Innovative design method for dual-Bell nozzles in rocket propulsion systems

The primary aim of a dual bell nozzle is to enhance performance by utilizing the principle of auto-adaptation based on altitude variations. This system offers the advantage of automatically adjusting the flow for two operational modes (low and high altitude) without the need for mechanical activation. Although the concept is theoretically straightforward, the structural forces involved can be considerable . This study presents a numerical approach for designing such nozzles. It utilizes transonic flow methods to establish the starting line for subsequent supersonic calculations, while the method of characteristics is employed to create the base nozzle profile. The base nozzle is modeled as a second-degree polynomial, with its constants derived from initial conditions. To reduce the weight of the nozzle, truncation is performed at a point that achieves the best compromise between weight and performance. The profile of the second curve is calculated to maintain a constant wall pressure, which is accomplished using the direct method of characteristics applied to a centered expansion wave with an intensity ratio of P2/P1 at the junction. After generating the profile, an analysis of the evolution of thermodynamic parameters (such as pressure and Mach number) and aerodynamic performance is conducted. To ensure accuracy, the results are compared with numerical databases from ONERA nozzle studies. Flow simulations in the nozzle are carried out using the Ansys 13.0 environment with various mesh types. Additionally, to mitigate boundary layer effects, the k-ω SST turbulence model is utilized in the simulations. The results obtained from the method of characteristics and numerical simulations are compared with computed results from the literature, showing good agreement and similarity.