Evaporation of polymeric solid fuels in backward facing step geometry subject to an inlet oxidizer flow at elevated temperatures is considered and convective heating of the fuel surface by the hot oxidizing inlet flow and subsequent mixing of the evaporated fuel with the oxidizer flow and its combustion is numerically studied. The objective of this work is to gain insight into the auto-ignition of the fuel and its controlling parameters in this configuration. The system of governing equations is solved with a finite volume approach using a structured grid in which the AUSM⁺ scheme is used to calculate the gas phase convective fluxes. The flowfield is turbulent and the Spalart–Allmaras turbulence model is used in these simulations. Special attention is paid to the coupling of gas and solid phase to study the ignition process. Distinct intervals in ignition delay time are studied and evaporation time, mixing time, and reaction time are individually estimated. We have demonstrated that for inlet oxidizer streams with high initial oxygen concentration levels and high enough inlet temperatures a diffusion-controlled ignition mechanism controls the ignition time delay independent of the inlet velocity. This ignition time delay is directly related to the solid fuel evaporation time delay.