The direct initiation of detonations in one-dimensional (1D) and two-dimensional (2D) cylindrical geometries is investigated through numerical simulations.In comparison of 1D and 2D simulations,it is found that cellular instability has a negative effect on the 2D initiation and makes it more difficult to initiate a sustaining 2D cylindrical detonation.This effect associates closely with the activation energy.For the lower activation energy,the 2D initiation of cylindrical detonations can be achieved through a subcritical initiation way.With increasing the activation energy,the 2D cylindricaJ detonation has increased difficulty in its initiation due to the presence of unreacted pockets behind the detonation front and usually requires rather larger source energy.