Fluid–structure-coupled simulations of heat transfer in hybrid rocket engine nozzles

In hybrid rocket engines, the nozzle experiences high temperatures that can lead to erosion, especially when passively cooled designs are used. The temperature at the fluid–structure interface plays a critical role in nozzle erosion, and understanding the heat transfer at this interface is essential for accurate erosion modelling. This study presents high- and low-fidelity coupled simulations of nozzle heating, combining computational fluid dynamics (CFD) and computational solid mechanics (CSM) to analyse transient heat transfer in hybrid rocket engine nozzles. In the high-fidelity case, axisymmetric simulations are performed. The reactive flow in the nozzle is simulated using the DLR TAU code, with RANS simulations and a chemical non-equilibrium model to capture the fluid flow. The thermal response of the nozzle structure is calculated using finite element analysis with Ansys, modelling the two-dimensional heating of the nozzle material. These two domains, computational fluid dynamics (CFD) and computational solid mechanics (CSM), are coupled using the DLR coupled numerical fluid, flight mechanics and structure simulations (CoNF ² 2 aS ² 2 ) tool, which facilitates the exchange of heat flux and temperature data between the fluid and structure simulations. The high-fidelity simulation results are compared with pre-design calculations. These low-fidelity calculations use thermodynamic relationships and semi-empirical methods to estimate the heat flux at the fluid–structure interface, and one-dimensional heat conduction in the structure to model the temperature response. The simulations are applied to the nozzle geometry of the AHRES-C hybrid rocket engine with a graphite insert. The low-fidelity simulations are validated by comparing their results with high-fidelity predictions, thereby providing an evaluation of the accuracy of pre-design methods. The methods show good agreement in the converging and throat nozzle sections in terms of heat flux and temperatures. This makes the low-fidelity method a good design tool for the sizing of thermal protection. In the diverging nozzle section, the low-fidelity approach underpredicts the heat fluxes and temperatures, as it cannot account for the formation of an oblique shock. This analysis enhances the understanding of heat transfer in hybrid rocket engine nozzles, further informing design decisions and optimization strategies.