Enthalpy Probe Measurements Research Articles

Diagnostics of Hybrid Water/Argon Thermal Plasma Jet with Water, Ethanol and Their Mixture Injection to Plasma

A plasma torch with the water/argon stabilization of an arc is characterized by extremely hot and high velocity plasma together with the relatively low flow rate of plasma, therefore, the torch might be properly used for plasma suspension spraying. Enthalpy probe and emission spectroscopy measurements were carried out at constant arc power with the combination of the injection of water, ethanol or their mixture to the plasma jet to diagnose the changes in temperature, heat flux and other characteristics.

Fluid-dynamic characterization of a radio-frequency induction thermal plasma system for nanoparticle synthesis

The fluid flow in a radio-frequency induction thermal plasma (RF-ITP) system for the synthesis of nanoparticles has been characterized using three- and two-dimensional modelling supported by enthalpy probe and calorimetric measurements in order to provide insights for the improvement of the process. The RF-ITP system is composed of a commercial inductively coupled plasma torch mounted on a reaction chamber that is equipped with viewports for diagnostics. The three-dimensional model predicted an almost axisymmetric temperature field in the reaction chamber in agreement with enthalpy probe measurements performed along two perpendicular scan axes, whereas recirculating flow patterns resulted in being strongly non-axisymmetric. Temperature profiles at two distances (60 mm and 100 mm) from the torch outlet have been calculated using two-dimensional modelling and compared with enthalpy probe measurements for different operating conditions with the aim of validating the predictive ability of the modelling tool. Calorimetric measurements have been performed in order to estimate the power coupled to the torch, which is usually an arbitrary input parameter for the models. Poor agreement was obtained between energy balances from modelling and from calorimetric measurements and, starting from this, a discussion on the uncertainties in the calculation of the radiative losses has been proposed. Finally, new insights for the improvement of the process of nanoparticle synthesis in the RF-ITP system are suggested.

Process diagnostics in suspension plasma spraying

Benefits and limitations of process diagnostics are investigated for the suspension plasma spraying of yttria-stabilized zirconia thermal barrier coatings. The methods applied were enthalpy probe measurements, optical emission spectroscopy, and in-flight particle diagnostic. It was proved that the plasma characteristics are not affected negatively by the injection of the ethanol based suspension since the combustion of species resulting from ethanol decomposition achieves a gain of plasma enthalpy. Furthermore, the conditions of the suspension injection into the plasma were found to be optimum as a significant content of evaporated powder material could be detected. Regarding the void content and segmentation crack density of the coatings, the in-flight particle diagnostic showed that the spray distance should be dimensioned in a way that the molten particles reach the substrates just before solidification starts.

Enthalpy probe measurements and three-dimensional modelling on air plasma jets generated by a non-transferred plasma torch with hollow electrodes

Thermal flow characteristics of air plasma jets generated by a non-transferred plasma torch with hollow electrodes are experimentally and numerically investigated in order to provide more reliable scientific and technical information, which has been insufficient for their practical applications to material and environmental industries. In this work, a thermal plasma torch of hollow electrode type is first designed and fabricated, and similarity criteria for predicting operational conditions for the scale-up to high-power torches are derived from the arc voltage characteristics measured with various operating and geometry conditions of the torch. The thermal flow characteristics of air plasma jets ejected from the torch are measured by enthalpy probe diagnostics and turn out to have relatively low temperatures of around 3000–7000 K, but show features of other unique properties, such as high energy flux, broad high temperature region and long plasma jet with moderate axial velocity, which are promising for their applications to material syntheses and hazardous waste treatments. Such high enthalpy at a relatively low temperature of air thermal plasma compared with the argon one is due to the high thermal energy residing in the vibrational and rotational states and oxygen dissociation, besides the translational states in monatomic gases such as argon. It is expected that this high specific enthalpy of the air plasma will enable material and environmental industries to treat a large amount of precursors and waste materials effectively at a lower temperature for a longer residence time by the low plasma velocity. It is also found from the measurements that the turbulence intensity influenced by the size of the electrode diameter has a significant effect on the axial and radial profiles of plasma jet properties and that a longer plasma jet is more readily achievable with a larger electrode diameter reducing the turbulence intensity in the external region of the torch. In the numerical studies based on magnetohydrodynamics (MHD) theory, a precise three-dimensional transient numerical model for the internal arc discharge plasma of the torch has been developed along with a practical two-dimensional stationary one for the external thermal plasma jet by considering highly localized distributions of arc roots with circumferential non-uniformity on the electrode wall surfaces, so that more reliable and realistic descriptions on the arc thermal plasma properties become feasible both inside and outside the torch. The numerical calculation results are compared with the experimental data obtained from the probe measurements and found to be in good agreement with them.

