Evaluation AC3-12: Difference between revisions

From KBwiki
Jump to navigation Jump to search
Line 43: Line 43:
|align="center" width=650px|'''Figure 11:''' Comparison between measurements and numerical calculations for the gas-phase in Case 2: (a) axial mean velocity; (b)  radial  mean velocity; (c) tangential mean velocity; (d) turbulent kinetic energy.
|align="center" width=650px|'''Figure 11:''' Comparison between measurements and numerical calculations for the gas-phase in Case 2: (a) axial mean velocity; (b)  radial  mean velocity; (c) tangential mean velocity; (d) turbulent kinetic energy.
|}
|}
The measured and calculated particle mean velocities and the associated
velocity fluctuations are compared in Figure 12 and 13 again  for  case
2. All mean velocity components are  generally  well-predicted,  except
for the radial velocity which is predicted to be positive at z = 315 mm
(i.e. the particles move towards the wall) whereas the experiments show
negative velocities. This implies that in the experiment the  particles
move on average away from the wall. Although, it might be expected that
the numerical results in the near-wall region are very  sensitive  with
respect to the modelling of the wall collision process,  it  was  found
that a variation in the  normal  restitution  ratio  and  the  friction
coefficient in the three-dimensional inelastic collision model did  not
result in considerable changes in the velocity profiles. The scattering
of the numerical results in the core region downstream of the location,
where the central recirculation bubble has its largest radial extension
is a result of  the  low  number  of  collected  particles,  since  the
majority of the particles have already moved out of the core region.
The particle velocity fluctuations are more or less under-predicted for
all three velocity components, which presumably is caused by the under-
prediction of the gas-phase turbulent kinetic  energy  in  the  initial
region of flow development (Figure  11(d)).  This  is  demonstrated  in
Figure 12(b)  for  the  fluctuation  of  the  axial  particle  velocity
component.
<br/>
<br/>
----
----

Revision as of 10:26, 12 February 2013

Front Page

Description

Test Data

CFD Simulations

Evaluation

Best Practice Advice

Particle-laden swirling flow

Application Challenge AC3-12   © copyright ERCOFTAC 2013

Comparison of Test Data and CFD

A rather good agreement between the experiments and predictions was obtained for gas and particle phase in both swirling cases considered. The comparison of the calculated streamlines of the gas flow with those obtained from the integration of the measured axial velocity shows that the flow field is predicted reasonably well for both conditions (Figure 10). The most obvious difference is that the axial extension of the central recirculation bubble is predicted to be larger at the top and downstream ends for both cases. The predicted width of the central recirculation bubble and the extension of the recirculation at the edge of the pipe expansion are in good agreement with the measured results.

AC3-12 fig10.png
Figure 10: Measured and calculated gas-phase streamlines (the upper parts of each figure corresponds to the calculations and the lower parts show the measurements); (a) Case 1; (b) Case 2.

The measured cross-sectional profiles of the three velocity components are compared with the calculations in Figure 11 for Case 2. The agreement is very good, except for the tangential velocity which is under-predicted in the region downstream of the location where the recirculation bubble has its largest radial extension. Although, the turbulent kinetic energy of the gas phase is considerably under- predicted in the initial mixing region between the primary and annular jets and within the recirculation at the edge of the pipe expansion (z = 52 mm), the agreement is reasonably good for the cross-sections further downstream. Similar results have been obtained for swirl Case 1 which was summarized in a previous publication (Sommerfeld et al. 1992).

AC3-12 fig11.png
Figure 11: Comparison between measurements and numerical calculations for the gas-phase in Case 2: (a) axial mean velocity; (b) radial mean velocity; (c) tangential mean velocity; (d) turbulent kinetic energy.

The measured and calculated particle mean velocities and the associated velocity fluctuations are compared in Figure 12 and 13 again for case 2. All mean velocity components are generally well-predicted, except for the radial velocity which is predicted to be positive at z = 315 mm (i.e. the particles move towards the wall) whereas the experiments show negative velocities. This implies that in the experiment the particles move on average away from the wall. Although, it might be expected that the numerical results in the near-wall region are very sensitive with respect to the modelling of the wall collision process, it was found that a variation in the normal restitution ratio and the friction coefficient in the three-dimensional inelastic collision model did not result in considerable changes in the velocity profiles. The scattering of the numerical results in the core region downstream of the location, where the central recirculation bubble has its largest radial extension is a result of the low number of collected particles, since the majority of the particles have already moved out of the core region.

The particle velocity fluctuations are more or less under-predicted for all three velocity components, which presumably is caused by the under- prediction of the gas-phase turbulent kinetic energy in the initial region of flow development (Figure 11(d)). This is demonstrated in Figure 12(b) for the fluctuation of the axial particle velocity component.



Contributed by: Martin Sommerfeld — Martin-Luther-Universitat Halle-Wittenberg

Front Page

Description

Test Data

CFD Simulations

Evaluation

Best Practice Advice


© copyright ERCOFTAC 2013