Ablation and radiation in hypervelocity earth-entry flows

Abstract

Atmospheric entry of spacecraft is an interesting and challenging problem in aerospace engineering as it involves a highly non-equilibrium aerothermodynamic shock layer environment. Re-entry of spacecraft into Earth’s atmosphere occurs at very high entry speeds ranging from 8–12 km · s−1, giving rise to a bow shock wave in front of the blunt body vehicle. The associated shock layer is characterised by a very rapid increase in pressure and temperature. The kinetic energy of the hypervelocity flow is converted into thermal and chemical energy, which is partially dissipated in the form of convective and radiative heat transfer. The gases traversing the shock layer undergo thermochemical changes such as dissociation, ionisation and recombination. To safeguard the re-entry vehicle from such a harsh thermal environment, a thermal protection system (TPS) is employed on the surface of the vehicle. The TPS material, when exposed to the re-entry heating, may undergo ablation due to the combined effect of heat flux and shear from the shock layer flow. The excited gas species in the shock and boundary layer react with the atomic/molecular species ablated from the solid TPS material. However, due to the uncertainties associated with the total heat flux estimation, large safety factors are currently being used in the TPS design, particularly for the afterbody region, compromising the payload mass and vehicle safety. The main objectives of this thesis were to experimentally simulate ablation product interactions with an expanding re-entry flowfield, to characterise the radiation of the entrained ablation products using ultra-violet emission spectroscopy targeting CN radicals, and to compare the ablating flowfields with non-ablating ones. The experiments were performed in the X2 expansion tube by using a stainless steel wedge model designed with a provision to mount a graphite ablation source on its compression face. The test flow condition, representative of a point in the Hayabusa capsule re-entry trajectory, was generated in X2 with a freestream velocity of 8.6 km · s−1 and a temperature of 2500 K, corresponding to an enthalpy of 38 MJ ·kg−1. As the test times available in X2 are limited, the ablating material cannot reach such wall temperatures by aero heating alone, and hence ablation in these experiments was created by electrically preheating the graphite strip to high wall temperatures representative of re-entry conditions. The wall temperatures realised in this work ranges from 1000–3000 K, which were measured by nonintrusive filtered image thermography using a dual-wavelength signal ratio technique. The hot graphite strip, upon exposure to the re-entry flow, ablates and mixes with the flow, and passes through the expansion fan and further into the afterbody region. The flowfield and interaction processes were optically diagnosed using a high frame rate video camera, two-dimensional filtered imaging and UV emission spectroscopy. CN emission measurements were made at various locations in the flowfield such as the forebody shock layer, the expansion region around the corner and on afterbody, with an aim to study the time evolution of CN species from the forebody to the afterbody via the expansion fan. The images from the high speed camera show the details of the flow features including the shock front and ablation layer. The signal intensities from these images illustrate the difference between the ablating and non-ablating flowfield. Twodimensional filtered images were recorded of CN emission for ablating and non-ablating cases. A bright ablation layer of CN was observed for the heated case from the intensity calibrated image, whereas for the unheated graphite and steel experiments, no significant ablation was observed. The importance of wall temperature for ablation to occur was verified from these results. The intensity of CN violet band emission was high for the heated cases in the ablation mixing layer behind the shock. At locations above the wedge surface, the intensity of CN radiation decreases as the height from the surface increases. Experimentally recorded CN emission spectra were compared with the synthetic spectra simulated using Specair to estimate the trans-rotational and vibro-electronic temperatures of CN at various spatial locations. The distribution of temperatures inside the ablation mixing layer immediately behind the shock front shows that the translational temperature is higher than the vibrational temperature as expected, whereas both translational and vibrational temperatures approach equilibrium near the wall. At a location immediately above the wedge surface, where the flow undergoes rapid expansion, the translational and vibrational temperatures estimated were observed to show an increase after the expansion fan interaction, which is possibly attributed to a non-Boltzmann population distribution in the upper energy levels. The study has validated the methodology of using expansion tubes to investigate the non-equilibrium ablation processes involved in re-entry flights. The experimental model currently has air gaps to avoid contact between steel and graphite parts, and the entrainment of ablated products into the air gap can complicate the interpretation of the data. Future work could be done on refined models that eliminate the effects of gap entrainment of ablation products. Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, financial support and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my higher degree by research candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis and have sought permission from co-authors for any jointly authored works included in the thesis. Publications included in this thesis Ranjith Ravichandran, David R. Buttsworth, Steven W. Lewis, Richard G. Morgan, and Timothy J. McIntyre, “Filtered Image Thermography for High Temperatures in Hypersonic Preheated Ablation Experiments”, Journal of Thermophysics and Heat Transfer (published online). Contributor Statement of contribution % Ranjith Ravichandran writing of text 100 proof-reading 80 theoretical derivations 90 preparation of figures 100 performing experiments 90 data analysis and interpretation 85 experimental instrument configuration 30 David R. Buttsworth proof-reading 5 data analysis and interpretation 2 experimental instrument configuration 70 Steven W. Lewis proof-reading 5 data analysis and interpretation 3 performing experiments 10 Richard G. Morgan proof-reading 5 supervision and guidance 50 theoretical derivations 5 data analysis and interpretation 5 Timothy J. McIntyre proof-reading 5 supervision and guidance 50 theoretical derivations 5 data analysis and interpretation 5 Submitted manuscripts included in this thesis No manuscripts submitted for publication. Other publications during candidature Journal articles Steven W. Lewis, Christopher M. James, Ranjith Ravichandran, Richard G. Morgan, and Timothy J. McIntyre, “Carbon Ablation in Hypervelocity Air and Nitrogen Shock Layers”, Journal of Thermophysics and Heat Transfer, Vol. 32, No. 2, 2018, pp. 449–468. Workshop abstracts/papers Ranjith Ravichandran, Steven W. Lewis, Christopher M. James, Richard G. Morgan, and Timothy J. McIntyre, “Interaction of Ablating Carbon with Expanding Earth Entry Flows in the X2 Expansion Tube”, 9th Ablation Workshop, The University of Kentucky [online abstracts], August 2017. Brian E. Donegan, Robert B. Greendyke, Ranjith Ravichandran, Steven W. Lewis, Richard G. Morgan, and Timothy J. McIntyre, “Preliminary Investigation of Ablating Hypersonic Radiating Wake Flows”, 9th Ablation Workshop, The University of Kentucky [online abstracts], August 2017. Ranjith Ravichandran, Steven W. Lewis, Christopher M. James, Richard G. Morgan, and Timothy J. McIntyre, “Expansion Tube Experiments of Graphite Ablation and Radiation in Hypervelocity Earth-entry Flows”, 8th International Workshop on Radiation of High Temperature Gases, Madrid, Spain, March 2019. Conference papers Brian E. Donegan, Robert B. Greendyke, Ranjith Ravichandran, Steven W. Lewis, Richard G. Morgan, and Timothy J. McIntyre, “Experimental Analysis of the Interaction of Carbon and Silicon Ablation Products with Expanding Hypersonic Flows”, 22nd AIAA International Space Planes and Hypersonic Systems and Technologies Conference, 2018. Technical reports Steven W. Lewis, Ranjith Ravichandran, Rowan J. Gollan, Richard G. Morgan, Peter A. Jacobs, Timothy J. McIntyre, and Anand Veeraragavan “Rapidly Expanding Non-Equilibrium Hypersonic Flow”, Annual Technical Report, AFOSR/AOARD (FA2386-16-1-40572017), 2017. C