V. L. ZIMONT ET AL.

Copyright © 2011 SciRes. EPE

623

Figure 10. 3D instantaneous surface with the temperature T

= 2200 K from the 3D animation.

the averaged fields, could strongly affect on NOx pro-

duction. So optimization of the injector system in the

context of the pollution problem is significant. We think

that this optimization would be more effective using nu-

merical data of the LES sub-problem. non-uniformity of

the temperature field especially at first part of burner

strongly depends on the used injector system. It is obvi-

ous that peculiarities of instantaneous non-uniformity of

the temperature, which have limiting effect on the aver-

aged fields, could strongly affect on NOx production. So

optimization of the injector system in the context of the

pollution problem is significant. We think that this opti-

mization would be more effective using numerical data

of the LES sub-problem.

5. Conclusions

1) We present the original timesaving joint RANS/LES

approach to simulate flameless combustion with separate

injection of gas fuel and strong exhaust gas recirculation.

It is based on successive RANS and LES numerical

modeling where some part of the information (stationary

average fields) is achieved by RANS simulations and

another part (instantaneous nonstationary image of the

process) by LES. The latter is performed using the

RANS field of the mean dissipation rate, which is used

for modeling of the subgrid turbulence and subgrid vis-

cosity in the context of the Kolmogorov theory of equi-

librium small-scale turbulen ce.

2) Timesaving is achieved 1) due to use of the subgrid

turbulent transport coefficient from the Kolmogorov the-

ory of small-scale turbulence instead of the traditionally

used Smagorinsky model (and it results in the possibility

to increase the time step in our LES approximately five

times) and 2) due to avoiding time averaging of LES data,

which need long-continued simulations, which are ine-

vitable in an approach “LES instead of RANS”. In our

RANS/LES approach ratio of necessary times for RANS

and LES sub-problems in practical application could be

the same order of magnit ude (practically ~1 - 5).

3) For validation of our approach we performed long-

continued large eddy simulations. Presented time aver-

aged LES data and RANS results are in reasonable agree-

ment.

4) Though RANS results for the flameless regime

demonstrate distributed combustion with smooth profiles

of the temperature and velocity, corresponding instanta-

neous LES fields show significant nonuniformity of the

temperature and clear documented large-scale eddies

especially at initial part of the burner. As these tempera-

ture surges can be significant for NOx emission the

burner and especially the injection system need optimi-

zation. We think that using for this optimization the re-

sults of the LES sub-problem could be more effective in

comparison with using of the RANS one.

6. References

[1] J. A. Wünning and J. G. Wünning, “Flamneless Oxida-

tion to Reduce Thermal NO-Formation,” Progress in En-

ergy and Combustion Science, Vol. 23, No. 1, 1997, pp.

81-94. doi:10.1016/S0360-1285(97)00006-3

[2] A. Cavaliere and M. De Joannon, “Mild Combustion,”

Progress in Energy and Combustion Science, Vol. 30, No.

4, 2004, pp. 329-366. doi:10.1016/j.pecs.2004.02.003

[3] T. Hasegawa and R. Tanaka, “High Temperature Air Co m-

bustion: Revolution in Combustion Technology,” JSME

International Journal, Series B, Vol. 40, No. 4, 1998, pp.

1079-1084

[4] J. G. Wünning, “FLOX®—Flameless Combustion,” Therm-

pr ocess Symposium VDMA, Düsseldorf, 2003.

http://www.flox.com/de_documents/Paper64%20tp03.pdf

[5] J. G. Wünning, “Flameless Combustion and its Applica-

tions,” 14th IFRF Members Conference, Noordwijkerhout,

2004. http://www.flox.com/de_documents/pra00045.pdf

[6] P. J. Coelho and N. Peters, “Numerical Simulation of a

Mild Combustion Burner,” Combustion and Flame, Vol.

124, No. 3, 2001, pp. 503-518.

doi:10.1016/S0010-2180(00)00206-6

[7] S. Murer, B. Perenti and P. Lybaert, “Simulation of Fla-

meless Combustion of Natural Gas in a Laboratory Scale

Furnace,” Turkish Journal of Engineering & Environ-

mental Sciences, Vol. 30, No. 3, 2006, pp. 135-143.

[8] V. Zimont and V. Battaglia, “Joint RANS/LES Numrical

Simulations of Premixed Combustion at Strong Tuibu-

lence,” Ninth International Conference on Numerical Com-

bustion (SIAM), Sorrento, 7-10 April 2002, Pap er No . 14 1,

pp. 225-226.

[9] V. L. Zimont and V. Battaglia, “Joint RANS/LES Ap-

proach to Premixed Flame Modelling in the Context of

the TFC Combustion Model,” Flow, Turbulence and Com-

bustion, Vol. 77, No. 1-4, 2006, pp. 305-331.