Diagnostic study of low pressure supersonic DC plasma jets by emission spectroscopy and enthalphy probe techniques

Abstract

In this work, the characteristics of plasma jets produced by a commercial DC plasma spray torch connected to a controlled pressure chamber are investigated. The plasma gas employed is a mixture of argon, nitrogen and hydrogen with the initial gas flow rates of 40, 10, and 1 SLPM respectively. The plasma torch is operated at chamber pressures of 6.5, 13, 26, 39, 53 and 101 kPa and a fixed input power of 17.5 kW. Optical emission spectroscopy is used to measure the temperatures and electron density profiles. The measurements of temperature are carried out by means of a Boltzmann plot of several isolated argon atom spectral lines. The electron density is measured from the Stark broadening of the H[subscript bêta] line 486.1 nm and the continuum emissivity. Partially resolved spectra of the N[subscript 2]+ molecular band are used for the rotational temperature evaluation. The rotational emission from the (0,0) band of the first negative system is compared to synthetic spectra to evaluate the rotational temperature within the flow field. Finally, the measurements of the gas kinetic temperature and the plasma velocity are performed by the enthalpy probe technique. The experimental results show the occurrence and the position of the different gas dynamics zones; i.e., supersonic expansion, stationary shock front and subsonic relaxation at low pressures (less than 40 kPa). The plasma flow is accelerated to its maximum velocity at the expansion where a minimum in the electron density and temperature is observed. At the end of the expansion a stationary shock front occurs at 4, 8, 12, and 15 mm downstream from the nozzle exit at pressures 39, 26, 13 and 6.5 kPa respectively. The electron density profiles show the variations along the plasma axis that coincide with the position of the shock waves. Good agreement between the electron density results obtained from the Stark broadening and from the continuum emissivity is observed. Enthalpy probe measurements on gas kinetic temperature and plasma velocity reveal the general features of low pressure plasma jets, i.e., higher flow velocity and longer heating zone of expanded plasma with lower temperature. The measurements also coincide with the rotational temperature obtained from emission spectroscopy. The temperature results confirm that the local thermodynamic equilibrium LTE exists at pressures of 100 and 53 kPa. However, at lower pressures where the supersonic shock waves are formed, the slow energy exchange between the heavy and light particles leads to significant deviations from the LTE especially in the shock region.