This paper proposes a novel structure for the quenching element of hydrogen flame arresters. The structure, named the radial slit structure, consists of two narrowly spaced parallel disks and solves the technical problem of establishing a narrow quenching gap for hydrogen. To obtain the structure's design criterion, its flow characteristics and quenching characteristics were experimentally studied. First, for the flow characteristics, the influences of the inner diameter and gap size of the radial slit structure were investigated. The results showed that the flow rate is more sensitive to gap size than inner diameter. Second, for quenching characteristics, behavior of the detonation in a hydrogen air mixture behind the flame arrester was investigated for conformity with ISO 16852. The experimental results showed that the maximum gap size of the quenching element was 0.13 mm, which is greater than the maximum experimental safe gap for hydrogen (0.08 mm). Based on these experimental results, a dimensional analysis was carried out to obtain the design criterion and the ratio of length-to-gap size of the radial slit structure. (C) 2011 Elsevier Ltd. All rights reserved.