Laser Doppler anemometry under plasma conditions. Part I. Measurements in a d.c. plasma jet

A study is presented on the use of laser Doppler anemometry (LDA) techniques for the measurement of the gas and particle velocities under plasma conditions. Experimental data is presented for a d.c. plasma jet in which alumina particles are injected under different operating conditions. The results reveal that the plasma velocity at the exit of the jet is of the order of 200–300 m/s. The intensity of turbulence is as high as 30 to 40% in the free shear layer and the particle velocity distribution is shown to be asymmetric, with particle dispersion in the plane of injection considerably more important than that in the perpendicular direction. The average particle velocity depends on the composition of the plasma gas, the torch current, and power.     

Plasma spraying of alumina: Plasma and particle flow fields

A comprehensive experimental examination of the interaction between a subsonic thermal plasma jet and injected alumine, particles is presented. Measurements of plasma velocity, temperature, and entrained air were obtained from an enthalpy probe and mass spectrometer combination. A diffusive separation, or demixing, of the Ar and He plasma gas was observed. Centerline plasma velocities and temperatures ranged from 1501500 m/s and 2000 to 14,000 K, respectively, in the region between the torch and a typical substrate location of 90 mm. Measurements of particle size, velocity, tempearture and local number density were obtained from a combination laser particle sizing system, Laser doppler velocimeter (LDV), and two-color pyrometer. Centerline temperatures and velocities for the nominally 30 m particles used were 2400–2800 K and 150–200 m/s, respectively.     

L.D.A. Simultaneous Measurements of Local Density and Velocity Distribution of Particles in Plasma Fluidized Bed at Atmospheric Pressure

Laser Doppler anemometry (L.D.A.) is an efficient and nonintrusive technique. Today, improved in its configuration, the L.D.A. has been applied even in flowing plasmas. ^(1,2) In-flight simultaneous measurements were performed for local density and velocity of particle distribution. The measurements provide an insight into thermal and mass transfer, chemical reactivity, and the distribution of residence times of particles in a plasma fluidized bed. The difficulties of L.D.A. in a plasma fludized bed such as high emission intensity of the plasma torch, high temperature, high particle density, and large distribution of particle granulometry were overcomed in the present investigation. The aims achieved were the characterization of the plasma fluidized bed distribution together with accurate measurements of local particle density and velocity as measured by L.D.A.     

Influence of velocity and surface temperature of alumina particles on the properties of plasma sprayed coatings

In this paper are described the main characteristics of the plasma spraying process of alumina deposits, i.e., the temperature and flow field of the plasma jets obtained with the classical spraying torches, the injection of the particles into the plasma jet, the particle surface temperature and velocities in the plasma (measured for calibrated alumina particles), and the coating generation. The measurements on the alumina particles are compared with the predictions of a mathematical model. The experimental and computed particle velocities are in rather good agreement. However, this is not the case for the particle surface temperature. Possible reasons for the discrepancy are proposed (influence of the carrier gas, thermophoretic forces, and poor penetration of the particles into the plasma core even for an injection velocity twice that of the optimal calculated one, as shown by recent measurements). Finally the correlations between the particle velocities and surface temperature, and the properties of the alumina coating (porosity, crystal structure, mechanical properties) are studied.     

Heat Generation and Particle Injection in a Thermal Plasma Torch

The operation of plasma guns used for plasma spraying involves a continuous movement of the anode arc root. The resulting fluctuations of voltage and thermal energy input introduce an undesirable element in the spray process. This paper deals with the effects of these arc instabilities on the plasma jet, and the behavior of particles injected in the flow. The first part refers to the formation of the plasma jet. Measurements show that the static behavior of the arc depends strongly upon the plasma-forming gas mixture, especially the mass flow rate, of the heavy gas, injection mode, nozzle diameter, and arc current. These parameters control the electric field in the arc column, the arc length, its stability, and the gas velocity and temperature. The dynamic behavior of the arc is examined to determine how the tempeature and velocity of the plasma gas vary with voltage variations. Relationships between the gas velocity at the nozzle exit and the lifetime of the arc roots, and the independent operating parameters of the gun can be established from a dimensional analysis. The second part discusses the interaction between the plasma jet and the particles injected into the flow. The parameters controlling particle injection and trajectory are examined to determine how injection velocity must vary with particle size and density to achieve a given trajectory. The effect of the transverse injection of the powder carrier gas is investigated using a 3-D computational fluid dynamics code. Finally, the effect of the jet fluctuations on particle trajectory is studied under the assumption that the jet velocity follows the voltage variation. The result is a continuous variation of the particle spray jet position in the flow. Experimental observations confirm the model predictions.     

