Investigation of an equilibrium condition boundary layer in front of a material probe in a subsonic plasma flow

Abstract

This work presents an investigation of the equilibrium state of an air plasma in front of a material probe by emission spectroscopic methods. A potential application of these boundary layer measurements is the determination of enthalpy as a main characteristic for material tests in the plasma wind tunnel. The investigation has been performed in a subsonic plasma wind tunnel of the Institut fur Raumfahrtsysteme (IRS) of the Stuttgart University, within the frame of a scientific cooperation project between the National Space Development Agency of Japan (NASDA) and the IRS. This project involves material tests with heat protection materials developed by NASDA with respect to later application to future space transportation systems such as HOPE. Since two different types of plasma wind tunnels are available at IRS and at the National Aerospace Laboratory (NAL) in Japan, another goal of the cooperation is a comparison of material test conditions at an overlapping operational regime of both facilities. The emission spectra were dominated by the emission of atomic nitrogen and oxygen close to the infrared region and by the emission of ionized nitrogen molecules N2 in the near UV-region. From high resolution measurements the rotational temperature of the ionized molecular nitrogen has been determined by a Boltzmann plot method. The vibrational temperatures have been inferred by comparing low resolution measurements with a self-developed simulation of the N2-radiation. All measurements have been performed for both water-cooled copper probes and SiC material probes, positioned in the air plasma flow. The investigated plasma state was mainly determined by the conditions of the NASDA material tests. The investigation of the assumed equilibrium layer in front of the material probe was performed by investigating the change in intensity ratios of the different species. The first 10 mm in front of the material probe have been considered as a temperature boundary layer with strongly varying plasma conditions. This temperature layer is followed by a region which has a plasma state at least close to equilibrium as can be seen from the experiments. The thickness of this region varies from about 20 mm up to 40 mm, depending on the plasma source parameters. Introduction This investigation accompanied material tests in At the Institut fur Raumfahrtsysteme (IRS) of the the course of a cooperation program between Stuttgart University four plasma wind tunnels NASDA and IRS [2]. The experiments with the PWK1-4 are in operation to simulate the thermal, NASDA-developed heat protection materials took aerodynamic and chemical loads on the surface of a P at the The P conditions were space vehicle [I]. Three different plasma sources determined by the requirements of the material tests have been developed for this purpose: an MPD and mainly defined by stagnation pressures of 0.8 and generator for the simulation of high enthalpy and low 3.5 kPa and material surface temperatures of 1700°C pressure conditions during the first phase of re-entry, and °' test condition in the overlapping a thermal arcjet device for the follow-on flight path at g of the P wind tunnels at NASDA and moderate specific enthalpies and higher stagnation IRS is defined X the stagnation pressure of 3.5 kPa. pressures, and an inductively heated generator for At the examined plasma conditions the plasma flow basic material experiments over a wide range of was subsonic. The two plasma conditions have been specific enthalpies and pressures. The wind tunnels investigated both in front of a glowing material probe are connected to a roots pump system with a total and a water-cooled pressure probe of the same shape suction power of more than 250,000 m/h. as the matenal P°be. 'Institut fur Raumfahrtsysteme, Universitat Stuttgart, Pfaffenwaldring 31, D-70550 Stuttgart, Germany Tohoku University, Aoba Aramaki, Aoba-ku, Sendai 980, Japan