旋转固体发动机燃烧室-喷管两相流数值仿真

针对旋转引起的固体发动机内弹道性能和热防护性能变化影响发动机推力性能,从旋转对发动机内部燃气流动影响出发,用Reynolds时均N-S方程、Reynolds应力方程湍流模型(RSM)和颗粒随机轨道模型,在FLUENT软件上对不同旋转转速的固体发动机燃烧室-喷管内气-固两相流动进行了一体化数值仿真,比较了有无旋转两种状态下的流场结构,研究了转速对流场结构和发动机热结构的影响,并进一步研究了发动机的工作特性.仿真结果表明,发动机旋转使燃烧室内部流场结构发生显著变化,流场结构呈组合涡形式,粒子严重偏离发动机对称轴,导致燃烧室压强升高,推进剂燃速增大,发动机工作时间变短,热防护环境恶化,工作性能降低.这些变化随转速的增加呈现加剧趋势.研究结果为发动机设计提供了一定的技术支持.     

旋转发动机燃烧室头部气-固两相流场结构分析

在固体火箭发动机性能的研究中,采用N-S方程、Reynolds湍流模型和颗粒随机轨道模型建立了旋转固体火箭发动机头部区域气-固两相流数学模型,针对安全性,使用FLUENT软件对不同转速发动机头部区域流场进行了数值仿真,分析和比较了有无旋转两种状态下燃烧室头部区域流场结构,研究了转速对燃烧室头部流场结构及热环境的影响.结果表明:发动机的旋转使燃烧室内部流动与传热发生显著变化,导致燃烧室头部压强升高,热防护环境恶化,而且,变化随着转速的增加而迅速加剧,影响发动机内弹道性能和头部热防护性能,为发动机设计提供安全保障的依据.     

基于CATIA的固体火箭发动机系统设计

发动机图形绘制与性能计算是火箭设计的关键步骤,通过对固体火箭发动机设计过程的分析,提出CATIA参数化图形绘制技术;以单孔管状装药发动机为例,建立了发动机燃烧室与喷管模型,在VC环境下利用CATIA Automation二次开发技术实现了燃烧室与喷管参数化设计,完成了两者装配与动态显示;利用Matcom实现内弹道压力时间曲线计算,发射段轨道分析;最后完成系统软件编制;实际使用情况表明,使用该软件可快速构造发动机,进行全面的性能分析,为固体火箭发动机方案论证与初步设计提供了有力的工具。     

整体式固体火箭冲压发动机转级过程数值仿真

为了研究整体式固体火箭冲压发动机从助推级向冲压发动机级的工况转换(转级)过程,采用双时间步推进方法,求解非定常、三维、有化学反应的Navier-Stokes方程组,得到了发动机进气道/燃烧室一体化流场在转级过程中的变化情况,分析了相关因素对转级过程的影响。仿真研究表明,进气道出口堵盖打开前,进气道内气流保持压力振荡状态,振荡频率主要与飞行高度相关;堵盖打开后,空气与一次燃气进入补燃室掺混并燃烧,过程中发动机内流场发生剧烈的变化;堵盖打开时燃烧室内的压力对于进气道的起动至关重要。上述研究对于整体式固体火箭冲压发动机设计具有指导意义。     

固体火箭发动机喷管两相流场与结构温度场一体化数值模拟与软件实现

为准确、高效地进行固体火箭发动机的喷管结构设计与性能分析,考虑两相流场和结构温度场之间的相互影响,采用有限差分方法求解二维轴对称两相欧拉方程组,进行喷管气体-颗粒两相无黏流动数值模拟;采用有限元法求解二维轴对称瞬态导热微分方程,进行喷管复合结构温度场数值模拟;根据面向对象编程思想,采用VC+ +以人机交互的工作方式实现喷管内型面和结构设计,并完成有限差分网格和有限元网格的自动划分和显示;通过数值模拟方法与面向对象软件设计方法的有效结合,实现二维轴对称喷管两相无黏流场和复合结构温度场的一体化数值模拟. 数值模拟结果表明,该方法有助于在统一的软件平台上充分利用计算机辅助技术完成喷管性能与结构的综合评估,可以用于固体火箭发动机喷管的工程设计.     

