Design Optimization of Nozzle and Second Throat Diffuser System for High Altitude Test using CFD

Nozzles are used to produce thrust in airplanes, rockets, and even satellites. Different designs are employed for various applications. For propelling satellites in space, High Area Ratio supersonic nozzles (A.R. > 100) are used. During ground tests of these nozzles, it is found that a shock wave is formed and flow tends to separate in divergent section. A second throat ejector diffuser system (STED) is employed to simulate ambient vacuum conditions at high altitudes, in sea level, around the nozzle. This vacuum level is maintained throughout the test times and is used for smooth expansion of the gas across the nozzle without flow separation. The Shockwaves formed in the diffuser help in maintaining the desired low-pressure levels and reduce the flow velocity and increase the pressure. A High Area Ratio nozzle (A.R.=114) and a Second Throat Diffuser are designed and modeled together. The flow analysis is carried out using CFD and the pressure, velocity and temperature variations in the nozzle and diffuser are studied. The diffuser design is then optimized, by varying different parameters of the diffuser, and analysis is carried out for each design in order to obtain full flow in the nozzle and the diffuser. The design of the diffuser must be such that the flow in the nozzle must be stable and should provide a suitable pressure recovery with the reduction in flow velocity. After the diffuser design is finalized, performance analysis of the diffuser is carried out, along with the nozzle, in order to achieve the working conditions of the diffuser.