Development of a unified model for flow-material interaction applied to porous ablators

During the atmospheric reentry phase of a spacecraft, a huge amount of heat is exchanged between the surrounding air and the thermal protection system of the vehicle. The ablation of carbon-resin composite materials used for the heat shield is a very complex multi-physic problem. The degradation phenomenon occurs mainly in two steps. First, the resin is progressively pyrolysed, producing pyrolysis gases and a char matrix. Then, the char is ablated simultaneously by chemical reactions, sublimation and spallation.

The objective of this work is to simulate the degradation of thermal protection materials inside the VKI Plasmatron facility by considering the contribution of the pyrolysis in the ablation process. The study is performed in two steps.

First, the ablation of a non-charring porous material (carbon preform) inside the Plasmatron facility is reproduced by means of the Argo code, developed at Cenaero in collaboration with the VKI. The free-stream boundary conditions corresponding to the Plasmatron test is first thoroughly reviewed for their implementation in the code. Then, the flow through and around the carbon preform sample is simulated.

The second step consists is the development and the implementation of a new module of the DGAblation branch of Argo to model the pyrolysis of the material. The code is validated on several test cases, and the simulation of a full pyrolysis and ablation problem of a carbon-phenolic material inside the Plasmatron is finally performed.

It is the first time that strongly coupled simulations of the degradation of a thermal protection material corresponding to those largely tested in the Plasmatron is performed.

Results of the simulations show good agreement with the surface temperature measurements. Numerically, a mesh fine enough, at least lower than the length of ablation, is required at the interface to catch the recession of the surface. The flow inside the material can also be extracted from the simulations. The Stokes flow percolates through the porous medium toward the shoulders of the material and at the same time products of reactions are released in the boundary layer. A simple Plasmatron simulation on carbon-phenol provides similar observations on the flow field.

These results demonstrate the ability of the unified approach together with a discontinuous galerkin discretization to solve ablation problems on simple carbon preform as well as on ablative composite materials. With the new model accounting for the pyrolysis, the code features a unique capability to simulate the flow around and within ablative composite material.