Investigating propellant flow in a converging-diverging nozzle with atmospheric outlet conditions using finite element simulation and validating the results using experimental tests

Back flow is a phenomenon that occurs due to the increase of upstream pressure compared to downstream pressure. In converging-diverging nozzles, by increasing the outlet pressure compared to the design pressure, the phenomenon of back flow is observed in the divergent part, which causes a decrease in the output velocity and thrust. In this paper, a nozzle with expansion ratio suitable for vacuum condition is selected. Then, finite volume simulation of the propellant flow, here butane, is conducted for two output vacuum pressure namely, vacuum and atmospheric pressure conditions. It has seen that in the atmospheric pressure, the back flow occurs in the nozzle. In order to determine the appropriate expansion ratio for the nozzle operating in atmospheric pressure, flow inside the nozzle is simulated for different expansion ratios and the average outlet axial velocity is obtained. According to the results, the most suitable expansion ratio has the highest average axial velocity. Furthermore, to verify the obtained results from the numerical simulation, four nozzles with different expansion ratios are manufactured and experimentally are being tested and the experimental thrust values are compared with counterparts obtained from the simulation.