Explore Different Heat and Mass Transfer Models and Develop Tool to Predict Droplet Evaporation

Abstract

The main pollutant emissions from diesel engine are Unburned hydrocarbon (HC), Carbon monoxides (CO), Nitrogen Oxides (NOx) and Particulate matters (PM). They have significant impact on environmental pollutions. Especially, diesel exhaust gas contains higher amount of particulate matter and NOx emissions that are responsible of severe environmental and health problems. Selective Catalytic Reduction (SCR) is one of the most promising techniques for the abatement of the nitrogen oxides (NOx) emissions. For reasons of storage, safety and mobility, urea is the preferred selective reducing agent for mobile SCR applications. Urea is typically being used as an aqueous solution at its eutectic composition (32.5%wt. urea, marketed as AdBlue). The liquid mixture of urea and water is also known as Diesel Exhaust Fluid (DEF). In a typical SCR system, urea-water-solution (UWS) is injected into the hot engine exhaust leading to atomization and subsequent spray processes. In this system it is commonly believed that water vaporizes first and then urea undergoes thermolysis and decomposes into gas phase ammonia (NH3) and isocyanic acid (HNCO). In this study a tool to predict evaporation rate of UWS droplet is developed. To achieve this, very first step for this project is to gain the knowledge regarding water droplet evaporation, various process involved in droplet evaporation, theory and assumptions related with modelling of droplet evaporation and become capable to model water evaporation process. In this study first evaporation model for pure water is developed and validated with published experimental data, for stationary as well as moving droplet. In same fashion evaporation model for UWS droplet is developed and validated. Developed tool gives better results for temperature below 573K. Above 573 K the phenomenon like micro explosion become dominant and that increases the total evaporation rate of UWS droplet. This effect is not included in this tool so, ambience temperature above 573 K there is deviation in actual experimental data and results by developed tool. Finally, this tool is implemented for sets of droplet. The developed tool gives evaporation rate of each droplet in set of droplets for validation there is no such a data is available. So, validation is done with percentage convergence efficiency of ammonia.