Prediction of Spray Behavior in Injected by Urea SCR Injector and the Reaction Products 2017-01-2375
In the urea SCR system, urea solution is injected by injector installed in the front stage of the SCR catalyst, and NOx can be purified on the SCR catalyst by using NH3 generated by the chemical reaction of urea. NH3 is produced by thermolysis of urea and hydrolysis of isocyanic acid after evaporation of water in the urea solution. But, biuret and cyanuric acid which may cause deposit are sometimes generated by the chemical reactions without generating NH3. Spray behavior and chemical reaction of urea solution injected into the tail-pipe are complicated. The purpose of this study is to reveal the spray behavior and NH3 generation process in the tail-pipe, and to construct the model capable of predicting those accurately. In this report, the impingement spray behavior is clarified by scattered light method in high temperature flow field. Liquid film adhering to the wall and deposit generated after evaporation of water from the liquid film are photographed by the digital camera. NH3 concentration is measured at 13 points of the cross section of the tail-pipe by FTIR, and NH3 concentration distribution is calculated. From the experiment, the influences of gas temperature on droplets scattering after wall impingement and impingement conditions on NH3 concentration distribution are clarified. It is suggested that the deposit after evaporation of liquid film may be an intermediate product such as biuret or cyanuric acid. Also, by comparing and verifying the results of calculation using CFD software FIRE v2014.2 and the experimental results, the phenomenon influencing the prediction accuracy of the NH3 concentration distribution is clarified. Factors influencing the prediction accuracy of calculation are atomization characteristics after wall impingement for NH3 concentration, gas flow in the tail-pipe for NH3 concentration distribution and liquid film generation due to the decrease of the plate temperature for NH3 generation process.