Experimental study of lean flammability limits of methane/hydrogen/air mixtures in tubes of different diameters

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 characterized 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% CH4 + 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% CH4 + 10% H-2)/air mixtures in the 8.9 mm ID tube, and all limit flames in 6.0 min 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% CH4 + 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. (C) 2009 Elsevier Inc. All rights reserved.