REFINED CHEMICAL-EQUILIBRIUM COMBUSTION MODELING FOR PROPULSION APPLICATIONS: NUMERICAL STABILIZATION, PHASE-BOUNDARY TREATMENT, AND NOZZLE-DESIGN CONSEQUENCES

This article presents the principal results of a refined chemical-equilibrium combustionmodeling framework developed for jet and rocket propulsion applications.The work focuses on the numerical and thermodynamic reliability of a from-scratch equilibrium solver that follows the classical Gibbs-free-energy minimization philosophy of NASA CEA while introducing additional treatment for condensed phases, phase-boundary crossings, triple-point neighborhoods, and polymorphic phase selection.The most important results are grouped into four technical outcomes.First, gas-only verification against NASA CEA confirms that the PT, PH, and PS formulations reproduce reference equilibrium states with average relative errors of order for representative H2/O2 and CH4/O2 combustion systems.Second, heterogeneous calculations show stable condensed-phase activation and agreement with NASA CEA at the sub-percent level, with the remaining discrepancies mainly associated with different conventions for heterogeneous internalenergy post-processing.Third, a phase-boundary module using composition-identical phase matching, inferred transition temperatures, coexistence tolerance, deterministic temperature snapping, and active-set augmentation resolves numerical oscillation near water solid-liquid coexistence and supports stable polymorph selection in metal-oxide systems.Fourth, coupling the refined equilibrium solver with a method-of-characteristics nozzle calculation changes the resulting RD-107 and RD-108 nozzle geometries in a physically interpretable way: both refined nozzles achieve the prescribed exit condition with larger initial turning, unchanged exit angle, shorter length, and smaller area ratio than the conventional fixed-property baseline.These results show that refined equilibrium modeling is not merely a post-processing improvement; it can directly alter propulsionsystem geometry when thermochemical response is embedded into design calculations.