Inclusion of Aerodynamic Non-Equilibrium Effects in Supersonic Plasma Jet Enthalpy Probe Measurements

Low pressure plasma spraying (LPPS) is a thermal spraying technique that has found a niche for low oxidation products. It uses a low pressure environment (i.e., chamber pressure between 2 and 90 kPa) and yields supersonic plasma jets. The enthalpy probe technique is a common measurement method in plasmas. However LPPS jets are difficult to diagnose as their supersonic nature forces the apparition of a shock wave in front of any measuring device inserted in the jet. Incomplete or erroneous assumptions are usually invoked to overcome the difficulties associated with this shock wave and carry out the LPPS jet diagnosis from enthalpy probe measurements. In this work, a new device is designed to gain access to an additional physical quantity, which is needed to assess the aerodynamic non-equilibrium state of the jet. It is combined with enthalpy probe measurements, and the resulting set of experimental data is used with a numerical procedure based on gas dynamics theory, yielding free-stream supersonic plasma jet values from the measurements behind the induced shock wave. The results agree well with the phenomenology of supersonic jets in aerodynamic nonequilibrium. However this new method is restricted by the local thermodynamic equilibrium assumption, which is directly linked with the pressure and temperature conditions of the plasma jet.

Entrainment in high-velocity, high-temperature plasma jets. Part I: experimental results

The development of a high-velocity, high-temperature argon plasma jet issuing into air has been investigated. In particular the entrainment of the surrounding air, its effect on the temperature and velocity profiles and the subsequent mixing and dissociation of oxygen has been examined in detail. The total concentration of oxygen and the velocity and temperature profiles in the jet were obtained from an enthalpy probe. High-resolution Thomson scattering provided an independent measure of plasma velocity and temperature, validating enthalpy probe measurements and providing non-intrusive measurements near the nozzle exit. The concentration of atomic oxygen was obtained from 2-photon laser induced fluorescence. Molecular oxygen concentration and temperature was obtained from coherent anti-Stokes Raman spectroscopy. It was found that both the incompleteness of mixing at the molecular scale and the rate of oxygen dissociation and recombination affects jet behavior.

Synthesis of Si–C–N coatings by thermal Plasmajet chemical vapour deposition applying liquid precursors

AbstractMaterials in the system Si–C–N feature excellent properties for wear protection applications, even at elevated temperatures, and an excellent thermal shock resistance. As these materials have no melting point, coatings have to be manufactured via a synthesis. Conventional chemical vapour deposition (CVD) processes have the disadvantage of low deposition rates. Thermal Plasmajet CVD processes with liquid feedstock feature the highest deposition rates among the gas‐phase synthesis processes. Single and triple DC torches and HF torches with supersonic nozzles have successfully been applied to produce Si–C(–N) coatings on different steel, aluminium, titanium and copper alloys, as well as on graphite. Besides chlorosilanes, hexamethyldisiloxane, tetramethyldisiloxane and hexamethyldisilazane have been used as liquid single precursors. Deposition rates up to 1500 μm h−1 have been achieved. The coatings show cauliflower, columnar or dense morphology and an amorphous or nanocrystalline structure. The formation of both α‐ and β‐Si3N4 has been verified by X‐ray diffraction. The application of chlorosilanes always results in chlorine‐containing coatings. The chlorine causes severe corrosion in the interface to mild carbon steel substrates. The processes are compared taking into account their characteristics concerning the injection modes, gas temperature and velocity profiles determined by enthalpy probe measurements. The process conditions are correlated to the coating microstructure and the adhesion to the substrates and guidelines for the optimum production of ­Si–C–N coatings by Plasmajet CVD are deduced. Emission spectroscopy is used to determine the mechanisms of the coating formation. Full dissociation of the liquid feedstock in the plasma jet has been verified. Copyright © 2001 John Wiley & Sons, Ltd.

