Laminar burning velocities and flame instabilities of butanol isomers–air mixtures

Abstract

Laminar burning velocities and flame instabilities of the butanol–air premixed flames and its isomers are investigated using the spherically expanding flame with central ignition at initial temperature of 428 K and initial pressures of 0.10 MPa, 0.25 MPa, 0.50 MPa and 0.75 MPa. Laminar burning velocities and sensitivity factor of n-butanol–air mixtures are computed using a newly developed kinetic mechanism. Unstretched laminar burning velocity, adiabatic temperature, Lewis number, Markstein length, critical flame radius and Peclet number are obtained over a wide range of equivalence ratios. Effect of molecular structure on laminar burning velocity of the isomers of butanol is analyzed from the aspect of C H bond dissociation energy. Study indicates that although adiabatic flame temperatures of the isomers of butanol are the same, laminar burning velocities give an obvious difference among the isomers of butanol. This indicates that molecular structure has a large influence on laminar burning velocities of the isomers of butanol. Branching ( CH3) will decrease laminar burning velocity. Hydroxyl functional group ( OH) attaching to the terminal carbon atoms gives higher laminar burning velocity compared to that attaching to the inner carbon atoms. Calculated dissociation bond energies show that terminal C H bonds have larger bond energies than that of inner C H bonds. n-Butanol, no branching and with hydroxyl functional group ( OH) attaching to the terminal carbon atom, gives the largest laminar burning velocity. tert-Butanol, with highly branching and hydroxyl functional group ( OH) attaching to the inner carbon atom, gives the lowest laminar burning velocity. Laminar burning velocities of iso-butanol and sec-butanol are between those of n-butanol and tert-butanol. The instant of transition to cellularity is experimentally determined for the isomers of butanol and subsequently interpreted on the basis of hydrodynamic and diffusion-thermal instabilities. Little effect on flame instability is observed for the isomers of butanol. Critical flame radii are the same for the isomers of butanol. Peclet number decreases with the increase in equivalence ratio.