Characterization of cellular structure appearance in ethanol expanding spherical flames

Abstract

The objective of this work is to characterize the combustion process of ethanol flames under cellular conditions. Ethanol is considered an alternative fuel and can be used to replace fossil fuels. To investigate the behavior of ethanol as a fuel, some of its combustion properties are measured and characterized, such as laminar burning velocity and flame front stability, which strongly depend on the appearance of cellularity on the flame. The study is developed in a cylindric constant volume combustion bomb instrumented with Schlieren technique to visualize ethanol flames and make an optical diagnosis of the combustion process. Some cellular parameters are proposed to characterize the cellular structure of the flame, which quantitatively define the appearance and cellularity development, such as cellular radius, the time for the cellular structure apparition on the flame and the influence of cellularity on the burning velocity. Other dimensionless parameters that can help to determine the influence of cellularity in the combustion process and compare between different flames. An I-Optimal design of experiments is proposed in this work to characterize the flame stability of ethanol, design the experimental testing and develop predictive models for the proposed cellular parameters. The proposed area of study to assure cellular flames is delimited by an initial temperature of 343K, initial pressures from 0.15 MPa to 0.30 MPa and equivalence ratios ranging from 0.8 to 1.4. Images confirm that models predict correctly the cellular radius and others cellular parameters, and the appearance of cellularity affects the burning velocity generating an auto-turbulence in the flame which enhance it. Predictions of cellular radius obtained with developed model are in accordance with the results obtained by other works.