Confidence interval analysis of urea water solution spray measurement data from Phase Doppler Anemometry using numerical validations from a commercial CFD code

Selective catalytic reduction (SCR) is the most promising technology being used to meet the increasingly stringent emission regulations. Urea water solution (UWS - 32.5% urea in water solution) is typically injected upstream the catalyst to release ammonia which reacts with the nitrogen oxides (NOx) in the exhaust gas to form harmless compounds. The efficiency of the whole process is highly dependent on the droplet characteristics of the injected UWS spray. To develop and optimize a good SCR system, it is essential to acquire the droplet characteristics using some experimental techniques and develop simulation models to assess the behavior of spray in different engine exhaust scenarios. Optically accessible test section of a purpose-built heated cross flow wind tunnel was used to record droplet characteristics using Phase Doppler Anemometry (PDA), which implies a fixed refractive index in the calculation of the droplet diameters when operating in refraction mode. CFD simulations were performed using a commercially available code, ANSYS FLUENT, and the results of hydrodynamic validations along with the methodology applied for recalculation of droplet diameters depending on the UWS droplet concentrations are discussed. The findings suggest that PDA can be used in 'first order refraction' mode to estimate the diameters of UWS droplets in cross flow within an underestimation of 5.8%.