽管掺入水蒸气能够使管内OH密度有显著提高,该方法对提高管外OH密度效果并不明显。选择合适的脉冲周期数是控制射流活性的有效手段。

Numerical simulation of fluid dynamics in rare earth chloride solution in jet-flow pyrolysis reactor

Rare earth oxide was prepared via direct pyrolysis of rare earth chloride solution. Based on this technique, a new-type jet-flow pyrolysis reactor was designed, and then the fluid dynamics (pressure and velocity) inside the reactor was numerically simulated using a computational fluid dynamics method. The self-produced pressure (p) and the fuel inlet velocity (v) satisfied a quadratic function,p=0.06v2+0.23v?4.49. To fully utilize the combustion-generated heat in pyrolysis of rare earth chloride, an appropriate external pressure p=v2+3v?4.27 should be imposed at the feed inlet. The 1.25- and 1.5-fold increase of feed inlet diameter resulted in decline of adsorption dynamic pressure, but the intake of rare earth chloride increased by more than 30% and 60%, respectively. The fluid flow in the reactor was affected by the feeding rate; the fluid flow peaked near the throat of venturi and gradually smoothed down at the jet-flow reactor’s terminal along with the sharp decline of feeding rate.

Mathematical modelling and numerical optimization of particle heating process in copper flash furnace

A mathematical model of the particle heating process in the reaction shaft of flash smelting furnace was established and the calculation was performed.The results indicate that radiation plays a significant role in the heat transfer process within the first 0.6 m in the upper part of the reaction shaft,whilst the convection is dominant in the area below 0.6 m for the particle heating.In order to accelerate the particle ignition,it is necessary to enhance the convection,thus to speed up the particle heating.A high-speed preheated oxygen jet technology was then suggested to replace the nature gas combustion in the flash furnace,aiming to create a lateral disturbance in the gaseous phase around the particles,so as to achieve a slip velocity between the two phases and a high convective heat transfer coefficient.Numerical simulation was carried out for the cases with the high-speed oxygen jet and the normal nature gas burners.The results show that with the high-speed jet technology,particles are heated up more rapidly and ignited much earlier,especially within the area of the radial range of R=0.3−0.6 m.As a result,a more efficient smelting process can be achieved under the same operational condition.

Using a Time-domain Higher-order Boundary Element Method to Simulate Wave and Current Diffraction from a 3-D Body

To study wave-current actions on 3-D bodies a time-domain numerical model was established using a higher-order boundary element method(HOBEM).By assuming small flow velocities,the velocity potential could be expressed for linear and higher order components by perturbation expansion.A 4th-order Runge-Kutta method was applied for time marching.An artificial damping layer was adopted at the outer zone of the free surface mesh to dissipate scattering waves.Validation of the numerical method was carried out on run-up,wave exciting forces,and mean drift forces for wave-currents acting on a bottom-mounted vertical cylinder.The results were in close agreement with the results of a frequency-domain method and a published time-domain method.The model was then applied to compute wave-current forces and run-up on a Seastar mini tension-leg platform.

Flow Behaviors of Gas-Solid Injector by 3D Simulation with Kinetic Theory of Granular Flow

A computational study on the flow behavior of a gas-solid injector by Eulerian approach was carried out. The gas phase was modeled with k-ε turbulent model and the particle phase was modeled with kinetic theory of granular flow. The simulations by Eulerian two-fluid model （TFM） were compared with the corresponding results by discrete element method （DEM） and experiments. It was showed that TFM simulated results were in reasonable agreement with the experimental and DEM simulated results. Based on TFM simulations, gas-solid flow pattern, gas velocity, particle velocity and the static pressure under different driving jet velocity, backpressure and convergent section angle were obtained. The results showed that the time average axial gas velocity sharply decreased and then slightly increased to a constant value in the horizontal conveying pipe. The time average axial particle velocity increased initially and then decreased, but in the outlet region of the convergent section the particle velocity remarkably increased once more to the maximal value. As a whole, the static pressure distribution change trends were found to be independent on driving gas velocity, backpressure and convergent section angle. However, the static pressure increased with increase of convergent section angle and gas jet velocities. The difference of static pressure to backpressure increased with increasing backpressure.

