Enthalpy probe performance in compressible thermal plasma jets

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Abstract
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.