Hypersonic Free Stream Characterization in LBK by Laser Induced Fluorescence and Diode Laser Absorption Spectroscopy

Abstract

In high enthalpy flows with complex non-equilibrium chemistry, like at entry conditions in the Martian atmosphere, the flow characterization is essential to estimate the aerodynamic behaviour and the TPS performance of the entry capsule. In the frame of the ESA TRP study 'CFD Validation in CO₂ Environment' non-intrusive laser based measurement techniques were applied to characterise the free stream properties in DLR's arc heated facility L2K in Cologne. 97% carbon dioxide and 3% nitrogen was used for simulation of the Martian atmosphere. Pitot pressure profile and cold wall heat flux measurements were carried out at stagnation enthalpy levels of 13.1 and 8.3 MJ/kg. In addition diode laser absorption spectroscopy (DLAS) on CO and laser-induced fluorescence (LIF) of nitrous oxide and carbon monoxide molecules was applied, in order to get flow velocities and translational and rotational temperatures and partial densities in free stream regions. Rotational temperatures of NO were extracted from measured LIF spectra by fitting theoretical spectra to the measured data. In the free stream flat temperature profiles with temperature levels of 488 K and 558 K were measured for the high and low enthalpy, respectively. This inverse dependency on free stream temperature on stagnation enthalpy was also result of the analytical estimation of the flow properties. Laser-induced fluorescence of the B¹∑⁺←X¹∑⁺ transition of carbon monoxide was applied to measure free stream density and temperature profiles. Compared to NO LIF the scattering of the temperature values from CO LIF is larger, due to the quadratic dependency of the LIF signal on excitation power and the high power densities required for the generation of an adequate signal. Diode laser absorption spectroscopy (DLAS) of the X¹∑⁺ (v=2←0,R7) transition was used to deduce the translational temperature, the velocity and the density of carbon monoxide in free stream flow. Velocities were measured with good accuracy, while the translational temperature measurements tended to be too high, because of the divergence of the flow and the influence of the shear layer flow region between the resting gas in the test chamber and the homogeneous flow core. The application of different sophisticated measurement techniques allowed to gain very valuable data for a better characterization of the high enthalpy flow representing Martian atmosphere and validation of numerical tools.