Laminar burning characteristics of ammonia and n-butanol blend fuels

Abstract

Blending n-butanol with ammonia has been shown to effectively improve the combustion performance of ammonia and enable carbon–neutral combustion systems. This study investigated the laminar burning speed and Markstein length of n-butanol/ammonia/air mixtures at various equivalence ratios (0.8–1.4) using the spherical flame method with the nonlinear extrapolation model (NQ) in a cylindrical constant-volume combustion bomb. The initial temperatures varied from 393 K to 453 K, and the initial pressures ranged from 0.1 MPa to 0.6 MPa, with ammonia fractions ranging from 50% to 80%. Results illustrate that the unstretched laminar propagating speed and laminar burning speed have a negative correlation with the ammonia fraction and initial pressure, but are positively correlated with the initial temperature, with peak values occurring at aroundϕ = 1.0 orϕ = 1.1 under all initial conditions. Adiabatic flame temperature is negatively related to the ammonia fraction and positively correlated with the initial temperature, but varies minimally with increasing initial pressure. Hydrodynamic instability is negatively related to the ammonia fraction and initial temperature but positively correlated with the initial pressure. When xNH3=80%, the Markstein length Lb and Markstein number Ma firstly decrease and then slightly increase as the equivalence ratios increase. The results of Lb and Ma indicate that the ammonia addition plays a primary role in the flame instability of ammonia/n-butanol blends at both ϕ≤0.9 and ϕ≥1.3, while the effect of n-butanol matters more at ϕ=1.0-1.2 under xNH3=80%. Both Rcr and Pecr increase with increasing ammonia fractions under Tin = 423 K, Pin = 0.6 MPa andϕ = 1.4. The findings indicate that the burning flux has a negative correlation with the ammonia fraction, whereas it is positively related to the initial pressure and initial temperature, with peak values occurring at around ϕ=1.0 under all tested conditions.