Nonlinear analysis of hydrodynamic instability in laminar flames—II. Numerical experiments

We consider a nonstationary nonlinear integrodifferential equation describing the evolution of a disturbed plane flame front. The following two-dimensional problems are solved numerically: (i) The formation of a wrinkled noncellular laminar flame front under conditions of spontaneous hydrodynamic instability. (ii) The formation of a turbulent cellular flame front under conditions of spontaneous diffusional-thermal instability. (iii) The formation of a turbulent wrinkled-cellular flame front under conditions of spontaneous hydrodynamic and diffusional-thermal instability. We compute the flame front velocity in laminar and turbulent propagation.     

同轴喷嘴自发激励高频燃烧不稳定性试验研究

为增进对液氧煤油火箭发动机同轴离心喷嘴燃烧不稳定性过程的理解,在大气环境下进行了同轴离心喷嘴的自发激励燃烧不稳定性试验。试验采用单喷嘴敞口模拟燃烧室,高温的氧气和空气混合物从同轴喷嘴的直流喷嘴喷注,高温的煤油蒸气从同轴喷嘴的离心喷嘴喷注。通过逐步改变氧化剂流量使模拟燃烧室内产生自发激励高频燃烧不稳定性,使用脉动压力传感器和黑白高速相机记录稳定和不稳定燃烧工况下的脉动压力和火焰。研究发现：气气同轴离心喷嘴的自发激励高频燃烧不稳定过程呈现“滞后”现象;不稳定工况下的火焰均为脱口火焰,火焰特征长度约等于喷嘴出口到脱口火焰团上沿的距离;气气同轴离心喷嘴燃烧不稳定性的发生原因可以被认为是因混合特征时间与声学特征时间相关。     

On a distorted flame front as a hydrodynamic discontinuity

The radius of curvature of a steady distorted flame often turns out to be considerably larger than the width of the thermal structure of the flame. Thus, even under finite deformations of the flame, its structure remains quasi-one-dimensional. This property enables the propagation velocity of a distorted flame front (relative to the gas) to be determined explicitly as a function of the hydrodynamic field and the physico-chemical parameters of the gaseous mixture. With the aid of this relationship one can try to determine the shape of the front in a model which is external with respect to the flame structure and treats the flame as a density jump in an ideal incompressible fluid. As applications, we consider (1) the structure of a Bunsen cone, (2) the extinction of a flame in a divergent flow and (3) the propagation velocity of a corrugated flame.     

突扩燃烧室在准稳态假设下的熄火过程对比分析

在准稳态假设条件下,运用数值模拟方法对火焰的边区驻留与旁侧驻留的两种突扩类型的亚燃冲压发动机燃烧室的熄火过程进行了分析。通过改变燃烧室进气流量和燃油当量比,对比分析了燃烧室在向熄火的压力下限和贫、富油边界转变过程中的流场特征和火焰形态变化。研究显示,中心突扩燃烧室在压力和燃油浓度下降过程中,熄火过程中表现为火焰锋面的中心收缩;旁侧突扩燃烧室则表现为突扩截面内温度的差异,以致火焰稳定功能的丧失。     

固体推进剂燃烧波温度分布测定

采用相对光强度法测定了双基推进剂、复合推进剂及NEPE（氧化剂为HNIW或HMX）推进剂的燃烧火焰温度分布。结果表明,用相对光强度法测得推进剂的最高燃烧火焰温度比热电偶法更接近推进剂的理论燃烧温度,测试压强越高,最高燃烧火焰温度与理论燃烧火焰温度的误差越小。     

