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Jean François Coudert - One of the best experts on this subject based on the ideXlab platform.

  • Influence of Time-Modulation of Applied Current on Arc Stability in DC Pulsed Plasma Spray Torch
    2017
    Co-Authors: F. Mavier, Vincent Rat, Jean François Coudert
    Abstract:

    Conventional direct current arc plasma Spray Torches generate highly unstable plasma jets. This leads to some difficulties with control of coating properties especially when injecting liquid feedstock for the elaboration of finely structured coatings. At constant applied current, a properly designed arc plasma Torch can work in self-sustained pulsed mode that produces large amplitude oscillations of the arc, which are interrupted by very repeatable restrikes. This paper investigates the influence of the modulation of the applied current on the arc stability. Electrical and acoustic diagnostics permit to show that amplification of arc oscillation and a stabilizing mechanism occur when the arc current frequency approaches the Torch resonance frequency. Time-resolved optical emission spectroscopy is used to determine temperature and enthalpy profiles close to the resonance and the enthalpy modulation.

  • Electric Arc Fluctuations in DC Plasma Spray Torch
    2017
    Co-Authors: Vincent Rat, F. Mavier, Jean François Coudert
    Abstract:

    Direct current plasma Torches for plasma Spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma Torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma Torch domain that can generate pressure fluctuations coupled with voltage ones.

  • Analytical interpretation of arc instabilities in a DC plasma Spray Torch: the role of pressure
    2016
    Co-Authors: Vincent Rat, Jean François Coudert
    Abstract:

    Arc instabilities in a plasma Spray Torch are investigated experimentally and theoretically thanks to a linear simplified analytical model. The different parameters that determine the useful properties of the plasma jet at the Torch exit, such as specific enthalpy and speed, but also pressure inside the Torch and time variations of the flow rate are studied. The work is particularly focused on the link between the recorded arc voltage and the pressure in the cathode cavity. A frequency analysis of the recorded voltage and pressure allows the separation of different contributions following their spectral characteristics and highlights a resonance effect due to Helmholtz oscillations; these oscillations are responsible for the large amplitude fluctuations of all the parameters investigated. The influence of heat transfer, friction forces and residence time of the plasma in the nozzle are taken into account, thanks to different characteristics' times. The volume of the cathode cavity in which the cold gas is stored before entering the arc region appears to be of prime importance for the dynamics of instabilities, particularly for the non-intuitive effect that induces flow-rate fluctuations in spite of the fact that the Torch is fed at a constant flow rate.

  • Resonant mode for a dc plasma Spray Torch by means of pressure-voltage coupling: application to synchronized liquid injection
    2013
    Co-Authors: Joanna Krowka, Vincent Rat, Jean François Coudert
    Abstract:

    Electric arc instabilities in dc plasma Torches result in non-homogeneous treatment of nanosized solid particles injected into the plasma jets. In the particular case of suspension plasma Spraying, large discrepancies in the particles trajectories and thermal histories make the control of coating properties more difficult to achieve. In this paper, a new approach of arc dynamics highlights the existence of different resonant modes and the possibility of their coupling. This study leads us to design a special plasma Torch working in a very regular pulsed regime. Then, an innovative injection system based on the drop-on-demand method synchronized with the plasma oscillations is presented as an efficient method to control the dynamics of plasma/particles interactions

  • Influence of configuration and operating conditions on the electric arc instabilities of a plasma Spray Torch: role of acoustic resonance
    2008
    Co-Authors: Jean François Coudert, Vincent Rat
    Abstract:

    A theoretical approach is proposed to explain the particularities of the power spectrum of a plasma Spray Torch voltage. This is founded on an acoustic resonance of the rear part of the plasma Torch which is occupied by the cold gas before it reaches the arc region. The spectrum is characterized by the presence of a sharp peak in the range 3–8 kHz, with an amplitude that represents up to ±30% of the voltage mean value. The peak frequency presents an evolution that is governed by the Torch operating conditions, the thermal properties of the plasma forming gas and the geometrical configuration of the electrode assembly. The theory is compared with experimental values of the peak frequency recorded for different operating conditions and Torch configurations. The injection ring through which the gas is injected also plays an important role. The Torch behaviour is carefully characterized in order to collect all the experimental data required for the analysis on the involved phenomena.

