Simulation of ablation layers in impulse facilities

D'Souza, Mary Gulrukh (2013). Simulation of ablation layers in impulse facilities PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2014.167

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Author D'Souza, Mary Gulrukh
Thesis Title Simulation of ablation layers in impulse facilities
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2014.167
Publication date 2013-01-01
Thesis type PhD Thesis
Supervisor Richard Morgan
Timothy McIntyre
Neil Mudford
Total pages 262
Language eng
Subjects 0901 Aerospace Engineering
0301 Analytical Chemistry
0205 Optical Physics
Formatted abstract
An experimental study investigating radiation in an ablating shocklayer over 1:13.5 scale Stardust and 1:10 scale Hayabusa forebody models in the X2 expansion tunnel in air, nitrogen and Mars atmospheres at 9 km/s was conducted at The University of Queensland. This speed has an effective flight velocity of 9.5 km/s, and enthalpy of 41 MJ/kg which closely simulates the forebody peak heating point on the Stardust re-entry trajectory (altitude 54 km, speed 9.6 km/s and enthalpy 45 MJ/kg). Models were manufactured from mild steel and coated with a low-pyrolysis temperature hydrocarbon coating, permitting thermal decomposition of the coating and the release of carbon-containing pyrolysis gas species into the shocklayer within the 80 μs test time. Thermochemical analyses of epoxy were conducted and conservative (upper) bounds for the epoxy pyrolysis onset temperature of 460 K and bulk pyrolysis temperature of 638 K were identified. Models were oriented at 0° to the stagnation streamline and were not instrumented; instead shocklayer radiation was visualised using a high speed camera, whilst the emission spectrum along the stagnation streamline was measured with ultraviolet and infrared spectrometers.

Shocklayer radiation is found to be strongest in the long-wave UV portion of the electromagnetic spectrum, in both ablating and non-ablating shocklayers. Shocklayer radiation close to the model surface is significantly greater when an epoxy coating is employed, implying an increase in radiative heat transfer to the model surface. Evidence of coating ablation is presented and shown to produce a 540% increase in radiation of combined CN (violet transitions) and N2+ radiation near the model surface. A method for estimation of the proportion of N2+ Δv = 0 transitions, which are buried within the CN Δv = 0 transitions, is conceived. This method predicts the proportion of radiation due to N2+ Δv = 0 transitions is 20 + 5% at 388.34 nm for the expansion tunnel experiments, which is close to and within tolerance of the Stardust flight value at 54 km altitude of 16 + 1%, observed by Winter and Herdich (2008). A similar method is used to determine the experimental vibration temperature as 8700 + 500 K, which is close to and within tolerance of the Stardust flight value at 54 km altitude for CN of 8500 + 500 K.

Ablation layer radiation is shown to vary with wavelength. The use of an epoxy coating results in a noticeable increase in radiation above 340 nm, corresponding to CN formation, and a possible reduction for atomic species below 320 nm, with transition in the 320 – 340 nm region. A possible second transition region may lie in the 562 – 870 nm region, whereby for greater wavelengths, ablation layer radiation is weaker with an epoxy coating than without.

All three instruments produce similar bow shock radiation distributions and show that there is no discernible increase in shock standoff distance when an epoxy coating is employed, which is consistent with theoretical predictions.

Species observed in the expansion tunnel also observed in flight include CN, N2+, C, H and possibly O and N. C2 Swan radiation is observed only when an epoxy coating is employed, developing initially in the ablation layer and continuing around the sides of the model, thus providing further evidence that the epoxy coating undergoes ablation during the experimental test period.

Shocklayer radiation is significantly increased in ablating shocklayers compared with non-ablating shocklayers, when both air and nitrogen test gases are used. Radiation may be greater in nitrogen than in air in both ablating and non-ablating shocklayers, although this is not determined conclusively. The effect of a carbon-rich test gas on the interaction between ablation and shocklayer radiation is also examined, however no substantial change in shocklayer radiation is observed when an epoxy coating is employed.

The utility of this experimental technique for investigating radiation in an ablating shocklayer is demonstrated in this Thesis.
Keyword Radiation
Spectral analysis
Expansion tunnels

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Created: Fri, 20 Jun 2014, 23:36:03 EST by Mary D'souza on behalf of Scholarly Communication and Digitisation Service