利用介质阻挡探针法测量大气压氦等离子体射流电子密度

分别利用电子的漂移速度和等离子体的传播速度计算了大气压下氦等离子体射流的电子密度。     

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The electron densities in the atmospheric pressure helium plasma were calculated by means of electron drift velocity and the jet velocity respectively. The electron velocity and jet velocity can be calculated by means of helium plasma jet current measured by a dielectric probe and plasma discharge current signal measured by voltage probes. The results show that the estimated electron densities of the helium plasma jet calculated from electron drift velocity and the jet velocity are in the order of 10 ¹¹ cm ^-3 and they increase with applied voltage. There is a little fluctuation in the value of the electron density along the jet axis of the plasma. This result is the same as the measured electron density in atmospheric pressure helium non-thermal plasma jet by using a Rogowski coil and a Langmuir probe. This is in one order lower than the electron density measured by microwave antenna.     

Influence of air pressure on the performance of plasma synthetic jet actuator

Plasma synthetic jet actuator(PSJA) has a wide application prospect in the high-speed flow control field for its high jet velocity.In this paper,the influence of the air pressure on the performance of a two-electrode PSJA is investigated by the schlieren method in a large range from 7 k Pa to 100 k Pa.The energy consumed by the PSJA is roughly the same for all the pressure levels.Traces of the precursor shock wave velocity and the jet front velocity vary a lot for different pressures.The precursor shock wave velocity first decreases gradually and then remains at 345 m/s as the air pressure increases.The peak jet front velocity always appears at the first appearance of a jet,and it decreases gradually with the increase of the air pressure.A maximum precursor shock wave velocity of 520 m/s and a maximum jet front velocity of 440 m/s are observed at the pressure of 7 k Pa.The averaged jet velocity in one period ranges from 44 m/s to 54 m/s for all air pressures,and it drops with the rising of the air pressure.High velocities of the precursor shock wave and the jet front indicate that this type of PSJA can still be used to influence the high-speed flow field at 7 k Pa.     

稠密颗粒射流倾斜撞击颗粒膜特征

采用高速摄像仪对稠密颗粒射流倾斜撞击形成的类液体颗粒膜特征进行实验研究,考察了颗粒粒径、射流速度以及射流含固率等因素对颗粒膜形态及动态特征的影响.结果表明:随着颗粒粒径增大,稠密颗粒倾斜撞击流由颗粒膜向散射模式转变;随着射流速度增加,气固不稳定增强,射流流量出现脉动,正面与侧面分别表现为颗粒膜的非轴对称振荡和表面波纹结构;颗粒膜非轴对称振荡的振荡频率和振荡幅度随射流速度的增大而增大;表面波纹速度和波长沿传播方向增大,波纹间存在叠加现象.颗粒膜出现非轴对称振荡主要是因为喷嘴出口由气固不稳定性引起的射流流量脉动,射流流量脉动频率与撞击面振荡频率基本相当.     

Flow Characteristics of Flapping Motion of a Plane Water Jet Impinging onto Free Surface

A submerged turbulent plane jet in shallow water impinging vertically onto the free surface will produce a large-scale flapping motion when the jet exit velocity is larger than a critical one. The flapping phenomenon is verified in this paper through a large eddy simulation where the free surface is modeled by volume of fluid approach. The quantitative results for flapping jet are found to be in good agreement with available experimental data in terms of mean velocity, flapping-induced velocity and turbulence intensity. Results show that the flapping motion is a new flow pattern with characteristic flapping frequency for submerged turbulent plane jets, the mean centerline velocity decay is considerably faster than that of the stable impinging jet without flapping motion, and the flapping-induced velocities are as important as the turbulent fluctuations.     

An experimental study of jet noise part I: Turbulent mixing noise

The characteristics, both spectral and directivity, of turbulent mixing noise in the far field from subsonic and fully-expanded supersonic jet flows have been studied experimentally over an extensive envelope of jet operating conditions (jet exit velocity and temperature). The measurements were conducted in an anechoic room which provides a free-field environment. The results are presented in a systematic manner, and the observed trends and dependencies are discussed in detail. In particular, the changes in detailed jet noise features with varying velocity and exhaust temperature are assessed independently. Empirical prediction schemes or comparisons with recent theoretical investigations are not attempted here. However, the isothermal jet noise results are compared with those predicted by the freely-convecting quadrupole theories (that is, in the absence of any mean flow shrouding effects). The discrepancies between this model and the measurements, many of which have been recently shown to occur due to the presence of mean velocity and temperature gradients surrounding the sources, are obtained accurately over all jet operating conditions of interest.     

稠密颗粒射流撞击壁面颗粒膜表面波纹特征

采用高速摄像仪对稠密颗粒射流撞击有限尺寸壁面的流动过程进行了实验研究,重点研究了颗粒膜及其表面波纹特征,考察了颗粒粒径、射流速度和固含率等因素对颗粒膜形态和表面波纹的影响.研究结果表明,随着颗粒粒径增大,稠密颗粒撞壁流由颗粒膜向散射模式转变.与液体射流撞壁液膜相比,颗粒膜扩展角较大,射流速度对其影响不显著.稠密颗粒射流撞壁颗粒膜表面波纹存在明显的叠加现象,颗粒膜表面波纹频率比液膜大约低一个数量级.颗粒膜表面波纹主要由射流脉动引起,表面波纹频率与射流脉动频率具有相同的数量级.     

