A novel method for supersonic plasma measurement using a double-jacketed enthalpy probe

Abstract

A novel method to characterize a supersonic plasma jet using a double-jacketed enthalpy probe is presented and applied to Ar supersonic plasma jets generated by a plasma torch operated at an input power level of 5.7 kW and a chamber pressure of 2.3 kPa. The basis of this method is to measure total stagnation pressure, total enthalpy, and static pressure at the backside of a shock wave formed in front of the probe with a single insertion of the probe. Once these three variables are known, normal shock relations before and after the shock wave can disclose information on static pressures, Mach numbers, temperatures, and velocities of the supersonic plasma jet under a calorically perfect gas assumption. For example, measurement experiments carried out with the proposed probe revealed that static pressure of Ar supersonic plasma jet oscillated around the chamber pressure of 2.3 kPa in a range of 1–5 kPa along the jet axis, clearly showing an aerodynamic non-equilibrium. Corresponding to the behaviors of static pressures, Mach numbers also oscillated in the range of 1.1–3.5 along the jet axis. In addition, oscillation patterns of static pressures and Mach numbers agreed well with those of compression and expansion wave zones observed in the photograph of an over-expanded Ar supersonic plasma jet. Although relatively large errors were accompanied due to a low input power level, plasma temperatures and velocities were measured to be decreasing and increasing, respectively, in the expansion wave zone while opposite behaviors were observed in the compression wave zone.