Modeling and Numerical Simulation of the Behavior of Charring Ablators During Atmospheric Re-Entry

During the atmospheric entry of a blunt body, a bow shock wave is generated on the nose of the vehicle, originating a zone at the wall where gaseous species are in thermochemical non-equilibrium. In fact, the large kinetic energy of the flow is converted through the shock into internal energy causing an increase in temperature, especially close to the nose. The temperature rise occurring near the stagnation region may excite the vibrational energy of the gas molecules and may also lead to dissociation and even to ionization. Thus, a hypersonic flow is frequently a chemically reacting flow. Another important high-temperature effect is the radiative heating from the flow to the body. The most important consequence of high temperatures, from a technological point of view, is the resultant high heat transfer rates to the surface. Thus, in order to protect the vehicle's substructure against the heat loads coming from the boundary layer, it is necessary to provide an accurate design of the heat shield that has the task to isolate the interior of the space vehicle from the high temperatures in the shock layer. Therefore, the accurate prediction of the thermal response of the Thermal Protection System (TPS) is essential to accurately design the heat shield with the aim of optimizing its thickness and shape. Thermal protection systems able to endure very high temperatures are made up of materials called surface ablators, that lose mass when subjected to high thermal loads. There can be different causes of mass loss such as phase change, chemical erosion, oxidation and mechanical removal. In general, the mass loss is part of a process called ablation through which the material rejects energy by means of a density variation. Another type of ablative materials are the so called charring or decomposing ablators that undergo both surface ablation and in-depth decomposition. The surface ablators are usually metals, graphite or carbon-carbon, while the charring ablators are resin/binder composites. The main goal of this research has been the development of a computational tool able to accurately simulate the behavior of a non-reusable heat shield during the atmospheric re-entry of a space vehicle. The final tool can study both the behavior of the ablative thermal protection system and its interaction with the boundary layer through a strong coupling with a CFD solver. The main features that can be considered during the computation are the decomposition of the resin and the consequent generation of pyrolysis gas inside the pore space, in addition to the ablation phenomenon. For instance, the pyrolysis effects related to the inner decomposition within a porous charring ablator can be studied, including in-depth gas flux, porosity and pore pressure. This information can be used to predict in-depth damage or mechanical removal caused by large pressure gradients inside the pore spaces. Our numerical simulation tool is also able to simulate the recession of the material and different ablation models can be used to evaluate the recession rate at the wall. Moreover the ablators (unlike the ceramic tiles) strongly affect the flow field through the ablation phenomenon and the pyrolysis gas injection into the boundary layer. Consequently, the coupling with a Navier-Stokes code allows us to study the gas/solid interaction at the wall.