In exhaust systems with selective catalytic reduction (SCR) a
fast conversion of liquid urea to gaseous ammonia and a uniform
distribution of the ammonia vapor upstream of the SCR catalyst are
essential to reduce the nitric oxides efficiently. For the
prediction of the mixing process and the transport of ammonia vapor
with the CFD method an accurate description of the turbulent flow
field is a basic requirement.
This paper presents the comparison of simulation results using
three different turbulence models (high-Re kε-RNG model, low-Re
kω-SST model, Reynolds stress model) with measurements of the
turbulent velocity field using Laser Doppler Anemometry (LDA). The
investigations were carried out for a SCR system with a swirl mixer
on a cold flow test bench for two different volume flows. From the
measured velocity signals different components of the
Reynolds-tensor were derived. All three turbulence models are
showing a similar mean velocity field as it was expected and there
is a good agreement with the LDA data. The turbulence intensity and
subsequently the turbulent mixing are underestimated for all
simulation setups in comparison to the LDA measurements. However,
the anisotropic formulation of the turbulence intensities with the
Reynolds stress model (RSM) resolves the flow field more accurately
than the two-equation turbulence models.
However, further work on a more detailed formulation of the
turbulent flow field (DES, LES) is necessary to improve the
prediction of the turbulent species transport.