Monopropellant droplet decomposition for large activation energies

The quasi-steady decomposition of a monopropellant droplet in a quiescent atmosphere is analyzed, in the limit of large activation energy, for a gas phase decomposition reaction of the Arrhenius type.An analytical relation is obtained for the pre-exponential rate constant, or the Damköhler number, as a function of the droplet vaporization rate. The curve giving the vaporization rate in terms of the Damköhler number has, for low values of the temperature at infinity, an S shaped form which exhibits ignition-extinction characteristics.With increasing values of the droplet vaporization rate above the pure vaporization value we pass from a nearly frozen regime, under which ignition conditions occur due to the large temperature sensitivity of the reaction rate, to a complete decomposition regime, in which practically all the fuel is decomposed in a thin reactive-diffusion zone, separated from the droplet surface by a transport region of lower temperature. For sufficiently large Damköhler numbers, the thin reaction zone is located close to the droplet surface, so that the flame structure becomes planar: a linear relation between the droplet radius and time is obtained in this case. The non planar effects are responsible for the extinction of the flame at low Damköhler numbers and ambient temperatures below the adiabatic flame temperature. For sufficiently low values of the ambient temperature, an intermediate regime exists in which the enhancement of the vaporization rate is due only to the fraction of the vaporized fuel decomposed at a thin reaction zone, not far from the droplet, where the temperature reaches its maximum value; the remaining fuel is decomposed very far from the droplet.     

Numerical modeling of unsteady flame propagation

The problem of the numerical modeling of unsteady flame propagation with large amounts of exothermic heat release has been addressed through the use of a model equation which contains many of the important nonlinearities and difficulties in the problem. The model equation has been solved for a significant variation in Damköhler number with a variety of conventional and new finite-difference procedures. One of the new procedures used was an adaptive grid method which places node points in the region of large temperature gradients where they are necessary for a proper simulation of the phenomena. This adaptive grid procedure does have significant advantages over conventional second and fourth order uniform grid methods for many aspects of the problem. It has been found that conventional methods with relatively few node points are inadequate for flames with high Damköhler number, because of the fact that too few nodes are located in the flame. Further refinement of the adaptive grid method seems feasible, but even in the present form it represents a significant improvement over conventional techniques.     

The turbulent combustion of a propane-air mixture

This paper models the combustion of a turbulent homogeneous mixture of propane and air within a duct having a stationary one-dimensional mean flow. The Bray-Moss model is applied to the closure of the chemical production terms, using a probability density function (pdf) of the temperature which is chosen as the characteristic variable. Under the conditions chosen for the study, chemical kinetic factors are important and the conventional assumption, that heat release is controlled by turbulent mixing, is not valid. The chemical model of Edelman and Fortune for the combustion of hydrocarbons is used and simplifying assumptions are made which reduce the systems of unknowns to that of the temperature alone. This leads to the introduction of two chemical production terms which are defined respectively in a “delay zone”, where the heat release is modest, and a “combustion zone”. The required equations for the Favre-averaged temperature, turbulence kinetic energy and the mean square fluctuation of the temperature are solved numerically. In the delay zone, a comparison is made between a second order Borghi type closure and the pdf closure. Good agreement is found in the case of relatively small turbulence intensity. It is shown that the pdf formulation does not require the two zones to be spatially distinct. Differing chemical source terms can be discriminated instantaneously by the reaction progress variable and contributions to the average production terms appropriately apportioned by its pdf. Predictions are made of the profiles of mean temperature and mean square fluctuation under different initial turbulence levels.     

