Understanding of nature and knowledge of the specific numerical values of the concentration limits for explosions in hydrogen-containing mixtures is important for existing nuclear power plants, future thermonuclear installations, multiple industrial applications in petrochemical and chemical industry, coming generation of the hydrogen-fueled vehicles and hydrogen energy infrastructure. In this paper a theoretical physics-based definition for the concentration limits of hydrogen-air gas explosions. Gas explosions have been and are attributed to the self-spreading frontal deflagration flames, whose evolution through flammable gas cloud after ignition results in the substantial baric effects. Specifically in the hydrogen-air mixtures within flammability limits two generic flames can propagate – self-propagating frontal deflagration flames (described by Zeldovich-Frank-Kamenetskii model) and confined in space the buoyant flame-balls (aka Zeldovich model). Fundamental concentration limit difference between these two generic types of flames could be regarded as a critical value for severity of hydrogen-air explosions. Within a Deflagration-to-Flame Ball Transition (DFBT) concentration range (7-12 vol% H2) three traceable candidates exist – two empirical and one theoretical concentration limits. Relations between these candidates and their specific physico-chemical features are described. Targets for three future studies are proposed – one for direct (empirical or computational) evidence in support of a fundamental character of concentration limits under consideration and two studies for accurate quantitative characterization of the ultimate lower concentration limit for hydrogen-air explosions.

International Seminar on Fire and Explosion Hazards (9; 2019; Saint Petersburg, Russia). Proceedings of the Ninth International Seminar on Fire and Explosion Hazards [Электронный ресурс]: 21-26 April 2019, Saint Petersburg, Russia. Vol. 2 / Peter the Great St. Petersburg Polytechnic University, Autonomous Non-Profit Organization "Fire and Explosion Safety", Gefest Holding Ltd ; [edited by A. Snegirev [et al.]. — Электрон. текстовые дан. (1 файл : 98,7 Мб). — Saint Petersburg, 2019. — Загл. с титул. экрана. — Свободный доступ из сети Интернет (чтение, печать, копирование). — Adobe Acrobat Reader 7.0. — <URL:http://elib.spbstu.ru/dl/2/k19-97.pdf>. — <URL:http://doi.org/10.18720/SPBPU/2/k19-97>.