凹腔超声速燃烧室氢气燃烧流场数值模拟

对带长深比为10的凹腔结构的燃烧室二维氢燃烧流场进行数值模拟,燃料喷注方式采用凹腔上游喷注加辅加凹腔前壁、底壁、后壁喷注。采用三阶MUSCL格式求解二维含组分守恒N-S方程组,湍流模型采用剪切修正的RNGk-ε湍流模型,对喷氢燃烧工况进行了计算研究,并分别分析了凹腔中不同燃料喷注方式对燃烧特性的影响。结果表明：凹腔是火焰驻留的主要区域;凹腔上游喷注氢,可以使燃料在凹腔中混合燃烧,辅加凹腔中喷氢的三种方式对燃烧状况产生一定的影响。在凹腔前壁、底面辅加喷氢,没有增强凹腔的稳焰特性,对整个燃烧状态影响不大;在凹腔后壁喷氢,能够增加凹腔中的燃料含量,加强了回流效果,对燃烧状态影响较大。三种喷注方式都没有从根本上改变凹腔燃烧流场的特性。     

流量调节器在泵压式供应系统中的动力学特性

针对某型流量调节器及泵压式供应系统,建立了描述其动态特性的频域分析模型,研究系统在出口压力扰动下的频率响应特性以及系统的固有稳定性.结果表明调节器在系统中的位置对系统高频范围内的频率特性影响很大.当供应系统总压降保持一定,增大出口局部流阻的压降能降低系统的谐振峰.当出口局部阻力较小,管路长度比例合适时,系统能够出现自发的不稳定.出口局部阻力越低,系统的总管路长度越大,则系统稳定性越差,不稳定的管路长度比例区间就越大.系统产生不稳定的机理是,在合适的管路长度比例下,调节器第二道节流口所分成的两截管路的声学频率相匹配,且流量调节器处于固有频率的压力波腹,滑阀始终受到频率一致、较大幅值的脉动压力的作用,使得滑阀在固有频率下产生明显的随动响应,对系统形成正反馈.在系统的阻尼耗散作用不足时,形成了耦合的不稳定系统.     

预燃室低频不稳定燃烧仿真研究

为弄清燃烧不稳定性激励机制,建立了氢氧预燃室低频燃烧稳定性分析数学模型,利用MATLAB/SIMULINK集成软件包搭建仿真平台,研究了与供应系统耦合的低频不稳定燃烧。结果表明:在各种不稳定激励因素中,氧的喷注压降对稳定性影响最为显著,当其值低于某一稳定边界时,出现极限环振荡,提高氧的喷注压降可以非常有效地抑制低频不稳定燃烧;当氧的喷注压降位于稳定边界附近时,混合比将对稳定性产生一定影响,降低混合比可以较好地抑制低频不稳定;氢的喷注压降对低频燃烧稳定性影响很小。     

白刚玉对微烟推进剂燃烧性能的影响

微烟推进剂不能含有铝粉,故必须加入其他燃烧稳定剂来抑制不稳定燃烧。本文提出了燃烧稳定剂选择的关键性指标——粒度及粒度分布的可控性,认为磨料微粉白刚玉(Al_2O_3)是微烟推进剂比较合适的燃烧稳定剂新品种,进而研究了白刚玉粒度、含量对推进剂燃速、压强指数和燃烧稳定性影响的规律性,探讨了燃烧前后白刚玉粒度变化的趋势,并由实测火焰结构证实,该推进剂与双基推进剂火焰结构相似,从而为该类推进剂选择催化剂提供了理论依据.     

Experiments on velocity augmentation of spherical flames by grids

Using spherical wire mesh screens with a wire diameter d and a mesh size l, an experimental study was carried out involving homogeneous mixtures of propane/air, ethylene/air, acetylene/air and with oxygen enriched air. Streak photographs of the processes were taken, yielding information on the flame propagation speeds inside and outside the screen. The ratio of the two speeds immediately before and after crossing the screen, α, can be interpreted as practically equivalent (for the same “heat release”) to the ratio of flame velocities relative to the unburned gas, i.e. Values of (α − 1) are correlated with the Reynolds number based on the wire thickness d, density and viscosity of the unburned gas and flame propagation speed inside the wire mesh. The experiments were repeated with two and three spheres of different diameters, e.g. inner screen 9 cm in diameter and outer screen 18 cm in diameter or three screens, 9, 13.5 and 18 cm, respectively. The second screen usually added a factor of 2 on the value of α. The third screen produced very little additional effect. The highest value of α we ever obtained for mixtures with air was about 12 (for acetylene/air). With oxygen enriched air the situation is quite different. If the flame velocity of the laminar flame relative to the unburned gas exceeds 2 m/sec, the value of α may increase up to a level corresponding to detonation.     

