A major design challenge for re-entry capsules lies in the modelling of convective and radiative heat transfer to the surface of the vehicle. At certain points on superorbital re-entry trajectories, up to 40% of the total radiative heat flux is contributed by the vacuum ultra-violet (VUV) spectral range and it is in this spectral range that the largest uncertainties lie. The high level of uncertainty in the VUV is a result of a lack of published experimental data due to difficulties encountered in measuring radiation in the VUV, such as strong absorption by most optical materials and air. Additional complexities of the VUV spectral range include its strongly self-absorbing nature and spectral line broadening.
The primary goal of this study was to obtain calibrated spectral measurements in the VUV that enable the investigation of physical processes occurring in the shock layer that influence the incident radiative heat flux. In particular, the issues to be investigated were the variation in spectral radiance observed across a shock layer compared to the spectral radiance measured through the surface, the effects of self-absorption on spectral line intensity and the broadening of spectral lines in the VUV as a function of depth of radiating flow field. The measurements made across and through the surface of a model provide the first set of calibrated experimental results for the validation of computational codes used to predict incident radiative heat flux. Measurements made with a varying depth of radiating flow field provide a unique set of experimental data for the validation of radiation transport models and broadening coefficients. This study also used computational simulations to investigate the accuracy of a flow field solver coupled with two reaction rate schemes and compared the spectra produced using Specair with experimentally measured values.
To achieve these goals, an optical system was designed to measure the VUV radiative emission produced around a blunt two-dimensional model in a spatially resolved manner across the shock layer. Spatial resolution allowed for spectral measurements to be made in both the equilibrium and non-equilibrium parts of the shock layer. A second optical system was designed to obtain measurements of VUV radiation incident on the surface of the model. This system incorporated a window in the surface with a mirror housed within the model to deflect the radiation out of the test section and into the detection system. To effectively vary the depth of the radiating flow field, the length of a two-dimensional model was varied, changing the depth of the shock layer being observed.
The X2 expansion tube was used to create the high enthalpy flows required to produce radiating shock layers. Two flow conditions were created for this study that represented flight equivalent velocities of 10.0 km/s and 12.2 km/s. The spectroscopy system utilized for this study consisted of an evacuated McPherson NOVA 225 spectrometer coupled to an Andor iStar VUV enhanced intensified charge coupled device. An evacuated light tube sealed with a magnesium fluoride window was required to extend the evacuated light path to the model and avoid any absorption by molecular oxygen. An in-situ calibration of the VUV spectroscopy system was conducted using a deuterium lamp located in the position of the radiating shock layer.
The integrated incident spectral radiance measured through the surface of the model between 115 nm and 180 nm was 0.744 W/cm2sr for the 10.0 km/s condition and 12.3 W/cm2sr for the faster 12.2 km/s condition. These values were 25% and 31% respectively of the integrated spectral radiance measured in the equilibrium region when viewing across the shock layer. The repeatability of the experimental results obtained by the VUV emission spectroscopy system was measured to be better than 15% for spectral line intensity and 20% for integrated intensity between 115 nm and 180 nm. All spectral lines measured were found to be self-absorbing to varying degrees for the 10.0 km/s condition and strongly self-absorbing for the 12.2 km/s condition. Spectral line broadening of the strongly self-absorbing lines was also observed.
Computational simulations of the flow field around each model were made using the in-house solver Eilmer 3 utilising the Park finite reaction rate scheme. Using the computed flow field as inputs into Specair, spectra were generated for both conditions and models. Simulations of the self-absorption experiments predicted self-absorption of all spectral lines investigated and showed good agreement with the measured values for most spectral lines. The computed broadened spectral line width was found to be an under prediction of the measured spectral line width.
An extension to this study was the creation of a VUV emission spectroscopy system for use on the plasma torch at Ecole Centrale Paris (ECP). The plasma torch produces air at thermochemical equilibrium and can therefore be used to validate Einstein coefficients used in the prediction of spectral line intensities without the complexities of the non-equilibrium regions observed in the expansion tube. The flow condition used for this study had a peak temperature of 6,600 K and the torch was operated at atmospheric pressure.
A nitrogen flush through the optical path was employed to reduce absorption by molecular oxygen. To maintain the structural integrity of the viewing window at the edge of the plasma, a water cooled copper housing assembly was designed. Simulations conducted using the COMSOL Multiphysics program predicted the thermal stresses on the window would not exceed the apparent elastic limit. Experiments were conducted to investigate variance in transmission of the viewing window due to surface degradation and absorption by molecular oxygen between the edge of the plasma and the viewing window. Through the observation of the 174 nm nitrogen doublet and the 777 nm oxygen triplet, it was concluded that surface degradation of the viewing window was occurring when the window was closer than 5 mm from the plasma edge. It was also found that there was molecular oxygen absorbing a fraction of the signal at this distance.
A further goal of the plasma torch VUV spectroscopy system was to establish a system capable of varying the depth of the radiating flow field to investigate self-absorption and spectral line broadening. A water cooled copper pipe was used as a fence and traversed through the plasma during operation. Spectral measurements of the nitrogen doublet at 174 nm and the oxygen triplet at 777 nm were made at 1 mm spacings. Self-absorption of the 174 nm nitrogen doublet was confirmed and no spectral line broadening was observed.
Calibration of the ECP VUV spectroscopy system was carried out using an argon mini-arc placed at the location of the plasma. The in-situ calibration was carried out after the experiment to account for any damage that occurred to the window during testing and any absorption through the optical path by molecular oxygen. Due to the observed degradation of the window surface during the test time and unquantified level of absorption between the viewing window and the plasma, a large uncertainty was estimated for the calibration resulting in an inability to validate the Einstein coefficients of the nitrogen doublet at 174 nm. Through this study two VUV spectroscopy systems were created on two separate facilities and a benchmark dataset was obtained viewing shock layers across and through the surface of a model. The levels of self-absorption and spectral line broadening were also quantified for varying depths of radiating flow fields providing a unique set of results for validation of radiation transport solvers. Computational simulations of the flow field conducted with Eilmer 3, in conjunction with the Park reaction rate scheme, were used to create spectra with Specair and the results were compared against experimentally measured values.