William L. Oberkampf - One of the best experts on this subject based on the ideXlab platform.

  • Computational fluid dynamics analysis of a wire-feed, high-velocity oxygen fuel (HVOF) thermal Spray Torch
    1998
    Co-Authors: A.r. Lopez, William L. Oberkampf, B. Hassan, R. A. Neiser, T.j. Roemer
    Abstract:

    The fluid and particle dynamics of a high-velocity oxygen fuel (HVOF) thermal Spray Torch are analyzed using computational and experimental techniques. Three-dimensional computational fluid dynamics (CFD) results are presented for a curved aircap used for coating interior surfaces such as engine cylinder bores. The device analyzed is similar to the Metco diamond jet rotating wire (DJRW) Torch. The feed gases are injected through an axisymmetric nozzle into the curved aircap. Premixed propylene and oxygen are introduced from an annulus in the nozzle, while cooling air is injected between the nozzle and the interior wall of the aircap. The combustion process is modeled using a single-step, finite-rate chemistry model with a total of nine gas species which includes dissociation of combustion products. A continually fed steel wire passes through the center of the nozzle, and melting occurs at a conical tip near the exit of the aircap. Wire melting is simulated computationally by injecting liquid steel particles into the flow field near the tip of the wire. Experimental particle velocity measurements during wire feed were also taken using a laser two-focus (L2F) velocimeter system. Flow fields inside and outside the aircap are presented, and particle velocity predictions are compared with experimental measurements outside of the aircap.

  • Computational Analysis of a Three-Dimensional High-Velocity Oxygen Fuel (HVOF) Thermal Spray Torch
    1998
    Co-Authors: B. Hassan, A.r. Lopez, William L. Oberkampf
    Abstract:

    An analysis of a high-velocity oxygen fuel thermal Spray Torch is presented using computational fluid dynamics (CFD). Three-dimensional CFD results are presented for a curved aircap used for coating interior surfaces such as engine cylinder bores. The device analyzed is similar to the Metco diamond jet rotating wire Torch, but wire feed is not simulated. The feed gases are injected through an axisymmetric nozzle into the curved aircap. Argon is injected through the center of the nozzle. Premixed propylene and oxygen are introduced from an annulus in the nozzle, while cooling air is injected between the nozzle and the interior wall of the aircap. The combustion process is modeled assuming instantaneous chemistry. A standard, two-equation, k-e turbulence model is employed for the turbulent flow field. An implicit, iterative, finite volume numerical technique is used to solve the coupled conservation of mass, momentum, and energy equations for the gas in a sequential manner. Computed flow fields inside and outside the aircap are presented and discussed.

  • Computational fluid dynamics analysis of a wire-feed, high-velocity oxygen-fuel (HVOF) thermal Spray Torch
    1996
    Co-Authors: A.r. Lopez, William L. Oberkampf, B. Hassan, R. A. Neiser, T.j. Roemer
    Abstract:

    The fluid and particle dynamics of a High-Velocity Oxygen-Fuel Thermal Spray Torch are analyzed using computational and experimental techniques. Three-dimensional Computational Fluid Dynamics (CFD) results are presented for a curved aircap used for coating interior surfaces such as engine cylinder bores. The device analyzed is similar to the Metco Diamond Jet Rotating Wire (DJRW) Torch. The feed gases are injected through an axisymmetric nozzle into the curved aircap. Premixed propylene and oxygen are introduced from an annulus in the nozzle, while cooling air is injected between the nozzle and the interior wall of the aircap. The combustion process is modeled using a single-step finite-rate chemistry model with a total of 9 gas species which includes dissociation of combustion products. A continually-fed steel wire passes through the center of the nozzle and melting occurs at a conical tip near the exit of the aircap. Wire melting is simulated computationally by injecting liquid steel particles into the flow field near the tip of the wire. Experimental particle velocity measurements during wire feed were also taken using a Laser Two-Focus (L2F) velocimeter system. Flow fields inside and outside the aircap are presented and particle velocity predictions are compared with experimental measurements outside of the aircap.