Solar wind inhomogeneity front inclination effect on properties of front-caused long-period geomagnetic pulsations

On the basis of data of two networks of Canadian stations and also of extra- and intra-magnetospheric satellites, daytime long-period geomagnetic pulsations related to sudden impulses of the dynamic pressure of the solar wind (SW) are studied. The influence of SW parameters, interplanetary magnetic field (IMF), and geomagnetic activity on the propagation direction, polarization, and amplitude of pulsations is discussed. It is shown that at arrival front of the solar wind inhomogeneity at the place of its tangency, surface oscillations within the range of Pc5 geomagnetic pulsations are excited on the magnetopause, and they run away from the tangency point to the nighttime side with increasing amplitude and opposite polarization. The pulsation properties and the position of the running-away point are explained by the mechanism of their excitation on the magnetopause by the inclined front of the inhomogeneity and also by the Kelvin-Helmholtz instability. Increases in SW density observed ahead of the shock front were able to cause pulsation excitation onsets prior to the sudden storms commencement (SSC) front arrival. The observed increase in geomagnetic activity after SSC could change the direction of pulsation propagation from anti-sunward to sunward. The analysis of oscillation spectra made it possible to assume that pulsations with a frequency of the order of 2.5 mHz are of a global character, they are not related to oscillations in SW and are excited by sharp SSC fronts.     

复合固体推进剂裂尖复合型变形场和约束场分析

用有限元法分析了不同Ⅰ-Ⅱ复合型裂端的变形场和约束场（裂端形状改变能和应力三维度水平Rσ）分布。结果表明：复合型裂端变形为不对称的相反变形,符合钝化-锐化模型。Ⅰ-Ⅱ复合加载时裂纹尖端出现负约束区域（该区Rσ〈0）,当Ⅱ型分量增加,负约束区域增大,最大约束水平Rσmax降低,且绕裂纹尖端顺时针转动。Rσmax出现距裂纹尖端有一定距离处。Ⅰ-Ⅱ复合载荷下偏离原裂纹方向的裂纹在钝化区内某处危险点开始萌生,并与应力三维度最大位置处产生的空穴或孔洞聚合向前扩展。     

超高速撞击声发射信号在Whipple结构中的传播时序特性研究

为研究屏/舱声发射信号传播时序特性,以典型Whipple防护结构为例,利用二级轻气炮对其进行超高速撞击实验。首先,利用独立于靶件的“遮挡板”阻挡弹丸击穿前板形成的二次碎片云撞击后板,利用布置在前、后板特定位置的超声换能器采集单纯的前板声发射信号,分析信号模态特征,结果表明：前板信号主要包括S0、A0和S2等三种模态板波,经圆柱支撑构件传播进入后板之后,全部转换为A0模态板波。在此基础上,建立了屏/舱声发射信号到达时序预测公式。其次,撤除遮挡板,利用布置在后板特定位置的超声换能器采集前、后板信号的混叠信号,分析两种信号的到达时序并与预测结果进行对比,结果验证了达时序预测公式的有效性。     

Effect of reflected ions on the formation of the structure of interplanetary quasi-perpendicular shocks for Mach numbers lower than the first critical mach number

