Role of Multiphase Modeling on Nozzle Thermochemical Erosion in Solid Rocket Motors

Because of the extremely severe thermal conditions faced in solid rocket nozzles, ablative thermal protection systems are employed to protect the underlying structures. Due to the harsh thermochemical environment in which they operate, ablative materials are eroded during motor firings, leading to performance losses. Modern high-energy solid propellants are loaded with high mass fractions of metal particles which, during combustion, burn to form liquid metal oxide droplets, leading to a complex two-phase flow. The presence of these particles substantially modify the flow features inside the nozzle, depending on the particle density and dimension. The present work aims to investigate the effect of the two-phase flow on the nozzle thermochemical erosion process, a coupled phenomena which has not received much attention in the open literature. To this goal, a finite-rate ablation model is integrated into a two-phase Navier–Stokes flow solver, including an Eulerian-Eulerian model able to deal with particles polydispersion. The HIPPO test motor is used as a test case. Multiphase effects are shown to have a relevant role on the nozzle thermochemical erosion only in case of particles smaller than 2 𝜇m. In fact, in case of larger particles, the nozzle erosion process can be reasonably represented by a single-phase CFD simulation, completely neglecting the presence of particles. For such a case, a maximum discrepancy of only 1.5% at throat is obtained with respect to a more general but also more computationally demanding multiphase solution. On the other hand, it is also shown how single-phase simulations employing the heavy-gas approximation are able to qualitatively predict the effects of very small particles on nozzle erosion.