Research On Modeling Of Critical State Of Deposits Formation On The Exhaust Pipe Wall Of The Diesel Engine Equipped With Urea-SCR System

Urea-SCR system CFD research model and deposit calculation model based on frequently-used droplet and wall interaction model can not effectively predict the critical state of deposit formation.A large number of studies have observed that the critical state is accompanied by the disappearance of the Leidenfrost phenomenon.Therefore,through exploring whether the critical state of deposits formation on exhaust pipe can be predicted effectively by calculating the Leidenfrost point,a critical state model of deposit formation on exhaust pipe can be established.(1)The Leidenfrost point model is established:? For the spray with low volumetric flux,the interaction between spray droplets can be neglected,which can be regarded as numerous single droplets impinging on pipe wall.At this time,the disappearance of the Leidenfrost phenomenon means the depositing of the single droplet.According to the force balance relationship of the interface between the droplet and the vapor layer,the thickness of the vapor layer of the Leidonfrost droplet can be calculated.Taking surface roughness of exhaust pipe as criterion,the single droplet Leidenfrost phenomenon can be judged and the single droplet Leidenfrost model is established.?For the spray with high volumetric flux,the interaction between droplets is enhanced,which can not be regarded as numerous single droplets impinging on pipe wall respectively.At this time,disappearance of the Leidenfrost phenomenon means that the spray directly contact the wall of exhaust pipe.Taking spray Leidonfrost temperature as criterion,which comparing with initial temperature of pipe wall,the dense spray Leidenfrost phenomenon can be judged and the dense spray Leidenfrost model is established.(2)A critical state calculation model for deposit generation on exhaust pipe of diesel engine Urea-SCR system was established:Based on a the bench selecting several typical operating conditions from usual range,the critical ablue flow rate which can represents the critical state was measured.And then experiment data of critical adblue flow are compared with simulation data of critical adblue flow,which is calculated using single Leidenfrost model and dense spray Leidenfrost model respectively taking the bench as the simulation object.Found:When spray volume flux is<7× 10-4 m3/(m2·s),single droplet Leidenforst model has a good imitate accuracy and critical state corresponding to disappear of dense spray Leidonfrost phenomenon;When spray volume flux is>7 × 10-4 m3/(m2·s),dense spray "Leidenforst" model has a low imitate precision.The reason for that low precision is single droplet deposited can not cause accumulation of wall film,which can be evaporated and decomposited by hot surface,so at this time critical state depend on capacity of evaporation and them oly sis of hot pipe wall.Accordingly,through considering evaporation and thermal themolysis when spray volume flux is<7 × 10-4 m3/(m2·s),the model of critical adblue flux is established and valid prediction of critical state is realized.Wall temperature curve proves that the relation between critical state and Leidenfrost phenomenon:For the operating condition whose critical volume flux is>7 × 10-4m3/(m2·s),critical state corresponds to disappear of Leidenfrost phenomenon with wall temperature drop;For the operating condition whose critical volume flux is<7 × 10-4 m3/(m2·s),there is no change in way of heat-exchange when critical state is reached.(3)The application of critical state model:The critical state calculation model is used to study the influence of the main factors affecting deposits formation critical ablue flux to provide the optimization direction for each factor.The results show that the critical ablue flux increases with the increase of exhaust temperature,space velocity and number of nozzle holes,but increases with the decrease of angle of injection direction and thermal conductivity of exhaust pipe wall.