Based on the data of the BMSW instrument installed on the of SPEKTR-R spacecraft, as well as according to the data of instruments of the WIND spacecraft, etc., using two examples, the paper has studied the role of ions reflected from the front and associated structural features of quasi-perpendicular interplanetary shocks (IS) with the Alfvén Mach number М _A lower than the first critical Mach number М _c1 . It has been shown that BSs with the finite parameter 0.1 < β₁ < 1 contain a small fraction of reflected protons, which play a significant role in forming the front structure (β₁ is the ratio of gas-to-magnetic pressure before the shock front). In particular, in the case of a perpendicular shock recorded on August 24, 2013 (the angle between the magnetic field direction and the normal to the front θ_Bn ≈ 85°), an IS with a small Mach number (М_A ≈ 1.4) and small β₁ ≈ 0.2 is shown that the interactions of reflected ions with inflowing solar wind may result in the collisionless heating of ions in front of and behind it. The case of the oblique (θ_Bn = 63°) IS on April 19, 2014 with a small Mach number (М ^A ≈ 1.2) and small β₁ ≈ 0.5 has been investigated. It has been found that, before the front, there is a sequence of trains of magnetosonic waves, the amplitude of which decreases to zero upon increasing their distance from the front. The mechanism of their formation is associated with the development of instability caused by the ions reflected from the front.     

A numerical study of the charge diameter effect on unsteady detonation

A numerical study for the unsteady detonation of an unconfined tetryl charge of small diameter, which is assumed to be homogeneous, was performed by using the two-dimensional Lagrangian hydrodynamic computer code, 2 DL, with the first order Arrhenius equation of reaction rate. Becker-Kistiakowsky-Wilson (BKW) and Kihara-Hikita (KH) equations of state have been applied to the detonation products.In the case of BKW, it is shown that the rarefaction waves propagating inward from the lateral surface make the reaction rate slow and give a curvature to the front. Then after an induction time, a strong initiation occurs in the reaction zone near the lateral surface and higher pressure zone moves to the axis. This higher pressure accelerates the detonation propagation near the lateral surface and the curvature of detonation front is reduced. Then, the reaction at the lateral surface again begins to decay by the rarefaction waves. Such a sequence of process is repeated periodically.The possibility of the occurrence of the strong initiation depends on the pressure and temperature in the shocked zone near the surface. In a small diameter charge, the delayed explosion becomes weaker near the surface, while its frequency increases. No shock interaction occurs because the direction of the particle flow is always divergent.In the case of KH equation of state, the temperature of detonation is higher than that obtained by BKW and the behaviour of instability becomes rather different from the previous result, i.e. in the axis the pressure oscillates repeating the overdriven and underdriven detonation similar with the case of BKW.     

煤油/氧气同轴离心喷嘴燃烧数值模拟

为了更好地了解同轴离心喷嘴的工作特性,基于DDES模型研究了油气比分别为0.5、1、1.5下以煤油/氧气为推进剂的喷嘴的流体动力学特性与非预混燃烧特征。研究结果表明:由于旋流离心作用,在喷嘴出口轴心处和燃烧室顶部分别存在一个驻定涡和角涡,驻定涡径向分布在0.9 R~1.4 R,轴向尺寸在-1 R~14 R,随着燃料流量增大,驻定涡会向喷嘴内部推进,并且径向尺寸也会扩大。燃烧计算结果表明,随着燃料流量增大,推进剂的掺混拖曳区变长、掺混效果变好;而由于油气比的增加,燃烧室更加富油因此燃烧温度有所下降,同时火焰前锋向喷嘴内移。     

壁面孔型对超声速气流中喷注特性的影响

超声速气流中,燃料与来流空气的高效混合是燃烧室实现点火、稳焰及高效燃烧组织的前提。国内外研究者已对比研究了不同壁面孔型对超声速气流中喷注、混合特性的影响,相比于最常见的圆形喷孔,菱形、楔形-半圆、箭形及针形等喷孔用于超声速气流燃料喷注时,不仅有利于降低喷孔前缘边界层的分离,而且也有利于提升射流穿透深度;相比于单孔喷注,组合型喷孔能进一步增强燃料与来流空气在射流远场的混合效果。通过综述各型喷孔的喷注特性,分析提出了适用于超声速燃烧组织的壁面喷注孔型及其工程应用条件。     

