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

COMBUSTION
the non-zero Mach number effects on the frequency of oscillation and growth rate of the thermo-acoustic
modes of a quasi-1D academic configuration [21] were obtained. A methodology to predict the transient
growth generated when several non-normal stable modes combine was also derived [CFD109].
2.3 Heat transfer and multi-physics
2.3.1 Radiation (J. Amaya, D. Poitou,T. Pedot, F. Duchaine, B. Cuenot, M. el Hafi)
Radiative heat transfer has a non-negligible impact on flames. In small burners, the radiated energy is small
compared to the combustion energy, and can be neglected in the energy balance, but it may locally modify
the gas temperature and the subsequent production of pollutants such as NOx or soot. In large burners or
fires, radiation becomes the main heat transfer process and controls the flame. In combustors, the radiative
heat transfer is not limited to wall exchanges, as hot products such as water vapor or CO2 have the capacity
to both absorb and emit thermal radiation. As a consequence, radiation calculations in combustors must
solve the Radiative Transfer Equation (RTE) in the gas, taking into account their spectral behavior.
In collaboration with M. el Hafi from EMAC, a radiation solver (PRISSMA) has been developed, using the
Discrete Ordinate Method (DOM) and several spectral models for the gas [CFD105]. This solver has been
used in the PhD of J. Amaya [CFD168] to study the impact of radiation on the temperature distribution at
the exit of the combustion chamber of a helicopter engine [CFD1]. A similar work was started by D. Poitou
(post-doc) in the framework of the STRASS project (FNRAE), and is now continued by F. Duchaine.
Finally, PRISSMA was also used by T. Pedot (PhD, defended in Feb. 2012) to calculate heat transfer in a
refinery furnace, in order to predict the occurrence of coking in the heating tubes (Fig. 2.9).
In all these problems, one critical parameter is the wall temperature, usually unknown. To determine this
wall temperature, coupling of the radiative, convective and conductive (in the solid wall) heat transfer is
performed (see Section 2.3.3).
Due to the non-local nature of the RTE and the complexity of gas spectra, radiation calculations are
extremely demanding in terms of CPU costs. Important efforts have been already made to reduce this cost,
mainly directed towards simplified spectral models and increased parallelism over the discrete directions of
the RTE and the frequencies of the spectra. Recently a significant step has been made, with the successful
implementation of parallel domain decomposition, which raises particular difficulties due to the sequential
nature of the RTE solving algorithm.
2.3.2 Fluid-structure interaction in Solid Rocket Motors (J. Richard, F. Nicoud)
Solid Rocket Motors (SRM) may be subjected to thrust oscillations which might jeopardize the integrity
of the payload due to vibrations. The phenomenon has been investigated extensively over the last decades.
This mechanism arises from a coupling between the acoustic mode and the hydrodynamic perturbation,
as represented in Fig. 2.10(left). An unstable shear layer in the mean flow produces vortices which are
convected until they impact the head of the nozzle. The acoustic wave generated by this impact can move
back upstream since the mean flow is subsonic. It then perturbs the unstable shear layer, intensifying the
generation of vortices. Such an aero-acoustic mechanism can lead to high amplitude fluctuations when the
underlying frequency is close to the frequency of an acoustic mode of the whole geometry. A numerical
chain was built in order to assess to what extent the coupling between the fluid flow and the engine structure
(Fig. 2.10 (right)) influences the amplitude of the aeroacoustic oscillations within the combustion chamber.
A particular attention was paid to the coupling algorithm between the fluid and the solid solvers (AVBP
and MARC respectively, coupled with Open-PALM) in order to ensure energy conservation through the
interface [CFD56, CFD57].
140 Jan. 2010 – Dec. 2011