2.4 Advanced RANS tools for combustion
Despite the progress of LES, many industrial applications are still easier to handle with classical RANS (Reynolds Averaged Navier-Stokes) methods. CERFACS develops activities in this field along three directions:
(1) hybrid numerical methods for RANS combustion codes, (2) coupling tools for shape and regime optimization using RANS tools
and (3) cross comparison of RANS and LES codes. RANS codes can be used to initialize LES for example or in an integrated design chain: this can be done efficiently only if both approaches give sufficiently close results. Therefore understanding the limits of RANS and LES is important.
2.4.1 Hybrid numerical methods for RANS (V. Auffray , L. Gicquel , G. Puigt )
The RANS code used by SAFRAN for RANS computations is N3S-Natur (developped by INCKA-SIMULOG). It is routinely employed for the evaluation of the next generation of aeronautical gas turbines. However, current N3S-Natur versions are limited to triangular and tetrahedric cell topologies. In an effort to reduce the associated computational cost when applied to ever more complex geometries, CERFACS participates in the development of advanced numerical schemes to handle various cell topologies. This work constitutes the bulk work of the PhD dissertation of Valérie Auffray conducted in conjunction with the INCKA specialists and under the supervision of the SAFRAN group. A strong link with the methods used at CERFACS for LES in AVBP is favored for future interaction between the two codes.
2.4.2 Shape and regime optimization for reacting flows (F. Duchaine , L. Gicquel )
Present operating conditions of combustion chambers are essentially dictated by mechanical limits of the components located downstream of the chamber; the chamber design must be tuned so as to satisfy pre-requisites on the temperature profile at the outlet of the chamber for example. The aim of this work (performed within the EC project INTELLECT DM) is to apply to combustion, closed loop optimisation methods already developed and validated in other fields (aircraft, structures and antenna designs). The coupling device PALM (develop within CERFACS by the Algo team) is used to develop an optimisation loop based on RANS calculations: N3S-Natur - SAFRAN's production code. Suitable test configurations have already been evaluated to assess the automatic chain and the optimisation algorithms implemented at CERFACS. Different cost functions and optimization methods are available
[Duchaine, 2005]
and 3D optimization tests on real combustors are currently conducted.
2.4.3 Comparison of RANS and LES (F. Duchaine , A. Roux , L. Gicquel , T. Poinsot )
Comparing RANS and LES results on the same burner is an interesting exercice. A direct comparison of the time averaged solutions obtained with the two approached and for the same configuration constitutes a first step toward the proper use of both methods by industry. Figure 2.21 presents solutions from RANS and LES computations (N3S-Natur and AVBP respectively) for an industrial gas turbine of SAFRAN. Such an assessment allows gauging of each approach when compared with the experiment but also evidences the necessity of a proper treatment if RANS is to be considered as an initial solution for LES.
(a) 
(b) 
Figure 2.21: Comparisons of (a) mean temperature and (b) mean axial velocity obtained with N3S-Natur and AVBP for an aircraft burner.
|
|
|
