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A Holistic View on Urea Injection for NOx Emission Control: Impingement, Re-atomization, and Deposit Formation

  • SPECIAL ISSUE: 2019 MODEGAT September 8-10, Bad Herrenalb, Germany
  • Published:
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Abstract

The injection of urea-water solution (UWS) sprays into engine-close selective catalytic reduction (SCR) systems faces developers with complex challenges regarding two-phase flow and deposit formation. Potential spray impact on the static mixing elements inside the exhaust duct can lead to accumulation of liquid film and solid deposits, which may result in detachment of large UWS droplets at the trailing edges of the mixer blades. Present work focuses on the mechanisms of film and deposit formation within the scope of a joint study on spray preparation in the mixing section of SCR systems. A production type SCR exhaust system is used for optical investigations on gas flow and UWS droplets at the mixing element using Laser Doppler Anemometry (LDA) and Long Distance Microscopy (LDM) with a high-speed camera. Measured shear flows inside the mixing element are up to three times higher than the flow velocity upstream the mixer. Here, a decrease of droplet diameters of detached droplets is revealed with increased shear flow velocity. Characteristic urea deposits are created in a laboratory test bench and their surface structure is measured by use of Confocal Microscopy. Results show highly rough surface structures, which are used as input parameters together with the flow measurements to define the computational domain at the trailing edge of the mixer blades. Smoothed Particle Hydrodynamics (SPH) method is adopted for the numerical simulation of the transient two-phase flow and validated by LDM high-speed recordings of droplet detachment. Comparisons reveal a distinct influence of solid depositions on the re-atomization of the liquid film at the rear edge of the mixer blade. By interface resolving simulations with a phase-field method, the wall impingement of a sequence of large secondary droplets and associated film formation are studied. The results indicate that UWS film formation at the exhaust pipe wall may be reduced by hydrophobic surfaces and high gas velocities near the wall.

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Notes

  1. Continuous light power of the laser system was measured with a Coherent Fieldmaster FM

  2. Frequency is calculated by counting shedding events in the raw pictures, considering recording frequency

  3. Frequency is calculated from an extended simulation for the clean blade—case A configuration. Number of shedding events are counted as 11 for 0.213 s of simulation time.

  4. 50 Hz corresponds to the measured air velocity of 50 m/s.

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Acknowledgements

We would like to thank Mr. Daniel Kümmel from the Institute for Applied Materials, KIT for his assistance with confocal microscopy measurements. We would also like to thank Shuo Wang from the Institute of Technical Chemistry and Polymer Chemistry, KIT for assistance in conducting some preliminary numerical simulations. SPH and phase-field simulations were performed on the computational resource bwUniCluster funded by the Ministry of Science, Research and Arts Baden-Württemberg.

Funding

This collaborative study is partially financially supported by the Friedrich and Elisabeth Boysen Foundation.

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Authors and Affiliations

  1. Karlsruhe Institute of Technology (KIT), Institute of Internal Combustion Engines, Rintheimer Querallee 2, 76131, Karlsruhe, Germany

    J. Dörnhöfer, J. Pfeil & T. Koch

  2. KIT, Institute for Chemical Technology and Polymer Chemistry, Karlsruhe, Germany

    M. Börnhorst & O. Deutschmann

  3. KIT, Institute of Thermal Turbomachinery, Karlsruhe, Germany

    C. Ates, R. Koch & H.-J. Bauer

  4. KIT, Institute of Fluid Mechanics, Karlsruhe, Germany

    N. Samkhaniani & B. Frohnapfel

  5. KIT, Institute of Catalysis Research and Technology, Karlsruhe, Germany

    M. Wörner & O. Deutschmann

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  1. J. Dörnhöfer
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  3. C. Ates
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Correspondence to J. Dörnhöfer.

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Dörnhöfer, J., Börnhorst, M., Ates, C. et al. A Holistic View on Urea Injection for NOx Emission Control: Impingement, Re-atomization, and Deposit Formation. Emiss. Control Sci. Technol. 6, 228–243 (2020). https://doi.org/10.1007/s40825-019-00151-0

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