Experimental study of lean ﬂammability limits of methane/hydrogen/air mixtures in tubes of different diameters Y.L. Shoshin * , L.P.H. de Goey 1 Eindhoven University of Technology, Mechanical Engineering, Eindhoven, The Netherlands article info Article history: Received 26 October 2009 Accepted 27 October 2009 Keywords: Flammability limit Flame ball Flame stretch Preferential diffusion abstract Lean limit ﬂames in methane/hydrogen/air mixtures propagating in tubes of internal diameters (ID) of 6.0, 8.9, 12.3, 18.4, 25.2, 35.0, and 50.2 mm have been experimentally studied. The ﬂames propagated upward from the open bottom end of the tube to the closed upper end. The content of hydrogen in the fuel gas has been varied in the range 0–40 mol%. Lean ﬂammability limits have been determined; ﬂame shapes recorded and the visible speed of ﬂame propagation measured. Most of the observed limit ﬂames in tubes with diameters in the range of 8.9–18.4 mm had enclosed shape, and could be character- ized as distorted or spherical ﬂame balls. The tendency was observed for mixtures with higher hydrogen content to form smaller size, more uniform ﬂame balls in a wider range of tube diameters. At hydrogen content of 20% or more in the fuel gas, limit ﬂames in largest diameters (35.0 mm and 50.2 mm ID) tubes had small, compared to the tube diameter, size and were ‘‘lens”-shaped. ‘‘Regular” open-front lean limit ﬂames were observed only for the smallest diameters (6.0 mm and 8.9 mm) and largest diameters (35.0 and 50.2 mm ID), and only for methane/air and (90% CH 4 + 10% H 2 )/air mixtures, except for 6 mm ID tube in which all limit ﬂames had open front. In all experiments, except for the lean limit ﬂames in methane/ air and (90% CH 4 + 10% H 2 )/air mixtures in the 8.9 mm ID tube, and all limit ﬂames in 6.0 mm ID tube, visible ﬂame speeds very weakly depended on the hydrogen content in the fuel gas and were close to- or below the theoretical estimate of the speed of a rising hot bubble. This observation suggests that the buoyancy is the major factor which determines the visible ﬂame speed for studied limit ﬂames, except that last mentioned. A decrease of the lean ﬂammability limit value with decreasing the tube diameter was observed for methane/air and (90% CH 4 + 10% H 2 )/air mixtures for tubes having internal diameters in the range of 18.4–50.2 mm. This effect has been attributed to the stronger combined effect of the preferential diffusion and ﬂame stretch in narrower tubes for ﬂames which resemble rising bubble. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Experimental studies on ﬂammability limits and limit ﬂames in combustible mixtures are important both from a fundamental and practical points of view. Knowledge of ﬂammability limits is essen- tial for safety standards and in the design of lean-combustion de- vices. On the other hand, new knowledge can be obtained, or theoretical predictions tested in such studies on ﬂame stretch and preferential diffusion effects on ﬂame propagation and extinc- tion in non-unity Lewis number mixtures. Practical ﬂammability tests are performed at normal gravity, in tubes or closed vessels. One of the standard ﬂammability tests utilizes a so-called standard ﬂammability tube: a 50 mm diameter and 1800 mm long tube open from the bottom end and closed at the top end [1]. In this test, the mixture is considered to be ﬂammable if, after the ignition near the bottom end, the ﬂame propagates upward throughout the en- tire tube length. The ﬂammability tests used in the present work, as described below, are similar to this standard one. The available experimental information on the effect of the tube diameter on ﬂammability lim- its, obtained in test conditions similar to ones used in the present work is rather limited. Tests of this kind have been performed in tubes of different diameters in [1–4]. Practically no difference in lean ﬂammability limits (LFL) for hydrogen/air mixtures (Lewis number, Le < 1) in tubes with 25 mm and 53 mm inner diameter (ID) was found in [1]. For methane/air mixtures, authors of [1] re- ported narrower LFL (Le < 1) for the smaller diameter tube (5.24 mol% of methane in 50 mm ID tube against 5.48 mol% in 25 mm ID tube). At the same time, the ﬂammability tests per- formed in [2] for methane/air mixtures in similar diameters tubes 0894-1777/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermﬂusci.2009.10.027 * Corresponding author. Address: TU/e, Mechanical Engineering, Combustion Technology, Den Dolech 2, WH 2.143, 5600 MB Eindhoven, The Netherlands. Tel.: +31 402475689; fax: +31 402433445. E-mail addresses: Y.S.Shoshin@tue.nl (Y.L. Shoshin), l.p.h.d.goey@tue.nl (L.P.H. de Goey). 1 Address: TU/e, Mechanical Engineering, Combustion Technology, Den Dolech 2, WH 3.133, 5600 MB Eindhoven, The Netherlands. Tel.: +31 402472140; fax: +31 402433445. Experimental Thermal and Fluid Science 34 (2010) 373–380 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs