NUMERICAL INVESTIGATION ON TEMPERATURE DEPENDENCE OF FLAME BEHAVIOR IN HIGH PRESSURE DIRECT INJECTION COMBUSTION OF DIESEL PILOTED LIQUID AMMONIA SPRAYS

Ammonia is a viable carbon-free alternative to fossil fuels used in large internal combustion engines (ICEs). However, it is characterized by its reluctance to combust due to a high heat of vaporization, low flame speeds and high auto-ignition temperature. High-pressure dual-fuel (HPDF) combustion of ammonia, piloted by diesel, has been shown to overcome these unfavorable combustion characteristics. The stability of ammonia diffusion flame under these conditions determines the overall efficiency of combustion and pollutant formation. Nevertheless, the mechanisms governing ammonia flame behavior in HPDF combustion remain under-examined in the scientific literature. This work investigates the temperature dependence of flame behavior in the combustion of diesel-piloted liquid ammonia sprays using detailed chemistry simulations. A three-dimensional computational fluid dynamics (CFD) model is developed to simulate ammonia spray combustion in a rapid compression expansion machine (RCEM). Simulation results for evaporating and com-busting sprays are validated against experimentally obtained optical and heat release rate (HRR) data. Subsequently, numerical investigations are conducted using the validated CFD model to isolate the effect of ambient mixture temperature on ammonia flame stability by varying the temperature while maintaining an operating pressure of 125 bar. The resulting structure of the flame is discussed in depth at several stages of combustion for the relevant temperatures. It is shown that ammonia flame does not stabilize up to a temperature of 1100 K and drifts downstream of the flow. The study reveals the mechanism through which a strong spatial and temporal interaction between ammonia spray and diesel combustion products promotes flame stabilization in the HPDF combustion of ammonia at high temperatures. High ambient mixture temperatures lead to increased NOx formation.