A fully coupled 2D fluid-solid direct numerical simulation (DNS) approach is used to simulate co-flow flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental measurements are conducted over a range of flame spread rates. Results show that the heat flux to the preheating region varies considerably in time - contradicting often employed assumptions used in established flame spread theories. Accounting for the time dependent behavior is essential in accurate predictions of flame spread, however, a universal characterization in terms of easily defined parameters is not found. Alternatively, a reaction progress variable based embedded flame model is developed using mixture fraction, total enthalpy and surface temperature. State maps of the gas-phase properties and surface heat flux are constructed and stored in pre-computed lookup tables. The resulting model Provides a computationally efficient and a local formulation to determine the flame heat flux to the surface resulting in excellent agreement to DNS and experiments for predictions of flame spread rate and position of the pyrolysis front. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.