Abstract: Numerical simulations of rectangular fire sources attached to PMMA wall surface with different aspect ratios and orientations were conducted using Computational Fluid Dynamics (CFD). To verify the accuracy of the numerical simulation method, the simulation results of wall-attached fire sources, with and without pyrolysis, were compared with experimental data. During the numerical simulation process, the study focused on flame morphology, solid temperature distribution, heat flux components and variations in mass loss rate. The results indicate that the numerical simulation effectively captures the flame morphology of wall-attached fire sources with different aspect ratios and orientations. Burner configurations influence flame thickness, thereby altering the incident radiative heat flux received by the PMMA wall surface. Moreover, burner configurations affect the pyrolysis process by influencing the contribution of the mass loss rate in the highest temperature range and the total volume percentage of temperature ranges above the pyrolysis temperature threshold.