大长径比固体火箭发动机点火瞬态过程性能散布分析

采用均匀设计方法设计数值试验,研究大长径比固体火箭发动机点火瞬态过程性能散布特性,利用逐步回归法得到堵盖打开时间、推进剂初焰时间、点火压强峰值及时间4个性能参数与散布影响因素的回归关系式,分析了各项性能散布指标与其主要影响因素的关系。结果表明,堵盖打开时间散布取决于堵盖强度,推进剂初焰时刻散布由其着火临界温度控制,点火瞬态压强峰值时刻散布由推进剂密度和喷喉直径控制,点火压强峰值散布主要受推进剂密度、着火临界温度和喷喉直径的影响;并提出了降低散布的工程措施。     

On gas-phase ignition of diffusion flame in the stagnation-point boundary layer

An analytical expression for gas-phase ignition is developed for a diffusion flame in the two-dimensional or axisymmetric stagnation-point boundary-layer flow of a hot oxidizing gas about a vaporizing condensed fuel surface. The analysis is based on the limit of large activation energy for a one-step, irreversible reaction describing the overall combustion process in the gas phase. The approach in this work, following that of our recent analysis on extinction for the same geometry, is to seek an exact correspondence of the parameters of the present problem with those of counterflow diffusion-flame problem of Liñán. Such a correspondence has been found in the frozen-flow regime and as a consequence, the asymptotic structure of the flame in the nearly frozen ignition regime is identical in both the problems. A particular result of this observation is the availability of an analytical criterion for ignition in the present problem. The analysis reveals that contrary to the case of extinction, fluid dynamic details do not have significant effect on the ignition criterion and that Liñán's results may be applied with good accuracy to the condensed fuel problem.     

An experimental study on the stability of jet diffusion flame

The stability behavior of jet diffusion flame developing in a coflowing high temperature air stream was studied experimentally, using city gas and hydrogen as fuel gases. The primary variables studied were temperature and velocity of the air stream. Two distinct types of stability limits were observed. The first is the blow-off of the rim-stabilized flame, which is familiar and has been studied by a number of investigators. The second is the break-off or extinction of the turbulent portion of the flame at the transition point from laminar to turbulent flow. The second limit is quite unfamiliar and is interesting in relation to the structure of turbulent diffusion flames. The important implications of the experimental findings concerning the stability and the structure of turbulent diffusion flames is discussed.     

Calculation of ignition and combustion of a hydrogen jet in air with finite chemical reaction rates

A comparison between two analytical models of a free turbulent mixing of reacting streams with finite chemical reaction rates is presented and the results of calculations compared with experimental data. The substance is a mixture of the inert non-dissociated nitrogen and oxygen that dissociates and reacts with hydrogen.For the first model the local values of flow parameters along the streamlines were provided by the numerical solution of the conservation equations in the boundary layer form for a multicomponent mixture of perfect gases in the von Mises coordinate system. In the course of this analysis the problem was treated by dividing the flowfield into a large number of regions and by solving the Cauchy problem of the conservation equations for each region.The second analytical model was based on the ignition delay to describe the chemistry in the region of pre-ignition. The external air stream was heated by combustion and the influence of the water vapor and the intermediate species on the reaction kinetics in the mixing zone was numerically investigated.The results of calculation are compared with the experimental data with respect to the location of a visible flame edge under combustion of hydrogen jet in the co-current flow.     

高空模拟试车台主被动引射方案数值研究

采用二维轴对称雷诺平均方程和Spalart-Allmaras湍流模型,对一大面积比火箭发动机高空模拟试车台在不同引射方案下发动机启动至稳定运行过程中不同室压下的流场结构进行了数值模拟.空间上采用二阶迎风格式进行耦合求解,时间上采用显式Runge-Kutta方法进行迭代推进.结果表明,采用启动段主动引射、主级段被动引射的...     

多饱和约束拦截弹参数时变系统控制器设计

针对拦截弹参数时变和执行机构饱和非线性影响控制系统性能,提出基于齐次多项式参数相关 Lyapunov 理论和非线性矩阵微分方程凸多面体转换算法相结合的鲁棒最优控制系统设计方案。 通过引入标准饱和约束算子,建立控制系统数学模型并给出控制系统结构;引入饱和归一化因子,把非线性参数时变微分方程转化成线性多胞型微分方程;基于齐次多项式参数相关Lyapunov理论和拓展Pólya’s定理,把参数时变多胞型微分方程转换成齐次多项式参数相关微分方程,并给出齐次多项式参数相关控制器结构;利用迭代算法把非线性矩阵不等式转换成线性矩阵不等式(LMIs),进而基于LMI凸优化理论获得控制器解,设计出的控制器是关于齐次多项式阶次g和Pólya’s松弛度d的函数;通过定点和制导控制系统仿真校验了该算法,仿真结果表明随着齐次多项式阶次g和Pólya’s松弛度d的增加,设计出的控制器在确保系统稳定的前提下改善控制系统性能。     

