Temperature and density measurements in a recombining CO ₂ plasma

Abstract Infrared emission spectroscopy is used to obtain measurements of temperature and chemical composition in a recombining CO 2 plasma. A CO 2 /argon mixture is first heated to high temperature using an inductively coupled plasma torch, and then passed through a water-cooled tube at high speed to induce chemical recombination. Different rotational temperatures are deduced when analyzing the CO 2 and CO emission bands. Unsteady thermal fluctuations in the flowfield, particularly at the exit of tubes of length 15 and 20 cm, are also observed. We attribute these fluctuations to the transition from a laminar to turbulent flow regime. We argue that the unsteady thermal fluctuations cause the discrepancy in rotational temperature between CO 2 and CO bands, and that the rotational temperature of the CO bands provides the correct average temperature. A power balance analysis is used as a consistency check. The measurements indicate that CO has a density equal to its equilibrium density. However, CO 2 is found to be underpopulated by a factor of about 3.5 with respect to its equilibrium density. This data set provides useful information to test the kinetic models of recombining CO 2 plasmas at high temperature, as will be analyzed in a companion paper.