  • Analysis of a high-velocity oxygen-fuel (HVOF) thermal Spray Torch part 1: Numerical formulation
    1996
    Co-Authors: William L. Oberkampf, M. Talpallikar
    Abstract:

    The fluid and particle dynamics of a high-velocity oxygen-fuel Torch are analyzed using computational fluid dynamic techniques. The thermal Spray device analyzed is similar to a Metco Diamond Jet Torch with powder feed. The injection nozzle is axisymmetric with powder and a carrier gas injected on the centerline, premixed fuel and oxygen fed from an annulus, and air cooling injected from an annulus along the interior surface of the aircap. The aircap is a conically converging nozzle that achieves choked flow conditions at the exit; a supersonic, underexpanded jet develops externally. A two-dimensional, axisymmetric geometry is assumed; the equations for mass, momentum, and energy conservation are solved for both the gas and the particle phases. The combustion process is modeled using approximate equilibrium chemistry with dissociation of the gas with a total of nine species. Turbulent flow is modeled by a two-equation model for turbulent kinetic energy and dissipation rate that includes compressibility effects on turbulent dissipation. Particles are modeled as a lumped-heat-capacity system and are considered to melt upon attaining the required latent heat of fusion. An iterative, implicit, finite-volume numerical method is used to solve the coupled gas and particle equations inside and outside the Torch. A companion paper presents the results of the numerical simulation and discusses in detail the gas and particle dynamics.

  • Analysis of a High Velocity Oxygen-Fuel (HVOF) thermal Spray Torch. Part 2, Computational results
    1996
    Co-Authors: William L. Oberkampf, M. Talpallikar
    Abstract:

    The fluid and particle dynamics of a high-velocity oxygen-fuel (HVOF) Torch are analyzed using computational fluid dynamic (CFD) techniques. The thermal Spray device analyzed is similar to a Metco Diamond Jet Torch with powder injection. The details of the CFD simulation are given in a companion paper. This paper describes the general gas dynamic features of HVOF Spraying and then discusses in detail the computational predictions of the present analysis. The gas velocity, temperature, pressure, and Mach number distributions are presented for various locations inside and outside the Torch. The two-dimensional numerical simulations show large variations in gas velocity and temperature both inside and outside the Torch due to flow features such as mixing layers, shock waves, and expansion waves. Characteristics of the metal Spray particle velocity, temperature, trajectory, and phase state (solid or liquid) are also presented and discussed. Particle velocities and temperatures are shown to be lower for this type of Torch than previously believed.

Vincent Rat - One of the best experts on this subject based on the ideXlab platform.

  • Control of the Arc Motion in DC Plasma Spray Torch with a Cascaded Anode
    2019
    Co-Authors: Rodion Zhukovskii, Christophe Chazelas, Armelle Vardelle, Vincent Rat
    Abstract:

    Two common concerns in DC plasma Torches are stability of plasma jet and anode erosion. The challenge is how to get a stable plasma jet with minimal anode erosion. This study tackles this question by using either a swirling gas injection or an external axial magnetic field applied to the Oerlikon SinplexPro™ plasma Torch. A 3-D, time-dependent MHD model of the plasma Torch operation was used to predict the value of the external magnetic field and its effect on the heat flux to the anode and plasma jet stability. The special feature of the model is to couple the gas phase and electrodes that makes it possible to follow the anode temperature evolution. For specific operation conditions (anode of Ø9 mm, 500 A, Ar 60 NLPM), the model predicted that the maximal value of the azimuthal self-magnetic field inducted by the arc current was 0.055 T; it also showed that an external magnetic field of 0.05 to 0.1 T could make it possible to limit the anode erosion without noticeably disturbing the plasma jet issuing from the plasma Torch. We expect this approach to help to better understand the arc behavior in commercial plasma Torches and control anode erosion.

  • Electric arc in a plasma Spray Torch under modulated current
    2018
    Co-Authors: F. Mavier, Fadi Zoubian, Vincent Rat
    Abstract:

    Plasma Spraying of liquid feedstock is a relevant process for the deposition of finely structured ceramic coatings. The control of arc instabilities makes obtaining the desired material properties challenging; hence efforts are devoted to developing segmented direct current Torches that intend to fix the length of the arc. An alternative method involves promoting the arc oscillations and associating a pulsed liquid injection with the Torch. This paper presents a study of the influence of the amplitude modulation of a direct current on the arc dynamics confined in a plasma Spray Torch. The dependence of the electrical features of the Torch on the current amplitude modulation is presented. Plasma speed and temperature are also measured. Time-resolved imaging of the arc attachment permits us to correlate its motion, particularly the arc reattachment on the anode surface, with arc current and voltage. Measurements are compared with a simplified model providing the time-dependence of the plasma properties. It is shown that the current modulation favors the arc restrike by modulating the arc radius, but also that the electric field strength of the arc column should be modified during current variations.