Thermochemical closure for high-speed flows

A model for the thermochemical closure in high-speed turbulent flows with nonpremixed reactants is presented, based on relatively lenient assumptions involving, inter alia, equilibrium chemistry, and the ratio ρ/p being pressure-independent. The proposed approach is based on the introduction of a presumed joint pdf of the conserved scalar and an appropriately defined nondimensional enthalpy gap, for which plausible shapes are assumed. The model is applied to a test case concerning a supersonic hydrogen/vitiated air flame for a check of its consistency; results indicate good performance for both the mean and rms of state quantities. The assumption of pressure independence is verified, and an indication of errors involved in overlooking thermochemical closure is also given. An extension to flows involving finite-rate chemistry and differential diffusion is also briefly presented, though requiring a quite extensive flamelet library.     

热声载荷下高温合金薄壁结构非线性动态响应特性

针对航空航天薄壁结构热声疲劳问题,采用解析法和有限元法相结合,研究矩形薄板样件的非线性动态响应特性。首先,推导了四边简支矩形板在热声载荷下的四阶非线性偏微分大挠度运动方程,并利用伽辽金法将其转化为模态坐标系下的二阶微分方程组;对该方程组进行单自由度简化,计算并分析了平面温度与温度梯度对矩形板振动响应的影响。然后,采用有限元方法计算出四边简支矩形钛合金板在不同温度下的模态频率,以及定常声压级、不同温度下的振动位移和应力响应,并着重对位移功率谱密度进行了详细分析。基于上述计算结果,对响应基频随温度变化关系进行了对比分析,并研究了矩形板屈曲和跳变响应特征。     

推进剂发火温度测定方法的改进

基于推进剂发火温度测试的需求,针对国军标测试方法和现有技术的发展,根据推进剂燃烧产生气体的基本性质,建立了推进剂点火温度测试方法和测试装置,测得定容燃烧器中压强突增点对应的温度为推进剂的发火温度.试验表明,以承压较高的金属容器作为试验器,增加了试验的安全性;以压强的骤增点作为推进剂样品的点燃点,可以实现点火温度的准确、...     

Radiant ignition of a combustible solid with gas-phase exothermicity

An asymptotic analysis is presented for the ignition of a solid fuel that gasifies endothermically then reacts exothermically in the gas phase through a one-step Arrhenius process. The endothermic gasification was analyzed in an earlier paper; the present paper employs those results in treating the gas-phase reaction. It is shown that depending on values of the chemical parameters, the gas-phase ignition reaction may begin either at the time that gasification begins or during the later stage of transport-controlled gasification. In the former case, results for the gas-phase ignition time are similar to those for condensed-phase ignitions. In the latter case, a mixing layer develops in the gas-phase and moves away from the surface, the exothermic reaction being negligible until thermal runaway occurs somewhere within the mixing layer. Both early-time and late-time ignitions within the mixing layer are analyzed by solving partial differential equations derived from perturbations for large activation energies and a small ratio of gas to solid thermal responsivity. The theory serves to indicate how ignition by gas-phase reactions may be distinguished experimentally from ignition by condensed-phase or heterogenous reactions.     

Prediction of turbulent diffusion flames with a four-equation turbulence model

For turbulent flame prediction a four-equation turbulence model consisting of the turbulent kinetic energy, the dissipation rate and the density-velocity correlations using unweighted statistics is suggested. The fluctuating concentrations are taken into account by using Spalding's (1971) equation for the variance of the mixture fraction f and by defining the probability density function of f as beta-function. The thermodynamic model for the combustion of H₂ with air assumes infinitely fast reaction in a global single step. Preliminary results are shown for the calculation of Kent and Bilger (1972) turbulent H₂-air diffusion flame. For this flame with fluctuating density the inclusion of the velocity-density correlations appears to be essential, because they have a strong effect especially on the development of the turbulent viscosity and the mean velocity and mass fraction profiles. Consequently the mean concentrations are in good agreement with the experimental results.     