Comparative investigation of microjetting generated from monocrystalline tin surface and polycrystalline tin surface under plane impact loading

With considering the scattering effect of grain boundary and the grain orientation, the molecular dynamics is used for the first time to comparatively investigate microjetting generated by monocrystalline tin surface and polycrystalline tin surface under plane impact loading in this work. The research results show that when the impact velocity is low, the scattering effect of grain boundary and different grain orientations in a polycrystalline tin will cause the sample to melt inhomogeneously, leading the shock wave front to attenuate, meanwhile, the inhomogeneous melting can result in jet deviating. Comparing with monocrystalline tin, the jet head velocity, jet velocity coefficient, and jet mass coefficient of polycrystalline tin at low impact velocity are all low. Moreover, as the impact velocity increases, this influence decreases and the microjetting results of polycrystalline tin and monocrystalline tin tend to be consistent with each other.     

A study in the developing region of square jet

The developing region of a turbulent square jet is investigated using high-resolution particle image velocimetry (PIV). The mean velocity and turbulence stresses are presented in various horizontal planes, along the jet centerline covering the initial region of the jet as well as the transition to the self-similar region. To study the flow structure away from the central plane, velocity measurements in two additional horizontal planes, one located halfway from the jet central plane toward the edge and the other at the edge of the square jet, are also examined. Analysis of the instantaneous velocity fields reveal the presence of an arrow-like feature in the square jet due to the higher instability generated in the jet shear layer compared with a round jet. To elucidate the imprints of the vortex structures present in the jets, a swirling strength-based vortex identification methodology is applied on a large ensemble of instantaneous velocity fields. Statistical analysis of the number of vortex cores, and their size and rotational strength in the measurement plane is undertaken. Vortex population at the edge was found to be very different compared with that in the central plane.     

PIV measurements of the flow and turbulent characteristics of a round jet in crossflow

The instantaneous and ensemble averaged flow characteristics of a round jet issuing normally into a crossflow was studied using a flow visualization technique and Particle Image Velocimetry measurements. Experiments were performed at a jet-to-crossflow velocity ratio, 3.3 and two Reynolds numbers, 1,050 and 2,100, based on crossflow velocity and jet diameter. Instantaneous laser tomographic images of the vertical center plane of the crossflow jet show that there exists very different natures in the flow structures of the near field jet due to Reynolds number effect even though the velocity ratio is same. It is found that the shear layer becomes much thicker when the Reynolds number is 2,100 because of the strong entrainment of the inviscid fluid by turbulent interaction between the jet and crossflow. The mean and second order statistics are calculated by ensemble averaging over 1,000 realizations of instantaneous velocity fields. The detail characteristics of mean flow field, streamwise and vertical rms velocity fluctuations, and Reynolds shear stress distributions are presented. The new PIV results are compared with those from previous experimental and LES studies.     

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     

Nonlinear oscillations of a charged jet of a conducting liquid under multimode initial deformation of its surface

The subject of consideration is a uniformly charged jet of an ideal incompressible conducting liquid moving with a constant velocity along the symmetry axis of an undisturbed cylindrical surface. An evolutionary expression for the jet shape is derived accurate to the second order of smallness in oscillation amplitude for the case when the initial deformation of the equilibrium surface is a superposition of a finite number of both axisymmetric and nonaxisymmetric modes. The flow velocity field in the jet and the electric field distribution near it are determined. The positions of internal nonlinear secondary combined three-mode resonances are found, which are typical of nonlinear corrections to the analytical expressions for the jet shape, flow velocity field potentials, and electrostatic field in the vicinity of the jet.     

Nonlinear corrections to the oscillation mode frequencies of an ideal liquid charged jet

An analytical asymptotic expression is derived for the time evolution of an ideal incompressible conducting liquid jet with a uniformly charged surface that moves with a constant velocity along the symmetry axis of an undisturbed cylindrical surface. The expression is obtained in the third order of smallness in jet oscillation amplitude under the conditions when the initial deformation of the equilibrium jet surface is specified by one (axisymmetric or nonaxisymmetric) mode. Analytical expressions are also found for the positions of internal nonlinear degenerate three-and four-mode resonances, which are typical of nonlinear corrections to the equilibrium shape of the jet, liquid velocity field potential in the jet, and electrostatic field near the jet, and also for a nonlinear frequency correction. It is established that the nonlinear oscillations of the jet take place about a surface depending on the type of initial (generally asymmetric) deformation rather than about the equilibrium cylindrical shape.     

