Comparative CFD Study of Small-Scale Rocket Nozzles (Bell vs. Conical): Impact of Chemical Species Transport Modeling on Flow Behavior and Performance Predictions

Abstract

This study investigates the internal and external flow behavior of conical and bell rocket nozzles through numerical simulations performed at two representative altitudes (0 ft and 30000 ft). The objective is to quantify how nozzle geometry, external shock structures, and working-fluid modeling strategies influence key performance metrics, particularly specific impulse, while also examining Mach number, static pressure, and specific heat ratio (gamma) distributions. The analysis considers simplified constant-gamma formulations, including air and a propellant-based model, as well as a multi-species thermochemical formulation. Newly observed features include internal convergent shock waves within the conical nozzle that produce measurable performance losses, in contrast to the bell nozzle’s stable kernel shock system, which does not generate comparable penalties and therefore yields higher efficiency. Results further show that simplified models can underpredict performance and misplace external shock structures when the effective gamma differs substantially from the true thermochemical state, whereas the multi-species formulation more reliably captures gamma evolution and the resulting internal and external flow behavior. Comparisons with quasi-onedimensional analytical solutions, frozen-equilibrium (FE) predictions, and static fire test data support these findings, demonstrating that nozzle geometry and gas-modeling approach jointly govern performance, shock behavior, and predictive reliability across the simulated altitude range.