A new simulation strategy for solid rocket motor ignition: coupling a CFD code with a 1D boundary flame model and verification against a fully resolved approach

A new framework is presented for the simulation of ignition transients in complete solid rocket motors. The methodology is based on the coupling of a CFD solver for the combustion chamber flow field and a 1D solver for the unsteady combustion of the propellant, locally at each propellant boundary face of the CFD domain. Two approaches are presented. The first approach solves the propellant flame with a 1D approach, encapsulating all the numerical and multiscale modelling difficulties (surface processes, kinetics for the flame) within the boundary model. It enables a dynamic and physics-based transition from the initial inert heating of the propellant by the igniter flow to established burning, whereas traditional ignition models switch from an inert material behaviour to a quasi-steady empirical burning behaviour once a predefined ignition temperature has been reached, restricting the ability to reproduce fine ignition dynamics and unsteady combustion response. The second approach solves the propellant flame within the CFD domain, while only the surface and solid phase thermal profile are solved within the boundary model. While being more demanding in terms of mesh refinement, it allows for a detailed interaction between the ignition and the internal flow field to be obtained. Both approaches are compared for the laser-induced ignition of a propellant in one-dimension, and on a simplified yet demanding 2D test case where a propellant surface is impinged by a hot igniter jet flow. Despite the sensitivity of the configuration, the results compare very favourably, showing that accounting for the propellant flame only in the boundary model is acceptable and leads to accurate first ignition times. Yet, potential limitations and pitfalls of this approach are discussed, as well as subsequent improvements.