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. KeywordsFlame front inversion • Deflagrations • Closed tubes • Ethanol List of symbols AR Aspect ratio c p Heat capacity at constant pressure (J/kg K) d Tube diameter (m) d f Distance of flattened flame formation (m) E a Activation energy (J/mol) * Andrés Z. Mendiburu

Experimental and Theoretical Analyses of Deflagrations of Mixtures Involving Hydrogen Enrichment and Helium Dilution Inside a Closed Duct

“…Note that the dimensionless position ξ tip,wall is measuring the distance traveled by the flame tip (from the ignition source) to the moment at which the planar flame front is starting to form. This distance has been discussed and measured in previous works. , The results of the integrations are given in eq . …”

“…This distance has been discussed and measured in previous works. 14,15 The results of the integrations are given in eq 30.…”

“…These results are in agreement with other experimental works available in the literature. 14,15,19,46,47 It is observed that the flame accelerates when traveling inside the first module. This means that the spherical and finger flame stages occur inside this module.…”

“…The flame propagation inside a closed duct free of obstacles can be divided into five stages: (a) the spherical flame propagation, (b) the finger flame propagation (or acceleration stage), (c) the planar flame formation (or deceleration stage), (d) the tulip flame formation (or flame front inversion stage), and (e) the cellular flame propagation . Moreover, depending on the initial pressure, the flame propagation can enter the turbulent regime, as observed by Mendiburu et al In their work, Ponizy et al showed experimentally that the tulip flame is formed due to the reverse flow of the burned gases that stablishes inside the duct during the deceleration stage.…”

Flammability Limits: A Comprehensive Review of Theory, Experiments, and Estimation Methods

“…Therefore, the flammable ranges tend to be larger for UFP than for DFP or HFP. Note that flames with HFP develop inverted flame fronts in closed vessels, also called tulip flames. , It is not clear how this phenomenon affects the experimental determination of FLs with HFP.…”