流过泰氟龙烧蚀表面的化学非平衡边界层数值研究

对流过泰氟龙烧蚀表面的化学非平衡边界层进行了数值分析．研究了气体模型、化学反应速率常数和壁画催化对边界层特性的影响．研究表明，气体模型对烧蚀产物组元浓度大小的排序有很大影响，但不同气体模型造成的电子数密度差最大仅一个数量级；不同的氟碳反应速率常数，虽在非催化壁条件下，对组元浓度剖面有强烈影响，但对电子数密度剖面，不管壁面催化特性如何，几乎都没有影响；表面烧蚀可能引起边界层分离．     

化学非平衡边界层壁面边界条件研究

通过理论分析导出了化学非平衡边界层，化学反应壁面边界条件的一般性提法，并对其具体应用进行了研究．在壁面完全催化时，可利用传递系数法使烧蚀壁面条件计算与边界层计算解耦，给出了一般计算方法．     

激波诱导含灰气体边界层中的粒子分布

本文研究当激波沿着一个固体表面等速地穿越含灰气体运动时所诱导的层流边界层特性。考虑了作用在气体边界层中球形粒子的 Saffman 升力,建议了一种计算近壁区中弥散相密度剖面的方法,并给出了数值计算结果。本文结果表明:在激波后方存在着一个弯曲的薄层区域,其中的粒子密度可以比其波前原始值增加许多倍。这种粒子聚集效应对于工业中粉尘爆炸等实际问题具有重要意义。     

含灰气体激波管侧壁层流边界层流动特征

水文采用数值方法求解含灰气体激波管中侧壁层流边界层,揭示了这类流动中呈现的三个特征不同的区域.为了研究两相边界层的发展,本文构造了一个包括六点和四点格式的隐式有限差分程序,引进了适当描述气固两相间相互作用的非斯托克斯关系,并分别给出了气体和粒子的速度与温度剖面。此外,文中还讨论了激波管压力比、粒子载荷比和粒子尺寸对侧壁边界层流动特性的影响。     

湍流水沙边界层扩散传质浓度和通量的精确解

为探索河道及消落带水沙传质效应对自然水体水质的影响，研究湍流条件下水沙边界层的传质机理，本文引入传质阻抗，在涡旋扩散封闭假设的基础上，通过水沙边界层阻抗分布预测物质浓度分布，数值模拟紊动对水沙边界层扩散传质浓度分布和传质通量的影响。研究结果表明：根据传质阻抗（mass-transfer resistance）分布预测物质浓度分布是研究水沙边界层传质的重要方法；由物质浓度分布推导的传质系数 mh 、剪切速度*u 、施密特数Sc的相互关系式( m 0.0827*h=u Sc?2/3)能为气液边界层传质通量的研究提供理论依据；由泥沙表面到自由水流的传质总阻抗关系式( R′=12.092 Sc u 2/3 T*)可得，总阻抗随施密特数的2/3次幂增长，且总阻抗与剪切速度成反比，即传质阻抗随剪切速度的增加而下降。     

稀土金属材料填充方式对预混气体爆炸特性抑制研究

利用20 L球形爆炸装置，探究了在多孔稀土金属材料两种不同的填充方式（球状和片状）下甲烷-空气预混气体爆炸特性的变化规律，考虑了留空率和填充密度对预混气体爆炸特性的影响，并利用压力传感器记录球体内爆炸压力曲线。研究结果表明:甲烷爆炸压力、最大压力上升速率和爆炸指数均与留空率呈正比例关系，与填充密度呈反比例关系，片状材料下最大压力下降幅度大于球状材料下的；片状材料抑爆性能优于球状材料的，片状材料的双重抑爆作用对爆炸威力的影响更甚；2种不同填充方式下阻火抑爆的主要机理存在差异。     

在1.4米航空风洞中模拟大气边界层

在风洞中正确模拟大气边界层的流动特性是风工程风洞试验结果可信的必要条件,本次试验研究的目的是在短试验段的航空风洞中建立大比例的大气边界层模拟流场。通过适当的方式延长1.4m×1.4m航空风洞的试验段长度,并利用尖塔、粗糙元等边界层发生装置,在该风洞中建立了边界层流场,测量了流场的平均风速剖面、湍流强度剖面、脉动风速的自相关系数、风谱等参数,讨论了湍流积分尺度的处理和大气边界层几何模拟比例的确定,用谱拟合法和自相关系数积分法求出了湍流积分尺度。结果分析表明:试验所得流场是合理的大气边界层模拟流场,其平均风速剖面幂指数α=0.3,大气边界层模拟比例为1∶500,为后续的建筑物模型动态风荷载试验提供了前提条件。     

