Flammability limits and laminar flame speed of hydrogeneair mixtures at sub-atmospheric pressures M. Kuznetsov a, *, S. Kobelt a , J. Grune b , T. Jordan a a Institute for Energy and Nuclear Energy, Karlsruhe Institute of Technology, Germany b Pro-Science GmbH, Germany article info Article history: Received 16 February 2012 Accepted 12 May 2012 Available online 30 June 2012 Keywords: Hydrogen Flammability limit Laminar velocity Markstein length abstract Hydrogen behavior at elevated pressures and temperatures was intensively studied by numerous investigators. Nevertheless, there is a lack of experimental data on hydrogen ignition and combustion at reduced sub-atmospheric pressures. Such conditions are related to the facilities operating under vacuum or sub-atmospheric conditions, for instance like ITER vacuum vessel. Main goal of current work was an experimental evalu- ation of such fundamental properties of hydrogeneair mixtures as flammability limits and laminar flame speed at sub-atmospheric pressures. A spherical explosion chamber with a volume of 8.2 dm 3 was used in the experiments. A pressure method and high-speed camera combined with schlieren system for flame visualization were used in this work. Upper and lower flammability limits and laminar flame velocity have been experimentally evaluated in the range of 4e80% hydrogen in air at initial pressures 25e1000 mbar. An extraction of basic flame properties as Markstein length, overall reaction order and acti- vation energy was done from experimental data on laminar burning velocity. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction The necessary strategies for the safe handling of hydrogen are developed by the research group “Hydrogen” of the Institute for Nuclear and Energy Technologies (IKET) of the Karlsruhe Institute of Technology (KIT). The focuses of the research are, for example, safety strategies against hydrogen accidents in nuclear reactors and as well as hydrogen and dust explosions in the planned fusion reactor ITER. The normal operating pressure of the ITER vacuum vessel (VV) is sub-atmospheric. The water leakage and followed by interaction with hot beryllium dust in the VV after an air ingress from ambient atmosphere may lead to the formation of combustible hydrogeneair mixture [1]. This was the reason why accident scenarios at the ITER operating conditions such as loss of vacuum (LOVA) accident consider formation of hydrogeneair mixtures at sub-atmospheric pressures [1e3]. Additionally, there are vacuum furnaces, where a hydrogen atmosphere is used for the heating treatment at sub-atmospheric pressures [4]. The flammability properties of such mixtures are well- known at atmospheric and elevated pressures as well [5e8]. Only one paper [4] is related to the flammability of hydro- geneair mixtures at sub-atmospheric pressures and ambient temperature. The lowest ignition pressure of 150 mbar was achieved in this work for the high energy spark and hot wire ignition of a hydrogeneair mixture (30% H 2 ). The Upper and Lower Flammability Limits (UFL and LFL) of hydrogeneair mixtures at sub-atmospheric pressures have not been inves- tigated in detail. The laminar burning velocity S L is an important integral characteristic of the reactivity of combustible mixtures. It is used as a measure of the combustion rate in numerical * Corresponding author. Tel.: þ49 72160 824716; fax: þ49 72160 824777. E-mail addresses: kuznetsov@kit.edu, kuznetsov@iket.fzk.de (M. Kuznetsov). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 37 (2012) 17580 e17588 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.05.049