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Description
During atmospheric entry high velocity free-stream gas decelerates rapidly through a shock near the vehicle surface. This sudden compression results in very high post-shock temperatures which are sufficient to cause thermochemical changes in the gas, including dissociation and ionisation. The internal energy modes of the flow also become excited, causing electromagnetic radiation to be emitted. For high entry velocities, large craft or high altitude aerobraking the heat-load from this radiation can exceed that due to convective effects [1]. Predictions of this radiative heating are essential to thermal protection system design, but their numerical simulation is challenging. The post-shock flow during peak heating for many planned missions exhibits thermochemical non-equilibrium, optically thick radiative effects and radiation/flow-field coupling [2]. Experimental datasets are therefore essential to provide validation cases for numerical codes, building confidence in the accuracy of their predictions.
A common method for the experimental investigation of radiative heating is to test using a scaled model, as has been presented by several authors (e.g. [3, 4]). However, sub-scale models cannot match both radiation scaling and binary scaling simultaneously [5, 6]. An alternate method employed in shock-layer radiation experiments involves the passing of a planar shock through a quiescent gas with density and composition matched to a trajectory point of interest (see for example [7, 8]). This produces conditions analogous to those along the stagnation line of an entry vehicle and allows reproduction of the exact conditions experienced in flight. This approach is taken when performing radiation experiments in the T6 tunnel’s shock tube modes, allowing the non-equilibrium processes which occur behind the shock to be analysed.
The T6 Stalker Tunnel has recently undergone initial commissioning at the University of Oxford [9]. T6 is a multi-mode facility, wherein a free-piston driver can be coupled to a range of downstream architectures to permit operation as a reflected shock tunnel, expansion tube or two types of shock tube. The first shock tube has been designed specifically for shock-layer radiation studies and possesses a large 225mm internal diameter for greater integration path length/test time, aluminium construction to prevent carbon contamination, diaphragms at either end to ensure chemical purity and CaF2 windows integrated into the tube wall. The aluminium shock tube will be commissioned later this year. The present work instead uses the smaller diameter steel shock tube, with an internal diameter of 96.3mm and predicted maximum shock speed capability of 18 km s-1 in air.
In these experiments the shock front is imaged as it exits the shock tube. An intensified sCMOS camera is coupled to a Princeton Instruments IsoPlane-320 spectrometer and a single image of the shock captured as it enters the test section. The shock image is focused onto the spectrometer slit using a combination of powered and flat UV-enhanced aluminium mirrors. The entire optical path can be calibrated in-situ for absolute spectral radiance using an integrating sphere. This arrangement results in a two-dimensional map of the variation in spectrally-resolved radiance with distance behind the shock. This data can subsequently provide insight into the sources of radiative emission, as well as the non-equilibrium thermochemical processes which occur in the shock front.
The full paper will present experimental UV/Visible spectra acquired using emission spectroscopy for a range of shock speeds in atmospheric air. A comparison will also be made to computational codes and data from existing facilities.
References
[1] Brandis, A.M. and Johnston, C.O., 2014. Characterization of stagnation-point heat flux for earth entry. In 45th AIAA Plasmadynamics and Lasers Conference (p. 2374).
[2] Boyd, I.D. and Jenniskens, P.M., 2010. Modeling of Stardust Entry at High Altitude, Part 2: Radiation Analysis. Journal of Spacecraft and Rockets, 47(6), pp.901-909.
[3] Sheikh, U. A., Morgan, R. G., & McIntyre, T. J., 2015. Vacuum ultraviolet spectral measurements for superorbital earth entry in X2 expansion tube. Aiaa Journal, 53(12), 3589-3602.
[4] Ishihara, T., Ogino, Y., Sawada, K., & Tanno, H., 2012. Computation of Surface Heat Transfer Rate on Apollo CM Test Model in Free-Piston Shock Tunnel HIEST. In 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition (p. 285).
[5] Morgan, R.G., McIntyre, T.J., Jacobs, P.A., Buttsworth, D.R., Macrossan, M.N., Gollan, R.J., Capra, B.R., Brandis, A.M., Potter, D., Eichmann, T. and Jacobs, C.M., 2006. Impulse facility simulation of hypervelocity radiating flows. European Space Agency (Special Publication), 629, pp.1-6.
[6] Andrianatos, A., Gildfind, D., & Morgan, R., 2015. A study of radiation scaling of high enthalpy flows in expansion tubes. In Asia-Pacific International Symposium on Aerospace Technology.
[7] Cruden, B., Martinez, R., Grinstead, J. and Olejniczak, J., 2009, June. Simultaneous vacuum-ultraviolet through near-IR absolute radiation measurement with spatiotemporal resolution in an electric arc shock tube. In 41st AIAA Thermophysics Conference (p. 4240).
[8] Brandis, A.M., Morgan, R.G., McIntyre, T.J. and Jacobs, P.A., 2010. Nonequilibrium radiation intensity measurements in simulated Titan atmospheres. Journal of Thermophysics and Heat Transfer, 24(2), pp.291-300.
[9] Collen, P.L., Doherty, L., McGilvray, M., Naved, I., Penty Geraets, R.T., Hermann, T.A., Morgan, R.G. and Gildfind, D., 2019. Commissioning of the T6 Stalker Tunnel. In AIAA Scitech 2019 Forum (p. 1941).
Summary
This work presents first measurements of radiation from air shock layers in a new high enthalpy pulse facility.