局部粗糙机翼表面气动特性影响研究

采用柱型粗糙元,以尖劈和机翼外形为基础,利用CFD数值模拟方法,研究机翼表面局部粗糙区域对其周围气动特性的影响。研究相同粗糙元高度下,粗糙元位于尖劈表面不同位置时局部边界层和气动特性的变化情况;基于流动分区理论,采用空气动力学理论分析与数值模拟结合的方法,分析F16机翼可接受的局部粗糙元高度;根据分析结果,在改进平板外形基础上,验证不同粗糙元高度对改进目标块区域气动特性的影响程度,并给出流经局部粗糙区域的流体发展状况。为了验证数值结果的准确性,采用S-A与SST湍流模型进行对比求解。本文工作对复杂大气环境引起的飞行器局部粗糙表面区域气动特性变化的研究具有指导意义。     

填充床中气体渗流与化学波耦合问题的研究

研究等温和显著气固反应条件下填充床内反应气体浓度(物质)波推进与混合气体渗流的相互作用,指出化学反应对流动的影响包括两个方面,反应过程中混合气体质量的变化和密度的变化。混合气体流动将反过来影响反应进程。分析表明,按耦合模型和非耦合模型得到的速度场完全不同;按耦合模型,反应气体的浓度(物质)波阵面的推进对混合气体的流场有显著影响,因此按耦合模型计算的混合气体流场强烈地依赖于时间;忽略化学反应引起的混合气体密度变化的耦合模型,将导致一个质量消失的汇(或质量生成的源),因此将引起混合气体渗流速度的明显变化,并可能导致物理上不合理的结果;按耦合模型和非耦合模型计算的浓度场也有很大差别;当反应气体与惰性气体摩尔质量相差较大时,不能忽略反应过程中混合气体密度的变化;研究表明对于显著气固反应不能忽略化学反应与气体渗流的相互作用。     

可侵蚀地表上方含尘大气运动的数值模型

在稀相气固两相流的双流体模型框架下，导出含尘大气运动的基本方程，其中的源项考虑相间双向耦合作用；通过对基本方程进行无量纲化，求得控制气相和弥散相动力学行为的相似准则，包括弗洛德数、罗斯贝数、颗粒雷诺数、颗粒惯性参数、颗粒剪切横移参数、颗粒旋转横移参数和沙尘质量载荷率等. 作为一个模型问题，研究可侵蚀地表上方充分发展湍流大气边界层流动对土壤颗粒的气动卷扬作用，其中计及气动阻力、萨弗曼升力和重力等因素的影响. 为了克服由于沙尘跃移运动轨迹交叉导致流动参数多值性所造成的困难，引进拉格朗日坐标下弥散相连续方程. 对两种不同风速和4种不同粒径条件下的沙尘运动进行数值模拟，给出相应情况中沙尘运动特性和密度分布剖面并讨论风速和粒径等参数的影响，还细致分析了含尘大气边界层中沙尘与气流之间的能量传递过程. 所得的方程、准则和方法可以为土壤风蚀和沙尘暴等自然灾害的预报提供理论基础.     

Kinetics of electron recombination in a molecular gas expanding into a cavity

The kinetics of recombination in a diatomic or polyatomic gas dispersing into a cavity is investigated in a model gas with one ionization potential and one electron affinity. In addition to the recombination reaction in triple collisions, which play the most important role in the case of a monatomic gas [1], dissociative recombination, ion-atom charge transfer, and reactions involving negative ions are considered. The qualitative differences in the kinetics of recombination of a molecular gas (in comparison with a monatomic gas) are due to the smallness of the relative electron concentrations at the instant of disturbance of ionization equilibrium and to the important contribution of dissociative recombination reactions and the kinetics of formation and recombination of negative ions.In addition, owing to the greater specific heat of a polyatomic gas and the corresponding lower rate of cooling on dispersion, recombination due to collision of three charged particles is not, as distinct from the case of a monatomic gas, decisive for the asymptotic values of the adiabatic exponent and residual ionization. For this reason the values of the adiabatic exponent can be assigned irrespective of a in the solution of the equations of the kinetics of recombination of diatomic and polyatomic gases. Expressions for the instant of failure of the equilibrium relationship between electrons and, respectively, positive and negative ions are obtained.The relationship between the charged-particle concentration in a gas in ionization nonequilibrium and the time for known values of the reaction rate constants is expressed by quadratures. The values of the rate constants of some ionization processes are known only in order of magnitude. However, available data on rate constants indicate that for practically any initial data for dispersion of the products of explosion or combustion of chemical compounds ionization equilibrium is upset at a time when there is still an equilibrium ratio of concentrations of electrons and negative ions.     

