Experimental investigation on laminar burning velocities and flame intrinsic instabilities of lean and stoichiometric H2/CO/air mixtures at reduced, normal and elevated pressures

Experiments were conducted in a constant-volume combustion chamber by using a Schlieren system to study the laminar burning velocities and intrinsic instabilities of lean and stoichiometric (equivalence ratio from 0.3 to 1.0) H2/CO/air premixed flames at various hydrogen fractions (30%, 50%, and 70%) at reduced, atmospheric, and elevated pressures (from 0.05 MPa to 0.4 MPa). The unstretched laminar burning velocities ( S u 0 ) and Lb are compared with data from previous literature. Results indicate that excellent agreements are obtained. The intrinsic instabilities of H2/CO/air flames were interpreted and evaluated in terms of hydrodynamic instability, diffusive-thermal instability, and body-force instability. Additionally, the critical flame radius and critical Peclet number were also measured. The results show that the cellular instabilities of H2/CO/air premixed flames are enhanced due to the weaker influence of curvature and the enhanced intensity of baroclinic torque on the flame with the increase in initial pressure. With the increase in equivalence ratio, the cellular instabilities are weakened with hydrogen fractions of 50% and 70%; however, the cellular instabilities are enhanced with a hydrogen fraction of 30% because of the influence of diffusive-thermal instability. An increase in hydrogen fraction causes an enhancement in cellular instabilities. Body-force instability is also observed at low equivalence ratios and has an inhibitory effect on the generation of irregular cracks on the flame surface.