调控燃烧室燃料初始分布建立稳定旋转爆轰波的方法

旋转爆轰发动机具有比传统航空航天发动机更高的燃烧效率，近年来引起人们的关注。其中，点火启动过程尤为重要。为达到一次点火就能在燃烧室内建立稳定旋转爆轰波的目的，本文提出通过控制点火前燃料初始分布来建立稳定旋转爆轰波的方法，并基于纳维-斯托克斯方程与10组分27可逆反应基元化学反应模型的数值模拟验证了该方法的可行性。对旋转爆轰波传播特性的研究表明，燃料在发动机燃烧室中的分布是影响旋转爆轰波建立的关键。在燃料喷注压力较低时此影响尤为明显，它决定了爆轰波发展第一周期内波前燃料层厚度。而波前燃料层与波的稳定传播密切相关。基于该方法，本文对燃烧室初始流速为360 m/s，喷注总压0.4 MPa的旋转爆轰发动机实现了点火至稳定爆轰，得到的爆轰波传播平均速度为1 604 m/s，频率为5 347.6 Hz。此外，燃料初始填充率作为燃料初始分布的量化指标，文中给出了它建立稳定旋转爆轰时的临界范围。     

液态燃料对连续旋转爆轰发动机爆轰特性的影响

为了研究液态燃料对连续旋转爆轰发动机爆轰特性的影响,采用CE/SE方法对以汽油/富氧空气为燃料的CRDE进行数值模拟,分析了不同液滴半径、当量比对爆轰性能参数的影响。计算结果表明:随着液滴半径增大,爆轰压力峰值、温度峰值以及爆轰波速度均降低,且压力峰值与温度峰值在爆轰波传播过程中出现不稳定现象;当增大到70 μm时,爆轰波将无法成功起爆。随着当量比的增大,CRDE爆轰波速度及平均推力增大,爆轰压力、温度以及气相周向速度的峰值均先增大后减小。在当量比1.1附近,爆轰压力与温度的峰值出现极大值;而气相周向速度峰值的极大值出现在当量比0.9附近。基于燃料的比冲随着当量比增大而减小。     

当量比对汽油燃料两相旋转爆轰发动机工作特性影响实验研究

为了研究当量比对汽油燃料两相旋转爆轰发动机工作特性的影响,开展了以高总温空气为氧化剂的气液两相旋转爆轰实验研究。旋转爆轰发动机环形燃烧室外径、内径和长度分别为202、166和155 mm。汽油和高温空气采用高压雾化喷嘴与环缝对撞喷注的方式混合,以此提高推进剂的掺混效果与活性,采用预爆轰管作为点火装置。实验通过改变汽油质量流量改变推进剂当量比,并基于燃烧室内测得的高频动态压力和平均静压,对气液两相旋转爆轰波的传播模态和传播特性以及发动机的工作特性进行了详细分析。实验结果表明:在当量比为0.79～1.25时,燃烧室内均实现了旋转爆轰波的连续自持传播,且随着当量比的增加,爆轰波传播模态从双波对撞/单波的混合模态转变为单波模态;降低当量比至0.61～0.66,爆轰波传播稳定性变差,传播模态表现为间断爆轰以及零星爆轰;进一步降低当量比至0.53,爆轰波起爆失败。此外,燃烧室平均绝对压力与爆轰波平均传播频率均随着当量比的增加呈先增大后减小的趋势,极大值出现在当量比1.19附近。在此工况下获得了最佳实验结果,旋转爆轰波的平均传播频率为1 900.9 Hz,平均传播速度为1 110.8 m/s,与高频压力信号经快速傅里叶变换得到的主频基本一致,爆轰波传播速度存在严重亏损。     

铝粉/氢气/空气混合爆轰现象试验研究

混合爆轰现象既包含气相反应又包含两相反应,具有复杂性和多样性.爆轰推进技术在新领域的突破性应用与发展,依赖对爆轰现象的深刻认识.文章采用卧式爆轰管开展铝粉/氢气/空气混合爆轰试验,将μm和nm量级的球形铝粉与当量比的氢气和空气通过扬尘充分混合,在长13 m和直径224 mm的管内直接起爆混合物.试验中观测到不同种类的混合爆轰波,包括双波面和单波面结构.通过对爆轰燃气中铝粉点火燃烧特性的分析,阐明了两相反应对铝粉/氢气/空气混合爆轰波结构的直接影响.粒径100 nm和1μm时,混合爆轰呈现单波面结构,对比气相爆轰爆速和压力峰值都有增加,铝粉点火释热开始于声速面之前.粒径20μm和40μm铝粉点火较慢,混合爆轰呈现出双波面结构,气相反应释热支持第一道波,而铝粉燃烧支持第二道波.粒径10μm时,测得爆轰波压力曲线是单波峰,峰值压力有大幅提高,但是爆速并没有增加.其本质是两波面距离很近的双波面结构,由于传感器空间辨识能力的不足而无法在压力曲线中区分.混合爆轰试验结果充分解释了铝粉/氢气/空气混合爆轰现象,反映了铝粉在复杂条件下的燃烧特性,并且明确了铝粉的点火燃烧特性对混合爆轰现象的影响机理.     