Numerical Simulation of Direct-contact Condensation from a Supersonic Steam Jet in Subcooled Water

The phenomenon of direct-contact condensation,used in steam driven jet injectors,nuclear reactor emergency core cooling systems and direct-contact heat exchangers,was investigated computationally by introducing a thermal equilibrium model for direct-contact condensation of steam in subcooled water.The condensation model presented was a two resistance model which takes care of the heat transfer process on both sides of the interface and uses a variable steam bubble diameter.The injection of supersonic steam jet in subcooled water tank was simulated using the Euler-Euler multiphase flow model of Fluent 6.3 code with the condensation model incorporated. The findings of the computational fluid dynamics（CFD） simulations were compared with the published experimental data and fairly good agreement was observed between the two,thus validating the condensation model.The results of CFD simulations for dimensionless penetration length of steam plume varies from 2.73-7.33,while the condensation heat transfer coefficient varies from 0.75-0.917 MW·（m ^2 ·K）^ -1 for water temperature in the range of 293-343 K.

Analysis of disagreement between numerically predicted and experimental heat transfer data of impinging jet

The method of numerical simulation was applied to investigate the effects of jet impinging plate thickness and its thermal conductivity on the local heat flux distribution along the impinging plate. The results show that the two factors have great effects on the heat flux distribution. The non-uniformity of the local heat-flux on the impinging plate surface gets more profound as the plate becomes thicker and thermal conductivity gets larger. When Reynolds number is 5000, the ratio of nozzle-to-plate spacing to nozzle diameter is 5 and thermal conductivity is 16W/(m·K), and even for the plate with only 25μm in thickness, the non-uniformity of the heat flux cannot be neglected. When the plate thickness is 50 μm, only when thermal conductivity is as small as 1W/(m·K), the heat flux curve can be approximately treated as an iso-heat-flux boundary. In the experimental research, a real non-iso-heat-flux boundary is treated as an iso-heat-flux boundary, which would result in under-estimated Nusselt number value in the stagnation zone and an over-estimated value outside. Such an experimental Nusselt number distribution is taken to evaluate turbulent model, and the conclusion would be drawn that the turbulent model over-predicts the stagnation heat transfer. This is one of the important reasons why many literatures reported that k-ε turbulent model dramatically over-predicts the impinging jet heat transfer in the stagnation region.

Numerical Study of the Mixing of Density-Stratified Fluid with a Jet

Density stratification of LNG （liquefied natural gas） is produced in a storage tank when one LNG is loaded on top of another LNG in the same tank. Mixing LNG by a jet issued from a nozzle on the tank wall is considered to a promising technique to prevent and eliminate stratification in LNG storage tanks. This study is concerned with the numerical simulation of a jet flow issued into a two-layer density-stratified fluid in a tank and the resultant mixing phenomena. The jet behavior was investigated with the laboratory-based experiment of the authors＇ previous study. A numerical method proposed by the authors is employed for the simulation. The upper and lower fluids are water and a NaCl-water solution, respectively, and the lower fluid is issued vertically upward from a nozzle on the bottom of the tank. The Reynolds number （Re） defined by the jet velocity and the nozzle diameter ranges from 95 to 2,378, and the mass concentration of the NaCl-water solution Co is set at 0.02 and 0.04. The simulation highlights the jet-induced mixing between the upper and lower fluids. It also clarifies the effects of Re and C0 on the height and horizontal spread of the jet.

Numerical simulation of flow field of single-hole oxygen lance jet

The numerical simulation of jet flow field in Laval tube was carried out first by commercial software CFX4.4, and it is used to determine inlet pressure condition out of nozzle in order to its numerical simulation. The decay rule of jet middle line velocity of axial symmetry turbulence jet and cross section＇s expand situation out of the tube in different stagnation pressure and surrounding temperature were analyzed systematically. The result showed that K-e model is suitable for numerical simulation. The conclusion will have important guide and referent function to research of two important craft parameters, impact depth and the impact area which are related to steel-making production practice.