基于机器学习建模的液体火箭发动机喷管内型面优化设计

喷管是液体火箭发动机产生推力的重要部件。喷管型面的结构将直接影响燃烧所产生的燃气在喷管中的流动情况,进而对发动机的性能产生影响。采用B样条曲线对抛物面型线进行参数化,计算样本集的流体动力学(Computational Fluid Dynamics, CFD)流场,以比冲为优化变量对喷管性能进行评估。研究表明,基于代理模型优化得到的喷管内型面结构与特征线法计算结果基本一致,比冲计算结果相当,最大误差为0.28%。通过代理模型和网格变形方法,可实现液体火箭发动机喷管内型面优化设计,提高优化效率。     

火箭防暴弹发动机的内弹道解析

研究防暴弹发动机燃烧室优化控制问题,为了实现对火箭防暴弹发动机燃烧室内压强的控制并确定其影响因素,解决燃烧室内各点压强分布不均衡的问题,提出了阶段分析法,并引用“零维”压强这一概念,在选用“零维”内弹道模型的基础上,采用龙格库塔法,利用MATLAB语言仿真技术进行编程测试,对“零维”条件下的内弹道进行数值仿真,并将模拟结果与实验结果进行分析比对,验证了内弹道模型和仿真结果的正确性,得出了在发动机工作过程中,燃烧室内压强随时间的变化规律,明确了影响压强的各个因素,为固体火箭发动机内弹道性能研究以及外弹道的计算提供了理论依据.     

固体火箭发动机金属壳体结构可靠性分析研究

固体火箭发动机壳体是固体火箭发动机的重要组成部分,壳体的可靠性设计水平高低直接影响发动机的性能.好的壳体可靠性设计方案是在保证总体要求的几何尺寸精度的同时,要做到结构质量轻,又要安全可靠.利用一阶可靠性分析法结合有限元法对固体火箭发动机金属壳体封头结构进行可靠性分析,结果表明,在相同受力条件下,不同的结构尺寸下,封头的最大应力可能都不会达到失效界限,但是可靠性指标不一定满足要求.分析结果可用于指导结构单元设计.     

燃气侧喷固冲发动机补燃室流场三维数值研究

采用标准κ-ε二方程模型和涡团耗散模型对固体火箭冲压发动机补燃室内流场进行了三维数值模拟研究.考虑燃气喷射角度,补燃室出口反压对燃烧流场的影响.得出了反应物和产物组分、温度等发动机参数的变化趋势.其中燃气喷射角度分别为30°,45°,60°.结果表明:增加喷射角度,补燃室头部温度升高,燃料完全燃烧所需补燃室长度减少;增大出口反压有利于补燃室内燃烧的进行.该结论对固冲发动机补燃室的理论设计和实验优化有重要作用.     

固体火箭冲压发动机三维数值计算

利用FLUENT中的概率密度函数非预混模型对固体火箭冲压发动机补燃室内的气相湍流燃烧进行数值模拟.主要目的是解决在固体火箭冲压发动机含镁铝推进剂的补燃室中,存在上百种中间及最终产物.复杂的反应机理使采用有限反应速率模型难以模拟补燃室中复杂的湍流燃烧的问题.模拟结果有助于提高对固体火箭冲压发动机补燃室内部流场流动的了解.模拟结果表明:补燃室内发生着复杂的三维化学反应流动,存在对掺混燃烧有重要影响的头部回流和轴向涡流.补燃室内温度分布与空气与燃气的掺混、燃烧及流动状态有密切关系.提高空燃比,可增强补燃室中燃气的回流和轴向涡流强度,加大掺混力度.提高燃烧效率.     

Thermal Investigation for Solid Rocket Motor Nozzle Inserts Material

A time-resolved numerical computational approach, involving the combustion of double-base propellant is performed on thermal protection material for SRM nozzle. An implicit Navier-Stokes Solver is selected to simulate two-dimensional axial-symmetric unsteady turbulent flow of compressible fluid. The governing equations are discredited by using the finite Volume method. S-A turbulence model is employed. CFD scheme is implemented to investigate the temperature distribution causes at nozzle throat inserts comp...     

Simulation of the effect of geometric parameters on tangentially injected swirling pipe airflow

A realizable k–ε turbulence model is employed to study compressible tangentially injected swirling flow in the nozzle of air–jet spinning. The effects of the nozzle geometric parameters (the injection angle, the diameter, number and position of the injector, nozzle length and chamber diameter) on both the flow and yarn properties are investigated. The simulation results show that some factors, such as velocity distribution, reverse flow in the upstream of the injector and vortex breakdown in the downstream caused by the nozzle geometric variation, are significantly related to fluid flow, and consequently to yarn properties. With increase in the injection angle or injector diameter or injector number, in the downstream of the injector, velocities will increase somewhat, and the locations of vortex breakdown move downward. As injector number increase with the total injection area being kept constant, the strength of vortex breakdown in the downstream of the injector will slightly increase. A larger reverse flow will be not helpful to draw the fibers into nozzle, as the injector position is closer to the nozzle inlet. The flow is more turbulent for a larger chamber diameter.     

