Experimental investigation of hydrous ethanol/air flame front instabilities at elevated temperature and pressures

Abstract

The present work experimentally investigates hydrous ethanol/air flame stability. The experimental data were obtained using spherically expanding flames in a constant volume bomb with optical access for high-speed schlieren photography. It explores the effect of flame parameters, such as thermal expansion rate, flame thickness, activation energy, and effective Lewis numbers, on flame dynamics at elevated pressures (2 to 6 bar) and temperatures (380 and 450 K), at various equivalence ratios (0.6 to 1.3) and water dilution contents (0, 5, 20 and 30% in volume). Adding water to the ethanol/air mixture and increasing its content leads to a significant decrease in flame instability, reducing the thermal expansion ratio while increasing the flame thickness and therefore reducing the propensity of hydrodynamic instability appearance on the flame front. The equivalence ratio has a significant effect on flame stability as well. Slightly rich mixtures present the maximum thermal expansion ratio and minimum flame thickness, therefore, presenting the highest propensity for hydrodynamic instability of the flame front. Besides, the effective Lewis number significantly decreases with equivalence ratio, showing a higher propensity of diffusional-thermal instability for rich mixtures. The flame front instability significantly increases with the mixture initial pressure, which results from the enhancement of the hydrodynamic instability due to the significant decrease in the flame thickness for all equivalence ratios. The initial temperature has a weaker effect on flame stability compared to the other thermo-chemical properties investigated. However, the flame front instability slightly increases with temperature.