Experimental observation of ethanol–air premixed flames propagating inside a closed tube with high aspect ratio

Abstract

The present article aims to experimentally observe the flame propagation of ethanol–air mixtures in a tube closed at both ends with an aspect ratio of 27.68. The mixtures were prepared with equivalence ratios ranging from 0.8 to 1.1. The tests were performed for initial pressures of 20, 40, and 60 kPa. The phenomenon of flame front inversion was observed in all experiments. This phenomenon is also known as tulip flame. It was also observed that the flame front inverted several times at the equivalence ratios of 1.0 and 1.1. After the initial deceleration, the velocity oscillated with a high amplitude at these equivalence ratios. An analysis of the available experimental data was performed to better understand the conditions that allow the flame velocity oscillations to occur. It was found that these oscillations manifest when the following conditions are met: (a) closed channels, (b) sufficiently high laminar flame velocity and (c) sufficiently high aspect ratio. Moreover, this phenomenon is coupled with pressure waves that develop inside the duct. The relationship between the distance for the formation of the flattened flame front and the laminar flame velocity was used to define a characteristic time that correlates with the available experimental data.