Velocity Measurements Of Sprayed Particles In Atmospheric And Low Pressnre Plasma Spraying Systems By A Low-cost Laser Interferometry

Abstract

Local Thermodynamic Equilibrium ( L E ) and n o n -L E numerical models on the basis of fluid and electromagnetic theories are analyzed to c h a r a c t k e the Inductively Coupled Plasma (ICP) of W torches. In an LTE model. the turbulent effects on an: plasma and the mixing mechanism of arc gases in the ICP torch are found by the calculations of plasma temperature and velocity distributions in the torch. A non-LTE model is applied to validate the assumption that ICPs are i n the state of LTE.A plasma-particle interaction model is used for inveshgatinp trajectories and time evolutions of tempemttm distributions inside the injected solid particles into the torch. Based on the numerical results of the turbulent L E model, the o p h u m ranges of design and operation parameta-s for an ICP torch are determined for the synthesis of ultrafme Ceramic poaders and the plasma spraying by either argon or nitrogen-mled argon plasma.Turbulences affect the flow velocity and tempaature of plasma mainly near the inlet and close to the wall. Argon and nitrogen gases fed into a e nitrogen-cooled argon or argon-cooled nitrogen plasma are mrsed each other to almost the same mass fractimz M o w $!e mi! r e x . The cdcula5ons for zrzm plasma in a n o n -L E model reveal that the maximum electron density is about 7.X102L m-3, which is close to the order of the threshold density of 1022 m-3, requird for LTE plasmas. Therefore, it is quite acceptable to regard ICPs as LTE plasmas. W h a an applied current is 103 .4, the & i d plasma velocity is 50 m/s at 0.1 atm, which is about five times as high as that at 1 aim. It is found that an alumina particle with a diameter of Xu m and an initial velocity above 1 m/s in the radial direction can not pene'rate into the argon plasma, but reflects on the isothermdl plasma region of XCO-3003 K.The computations in a turbulent LTE model show the various design and opaational characteristics of ICP torches. The effects of electromagnetic fields on the plasma temperatures are negligible outside the torch length of M) cm if the coil is located between the distances of 7 and 10 cm from the inlet An increase of coil bxns favors the increase of plasma temperatures, but the recirculating flow occurs near the wall when the coil turns exceed five. An elevation of the carrier gas flowrate prevents the recirculation of plasma near the inlet and helps injected particles to penetrate deeply into the plasma However, the plasma t e m p e r a~e~ are rgeatly lowered with the carrier gas flowrates of above 8 lpm As long as the plasma volume is not too small in the torch, an increase of the sheath gas flowrate keeps the plasma from rezirculating near the wall and protects the tube wall from the hot plasma. Computations of plasma temPaatures by varying the nurent and frequency show that more than 90 A and 1 MHz from an RF s m c e are required to sustain a sufficiently high -&re plasma in the brdL 1 P28 The velocities of powder particles spayed by D.C. thermal plasma jets are measured in amnospheric and low pressure plasma...