Drag force acting on an evaporating particle immersed in a rarefied plasma flow

Analytical expressions are presented for the drag force acting on an evaporating or nonevaporating particle immersed in a plasma flow for the extreme case of free-molecule flow regime and thin plasma .sheath. It is shown that the drag force on a spherical particle is proportional to the square of the particle radius and to the relative velocity between the particle and the bulk plasma at low speed ratios. The existence of a relative velocity between the particle and the plasma results in a nonuniform heat flux distribution with its rnaximum value at the frontal stagnation point of tire sphere. This nonuniform distribution of the local heat fux density causes a nonuniforrn distribution of the local evaporated-mass flux and vapor reaction force around the surface of an evaporating particle, and thus induces an additional force on the particle. Consequently, the drag force acting on art evaporating particle is always greater than that on a nonevaporating one. This additional drag force due to particle evaporation is more significant for nonmetallic particles and for particle materials with lower latent heat of evaporation and lower vapor molecular mass. It increases with increasing plasma temperature and with decreasing gas pressure at the high plasma temperatures associated with appreciable gas ionization. The drag ratio increases with increasing electron/heavy-particle temperature ratio at high electron temperatures for a two-temperature plasma.     

The influence of laser-particle interaction in laser induced breakdown spectroscopy and laser ablation inductively coupled plasma spectrometry

Particles produced by previous laser shots may have significant influence on the analytical signal in laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma (LA-ICP) spectrometry if they remain close to the position of laser sampling. The effects of these particles on the laser-induced breakdown event are demonstrated in several ways. LIBS-experiments were conducted in an ablation cell at atmospheric conditions in argon or air applying a dual-pulse arrangement with orthogonal pre-pulse, i.e., plasma breakdown in a gas generated by a focussed laser beam parallel and close to the sample surface followed by a delayed crossing laser pulse in orthogonal direction which actually ablates material from the sample and produces the LIBS plasma. The optical emission of the LIBS plasma as well as the absorption of the pre-pulse laser was measured. In the presence of particles in the focus of the pre-pulse laser, the plasma breakdown is affected and more energy of the pre-pulse laser is absorbed than without particles. As a result, the analyte line emission from the LIBS plasma of the second laser is enhanced. It is assumed that the enhancement is not only due to an increase of mass ablated by the second laser but also to better atomization and excitation conditions favored by a reduced gas density in the pre-pulse plasma. Higher laser pulse frequencies increase the probability of particle-laser interaction and, therefore, reduce the shot-to-shot line intensity variation as compared to lower particle loadings in the cell. Additional experiments using an aerosol chamber were performed to further quantify the laser absorption by the plasma in dependence on time both with and without the presence of particles. The overall implication of laser-particle interactions for LIBS and LA-ICP-MS/OES are discussed.     

Laser Doppler anemometry under plasma conditions. Part II. Measurements in an inductively coupled r.f. plasma

Laser Doppler anemometry is used for the measurements of the plasma and particle velocity profiles in the coil region of an inductively coupled r.f. plasma. Results are reported for a 50 mm i.d. induction plasma torch operated at atmospheric pressure with argon as the plasma gas. The oscillator frequency is 3 MHz and the plate power is varied between 4.6 and 10.5 kW. Plasma velocity measurements are obtained using a fine carbon powder as a tracer. Measurements are also given for larger silicon particles ( ) centrally injected into the discharge under different operating conditions.Nomenclature d _p particle diameter - P ₀ plasma power - Q ₁ powder carrier gas flow rate - Q ₂ plasma gas flow rate - Q ₃ sheath gas flow rate - r distance in the radial direction - V axial plasma velocity - V _p axial particle velocity - Z distance in the axial direction - standard deviation     

A Proposed Process Control Chart for DC Plasma Spraying Process

A process control chart is proposed for DC plasma spraying process based on the in-flight simulation of the injected states of the particles determined by computational fluid dynamics analysis (via FLUENT V4.3). The chart consists of five regions, i.e., the unmelted, melted, vaporized, escaped, and rebounded, which represent the various states of the particles at impact on the substrate. The X and Y axes of the chart are particle entry conditions, i.e., diameter (ranging from 20 to 100 m) and injection velocity (between 10 to 50 m/s), respectively. The regions indicate the fate of the particle on impact. A grid-array of (14×11) entry conditions is simulated in developing the chart. The proposed chart is aimed at providing a general guideline for plasma spraying process in achieving a thoroughly melted particle on arrival at the substrate to be coated.     

Powder Loading Effects of Yttria-Stabilized Zirconia in Atmospheric dc Plasma Spraying

Powder loading effects have been reexamined for various yttria-stabilized zirconia powders under atmospheric dc plasma spraying. A laser illumination method was utilized to observe powder injection into the plasma jet, while single particle and ensemble methods to measure particle state parameters. Statistical temperature distributions of in-flight particles suggested a rapid increase in the number of semi-molten particles above a certain powder loading rate. Despite drops in the particle temperature and velocity due to the powder loading effect, the deposition efficiency tends to have increased in some cases. Reliability of the single particle and ensemble methods has also been examined at various powder feed rates. Particle temperature measurement by the ensemble method at low powder feed rates could cause a significant error, which may affect powder injection optimization. Particle plume trajectory was not affected as much by the powder loading, which hence had only a limited effect on the particle diagnostics.     

