Optimization Of Mixing Flow Distribution Of Low Pressure Urea-SCR Reducing Agent

In the urea selective catalytic reduction(Urea-SCR)system,the mixing quality of reducing agent and exhaust gas is directly related to NOx conversion rate,ammonia escape rate,catalyst life and formation of sediments.Due to the limited space of the ship engine room,the SCR reactor and its piping arrangement are difficult,this problem is particularly prominent for the two stroke diesel engine SCR reactor.To achieve reducing agent and smoke in a short distance at lower cost to achieve a good mixture of reductant and smoke is the main research direction.The paper use computational fluid dynamics(CFD)commercial software FLUENT of internal air assisted atomization nozzle flow state,the atomization process of diesel engine and 100%load reduction of mixed agent and mixing characteristics of exhaust gas and part of the SCR reactor in the pipeline.Firstly,the gas-liquid mixing rule inside the nozzle is studied by studying the gas-liquid mixing state inside the nozzle,and the boundary conditions of nozzle atomization are obtained.The results show that the vapor-liquid interface required for stability time is about 70ms;internal mixing type air assisted atomization nozzle internal flow process is divided into:the liquid pressure energy into kinetic energy of the accelerated phase,between gas phase and liquid phase exchange the momentum again accelerated stage.Secondly,according to the experimental data to verify the Discrete Phase Model(DPM)compressed air assisted atomization nozzle was simulated with the feasibility and characteristics of atomization flue pipe shape optimization,the results showed that urea nozzle angle of 70 degrees of fog reduction agent can obtain the ideal shape.Thirdly,the mixer and the diverter are added to realize the good mixing of the reducing agent and the smoke exhaust in a short distance,and the structure and the position are optimized.Comparing the mixer with different structure can get the best right angle mixer.Orthogonal test was performed to obtain the most mixed blade angle,and the optimum mixer structure and position were determined.At the same time of diversion angle and expansion angle,the diverter has the best mixing effect.The combined use of mixer and guide the catalyst entrance section of reductant concentration evenness index was 0.892 higher than 0.85;mixture velocity uniformity index was 0.921,to meet the requirements of speed deviation is less than 10%;the mixer and the diffuser pressure drop is 225Pa,meet the requirement of less than 280Pa.When the mixer and the diverter are installed,the mixing distance is greater than or equal to 3.5 times the pipe diameter to meet the mixing requirements.Finally,according to the simulation results,the Urea-SCR system was tested.The results showed that the average NOx emission reduction rate of the SCR was 85%and the emission rate was 2.68 g/Kw·h,which could meet the requirements of Tier ? less than 3.4 g/Kw·h.