Chemical vapor deposition of an aluminum nitride–diamond composite in a triple torch plasma reactor

An aluminum nitride–diamond composite has been produced by sequential deposition of AlN and diamond in a triple torch plasma reactor (TTPR). AlN was deposited from AlN powder by injection into the argon–nitrogen, converging plasma plume of a TTPR. Velocity and temperature profiles of the converging plasma plume, obtained by enthalpy probe measurements, were used to show that the powder decomposed prior to reaching the substrate. Diamond was deposited in an argon–hydrogen–methane system onto the existing AlN film. Characterization of an AlN–diamond–AlN composite indicated a Vickers hardness of 18.6 GPa and a modulus of elasticity of 245–282 GPa.

Characterization of the Converging Jet Region in a Triple Torch Plasma Reactor

Enthalpy probe measurements were taken of the converging plasma plume in a triple torch plasma reactor and related to substrate heat flux measurements. Results show excellent entrainment of process gases injected into the converging plasma plume by way of the central injection probe. At lower pressures (≤40 kPa), the plasma volume is equivalent to at least a 3 cm diameter, 4 cm long cylinder, with relatively uniform temperature, velocity, and substrate heat flux profiles when compared to a typical dc arc jet. Converging plasma plume size, substrate heat flux, and enthalpy profiles are also shown to be a strong function of applied system power. Substrate heat flux measurements show smaller radial gradients than enthalpy probe measurements, because of the high radial velocity component of gases above the substrate boundary layer. Enthalpy probe measurements were also conducted for diamond deposition conditions and approximate temperature and velocity profiles obtained. Problems with the uniform gas mixture assumption prohibited more accurate measurements. Reproducibility of enthalpy measurement results was shown with an average standard deviation of 11.8% for the velocity and 7.6% for the temperature measurements.

Enthalpy probe diagnostics of an atmospheric pressure argon microwaveinduced plasma

An argon microwave-induced plasma (MIP) at atmospheric pressure wasgenerated in a TE10 mode waveguide. The discharge was sustained bya relatively low absorbed power of around 215 W. An enthalpy probe was used tocharacterize the argon MIP discharge, yielding values of enthalpy, velocityand heavy particle temperature along the jet axis. The theory of the enthalpyprobe operation and calibration is reviewed and the system for itsimplementation is described. The heavy particle temperature is found to beabout 1600 K and the velocity to be about 150 m s-1 at theexit plane of the discharge tube for an argon flow rate of 1.6 l min -1. Although the assumption of local thermodynamic equilibrium isrequired to determine the temperature and velocity in the jet from the enthalpyprobe measurements, it is argued that the values obtained should be reasonableeven if, as expected in MIPs, the electron temperature issignificantly higher than the heavy particle temperature.

Enthalpy Measurements of Direct Current Plasma Jets Used for ZrO<SUB>2</SUB> · Y<SUB>2</SUB>O<SUB>3</SUB> Thermal Barrier Coatings

This study combines enthalpy probe measurements in a direct current (DC) plasma jet under atmospheric conditions and the characterization of sprayed coatings. Two different nozzle diameters were used as anode profiles. A detailed description of the experimental setup is given together with preliminary results. These results emphasize the usability of enthalpy probe measurements in thermal plasma jets for obtaining temperature distributions. For zirconia, the deposition efficiency, density, and structure of the coatings formed with the different anode profiles were compared.

Computational study of high-speed plasma flow impinging on an enthalpy probe

Enthalpy probe measurements in supersonic plasma flows are subject to various sources of error which are difficult to quantify experimentally v. The relative importance of several such errors has been assessed by means of detailed two-dimensional numerical simulations of high-speed plasma flow impinging on an enthalpy probe. The simulations show that moderate uncertainties in upstream pressure and composition (i.e., degree of ionization) can lead to significant errors in the velocity and temperature inferred from the measurements. These errors tend to be larger in velocity than temperature A second potential source of error is that enthalpy probe data are generally interepreted by means of simplified analytical relations which neglect the effects of finite-rate ionization, internal electronic excitation, thermal radiation, probe cooling, and probe sampling. The importance of these effects was also assessed, and the resulting errors were not ,significant under the conditions examined. We conclude that enthalpy probe measurements in supersonic plasma flows are use f d in situations where the upstream pressure and degree of ionization are known to reasonable accucary.

