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In the present work theoretical models for determining optical density in smoke (visibility) with Computational Fluid Dynamics (CFD) simulations are compared to experimental data. CFD models do not give the smoke concentration directly in the calculation domain. Two different methods are used in the paper to convert the CFD results into an optical density. The first model analyzed here assumes that the optical density is dependent on the local conditions in the flow field and the mass optical density of the burning material. Good correlation was found between calculated results based on the model and the experimentally obtained results. A second model is based on the assumption that there is a correlation between the gas temperature rise and the optical density. This model produced poorer results, especially far away from the fire source. The experimental test rig consisted of a 20 m long tunnel (corridor/culvert) with different types of longitudinal ventilation and exhaust ventilation arrangements. The optical density was measured at three different distances downstream of a kerosene fire source.