Sound amplification in inhomogeneous flows of nonequilibrium gas

The conditions of the acoustic instability of flows of thermodynamically nonequilibrium gas are determined. It is shown that the growth increments of the velocity, temperature, pressure, and density disturbances are different. When the Mach numbers are small, only the velocity and temperature disturbances grow along the flow.     

Gas flow characteristics of argon inductively coupled plasma and advections of plasma species under incompressible and compressible flows

In this work, incompressible and compressible flows of background gas are characterized in argon inductively coupled plasma by using a fluid model, and the respective influence of the two flows on the plasma properties is specified. In the incompressible flow, only the velocity variable is calculated, while in the compressible flow, both the velocity and density variables are calculated. The compressible flow is more realistic; nevertheless, a comparison of the two types of flow is convenient for people to investigate the respective role of velocity and density variables. The peripheral symmetric profile of metastable density near the chamber sidewall is broken in the incompressible flow. At the compressible flow, the electron density increases and the electron temperature decreases. Meanwhile, the metastable density peak shifts to the dielectric window from the discharge center, besides for the peripheral density profile distortion, similar to the incompressible flow.The velocity profile at incompressible flow is not altered when changing the inlet velocity, whereas clear peak shift of velocity profile from the inlet to the outlet at compressible flow is observed as increasing the gas flow rate. The shift of velocity peak is more obvious at low pressures for it is easy to compress the rarefied gas. The velocity profile variations at compressible flow show people the concrete residing processes of background molecule and plasma species in the chamber at different flow rates. Of more significance is it implied that in the usual linear method that people use to calculate the residence time, one important parameter in the gas flow dynamics, needs to be rectified. The spatial profile of pressure simulated exhibits obvious spatial gradient. This is helpful for experimentalists to understand their gas pressure measurements that are always taken at the chamber outlet. At the end, the work specification and limitations are listed.     

Ar流速对多级弧放电装置中Ar等离子特性的影响

采用PLASIMO程序模拟了入口处Ar流速对多级弧放电产生的非热平衡Ar等离子体特性的影响。模拟结果发现：从入口处到出口处,沿中心轴线,压强逐渐降低,电子平均能量基本保持不变。当流速一定时,从器壁到中心轴线处,电子数密度呈增大趋势;从入口处到出口处,电子数密度呈先增大后减小的趋势;当流速分别为50,100,150和200 cm3/s时,电子数密度最大值分别为10.131021,16.311021,18.981021和26.331021 m-3;随着流速的增大,其电子数密度逐渐增大。当流速一定时,从器壁到中心轴线处,电子温度逐渐增大;从入口处到出口处,电子温度呈先增大后减小再增大的趋势,并在中心轴线处距入口55～60 mm有最大值,当流速分别为50,100,150和200 cm3/s时,其最大值分别为1.299,1.234,1.157和1.132 eV;由于入口处和器壁处的电子温度都为0.517 eV,所以随着Ar流速的增大,其电子温度逐渐减小。当Ar流速一定时,从器壁到中心轴线处,离子温度逐渐增大;从入口处到出口处,离子温度呈先增大后减小的趋势,并且在中心轴线距入口20～30 mm离子温度取得最大值,当流速分别为50,100,150和200 cm3/s时,离子温度最大值分别为0.815 6,0.907 02,0.975 2和1.014 eV。随入口处流速的增大,电弧腔体内的离子温度逐渐增大。     

Effects of dimensional wall temperature on velocity-temperature correlations in supersonic turbulent channel flow of thermally perfect gas

Direct numerical simulations of temporally evolving supersonic turbulent channel flows of thermally perfect gas are conducted at Mach number 3.0 and Reynolds number 4800 for various values of the dimensional wall temperature to study the influence of the latter on the velocity-temperature correlations. The results show that in a fully developed turbulent channel flow, as the dimensional wall temperature increases, there is little change in the mean velocity, but the mean temperature decreases. The mean temperature is found to be a quadratic function of the mean velocity, the curvature of which increases with increasing dimensional wall temperature. The concept of "recovery enthalpy" provides a connection between the mean velocity and the mean temperature, and is independent of dimensional wall temperature. The right tails of probability density function of the streamwise velocity fluctuation grows with increasing dimensional wall temperature. The dimensional wall temperature does not have a significant influence on the Reynolds analogy factor or strong Reynolds analogy(SRA). The modifications of SRA by Huang et al. and Zhang et al. provide reasonably good results, which are better than those of the modifications by Cebeci and Smith and by Rubesin.     

