Experimental investigations were performed to examine the effects of flame interactions with different multiple obstacles within chambers of different L/D ratios. The basic chamber consists of a rectangular box, 200 mm in height, with a 700 nun x 700 mm cross-section and a large top-venting area of 700 mm x 2 10 mm. The chambers were designed at 200 mm height intervals, and the maximum height of the chambers was 1000 mm. Five chambers were employed to examine the flame interaction with multiple bars in each chamber. Three different multiple obstacles were used: square, triangular and circular cross-sections with blockage ratios of 30% and 43%. The results of flame speed and pressures of different LID ratios were discussed. The flame speeds and pressures associated with the obstacles showed little significant difference in chambers with the LID ratios of 0.29 and 0.57. However, chambers over a ratio of 0.86 became sensitive to the obstacle types, the L/D ratio and the blockage ratios. The highest overpressures were observed with the triangular obstacles while the lowest overpressures occurred with the cylindrical obstacles. The flame development was correlated with pressure development in the chambers having L/D ratios of 0.86-1.43. (c) 2007 Elsevier B.V. All rights reserved.