A numerical study on the interaction of flow and combustion in solid propellant rocket motors

A computational model is developed for the simulation of reactive fluid flow in a solid rocket motor (SRM) chamber involving variable internal geometry associated with pressure dependent propellant burning surface regression. Complete conservation equations of mass, momentum, energy and species with finite rate chemistry are solved. Spatial discretization is obtained using second order upwind differencing scheme and temporal discretization is achieved using first order implicit method. Stiff chemistry solver is used to avoid the stiffness problem due to the high temperature gradient as well as species mass fraction gradients. Cells are highly clustered in the neighborhood of the burning part of the motor to better capture these rapid variations of flow properties within the flame zone. The computational model is capable to represent the flow with wide range of Mach number, from subsonic inlet on the burning boundary to supersonic outlet at the exit of the nozzle. The pressure and time dependent dynamic boundary is established on the burning regressive boundary. To do this, User Defined Functions (UDF's) are developed and used by means of C++ which are implemented in ANSYS Fluent 15.0. The regressive boundary, which increases the volume of the combustion chamber in time, is treated by use of remeshing techniques. Time dependent cold and reactive flows were simulated successfully. Cold flow both with non-regressive and regressive boundary model results agreed with the numerical solutions in the literature. Reacting flow with stagnant wall simulation showed good agreement when compared with two different geometry models available in the literature. These results indicate that, the current simulation are accurately developed. Hydrogen and propane combustion processes and thrust variations in time are examined for the constant rate boundary regression. Temperature and species mass fraction variations are obtained within the flame zone. Time dependent pressure and burning rate changes are illustrated for pressure dependent burning rate calculations. Temperature, velocity and density distributions are compared for both constant burning rate and pressure dependent burning rate simulations for different geometries. The developed model can simulate reactive fluid flow in a solid rocket motor (SRM) chamber with variable internal geometry due to the pressure dependent propellant burning surface regression successfully.