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     

Spectral and Electrical Properties of a Medium Pressure,Low-Power,RF-Generated Postdischarge Nitrogen Plasma Jet

A method for obtaining a medium pressure (p ～ 1 torr), low power (< 100 W), RF-generated plasma jet, suitable for chemical kinetics studies and technological applications is described. Though various supply gases can be used to create the plasma jet, only results on the properties of a nitrogen jet are given in this paper with an emphasis on some spectral and electrical characteristics. The emission spectra of the nitrogen jet are discussed and spatial distributions of the relative concentration of nitrogen or radical impurities emitting species such as N₂(C³Π_u, v' = 0), NH(A³Π_g, v'=0) in the bulk of the postdischarge plasma are calculated from the intensity of their vibrational bands. Electrical space resolved double probe measurements and the spatial distributions of the electron density and temperature in the plasma jet are also presented. Possible applications are shortly discussed.     

An experimental study of jet noise part II: Shock associated noise

The characteristics of the sound field of shock-containing under-expanded jet flows are studied by measuring the noise from a convergent nozzle operated over an extensive envelope of supercritical jet operating conditions. The measurements were conducted in an anechoic facility. They are complementary to the turbulent mixing noise experiments (described in Part I) for subsonic and fully-expanded (shock-free) supersonic jets. The overall results from shock-containing jets are compared directly with the corresponding results from shock-free jets, and the effects of nozzle pressure ratio and jet exhaust temperature on broadband shock-associated noise are assessed independently. For a supersonic jet, the regimes of jet operating conditions, observer angles, and frequencies over which the sound field is dominated by shock-associated noise are identified. Finally, the spectral results are compared in a preliminary manner with the spectra predicted by an existing theoretical model, and good agreement is obtained in most cases.     

Use of perturbing probes for plasma-jet diagnostics

The experience gained in using an array of cylindrical Langmuir probes as a perturbing body for plasma-jet diagnostics is described. The plasma potential on a part of the probe is determined from the ion-scattering minimum at that part. The potential values thus obtained are compared with the results of an analysis of the probe characteristics. The conditions for applicability of the method are discussed. A density marker is created by applying a voltage pulse with a duration of 30 ns to the probes, and time-of-flight probing of the plasma jet is performed. Zh. Tekh. Fiz. 67, 24–27 (July 1997)     

Characterization of an atmospheric pressure plasma jet for surface modification and thin film deposition

In this paper, an experimental study is presented to characterize a commercially available atmospheric pressure plasma jet (APPJ) kINPen which can be used for local surface modification, e.g. changing the wettability as well as for thin film deposition with silicon-organic and metal-organic precursors to enhance scratch resistance or to lower the gas permeability. Characterization of the jet discharge has been carried out by three methods: (i) measurement of the energy influx from the jet plasma to a substrate by a calorimetric probe, (ii) spatial resolved investigation of the plasma beam by optical emission spectroscopy (OES) and (iii) observation of the plasma jet by video imaging. The deposited SiO _x and AlO _x films were analyzed by XPS measurements.     

Corona discharge as a temperature probe of atmospheric air microwave plasma jet

We developed and tested a new method for temperature measurements of near-LTE air plasmas at atmospheric pressure. This method is specifically suitable for plasmas at relatively low gas temperature (800–1700 K) with no appropriate radiation for direct spectroscopic temperature measurements. Corona discharge producing cold non-equilibrium plasma is employed as a source of excitation and is placed into the microwave plasma jet. The gas temperature of the microwave plasma jet is determined as the rotational temperature of N₂? produced in the corona discharge. The corona probe temperature measurement was tested by the use of a thermocouple. We found a fairly good agreement between the two methods after correcting the thermocouple measured temperatures for radiative losses. The corona probe method can be generally applied to determine the temperature of the near-LTE plasmas and contrary to the thermocouple it can be used for higher plasma temperatures and is not affected by radiative losses and problems of interaction with the microwave plasma and electromagnetic fields.     

Measurement of wind noise levels in streamlined probes

This paper investigates the wind noise pressure spectra measured by aerodynamically designed devices in turbulent flow. Such measurement probes are often used in acoustic measurements in wind tunnels to reduce the pressure fluctuations generated by the interaction of the devices with the incident flow. When placed in an outdoor turbulent environment however, their performance declines noticeably. It is hypothesized that these devices are measuring the stagnation pressures generated by the cross flow components of the turbulence. Predictions for the cross flow contribution to the stagnation pressure spectra based on measured velocity spectra are developed, and are then compared to the measured pressure spectra in four different probe type devices in windy conditions outdoors. The predictions agree well with the measurements and show that the cross flow contamination coefficient is on the order of 0.5 in outdoor turbulent flows in contrast to the published value of 0.15 for measurements in a turbulent jet indoors.     

Flow field and acoustic properties of a Mach number 0·9 jet at a low Reynolds number

An experimental study of the flow field and acoustic properties of a low Reynolds number (Re ? 3600), M = 0·9 jet has been performed in our low pressure anechoic test chamber. The mean flow field was surveyed with a conventional Pitot pressure probe and flow fluctuations were detected with a normal hot wire probe. Also, condenser microphone measurements were made in the acoustic field. The major goal of the study was to develop a better understanding of the noise generation mechanisms of subsonic jets. The flow fluctuations within the jet were found to be dominated initially by a relatively discrete, large-scale, wave-like instability centered around a Strouhal number of 0·44. The axial wavelength of this instability was determined to be 1·45 jet diameters and its azimuthal character includes the n = 0 and n = ± 1 modes. The growth of this instability coupled with its non-linear breakdown are major contributors to the termination of the potential core region of the jet. The acoustic field of the jet, in contrast to the flow field, has a broad frequency spectrum with a peak amplitude near a Strouhal number of St = 0·2. The results indicate that a non-linear mechanism involving the large scale flow instability is responsible for a dominant portion of the noise generated from this jet.     

