Investigation of NH3/Air Laminar Burning Speed and Flame Structure at High Pressures

Abstract

Abstract Understanding the effect of fuel properties on ammonia (NH3) flame characteristics is important for practical applications. This study investigated NH3/air mixture ignitability, flame morphology, and propagation rate in a constant volume combustion chamber (CVCC) at high-pressure conditions using high-speed schlieren imaging. Experiments were performed at an equivalence ratio, ϕ, from 0.7 to 1.3, initial pressures from 1 atm to 10 atm, and an initial temperature of 298 K. The study also used a novel filling method that accounts for the NH3 adsorption on the stainless steel CVCC surfaces. While the data analysis suggested that the NH3 adsorption was negligible, the CVCC passivation before the start of the experiments (i.e., allowing enough time for NH3 to adsorb on the chamber walls) reduced the experimental uncertainty with respect to the actual NH3/air (ϕ) inside the CVCC. More, the low speed of NH3/air flames can negatively affect flame stretch and laminar burning speed (LBS) calculations for mixtures at high initial pressures due to the significant buoyancy effects. Hence, a special method accounting for buoyancy was implemented to calculate the volume of the combustion products and, subsequently, to compute the LBS of the non-spherical NH3/air flames. The high-speed schlieren imaging showed that, as the pressure increased, the flames got smaller, indicating a decrease in speed and therefore an increase in buoyancy. Cellular structures were absent during the period of flame propagation inside the chamber. The conventional constant pressure methodology was used for the LBS calculations. Results indicated that for the evaluated conditions, the LBS increased with ϕ until reaching a maximum value for ϕ = 1.1. In addition, it was shown that as the initial pressure increased, the LBS value decreased, similar to the conventional trend seen for hydrocarbons LBSs.