Thermal cracking of hydrocarbons has gone through a significant evolution over the past 20 years. Improved metallurgical properties together with a better understanding of the chemical aspects have led to new configurations for furnace and reactor, all aiming for high severity cracking. A full 3D CFD model containing transport equations for mass, momentum and energy has been implemented in the software code FLOWSIM, together with the kappa-epsilon turbulence model. It has been coupled with the appropriate kinetic models (the radical reaction scheme CRACKSIM for the reactor and combustion kinetics for the furnace) and an overall iteration scheme has been developed for a coupled furnace-reactor simulation allowing to simulate industrial units. This approach has been applied for a propane cracking furnace, providing detailed understanding of the transport mechanisms taking place.