PMMA激光点火特性研究

采用CO2激光点火实验系统,研究了常压下PMMA的点火过程以及点火延迟时间与点火热流密度的关系。研究结果表明,随点火热流密度的增加,PMMA的点火延迟时间减少;当点火热流密度大于286 W/cm2时,热流密度对点火延迟时间的影响变小;当点火热流密度较低时,点火延迟时间随着热流密度的减小而急剧增加,且存在点火的最低能量阈值。最后,给出了常压下热流密度范围为70～881 W/cm2内的PMMA点火延迟时间与点火热流密度的数学关系式。     

Nonisotropic turbulence in swirling flows

Recent experimental, inverse and prediction studies have disputed isotropy assumptions for turbulent swirling flows. To cater for the simulation (and hence solution) of these flows, simple turbulence models may be modified or more advanced models developed. This paper reviews recent work in this area, clarifies the entent of nonisotropy and recommends suitable turbulence models to effect closure of the governing Reynolds equations.     

小型固体火箭发动机尾部点火器设计方法

以尾部点火装置安装位置为分类方法,以点火装置燃烧产物与发动机装药的不同换热方式为基础,对典型的小型固体火箭发动机的尾部点火设计方法和设计中需要重点考虑的问题作了较为全面的介绍。以某型号发动机尾部点火设计为研究对象,对点火序列设计、点火药量的估算、喷管堵片的设计方法进行了阐述。通过调整点火药量、喷管堵片设计状态,使点火压强逐步提高,解决了发动机研制初期出现的点火延迟、不点火等技术问题。     

某固体火箭发动机点火启动过程三维流场一体化仿真

以某固体发动机的燃烧室和喷管为一体化研究对象,采用三维流场控制方程,应用有限体积法计算了发动机点火启动过程中燃烧室和喷管内燃气的流场特性。发动机药柱上的着火点最初出现在药柱星角尖上,然后向四周扩展;在药柱点火初期,燃气压力波先于火焰峰到达喷管;随着燃烧室内燃气压力升高,压力沿轴向分布逐渐平缓;当喷管进口压力与出口背压比达到某一值时,喷管扩张段内出现一道激波,随着压力比的升高,激波最终移出喷管,燃气流速在喷管出口处达到最大值。     

双脉冲发动机点火过程三维数值模拟

以喷射棒式双脉冲发动机燃烧室、级间隔离装置和喷管一体化为研究对象,采用数值仿真技术对Ⅱ脉冲点火过程三维流场特性进行分析研究。计算结果表明,点火初期燃气压力波峰超前于火焰峰到达级间隔离装置,并以压强冲击波形式传播,Ⅱ脉冲燃烧室相对高压区位置不断发生改变;级间孔打开过程对药柱末端压强影响较大,但对Ⅱ脉冲燃烧室压强整体上升过程影响较小;级间孔打开后,燃气经级间孔加速后形成高度欠膨胀射流,并在Ⅰ脉冲燃烧室内形成非对称带状低压区;级间孔分布的非对称性,导致压强及温度在发动机燃烧室中呈现显著的三维分布特性;高温区出现在隔板附近,而在装药前端、装药末端及外围级间孔轴线附近出现低温区。     

固体火箭发动机药柱裂纹腔内三维流场瞬态特性分析

以固体发动机药柱内存在的楔形裂纹为研究对象,采用三维流场控制方程,应用有限体积法计算了发动机点火启动阶段裂纹腔内的对流燃烧过程。在裂纹腔侧壁被点燃前,裂纹腔内的燃气压力基本呈均匀分布,且约等于燃烧室燃气压力;在裂纹腔侧壁被点燃后,燃气压力逐渐呈现出上部低、下部高的分布,且腔内平均压力远高于燃烧室内燃气压力;裂纹腔侧壁开口边缘处的推进剂首先达到点火温度开始燃烧,燃面迅速向内推进,燃气以非常高的速度向外流出裂纹腔。