Numerical simulation of charring ablation coupled with computational fluid dynamics

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2019
Alanyalıoğlu, Çetin Ozan.
Usage of charring ablators as nozzle liners is a common practice in the field of solid rocket motor industry. Among them, silica-phenolic is a commonly employed material due to its excellent insulation capability. During the design of a solid propellant rocket motor employing silica-phenolic as a nozzle liner, it is desired to have an accurate thermal analysis along with throat recession rate estimation, as the interior ballistics of a solid rocket motor is tightly coupled with throat diameter. This work presents two tools with different levels of fidelity to fulfill these requirements. A one-dimensional tool named as KAYMAK is developed to perform in-depth analysis involving decomposition reactions and pyrolysis gas effects.A built-in simple interior ballistics solver is also included in KAYMAK to serve as a rapid computational tool during earlier phases of design. The governing equations for melting ablation surface energy balance, in-depth charring and pyrolysis flow and injection are implemented in commercial CFD solver FLUENT along with a boundary condition coupled to interior ballistics analysis to perform conjugate, transient analysis of charring ablation for axisymmetrical geometries. A new boundary condition for inclusion of blowing with source terms has been introduced and validated against analytical results, and shape-change instability found in coupled ablation simulations is studied. Turbulence models used in nozzle heat transfer analysis are examined and compared against commonly used Bartz correlation for nozzle heat transfer. Validation of KAYMAK is performed with available data found in literature, and FLUENT implementation is verified against results obtained with KAYMAK. A static firing has been conducted with a small scale motor employing a silica-phenolic nozzle insert and results are compared against interior ballistics coupled conjugate analysis performed with FLUENT implementation. Although a large uncertainty is present with the material characterization, promising results are obtained showing that all relevant physics are effectively captured, and it is illustrated that these effects cannot be captured with a lower fidelity analysis done with KAYMAK.