气体-燃料液滴两相系统爆轰的数值模拟

用两相流体力学模型对气体 燃料液滴系统进行了研究。数值模拟了点火后两相系统爆轰波的发展过程，得到爆轰波的结构和参数。数值模拟结果表明气体 燃料液滴系统爆轰波有较宽的反应区，因而两相爆轰波的曲率对爆速的影响效应十分明显。进行了燃料液滴尺寸对爆轰波的结构和参数的影响的数值模拟。除了很小的液滴外，燃料液滴在爆轰波前导激波面和ＣＪ面间不能完全气化。随着液滴尺寸的增加，燃料液滴在爆轰波前导激波面和ＣＪ面间释放出的能量随之减少，爆轰参数也随之下降。     

基于液体碳氢燃料的旋转爆轰燃烧特性研究

为了探索液体碳氢燃料参与旋转爆轰所产生的不完全燃烧现象,采用守恒元与求解元方法,开展柱坐标系下的汽油/空气两相旋转爆轰燃烧室三维数值模拟研究,针对燃料喷注压力和反应物当量比对旋转爆轰流场结构及燃烧室性能的影响进行分析。分析结果表明:保持总当量比为1.00,随着燃料喷注压力的上升,燃烧室内燃料不均匀分布增强,产生局部富燃区,燃料在燃烧室未能完全反应,导致燃烧室燃料比冲下降;保持喷注压力不变,减小当量比,在贫燃工况下依然存在局部富燃区,导致燃烧室内出现不完全燃烧现象,降低燃烧室比冲性能。由此可知,反应物喷注方案对气液两相旋转爆轰的不完全燃烧有显著影响。     

悬浮RDX炸药和铝颗粒混合粉尘爆轰的数值模拟

采用两相流方法对炸药颗粒直径为20.0 m时与铝颗粒混合物的爆轰波的发展与传播过程及爆轰波参数进行了数值计算。结果表明,在炸药粉尘中加入铝颗粒,可以大大提高爆轰波参数。当铝颗粒直径为3.4 m时,尽管铝颗粒的直径较炸药颗粒直径小,但由于炸药颗粒的点火温度低,二者的点火时间相差不多。如果铝颗粒的直径为7.0 m,由于铝颗粒的点火滞后于炸药颗粒的点火,混合颗粒粉尘中可能形成双波阵面的爆轰波。     

不同当量比下喷管对旋转爆震特性的影响研究

为研究不同当量比下喷管构型对旋转爆震特性的影响，以煤油预燃裂解气为燃料，氧气体积分数为30%的富氧空气为氧化剂，开展了无喷管、收敛喷管、扩张喷管和收敛扩张喷管等工况下旋转爆震特性实验研究。实验发现，当量比为0.73～1.30时旋转爆震可稳定工作。随着当量比和喷管构型的变化，爆震波出现了单波、不稳定的对撞双波和稳定的对撞双波等3种传播模态。喷管构型对模态转换和旋转爆震波速有重要影响，收敛和收敛扩张喷管会促使新波头的产生，导致爆震波主要以双波对撞模态传播；而扩张喷管工况下，爆震波主要以单波模态传播。收敛喷管和收敛扩张喷管会使得波速最大值偏离化学恰当比，收敛扩张喷管可以提升爆震波速。     

爆轰理论与实践

爆轰是一个带有化学反应的冲击波传播过程.能够传播爆轰过程的系统可以是气相、液相、固相或者气-液、气-固和液-固等混合相.人们通常把液、固相的爆轰系统称为炸药.炸药的爆轰当然是十分重要的,但是,气相及混合相的爆轰也是很重要的.例如天然气与空气的混合物,金属 ...     

竖直下抛液体燃料爆炸抛撒的初步数值研究

对具有轴向初始速度即竖直下抛液体燃料的爆炸抛撒云雾形成过程进行了数值研究。近场的数据来自丁珏等的工作,以此数据作为远场初始参数。远场是燃料液滴与空气、燃料蒸汽、不同尺寸的液滴颗粒组成的多相体系之间的相互作用的过程。液滴的直径比较小,把液滴看成连续介质,且相互作用只考虑液滴的蒸发、破碎、碰撞聚合。轴向初速为0(静爆)时计算结果与实验结果相吻合。这说明本模型可以模拟爆炸抛撒云雾的形成过程,进而可以预估高速运动下云雾的形成过程。用以上模型计算了竖直下抛初速为100m/s、装填5kgPO的FAE装置的爆炸抛撒过程。所得的结果表明,100m/s的初速将影响云雾的最终形状及云雾内部的浓度分布。     

