During the silo discharging progress, if the orifice size is not large enough, the orifice of the silo would be jammed by granules due to the formation of an arch. The silo with a curved hopper can effectively prevent jamming because the contraction rate is constant. To clarify the critical orifice size of the curved hopper which does not affect the outpouring of cohesionless granules, the jamming at different orifice sizes was analyzed. It was found that the change in jamming probability is similar at different contraction rates. Moreover, the relationship between the structure of the arch and the critical orifice size was clarified. Later on, a prediction model for the critical orifice size of the curved hopper was proposed. Finally, the effectiveness of the model was verified by experiments. Overall, the results are useful for understanding the jamming during discharging and further providing guidelines for the appropriate design of curved hoppers. (c) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.