  • Electric Arc Fluctuations in DC Plasma Spray Torch
    2017
    Co-Authors: Vincent Rat, F. Mavier, Jean François Coudert
    Abstract:

    Direct current plasma Torches for plasma Spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma Torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma Torch domain that can generate pressure fluctuations coupled with voltage ones.

  • Influence of Time-Modulation of Applied Current on Arc Stability in DC Pulsed Plasma Spray Torch
    2017
    Co-Authors: F. Mavier, Vincent Rat, Jean François Coudert
    Abstract:

    Conventional direct current arc plasma Spray Torches generate highly unstable plasma jets. This leads to some difficulties with control of coating properties especially when injecting liquid feedstock for the elaboration of finely structured coatings. At constant applied current, a properly designed arc plasma Torch can work in self-sustained pulsed mode that produces large amplitude oscillations of the arc, which are interrupted by very repeatable restrikes. This paper investigates the influence of the modulation of the applied current on the arc stability. Electrical and acoustic diagnostics permit to show that amplification of arc oscillation and a stabilizing mechanism occur when the arc current frequency approaches the Torch resonance frequency. Time-resolved optical emission spectroscopy is used to determine temperature and enthalpy profiles close to the resonance and the enthalpy modulation.

  • Analytical interpretation of arc instabilities in a DC plasma Spray Torch: the role of pressure
    2016
    Co-Authors: Vincent Rat, Jean François Coudert
    Abstract:

    Arc instabilities in a plasma Spray Torch are investigated experimentally and theoretically thanks to a linear simplified analytical model. The different parameters that determine the useful properties of the plasma jet at the Torch exit, such as specific enthalpy and speed, but also pressure inside the Torch and time variations of the flow rate are studied. The work is particularly focused on the link between the recorded arc voltage and the pressure in the cathode cavity. A frequency analysis of the recorded voltage and pressure allows the separation of different contributions following their spectral characteristics and highlights a resonance effect due to Helmholtz oscillations; these oscillations are responsible for the large amplitude fluctuations of all the parameters investigated. The influence of heat transfer, friction forces and residence time of the plasma in the nozzle are taken into account, thanks to different characteristics' times. The volume of the cathode cavity in which the cold gas is stored before entering the arc region appears to be of prime importance for the dynamics of instabilities, particularly for the non-intuitive effect that induces flow-rate fluctuations in spite of the fact that the Torch is fed at a constant flow rate.

Mahrukh Mahrukh - One of the best experts on this subject based on the ideXlab platform.

  • effects of angular injection and effervescent atomization on high velocity suspension flame Spray process
    2016
    Co-Authors: Mahrukh Mahrukh, Arvind Kumar, Sai Gu
    Abstract:

    This work presents the nanostructured coating formation using suspension thermal Spraying through the HVOF Torch. The nanostructured coating formation requires nanosize powder particles to be injected inside a thermal Spray Torch using liquid feedstock. The liquid feedstock needs to be atomized when injected into the high-velocity oxygen fuel (HVOF) Torch. This paper presents the effects of angular injection and effervescent atomization of the liquid feedstock on gas and droplet dynamics, vaporization rate, and secondary breakup in the high-velocity suspension flame Spray (HVSFS) process. Different angular injections are tested to obtain the optimum value of the angle of injection. Moreover, effervescent atomization technique based on twin-fluid injection has been studied to increase the efficiency of the HVSFS process. Different solid nanoparticle concentrations in suspension droplets are considered. In angular injection the droplets are injected into the core of the combustion zone; this immediately evaporates the droplets, and evaporation is completed within the Torch. The value of 10°–15° is selected as the optimal angle of injection to improve the gas and droplet dynamics inside the Torch, and to avoid the collision with the Torch's wall. The efficiency of the effervescent atomization can be enhanced by using high gas-to-liquid mass flow rate ratio, to increase the Spray cone angle for injecting the suspension liquid directly into the combustion flame. It is also found that the increment in the nanoparticle concentration has no considerable effects on the droplet disintegration process. However, the location of evaporation is significantly different for homogeneous and non-homogeneous droplets.

