Mars entry afterbody radiative heating: an experimental study of nonequilibrium CO2 expanding flow

Gu, Sangdi (2018). Mars entry afterbody radiative heating: an experimental study of nonequilibrium CO2 expanding flow PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2018.200

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Author Gu, Sangdi
Thesis Title Mars entry afterbody radiative heating: an experimental study of nonequilibrium CO2 expanding flow
Formatted title
Mars entry afterbody radiative heating: an experimental study of nonequilibrium CO2 expanding flow
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2018.200
Publication date 2018-02-27
Thesis type PhD Thesis
Supervisor Richard Morgan
Tim McIntyre
Aaron Brandis
Total pages 253
Language eng
Subjects 0901 Aerospace Engineering
Formatted abstract
This thesis presents an experimental study of high temperature CO2 flows – focusing on CO2 flows subjected to rapidly expanding conditions relevant to Mars entry and representative of the corner expansion around the shoulder, between the windward and leeward flows, on an entry capsule. This is an important but poorly understood aspect of spacecraft design. Past numerical research showed that, during Mars entry, the CO2 4.3 µm and 2.7 µm band radiation from the aforementioned expanding flow produce non-negligible contributions to the heating of the capsule afterbody. Consequently, this radiative heating should be considered in the sizing of the afterbody thermal protection systems (TPS). However, due to a lack of experimental research, the nonequilibrium characteristics of CO2 expanding flows are not well understood. Therefore, to help with the design of future Mars entry vehicles, it is necessary to investigate such flows.

The X2 expansion tube was used to facilitate the current study. Three expansion tube test conditions with differing velocities – nominally 2.8 km/s, 3.4 km/s, and 4.0 km/s – were developed. Using a two-dimensional wedge model with a 54° convex corner along with the new conditions, flows with similarities to the expanding flow around the shoulder of an aeroshell at certain Mars entry conditions were created. Mid-wavelength infrared emission spectroscopy of the 4.3 μm and 2.7 μm bands were performed on the flow. The spectroscopic measurements were used to estimate the rotational and vibrational temperatures and the CO2 number densities using a spectral fitting method. Supplementing the spectroscopic measurements, filtered images of the 4.3 μm and 2.7 μm bands were taken to provide a two-dimensional spatial map of the band radiance in the flow around the wedge. Estimates of the experimental inflow conditions were produced by solving for the intermediate test gas states using measurements of various properties of the operating condition. CO2 spectroscopic measurements of the inflow along with measurements of the wedge model shock location and deduced post-shock conditions were also used to help characterize the test conditions. Using estimated inflow conditions, three-dimensional CFD simulations of the experiments were conducted using a two-temperature model. The computed results were compared to the available measurements to examine the appropriateness of the current numerical model at simulating the CO2 flow.

Important qualitative results were obtained in this thesis in regards to the high temperature CO2 expanding flow. The temperatures estimated in the wedge flow using both the 2.7 and 4.3 µm spectroscopic measurements were found to be the same. This provides some confidence to the validity of NEQAIR, using CDSD-4000, at predicting CO2 radiation under gas-dynamic conditions which are more relevant to Mars entry. Using the estimated rotational and vibrational temperatures, the CO2 thermal non-equilibrium in the expanding flow was shown to be small – less than 10%. As the experiments simulate the expanding flow around the shoulder of an entry vehicle, this result is consistent with previous state-specific numerical computations done for such flow. Using vibrational relaxation rates tuned for post-shock conditions, CFD simulations significantly over-estimated the thermal non-equilibrium in the expanding flow, indicating the CO2 vibrational relaxation is faster in expanding conditions than in post-shock conditions. This result is consistent with past results for N2 and CO, as well as CO2 at lower temperatures.

The qualitative results seem to indicate that no complex thermal non-equilibrium effects occur in the expanding flow. This means that multi-temperature models, using appropriate thermochemical rates, along with the NEQAIR radiation code should be capable of accurately predicting the CO2 radiation in the current experiment. However, this is not a general result as the experiments concern the flow around the shoulder of an entry vehicle. In fact, based on the current results as well as results from past studies of CO2 expanding flows, it is speculated that complex non-equilibrium states form far downstream of the initial expansion rather than near the start of the expansion.

Lastly, some interesting anomalies were discovered from the analyses on characterizing the experimental test conditions. While no clear anomalies were observed in the 4.0 km/s experiments, the 2.8 and 3.4 km/s experiments did have obvious anomalies. For the 2.8 and 3.4 km/s conditions, the experimental freestream CO2 number densities estimated from pitot pressure measurements were factors of 4 – 5 greater than that estimated from spectroscopic measurements. Additionally, for the 2.8 and 3.4 km/s conditions, the measured post-shock temperature of the wedge was found to be approximately 30 % lower than that computed using the available freestream estimates. The difficulty in characterizing similar CO2 test conditions has been reported for other impulse facilities. This difficulty is believed to be due to the fact that generating test conditions in these impulse facilities involves nonequilibrium CO2 expanding flow processes, which are poorly understood and which need to be studied in the first place.
Keyword Expansion tubes
Mars
Carbon dioxide
Radiation
Expanding flow
Hypersonics
Spectroscopy
Aerothermodynamics

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Created: Thu, 22 Feb 2018, 20:33:42 EST by Sangdi Gu on behalf of Learning and Research Services (UQ Library)