Characterization of the flame front inversion of Ethanol–Air deflagrations inside A closed tube

Ethanol–air deflagrations were experimentally studied inside a closed tube by varying the equivalence ratio (0.9–1.4) and the initial pressure (20–60 kPa). The flame front inversion phenomenon was observed in all tests. It was determined that the ratio of the distance of flat flame formation to the hydraulic diameter (df/dh) is a measurable parameter that allows the characterization of the phenomenon. For ethanol–air mixtures two dimensionless parameters can be used to correlate df/dh, namely, the ratio of the laminar flame speed to the speed of sound of the reactants (SL/CS,0) and the dimensionless distance travelled by the flame tip up to the time when the flame touches the lateral walls of the tube (ξtip,wall). Two correlations were obtained and the averages of the absolute values of the relative errors (AARE) were less than 1.2%. Another correlation was obtained considering published data of different fuel–air mixtures. A statistical analysis applied considering experimental data from published literature and from this work, showed an AARE of 8.95% and a coefficient of multiple determination (R2) of 0.79. The p-values of the regression coefficients were lower than 0.001. Therefore, the correlations present good accuracy and can be used for prediction.