Modelling of liquid injection of ammonia in a direct injector using Reynolds-averaged Navier–Stokes simulation

Abstract

Ammonia as a fuel has gotten attention in the past years to enable decarbonization for internal combustion engines. There is a need to understand the behavior of the liquid fuel in direct injection engines, a crucial step of the engine cycle. Injection impacts the mixture formation in the cylinder, equivalence ratio, combustion and pollutants formation. Liquid ammonia is expected to behave significantly different than traditional fuels during the injection phase and hence requires to be investigated. Indeed, recent experimental research has highlighted the appearance of flash boiling during injection of ammonia spray under engine-relevant conditions. The high volatility is also expected to influence the cavitation behavior. Cavitation is the partial vaporization of the liquid typically caused by locally increased velocity resulting in a pressure drop, when the fluid enters an orifice with sharp edges. One parameter controlling cavitation is therefore the geometry, but cavitation is also influenced by the fuel’s property and the boundary conditions. This study presents 3-D RANS simulations performed with CONVERGE CFD of the internal flow of a Gasoline Direct Injector (GDI), operating with liquid ammonia. The transient simulations account for the injector needle movement. Simulations capture the presence of both vapor and liquid in the nozzle head. These results from the simulations will provide input data for separate spray simulations with the same engine geometry, as well as support the development of a 0-D model that will be important for design purposes. Preliminary results predict a liquid discharge coefficient of less than 0.1 at the outlet for injection in atmospheric conditions.