Nanoparticle formation using a plasma expansion process

We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.

Diffusion phenomena of a cold gas in a thermal plasma stream

The present study involves both experimental investigation and mathematical modeling of the diffusion process of a cold gas injected into a main plasma stream. The cold gas (nitrogen or helium) was injected axially through a water cooled tube located along the centerline of an induction plasma torch. The 2-D distribution of the temperature, velocity and concentration profiles in the plasma flow were measured using enthalpy probe techniques. The results are compared with the predictions of a 2-D, LTE, turbulent mathematical model. The effects of the nature (composition) of the injected gas and its mass flow rate are investigated. The enthalpy probe measurements and the predictions of the model are in good agreement. The effective (turbulent and molecular) transport properties are estimated from a comparison of the measured and calculated profiles of the temperature, velocity and concentration fields. This study sheds light on the basic diffusion mechanisms involved in a widely used configuration of induction plasma reactors, i.e. in which the material to be treated is injected axially into the plasma, through a central water cooled tube.

A two-fluid model of turbulence for a thermal plasma jet

In this paper, a turbulent argon plasma jet issuing into a stagnant argon environment at 1 ATM is .studied by applying a two-fluid turbulence model, in order to advance our understanding of thermal plasma jets. The mathematical model has some similarities to the models of two-phase flows, so that the turbulent plasma jet is treated as a two-phase mixture. The governing equations include the transport equations far mass, momentum, and energy far two different fluid parcels (in-moving parcels and out-moving parcels). Auxiliary relations that govern the physical phenomena of the interfluid mass, momentum, and energy exchange are preserved together with a description of the mechanisms that control the growth or diminution of the fragment size. The results are presented with conditional- and unconditional-averaged forms and compared with experimental results from enthalpy-probe measurements. A well-known nondimensional farm (a Gaussian error function) can represent the radial distributions of the measured- and predicted-unconditional mean axial velocity and temperature in consecutive sections (20–45 mm from the nozzle exit). Further insight into the behavior of turbulent plasma jets can be gained by looking at the conditional fluid properties. The results show that this model can predict phenomena that escape more conventional models, e.g., the uninixing phenomenon.

Diagnostics of a thermal plasma jet by optical emission spectroscopy and enthalpy probe measurements

An accurate determination of electron density, temperature, and velocity distributions is of primary interest for the characterization of steady-state thermal plasma spray jets. Our diagnostic capabilities based on optical emission spectroscopy include measurements of absolute emission coefficients and Stark broadening. In addition, enthalpy probe diagnostics has also been used for temperature and velocity measurements. Observation of large discrepancies between temperatures derived from absolute emission coefficients, Stark broadening, and from enthalpy probe measurements indicate that severe deviations from LTE (local thermal equilibrium) exist in various regimes of plasma spray jets. Nonequilibrum characterization of such turbulent thermal plasma jets suggests that diffusion of high-energy electrons into the fringes of plasma jets and deviations from chemical equilibrium due to high velocities in the core of plasma jets and entrainment of cold gas, are the main reasons for these discrepancies. The establishment of a reliable data base, taking these nonequilibrium effects into account, is a prerequisite for meaningful modeling of real plasma jets.

Enthalpy probe performance in compressible thermal plasma jets

Enthalpy probe measurements in compressible argon/helium thermal plasma jets are compared with results from high spectral resolution laser light scattering. In the laser scattering measurement the plasma temperature and velocity are determined directly from high-resolution line-shape analysis of light scattered by the plasma. The technique yields an unambiguous determination of gas or kinetic temperature without the assumption of local thermodynamic equilibrium. Velocity is determined directly from the measured Doppler shift. The enthalpy probe is a combination stagnation probe and flowing calorimeter. Gas temperatures and velocities are calculated from measured values using both frozen flow and equilibrium flow assumptions for the stagnation process. Over the Mach number range examined, the assumption of isentropic, frozen composition stagnation best matches the laser results. In the jet periphery a significant streamline displacement is caused by the presence of the probe. The displacement, which is a function of the nondimensional radial velocity gradient, distorts the velocity and temperature results in the wings of the profiles. In the core of the jets examined, probe measurements in both subsonic and supersonic compressible flows show excellent agreement with laser scattering results.