A Posteriori Assessment of Algebraic Scalar Dissipation Models for RANS Simulation of Premixed Turbulent Combustion

This paper examines the effects of scalar dissipation rate modelling on mean reaction rate predictions in turbulent premixed flames. The sensitivity of the mean reaction rate is explored by using different closures for scalar dissipation and the sensitivity of the scalar dissipation models themselves is also examined with respect to their defining constants. The influence of different scalar dissipation models on the flame location and mean velocities is reported and compared with experimental results. The predicted reaction rate is found to be sensitive to the choice of closure used for scalar dissipation and also to the respective constants used in the scalar dissipation models. It is also found that the scalar dissipation models involving chemical and turbulent time scales yield a more physically plausible reaction rate when compared with the scalar dissipation models relying only on the turbulent time scale.     

Effects of gas and droplet characteristics on drop-drop collision outcome regimes

Droplet-droplet collisions occur in many spray systems. The collision of two spherical droplets in a gas is considered in terms of the five primary phenomenological outcomes: slow coalescence (SC), bounce (B), fast coalescence (FC), reflexive separation (RS), and stretching separation (SS). The boundaries that separate these outcomes were investigated herein in terms of droplet viscosity and surface tension as well as gas pressure and density. Gas effects are not accounted for previous models, but can be important for hydrocarbon drops in pressurized sprays associated with many fuel systems. Based on a comprehensive review of available drop-drop collision data, phenomenological models were proposed herein for a wide variety of test conditions. For slow coalescence/bouncing (SC/B), increasing droplet viscosity and gas pressure were found to increase the probability of a bouncing outcome of the collision. For the B/FC boundary, increasing droplet viscosity and gas density were also found to increase bouncing probability. In both cases, the variations can be explained in terms of the stability of the gas layer that develops between the droplets. Additionally, the Brazier-Smith model for the FC/SS boundary was modified to increase robustness for a wide range of droplet viscosities. In general, the present models reasonably predicted collision outcomes for a large variety of gas pressures and densities as well as droplet viscosities and surface tensions. These are also the first models to include gas effects and the first models of the SC/B boundary. However, the droplet diameters of the data set were limited in range from 200 to 400 microns. Significantly larger droplet collisions may include effects on initial non-sphericity while significantly smaller drop collisions may include effects on non-continuum flow and gas viscosity.     

高速双锥绕流中热化学与输运模型影响研究

激波与边界层之间相互作用是高超声速飞行中的常见现象,对飞行器气动性能与飞行安全至关重要.对于高焓来流,流场中通常存在复杂的物理化学现象,此时准确模拟流场中激波边界层相互作用的难度大,相关物理化学建模仍有待进一步考察和研究.本文针对最近文献中纯净空气高超声速双锥绕流实验开展数值研究,分别研究了不同热化学模型与输运模型对壁面压力与热流的影响.热力学模型包括完全气体、热力学平衡和非平衡模型,化学模型包括冻结和非平衡化学模型,输运模型包括经典的Wilke/Blottner/Eucken模型与更加复杂的Gupta/SCEBD模型,以及考虑壁面催化/非催化影响的模型.计算了6个不同算例,涵盖了低焓至高焓来流等不同工况.壁面压力与热流的数值计算结果与实验结果符合较好;对于低焓来流,计算结果主要受到分子内能分布的影响,输运模型对计算结果的影响不大;对于高焓来流,一方面计算结果受到化学反应与壁面催化的影响较大,另一方面不同输运模型对计算结果的影响也更加明显.      