Vorticity generation and acoustic resonance of simulated solid rocket motor chamber with high wave number wall injection

An asymptotic technique is integrated with computational solution development to describe the generation and evolution of intense unsteady vorticity and the accompanying temperature response in a model of solid rocket motor (SRM) chamber with low Mach number, weakly viscous internal flow. The chamber considered here has a rectangular cross-section, has to equally permeable walls, closed at the head end, and is opened at the downstream end. An initially steady internal flow from steady sidewall injection is generated. Then an additional transient sidewall mass injection with different wave numbers at non-resonance and resonance frequencies is imposed on the generated steady one. This mechanism is used to simulate the transient propellant combustion occurring on the sidewall in a real SRM chamber. The analytical approach is based on the reduced form of the Navier–Stokes using asymptotic technique. The results show that unexpectedly large transient shear stresses and temperature gradients are created at the sidewall of the chamber at resonance frequencies and low wave numbers. A comparison between the analytical, computational and experimental results is performed.     

Investigation of the influence of swirl on a confined coannular swirl jet

Large Eddy Simulations are used to model a turbulent confined coannular combustor and examine the effects of swirl on the flow field and mixing. Three separate simulations with relatively high mesh resolutions and different swirl numbers have been carried out using a finite volume method on a Cartesian non-uniform structured grid. A localised dynamic Smagorinsky model is used to parameterize the sub-grid scale turbulence. The snapshots of the axial and swirl velocities and velocity vector fields show the complex flow patterns developing with increased swirl number and the rapid decay of axial momentum. Precessing vortex cores (PVC) were identified for all three cases and the mean axial velocity plots indicate that the upstream extremity of the vortex breakdown bubble shifts towards the inlet as the swirl number increases. The calculated power spectra indicate the distinct precession frequency for high swirl number. Probability density functions of axial velocity showed the changes of their distributions from approximately Gaussian to non-Gaussian with increased swirl number. The swirl has a large effect on the rate of decay of the axial velocity throughout the domain, whereas only has a significant effect on the decay of swirl velocity in the near field close to the jet inlet. The relation between swirl number and the axial extent of the recirculation zone is approximately linear. Radial plots of mean passive scalar and its variance also demonstrate an increase in the rate of mixing with increasing swirl number.     

Effect of inlet and outlet boundary conditions on swirling flows

Numerical simulations are conducted for both three-dimensional, turbulent flow in a multi-channel swirler and axisymmetric, isothermal, turbulent flow in combustion chambers using the standard κ−ε turbulence model. Calculations are first carried out for three-dimensional, isothermal and turbulent flow inside the swirler channels in order to derive the velocity profiles of both air and gas at the swirler outlets, which are used as inlet boundary conditions of the model combustor and can also be used in future studies for different combustors with the same type of swirler. In order to study the sensitivity of swirling flow inside the chamber to the inlet and outlet boundary conditions, different inlet velocity profiles and outlet boundary conditions are also employed. The results show that in the cases considered, the flow behaviour in the chamber is not very sensitive to the actual shape of the inlet velocity profiles provided the averages of the inlet axial, radial and azimuthal velocity components are separately preserved. Other conditions being equal, we find that the swirling flow performance in the combustor depends not only on the inlet swirl number, but also strongly on the relative magnitude of the radial velocity component at inlet and introduce a new dimensionless number N_r, analogous to the swirl number, to measure the relative importance of this quantity. Outlet boundary conditions have some influence near the outlet, but nearly no effect further upstream for the cases investigated.     

Simulations of confined turbulent vortex flow

Large-eddy simulations (LES) of the turbulent flow in a swirl tube with a tangential inlet have been performed. The geometry, and flow conditions were chosen according to an experimental study by [Escudier MP, Bornstein J, Zehnder N. Observations and LDA measurements of confined turbulent vortex flow. J Fluid Mech 1980;98:49-63]. Lattice-Boltzmann discretization was used to numerically solve the Navier-Stokes equations in the incompressible limit. Effects of spatial resolution and choices in subgrid-scale modeling were explicitly investigated with the experimental data set as the testing ground. Experimentally observed flow features, such as vortex breakdown and laminarization of the vortex core were well represented by the LES. The simulations confirmed the experimental observations that the average velocity profiles in the entire vortex tube are extremely sensitivity to the exit pipe diameter. For the narrowest exit pipe considered in the simulations, very high average velocity gradients are encountered. In this situation, the LES shows the most pronounced effects of spatial resolution and subgrid-scale modeling.     