Numerical simulation of sandy bed erosion by 2D vertical jet

A two-dimensional numerical model of vertical jet scour was developed based on the turbulent flow theory and jet scour mechanism. In this model, drag force acts as the main reference variable and the critical Shields number acts as the incipient motion criteria of sediment. The morphological change in the bed caused by bed-load transport is simulated using the moving mesh method and the suspended-load is not considered. An experiment investigating vertical clear water jet scour was conducted in the laboratory, and some effective experimental results, such as flow pattems and distribution characteristics of scour pits, were obtained. Numerical simulation of the 2D jet scour was conducted using the same parameters as were used in the flume experiment. The evolution process of the jet scour observed in the experiment was simulated by the new model; validation of the numerical model and the algorithm was conducted. Semi-empirical formulas of the characteristic length of the equi-librium scour hole were deduced based on the results of the experiment and simulation.

Experimental and numerical study on the flapping motion of submerged turbulent plane jet

A submerged,vertical turbulent plane water jet impinging onto a free surface will be self-excited into a flapping oscillation when the jet velocity,leaving the jet orifice,exceeds a critical value.The flapping phenomenon was verified simultaneously in this paper by laser Doppler velocimeter measurement and numerical analyses with volume of fluid approach coupled with a large eddy simulation turbulent model.The general agreement of mean velocities between numerical predictions and experimental results in self-similar region is good for two cases:Reynolds numbers 2090 and 2970,which correspond to the stable impinging jet and flapping jet.Results show that the flapping jet is a new flow pattern for submerged turbulent plane jets with characteristic flapping frequency,and that the decay of the mean velocity along the jet centerline is considerably faster than that of the stable impinging state.

The measurement error analysis when a pitot probe is used in supersonic air flow

Pitot probes enable a simple and convenient way of measuring mean velocity in air flow. The contrastive numerical simulation between free supersonic airflow and pitot tube at different positions in supersonic air flow was performed using Navier-Stokes equations, the ENN scheme with time-dependent boundary conditions （TDBC） and the Spalart-Allmaras turbulence model. The physical experimental results including pitot pressure and shadowgraph are also presented. Numerical results coincide with the experimental data. The flow characteristics of the pitot probe on the supersonic flow structure show that the measure- ment gives actually the total pressure behind the detached shock wave by using the pitot probe to measure the total pressure. The measurement result of the distribution of the total pressure can still represent the real free jet flow. The similar features of the intersection and reflection of shock waves can be identified. The difference between the measurement results and the actual ones is smaller than 10%. When the pitot probe is used to measure the region of L=0-4D, the measurement is smaller than the real one due to the increase of the shock wave strength. The difference becomes larger where the waves intersect. If the pitot probe is put at L=SD-10D, where the flow changes from supersonic to subsonic, the addition of the pitot probe turns the original supersonic flow region subsonic and causes bigger measurement errors.

A decoupled procedure for convection-radiation simulation in scramjets

Following an order analysis of key parameters, a decoupled procedure for simulation of convection-radiation heat transfer problems in supersonic combustion ramjet(scramjet) engine was developed. The radiation module of the procedure consisted of Perry 5GG weighted sum gray gases model for spectral property calculation and discrete ordinates method S4 scheme for radiative transfer computation, while the flow field was computed using the Favrè average conservative Navier-Stokes(N-S) equations, in conjunction with Menter's k-ω SST two-equation model. A series of 2D supersonic nonreactive turbulent channel flows of radiative participants with selective parameters were simulated for validation purpose. Radiative characteristics in DLR hydrogen fueled and NASA SCHOLAR ethylene fueled scramjets were numerically studied using the developed procedure. The results indicated that the variations of spatial distributions of the radiative source and total absorption coefficient are highly consistent with those of the temperature and radiative participants, while the spatial distribution of the incident radiation spreads wider. It also demonstrated that the convective heating is significantly affected by the complexity of the flow field, such as the shock wave/boundary layer interactions, while the radiative heating is simply an integral effect of the whole flow field. Although the radiative heating in the combustion chambers reaches a certain level, an order of magnitude of 10 k W/m2, it still contributes little to the total heat transfer(<7%).

