In the present work, CFD simulations of a large scale open deflagration experiment are performed. Stoichiometric hydrogen-air mixture occupies a 20 m hemisphere. Two main combustion models are compared and are evaluated against the experiment. The Eddy Dissipation Model and a multi-physics combustion model which is based on Yakhot's equation for the turbulent flame speed. The values of models' critical parameters are investigated. The effect of the turbulence model on the results is also examined. LES approach and k- model were tested. The multi-physics combustion model using the RNG LES turbulence model achieved the best agreement with the experiment.