Modelling of carbon-carbon composite ablation in rocket nozzles (8)

The work has presented a modelling analysis of the surface evolution of C/C composites under ablation in rocket nozzle conditions. Previous studies had shown that the morphological parameters are directly linked to the material structure (reactivity contrast between constituents) and to the physicochemical regimechiefly the reaction/diffusion ratio translated by the Da number.

The ablative behavior of a complex twoscale architecture has been successfully performed, thanks to an inverse approach for the determination of the reactivity contrasts between phases.

The extension of the direct simulation to complex woven architectures confirm the impact of the Damkohler number on the recessive evolution that had been determined on ideal architectures. In the case of a reactive porous interface, the numerical simulations show that, when the heterogeneous reaction becomes faster, the reaction front becomes thinner and the global recession depends less on the pore space structure.

Another conclusion is that the effective recession rate for the composite is not given by a simple rule of mixturesrather, in many conditions, a weakestlink rule applies. Reconstituting the morphology helps in identifying the weakest phase properties, even though they are not available as separate components.

These encouraging results have been obtained is spite of very crude approximations on the description of the gas flow surrounding the material. This very fact suggests that the material architecture is the primary element that determines its morphology after albation, though the flow has an importance in some cases.

These results are of importance for the rocket nozzle designer, since the roughness and effective mass transfer coefficients may be assessed directly. Entering these quantities in a rocket nozzle CFD code allows a better prediction of the final protection thicknesses and throat radius.

Depending on the application, it could be necessary to extend the presented work by taking into account stronger couplings with other transport mechanisms, in particular heat transfer, under the form of converction, conduction and radiation. Also, mass convection is a phenomenon that has been neglected in this first approach, and further work is ongoing ro remove this rather strong hypothesis.

 

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