CERFACS CERFACS Scientific Activity Report Jan. 2010 – Dec. 2011

TURBOMACHINERY
(a)
(b)
FIG. 3.7: Application of LES to the MUR235 test case : (a) instantaneous flow field colored by density
gradient gradρ/ρ (mark 1 is related to the normal shock and mark 2 to vortices produced by the
impact of freestream turbulence) and (b) wall heat transfer coefficient predicted with structured (elsA) and
unstructured (AVBP) flow solvers.
LES with wall laws is expected to decrease the computational cost by about one order of magnitude,
making the computation of the whole blade feasible. A first attempt of LES with wall law around the
turbine vane was performed with elsA. The same domain as for the wall-resolved LES is computed and
simple wall laws, namely the logarithmic law and Kader law, are used respectively for the velocity and
temperature fields. The computational cost is decreased by a factor close to 5. However, this conclusion
needs further investigations since several meshes and wall laws still need to be evaluated. Despite this
simple modelling, reasonable results are obtained on the second half of the pressure side and suction side.
One difficulty raised by this configuration is the laminar to turbulent transition of the boundary layer on
the suction side : wall laws, derived for fully turbulent flow, overpredict the wall heat flux in laminar regions.
3.2.8 Turbine flows : simulation of internal blade cooling devices
(R. Fransen, L. Gicquel, N. Gourdain)
The efficiency of aeronautical engines can be increased by raising the combustor outlet temperature, but this
rise in temperature can decrease the blade life duration if the cooling system is poorly designed. Today, for
the fluid in the main vein and within the cooling ducts, RANS modeling is routinely used. In that context,
LES can greatly improve the predictive capability of flow solvers.
LES of blade cooling ribbed channels have been computed with AVBP and compared with elsA RANS
results. PIV measurements performed by Casarsa [2] allow validations in several planes in the channel.
Comparison with experiment shows that LES gives very good agreement with experimental data, Fig. 3.8.
In contrast, RANS is not able to capture large-scale unsteadiness of the flow produced by the ribs in the
cooling channel. The thermal efficiency on the wall of the channel has also been computed yielding to
better LES predictions of cooling than RANS. Interestingly, the methodology developped here will be
used in 2012 to compute heat transfer and pressure losses in ribbed ducts for petrochemical applications
(partnership with Total).
154 Jan. 2010 – Dec. 2011