Nonequilibrium Surface Chemical Processes

The chemical interactions between hypersonic gas flow and the surface of a planetary entry vehicle can have a significant effect on the vehicle's aerothermodynamic properties. These vehicles are required to sustain very high rates of heat transfer to the surface when entering an atmosphere. For this reason, many vehicles employ a thermal protection system (TPS). Accurate simulation of the aerothermal environment expected during atmospheric entry requires models for complicated physical processes, such as nonequilibrium surface chemistry, material response including nonequilibrium pyrolysis chemistry for ablative heat-shields, and radiation due to the strong shock that develops around the entry capsule. These models will have to be incorporated in a coupled manner into a computational framework comprising a hypersonic flow solver that can simulate finite-rate chemical processes at the surface of planetary entry vehicles, as well as material response and radiation codes.

The current ongoing project is a cooperative effort by researchers at the University of Michigan, the University of Kentucky, and the CFD Research Corporation (CFDRC) to develop a computational framework to accurately model the aerothermal environment around ablation-cooled planetary entry vehicles. At the University of Michigan, a finite-rate surface chemistry module originally developed by Marschall and MacLean [1,2] has been implemented in the computational fluid dynamics (CFD) code LeMANS. This module allows for the specification of several different finite-rate surface reaction processes, including adsorption/desorption, Eley-Rideal recombination, and oxidation/nitridation. The module calculates the species production rates at the surface based on the pressure, temperature, and species concentrations at the wall. These production rates are then used in LeMANS to calculate the densities, pressure and temperature at the surface by solving the mass, momentum, and energy conservation equations.

Figure 1 below shows temperature contours along the Stardust sample-return capsule (SRC) calculated using LeMANS at three different altitude conditions during its Earth re-entry. The Stardust payload was launched in 1999 on a mission to collect samples from interstellar dust and the tail of the Comet Wild-2, and return them to Earth. The Stardust SRC landed in the Utah desert in January of 2006. The Stardust spacecraft then continued its travel through the solar system, on a mission to image Comet Tempel-1. It was decommissioned after completing that final mission in March 2011. The Stardust mission represents the first ever return of a sample from a comet; a significant milestone in the human exploration of space. With an entry velocity of 12.6 km/s, the capsule was also the fastest man-made object ever to enter the Earth's atmosphere, providing a unique test case to evaluate numerical simulations. In order to protect the vehicle from the extreme entry conditions, the thermal protection system for the Stardust capsule used the lightweight phenolic-impregnated carbon ablator (PICA).

The chemical mechanisms that are assumed to occur at the surface of the Stardust SRC are:

O + C(b) → CO
O2 + 2C(b) → 2CO
N + N → N2
These mechanisms were developed by Driver et al. [3,4] by comparing CFD predictions to heat transfer and recession rate measurements obtained at an arc-jet facility for PICA. The mechanisms found by Driver et al. include the oxidation of bulk carbon by atomic oxygen with a constant reaction efficiency of 0.9, and the oxidation of bulk carbon by molecular oxygen with a constant efficiency of 0.01. These mechanisms also include the recombination of atomic nitrogen at the surface with a constant recombination efficiency of 0.05.

Figure 1: Temperature contours along the Stardust forebody at entry trajectory altitudes of 71 km (left), 62 km (center), and 51 km (right).

Figure 2a below shows the stagnation point heat transfer to the Stardust capsule as a function of freestream velocity for four different trajectory points. The heat transfer peaks at an altitude of 62 km and then decreases as the capsule slows down. The figure also shows the results assuming no chemical reactions occurring at the surface (i.e. non-catalytic). These results show that the surface reactions described earlier increase the heat transfer to the vehicle. The removal of bulk carbon by the two oxidation reactions results in an effective mass blowing at the surface. The rate of this blowing at the stagnation point is shown in Fig. 2b as a function of freestream velocity. The results also show that the mass blowing rate peaks at an altitude of 62 km, and then decreases as the temperature along the Stardust surface decreases.

Figure 2: Heat transfer (left) and mass blowing rate (right) at the stagnation point of the Stardust capsule.

Acknowledgments

This work is funded by a NASA SBIR Phase II Contract.

References

  1. Marschall, J. and MacLean, M., "Finite-Rate Surface Chemistry Model, I: Formulation and Reaction System Examples," AIAA Paper 2011-3783, June 2011.
  2. MacLean, M., Marschall, J., and Driver, D. M., "Finite-Rate Surface Chemistry Model, II: Coupling to Viscous Navier-Stokes Code," AIAA Paper 2011-3784, June 2011.
  3. Driver, D. M., Olsen, M. W., Barnhardt, M. D., and MacLean, M., "Understanding High Recession Rates of Carbon Ablators Seen in Shear Tests in an Arc Jet," AIAA Paper 2010-1177, January 2010.
  4. Driver, D. M. and MacLean, M., "Improved Predictions of PICA Recession in Arc Jet Shear Tests," AIAA Paper 2011-141, January 2011.

Recent Publications

  1. Alkandry, H., Boyd, I. D., and Martin, A., "Comparison of Models for Mixture Transport Properties for Numerical Simulations of Ablative Heat-Shields," 51st AIAA Aerospace Sciences Meeting, 7-10 January 2013, Grapevine, Texas, AIAA Paper 2013-0303.
  2. Alkandry, H., Farbar, E. D., and Boyd, I. D., "Evaluation of Finite-Rate Surface Chemistry Models for Simulation of the Stardust Reentry Capsule," 43rd AIAA Thermophysics Conference, 25-28 June 2012, New Orleans, Louisiana, AIAA Paper 2012-2874.