Implementation of isoconversional pyrolysis kinetics in a finite-element model of charring ablation

Abstract

Constant-parameter Arrhenius models for phenolic-impregnated carbon ablator (PICA) often match surface temperature, yet systematically overstate char recession. This study implements isoconversion kinetics extracted from multi-rate TGA via Flynn–Wall–Ozawa and embedded in UMATHT for PICA, with tabulated activation energy and pre-exponential factor {E(α), A(α)} advanced at integration points and coupled to conduction, pyrolysis-gas enthalpy, radiative and convective exchange with blowing, and mesh motion. Validation against oxy-acetylene tests shows two main improvements: surface-temperature plateaus remain within the measured 2250–2500 K band while avoiding the Arrhenius overshoot, and mid-range predictions fall by 60–120 K toward the data. Recession bias is reduced by ∼ 39%, from up to fourteen-fold overprediction to ∼ 0.7–3.5× the experimental mean (≈ 0.24 mm s−1). Joint interpretation of temperature history, recession-depth kinetics, and heating-rate fields shows a traveling subsurface maximum that weakens with time and reduces char consumption. Because a single convective coefficient cannot match both temperature and recession, boundary closure limits fidelity.