Correction of LIF temperature measurements for laser absorption and fluorescence trapping in a flame

A computational method is described in order to correct OH LIF temperature measurements for absorption of laser energy and trapping of fluorescence. Calculations are performed in a large range of flame conditions and can be used as a correction data base both in case of (0-0) and (1?0) excitations. Comparison of corrected temperatures profiles obtained in a 40 Torr methanol/air flame, for both kinds of Laser-Induced Fluorescence (LIF) excitations shows a very good agreement. This method is applied to measure the temperature profile of a methanol flame perturbed by a sampling probe. The LIF collection volume is located at the actual probe sampled volume using an experimental procedure already described. Spatial resolution and sensitivity of temperature measurements are sufficiently efficient to highlight, for the first time by LIF, an indubitable cooling effect due to the probe presence that induces important OH profile change. According to flame chemical modelling, it is shown that both effects are strongly correlated.     

Magnetic field influence on coflow laminar diffusion flames

The behavior of a laminar methane air flame with a central methane jet and a surrounding air coflow is analyzed in a large range of fuel and air flow rates. Different regimes of flame stability are described from an anchored flame to a stable lifted flame which is destabilized before extinction. Influence of an upward increasing magnetic field generated by an electromagnet is then studied. Experimental measurements at different values of methane and air flow rates show that the flame lift-off height is decreased by the magnetic gradient. These effects are attributed to the magnetic force which develops on air via its action on the paramagnetic oxygen molecules. The magnetic force interacts with the air jet structures upstream of the flame and then influences the flame stability.     

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.     

Flow visualization of vortex structure of an excited coaxial jet

This paper describes the results of experimental and numerical studies concerning the near-field vortical structure and dynamics of a coaxial jet. The effect of excitation of annular and circular jets on the vortical structure in the inner and outer shear-layers was studied. The flow visualization and the measurements of mean and fluctuating velocities, by a hot-film probe and the particle image velocimetry (PIV) technique, were carried out in an open water tank. The results of flow visualization and numerical simulation are in good agreement. With the excitation of a half of the initial vortex frequency, it is found that the vortex-pairing event is promoted by the forced excitation and it results in the rapid expansion of the jet width and the increase of velocity fluctuation and the entrainment rate. For the velocity ratio near unity, the dominant peak frequency coincides with the initial vortex frequency, i.e., the lip wake vortex frequency, independent of the forced excitation frequency.     

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.     

Turbulent structure in supersonic jets with a high Reynolds number

The turbulent properties of a supersonic jet were studied related to a high level of pressure pulsation found in model jets of a reentry flight vehicle approaching the landing ground. This study comprised measurements of total pressure at a small-size target using a dynamic pressure probe placed in a free jet. The most comprehensive data about jet turbulence can be obtained by direct transformation of the pressure reading at the stagnation point near the target into the normalized velocity. The oscillogram of normalized velocity produces the velocity average value, root-mean-square value as well as turbulence intensity and turbulence spectrum. It was demonstrated that a high level of turbulence for a high-head jet retains along the supersonic core length and at the beginning of subsonic interval.     

New scaling laws for hot and cold jet mixing noise based on a geometric acoustics model

New scaling laws are presented for hot turbulent jet mixing noise outside the cone of silence. These account for mean flow field effects on sound radiation via an analytical high frequency approximate solution to Lilley's equation. Numerical calculations for sound radiation from sources in a cylindrical shear flow are used to test the validity of the approximation. The proposed scaling laws yield an excellent collapse of jet noise measurements over a wide range of conditions. The resulting information has been incorporated into a jet mixing noise prediction scheme which, with appropriate modifications to the analytical high frequency approximation, can be applied both inside and outside the cone of silence. The prediction scheme for angles inside the cone of silence will be described in a subsequent paper.     

层流氩等离子体射流温度的测量

 采用给水冷管状静电探针施加负偏置电压、并使探针以一定速度垂直于射流轴线扫过层流氩等离子体射流的方法,测量探针所收集到的累积离子饱和电流随侧向位置的变化,利用Abel变换推导出了局部离子饱和电流密度沿射流径向的分布;采用自制的水冷动压探针,以动态扫描法测量了射流动压沿射流径向的分布;根据局部离子饱和电流密度和射流动压的测量数据,由理论关系式推导出了等离子体射流横截面上的温度分布,同时,采用谱线相对强度法测量了等离子体射流的激发温度。结果表明：两种方法得到的等离子体射流中心温度吻合较好,所得到的射流中心温度随弧电流加大而增大的变化趋势也一致。     

ACOUSTIC IDENTIFICATION OF COHERENT STRUCTURES IN A TURBULENT JET

Fluid-dynamic events associated with noise generation in a subsonic jet are educed by conditioning in-flow velocity and pressure signals on farfield sound measurements. The jet is located in an anechoic chamber, and farfield noise measurements are performed simultaneously with in-flow anemometric and acoustic measurements at a number of distances x from the nozzle (0?x/D?20, with D the jet diameter). The experimental data are then analyzed with a conditional averaging procedure using peaks in the acoustic signal as a trigger. An analysis of the method is developed and supported by numerical simulations. The averaging procedure permits the identification of the average time signatures of in-flow velocity and pressure associated with noise-generating coherent structures in the flow, their position at the emitting instants and their temporal statistics. The physical properties of the events associated with the averaged time signatures are then discussed.     

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。     

���ý����赲̽�뷨��������ѹ�������������������ܶ�

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.