Influence des conditions d'émission sur un écoulement de type jet plan laminaire isotherme ou chauffé

We propose numerical solutions for a laminar jet, accounting for emission conditions at the exit of the nozzle. Two emission cases are considered in this study: velocity and temperature profiles are uniform or parabolic, respectively. A finite difference scheme is developed for the resolution of the equations governing the isothermal and non-isothermal free jet and wall jet developing tangentially along an adiabatic flat plate. The analysed results are the centerline velocity and centerline temperature for the free jet, and wall temperature and shear stress for the wall jet. The results obtained are compared to another method that is based on two constraints of integration, i) conservation of momentum and ii) conservation of energy, to replace the emission conditions at the exit of the nozzle for the resolution of equations. Our results of the velocity and temperature profiles compare well with those obtained by the latter method solely in the plume region, where buoyancy forces are responsible for flow.     

Interaction of two parallel plane jets of different velocities

The interaction of two parallel plane jets of different velocities is studied by flow visualization and PIV measurement to examine the influence of velocity ratio on the development ofjets in the initial region. It is found that the parallel plane jets develop toward the high velocity side and the jet width is reduced with a decrease in the jet velocity ratio. Corresponding to the variation of mean velocity field to the velocity ratio, the magnitudes of turbulence intensities, Reynolds stress and static pressure are weakened in the merging region of the jets and their peak locations of the properties are shifted to the high velocity side. These results indicate that the interaction of two parallel jets is weakened with a decrease in the velocity ratio of the jets.     

两电极等离子体合成射流激励器工作特性研究

采用放电测量和高速阴影技术对两电极等离子体合成射流激励器工作特性进行了系统实验研究.实验表明:激励器工作击穿电压和放电峰值电流随激励器所处环境压强的降低和放电频率的增大而减小,激励器腔体内的放电过程为火花电弧放电.典型的等离子体合成射流流场包含有一道前驱激波和一股呈蘑菇状的高速射流.在整个射流发展过程中,前驱激波以当地声速恒速传播,不随激励器条件的改变而变化,波的强度则随着激励器出口直径的减小、腔体体积的增大、环境压强的降低和放电频率的升高而减小.激励器腔体体积和放电频率的增加会降低腔内气体的加热效果,并减小射流速度.激励器出口直径和环境压强对射流速度的影响按规律变化且存在最佳值.本文实各验条件下激励器都产生了明显的前驱激波和高速射流,具有实现高速流场主动流动控制的应用潜能.     

直接模拟离散颗粒对湍流射流的影响

发展了一种基于双向耦合的直接数值模拟方法,研究了两相湍流射流中离散颗粒对气相射流的影响。流场采用高分辨率的算法直接耦合求解,颗粒相采用拉格朗日方法跟踪。结果表明不同Stokes数的颗粒对射流的影响不同。所有的颗粒都降低了射流速度半宽,但对流向平均速度剖面的影响比较复杂.Stokes数为0.01和50的颗粒降低了横向平均速度,使得湍流强度剖面变的更宽更低;而Stokes数为1的颗粒则增加了横向平均速度,降低了湍流强度。     

Experimental study of corona jet produced from a circular tube fitted with a nozzle

The flow characteristics of a corona jet, which is produced from a single needle electrode positioned at the centerline of a circular tube fitted with a grounded stainless-steel nozzle at one end of the tube, is experimentally evaluated. Six nozzles with two diameter ratios and three taper angles are evaluated for their effectiveness in accelerating the jet produced by corona discharge with positive polarity. To determine the maximum jet velocity and volume flow rate, experiments have been conducted at a voltage ranging from corona onset (5 kV) to sparkover (approximately 12.5 kV) at an increment of 2.5 kV. The results show that the jet velocity increases with the applied voltage. The maximum velocity occurs at the center line but its value decreases as the jet expands downstream. In addition, the results show that a nozzle with a smaller diameter ratio does not always perform the best in accelerating the flow or producing the maximum volume flow rate. The nozzle's taper angle further accentuates the result produced by the diameter ratio. The implications from the present results for actual applications are provided.     

Experimental and computational visualization and frequency measurements of the jet oscillation inside a fluidic oscillator

PIV measurements and computational simulations (2D, unsteady Navier-Stokes) are performed to visualize the inherently unsteady jet oscillation inside a fluidic oscillator. Both the measurements and computations are obtained for a jet exit Reynolds number of 321, based on the maximum velocity and the nozzle width at the jet exit plane. The computed jet oscillation frequency is in close agreement with the measured PIV frequency. Formation of a pressure gradient across the jet is observed from the computations. The variation of the jet oscillation frequency with jet exit Reynolds number is also determined by single sensor hot-wire measurements inside the oscillation chamber.     

浮力对矩形射流扩散火焰影响的大涡模拟

本文对浮力作用下的矩形射流扩散燃烧过程进行了三维大涡模拟。数值模拟结果展示了浮力作用下矩形射流扩散火焰的动态弯曲过程,比较分析了射流速度对火焰刚性的影响,发现射流速度越高火焰弯曲程度越小、燃料喷射距离越远。对浮力作用下的水平射流横截面流动分析结果表明,由于流向涡的卷吸作用在局部区域存在逆着浮力方向的流动。