Shape of burning surface for solid propellant samples ignited with low laser heat flux

Unlike classical pyrotechnical solutions such as black powders, laser ignition is a non-intrusive, scalable method making it an interesting ignition technique for solid propellants in academia. When studies focus on the solid propellant gas flow, the shape of the burning surface becomes crucial. It is usually necessary to have a burning surface as flat as possible. The laser ignition delay of solid propellants follows a power law function of the laser heat flux. Because this law is not linear, ignition time varies strongly even for small laser irradiance variations. More precisely, the lower the laser irradiance, the more difficult it is to obtain a plane ignition. In this paper we present a calculation method to estimate the shape of the burning surface for a solid propellant sample based on the laser irradiance spatial distribution. For this purpose, a one-dimensional surface heating model is implemented together with a solid-propellant ignition model. This allows to calculate a local ignition delay as a function of the incident heat flux. The Gaussian spatial heat flux distribution has been discretised over the beam radius, and an ignition time calculation is performed for each spatial discretisation point. Considering that the solid propellant is a good thermal insulator, the ignition delay is independently calculated for each radius. Finally, the temporal evolution of the burning surface is determined, taking into account the flame propagation over the propellant surface. These results show the evolution of the surface with different laser parameters (i.e. beam size and power). This allows us to define, based on the laser irradiance distribution, a radius limit for which planar ignition can be assumed. An experimental setup is presented to measure the shape of the burning surface as a function of heat flux distribution for low laser irradiance. The calculation method is demonstrated with physical properties representative of a classical AP/HTPB research propellant composition. The approach is very promising to optimize ignition conditions for future experiments that require planar ignition.