Predictions of Urea Deposit Formation with CFD using Autonomous Meshing and Detailed Urea Decomposition

Pengze  Yang, Convergent Science Inc.

Urea-water solution (UWS) injection combined with Selective Catalytic Reduction (SCR) has developed as an effective method of meeting EPA and EURO NOx emissions regulations for diesel engines. Urea/SCR systems encompass a wide range of engine sizes from light duty vehicles to large ships or power generation engines. One key challenge faced by modern urea/SCR systems is the formation of solid deposits of urea decomposition by-products that are difficult to remove. These deposits are proven to be detrimental to urea/SCR systems by clogging injector nozzles and increasing pressure drop. Urea deposits only form in a narrow range of wall temperatures and take many minutes to hours to form. The decomposition of urea into deposits begins with the formation of biuret and then progresses into the crystalline species cyanuric acid (CYA), ammelide and ammeline. Computational Fluid Dynamics (CFD) has the potential of predicting urea deposit formation provided the simulation time to achieve deposit formation can be shortened to a time-frame acceptable to modern design cycles. The autonomous CFD meshing code used in this study incorporates a detailed urea decomposition mechanism with Conjugate Heat Transfer (CHT) and spray-wall interaction models to predict wall temperatures with filming. The CFD code also takes advantage of the fixed flow approach and spray evaporation sub-cycling to extensively accelerate the flow and spray simulation to reach the time scale required for appreciable deposit formation. The prediction approach is applied to a practical exhaust system urea deposit test bench published by Brack et al, 2016. Three operating points of differing exhaust temperatures and flow rates are evaluated with the CFD code. The three operating points represent temperatures that are below, inside and above the urea deposit temperature range. The results include predictions of exhaust pipe wall temperatures, film shape and deposit composition.

Reference:
1.Sun, Y., Sharma, S., Vernham, B., Shibata, K. and Drennan, S., “Urea Deposit Predictions on a Practical Mid/Heavy Duty Vehicle After-Treatment System,” SAE Technical Paper 2018-01-0960, 2018, doi:10.4271/2018-01-0690.

2.Maciejewski, D., Sukheswalla, P., Wang, C., Drennan, S.A. and Chai, X., “Accelerating Accurate Urea/SCR Film Temperature Simulations to Time-Scales Needed for Urea Deposit Predictions,” SAE Technical Paper 2019-01-0982, 2019, doi:10.4271/2019-01-0982.

3.Brack, W., Heine, B, Birkhold, F., Kruse, M. and Deutschmann, O., “Formation of Urea-Based Deposits in an Exhaust System: Numerical Predictions and Experimental Observations on a Hot Gas Test Bench,” Emission Control Science and Technology (2016) 2:115–123