大气压氩气射流等离子体的光谱特性研究

等离子体喷枪是一种重要的等离子体源,已成为近几年低温等离子体研究的一个重要课题。本文利用钨针-钨丝网电极制作了直流喷枪装置,在大气压空气中产生了稳定的等离子体羽,并采用发射光谱的方法,对等离子体羽的等离子体参数进行了研究。在钨针电极与钨丝网电极之间放出耀眼的白光,钨丝网电极出口的气流下游有火苗形状的等离子体羽喷出。在电压保持不变的条件下(13.5 kV),等离子体羽长度随气体流量增加而增大;在气体流量保持不变的条件下(10 L·min-1),羽长度随外加电压的增大而增大。在气体流量一定的条件下,放电电压和放电电流呈反比例关系,即电压随着电流的增大而减小,说明放电属于辉光放电。采集了该喷枪在300～800 nm范围内的放电发射光谱,通过玻尔兹曼方法对放电等离子体电子激发温度进行了测量。结果表明,电子的激发温度随外加电压的增大而降低,随着工作气体流量的减小而升高。利用放电的基本理论对上述现象做了解释。这些研究结果对大气压均匀放电等离子体源的研制和工业应用具有重要意义。     

非对称纳米通道内流体流动与传热的分子动力学

采用分子动力学方法研究了流体在非对称浸润性粗糙纳米通道内的流动与传热过程,分析了两侧壁面浸润性不对称对流体速度滑移和温度阶跃的影响,以及非对称浸润性组合对流体内部热量传递的影响.研究结果表明,纳米通道主流区域的流体速度在外力作用下呈抛物线分布,但是纳米通道上下壁面浸润性不对称导致速度分布不呈中心对称,同时通道壁面的纳米结构也会限制流体的流动.流体在流动过程中产生黏性耗散,使流体温度升高.增强冷壁面的疏水性对近热壁面区域的流体速度几乎没有影响,滑移速度和滑移长度基本不变,始终为锁定边界,但是会导致近冷壁面区域的流体速度逐渐增大,对应的滑移速度和滑移长度随之增大.此时,近冷壁面区域的流体温度逐渐超过近热壁面区域的流体温度,流体出现反转温度分布,流体内部热流逆向传递.随着两侧壁面浸润性不对称程度增加,流体反转温度分布更加明显.     

Study on unsteady molten insulation volume change during flame spreading over wire insulation in microgravity

Tests with flames spreading over wire insulation in microgravity were performed at varying external opposed flow conditions to examine the influence of flow velocity in the time dependent volume change of molten insulation. In the experiments, low density polyethylene insulated Nickel–chrome wire specimens were used and the oxygen concentration was fixed at 30% (N₂ balanced). The results show that the time dependent changes in molten insulation volume are related to the opposed flow velocity. Further, as opposed flow velocity increases, the volume change rate decreases monotonically. By subtracting the volume change rate from the volume supply rate from the solid part to the molten part, which is calculated by multiplying the rate of spreading of molten insulation at the leading edge by the cross sectional area of the insulation, a pyrolysis volume rate for the polyethylene was established. The pyrolysis volume rate is defined as the amount of consumed molten insulation volume per unit time. After these calculations, it was found that the pyrolysis volume rate increases monotonically with increases in the opposed flow velocity. Further, numerical calculations of time dependent volume change in the molten insulation at different flow velocities were made. The numerical results show good agreement with the experimental results of the molten insulation volume change during the 0–4.5 s of microgravity measured here. By using the numerical calculations for this initial short period, the time dependent volume change in molten insulation during longer-term microgravity is predicted. The calculated results show that the volume finally reaches a steady state value in flow velocities of 10–250 mm/s investigated here. These results provide insight into the mechanism of flame spreading over wire insulation, especially the unsteadiness of the flame in flame spreading events.     

玻璃包覆纯铜丝快速冷却温度场数值模拟

建立了玻璃包覆纯铜丝快速冷却过程温度场的有限元计算模型,模拟得出了冷却过程丝线横断面的温度分布图及节点温度随时间的变化关系。研究了冷却水温度、水流速度对玻璃包覆纯铜丝冷却效果的影响。结果表明:快冷时玻璃层内的温差最大可达45 K/m,铜芯内温差最大为0.12 K/m; 随着冷却水流速增大,冷却效果增强,流速达到0.5 m/s时冷却能力饱和; 冷却水温在283～303 K范围内变化时,冷却效果基本不变。     

Comparative measurements on thermal plasma jet characteristics inatmospheric and low pressure plasma sprayings

