Analysis of explosion and laminar combustion characteristics of premixed ammonia-air/oxygen mixtures

Abstract

Ammonia is suggested to be a potential carbon-free energy source because of its high volumetric energy density and has attracted attention recently. However, systematic investigations into the explosion characteristics and flame instability of ammonia are still lacking, particularly for low-pressure ammonia-oxygen mixture. The use of low-pressure ammonia-oxygen mixtures could not only benefit for increasing engine's efficiency but also enhance its operational safety. In this study, explosion experiments of premixed ammonia-air/oxygen mixtures are carried out in a 20-L spherical chamber. By adjusting the initial pressure (P0) and equivalence ratio (ϕ), the maximum explosion pressure (Pmax) and maximum rate of pressure rise ((dP/dt)max) are measured. The laminar flame characteristics are analysed simultaneously. The results show that for the ammonia-air mixture, Pmax increases as P0 increases and reaches the peak at ϕ = 1.1; the unstretched laminar burning velocity (SL), Markstein length (Lb), and laminar flame thickness (δ) decrease as the initial pressure increases and reach the peak at ϕ = 1.1. For the ammonia-oxygen mixture, the explosion parameters linearly increase as P0 increases, while the maximum values of the parameters are obtained at ϕ = 1.0; the unstretched laminar burning velocity increases first and then gradually decreases as P0 increases; the maximum SL is 1.14 m/s at P0 = 0.1 MPa and ϕ = 1.0, which is 18 times that of the premixed ammonia-air mixture under the same condition. The schlieren images demonstrate that the buoyancy instability has significant impacts on the premixed ammonia-air flame propagation, while it has negligible impacts on the premixed ammonia-oxygen flame propagation. The effect of hydrodynamic instability on flame propagation could be observed in both the premixed ammonia-air flame at ϕ > 1.0 and the premixed ammonia-oxygen flame at P0 > 0.1 MPa and ϕ < 1.0. In addition, it is proposed that for the premixed ammonia-air flame, the buoyancy instability would remarkably influence the flame propagation when the unstretched laminar burning velocity is < 5 cm/s. The fundamental research on the combustion characteristics of ammonia-air and ammonia-oxygen in this study can provide an experimental reference for the later development of chemical reaction mechanisms, especially for the mechanism suitable for ammonia combustion in oxygen-rich environments. Meanwhile, the combustion characteristics of the ammonia-oxygen mixture at ultra-low pressure also provide a data basis for the possibility of using ammonia fuel as a propellant for hypersonic vehicles in the future.