Experimental and numerical study on the critical initiation radii of 1,3-Butadiene/Oxygen/Helium flame under elevated pressures

A numerical and experimental study investigated the first and second critical initiation radii of 1,3-butadiene/ oxygen/helium flame initiation. A minimum critical flame radius was measured and compared with a calcu-lated first critical initiation radius since the first radius is hard to obtain experimentally. Experiments were conducted in a constant-pressure combustion chamber under pressure up to 1.5 MPa at equivalence ratios of 0.8-1.5 and a temperature of 298 K. The effects of equivalence ratio and pressure on the critical initiation radii and critical stretch rates were studied. Both critical initiation radii first decrease with increasing equivalence ratio and then increase. In contrast, with higher pressure, both critical initiation radii reduce. The critical stretch rates, which correspond to the stretch rates at the critical initiation radii, vary in the opposite direction with changing pressure and equivalence ratio. A nonlinear model and a detailed model were used for a quick esti-mation of critical initiation radii and theoretical analysis, respectively. The calculated results obtained using the two models demonstrated strong agreement with the experimental data. Additionally, the minimum radius and critical radius were highly linearly related to the flame thickness. The detailed model was used to analyze the effects of density ratio, Zel'dovich number, and effective Lewis number on critical initiation radii. A sensitivity analysis revealed that the Zel'dovich number dominates the trends of critical initiation radii. The results also indicated that the critical initiation radii become more sensitive to the Lewis number at low values of the Lewis number. Comparisons between the theoretical results of a detailed model and a nonlinear model were conducted. The nonlinear model was capable of accurately predicting the second critical radius well but only matched the first critical radius at smaller Markstein numbers. An empirical remedy equation of the first critical radii at large Markstein number was also presented.