The ion and electron temperatures and plasma flow velocities are measured and compared between atmospheric and low pressure plasma spraying systems. The measurements of ion temperature for two systems are carried out by an optical emission spectroscopy which uses the relative emissivities of isolated Ar I emission lines. The electron density and temperature are measured by a Langmuir probe rotating across the plasma jets. The ion saturation currents collected by a Mach probe at two orientations, perpendicular and parallel to the plasma jet, determine the flow velocity. The spatial distributions of electron density, plasma flow velocity, and the associated shock activity in thermal plasma jets are discussed in conjunction with their direct dependency upon the ambient pressures as well as the torch powers. Measurements on temperatures and velocity profiles of thermal plasma jets reveal the general features of the LPPS jet characteristics, i.e., higher velocity flow with lower temperature, longer heating zone of expanded flame, and more extended accelerating zone compared with those of the APS jets. The shock activity clearly exists in the form of standing shock waves in the plasma jet of LPPS in view of flow compression and abrupt velocity drop which are appeared in the results of measurements on the variations of electron density and flow velocity along the plasma jet. In the center of the plasma jet of APS, the electron density is high enough to reach the LTE criterion, and the difference between ion and electron temperatures becomes insignificant as the torch input power increases     

INVESTIGATION OF BOILING OF REFRIGERATING MEDIUM UNDER CONDITIONS OF IMPULSE DISTURBANCES

This article describes investigations of bubbly boiling in flow of a refrigerating medium under conditions of impulse disturbances occurring in refrigeration engineering systems. The development and decay of boiling is induced by impulsively changing parameters, such as pressure and mass flux density, of the two-phase system in a tubular channel. It is confirmed that the two-phase systems exhibit wave properties and the induced disturbances propagate with a finite velocity. Correlations are elaborated to evaluate the propagation velocity of disturbance signals during the development and decay of boiling in a coil tube. Investigations are conducted by feeding under pressure a heat exchanger with an environment-friendly refrigerant R404A.     

玻璃包覆纯铜丝快速冷却温度场数值模拟

建立了玻璃包覆纯铜丝快速冷却过程温度场的有限元计算模型,模拟得出了冷却过程丝线横断面的温度分布图及节点温度随时间的变化关系。研究了冷却水温度、水流速度对玻璃包覆纯铜丝冷却效果的影响。结果表明:快冷时玻璃层内的温差最大可达45 K/m,铜芯内温差最大为0.12 K/m; 随着冷却水流速增大,冷却效果增强,流速达到0.5 m/s时冷却能力饱和; 冷却水温在283～303 K范围内变化时,冷却效果基本不变。     

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采用两相流的流动换热理论,建立二维几何模型,运用FLUENT对稳定入口流速下注入气体的铅铋流动段作了模拟。模拟研究注入气体的体积份额或速度改变对压力和铅铋与注入气体之间换热与影响,得到了不同条件下的温度与压力分布。分析结果发现,体积份额减小,铅铋流体的径向温度分布更加均匀,中心温度更低;随着体积份额的减小,铅铋的总压呈现出一种下降的趋势。注入气体速度不同对铅铋整体的换热影响不大;中心处的动压有较大增加,总压改变甚微。     

Simultaneous effects of temperature and pressure on the donor binding energy in a V-groove quantum wire

The influence of temperature and pressure, simultaneously, on the binding energy of a hydrogenic donor impurity in a ridge GaAs/Ga_1−xAl_xAs quantum wire is studied using a variational procedure within the effective mass approximation. The subband energy and the binding energy of the donor impurity in its ground state as a function of the wire bend width and impurity location at different temperatures and pressures are calculated. The results show that, when the temperature increases, the donor binding energy decreases for a constant applied pressure for all wire bend widths. Also, the binding energy increases by increasing the pressure for a constant temperature for all wire bend widths. In addition, when the temperature and pressure are applied simultaneously the binding energy decreases as the quantum wire bend width increases. On the whole, it is deduced that the temperature and pressure have important effects on the donor binding energy in a V-groove quantum wire.     

Theoretical Studies of the Laser Induced Plasma Profile Near the Critical Density Region for an Adiabatic Plasma Model

A theoretical study of the variation of density and flow velocity due to pondermotive force, near critical density region of a laser – produced plasma, obeying an adiabatic pressure law is reported. The ion-continuity, momentum and wave equations are solved in a steady state. The density and flow velocity as a function of incident laser intensity are calculated and plotted for both underdense and overdense regions for adiabatic exponent γ = 4/3, 5/3, 2. The velocity decreases and density increases in the overdense region and velocity increase and density decreases in the underdense region with the increase of larger power at sonic point. It is found that the velocity is subsonic in the overdense region and supersonic in the underdense region. The nature of variation is similar to the isothermal model.     

非均匀加热条件下内插扭带管强化传热模拟分析

以水为工作介质,采用欧拉多相流模型和非平衡沸腾模型,当流速在0.3～0.7m·s-1范围内、工作压力为4.5MPa、热流密度为2MW·m-2时,数值模拟了内插扭带管和光管管内流动过冷沸腾传热.对比了两种管道的换热系数、气泡份额、流动速度、流场流线、固体组件温度和压降,分析了内插扭带管的综合性能.结果表明,与光管相比较,...     

