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Description
The atmospheric entry of spacecraft is characterized by very high entry speeds and the formation of a bow shock in front of the blunt body. The sudden deceleration of the vehicle converts the kinetic energy of the vehicle into thermal energy of the gas which gets dissipated in the form of convective and radiative heat transfer. The gas species trapped inside the shock layer undergo a very high temperature rise in the order of thousands of kelvin that brings about dissociation, ionization and recombination reactions. To safeguard the vehicle and the scientific returns it may contain from the extreme heating conditions during entry, a thermal protection system (TPS) is employed. The basic constituent of the TPS in most cases is carbon and carbon based ablative materials. In this work, the ablation of carbon is experimentally studied under hypervelocity Earth-entry conditions generated in the X2 expansion tube facility at The University of Queensland, Australia.
A two-dimensional wedge model has been designed with an ablation source [1] mounted onto the compression face of the wedge, connected by copper electrodes. Graphite samples in the shape of rectangular strips have been used as the ablation sources which were resistively pre-heated using a DC power supply to simulate representative re-entry wall temperatures. The temperature of the pre-heated graphite strips was measured by a non-intrusive diagnostic technique called dual-wavelength signal ratio thermography [2]. The signal ratio between the two optically filtered images was used to obtain the two-dimensional temperature map of the heated strips.
The pre-heated graphite strip, upon exposure to the high enthalpy test gas flow, starts ablating and the ablated products mix with the flow which then pass through the expansion fan and further into the afterbody region. The evolution of the entrained ablation products and their interaction with the flowfield is experimentally investigated in this work. Optical diagnostics such as high speed video imaging and ultraviolet (UV) emission spectroscopy were used. The ablated carbon interacts with the nitrogen species available in the test gas flow and results in the formation of the cyanogen radical (CN) – a well-known radiator. The CN violet band emission is investigated for different wall temperature cases and the spatial and spectral distribution of radiance will be discussed. From the shapes of the vibrational transitions of the CN violet band, we have also attempted to extract the vibrational and rotational temperatures of the species by fitting the experimentally recorded spectra with the synthetic spectra.
REFERENCES
[1] Ravichandran R., Lewis S. W., James C. M., Morgan R. G. and McIntyre T. J., Interaction of Ablating Carbon with Expanding Earth Entry Flows in the X2 Expansion Tube, 9th Ablation Workshop [online abstracts], Montana State University, Bozeman, MT, 2017, pp. 38–39, URL: http://ablation2017.engineering.uky.edu/files/2017/08/9th-Ablation-Workshop-book-of-abstracts-Montana-State-University.pdf
[2] Ravichandran R., Buttsworth D. R., Lewis S. W., Morgan R. G. and McIntyre T. J., Filtered Image Thermography for High Temperatures in Hypersonic Pre-Heated Ablation Experiments, Journal of Thermophysics and Heat Transfer (under review).
Summary
Interaction of ablating carbon, from preheated graphite strip, with re-entry flows was experimentally studied in expansion tube X2. The flowfield was investigated using UV emission spectroscopy and CN violet band was studied for different cases from which we have attempted to estimate temperatures.