热喷干扰气体模型对飞行器气动特性影响分析

针对不同气体模型对高超声速飞行器喷流反作用控制系统(RCS)热喷干扰流场模拟的计算效率和准确性问题, 基于喷流燃气物理化学模型, 通过数值求解含化学反应源项的三维N-S方程, 建立了飞行器RCS热喷干扰流场数值模拟方法, 分别采用化学反应流、反应冻结流、二元异质流以及空气喷流四种气体模型开展了典型外形热喷干扰流场的数值模拟, 研究了不同气体模型对热喷干扰流场结构、飞行器气动力热特性的影响, 分析了不同马赫数、飞行高度下的变化规律. 研究表明: 化学反应流模型计算精度较高, 计算与风洞试验数据的吻合程度优于其他三种简化模型; 在本文的低空条件下, 采用简化模型进行热喷干扰流场数值模拟, 会低估分离区大小, 使飞行器气动力特性预测出现偏差, 同时也会低估表面热环境, 对防热系统设计不利, 随着马赫数增加, 简化模型对气动力热特性预估的误差进一步增大, 同时不同简化模型之间的差异也进一步增大; 飞行高度较高时, 模型之间的差异减小, 此时可采用简化模型进行计算以提高计算效率. 本文的研究结果可为飞行器热喷干扰流场数值模拟及喷流反作用控制系统设计提供参考.      

Analysis of Nonequilibrium Dissociation Kinetics in a Boundary Layer Using Various Reaction Models

Values of the nonequilibrium macroscopic reaction rate for a nonisothermal boundary layer of a monatomic diluent gas are calculated using a number of models for thermal dissociation of diatomic molecules — anharmonic Morse oscillators. Analysis is performed for conditions where the diffusive transfer of excited molecules has a significant effect on the population of their upper vibrational levels, which does not only amount to change in vibrational temperature. Under the joint influence of diffusive transfer of molecules, vibrational exchanges, and reactions involving vibrationally excited particles, the local vibrational distribution functions are substantially nonequilibrium. The kinetic models considered take into account the possible contribution of the energy of molecular translational and rotational degrees of freedom to the energy required to overcome the reaction threshold. The effect of multiquantum vibrational—translational exchanges on the distribution of dissociating molecules in their upper vibrational levels is taken into account approximately.     

Laminar slip flow of an electrically conducting incompressible rarefied gas in a channel with a transverse magnetic field

Summary The effects of a constant external magnetic field on the laminar, fully developed flow of an electrically conducting incompressible rarefied gas in a nonconducting parallel-plate channel are studied. Consideration is given to the slip-flow regime, wherein a gas velocity discontinuity occurs at the channel walls. It is found that the magnitude of the slip velocity is unaffected by the magnetic-field strength for a given pressure drop, but that the mean gas velocity and wall friction coefficient are functions of both the velocity slip coefficient and the magnetic-field strength. The effect of a second-order slip-flow boundary condition is briefly discussed.     

Two-phase nonlocal integral models with a bi-Helmholtz averaging kernel for nanorods

In this work, the static tensile and free vibration of nanorods are studied via both the strain-driven (StrainD) and stress-driven (StressD) two-phase nonlocal models with a bi-Helmholtz averaging kernel. Merely adjusting the limits of integration, the integral constitutive equation of the Fredholm type is converted to that of the Volterra type and then solved directly via the Laplace transform technique. The unknown constants can be uniquely determined through the standard boundary conditions and two constrained conditions accompanying the Laplace transform process. In the numerical examples, the bi-Helmholtz kernel-based StrainD (or StressD) two-phase model shows consistently softening (or stiffening) effects on both the tension and the free vibration of nanorods with different boundary edges. The effects of the two nonlocal parameters of the bi-Helmholtz kernel-based two-phase nonlocal models are studied and compared with those of the Helmholtz kernel-based models.

     

Wall effects on close-to-equilibrium flow in a tubular reactor

The deviation from local homogeneous chemical equilibrium flow due to non-catalytic heterogeneous reactions in a tubular reactor is analysed by an asymptotic expansion for large Damköhler numbers. Close to the reacting surface a boundary layer is shown to exist where the effects of the homogeneous and the heterogeneous reaction have an influence of the same order of magnitude. The extension of this boundary layer into the equilibrium flow is proportional to the inverse of the square root of the Damköhler number. The boundary layer equation is derived and solved for a particular example.