Speaker
Description
The study of the convective and radiative heat fluxes to the capsule surface during its atmospheric entry is critical for the design of the thermal protective system. For Mars entry scenarios, where CO2 represents 96% of the atmosphere, the radiative heat flux to the afterbody suffers from large uncertainties - up to 260%. The rapid hydrodynamic expansion of the plasma into the afterbody region results in rapid cooling, chemical recombination, and a departure from equilibrium. This chemical non-equilibrium and the associated radiation are still not accurately modeled, and our goal is to provide experimental data for model validation. Our experiments focus on a fundamental study of the recombination kinetics of CO2 plasmas.
The inductively coupled plasma torch at laboratoire EM2C was used to produce a CO2 plasma at atmospheric pressure. More details of the plasma torch facility can be found in Ref. [4] (see attachment). The CO2 plasma studied here exits the torch through a 1-cm diameter nozzle and is composed of 10% of CO2 and 90% of argon (argon required for stable operating conditions). The plasma is then passed through a water-cooled test-section of various lengths at high speed (~ 500 m/s) to force rapid cooling and chemical recombination. The IR spectra obtained by OES are calibrated in absolute intensity using a tungsten lamp traceable to NIST standards. The calibration procedure considered absorption from cold CO2 and H2O present in the optical path. The complete calibration procedure is described in [5] (see attachment). Figure 1 shows the calibrated and abel-inverted spectrum at the exit of the 35-cm test-section. This corresponds to the local emission at the center of the jet. Emission from both CO and CO2 is present. Several CO/CO2 spectra at different rotational and vibrational temperatures were calculated using the RADIS line-by-line radiative code, in conjunction with the HITEMP-2010 database. The best fit achieved is shown in red. The complete fitting procedure is described in [5] (see attachment).
A 0D chemical kinetic simulation was realized using the Cantera code in conjunction with the Park 1994 kinetics model. The temperature, as measured above using the CO molecular band, was converted into a time-dependent temperature profile, and put into Cantera which then calculates the evolution of the chemical composition. Figure 2 shows the evolution of CO in black and CO2 in red, the dashed lines represent the equilibrium. CO density prediction at 35 cm is in good agreement with our measurement at the exit of the 35-cm test-section. However, the CO2 density is underpredicted by a factor of about 10.
Summary
An ICP torch was used to produce a CO2/Ar plasma jet at atmospheric pressure. The plasma jet is close to LTE conditions at a temperature of 6650 K. This plasma is then passed at high velocity through a water-cooled test-section that forces rapid cooling and recombination. The thermochemical evolution of the plasma is studied using infrared OES. The measured spectra at the exit of the torch and at the exit of the test-sections are calibrated in absolute intensity and compared with calculations done using the RADIS radiation code. The measurements of temperature and CO/CO2 densities that result provide a test case for comparison with kinetic modeling and CFD predictions. A 0D chemical kinetic simulation was realized using the Cantera code and the Park 1994 kinetics model. CO density prediction is in good agreement with our measurement, however, the CO2 density is slightly underpredicted at the exit of the recombining tube.
