Experimental and CFD simulation study of heated and cold Diesel Exhaust Fluid Spray characteristics

This paper investigates heated and cold Diesel Exhaust Fluid (DEF) sprays with the aim of establishing the effect of temperature on the resulting spray characteristics. The work is motivated by the need to optimize active Selective Catalytic Reduction (SCR) systems to meet more stringent NOx emission regulations for internal combustion engines. Pre-heating DEF has the potential to improve evaporation of the injected fluid, increasing the NOx conversion efficiency of the SCR at low exhaust temperatures. Experiments are carried out using the MAHLE SmartHeat fluid heater and mounted atop a DEF injector, with an incorporated thermocouple for fluid temperature. The fluid temperature established by the heater in this configuration was about 130 °C. The fluid is injected into an atmospheric environment and Schlieren imaging is used to visualize the spray evolution. CFD Simulations are also carried out to validate the experimental observations and further shed light on the associated velocity field. With respect to the spray characteristics, the results show that the most significant difference between cold and heated DEF sprays occurs when the heated fluid experiences flash boiling prior to injection. This flash boiling is thought to arise from occasional overheating near the heating surfaces. Under conditions of normal hot liquid DEF injection, only a slight improvement in the atomization pattern is observed. The merit of the heating is expected to be more pronounced in the subsequent evaporation of the droplets. With respect to the simulations, it was found that the predicted penetration depths agree with the experimental observations. The observed pronounced differences between the characteristics of the cold and hot spray with flash boiling as well as the limited improvements of the hot liquid spray characteristics are explainable by considering their Reynolds and Weber numbers.