Numerical research on mixing characteristics of different injection schemes for supersonic transverse jet

A numerical method using AUSMDV scheme and k-ω SST turbulence model with an explicit compressibility correction was developed,and a 3-D numerical simulation of a supersonic flow field with a vertical sonic jet of hydrogen was performed.Good agreement between numerical results and experimental data validated the reliability of the numerical method.Whereafter,two parameters,mass-weighted average total pressure and mixing efficiency,were defined to evaluate the mixing performance of different injection schemes.Based on the numerical method and evaluation criterion,the mixing characteristics of different injection schemes were studied in detail.It was found that for the mixing field of supersonic transverse jet,the near-field mixing is controlled by convection transport while the far-field mixing is controlled by mass diffusion;the circular-hole injection causes a loss of total pressure comparable to the slot injection,but can induce a much higher mixing efficiency because of its 3-D flow characteristic;the variation of injection angle under circular-hole injection mainly affects the near-field mixing degree,and among the five injection angles studied in the present paper,angle 120° is the optimal one;with the increase of the ratio between injector space and diameter,the induced mixing efficiency increases while the caused loss of total pressure can grow greatly;the two-stage injection method designed through reducing the injector area to keep the same hydrogen mass flowrate can induce a much higher mixing efficiency while only a bit larger loss of total pressure when compared to the single-stage injection,and hence the two-stage injection is superior to the single-stage injection.The research results can direct the design of the fuel injection method in the combustor of scramjet engine.

The influence of synthetic jet excitation on secondary flow in compressor cascade

The detailed numerical simulation has been carried out to investigate the effect of synthetic jet excitation on the secondary flow at 5° incidence in a compressor cascade, in which the synthetic jet actuation is equipped on the suction surface. The influence of excitation position, one fixed near the trailing edge and the other fixed a little far from the trailing edge, has also been studied. The results show that unsteady disturbance of desirable synthetic jet effectively enhances the mixing of the fluid inside the separation region, which reduces the vortex intensity and the energy loss, improves the flow status in the cascade, and also suppresses velocity fluctuation near the trailing edge. Additionally, the actuation fixed near the separation region proves to be more effective and exit load distribution is more uniform due to the employment of the synthetic jet.

Turbine Blade Boundary Layer Separation Suppression via Synthetic Jet: an Experimental and Numerical Study

The present paper focuses on the analysis of a synthetic jet device (with a zero net massflow rate) on a separated boundary layer. Separation has been obtained on a flat plate installed within a converging-diverging test section specifically designed to attain a local velocity distribution typical of a high-lift LPT blade. Both experimental and numerical investigations have been carried out. Unsteady RANS results have been compared with experiments in terms of time-averaged velocity and turbulence intensity distributions. Two different Reynolds number cases have been investigated, namely Re = 200,000 and Re = 70,000, which characterize low-pressure turbine operating conditions during take-off/landing and cruise. A range of synthetic jet aerodynamic parameters (Strouhal number and blowing ratio) has been tested in order to analyze the features of control-separated boundary layer interaction for the aforementioned Reynolds numbers.

Investigation of Phase Excitation Effect on Mixing Control in Coaxial Jets

Detailed unsteady numerical simulation has been carried out to investigate the mechanism of adjacent syntheticjets and the influence of different phases on the mixing of coaxial jets. The results show the combined jet, formedby coupling the vortex pairs at the orifice of two adjacent actuators, can exhibit better controlling effect. Spanwisepressure difference appears because of the existence of phase difference between the left jet and right jet, whichresults in the variation of the combined jet. When the phase difference is greater than zero, mixing enhancementof coaxial jets can be achieved, but there are maximum phase difference and optimal phase difference. On thecontrary, application of adjacent synthetic jets always leads to the mixing reduction when phase difference is lessthan zero.