Gas temperature in the trace of water droplets streamlined by hot air flow

In order to obtain the knowledge necessary for developing new effective fire extinguishing technologies, we determined experimentally the gas temperature in the trace of water droplets streamlined by hot air flow. It was important to establish how much the temperature in the droplet trace decreases and how fast it recovery to the initial temperature field after the droplet evaporation. The following parameters were varied: droplet size from 1.3 mm to 1.7 mm, velocity from 1 m/s to 5 m/s, initial airflow temperature from 473 K to 773 K, number of droplets (one or two), and the arrangement of droplets relative to the hot inflow (serial or parallel). The study proves the theoretical hypothesis about a significant influence of evaporation on the temperature in the water droplet trace. When a temperature trace of water droplets is formed, irrespective of their arrangement, the role of the evaporation process strengthens with the gas flow temperature rising. Furthermore, the study specifies typical longitudinal dimensions of the aerodynamic and temperature traces of water droplets. It has been established that when droplets are located in series and in parallel, their combined impact on the temperature and velocity of the gas flow in the medium differs rather considerably.     

After fogging process in water injected gas turbine systems

In gas turbine system with after fogging, water droplets are injected after compressor. After fogging could have more significant potential for enhancement of specific power production compared to inlet fogging alone, since a larger water injection rate is possible. Transient analysis of after fogging process is carried out by using a heat and mass transfer modeling on water droplet evaporation. Transient variables such as droplet diameter and air temperature are evaluated as the droplet evaporation proceeds for different values of initial droplet diameter, pressure ratio of compressor, and water injection ratio. The evaporation time for injected droplets are also estimated. Present results show that the evaporation time decreases sensitively with increasing pressure ratio or initial droplet diameter. However, the effect of water injection ratio on evaporation time is relatively insignificant unless water injection ratio is near the critical ratio.     

An experimental study of the evaporation characteristics of emulsified liquid droplets

This paper presents the results of an experimental investigation, into the effect of water in diesel and kerosene emulsions, on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum). Experiments are performed at atmospheric pressure, and initial water volume concentrations of 10, 20, 30, and 40%. The wall temperatures ranging from 100–460 °C, to cover the entire spectrum of heat transfer characteristics from evaporation to film boiling. Results show that, qualitatively, the shapes of emulsion evaporation curves are very similar to that of pure liquids. Quantitavely, there are significant differences. The total evaporation time, for the emulsion droplets is lower than that for diesel and kerosene fuels, and decreased as water initial concentration increases, up to surface temperatures less than the critical temperature. The value of the critical surface temperature (maximum heat transfer rate), decreases as initial concentration of water increases. In the film-boiling region, the evaporation time for the emulsion droplets is higher than for diesel and kerosene droplets, at identical conditions.List of Symbols h_fg latent heat of vaporization, KJ/kg - m mass of the droplet, gm - T_b boiling temperature, °C - T_c critical temperature, °C - T_L Leidenfrost temperature, °C - T_s initial surface temperature of the hot surface, °C     

Aluminum Particles–air Detonation at Elevated Pressures

The effect of initial pressure on aluminum particles–air detonation was experimentally investigated in a 13 m long, 80 mm diameter tube for 100 nm and 2 μm spherical particles. While the 100 nm Al–air detonation propagates at 1 atm initial pressure in the tube, transition to the 2 μm aluminum–air detonation occurs only when the initial pressure is increased to 2.5 atm. The detonation wave manifests itself in a spinning wave structure. An increase in initial pressure increases the detonation sensitivity and reduces the detonation transition distance. Global analysis suggests that the tube diameter for single-head spinning detonation or characteristic detonation cell size would be proportional to (d ₀: aluminum particle size, p ₀: initial pressure). Its application to the experimental data results in m ~ O(1) and n ~ O(1) for 1 to 2 μm aluminum–air detonation, thus indicating a strong dependence on initial pressure and gas-phase kinetics for the aluminum reaction mechanism in detonation. Hence, combustion models based on the fuel droplet diffusion theory may not be adequate in describing micrometric aluminum–air detonation initiation, transition and propagation. For 2 μm aluminum–air mixtures at 2 atm initial pressure and below, experiments show a transition to a “dust quasi-detonation” that propagates quasi-steadily with a shock velocity deficit nearly 40% with respect to the theoretical C–J detonation value. The dust quasi- detonation wave can propagate in a tube with a diameter less than 0.4–0.5 times the diameter required for a spinning detonation wave.     

Numerical investigation of unsteady shock waves in vapor-gas-droplet mixtures

The results of mathematical modeling of the evolution of unsteady shock waves in two-phase mixtures of inert gas, vapor and suspended liquid droplets with allowance for dynamic, thermal and mass phase interaction processes are presented. The influence of interphase mass transfer effects (droplet breakdown and evaporation, vapor condensation) on the structure of unsteady shock waves in vapor-gas-droplet mixtures is analyzed. The important influence of phase mass transfer and, in particular, droplet breakdown as a result of surface layer stripping by the gas flow on the distribution of the parameters of the carrier and dispersed components of the mixture behind the shock front is demonstrated. The effect of the principal governing parameters of the two-phase mixture on the unsteady shock wave propagation process is analyzed.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.4, pp. 67–75, July–August, 1992.