Three-dimensional distribution of signal from single monodisperse aerosol particles in a laser induced plasma: Initial measurements

Evidence is provided for a spatial distribution of signal in laser-induced breakdown spectroscopy (LIBS) measurements of single aerosol particles. Monodisperse 1.5-μm diameter silica microspheres are sampled, and particle emission is measured as a function of three-dimensional (3-D) particle location. A combination of scattered-light imaging and spatially resolved spectroscopy is used to determine 3-D particle location. Particle signal is found to be optimized in a region near the major axis of the plasma, on the end of the plasma nearest the incident plasma-forming laser beam. The demonstration of an optimal location for particle signal motivates the development of techniques that account for the effect of particle location to increase measurement precision.     

Laser-induced particulate as carrier of analytical information in LA-ICPMS direct solid microanalysis

Laser ablation in combination with plasma spectrochemistry is an ideal technique for depth profiling analysis, based on signal profiles. However, signal profiles were found to be critically influenced by the characteristics of the ablated particles, especially their composition and size distribution, and consequently transport mechanism and plasma-assisted vaporization efficiency. Even for a refractory material like ceramic, relics of melting following laser irradiation were found, so that particles were non-stoichiometric as compared to the parent material. Estimates of transport efficiency showed that this is highly variable as a function of particle size. Large particles are likely to be lost in the sample chamber. Fine particles are prone to wall reaction, especially in Ar ambient. Variability in particle delivery to the ICP-MS was suspected to be the cause for an element-dependent analyte signal response. Fluctuation in particle vaporization degree as a consequence of plasma temperature instability was also responsible for element-dependent signal profile deviation. However, for a 10-fold higher mass load into the plasma, no direct fractionation effects were observed. Differential transport of chemically-differentiated analyte-carriers is suggested to be primary cause for element-dependent signal structure.     

A contribution to non-equilibrium chemical kinetics. II. Strongly non-equilibrium “hot” reactions in liquids and solids

The method proposed in part I for non-equilibrium chemical kinetics is applied to processes provoked by non-equilibrium assemblies of energetic particles in liquids and solids. The movement of such an energetic particle belonging to a certain energy group is considered as a stochastic process when the direction of the velocity is changed stochastically at each step. On the ground of this consideration a simplified model of such a process is introduced: the stochastic movement of a particle is replaced by the deterministic movement of the corresponding quasi-particle having parameters determined through corresponding averages of the stochastic process. By use of this model, group constants of kinetic equations of our abovementioned work were expressed through parameters of microscopic processes in solids and liquids, and systems of non-equilibrium chemical kinetics' equations were written for different case. The proposed approach also permits us to consider the non-equilibrium of the crystalline lattice created by energetic particles. “Hot spot” reactions were considered as an example and a method to distinguish between direct and “hot spot” reactions was indicated. The proposed approach and obtained kinetic equations can be applied to recoil atoms (ions), fission products, hot particles produced in radiation chemistry, photochemistry, by laser beams, flash-photolysis etc. The destruction of the crystalline lattice by laser beams can also be considered by use of these equations.     

System for <Emphasis Type="Italic">In Situ</Emphasis> Characterization of Nanoparticles Synthesized in a Thermal Plasma Process

We have designed a particle diagnostic system that is able to measure particle size and charge distributions from low stagnation pressure (≥746 Pa) and high temperature (2000–4000 K) environments in near real time. This system utilizes a sampling probe interfaced to an ejector to draw aerosol from the low pressure chamber. Particle size and charge distributions are measured with a scanning mobility particle sizer. A hypersonic impactor is mounted in parallel with the scanning mobility particle sizer to collect particles for off-line microscopic analysis. This diagnostic system has been used to measure size and charge distributions of nanoparticles (Si, Ti, Si–Ti–N, etc.) synthesized with our thermal plasma reactor. We found that the mean particle size increases with operating pressure and reactant flow rates. We also found that most particles from our reactor are neutral for particles smaller than 20 nm, and that the numbers of positively and negatively charged particles are approximately equal.     

Parameters controlling the generation and properties of plasma sprayed zirconia coatings

D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H₂ (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.     

Effect of particle charging on momentum and heat transfer from rarefied plasma flow

The features of interaction of a spherical metallic particle with a rarefied thermal plasma flow due to the presence o charges-electrons and ions in the gaseous phase-are considered. Analytical expressions describing charge, momentum, and energy exchange between the plasma and the particle für the cases of strong and weak Debye screening are obtained. It is illustrated that the efficiency of particle heating in the plasma considerably grows as compared with a hot molecular gas due to participation of electrons and ions in file transfer processes.     