黎开管振荡的简单理论

根据瑞利定性解释的黎开管振荡的机理,推出了黎开管的波动方程,并取得了严格解。基本要领是把黎开管看做两端开口的均匀管,其中有一点的温度较高。冷空气进入后,通过热点,密度突变,成为声源。据此列出的波动方程可严格求解,并求得黎开管振荡的特性。首先黎开管是反馈系统。热点在其节点或腹点部不发生反馈,也不产生振荡,热点位置决定有无反馈。黎开管振荡必须有气流,所产生的质点速度与气流平均速度成正比。热点温度决定反馈大小.温度低时,反馈基本不影响质点速度的大小,它小于平均速度,并和温度成正比。温度高时,反馈迅速增加。在另一方面,在刚加上温度或气流时,振荡的建立要靠反馈逐渐积累,所以很缓慢。但气流停止后,振荡立刻停止,与室内声场的建立和混响完全不同。如果用阀门或盖板在管的一端控制气流时,发声很有特色。     

Parameters of an erosion carbon plasma in the channel of a railgun

The working current dependences of the thermodynamic and electrophysical parameters of a free plasma piston moving with a near-maximal velocity in the channel of an electromagnetic rail launcher with graphite electrodes are obtained. The composition and weight of the plasma depend on the degree of electrode erosion due to discharge current passage (i = 40–80 kA). It is shown that the mean temperature of the plasma piston only slightly depends on the plasma mean pressure and plasma piston weight and increases with current by a near-power law. The measured values of the maximal velocity of the plasma piston front are compared with the calculated value of the sound velocity inside the piston. With the working current and cross-sectional area of the channel fixed, the initial gas density in the channel is found to influence the ratio of the piston maximal velocity to the sound velocity in the plasma. If the initial gas density is low (lower than some critical value), the maximal velocity of the plasma piston front exceeds the sound velocity in the plasma.     

超临界CO_2在岩石孔隙内流动状态的分子模拟

本文采用分子模拟的方法研究了地质封存条件下超临界CO_2在镁橄榄石孔隙内的流动,分析了孔隙尺寸、温度、压强对超临界CO_2的密度分布和流动速度的影响.研究结果表明,只有当孔隙尺寸大于5.0 nm时,超临界CO_2分子在岩石孔隙内的流动才符合Poiseuille流动;同时,超临界CO_2分子在岩石近壁面存在1.5 nm的密度震荡,当孔隙尺寸小于15 nm时,密度振荡现象会影响CO_2分子在岩石孔隙内的平均密度;升高温度、降低压强的方法可以减小密度震荡的第一峰值,减弱岩石壁面与CO_2的相互作用,使得CO_2的流动速度增加.     

Thermal hardening of Ni3Ge alloy single crystals with an L12 superstructure at low temperatures

The mechanical properties and the dislocation structure of Ni₃Ge alloy single crystals have been experimentally studied at low temperatures. It is found that the flow stresses increase beginning with 4.2 K, and the observed rise in the stresses depends on the orientation of the strain axes of the crystals. The dislocation structure is investigated thoroughly. It is revealed that the mean density of dislocations and the interdislocation interaction parameter α anomalously increase as the temperature increases in the range 4.2–293 K. The mechanisms providing an explanation for the temperature anomaly of flow stresses and the α parameter are considered. The activation energy of thermal hardening is evaluated. It is assumed that the low activation energies of thermal hardening are due to the motion of dislocations at velocities close to the velocity of sound at these temperatures.     

Längsfeldstärke,Elektronentemperatur, mittlere Neutralgasdichte,Ionenstromdichte auf die Wand und mittlere Entladungsstromdichte in der Freifallsäule bei hohem Ionisationsgrad

On the basis of a foregoing paper new theoretical results for the positive column at low pressure and strong ionization, especially for discharges in noble gas ion lasers, are given. The mean velocity v_n0 of the neutral atoms reemitted from the wall is taken into account. The electric conductivity is calculated for an argon plasma. The formulas connecting the electron temperature, the mean neutral gas density, and the electric field strength are derived. The electron temperature, the axial electric field intensity, the degree of ionization, the axial electron drift velocity, the ion flux to the wall, and the force density causing the main part of gas pumping along the column are calculated as functions of the product of the mean current density and the tube radius, and of v_n0 for argon. The axial drift velocity of the electrons is still smaller than the mean thermal electron velocity for high discharge currents, except at very low gas pressures. In general, the ion flux to the wall is not directly proportional to the discharge current. The factor for the determination of the charged particle density by means of probe measurements at the wall is discussed. The self-magnetic field affects the discharge only at high electron temperature, high degree of ionization, and relatively large tube radius, i.e. at high current density and low gas pressure in not too narrow discharge channels.