航天飞行器防热部件烧蚀行为的数值模拟

对航天飞行器防热部件在氧－煤油发动机火焰喷吹下的烧蚀行为进行了有限元数值模拟。利用“杀死”单元的方法建立防热部件瞬态温度场的有限元模型，实现了烧蚀边界的退缩，从而完成了对烧蚀尺寸变化的定量描述。烧蚀开始于4．59s，到12s时线烧蚀量为1．47mm。计算结果与试验的实测结果一致。     

Comparison of the roles of nucleotide synthesis, polymerization, and recombination in the origin of autocatalytic sets of RNAs

Ribozymes that act as polymerases and nucleotide synthases are known experimentally, even though no fully self-replicating system has yet been found. If the RNA World hypothesis is true, ribozymes must have arisen initially from within a random abiotic polymerization system. To investigate the origin of the RNA world, we studied a mathematical model of a chemical reaction system describing RNA polymerization. It is supposed that, in absence of ribozymes, polymerization occurs at a small spontaneous rate, and that in the presence of polymerase ribozymes, polymerization occurs at a faster rate that is proportional to the ribozyme concentration. Chains must be longer than a minimum threshold length in order to have the possibility of acting as ribozymes. The reaction system has two stable states that we term dead and living. The dead state is controlled by the small spontaneous rate and has negligible concentration of ribozymes. The living state has high concentration of ribozymes, and the reaction rates are determined by the ribozymes; thus, the system is autocatalytic. Concentration fluctuations in a finite volume can cause a transition to occur from the dead to the living state, that is, an origin of life occurs within this model. We also consider ribozymes that catalyze nucleotide synthesis. We show that living and dead states arise in the presence of synthase ribozymes in the same way as for polymerases. It has been proposed that recombination reactions are a way of generating long RNA chains in the early stages of life. We show that if the possibility of random reversible recombination reactions is added to our model, this does not lead to an increase in long polymer concentration. Thus, if recombination is fully reversible, there is no autocatalytic state controlled by recombination. Nevertheless, recombination can play an important role in ribozyme synthesis if there is an additional process that keeps the recombination reactions out of equilibrium. We modeled a case studied experimentally in which building block strands of moderate length associate due to RNA secondary structure formation. A recombination reaction then occurs between these strands to form a longer sequence that catalyzes its own formation via the recombination reaction. This system has an autocatalytic state, and it is possible for it to arise within our random polymerization system. If complexes formed by associations of shorter strands can act as catalysts without the requirement that the strands be covalently linked, this would alleviate the need for synthesis of very long strands; hence, it makes the emergence of an autocatalytic system from an abiotic random polymerization system much more likely.     

Broadband light source for fiber-optic measurement system in spaceborne applications

Measuring temperatures, mechanical loads and derived quantities precisely and reliably play an important role in spaceflight. With spacecraft becoming increasingly complex, upscaling of present telemetry techniques can become cumbersome. Additionally, there are entirely new sensory requirements, resulting from emerging technologies such as smart structures, active vibration damping and composite material health monitoring. It has been demonstrated in preceding studies that these measurements can be advantageously and efficiently carried out by means of fiber-optic systems. The most prominent fiber-optic strain and temperature sensor is the fiber Bragg grating. Typically, multiple fiber Bragg gratings are used to translate entire temperature and strain fields into an optical wavelength information. For the interrogation of these sensors, a broadband or scanning light source is required. Additional requirements with respect to the light source are high intensity and unpolarized illumination of the gratings. These constraints can be met by a light source that is based on amplified spontaneous emission in a rare-earth-doped fiber. In the presented work, a compact light source, adapted for measurement applications and targeted towards space applications, has been developed. The design of this light source is presented, as well as its implementation. The light source has been designed and tested for selected core aspects of space robustness and the results of these tests are summarized.