Two Dimensional Spatially Resolved Two Photon Oxygen Atom Laser Induced Fluorescence Measurements In The Flow Field Of The Arc Heated Facility L3K

Abstract

The arc heated facility LBK in Cologne is designed to characterise thermal protection materials for reentry vehicles in high enthalpy flow fields. Models with dimensions of 300 x 300 mm can be studied with stable flow conditions for 7200 s in the L2K and 1800 s in the L3K. Typical model surface temperatures are 2000 K at pressure levels of up to 350 hPa and enthalpy levels up to 25 MJ/kg. Infrared thermography, pyrometry, emission spectroscopy, absorption and diode laser absorption spectroscopy, CARS and 2D-NO-LIF have been used to study the flow conditions in the free stream and in shock layers at moderate enthalpy levels. At high enthalpy levels above 12 MJ/kg it will not be possible to apply the well established NO-LIF technique to characterise flow conditions, because the NO-density in these flow fields becomes too low and the NO formed in the shock regions near the models appears in rotational non equilibrium. Two photon O-atom-LIF-technique was applied to study the relative O-atom density in the flow field of the arc heated facility at high enthalpy levels. The free stream electronic temperature of O-atoms agrees with roto-translational temperature from CFD calculations. Different flow velocities could be detected in the free stream and in the shock region by scanning the excitation wavelength. Measured flow velocities perpendicular to the model surface are in good agreement with the results from CFD-calculations. It has been noticed that the accuracy of the velocity values can be improved by lowering the line width of the excitation laser light with an etalon. The shock standoff distances for the IMENS model were determined from the intensity profiles for different angles of attack and different flow conditions. The results have been compared with CFD-calculations. The shock standoff distances from the measurements and the calculation agree very well. The determination of the absolute oxygen atom concentration using a calibration method based on a titration method combined with an absolute sensitivity determination of the detection system using a Rayleigh technique is to be improved.