Experimental investigation of powder vaporization in thermal plasma jets

An experimental study of the vaporization of metallic and ceramic particles in a thermal do plasma jet has been initiated and two series of experiments have been performed: (1) measurement of the vapor concentration within the plasma jet by absorption spectroscopy. (2) Investigation of the vapor cloud surrounding a single particle in flight by emission spectroscopy. The temperature within this cloud is determined by the intensity ratio of two lines which are simultaneously measured. The cloud radius is deduced /torn measurement of the particle velocity by laser doppler anemometry, and the vapor concentration is calculated from the line intensity profile, once the temperature is known. Results on iron and alumina particles injected in argon or argon-hydrogen plasma jets are given and discussed.     

Photophoresis of small-size particles in a gas excited by resonance radiation

The motion of small aerosol particles in uniform gas excited by a resonance radiation is caused by the specific mechanism. In the case of a small deviation of the laser radiation frequency from that of the electronic or the vibration-rotation transition, due to the Doppler effect, only the molecules whose velocity projection on the radiation propagation direction lies within a certain velocity range can be excited. For this reason, the relationship between the excited and the nonexcited molecules in the aforesaid velocity range differs from the corresponding relationship in the velocity range, which is symmetrical with regard to the center of the Maxwell distribution. A resultant force, which is applied by the gas to aerosol particles, arises due to the different scatterings of the excited and nonexcited molecules by the surface of aerosol particles.     

Experimental and Theoretical Study of the Impact of Alumina Droplets on Cold and Hot Substrates

A complex experimental set-up was built to study the impact of liquid alumina droplets on different substrates (stainless steel 304L, sintered alumina, carbon–carbon) kept at temperatures up to 2100 K. The impact behavior: rebound, deposition, splashing, spattering was systematically studied as well as the resulting splat shapes. The set-up consists in a controlled atmosphere chamber where molten alumina particles with diameters between 10 and 90 m, are produced by a d.c. plasma torch, substrates being heated by a second d.c. plasma torch. In such conditions, it was possible to achieve particle temperatures between 2300 and 4200 K with velocities in the range 50 to 300 m/s. The particle behavior at impact was characterized by the Sommerfeld parameter K (K=We^1/2 Re^1/4 We and Re being respectively the Weber and Reynolds numbers of impacting particles). It was possible to vary K between 3 and 1300. Low K values were obtained by tilting the substrate up to 60°. The parameters of a single particle at impact were measured: its velocity v_p and diameter d_p by Phase Doppler Anemometry (v_p=5%, d_p=10%) and its temperature T_p by fast (100 ns) two color pyrometry (T_p=15%). The particle impact was visualized by a fast camera coupled to a microscope (exposure delay time 50 ns . . .100 ms) with complex synchronization and light intensity problems. To solve the latter, the impacting particle had to be illuminated with a 2 W c.w. Ar⁺ laser at 488 nm. Unfortunately, the controlled atmosphere chamber did not allow to change the substrate after each particle impact. Starting from a smooth surface for the first impact, due to the successively deposited splats, rapidly droplets impacted on a rough surface (Ra5 m). For splats collected on a hot alumina substrate (2100 K), where flattening is completed before solidification starts (case similar to that of ethanol droplets on cold copper) deposition occurs for K between 4 and 90 while splashing occurs for K as low as 30. These results are slightly different from those related to the ethanol droplet for which deposition occurs for 357.7. This could be due to the precision of measured values and the rough surface. For splats collected in spraying conditions splashing is always the rule K values up to 1400) especially on rough surfaces. However the particle impact velocity and temperature, the substrate temperature and tilting plays an important role on the resulting splat diameters, distortion and elongation rates. The question which is still pending is which quantity of splashed material is incorporated within the constructing coating and how does it affect its thermophysical properties.     

Diffusiophoresis in a concentrated suspension of colloidal spheres in nonelectrolyte gradients

The diffusiophoretic motion of a homogeneous suspension of identical spherical particles is considered under conditions of small Reynolds and Peclet numbers. The effects of interaction of the individual particles are taken into explicit account by employing a unit cell model which is known to provide good predictions for the sedimentation of monodisperse suspensions of spherical particles. The appropriate equations of conservation of mass and momentum are solved for each cell, in which a spherical particle is envisaged to be surrounded by a concentric shell of suspending fluid, and the diffusiophoretic velocity of the particle is calculated for various cases. Analytical expressions of this mean particle velocity are obtained in closed form as functions of the volume fraction of the particles. Comparisons between the ensemble-averaged diffusiophoretic velocity of a test particle in a dilute suspension and our cell-model results are made. Received: 30 June 1999 Accepted: 8 December 1999