M. Talpallikar - One of the best experts on this subject based on the ideXlab platform.

  • Analysis of a high-velocity oxygen-fuel (HVOF) thermal Spray Torch part 1: Numerical formulation
    1996
    Co-Authors: William L. Oberkampf, M. Talpallikar
    Abstract:

    The fluid and particle dynamics of a high-velocity oxygen-fuel Torch are analyzed using computational fluid dynamic techniques. The thermal Spray device analyzed is similar to a Metco Diamond Jet Torch with powder feed. The injection nozzle is axisymmetric with powder and a carrier gas injected on the centerline, premixed fuel and oxygen fed from an annulus, and air cooling injected from an annulus along the interior surface of the aircap. The aircap is a conically converging nozzle that achieves choked flow conditions at the exit; a supersonic, underexpanded jet develops externally. A two-dimensional, axisymmetric geometry is assumed; the equations for mass, momentum, and energy conservation are solved for both the gas and the particle phases. The combustion process is modeled using approximate equilibrium chemistry with dissociation of the gas with a total of nine species. Turbulent flow is modeled by a two-equation model for turbulent kinetic energy and dissipation rate that includes compressibility effects on turbulent dissipation. Particles are modeled as a lumped-heat-capacity system and are considered to melt upon attaining the required latent heat of fusion. An iterative, implicit, finite-volume numerical method is used to solve the coupled gas and particle equations inside and outside the Torch. A companion paper presents the results of the numerical simulation and discusses in detail the gas and particle dynamics.

  • Analysis of a High Velocity Oxygen-Fuel (HVOF) thermal Spray Torch. Part 2, Computational results
    1996
    Co-Authors: William L. Oberkampf, M. Talpallikar
    Abstract:

    The fluid and particle dynamics of a high-velocity oxygen-fuel (HVOF) Torch are analyzed using computational fluid dynamic (CFD) techniques. The thermal Spray device analyzed is similar to a Metco Diamond Jet Torch with powder injection. The details of the CFD simulation are given in a companion paper. This paper describes the general gas dynamic features of HVOF Spraying and then discusses in detail the computational predictions of the present analysis. The gas velocity, temperature, pressure, and Mach number distributions are presented for various locations inside and outside the Torch. The two-dimensional numerical simulations show large variations in gas velocity and temperature both inside and outside the Torch due to flow features such as mixing layers, shock waves, and expansion waves. Characteristics of the metal Spray particle velocity, temperature, trajectory, and phase state (solid or liquid) are also presented and discussed. Particle velocities and temperatures are shown to be lower for this type of Torch than previously believed.

  • analysis of a high velocity oxygen fuel hvof thermal Spray Torch part 1 numerical formulation
    1994
    Co-Authors: William L. Oberkampf, M. Talpallikar
    Abstract:

    The fluid and particle dynamics of a High Velocity Oxygen-Fuel (HVOF) Torch are analyzed using computational fluid dynamic (CFD) techniques. The thermal Spray device analyzed is similar to a Metco Diamond Jet Torch with powder injection. The Spray nozzle is axisymmetric with powder injection on the centerline, premixed fuel and oxygen fed from an annulus, and air cooling injected along the interior surface of the aircap. Choked flow conditions occur at the exit of the aircap and a supersonic, under-expanded jet develops externally. The CFD simulation assumes three injection streams (solid metal particles with argon as a carrier gas, premixed oxygen/fuel, and air) inside the aircap and solves the combusting two-phase flow until the external Spray stream decays to sonic conditions. The numerical formulation solves the mass, momentum, and energy transfer for both the gas and particle phase and strongly couples each phase. The combustion process is modeled using approximate equilibrium chemistry with dissociation of the gas with a total of nine species. Melting and re-solidification of the metal panicles is modeled as a lumped-mass system. Turbulent flow is modeled by a two equation k-{epsilon} turbulence model, including compressibility effects on turbulent dissipation. A time iterative, implicit, finite volume numerical method is used to solve the partial differential equations. A companion paper [10] presents the results of the numerical simulation and gives a detailed discussion of the gas and panicle dynamics.