CFD modeling of the in-cylinder flow in direct-injection Diesel engines

Three-dimensional flow calculations of the intake and compression stroke of a four-valve direct-injection Diesel engine have been carried out with different combustion chambers. A limited number of validation calculations of the compression stroke were first performed in order to explore the limits of CFD representation of the in-cylinder flow. The calculated flow field in three different combustion chambers was compared with laser Doppler velocimetry measurements; the comparison shows that the three-dimensional model is reasonably accurate for crank-angles around top dead center (TDC). In general, it performs better for low swirl combustion chambers while turbulence velocities are under-predicted when squish effects are important.In the main study, the flow characteristics inside the engine cylinder equipped with different piston configurations were compared. For this, complete calculations of the intake and compression strokes were performed under realistic operating conditions and the ensemble-averaged velocity and turbulence flow fields obtained in each combustion chamber analyzed in detail. The results confirmed that the piston geometry had little influence on the in-cylinder flow during the intake stroke and the first part of the compression stroke. However, the bowl shape plays a significant role near TDC and in the early stage of the expansion stroke by controlling both the ensemble-averaged mean and the turbulence velocity fields.     

海水淡化双级真空引射器的射流和混合特性研究

引射器是实现热法海水淡化系统高效节能的重要装置,该文通过三维数值模拟方法研究了双 级引射器内部流动和传输特性,分析了引射和工作流体的混合与扩散过程,探讨了降低能耗的方法。 研究结果表明,喷射器内速度最大值均在轴线处,流体在喷嘴与混合室内流线均匀,而在吸入室内产 生一个或多个涡流。不同引射流体速度下,其混动迹线为不对称分布,随着引射流体速度的增大,引 射流体入口处负压先增加后减小,漩涡量减少且漩涡变小。因此,为改善引射器运行效率,可适当提 高引射流体速度。随着偏离喷嘴中心距离的增大,湍动耗散率明显增大,而湍动能略微增大,漩涡消 散加快。     

Numerical Analysis of a Vortex Tube: A Review

Ranque–Hilsch vortex tube is a simple devise with no moving parts which could generate cold and hot air/gas streams simultaneously with compressed air/gas as a working fluid. The energy and flow separation in a vortex tube is highly depends on factors like nozzle shape, nozzle number, diameter and length of the vortex tube, inlet pressure, control valve, diaphragm hole size and cold mass fraction. As the energy separation and flow patterns in a vortex tube are highly complex and were not explained successfully by any researcher, a computational study of vortex tube flow and energy separation will give a better understanding about the physics and mechanism involved. Many researchers conducted computational fluid dynamic analysis of the vortex to have a deep insight about the process of flow separation. In this paper computational analysis of vortex by many researchers were presented along with the results obtained and suggestions to improve the performance of the vortex tube. Researchers considered Turbulence models which predict the performance precisely were discussed in the present paper. Researchers considered turbulence models like LES, k–ε, k–ω and RMS to predict the energy separation in vortex tube. Some researchers considered artificial neural networks (ANN) and Taguchi methods for their analysis. Comparison of the predictions with simulation results were also presented to give a clear idea for the reader about the CFD models prediction capabilities.     

Numerical prediction of confined swirling jets

The axisymmetric flow of a swirling viscous, incompressible fluid jet inside confining cylindrical boundaries has been numerically investigated using the well-known implicit finite-difference scheme. For a swirling jet confined by cylindrical tube, the vortex-breakdown or the formation of an axisymmetric isolated eddy occurs at high values of swirl ratios at a moderate flow rate or Reynolds number. With the introduction of artificial adverse pressure gradients, such as one studied in the case of a step-up cylindrical tube, the vortex-breakdown occurs at a relatively lower swirl ratio at a given flow rate. For a swirling jet discharging in a coaxial non-rotating surrounding stream enclosed by a cylindrical tube, the vortex-breakdown and its structure depend on various parameters such as the flow rate of the jet, surrounding stream velocity, the swirl of the jet and on the radius of the enclosing cylindrical tube. In general increasing Reynolds numbers, swirl ratio, decreasing surrounding stream velocity and increasing size of the cylindrical tube enhance the occurrence and size of the vortex-breakdown.